Structural Determinants of Alkyne Reactivity in

molecules Article

Structural Determinants of Alkyne Reactivity in Copper-Catalyzed Azide-Alkyne Cycloadditions Xiaoguang Zhang, Peiye Liu and Lei Zhu * Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306-4390, USA; [email protected] (X.Z.); [email protected] (P.L.) * Correspondence: [email protected]; Tel.: +1-850-645-6813 Academic Editor: James Crowley Received: 2 November 2016; Accepted: 5 December 2016; Published: 9 December 2016

Abstract: This work represents our initial effort in identifying azide/alkyne pairs for optimal reactivity in copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions. In previous works, we have identified chelating azides, in particular 2-picolyl azide, as “privileged” azide substrates with high CuAAC reactivity. In the current work, two types of alkynes are shown to undergo rapid CuAAC reactions under both copper(II)- (via an induction period) and copper(I)-catalyzed conditions. The first type of the alkynes bears relatively acidic ethynyl C-H bonds, while the second type contains an N-(triazolylmethyl)propargylic moiety that produces a self-accelerating effect. The rankings of reactivity under both copper(II)- and copper(I)-catalyzed conditions are provided. The observations on how other reaction parameters such as accelerating ligand, reducing agent, or identity of azide alter the relative reactivity of alkynes are described and, to the best of our ability, explained. Keywords: alkyne; click chemistry; copper; reactivity; CuAAC

1. Introduction The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction [1,2] has been used as a principal molecular conjugation strategy by many in- and outside the chemistry community [3–14]. Identifying highly reactive azide and alkyne substrates of CuAAC carries the obvious benefit of increasing the efficiency of molecular conjugation. In bioconjugation applications, the additional benefit of applying reactive substrates is the reduced loading of copper catalyst that might be detrimental to cellular processes [15–17]. Our group reported the high reactivity of chelating azides in CuAAC [18–21]. These azides have subsequently been developed as bioconjugation tools applicable in living cells [22–26]. Despite the emphasis placed upon copper acetylide in the mechanistic description of CuAAC [27,28], there have not been as many studies on the reactivity of terminal alkynes in CuAAC reactions. Isolated examples of reactive alkynes appeared soon after the discovery of CuAAC. Ju and coworkers found that methyl propiolate (1, Figure 1) has high reactivity in both thermal and copper-catalyzed cycloadditions with azides [29]. Propiolamide 2 was the most reactive in a 18 F-radiolabeling study by Årstad and coworkers [30]. In the only systematic study on alkyne reactivity in CuAAC thus far, Finn and coworkers ranked the reactivity of thirteen alkynes under either bioconjugation or organic preparative conditions in the presence of an accelerating ligand [31]. Propiolamides (e.g., 3 and 4) were found to be more reactive than other tested alkynes [31]. Our group found that the reactivity of para-substituted phenylacetylene increases as the substituent becomes more electron-withdrawing in copper(II) acetate-catalyzed CuAAC reaction in acetonitrile [20]. p-Nitroethynylbenzene (5) was the fastest-reacting alkyne in that regard [20].

Molecules 2016, 21, 1697; doi:10.3390/molecules21121697

www.mdpi.com/journal/molecules

Molecules 2016, 21, 1697 Molecules 2016, 21, 1697 Molecules 2016, 21,21, 1697 Molecules 2016, 1697

2 of 17 2 of of 17 17 2 of217

Figure 1.Reported Reported fast-reacting alkynes in CuAAC. Figure Figure 1. 1. Reported fast-reacting fast-reacting alkynes alkynes in in CuAAC. CuAAC.

Compounds 1–5are are alkynes bearing bearing electron-withdrawing groups. Given the mechanistic Compounds mechanistic Compounds 1–5 1–5 are are alkynes alkynes bearing bearing electron-withdrawing electron-withdrawing groups. groups. Given Given the the mechanistic mechanistic Compounds 1–5 Given the revelations that the alkynealkynes deprotonation to electron-withdrawing afford copper(I) acetylidegroups. is turnover limiting in CuAAC revelations that the alkyne deprotonation to afford copper(I) acetylide is turnover limiting revelations that that the the alkyne alkyne deprotonation deprotonation to to afford afford copper(I) copper(I) acetylide acetylide is is turnover turnover limiting limiting in in CuAAC CuAAC revelations in CuAAC to the reactions [20,32], an electron-withdrawing group that would give a relatively low pKa value aa value to the reactions [20,32], an electron-withdrawing group that would give a relatively low pK value to the the reactions [20,32], an electron-withdrawing group that would give a relatively low pK reactions [20,32], an electron-withdrawing group that give a relatively low pK to a valuehigh Csp-H bond should enhance the alkyne reactivity. Bywould this reasoning, the recently reported C sp -H bond should enhance the alkyne reactivity. By this reasoning, the recently reported high Csp sp-H should enhance alkyne reactivity. By reasoning, recently high reported high C -H bond bond should of enhance the alkyne By this this reasoning, the recently reported high CuAAC reactivity alkyne 6the [33], in ourreactivity. opinion, could be attributed to the its relatively acidity. CuAAC reactivity of alkyne 6 [33], in our opinion, could be attributed to its relatively high acidity. CuAAC reactivity of alkyne 6 [33], in our opinion, could be attributed to its relatively high acidity. CuAAC reactivity of can alkyne 6 [33], to in explain our opinion, could be to itsreported relatively high7 acidity. The same rationale be applied the reactivity of attributed another newly alkyne [34]. The same rationale can applied to the of newly reported 777 [34]. The same rationale can be be applied to explain the reactivity of another newly reported alkyne [34]. The same rationale can be to explain explain the reactivity reactivity of another anotherin newly reported alkyne alkyne [34]. The lone-pair electrons onapplied the Csp-attached nitrogen in 7 is delocalized the benzimidazolyl group, The lone-pair electrons on the C sp -attached nitrogen in 7 is delocalized in the benzimidazolyl group, The lone-pair electronsacts onas the Celectron-withdrawing sp-attached nitrogen in 7 is delocalized in the benzimidazolyl group, sp -H bond. which consequently anC group to increase the acidity of the C The lone-pair electrons on the -attached nitrogen in 7 is delocalized in the benzimidazolyl group, sp sp -H which consequently acts as an electron-withdrawing group to increase the acidity of the C sp -H bond. which consequently consequently acts as an an electron-withdrawing electron-withdrawing group to increase increase the acidity acidity of the C Csp The purpose ofacts the as current work is to verify the high reactivity of relatively acidicof terminal alkynes which group to the the -H bond. bond. The purpose of the current work is to verify the high reactivity of relatively acidic terminal alkynes The purpose of the current work is to verify the high reactivity of relatively acidic terminal alkynes under copper(II) copper(I)-mediated CuAAC conditions, and of to relatively identify new classes of The both purpose of theand current work is to verify the high reactivity acidic terminal under both copper(II) and copper(I)-mediated CuAAC conditions, and to identify new classes of under both copper(II) and copper(I)-mediated CuAAC conditions, and to identify new classes of reactive alkynes. The azide reaction partner is chosen purposefully to be the chelating 2-picolyl azide. alkynes under both copper(II) and copper(I)-mediated CuAAC conditions, and to identify new classes reactive alkynes. The azide reaction partner is chosen purposefully to be the chelating 2-picolyl azide. Therefore, the identified fast-reacting terminal alkynes may work together with 2-picolyl azide to reactive alkynes. The azide reaction partner is chosen purposefully to be the chelating 2-picolyl azide. of reactive alkynes. The azide reaction partner is chosen purposefully to be the chelating 2-picolyl Therefore, the fast-reacting terminal alkynes may work together with 2-picolyl azide to afford a CuAAC substrate pairfast-reacting with optimal coupling efficiency. Therefore, the identified identified fast-reacting terminal alkynes maymay work together with 2-picolyl azide to azide. Therefore, the identified terminal alkynes work together with 2-picolyl azide

afford aa CuAAC substrate pair with optimal coupling efficiency. afford CuAAC substrate pair with optimal coupling efficiency. to afford a CuAAC substrate pair with optimal coupling efficiency. 2. Results and Discussion 2. 2. Results Results and and Discussion Discussion 2.1. Choice of Alkynes 2.1. Alkynes 2.1. Choice Choice oftested Alkynes Theof alkynes are separated in three categories: (a) electron-withdrawing group-bearing alkynes (Figure 2). These alkynes have relatively acidic Csp-H bonds. Therefore, deprotonation to afford The separated in three categories: electron-withdrawing The tested tested alkynes alkynes are are separated separated in in three three categories: categories: (a) (a) electron-withdrawing electron-withdrawing group-bearing group-bearing acetylide, which is rate-determining in both alkyne oxidative homocoupling and CuAAC reactions, alkynes alkynes have relatively acidic C Therefore, deprotonation to alkynes (Figure (Figure2). 2).These These alkynes have relatively acidic Cspbonds. -H bonds. Therefore, deprotonation to (Figure 2). These alkynes have relatively acidic Csp sp-H -H bonds. Therefore, deprotonation to afford afford would be fast. The known examples of highly reactive alkynes in CuAAC (e.g., 1–7) all fall into this acetylide, which rate-determining in homocoupling and afford acetylide, which is rate-determining in bothoxidative alkyne oxidative homocoupling andreactions, CuAAC acetylide, which is is rate-determining in both both alkyne alkyne oxidative homocoupling and CuAAC CuAAC reactions, category; (b) Alkynes with potentially copper-binding moieties (Figure 3). There have not been any would The known of reactive alkynes in (e.g., all into this reactions, would fast. examples The known examples of highly reactive alkynes in1–7) CuAAC would be be fast. fast. Thebe known examples of highly highly reactive alkynes in CuAAC CuAAC (e.g., 1–7) all fall fall(e.g., into 1–7) this studies on the reactivity of copper-binding alkynes. We are interested in this class of compounds category; (b) Alkynes with potentially (Figure 3). There have not been any all fall into this category; (b) Alkynes copper-binding with potentiallymoieties copper-binding moieties (Figure 3). There category; (b) Alkynes with potentially copper-binding moieties (Figure 3). There have not been any because the alkyne themselves or their CuAAC products may be accelerating ligands. For example, the studies the of on copper-binding We interested this class of have noton any studies the reactivityalkynes. of copper-binding alkynes.in interested in this studies onbeen the reactivity reactivity We are are interested in We thisare class of compounds compounds product between alkyneof 21copper-binding and benzyl azidealkynes. in a CuAAC reaction is tris[(1-benzyl-1H-1,2,3-triazolbecause the alkyne themselves or their CuAAC products may be accelerating ligands. For example, the class of compounds because the alkyne themselves or their CuAAC products may be accelerating because the alkyne themselves or their be accelerating ligands. example, 4-yl)methyl]amine (TBTA), which is aCuAAC known products ligand formay accelerating CuAAC [35]; (c)For Alkynes thatthe product between alkyne 21 and benzyl azide aa CuAAC reaction tris[(1-benzyl-1H-1,2,3-triazolligands. Forneither example, the product between alkyne 21 and benzylis in a CuAAC reaction is product alkyne 21 and(Figure benzyl azide in in CuAAC reaction isazide tris[(1-benzyl-1H-1,2,3-triazolbelongbetween to group above 4). 4-yl)methyl]amine (TBTA), which is a known ligand for accelerating CuAAC [35]; that tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA), which is a known forAlkynes accelerating 4-yl)methyl]amine (TBTA), which is a known ligand for accelerating CuAACligand [35]; (c) (c) Alkynes that belong to neither group above (Figure 4). CuAAC (c) Alkynes that belong neither group above (Figure 4). belong to[35]; neither group above (Figureto4).

Figure 2. Alkynes bearing electron-withdrawing groups in addition to compounds 1–7.

Figure addition to to compounds compounds 1–7. 2. Alkynes groups in in addition Figure 2. Alkynes bearing bearing electron-withdrawing electron-withdrawing groups compounds 1–7. 1–7.

Figure 3. Alkynes with pendant coordination ligands.

Figure with ligands. Figure 3. 3. Alkynes Alkynes with pendant pendant coordination coordination ligands. ligands. Figure 3. Alkynes with pendant coordination

Molecules 2016, 21, 1697 Molecules 2016, 21, 1697 Molecules 2016, 21, 1697

Molecules 2016, 21, 1697

3 of 17 3 of 17 3 of 17

Figure 4. Alkynes group above. above. Figure 4. to neither neither group Alkynes that that belong belong to

3 of 17

2.2. The 1H-NMR Assay 2.2. The 11H-NMR H-NMR Assay Assay The reactivity of alkynes was compared in a copper(II) acetate (5 mol % of the limiting reagent The reactivity of alkynes was compared in a copper(II) acetate (5 mol % of the limiting reagent alkyne) catalyzed reaction [18,19] with the 2-picolyl azide (Scheme alkyne) catalyzed catalyzed reaction reaction [18,19] [18,19] with the chelating chelating 2-picolyl azideabove. (Scheme 1). 1). The The reactive reactive 2-picolyl 2-picolyl (Scheme Figurewith 4. Alkynes that belong2-picolyl to neither azide group azide was selected so that the concentrations of substrates can be lowered to NMR-manageable values azide was selected so that the concentrations of substrates can be lowered lowered to NMR-manageable NMR-manageable values (i.e., 10 mM or lower) for online continuous monitoring [20]. If no product formed after 3 h of reaction 1 2.2.mM The or H-NMR (i.e., 10 continuous monitoring monitoring [20]. [20]. If no product formed after 3 h of reaction (i.e., lower)Assay for online continuous time, ascorbate (13 mol %) was added to completely reduce copper(II) to copper(I), which time, sodium sodium ascorbate (13 mol %) was added to completely reduce tolimiting copper(I), which in in The reactivity of alkynes was compared in a copper(II) acetate (5 copper(II) mol % of the reagent most cases kick-started the reaction. reaction. mostalkyne) cases kick-started the reaction. catalyzed reaction [18,19] with the chelating 2-picolyl azide (Scheme 1). The reactive 2-picolyl azide was selected so that the concentrations of substrates can be lowered to NMR-manageable values (i.e., 10 mM or lower) for online continuous monitoring [20]. If no product formed after 3 h of reaction time, sodium ascorbate (13 mol %) was added to completely reduce copper(II) to copper(I), which in most cases kick-started the reaction. 1H-NMR time course assay to evaluate alkynes. The actual concentrations Scheme used in 1 Scheme 1. 1.1.Reaction Reaction used in the the time course to evaluate Thealkynes. actual concentrations Scheme Reaction used in H-NMR the 1 H-NMR time assay course assay toalkynes. evaluate The actual of reaction ingredients are in parentheses. of reaction ingredients are in parentheses. concentrations of reaction ingredients are in parentheses.

Among all alkynes tested, six achieved significant within Among all the the alkynes tested, six of of them them achieved significant conversion conversion within the the first first 33 h h without without Scheme 1.the Reaction used intested, the 1H-NMR time course achieved assay to evaluate alkynes. The actual concentrations Among all alkynes six of them significant conversion within the first the addition of sodium ascorbate (Figure 5). p-Nitrophenylacetylene (5) has the shortest induction period, the addition of sodium ascorbate (Figure 5). p-Nitrophenylacetylene (5) has the shortest induction period, of reaction ingredients are in parentheses. 3followed h withoutthat the addition of sodium ascorbate (Figure 5). p-Nitrophenylacetylene (5) has the followed by by that of of ethynyl ethynyl methyl methyl ketone ketone (8), (8), ethyl ethyl propiolate propiolate (9), (9), and and propiolamide propiolamide 4. 4. Alkynes Alkynes 20 20 shortest induction period, followed by that of ethynyl methyl ketone (8), ethyl propiolate (9), and and 21 that contain the copper-binding triazolylmethyl group have longer induction periods. The Among all the tested, six of them achieved significant conversion within the first 3 h without and 21 that contain thealkynes copper-binding triazolylmethyl group have longer induction periods. The length length propiolamide 4.and Alkynes 20 andformation 21(Figure that contain theappear copper-binding triazolylmethyl group have longer of induction the efficiency the the protons. addition of sodium ascorbate 5). p-Nitrophenylacetylene (5) has theacidity shortestof period, of the thethe induction and the triazole triazole formation efficiency appear to to followed followed the acidity ofinduction the ethynyl ethynyl protons. induction periods. The of the induction and thetriazole triazoleformation formation efficiency appear to followed The 4, 99 have the highest efficiency—the reactions followed byderivatives that of length ethynyl ketone ethyl propiolate (9), and propiolamide 4. Alkynes 20 are The propiolyl propiolyl derivatives 4, 8, 8,methyl have the (8), highest triazole formation efficiency—the reactions are and 21within that contain the copper-binding triazolylmethyl group have induction periods. The length the acidity of the ethynyl protons. The propiolyl derivatives 4, 8,longer 9 have the highest triazole formation completed 5–10 min after the induction. This type of alkynes however is susceptible to conjugate completed within 5–10 min after the induction. This type of alkynes however is susceptible to conjugate of the induction and the triazole formationwithin efficiency appear to followed acidity of the ethynyl efficiency—the reactions are completed 5–10 min afterare thethe induction. This typeprotons. of alkynes nucleophilic nucleophilic addition addition at at the the terminal terminal carbon. carbon. Alkynes Alkynes 20 20 and and 21 21 are precursors precursors of of CuAAC-accelerating CuAAC-accelerating The propiolyl derivatives 4, 8, 9 have the highest triazole formation efficiency—the reactions however is susceptible to conjugate nucleophilic additionhigh at the terminal carbon.ofAlkynes 20are and 21 ligands. They also be self-accelerating. The relatively reactivity 21 ligands. They may may also bemin self-accelerating. TheThis relatively high CuAAC CuAAC reactivity of 20 20toand and 21 with with no no completed within 5–10 after the induction. type of alkynes however is susceptible conjugate are precursors of CuAAC-accelerating ligands. Theysubstrate may alsomanifolds be self-accelerating. The relatively high conjugate addition reactivity makes them attractive to build reactive and selective conjugate addition reactivity makes them attractive substrate manifolds to build reactive and selective nucleophilic addition at the terminal carbon. Alkynes 20 and 21 are precursors of CuAAC-accelerating CuAAC reactivity 20 and 21 with no conjugate addition various reactivity makes them attractive substrate conjugation linkersof in complexed environments containing nucleophilic species. conjugation in also complexed environments various nucleophilic ligands. linkers They may be self-accelerating. The containing relatively high CuAAC reactivity ofspecies. 20 and 21 with no manifolds to build reactive and selective conjugation linkers in complexed environments containing conjugate addition reactivity makes them attractive substrate manifolds to build reactive and selective various nucleophilic species. conjugation linkers in complexed environments containing various nucleophilic species.

Figure 5. values of the limiting reagent alkyne over time before the 3-h mark as Figure 5. Conversion Conversion values ofof the alkyneover overtime time before mark as acquired acquired Figure 5. Conversion values thelimiting limitingreagent reagent alkyne before thethe 3-h 3-h mark as acquired 1H-NMR via continuous monitoring by at r.t. The spectra were taken every 5 min. Alkyne (10 mM), Figure 5. Conversion values of the limiting reagent alkyne over time before the 3-h mark as acquired 1 via continuous monitoring by H-NMR at r.t. The spectra were taken every 5 min. Alkyne (10 1 via continuous monitoring by H-NMR at r.t. The spectra were taken every 5 min. Alkyne (10 mM),mM), 1 H-NMR 2-picolyl azide (10 mM), Cu(OAc) 2·H2O (0.5 mM). via continuous monitoring by at r.t. The spectra were taken every 5 min. Alkyne (10 mM), 2-picolyl azide (10 (10 mM), Cu(OAc) 2·H 2O 2-picolyl azide mM), Cu(OAc) 2·H 2O(0.5 (0.5mM). mM). 2-picolyl azide (10 mM), Cu(OAc)2 ·H2 O (0.5 mM). 1H-NMR data from these reactions are discussed herein. Some information on Representative 1H-NMR data from these reactions are discussed herein. Some information on 1H-NMR Representative Representative data from these reactions are discussed herein. Some information on how azide and/or alkyne catalyst pre-catalyst obtained how azide and/or alkyne interactedwith withthe the copper copper catalyst waswas obtained fromfrom thesethese how azide and/or alkyne interacted interacted with the copper catalystoror orpre-catalyst pre-catalyst was obtained from these data. The spectra shown in Figure 6 were acquired during the reaction between 2-picolyl azide data. The spectra shown in Figure 6 were acquired during the reaction between 2-picolyl azide and and data. The spectra shown in Figure 6 were acquired during the reaction between 2-picolyl azide and

Molecules 2016, 21, 1697

4 of 17

Representative 1 H-NMR data from these reactions are discussed herein. Some information on how azide and/or alkyne interacted with the copper catalyst or pre-catalyst was obtained from 4these Molecules 2016, 21, 1697 of 17 data. The spectra shown in Figure 6 were acquired during the reaction between 2-picolyl azide and ethynyl The mixture mixture of CN in in the the absence absence of of the the ethynyl methyl methyl ketone ketone 8. 8. The of azide azide and and alkyne alkyne substrates substrates in in CD CD33CN · copper catalyst gave the spectrum at the bottom. Upon addition of Cu(OAc) H O at 5 mol % of the 2 at 5 mol % of the copper catalyst gave the spectrum at the bottom. Upon addition of Cu(OAc)22·H2O limiting reagentalkyne, alkyne, pyridyl protons are ortho (proton and meta (protons d) to limiting reagent thethe pyridyl protons thatthat are ortho (proton a) anda) meta (protons b and d)btoand nitrogen nitrogen were broadened (spectrum ‘0 min’), suggesting coordination of copper at the pyridyl nitrogen were broadened (spectrum ‘0 min’), suggesting coordination of copper at the pyridyl nitrogen of 2-picolyl of 2-picolyl azide. Line-broadening wasfor notthe observed for thepeaks alkyne protong peaks g in and f). azide. Line-broadening was not observed alkyne proton (protons and f). (protons Therefore, this Therefore, in this case when the azide is chelating, copper (pre)catalyst is bound with the azide case when the azide is chelating, copper (pre)catalyst is bound with the azide substrate at the outset substrate at the before outset the of the reaction,ofbefore theacetylide. formation of copper acetylide. of the reaction, formation copper

1H-NMR 1 H-NMR Figure 6. of of of the between 2-picolyl azide azide and ethynyl methylmethyl ketone Figure 6. The Theevolution evolution of reaction the reaction between 2-picolyl and ethynyl 2 ·H 2 O was added at ‘0 min’. (8). Cu(OAc) ketone (8). Cu(OAc)2 ·H2 O was added at ‘0 min’.

The 1H-NMR data of the reaction between 2-picolyl azide and alkyne 21 are shown in Figure 7. The 1 H-NMR data of the reaction between 2-picolyl azide and alkyne 21 are shown in Figure 7. In this case, after the addition of Cu(OAc)2·H2O (spectrum ‘0 min’), the signals of protons g, h, and j on In this case, after the addition of Cu(OAc)2 ·H2 O (spectrum ‘0 min’), the signals of protons g, h, and j 21 completely disappeared (i.e., broadened down to the baseline), indicative of binding of the alkyne to on 21 completely disappeared (i.e., broadened down to the baseline), indicative of binding of the copper(II) at the outset of the reaction. Therefore, the high reactivity of alkyne 21, which shall have a alkyne to copper(II) at the outset of the reaction. Therefore, the high reactivity of alkyne 21, which shall relatively high pKa value in the realm of aliphatic alkynes, can be at least partially attributed to its have a relatively high pKa value in the realm of aliphatic alkynes, can be at least partially attributed ability to recruit copper, even in the pre-catalyst form. As the spectrum transitioned during the to its ability to recruit copper, even in the pre-catalyst form. As the spectrum transitioned during induction period from ‘0 min’ to ’65 min’, the peaks of protons g, h, and j reappeared as broad singlets. the induction period from ‘0 min’ to ’65 min’, the peaks of protons g, h, and j reappeared as broad The reappearances of these protons on the alkyne as the triazole formation is imminent suggest that singlets. The reappearances of these protons on the alkyne as the triazole formation is imminent the (partial) reduction of paramagnetic copper(II) to diamagnetic copper(I) is prerequisite to the suggest that the (partial) reduction of paramagnetic copper(II) to diamagnetic copper(I) is prerequisite triazole formation. to the triazole formation. Triazole production was not observed with other alkynes during the first 3 h under the conditions Triazole production was not observed with other alkynes during the first 3 h under the conditions listed in the caption of Figure 5 [36]. For these reactions, at the 3-h mark, sodium ascorbate (13 mol % listed in the caption of Figure 5 [36]. For these reactions, at the 3-h mark, sodium ascorbate (13 mol % of of the limit reagent alkyne) was delivered to the reaction mixture in the NMR tube, and the reaction the limit reagent alkyne) was delivered to the reaction mixture in the NMR tube, and the reaction was was monitored for an additional 5 h. Most alkynes except two (18 and 19 in Figure 3) started conversion monitored for an additional 5 h. Most alkynes except two (18 and 19 in Figure 3) started conversion to to triazole immediately after the addition of sodium ascorbate and achieved various levels of completion triazole immediately after the addition of sodium ascorbate and achieved various levels of completion after 5 h. The alkynes are divided into three groups in Figure 8 based on reactivity. Group I consists after 5 h. The alkynes are divided into three groups in Figure 8 based on reactivity. Group I consists of of alkynes 10–13, all ethynyl arenes with electron-withdrawing substituents, which had achieved alkynes 10–13, all ethynyl arenes with electron-withdrawing substituents, which had achieved over over 80% conversion. Phenylacetylene (22), propargylic alcohol (14) and amines 15, 16 constituted 80% conversion. Phenylacetylene (22), propargylic alcohol (14) and amines 15, 16 constituted Group II, Group II, while the aliphatic alkynes 23, 24, and the acylated propargyl amine 17 have the lowest while the aliphatic alkynes 23, 24, and the acylated propargyl amine 17 have the lowest conversion conversion values and therefore are in Group III. The 2-picolyl-functionalized propargyl amines 18 values and therefore are in Group III. The 2-picolyl-functionalized propargyl amines 18 and 19 failed and 19 failed to react. to react.


5 of 17 5 of 17 5 of 17

Figure The evolution evolution of of 11H-NMR H-NMR of 21.21. Figure 7. 7. The of the the reaction reaction between between2-picolyl 2-picolylazide azideand andalkyne alkyne 1 Figure 7. The evolution of the reaction between 2-picolyl azide and alkyne 21. min’. Py of = 2-pyridyl. Cu(OAc)2·H2O was added at ‘0H-NMR Cu(OAc)2 ·H2 O was added at ‘0 min’. Py = 2-pyridyl. Cu(OAc)2·H2O was added at ‘0 min’. Py = 2-pyridyl.

Figure 8. Conversion values over time beyond the 3-h mark at which sodium ascorbate (1.3 mM, Figure 8. Conversion values over time beyond the 3-h mark atbywhich sodium ascorbate mM, 1H-NMR 13 was added as acquired via continuous at r.t. Alkyne (1.3 (10(1.3 mM), Figuremol 8. %) Conversion values over time beyond themonitoring 3-h mark at which sodium ascorbate mM, 1H-NMR at r.t. Alkyne (10 mM), 13 mol %) was added as acquired via continuous monitoring by 1 ·H2O (0.5 mM, 5 monitoring mol %). Sodium mM) was added at azide (10 mM), Cu(OAc)2via 13 2-picolyl mol %) was added as acquired continuous by ascorbate H-NMR (1.3 at r.t. Alkyne (10 mM), 2-picolyl azide (10 mM), Cu(OAc)2·H2O (0.5 mM, 5 mol %). Sodium ascorbate (1.3 mM) was added at the 3-hazide mark. 2-picolyl (10 mM), Cu(OAc)2 ·H2 O (0.5 mM, 5 mol %). Sodium ascorbate (1.3 mM) was added at the 3-h mark. the 3-h mark.

We attribute the low reactivity of alkynes 18 and 19 to their abilities in forming strong complexes attribute low reactivity of alkynes 18potency and 19 tooftheir in forming 21 strong withWe copper(II or the I), which reduce the catalytic the abilities metal. Compound is ancomplexes analog of We attribute the low reactivity of alkynes 18 and 19 to their abilities in forming strong complexes with copper(II or I), which reduce the catalytic potency of the metal. Compound 21 is of compound 19; yet 21 is reactive without the participation of sodium ascorbate (Figure 5). an Theanalog addition compound 19; yet 21 is reactive without the participation of sodium ascorbate (Figure 5). The addition with copper(II or I), which reduce the catalytic potency of the metal. Compound 21 is an analog of an equal molar amount of alkyne 19 in the reaction mixture of 21 and 2-picolyl azide resulted in of of equal of alkyne 19the in the reaction mixture of 21 and 2-picolyl azide in compound 19;molar yet 21amount is2), reactive without participation of sodium ascorbate (Figure 5). resulted Thethan addition noan reaction (Scheme which is consistent with the argument that 19 binds copper stronger 21, (Scheme 2), which is consistent the argument that binds copper stronger than 21, of no an equal molar amount of alkyne 19 in participating thewith reaction mixture of 2119and 2-picolyl azide resulted in no andreaction consequently it hijacks copper from in the reaction between 21 and 2-picolyl azide. and consequently hijacks fromwith participating in thethat reaction between 21 and 2-picolyl reaction (Scheme 2),itwhich iscopper consistent the argument 19 binds copper stronger thanazide. 21, and consequently it hijacks copper from participating in the reaction between 21 and 2-picolyl azide.

Molecules 2016, 21, 1697

6 of 17

Molecules 2016, 21, 1697

6 of 17

Molecules 2016, 21, 1697

6 of 17

Scheme 2. The effect of alkyne 19 on the reaction between alkyne 21 and 2-picolyl azide.

Scheme 2. The effect of alkyne 19 on the reaction between alkyne 21 and 2-picolyl azide. From the data in Figures 5 and 8, it can be concluded that: (a) electron-deficient alkynes such as

From theketones, data inpropiolates Figures 5and andpropiolamides, 8, it can be concluded that: (a) electron-deficient alkynes ethynyl and electron-withdrawing group-substituted ethynylsuch as arenes are reactive in both copper(II)and copper(I)-catalyzed conditions; (b) N-triazolylmethylethynyl ketones, propiolates and propiolamides, and electron-withdrawing group-substituted ethynyl Scheme 2. The effect ofare alkyne on the reaction Those between alkyne 21 and 2-picolyl azide. propargyl also19 fast reacting. compounds might act accelerating arenessubstituted are reactive in bothamines copper(II)and copper(I)-catalyzed conditions; (b)asN-triazolylmethylligands for their own conversion to triazoles; (c) Inclusion of amino or hydroxyl groups in an aliphatic substitutedFrom propargyl amines are also fast Those compounds might act as such accelerating the data 5 and 8, it canreacting. be concluded that:reactivity; (a) electron-deficient alkyne adjacent to in theFigures ethynyl group increases its CuAAC and (d) thealkynes presence of as a ligandsmultidentate for their own conversion to triazoles; (c) Inclusion of amino or hydroxyl groups in an aliphatic ethynyl ketones, propiolates and propiolamides, and electron-withdrawing group-substituted ethynyl ligand of a strong copper affinity, such as di(2-picolyl)amino [37], slows the reaction down. arenes are reactive in both copper(II)and copper(I)-catalyzed conditions; (b) alkyneIt adjacent to the group increases its CuAAC reactivity; (d) 21 the“chelating presence of a was tempting to ethynyl call the N-triazolylmethyl-substituted propargyl aminesand 20 N-triazolylmethyland substituted propargyl amines are also fast reacting. Those compounds might act as accelerating multidentate of a strong affinity, suchwhich as di(2-picolyl)amino slows the reaction alkynes”ligand in the same manner copper as “chelating azides”, would be suggestive[37], of high reactivity. ligands forwhen their own toligand triazoles; (c) Inclusion or hydroxyl groups an a stronger copper is used in place of of amino a triazolylmethyl group such asaliphatic in down. However, It was tempting to conversion call the N-triazolylmethyl-substituted propargyl amines 20inand 21those “chelating alkyne adjacent to the ethynyl group increases its CuAAC reactivity; and (d)the theuse presence of a 18 and 19, same the reactivity precipitously. Therefore, we would do not advocate of high the term alkynes” in the mannerdrops as “chelating azides”, which be suggestive of reactivity. multidentate ligand ofwhich a strong copper affinity, such asexpectation di(2-picolyl)amino [37], slows the reaction down. “chelating alkynes”, would likely give a false of high reactivity. However, when a stronger copper ligand is used in place of a triazolylmethyl group such as in It was tempting to call the N-triazolylmethyl-substituted propargyl amines 20 and 21 “chelating those 18 and 19, the reactivity drops precipitously. Therefore, we do not advocate the use of the term alkynes” in the manner as “chelating azides”, which would be suggestive of high reactivity. 2.3. Reaction withsame the Non-Chelating Benzyl Azide “chelating alkynes”, would likely give a false expectation of high reactivity. However, whenwhich a stronger copper ligand is used in place of a triazolylmethyl group such as in those For practical reasons, we focus mostly on the six fast-reacting alkynes shown in Figure 5 in the 18 and 19, the reactivity drops precipitously. Therefore, we do not advocate the use of the term

following We would like to know whether they would also engage azides without 2.3. Reaction withsubsections. the Non-Chelating Benzyl “chelating alkynes”, which would likely Azide give a false expectation of high reactivity.

chelation activation, such as benzyl azide, under the conditions depicted in Figure 5. Different from

For reasons, we azide, focusBenzyl mostly onoccurred the six between fast-reacting alkynes shown Figure 5 in the reactions with 2-picolyl no reaction propiolamide 4 or ethynylinmethyl 2.3.practical Reaction with the Non-Chelating Azide ketone 8 and benzyl azide within 3 h, while alkyne 21, which is the second slowest among the “lead the following subsections. We would like to know whether they would also engage azides without For practical reasons, we focus mostly on the six fast-reacting alkynes shown in Figure 5 in the pack” in Figure 5, started conversion to triazole at 100-min mark (orange trace in Figure 9). 2-Picolyl chelation activation, such as benzyl azide, under the conditions depicted in Figure 5. Different from the following subsections. would to know whether they would engage azides(Figure without azide, on the contrary, We initiated thelike reaction at 65-min mark under thealso same 9, reactions with 2-picolyl azide, nobenzyl reaction occurred between propiolamide 4Figure orconditions ethynyl methyl ketone 8 chelation activation, such as azide, under the conditions depicted in 5. Different from blue, reproduced from Figure 5). From these data, it can be concluded that (a) the nature of the azide and benzyl azide within 3 h, while alkyne 21, which is the second slowest among the “lead pack” in the reactions with 2-picolyl noofreaction occurred 4 or ethynyl methyl has an overriding effect on azide, the rate CuAAC, as the between chelatingpropiolamide 2-picolyl azide reacts faster than Figurebenzyl 5, started to triazole at 100-min mark (orange trace in Figure 9). 2-Picolyl ketone 8azide andconversion benzyl azide within 3 h, while alkyne 21, which is the second slowest among the “lead regardless of the identity of alkyne; (b) the reactivity rank of alkyne depends on the azide, pack” in Figure 5, started triazole atmark 100-min mark4/8 (orange trace in Figure 9).(Figure 2-Picolyl on thenature contrary, initiated theconversion reaction at 65-min under the same conditions of azide—reactivity reversion to was observed for alkynes and 21 when they pair up with9, blue, azide, on the contrary, initiated the reaction at 65-min mark under the same conditions (Figure 9, 2-picolyl azide or benzyl azide. these data, it can be concluded that (a) the nature of the azide reproduced from Figure 5). From has blue, reproduced from Figure 5). From these data, it can be concluded that (a) the nature of the azide an overriding effect on the rate of CuAAC, as the chelating 2-picolyl azide reacts faster than benzyl has an overriding effect on the rate of CuAAC, as the chelating 2-picolyl azide reacts faster than azide regardless of the identity of alkyne; (b) the reactivity rank of alkyne depends on the nature of benzyl azide regardless of the identity of alkyne; (b) the reactivity rank of alkyne depends on the azide—reactivity reversion was observed alkynes and4/8 21 and when they pair up with 2-picolyl nature of azide—reactivity reversion wasfor observed for4/8 alkynes 21 when they pair up with azide or benzyl azide. 2-picolyl azide or benzyl azide.

Figure 9. Conversion values of alkyne 21 over time as acquired via continuous monitoring by 1H-NMR at r.t. The spectra were taken every 5 min. alkyne 21 (10 mM), 2-picolyl azide or benzyl azide (10 mM), Cu(OAc)2·H2O (0.5 mM).

Figure 9. Conversion values of alkyne 21 over time as acquired via continuous monitoring by

Figure 9. Conversion values of alkyne 21 over time as acquired via continuous monitoring by 1 H-NMR 1H-NMR at r.t. The spectra were taken every 5 min. alkyne 21 (10 mM), 2-picolyl azide or benzyl azide at r.t. The spectra were taken every 5 min. alkyne 21 (10 mM), 2-picolyl azide or benzyl azide (10 mM), (10 mM), Cu(OAc)2·H2O (0.5 mM). Cu(OAc)2 ·H2 O (0.5 mM).

Molecules 2016, 21, 1697 Molecules 2016, 21, 1697

7 of 17 7 of 17

2.4. Comparison of Fast-Reacting Alkynes Alkynes with with aa Normalized Normalized Amount Amount of of Copper(I) Copper(I) As shown shown in in Figure Figure 5, 5, the the triazole triazole formation phases for four of the alkynes (compounds 4, 5, 8, 9) are exceptionally fast, fast, being beingcomplete completewithin within5–10 5–10min minafter afterthe theinduction induction period. Because there is period. Because there is no no information how much copper(I) catalystwas wasproduced producedduring duringthe the induction induction periods periods of these information on on how much copper(I) catalyst these reactions, inin reaction rates of these fourfour compounds solely to the reactions, one oneshall shallnot notattribute attributethe thedifferences differences reaction rates of these compounds solely to differences in their inherent reactivity. For comparing their reactivity, another set of experiments with the differences in their inherent reactivity. For comparing their reactivity, another set of experiments lower substrate concentrations and in the of an excess amount sodiumof ascorbate to with lower substrate concentrations andpresence in the presence of an excessofamount sodium relative ascorbate copper amountthe of copper(I) were conducted 10).(Figure 10). relativetotonormalize copper to the normalize amount ofcatalyst copper(I) catalyst were (Figure conducted

Figure 10. 10. Conversion Conversion values alkynes 4, 5, 8, 8, 9, 9, 20, 20, and and 21 21 over over time time as as acquired acquired via via continuous continuous Figure values of of alkynes 4, 5, 1H-NMR at r.t. The spectra were taken every 5 min. Concentrations: alkyne (3.0 mM), 1 monitoring by monitoring by H-NMR at r.t. The spectra were taken every 5 min. Concentrations: alkyne (3.0 mM), 2-picolyl azide azide (3.2 (3.2 mM), mM), Cu(OAc) Cu(OAc)22··H 2-picolyl H22O O (0.15 (0.15 mM), mM), and and sodium sodium ascorbate ascorbate (0.39 (0.39 mM). mM).

The reactions were run at lower concentrations than those employed in the last subsection. The reactions were run at lower concentrations than those employed in the last subsection. The limiting reagent alkyne was set at 3.0 mM instead of 10 mM. As expected, the reactions were slower The limiting reagent alkyne was set at 3.0 mM instead of 10 mM. As expected, the reactions were so that the differences between the reactivity of these alkynes can be unambiguously observed. The slower so that the differences between the reactivity of these alkynes can be unambiguously observed. induction periods were eliminated in the presence of sodium ascorbate, and the propiolyl type alkynes The induction periods were eliminated in the presence of sodium ascorbate, and the propiolyl type 4, 8, and 9 reacted faster than 5, 20, and 21 (Figure 10). Among the propiolyls, propiolamide 4 has the alkynes 4, 8, and 9 reacted faster than 5, 20, and 21 (Figure 10). Among the propiolyls, propiolamide 4 highest reactivity, followed by ethynyl ketone 8 and propiolate 9. has the highest reactivity, followed by ethynyl ketone 8 and propiolate 9. 2.5. Reduction Reduction of of Copper(II) Copper(II) Acetate Acetate by by Ascorbate Ascorbate Alters Alters the the Pathway Pathway 2.5. Although the the addition addition of of sodium sodium ascorbate ascorbate eliminated eliminated the the induction induction period, period, it it might might also also have have Although altered the the structure structure of of the the active active catalyst catalyst formed formed under under the thenon-reductive non-reductive conditions. conditions. The The reaction reaction altered between propiolamide 4 and 2-picolyl azide was used to examine this possibility. After a long induction between propiolamide 4 and 2-picolyl azide was used to examine this possibility. After a long induction period in in the the reaction reaction without without sodium sodium ascorbate, ascorbate, the the product product formation formation is is steeper steeper (i.e., (i.e., faster) faster) than than period the reaction with copper(II) fully reduced by sodium ascorbate at the outset (Figure 11). Considering the reaction with copper(II) fully reduced by sodium ascorbate at the outset (Figure 11). Considering that only only aa (small) (small) fraction fraction of of copper copper was was reduced reduced to to copper(I) copper(I) in in the the former former case, case, the the activity activity of of the the that catalyst formed formed after after the the induction induction reaction Structural identification identification of catalyst reaction was was extraordinary. extraordinary. Structural of that that catalyst, catalyst, likely a 2-picolyl azide coordinated dinuclear copper(I) acetate [20], is a target of future study. likely a 2-picolyl azide coordinated dinuclear copper(I) acetate [20], is a target of future study.


8 of 17 8 of 17 8 of 17

Figure 11. The kinetic traces of the reaction between 2-picolyl azide and alkyne 4 to show the effect of

Figure 11. The kinetic traces of the reaction between 2-picolyl azide and alkyne 4 to show the effect of sodium (orange) and TBTA (gray). Py = 2-pyridyl. Figure 11.ascorbate The kinetic traces ofTBTA the reaction 2-picolyl azide and alkyne 4 to show the effect of sodium ascorbate (orange) and (gray).between Py = 2-pyridyl. sodium ascorbate (orange) and TBTA (gray). Py = 2-pyridyl. 2.6. The Effect of TBTA on the Slow Alkyne Substrates

2.6. The Effect of TBTA on the Slow Alkyne Substrates 2.6. The Effect[35] of TBTA on the that SlowisAlkyne TBTA is a ligand knownSubstrates to accelerate the triazolyl ring formation step in CuAAC. TBTA [35] is a ligand that is known to accelerate ringwithin formation step3 h, in three CuAAC. Among the alkynes listed in Figure 8, which are those the thattriazolyl did not react the first TBTA [35] is a ligand that is known to accelerate the triazolyl ring formation step in CuAAC. Among the alkynes listed in Figure 8, which are those that did not react within the first 3 h, three alkynes alkynes (10, 22, 23) were selected from each group to be subjected under copper(II) acetate only Among the alkynes listed in Figure 8, which are those that did not react within the first 3 h, three in selected the presence ofeach TBTAgroup (10 mol TBTA was copper(II) shown to enhance reactivity of in (10, conditions 22, 23) were from to %) be [19,38]. subjected under acetate the only conditions alkynes (10, 22, 23) were selected from each group to be subjected under copper(II) acetate only the relatively acidic(10 p-ethynylbenzonitrile (10) and phenylacetylene (22) (Figure 12). TBTA therefore theconditions presence of TBTA mol %) [19,38]. TBTA was shown to enhance the reactivity of the relatively in the presence of TBTA (10 mol %) [19,38]. TBTA was shown to enhance the reactivity of likely shortens the induction period accelerating the(22) alkyne oxidative homocoupling reaction. This acidic p-ethynylbenzonitrile (10) and by phenylacetylene (Figure 12). therefore likely shortens the relatively acidic p-ethynylbenzonitrile (10) and phenylacetylene (22)TBTA (Figure 12). TBTA therefore function of TBTA was also shown in the reaction involving the more reactive alkyne 4 at a lower thelikely induction period by accelerating alkyne oxidative reaction.reaction. This function shortens the induction period bythe accelerating the alkynehomocoupling oxidative homocoupling This substrate concentration (Figure 11). No reaction occurred to 1-hexyne 23, the alkyne in the lowest of function TBTA was also shown in the reaction involving the more reactive alkyne 4 at a lower of TBTA was also shown in the reaction involving the more reactive alkyne 4 at asubstrate lower reactivity category (Group III in Figure 8), under the same conditions. concentration (Figure 11).(Figure No reaction 1-hexyne 23, the alkyne the lowest substrate concentration 11). Nooccurred reaction to occurred to 1-hexyne 23, theinalkyne in thereactivity lowest category (Group III in(Group FigureIII8),inunder reactivity category Figurethe 8), same underconditions. the same conditions.

Figure 12. The kinetic traces of reactions between 2-picolyl azide and alkynes 10 (orange), 22 (blue), and 23 (green) in the presence of TBTA (filled circles) or TEA (crosses).

Figure The kinetictraces tracesofofreactions reactions between between 2-picolyl 2222 (blue), Figure 12.12. The kinetic 2-picolylazide azideand andalkynes alkynes1010(orange), (orange), (blue), and 23 (green) in the presence of TBTA (filled circles) or TEA (crosses). and 23 (green) in the presence of TBTA (filled circles) or TEA (crosses).

Molecules 2016, 21, 1697

9 of 17

Molecules 2016, 21, 1697

9 of 17

For the sake of completeness, the reactivity of all alkynes 1–24 were compared in the presence of 10For mol TBTA. For the highly reactive of alkynes 4, 5, 1–24 8, 9,were 20, compared and 21 listed Figure of 5, the% sake of completeness, the reactivity all alkynes in theinpresence themol concentrations substrates cut 4, to5,58,mM. (HCin ≡C-(C=O)-) 4, 8, 10 % TBTA. Forof thethe highly reactivewere alkynes 9, 20,Propiolyl and 21 listed Figure 5, thederivatives concentrations and 9 maintained their superior reactivity, while p-nitrophenylacetylene (8) was falling far behind, of the substrates were cut to 5 mM. Propiolyl (HC≡C-(C=O)-) derivatives 4, 8, and 9 maintained their and N-triazolylmethyl-substituted propargyl amines 20 falling and 21far failed to produce triazole in 2 h superior reactivity, while p-nitrophenylacetylene (8) was behind, and N-triazolylmethyl(Figure 13a).propargyl Alkynes amines 20 and 20 21 and were21reexamined with the rest of at the original substrate substituted failed to produce triazole inalkynes 2 h (Figure 13a). Alkynes 20 and concentrations (10 mM each for alkyne and 2-picolyl azide) in the presence of 10 mol % TBTA. 21 were reexamined with the rest of alkynes at the original substrate concentrations (10 mM each for The reactivity of these alkynes in broadofstrokes matches the reactivity conclusions fromalkynes the ligand-free alkyne and 2-picolyl azide) in the presence 10 mol % TBTA. The of these in broad experiments (Figures 5 and 8)—ethynyl arenes areexperiments more reactive than 5aliphatic terminal alkynes strokes matches the conclusions from the ligand-free (Figures and 8)—ethynyl arenes are (Figure 13b). Within ethynyl arenes, electron-withdrawing substituents increase the reactivity. more reactive than aliphatic terminal alkynes (Figure 13b). Within ethynyl arenes, electron-withdrawing The N-triazolylmethyl-substituted propargyl amines 20 and 21 still show rapid triazole-forming substituents increase the reactivity. The N-triazolylmethyl-substituted propargyl amines 20 and 21 still phases, with relatively long induction periods. The pyridyl analogs of both compounds—18 and show rapid triazole-forming phases, with relatively long induction periods. The pyridyl analogs of 19—failed to react under the same conditions. Several alkynes jumped up on the order of reactivity both compounds—18 and 19—failed to react under the same conditions. Several alkynes jumped up with the assistance of TBTA. 2-Ethynylpyridine (13) was identified for its relatively high reactivity; on the order of reactivity with the assistance of TBTA. 2-Ethynylpyridine (13) was identified for its this compound was reported as an accelerating additive for CuAAC reactions [39]. Alkyne 24 was relatively high reactivity; this compound was reported as an accelerating additive for CuAAC reactions another one that showed surprising in the presence of 10in mol TBTA. This also [39]. Alkyne 24 was another one thatreactivity showed surprising reactivity the%presence of 10compound mol % TBTA. exhibited high reactivity in thehigh study by Finn in and [31]. The propargyl 17 This compound also exhibited reactivity thecoworkers study by Finn and acylated coworkers [31]. Theamine acylated saw the uptick of its reactivity aided by TBTA. We do not have a model to relate the structures of these propargyl amine 17 saw the uptick of its reactivity aided by TBTA. We do not have a model to relate alkynes to the of observations on their The rest of thereactivity. alkynes not in the structures these alkynes to theTBTA-dependent observations on reactivity. their TBTA-dependent Thelisted rest of Figure 13 did not react under the given conditions. the alkynes not listed in Figure 13 did not react under the given conditions.

Figure 13. (a) (a)Triazole Triazoleformation formation over time of alkynes 5, 10, 8, 9, 21 each) (5 mM each) with Figure 13. over time of alkynes 4, 5, 4, 8, 9, 20,10, and20, 21 and (5 mM with 2-picolyl 2-picolyl azidein(5the mM) in the presence of Cu(OAc) 2(H2O) (0.25 mM) and TBTA (0.5 mM) in CD3CN at azide (5 mM) presence of Cu(OAc) 2 (H2 O) (0.25 mM) and TBTA (0.5 mM) in CD3 CN at r.t. for r.t. for 2 h. *: substrates reexamined at a higher concentration; triazole formationover overtime timeof of alkynes alkynes 2 h. *: substrates reexamined at a higher concentration; (b)(b) triazole formation 10–13, 17, 20–22, and 24 (10 mM each) with 2-picolyl azide (10 mM) in the presence of Cu(OAc) (H2O) 10–13, 17, 20–22, and 24 (10 mM each) with 2-picolyl azide (10 mM) in the presence of Cu(OAc)22(H 2 O) (0.5 mM) and TBTA (1.0 mM) in CD 3 CN at rt for 2 h. Compound numbers in red: propiolyl derivatives; (0.5 mM) and TBTA (1.0 mM) in CD3 CN at rt for 2 h. Compound numbers in red: propiolyl derivatives; orange: orange: ethynyl ethynyl aromatics; aromatics; blue: blue: ethynyl ethynyl aliphatics; aliphatics; and and green: green: multidentate multidentate copper-binding copper-binding ligands. ligands.

The rank of reactivity of alkyne 1–24 are summarized in Figure 14. Again, propiolyl derivatives are The rank of reactivity of alkyne 1–24 are summarized in Figure 14. Again, propiolyl derivatives are the most reactive, followed by ethynyl arenes that are activated by electron-withdrawing substituents. the most reactive, followed by ethynyl arenes that are activated by electron-withdrawing substituents. Aliphatic alkynes have relatively low reactivity. Copper-binding neighboring groups may elevate Aliphatic alkynes have relatively low reactivity. Copper-binding neighboring groups may elevate (e.g., 20, 21) or attenuate (e.g., 18, 19) the reactivity of terminal alkynes. The structure-function (e.g., 20, 21) or attenuate (e.g., 18, 19) the reactivity of terminal alkynes. The structure-function relationship of these alkynes is a topic of ongoing studies. relationship of these alkynes is a topic of ongoing studies.


10 of 17 10 of 17 10 of 17

Figure 14. Summary of reactivity rankingsof ofalkyne alkyne 1–24 1–24 in Figures 5, 8,5, and 13. Figure 14. ofofreactivity rankings in CuAAC determined in Figures Figures 5, and13. 13. Figure 14.Summary Summary reactivity rankings of alkyne 1–24in inCuAAC CuAACdetermined in 88,and

is the mechanistic basis for the accelerating effect of TBTA, and other poly(triazolyl) WhatWhat is the mechanistic basis for the accelerating effect of TBTA, and other poly(triazolyl) What theCuAAC mechanistic effect of TBTA,precatalyst? and other Itpoly(triazolyl) ligands,ison reactionsbasis that for startthe withaccelerating either copper(I) or copper(II) has been ligands, on CuAAC reactions that start with either copper(I) or copper(II) precatalyst? It has been ligands, on CuAAC that start with copper(I) or copper(II) precatalyst? hypothesized thatreactions TBTA stabilizes copper ineither the active +1 oxidation state [35,40]. However, Itinhas ourbeen hypothesized that TBTA stabilizes copper in the[41], active +1 oxidation state [35,40]. However, in our opinion, which we presented in a copper review article there no definitive evidence to fully establish hypothesized that TBTA stabilizes in the active +1is oxidation state [35,40]. However, in our opinion, which we presented in a review article [41], there is no definitive evidence to fully establish this copper(I)-stabilizing ability. From the data of TBTA-accelerated reactions in this work, TBTA likely opinion, which we presented in a review article [41], there is no definitive evidence to fully establish this copper(I)-stabilizing ability. From the data of TBTA-accelerated reactions in this TBTA likely accelerates the inducting alkyne oxidative homocoupling reaction. This argument waswork, substantiated this copper(I)-stabilizing ability. From the data of TBTA-accelerated reactions in this work, TBTA likely accelerates the inducting alkyneeffect oxidative reaction. This argument(Table was substantiated as TBTA showed accelerating on the homocoupling oxidative homocoupling of phenyacetylene 1). Under accelerates the inducting alkyne oxidative homocoupling reaction. This argument was substantiated 1H-NMR, entry 1) of phenylacetylene was converted to diyne in the given conditions, 57% (based as TBTA showed accelerating effect on the oxidative homocoupling of phenyacetylene (Table 1). Under as TBTA showed accelerating effect on the oxidative homocoupling of phenyacetylene (Table 1). Under 1 the presence of TBTA (10 mol %), while without TBTA nophenylacetylene oxidative reactionwas occurred (entry 2). The in the given conditions, 57% (based on H-NMR, 1) of converted to diyne 1 H-NMR,entry the given conditions, 57% (based onalkyne entry 1) of phenylacetylene was converted to diyne accelerating effect of TBTA on the oxidative homocoupling reaction (the Glaser reaction) was the presence of TBTA (10 mol %), while without TBTA no oxidative reaction occurred (entry 2). The in thealso presence of TBTA (10 mol %),ofwhile without no (entry oxidative reaction occurred (entry 2). reported previous our group [38]. TBTA 2-Picoline 3), which resembles 2-picolyl accelerating effect in ofaTBTA on work the alkyne oxidative homocoupling reaction (the Glaser reaction) was The accelerating effect of modest TBTA on the alkyne oxidative homocoupling reaction Glaser azide, showed a more accelerating effect, while triethylamine (TEA) did not(the (entry 4). reaction) also reported in a previous work of our group [38]. 2-Picoline (entry 3), which resembles 2-picolyl was also reported in a previous work of our group [38]. 2-Picoline (entry 3), which resembles 2-picolyl azide, showed a1.more modest accelerating effect, while triethylamine (TEA) did not (entry 4). Tablea The effect of TBTA, TEA, and 2-picoline on oxidative homocoupling of phenylacetylene azide, showed more modest accelerating effect, while triethylamine (TEA) did not (entrya. 4). Table 1. The effect of TBTA, TEA, and 2-picoline on oxidative homocoupling of phenylacetyleneaa. Table 1. The effect of TBTA, TEA, and 2-picoline on oxidative homocoupling of phenylacetylene . 1H-NMR Yield b Entry Additive 1 TBTA (0.02 mmol) 57% 1 2 None 0 Yield b Entry Additive 1H-NMR Entry Additive H-NMR Yield b 3 2-picoline (0.02 mmol) 14% 1 TBTA (0.02 mmol) 57% 14 TBTA (0.02mmol) mmol) 57% TEA (0.02 00 22 None None 0 (1 mL), r.t., air, 18 h; b Yields a Conditions: phenylacetylene (0.2 mmol), Cu(OAc)2·H2O (0.2 mmol), t-BuOH 33 2-picoline 14% 2-picoline(0.02 (0.02 mmol) mmol) 14% 1 of 1,4-diphenylbuta-1,3-diyne were obtained from the H-NMR spectra of the reaction mixtures. TEA(0.02 (0.02 mmol) mmol) 00 44 TEA a a

b Yields Conditions: (0.2 mmol), Cu(OAc) 2H ·H2 O 2O (0.2mmol), mmol), t-BuOH (1 mL), 18bmodel h; Conditions: phenylacetylene mmol), Cu(OAc) (0.2 t-BuOH (1 mL), r.t.,r.t., air, air, 18 h; Yields of Being a phenylacetylene tertiary amine, TBTA is also a base.2 ·We therefore used triethylamine (TEA) to the 1 H-NMR spectra of the reaction mixtures. 1,4-diphenylbuta-1,3-diyne were obtained from the 1 spectra of the reaction mixtures. of 1,4-diphenylbuta-1,3-diyne were obtained from theof H-NMR basicity of TBTA to gauge how much the basicity TBTA contributes to the accelerating effect on CuAAC. When TEA was used in place of TBTA, the reactions with both alkynes 10 and 22 were Being a tertiary amine, TBTA is also a base. We therefore used triethylamine (TEA) to model Being a tertiary amine, TBTA is also a base. We therefore used triethylamine (TEA)yet to model accelerated. For both alkynes, TEA-involved reactions experienced longer induction periods faster the the basicity of TBTA tothan gauge how much the basicity ofwhich TBTAmeans contributes toisthe accelerating triazole production those with TBTA participation, that TEA not as effective aseffect basicity of TBTA to gauge how much the basicity of TBTA contributes to the accelerating effect on on CuAAC. TEAthe was used in place of TBTA, the homocoupling) reactions withbut both alkynes 10 and 22 were TBTA inWhen catalyzing Glaser reaction oxidative may form a 10 more potent CuAAC. When TEA was used in place(alkyne of TBTA, the reactions with both alkynes and 22 were accelerated. For both alkynes, TEA-involved reactions experienced longer induction periods yet faster copper(I) catalyst of the CuAAC reaction. reactions accelerated. For both alkynes, TEA-involved experienced longer induction periods yet faster triazole production than those with TBTA participation, which that TEA acetate-mediated is not as effective as Based on the comparison of the effects of TBTA and TEA means on the copper(II) triazole production than those with TBTA participation, which means that TEA is not as effective as is possible one of(alkyne the functions of TBTA is the kinetic activation copper(II) TBTAcycloaddition, in catalyzingitthe Glaser that reaction oxidative homocoupling) but may of form a morefor potent TBTA in catalyzing the Glaser reaction (alkyne oxidative homocoupling) but may form a more potent oxidative chemistry, re-engages oxidized copper catalyst in CuAAC reaction (Scheme 3a). This copper(I) catalyst of the which CuAAC reaction. copper(I) catalyst of the CuAAC reaction. argument from the thermodynamic model that TBTA the +1 oxidation state of Based on differs the comparison of the effects of TBTA and TEAstabilizes on the copper(II) acetate-mediated

Based on the comparison of the effects of TBTA and TEA on the copper(II) acetate-mediated cycloaddition, it is possible that one of the functions of TBTA is the kinetic activation of copper(II) cycloaddition, it is possible that one of the functions of TBTA is the kinetic activation of copper(II) for for oxidative chemistry, which re-engages oxidized copper catalyst in CuAAC reaction (Scheme 3a). oxidative chemistry, which re-engages oxidized copper catalyst in CuAAC reaction (Scheme 3a). This This argument differs from the thermodynamic model that TBTA stabilizes the +1 oxidation state of argument differs from the thermodynamic model that TBTA stabilizes the +1 oxidation state of

Molecules 2016, 21, 1697

11 of 17

Molecules 2016, 21, 1697

11 of 17

copper. TBTA ligand might also increase the longevity of the copper catalyst (e.g., by reducing Molecules 2016,as 21,a1697 11 of 17 the copper. TBTA as a ligand might also increase the longevity of the copper catalyst (e.g., by reducing the likelihood of copper(I) acetylide polymer formation, Scheme 3b). However, the TBTA/copper complex likelihood of copper(I) acetylide polymer formation, Scheme 3b). However, the TBTA/copper complex copper. TBTA a ligand might also increase the longevity copper catalystAs (e.g., by reducing the 12, might not be the as most reactive copper(I) catalytic species of inthe these reactions. shown in Figure might not be the most reactive copper(I) catalytic species in these reactions. As shown in Figure 12, of copper(I) acetylide polymer formation, Scheme 3b). However, the TBTA/copper complex the likelihood copper catalyst produced in the TEA-involved reactions was more potent. In Figure 11, the copper the copper catalyst produced in the TEA-involved reactions was more potent. In Figure 11, the copper mightproduced not be thefrom mostligand-free reactive copper(I) catalytic species theseisreactions. Asreactive shown in Figure catalyst induction (bluein trace) least as one the with one12,with catalyst produced from ligand-free inductionperiod period (blue trace) is atat least as as reactive as the the copper catalyst produced in the TEA-involved reactions was more potent. In Figure 11, the copper the participation of TBTA (gray trace). the participation of TBTA (gray trace). catalyst produced from ligand-free induction period (blue trace) is at least as reactive as the one with the participation of TBTA (gray trace).

Scheme 3. Possible functions of TBTA in CuAAC.

Scheme 3. Possible functions of TBTA in CuAAC.

2.7. Competition ExperimentsScheme 3. Possible functions of TBTA in CuAAC.

2.7. Competition Experiments

Several combinations of the alkynes (5, 8, 10, 21) were used to react with 2-picolyl azide. The 2.7. Competition Experiments Several of the alkynes are (5, to 8,(a) 10, 21) the were used ranking to reactthat with purposescombinations of these competition experiments verify reactivity was2-picolyl establishedazide. Several combinations the alkynes (5, 8,(b)10, 21) to were react with 2-picolyl azide. Thewas using the individual NMRofexperiments; and observe how theytomay interfere with each other’s The purposes of these competition experiments are (a) used verify the reactivity ranking that reaction pathway. When p-nitrophenylacetylene (5), ethynyl methyl ketone (8) and 2-picolyl azide were purposes of these competition experiments are to (a) verify the reactivity ranking that was established established using the individual NMR experiments; and (b) observe how they may interfere with each mixed at individual 10 mM each,NMR after an induction period of 20 min, the reaction rapidly that involved using the experiments; and (b) observe how they proceeded may interfere with other’s other’s reaction pathway. When p-nitrophenylacetylene (5), ethynyl methyl ketone (8)each and 2-picolyl primarily alkyne 8 (Figure 15). Comparing to the reactivity ranking and the length of induction periods reaction p-nitrophenylacetylene (5), ethynyl ketone (8)reaction and 2-picolyl azide were azide were pathway. mixed at When 10 mM each, after an induction periodmethyl of 20 min, the proceeded rapidly acquired from the individual experiments, which had alkyne 5 more proceeded reactive than alkyne 8 without mixed at 10 mM each, after an induction period of 20 min, the reaction rapidly that involved that involved primarily alkyne 8 (Figure 15). Comparing to the reactivity ranking and the length sodium ascorbate (Figure15). 5), Comparing the observation from the competition experiment that alkyne primarily alkyne 8 (Figure to the reactivity ranking and the lengthsuggests of induction periods of induction periods acquired fromprecatalyst the individual experiments, which had alkynethe 5 more reactive 8 interacts with copper(II) acetate stronger than alkyne 5, which lengthens acquired from the individual experiments, which had alkyne 5 more reactive than alkyneinduction 8 without than alkyne 8 without ascorbate (Figure 5), the observation from thestep, competition experiment period.ascorbate Yet alkyne(Figure 8sodium is more in the post-induction, triazole-production as it outcompetes sodium 5),reactive the observation from the competition experiment suggests that alkyne suggests that alkyne 8 interacts with copper(II) acetate precatalyst stronger than alkyne 5, which alkyne 5 (Figure 15). 8 interacts with copper(II) acetate precatalyst stronger than alkyne 5, which lengthens the induction lengthens the induction period. Yet alkyne 8 is more reactive in the post-induction, triazole-production period. Yet alkyne 8 is more reactive in the post-induction, triazole-production step, as it outcompetes step, as it outcompetes alkyne 5 (Figure 15). alkyne 5 (Figure 15).

Figure 15. The evolution of the 1H-NMR spectra of the reaction between alkynes 5 (10 mM), 8 (10 mM), and 2-picolyl azide (10 mM). Cu(OAc)2·H2O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl.

Figure The evolutionofofthe the1 H-NMR H-NMR spectra spectra of alkynes 5 (10 mM), 8 (10 mM), Figure 15. 15. The evolution of the thereaction reactionbetween between alkynes 5 (10 mM), 8 (10 mM), and 2-picolyl azide (10 mM). Cu(OAc) 2·H2O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl. and 2-picolyl azide (10 mM). Cu(OAc)2 ·H2 O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl. 1


12 of 17

12 of 17

Mixing alkynes 10 mM mM each eachreplicated replicatedthe theshort shortinduction induction period Mixing alkynes5,5,20, 20,and and2-picolyl 2-picolyl azide azide at 10 period (5 (5 min) that was (Figure 16). 16). Compound Compound55was wasconsumed consumed faster than min) that wasobserved observedwith withalkyne alkyne 55 alone (Figure faster than compound 20,20,therefore of alkyne alkyne55than thanthat thatofof20.20.The The reaction compound thereforeconfirming confirmingthe the higher higher reactivity reactivity of reaction between alkyne2020and and2-picolyl 2-picolylazide azide was was accelerated by thethe between alkyne by the the presence presenceofofalkyne alkyne5, 5,because because relatively fast homocoupling of alkyne 5 provided copper(I) that was shared with alkyne 20, effectively relatively fast homocoupling of alkyne 5 provided copper(I) that was shared with alkyne 20, effectively cutting inductionperiod periodfrom frompreviously previously measured measured 80 cutting itsits induction 80min mintoto55min. min.

11 Figure 16.16. The evolution betweenalkynes alkynes5 5(10 (10mM), mM), mM), Figure The evolutionofofthe theH-NMR H-NMRspectra spectraof ofthe the reaction reaction between 2020 (10(10 mM), and 2-picolyl azide (10 mM). Cu(OAc) 2 ·H 2 O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl. and 2-picolyl azide (10 mM). Cu(OAc)2 ·H2 O (0.5 mM) was in (‘0 min’). Py = 2-pyridyl.

In In thethe last reactedwith with2-picolyl 2-picolylazide azidewithin within first lastcompetition competitionexperiment, experiment,alkyne alkyne 20, 20, which which reacted thethe first 3 h3 (Figure 5),5), and ascorbatewas wasadded addedatat3-h 3-hmark mark(Figure (Figure h (Figure andalkyne alkyne10, 10,which whichdid didnot not until until sodium sodium ascorbate 8),8), were mixed at 5 mM each with 2-picolyl azide (10 mM). Interestingly, after a relatively short induction were mixed at 5 mM each with 2-picolyl azide (10 mM). Interestingly, after a relatively short induction period ofof 2020min experienced an an80-min 80-mininduction inductionperiod), period), the triazole period min(alkyne (alkyne20 20alone alone at at 10 10 mM mM experienced the triazole formation of both alkynes started (Figure 17). Alkyne 10, which by itself was unreactive during formation of both alkynes started (Figure 17). Alkyne 10, which by itself was unreactive during the firstthe first h under otherwise same conditions, reacted faster than alkyne when they were mixed together. 3 h3 under otherwise same conditions, reacted faster than alkyne 2020 when they were mixed together. Presumably, acceleratethe thehomocoupling homocouplinginduction induction reaction Presumably,alkyne alkyne2020acted actedas asaaligand ligand for for copper copper to accelerate reaction alkyne soonasascopper(I) copper(I)isisproduced produced via the induction alkyne 10 10 of of alkyne 10.10. AsAssoon inductionreaction, reaction,the themore moreacidic acidic alkyne dominates the triazoleformation formationphase. phase. dominates the triazole


13 of 17 13 of 17

1 Figure 17. Figure 17. The The evolution evolution of of the the 1H-NMR H-NMR spectra spectra of of the the reaction reaction between between alkynes alkynes 10 10 (5 (5 mM), mM), 20 20 (5 (5 mM), mM), and 2-picolyl azide (10 mM). Cu(OAc) 2 ·H 2 O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl. and 2-picolyl azide (10 mM). Cu(OAc)2 ·H2 O (0.5 mM) was added in (‘0 min’). Py = 2-pyridyl.

3. Experimental 3. Experimental Section Section 3.1. Materials Methods 3.1. Materials and and General General Methods Reagents and and solvents from various various commercial commercial sources sources and and used used without without further Reagents solvents were were purchased purchased from further purification unless otherwise stated. Analytical thin layer chromatography (TLC) was performed using purification unless otherwise stated. Analytical thin layer chromatography (TLC) was performed precoated TLC plates with silica 60gel F254. Flash column chromatography was performed using using precoated TLC plates with gel silica 60 F254. Flash column chromatography was performed 1H- and 13C-NMR 1 13 40–63 μm (230–400 mesh ASTM) silica gel as the stationary phase. spectra were using 40–63 µm (230–400 mesh ASTM) silica gel as the stationary phase. H- and C-NMR spectra recorded at 500 and 125 MHz respectively, at 295 K unless otherwise noted. The chemical shifts (δ) were recorded at 500 and 125 MHz respectively, at 295 K unless otherwise noted. The chemical were recorded ppm relative to the residual 3 or CHD 2CN asor internal High standards. resolution shifts (δ) werein recorded in ppm relative to CHCl the residual CHCl CHD2standards. CN as internal 3 mass spectra (HRMS) were obtained under electrospray ionization (ESI) using a time-of-flight (TOF) High resolution mass spectra (HRMS) were obtained under electrospray ionization (ESI) using a analyzer. Alkynes [31], 17 [42], 18 [43], and 17 19 [42], [44] were prepared using reported procedures. Other time-of-flight (TOF)4 analyzer. Alkynes 4 [31], 18 [43], and 19 [44] were prepared using reported alkynes wereOther acquired from were commercial sources. For ensuringsources. reproducibility, alkynes 8, 14–16, and procedures. alkynes acquired from commercial For ensuring reproducibility, 22–24 were re-purified via distillation, while alkynes 5, 10, 11, and 13 were re-purified alkynes 8, 14–16, and 22–24 were re-purified via distillation, while alkynes 5, 10, 11, andvia 13 silica were column chromatography. re-purified via silica column chromatography. 3.2. Synthesis of Compound 20 (Scheme 4) In a round-bottom flask compound 25 [44] (303 mg, 1.2 mmol) was dissolved dissolved in in THF THF (4.8 (4.8 mL). mL). K22CO33 (3.2 mmol, 442 (84(84 µL,μL, 1.0 1.0 mmol) were added. The The flaskflask was 442mg) mg)and andN-methyl N-methylpropargylamine propargylamine mmol) were added. equipped withwith an argon balloon and the was let r.t.atfor h.24 The mixture was was equipped an argon balloon andreaction the reaction wasgoletatgo r.t.24for h. reaction The reaction mixture then diluted with with ethyl ethyl acetate, and passed through a pad of K2 CO . 2The was removed under was then diluted acetate, and passed through a pad of 3K CO3solvent . The solvent was removed under reduced pressure tothe afford the product as aoil yellow oil (144 60% yield). FTIR (neat, reduced pressure to afford product as a yellow (144 mg, 60% mg, yield). FTIR (neat, νmax (cm−ν1max )): 1 −1 1 (cm (C )):≡ 3293 (C≡C-H), 2104 NMR3 )(CDCl 3) δ/ppm 1H), 7.39–7.33 3H), 7.27–7.25 3293 C-H), 2104 (C≡ C); (C≡C); H-NMRH(CDCl δ/ppm 7.41 (s, 7.41 1H),(s, 7.39–7.33 (m, 3H),(m, 7.27–7.25 (m, 2H), 13 C-NMR (m, 2H), 5.513.71 (s, 1H), 3.713.31 (s, 2H), 2.4 Hz, 2.332.25 (s, 3H), 2.5 Hz, 1H); 13(CDCl C NMR 5.51 (s, 1H), (s, 2H), (d, J 3.31 = 2.4(d, Hz,J =2H), 2.33 2H), (s, 3H), (t, J =2.25 2.5 (t, Hz,J =1H); 3) (CDCl3) δ/ppm 145.4, 134.7, 129.2,128.9, 128.2, 122.7, 78.5, 73.7, 54.3, 50.7, 45.3, 41.7; HRMS(ESI): m/z [M + Na+] calcd for C14H16N4Na, 263.1273; found, 263.1269.

Molecules 2016, 21, 1697

14 of 17

δ/ppm 145.4, 134.7, 129.2,128.9, 128.2, 122.7, 78.5, 73.7, 54.3, 50.7, 45.3, 41.7; HRMS(ESI): m/z [M + Na+ ] calcd for2016, C1421, H161697 N4 Na, 263.1273; found, 263.1269. Molecules 14 of 17 Molecules 2016, 21, 1697

14 of 17

Scheme 4. 4. Synthesis Synthesis of of compound compound 20. 20. Reagents Reagents and and conditions: conditions: (a) K2CO Scheme (a) K CO3,,THF, THF, r.t., r.t., 24 24 h, h, 60%. 60%. Scheme 4. Synthesis of compound 20. Reagents and conditions: (a) K22CO33, THF, r.t., 24 h, 60%.

3.3. Synthesis (Scheme 5) 5) 3.3. Synthesis of of Compound Compound 21 21 3.3. Synthesis of Compound 21 (Scheme (Scheme 5) In aa round-bottom round-bottom flask In flask compound compound 26 26 [45] [45] (359 (359 mg, mg, 1.0 1.0 mmol) mmol) was was dissolved dissolved in in THF THF (5 (5 mL). mL). In a round-bottom flask compound 26 [45] (359 mg, 1.0 mmol) was dissolved in THF (5 mL). K2CO 3 (520 mg, 3.8 mmol) and propargyl bromide (131 μL, 1.2 mmol, in 80 wt % toluene solution) were K CO (520 mg, 3.8 mmol) and propargyl bromide (131 µL, 1.2 mmol, in 80 wt % toluene solution) were 2 3 K2CO3 (520 mg, 3.8 mmol) and propargyl bromide (131 μL, 1.2 mmol, in 80 wt % toluene solution) were added. The reaction was run at at r.t. forfor 24 h, followed by added. The flask flaskwas wasequipped equippedwith withan anargon argonballoon. balloon.The The reaction was run h, followed added. The flask was equipped with an argon balloon. The reaction was run at r.t.r.t. for 24 24 h, followed by dilution with EtOAc (50 mL), and and was passed through a pada of K2CO 3. Solvent was removed under by dilution with EtOAc (50 mL), passed through pad CO3 . Solvent was removed dilution with EtOAc (50 mL), and was was passed through a pad of K2of COK3.2Solvent was removed under reduced pressure to afford a yellow oil oil crude product, which under reduced pressure to afford a yellow crude product, whichwas waspurified purifiedby by recrystallization recrystallization reduced pressure to afford a yellow oil crude product, which was purified by recrystallization (CH 2Cl2/diethyl ether) to a white solid (167 mg, 42% yield). 11H-NMR (CDCl3): δ/ppm 7.48 (s, 2H), (CH Cl /diethyl ether) to a white solid (167 mg, 42% yield). H-NMR (CDCl ): δ/ppm 7.48 (s, (s, 2H), 2H), (CH22Cl22/diethyl ether) to a white solid (167 mg, 42% yield). 1H-NMR (CDCl33): δ/ppm 7.48 7.38–7.33 (m, 6H), 7.26–7.25 (m, 2H), 5.50 (s, 4H), 3.81 (s, 4H), 3.32 (d, J = 2.4 Hz, 2H), 2.23(t, J = 2.5 7.38–7.33 (t, JJ == 2.5 2.5 Hz, Hz, 7.38–7.33 (m, (m, 6H), 6H), 7.26–7.25 7.26–7.25 (m, (m,2H), 2H),5.50 5.50(s, (s,4H), 4H),3.81 3.81(s, (s,4H), 4H),3.32 3.32(d, (d,JJ== 2.4 2.4 Hz, Hz,2H), 2H),2.23 2.23(t, Hz, 1H). This compound was reported [46]. 1H). This compound was reported [46]. 1H). This compound was reported [46].

Scheme 5. Synthesis of compound 21. Reagents and conditions: (a) K2CO3, THF, rt, 24 h, 42%. Scheme 5. 5. Synthesis Synthesis of of compound compound 21. 21. Reagents Reagents and and conditions: conditions: (a) (a) K K22CO Scheme CO33,, THF, THF, rt, rt, 24 24 h, h, 42%. 42%.

3.4. General Procedure for 1H-NMR Reaction Monitoring Experiments 3.4. General for 11H-NMR 3.4. General Procedure Procedure for H-NMR Reaction Reaction Monitoring Monitoring Experiments Experiments The stock solutions were prepared in CD3CN for all reaction components except Cu(OAc)2·H2O The stock stock solutions solutions were were prepared preparedin inCD CD3CN CN for for all all reaction reaction components componentsexcept exceptCu(OAc) Cu(OAc)2··H 2O The and sodium ascorbate. The Cu(OAc)2·H 2O stock solution was prepared in CH 3CN. This solution 3 2 H2 O and sodium ascorbate. The Cu(OAc) 2·H2O stock solution was prepared in CH3CN. This solution and sodium The Cu(OAc) O stock solution was experiments prepared in CH Thisreproducibility. solution needs needs to be ascorbate. freshly made before the monitoring to3 CN. ensure 2 ·H12H-NMR 1H-NMR monitoring experiments to ensure reproducibility. needs to be freshly made before the 1 to be freshly madestock before the H-NMR monitoring experiments ensure reproducibility. Sodium ascorbate solution was prepared in deionized water.toCD 3CN (372.5 μL), alkyneSodium (50 μL, Sodium ascorbate stock was solution was prepared in deionized water. CD3CN (372.5 μL), alkyne (50 μL, ascorbate stock solution prepared in deionized water. CD CN (372.5 µL), alkyne (50 µL, 100 mM), 100 mM), and 2-picolyl azide or benzyl azide (52.5 μL, 100 3mM) were added sequentially into an 100 mM), andazide 2-picolyl azide azide or benzyl azide (52.5 μL, 100 added mM) were added sequentially into an and or by benzyl µL,NMR 100 mM) sequentially into of anthe NMR tube NMR2-picolyl tube and mixed flipping the(52.5 sealed tube were 6 times. A 1H-NMR spectrum mixture 1H-NMR spectrum of the mixture NMR tube and mixed by flipping the sealed NMR tube 6 times. A 1 and by flipping the sealed2NMR tube 6 times. Awas H-NMR of thetube mixture taken. was mixed taken. Afterwards, Cu(OAc) ·H2O (25 μL, 10 mM) addedspectrum into the NMR and was the sealed was taken. Afterwards, Cu(OAc) 2·H2O (25 μL, 10 mM) was added into the NMR tube and the sealed Afterwards, Cu(OAc) · H O (25 µL, 10 mM) was added into the NMR tube and the sealed tube was tube was flipped 6 times 2 to 2 obtain a homogenous solution. The sample was inserted and the reaction tube was flipped 6 times to obtain a homogenous solution. The sample was inserted and the reaction flipped 6 timesunder to obtain a homogenous The sample inserted theIfreaction was monitored a multiple scan modesolution. for 3 h. The interval ofwas the scans wasand 5 min. there waswas no was monitored under a multiple scan mode for 3 The h. The interval of thescans scanswas was55min. min.IfIfthere there was was no no monitored under a multiple scan mode for 3 h. interval of the reaction after 3 h, sodium ascorbate (3.2 μL, 200 mM) was added into the NMR tube and mixed in the reaction after 3 h, sodium ascorbate (3.2 μL, 200 mM) was added into the NMR tube and mixed in the 1H-NMR reaction after 3 h, sodium ascorbate 200 mM) addedbyinto the NMR tubethe and mixed in the same way to generate Cu(I) in situ. (3.2 The µL, reaction was was followed under same multiple 1H-NMR under the same multiple same way to generate Cu(I) in situ. The reaction was followed by 1 same way to scan mode forgenerate anotherCu(I) 5 h. in situ. The reaction was followed by H-NMR under the same multiple scan mode mode for for another another 55 h. h. scan For other experiments, in addition to adjusting the concentrations of the abovementioned reaction For other other experiments, experiments, in addition to adjusting the of reaction For addition to2O, adjusting the concentrations concentrations of the the abovementioned abovementioned reaction components (azide, alkyne, in Cu(OAc) 2·H and sodium ascorbate), appropriate amounts of the stock components (azide, alkyne, Cu(OAc) 2·H2O, and sodium ascorbate), appropriate amounts of the stock components (azide,oralkyne, Cu(OAc) · H O, and sodium ascorbate), appropriate amounts of the stock solutions of TBTA TEA (both in CD 3 CN) were added as required. 2 2 solutions of of TBTA TBTA or or TEA TEA (both (both in in CD CD3CN) CN) were were added added as as required. required. solutions All the concentrations listed in the 3 procedure are stock solution concentrations of individual All the the concentrations concentrations listed listed in in the the procedure procedure are are stock stock solution solution concentrations of of individual individual All reaction participants. The final concentrations of reaction componentsconcentrations are listed in the figures and reaction participants. The final concentrations of reaction components are listed in the figures and reaction participants. The final concentrations of reaction components are listed in the figures and figure captions. figure captions. figure captions. 3.5. Oxidative Homocoupling of Phenylacetylene (Table 1) 3.5. Oxidative of Phenylacetylene Phenylacetylene (Table (Table 1) 1) 3.5. Oxidative Homocoupling Homocoupling of To a 10-mL round-bottom flask t-BuOH (1 mL) was added. Phenylacetylene (46.1 μL, 0.42 mmol), To aa 10-mL 10-mL round-bottom round-bottom flask flask t-BuOH (1 (1 mL) was was added. added. Phenylacetylene (46.1 μL, mmol), To Phenylacetylene (46.1reaction µL, 0.42 0.42mixture mmol), additive (0.02 mmol), Cu(OAc)2·H2Ot-BuOH (40 mg, 0.2mL) mmol) were added in this order. The additive (0.02 mmol), Cu(OAc) 2 ·H 2 O (40 mg, 0.2 mmol) were added in this order. The reaction mixture additive (0.02 mmol), Cu(OAc) (40reaction mg, 0.2 finished, mmol) were in mixture this order. The reaction mixture 2 ·H2 Othe was stirred under air for 18 h. After theadded reaction was diluted with ethyl was stirred stirred under under air air for for 18 h. After the reaction finished, the reaction mixture was diluted with ethyl ethyl was 18 h. After the reaction finished, the reaction mixture was diluted with acetate (50 mL), then washed with saturated brine three times. The organic layer was dried over acetate (50 mL), mL), then then washed washed with with saturated saturated brine brine three times. times. The organic layer dried over over acetate organic layer was was dried 1H-NMR Na2SO4,(50 and the solvent was removed under reducedthree pressure. TheThe spectrum of the residue 11H-NMR spectrum of the residue Na 2 SO 4 , and the solvent was removed under reduced pressure. The Na the solvent wasconversion removed under reduced pressure. The spectrum the residue 4 , and was2 SO taken, from which the to diyne was calculated fromH-NMR the analysis of theof7.4–7.6 ppm was taken, from which the conversion1to diyne was calculated from the analysis of the 7.4–7.6 ppm region (underlined). Phenylacetylene: H-NMR (CDCl3) δ/ppm 7.51–7.49 (m, 2H), 7.38–7.31 (m, 3H), region (underlined). Phenylacetylene: 1H-NMR (CDCl3) δ/ppm 7.51–7.49 (m, 2H), 7.38–7.31 (m, 3H), 3.08 (s, 1H); 1,4-Diphenylbuta-1,3-diyne data reported in literature [47]: 1H-NMR (270 MHz, CDCl3) 3.08 (s, 1H); 1,4-Diphenylbuta-1,3-diyne data reported in literature [47]: 1H-NMR (270 MHz, CDCl3) δ/ppm 7.51–7.55 (m, 4H), 7.30–7.40 (m, 6H). δ/ppm 7.51–7.55 (m, 4H), 7.30–7.40 (m, 6H).

Molecules 2016, 21, 1697

15 of 17

was taken, from which the conversion to diyne was calculated from the analysis of the 7.4–7.6 ppm region (underlined). Phenylacetylene: 1 H-NMR (CDCl3 ) δ/ppm 7.51–7.49 (m, 2H), 7.38–7.31 (m, 3H), 3.08 (s, 1H); 1,4-Diphenylbuta-1,3-diyne data reported in literature [47]: 1 H-NMR (270 MHz, CDCl3 ) δ/ppm 7.51–7.55 (m, 4H), 7.30–7.40 (m, 6H). 4. Conclusions In this work, it is confirmed that C-H acidic terminal alkynes have relatively high reactivity in CuAAC reaction. It is also discovered that alkynes that are precursors of CuAAC-accelerating poly(triazolylmethyl) ligands have relatively high reactivity despite having a low acidity on par with the typical aliphatic terminal alkynes. Alkyne deprotonation and azide binding to copper are the slow steps in the CuAAC pathway, which considerably influence the rate of the overall reaction [41]. Consequently, it is understandable that acidic alkynes (i.e., 1–13) and chelating azides such as 2-picolyl azide have high reactivity in CuAAC reactions. The fact that aliphatic alkynes 20 and 21 with relatively low C-H acidity are also fast-reacting is somewhat a myth. Indeed these compounds, or their triazole products from CuAAC, bear resemblance to the poly(triazolylmethyl) ligands such as TBTA that are known to accelerate CuAAC, yet it has not been entirely understood how TBTA accelerates the overall CuAAC reaction. It should also be noted that in this work the reactivity of these alkynes was compared in the presence of 2-picolyl azide, the most thoroughly studied chelating azide to date that delivers reliably high reactivity in CuAAC reactions. When the non-chelating benzyl azide was used in its place, the reactions were drastically deaccelerated, reaffirming that both azide and alkyne components have tremendous influence on the overall rate of the CuAAC reaction. Acknowledgments: This work was supported by the National Science Foundation (CHE1213574) and the Florida State University. Author Contributions: L.Z., X.Z., and P.L. conceived and designed the experiments; X.Z. and P.L. performed the experiments and analyzed the data; L.Z. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2.

3. 4. 5. 6. 7. 8. 9. 10.

Tornøe, C.W.; Christensen, C.; Meldal, M. Peptidotriazoles on solid phase: [1,2,3]-Triazoles by regiospecific copper(I)-catalyzed 1,3-dipolar cycloadditions of terminal alkynes to azides. J. Org. Chem. 2002, 67, 3057–3064. [CrossRef] [PubMed] Rostovtsev, V.V.; Green, L.G.; Fokin, V.V.; Sharpless, K.B. A stepwise huisgen cycloaddition process: Copper(I)-catalyzed regioselective “ligation” of azides and terminal alkynes. Angew. Chem. Int. Ed. 2002, 41, 2596–2599. [CrossRef] Finn, M.G.; Fokin, V.V. Click chemistry: Function follows form. Chem. Soc. Rev. 2010, 39, 1231–1232. [CrossRef] [PubMed] Košmrlj, J. Click Triazoles; Springer: Berlin/Heidelberg, Germany, 2012; Volume 28. Ramil, C.P.; Lin, Q. Bioorthogonal chemistry: Strategies and recent developments. Chem. Commun. 2013, 49, 11007–11022. [CrossRef] [PubMed] Yang, M.; Li, J.; Chen, P.R. Transition metal-mediated bioorthogonal protein chemistry in living cells. Chem. Soc. Rev. 2014, 43, 6511–6526. [CrossRef] [PubMed] Lang, K.; Chin, J.W. Bioorthogonal reactions for labeling proteins. ACS Chem. Biol. 2014, 9, 16–20. [CrossRef] [PubMed] Lang, K.; Chin, J.W. Cellular incorporation of unnatural amino acids and bioorthogonal labeling of proteins. Chem. Rev. 2014, 114, 4764–4806. [CrossRef] [PubMed] Tra, V.N.; Dube, D.H. Glycans in pathogenic bacteria—Potential for targeted covalent therapeutics and imaging agents. Chem. Commun. 2014, 50, 4659–4673. [CrossRef] [PubMed] McKay, C.S.; Finn, M.G. Click chemistry in complex mixtures: Bioorthogonal bioconjugation. Chem. Biol. 2014, 21, 1075–1101. [CrossRef] [PubMed]

Molecules 2016, 21, 1697

11. 12. 13. 14. 15.

16.

17.

18. 19. 20.

21.

22.

23.

24.

25. 26.

27. 28. 29. 30. 31. 32.

16 of 17

Patterson, D.M.; Nazarova, L.A.; Prescher, J.A. Finding the right (bioorthogonal) chemistry. ACS Chem. Biol. 2014, 9, 592–605. [CrossRef] [PubMed] Boutureira, O.; Bernardes, G.J.L. Advances in chemical protein modification. Chem. Rev. 2015, 115, 2174–2195. [CrossRef] [PubMed] Patterson, D.M.; Prescher, J.A. Orthogonal bioorthogonal chemistries. Curr. Opin. Chem. Biol. 2015, 28, 141–149. [CrossRef] [PubMed] Mohan, K.; Weiss, G.A. Chemically modifying viruses for diverse applications. ACS Chem. Biol. 2016, 11, 1167–1179. [CrossRef] [PubMed] Kennedy, D.C.; McKay, C.S.; Legault, M.C.B.; Danielson, D.C.; Blake, J.A.; Pegoraro, A.F.; Stolow, A.; Mester, Z.; Pezacki, J.P. Cellular consequences of copper complexes used to catalyze bioorthogonal click reactions. J. Am. Chem. Soc. 2011, 133, 17993–18001. [CrossRef] [PubMed] Gary R Abel, J.; Calabrese, Z.; Ayco, J.; Hein, J.E.; Ye, T. Measuring and suppressing the oxidative damage to DNA during Cu(I)-catalyzed azide-alkyne cycloaddition. Bioconjugate Chem. 2016, 27, 698–704. [CrossRef] [PubMed] Li, S.; Cai, H.; He, J.; Chen, H.; Lam, S.; Cai, T.; Zhu, Z.; Bark, S.J.; Cai, C. Extent of the oxidative side reactions to peptides and proteins during the cuaac reaction. Bioconjugate Chem. 2016, 27, 2315–2322. [CrossRef] [PubMed] Brotherton, W.S.; Michaels, H.A.; Simmons, J.T.; Clark, R.J.; Dalal, N.S.; Zhu, L. Apparent copper(II)-accelerated azide-alkyne cylcoaddition. Org. Lett. 2009, 11, 4954–4957. [CrossRef] [PubMed] Kuang, G.-C.; Michaels, H.A.; Simmons, J.T.; Clark, R.J.; Zhu, L. Chelation-assisted, copper(II) acetate-accelerated azide-alkyne cycloaddition. J. Org. Chem. 2010, 75, 6540–6548. [CrossRef] [PubMed] Kuang, G.-C.; Guha, P.M.; Brotherton, W.S.; Simmons, J.T.; Stankee, L.A.; Nguyen, B.T.; Clark, R.J.; Zhu, L. Experimental investigation on the mechanism of chelation-assisted, copper(II) acetate-accelerated azide-alkyne cycloaddition. J. Am. Chem. Soc. 2011, 133, 13984–14001. [CrossRef] [PubMed] Brotherton, W.S.; Guha, P.M.; Hoa, P.; Clark, R.J.; Shatruk, M.; Zhu, L. Tridentate complexes of 2,6-bis(1,2,3-triazol-1-ylmethyl)pyridine and its organic azide precursors—An application of the copper(II) acetate-accelerated azide-alkyne cycloaddition. Dalton Trans. 2011, 40, 3655–3665. [CrossRef] [PubMed] Uttamapinant, C.; Tangpeerachaikul, A.; Grecian, S.; Clarke, S.; Singh, U.; Slade, P.; Gee, K.R.; Ting, A.Y. Fast, cell-compatible click chemistry with copper-chelating azides for biomolecular labeling. Angew. Chem. Int. Ed. 2012, 51, 5852–5856. [CrossRef] [PubMed] Jiang, H.; Zheng, T.; Lopez-Aguilar, A.; Feng, L.; Kopp, F.; Marlow, F.L.; Wu, P. Monitoring dynamic glycosylation in vivo using supersensitive click chemistry. Bioconjugate Chem. 2014, 25, 698–706. [CrossRef] [PubMed] Bevilacqua, V.; King, M.; Chaumontet, M.; Nothisen, M.; Gabillet, S.; Buisson, D.; Puente, C.; Wanger, A.; Taran, F. Copper-chelating azides for efficient click conjugation reactions in complex media. Angew. Chem. Int. Ed. 2014, 53, 5872–5876. [CrossRef] [PubMed] Machida, T.; Winssinger, N. One-step derivatization of reducing oligosaccharides for rapid and live-cellcompatible chelation-assisted cuaac conjugation. ChemBioChem 2016, 17, 811–815. [CrossRef] [PubMed] Uttamapinant, C.; Sanchez, M.I.; Liu, D.S.; Yao, J.Z.; White, K.A.; Grecian, S.; Clark, S.; Gee, K.R.; Ting, A.Y. Site-specific protein labeling using prime and chelation-assisted click chemistry. Nat. Protocols 2013, 8, 1620–1634. [CrossRef] [PubMed] Hein, J.E.; Fokin, V.V. Copper-catalyzed azide-alkyne cycloaddition (cuaac) and beyond: New reactivity of copper(I) acetylides. Chem. Soc. Rev. 2010, 39, 1302–1315. [CrossRef] [PubMed] Berg, R.; Straub, B.F. Advancements in the mechanistic understanding of the copper-catalyzed azide–alkyne cycloaddition. Beilstein J. Org. Chem. 2013, 9, 2715–2750. [CrossRef] [PubMed] Li, Z.; Seo, T.S.; Ju, J. 1,3-dipolar cycloaddition of azides with electron-deficient alkynes under mild condition in water. Tetrahedron Lett. 2004, 45, 3143–3146. [CrossRef] Glaser, M.; Årstad, E. “Click labeling” with 2-[18 F]fluoroethylazide for positron emission tomography. Bioconjugate Chem. 2007, 18, 989–993. [CrossRef] [PubMed] Kislukhin, A.A.; Hong, V.P.; Breitenkamp, K.E.; Finn, M.G. Relative performance of alkynes in copper-catalyzed azide–alkyne cycloaddition. Bioconjugate Chem. 2013, 24, 684–689. [CrossRef] [PubMed] Jin, L.; Tolentino, D.R.; Melaimi, M.; Bertrand, G. Isolation of bis(copper) key intermediates in cu-catalyzed azide-alkyne “click reaction”. Sci. Adv. 2015, 1, e1500304. [CrossRef] [PubMed]

Molecules 2016, 21, 1697

33.

34.

35. 36. 37. 38. 39. 40.

41. 42.

43.

44.

45.

46. 47.

17 of 17

Monasterio, Z.; Sagartzazu-Aizpurua, M.; Miranda, J.I.; Reyes, Y.; Aizpurua, J.M. Cationic 1,2,3-triazolium alkynes: Components to enhance 1,4-regioselective azide−alkyne cycloaddition reactions. Org. Lett. 2016, 18, 788–791. [CrossRef] [PubMed] Hatit, M.Z.C.; Sadler, J.C.; McLean, L.A.; Whitehurst, B.C.; Seath, C.P.; Humphreys, L.D.; Young, R.J.; Watson, A.J.B.; Burley, G.A. Chemoselective sequential click ligations directed by enhanced reactivity of an aromatic ynamine. Org. Lett. 2016, 18, 1694–1697. [CrossRef] [PubMed] Chan, T.R.; Hilgraf, R.; Sharpless, K.B.; Fokin, V.V. Polytriazoles as copper(i)-stabilizing ligands in catalysis. Org. Lett. 2004, 6, 2853–2855. [CrossRef] [PubMed] The reactions would proceed to completion under preparative conditions, and many of these alkynes have been reported engage in copper(II) acetate-catalyzed CuAAC reactions. See 19. Romary, J.K.; Barger, J.D.; Bunds, J.E. New multidentate alpha-pyridyl ligand. Coordination of bis(2-pyridylmethyl)amine with transition metal ions. Inorg. Chem. 1968, 7, 1142–1145. [CrossRef] Michaels, H.A.; Zhu, L. Ligand-assisted, copper(II) acetate-accelerated azide–alkyne cycloaddition. Chem. Asian J. 2011, 6, 2825–2834. [CrossRef] [PubMed] Hiroki, H.; Ogata, K.; Fukuzawa, S.-I. 2-ethynylpyridine-promoted rapid copper(I) chloride catalyzed azide-alkyne cycloaddition reaction in water. Synlett 2013, 24, 843–846. [CrossRef] Donnelly, P.S.; Zanatta, S.D.; Zammit, S.C.; White, J.M.; Williams, S.J. “Click” cycloaddition catalysts: Copper(I) and copper(II) tris(triazolylmethyl)amine complexes. Chem. Commun. 2008, 2459–2461. [CrossRef] [PubMed] Zhu, L.; Brassard, C.J.; Zhang, X.; Guha, P.M.; Clark, R.J. On the mechanism of copper(I)-catalyzed azide-alkyne cycloaddition. Chem. Rec. 2016, 16, 1501–1517. [CrossRef] [PubMed] Bradbury, B.J.; Baumgold, J.; Paek, R.; Kammula, U.; Zimmet, J.; Jacobson, K.A. Muscarinic receptor binding and activation of second messengers by substituted N-methyl-N-[4-(1-azacycloalkyl)-2-butynyl]acetamides. J. Med. Chem. 1991, 34, 1073–1079. [CrossRef] [PubMed] Michaels, H.A.; Murphy, C.S.; Clark, R.J.; Davidson, M.W.; Zhu, L. 2-anthryltriazolyl-containing multidentate ligands: Zinc-coordination mediated photophysical processes and potential in live-cell imaging applications. Inorg. Chem. 2010, 49, 4278–4287. [CrossRef] [PubMed] Simmons, J.T.; Allen, J.R.; Morris, D.R.; Clark, R.J.; Levenson, C.W.; Davidson, M.W.; Zhu, L. Integrated and passive 1,2,3-triazolyl groups in fluorescent indicators for zinc(II) ions—thermodynamic and kinetic evaluations. Inorg. Chem. 2013, 52, 5838–5850. [CrossRef] [PubMed] Presolski, S.I.; Hong, V.; Cho, S.-H.; Finn, M.G. Tailored ligand acceleration of the cu-catalyzed azide-alkyne cycloaddition reaction: Practical and mechanistic implications. J. Am. Chem. Soc. 2010, 132, 14570–14576. [CrossRef] [PubMed] Chan, T.R.; Fokin, V.V. Polymer-supported copper(i) catalysts for the experimentally simplified azide–alkyne cycloaddition. QSAR Comb. Sci. 2007, 26, 1274–1279. [CrossRef] Kamata, K.; Yamaguchi, S.; Kotani, M.; Yamaguchi, K.; Mizuno, N. Efficient oxidative alkyne homocoupling catalyzed by a monomeric dicopper-substituted silicotungstate. Angew. Chem. Int. Ed. 2008, 47, 2407–2410. [CrossRef] [PubMed]

Sample Availability: Not available. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

Structural Determinants of Alkyne Reactivity in

Structural Determinants of Alkyne Reactivity in

Suggest Documents

Determinants of Molecular Reactivity as - BioMedSearch

Structural features and catalytic reactivity in the

Structural Dependence of Reactivity of Alkylcarboxylate-Stabilized ...

structural determinants of police effectiveness in

Structural Determinants Of Customer Satisfaction In ... - ScienceDirect

Structural Determinants Of Customer Satisfaction In ... - ScienceDirect

Structural Characteristics and Reactivity Relationships of ... - MDPI

Structural determinants of limited proteolysis

Structural Determinants of Antiretroviral Therapy

reactivity of adsorbed and structural iron in hectorite as ... - CiteSeerX

Astrocyte structural reactivity and plasticity in models of retinal ... - Core

Structural Insights into Autoreactive Determinants in Thyroid ...

Structural Effects on Stability and Reactivity - Springer

Recent advances in the development of alkyne

Structural Determinants of Unique Properties of ... - CyberLeninka

Structural Determinants of the Capacity of Heparin

Genetic Determinants of On-Aspirin Platelet Reactivity - CiteSeerX

Neighborhood Boundaries and Structural Determinants of Social ...

Structural Determinants of the Agonist-independent ... - oppi.uef.fi

Structural Determinants of Conformationally Selective, Prion-binding ...

Structural determinants of 5â²,6 ... - Repositorio UNAB

The Structural Determinants of the Immunoglobulin ...

Structural Determinants of the Dictyostatin ... - ACS Publications

The structural determinants of checkpoint activation - Genes ...

Structural Determinants of Alkyne Reactivity in