Tetrahedron Letters 56 (2015) 963–971
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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Digest Paper
Recent advances in transition metal-mediated functionalization of o-carboranes Zaozao Qiu ⇑ Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e
i n f o
Article history: Received 10 November 2014 Revised 22 December 2014 Accepted 2 January 2015 Available online 12 January 2015 Keywords: Carborane Carboryne Cycloaddition Coupling reaction Transition metal
a b s t r a c t Icosahedral carboranes constitute a class of structurally unique molecules with exceptionally thermal and chemical stabilities, which limits the derivatization of these clusters. In view of the spectacular role of transition metals in synthetic chemistry, novel o-carborane functionalization methods for both cage carbon and boron vertices have been developed by using transition metal-promoted reactions. These methods offer a series of exceptionally efficient synthetic routes to a wide range of functionalized carboranes from readily available starting materials. An overview of recent advances in this field is presented in this digest. Ó 2015 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cage carbon functionalization of o-carborane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Copper-mediated reaction of lithiocarborane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition metal-promoted reaction of carboryne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction with alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reaction with alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cage boron functionalization of o-carboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cage B–I bond activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cage B–H bond activation and functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction Carboranes are a class of boron hydride clusters in which one or more BH vertices are replaced by CH units. They have many characteristics such as spherical geometry, remarkable thermal and chemical stability, and a hydrophobic molecular surface, leading to many applications in medicinal,1 materials,2 and coordination chemistry.3 The synthesis and properties of icosahedral carboranes were first reported in 1963,4 which has been the most extensively ⇑ Tel.: +86 21 54925569; fax: +86 21 54925383. E-mail address:
[email protected]
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investigated, of all known carboranes, during the last 50 years. o-Carborane was obtained by the reaction of acetylene with decaborane.5 Recently, it was reported that the addition of a catalytic amount of silver salt to the above reaction significantly enhances the yield of carborane formation.6 Unlike boron hydrides, o-carborane is stable in the presence of oxidizing agents, alcohols, and strong acids. It exhibits phenomenally thermal stability up to 400 °C. Under an inert atmosphere, it rearranges to m-carborane between 400 and 500 °C and to p-carborane between 600 and 700 °C (Scheme 1).7 One of the most important features of carborane is its ability to enter into substitution reactions at both the cage carbon and boron
http://dx.doi.org/10.1016/j.tetlet.2015.01.038 0040-4039/Ó 2015 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 H H HH + HC CH
unmarked H
H
Lewis base
I F
H o-Carborane
C BH
NO2
400 ~ 500 oC
H
600 ~ 700 oC
H n
H
H
H
NO 2
BuLi CuCl
I
H
m-Carborane
p-Carborane
NaH
H concentration time yield 0.1 M 48 h 41% 0.3 M 5 h 84%
Scheme 1. Synthesis and thermal isomerization of o-carborane.
Scheme 3. Synthesis of mono- and diaryl-o-carboranes.
atoms without degradation of the cage. The stability of the carborane cage is demonstrated under many reaction conditions used to prepare a wide range of C- and B-substituted carborane derivatives. In view of the important role of transition metals in chemical transformations, transition metal promoted carborane functionalization methodology has experienced tremendous growth to afford various novel carborane derivatives which cannot be generated by other conventional methods. Despite the relatively weaker inductive electron attraction in m- and p-carboranes, which leads to reduced C–H reactivity toward metallation at carbon atoms, many o-carborane derivatization methods are applicable to m- and p-carboranes.7a As the icosahedral o-carborane has been most extensively investigated and is commercially available, this digest focuses on recent advances in transition metal-mediated/-catalyzed functionalization of o-carboranes.
H I H
I
H
I
Cu
I H
Cu I
Cage carbon functionalization of o-carborane
H
Copper-mediated reaction of lithiocarborane I
During the past decades, investigations in the field of carbonsubstituted carboranes were directed at improving the synthetic methods for the preparation of organic and organometallic
H
0
Scheme 4. Reaction of C,C -dicopper(I) o-carborane with ortho-, meta- or paradiiodobenzene.
H H n
BuLi CuCl
X
R Ph
H
H
N
H
N Ph
Zn N
N
Fe Ph
I
H N I
I
N
H H
Scheme 2. Cu(I)-mediated coupling of o-carboranyl with arylhalides.
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Z. Qiu / Tetrahedron Letters 56 (2015) 963–971
Li
Cu(Tol)
C B BH
R Li
unmarked
Cu(Tol) carboryne 1,3-dehydroo-carborane
H
(Tol)Cu
[M]
[M]
metal-carboryne H
Cu(Tol)
Chart 1. Carboryne, 1,3-dehydro-o-carborane, and metal–carboryne. Scheme 5. Synthesis of 1,10 -bis(o-carborane) from Cu-mediated reaction. R1
carboranyl compounds used in materials as well as in biological and medical applications. There are two conventional synthetic methods leading to carbon-substituted carboranes: the reaction of substituted acetylenes with decaborane and electrophilic substitutions of lithiocarborane.7 The latter is limited only to electrophiles with sp3 carbons. Copper-mediated coupling reaction is developed for the synthesis of alkenyl, alkynyl, and aryl carboranes.8 Selected examples are illustrated bellow. Direct Cu(I)-mediated coupling of o-carborane is used to introduce various aryl containing functional groups (Scheme 2). For examples, carborane–ferrocene conjugated dyads are synthesized to investigate their nonlinear optical (NLO) properties;9 direct coupling of o-carborane with a porphyrin via a carbon–carbon bond is achieved;10 and Cu-mediated method is used to introduce a styrene conjugation bridge in the carborane-based tribranched compounds to study the influence of isomeric carboranyl units as well as their push and pull electronic effects on photophysical properties in bioimaging.11 As the aforementioned reactions can afford only monoaryl-ocarboranes, the aromatic nucleophilic substitution (SNAr) reaction of 1-aryl-o-carboranes with 4-nitrofluorobenzene in the presence of NaH or KOtBu is then developed to yield various 1,2-diaryl-o-carboranes that are useful precursors for macromolecular construction and drug design (Scheme 3).12 On the other hand, Ullmann-type coupling reaction of 1-iodonaphthalene with o-carboranyl copper gives efficiently 1-(o-carboranyl)-naphthalene (Scheme 3). It is found that the concentration is crucial for this reaction. When the reaction is conducted at a high concentration, such as 0.3 M, it affords the desired compound in excellent yield (84%), which is much better than that of 41% at 0.1 M concentration.13 Reactions between C,C0 -dicopper(I) derivative of o-carborane and ortho-, meta-, or para-diiodobenzene are investigated. The reactions with 1,3- or 1,4-C6H4I2 provide 1,3-bis(10 -o-carboranyl)benzene and 1,4-bis(10 -o-carboranyl)benzene, respectively, whereas reaction with 1,2-C6H4I2 afford unexpectedly 2,20 -bis(10 o-carboranyl)biphenyl, [HCB10H10CC6H4]2 (Scheme 4). Electronic communication between the carborane cages via the para-phenylene bridge is found only in the para-substituted benzene compound.14 CuCl-mediated C–C coupling of dilithiocarborane in toluene is a facile and practical method for the preparation of 1,10 -bis(o-carborane) (Scheme 5). The reaction intermediate 1,10 :2,20 -[Cu(toluene)]2(C2B10H10)2 is isolated from the reaction system before hydrolysis, which confirms the transmetalation process.15 Transition metal-promoted reaction of carboryne Carboryne, 1,2-dehydro-o-carborane, is a three-dimensional relative of benzyne (Chart 1).16 It can react with alkenes, dienes, and alkynes in [2+2], [4+2] cycloaddition and ene-reaction patterns,17 similar to those of benzyne.18 Although these reactions show potential for the preparation of functionalized carboranes in a single operation, they are complex and do not proceed in a
δ−
Cp 2Zr
δ+
R2
R1 Cl Cp2Zr
Li
OEt2 R1
OEt2
R2
R2
Cp 2Zr
toluene (R 1, R 2 = alkyl, aryl)
Scheme 6. Reaction of zirconocene–carboryne precursor with alkynes.
Et
Et H H
Et
N
Et
H3+O
XylNC Et
Et I Et
Cp2Zr
H
Et
CuCl I2 19a CuCl2 Et
I
Et
Et
I CuCl
Et
Scheme 7. Transformations of zirconacyclopentene incorporating a carboranyl unit.
controlled manner. It has been reported that the M–Ccage (Ccage: hypervalent cage carbon) r bonds in metal–carboranyl complexes (Chart 1) are generally inert toward various electrophiles due to steric reasons.19 To overcome this problem, metal–carboryne complexes with a metallacyclopropane structure are constructed to reduce the steric hindrance around the M–Ccage bond and create the ring strains, thus facilitating the reactivity of M–Ccage bonds.20 The first example of metal–carboryne, (g2-C2B10H10)Ni(PPh3)2, was communicated in 1973,21a much earlier than the parent carboryne intermediate.17a DFT calculations suggest that bonding interactions between metal and carbon atoms are best described as a resonance hybrid of both the M–C r and M–C p bonding forms,22 similar to those described for metal–benzyne complexes.23 Reactivity patterns of these complexes are dependent on the nature of transition metals.24 Reaction with alkynes Ate-complex Cp2Zr(l-Cl)(l-C2B10H10)Li(OEt2)2, prepared from the reaction of Cp2ZrCl2 with 1 equiv of Li2C2B10H10 in ether, can
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Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 R1 PPh3 2 R1
R
R2 R2
2
1
δ+
δ−
R
δ+
+
R2 R1, R2 = alkyl, aryl
2
R1
δ+
δ−
[Ni(II)]
R2
R2
R R1
Ni
Ni
PPh3
Ph3P
δ−
R2
- [Ni] R1
R1 R2
R1
2
R1
PPh3
R
OEt2 Cl ZrCp2 Li OEt
R1
ZrCp2
Ni
Fe(II)
2
R1
[Ni]
Fe(III) [Ni(0)]
R2
R3 R
Scheme 8. Reaction of nickel–carboryne with internal alkynes.
R3
3
R
R4
δ+
be viewed as a precursor of zirconocene–carboryne Cp2Zr (g2-C2B10H10).25 Treatment of this complex with various kinds of alkynes in refluxing toluene affords the carborane version of zirconacyclopentenes 1,2-[Cp2ZrC(R1)@C(R2)]-1,2-C2B10H10 (Scheme 6). X-ray structures show that such zirconacyclopentene ring adopts a planar geometry, resembling those of zirconacyclopentadienes. This reaction cannot proceed in donor solvents such as Et2O and THF, suggesting that the coordination of alkyne to the Zr atom is essential for the subsequent insertion. No double insertion products are observed even under forced reaction conditions in the presence of an excess amount of alkynes. For unsymmetrical alkyne substrates, excellent regioselectivity of the insertion is generally determined by the polarity of alkynes.26 Zirconacyclopentene incorporating a carboranyl unit can be converted to a variety of functionalized carboranes via Zr–Cvinyl bond insertion, hydrolysis, as well as Cu-mediated iodination and coupling reaction (Scheme 7).27 Treatment of 1,2-[Cp2 ZrC(Et)@C(Et)]-1,2-C2B10H10 with CuCl2 gives the C–C coupling product with a four-membered ring 1,2-[C(Et)@C(Et)]-1, 2-C2B10H10.27,28 These results indicate that zirconacyclopentenes incorporating a carboranyl unit resemble their analogues zirconacyclopentadienes Cp2Zr[C(R)@C(R)–C(R)@C(R)] in some reactions, while they have some unique properties of their own due to the presence of highly sterically demanding carboranyl unit. Treatment of nickel–carboryne complex (g2-C2B10H10)Ni(PPh3)2 with internal alkynes affords highly substituted 1,2benzo-o-carboranes 1,2-[C(R1)@C(R2)C(R1)@C(R2)]-1,2-C2B10H10 via a [2+2+2] cycloaddition (Scheme 8), resembling the reaction between nickel–benzyne and alkynes.29 The localized double bonds in the products suggest there is no substantial p-delocalization in the six-membered ring. Excellent regioselectivity can be obtained with the exclusive formation of the head-to-tail products in the reaction with unsymmetrical aryl alkynes, which proves the high reactivity of the resultant Ni–Cvinyl bond in the intermediate nickelacyclopentene. In the above reaction, a stoichiometric amount of the Ni reagent is necessary. On the other hand, when 1-iodo-2-lithiocarborane is used as the carboryne precursor, oxidative addition on Ni(0) followed by an elimination of LiI can afford the Ni–carboryne complex required for a catalytic cycle.30 Screening experiments suggest that
δ
−
4
R2 R1
R2 R1
[Ni]
R4
Scheme 10. Synthesis of unsymmetric benzo-o-carboranes.
Ph
Ph
Ph Ph
NiCl2(dppe)
Zr Cp2
Ph P Ph
Ni P
Ph Ph
Et
Ph
Et
Et
Ph
Et
Scheme 11. Key transmetalation intermediate nickelacyclopentene.
NiCl2(PPh3)2 is the best catalyst among a variety of metal complexes, although Ni(0) complexes show some catalytic activities in the reactions. In sharp contrast, palladium, iron, and cobalt complexes are inactive. In the presence of 20 mol % of NiCl2(PPh3)2, reaction of 1-I-2-Li-1,2-C2B10H10 with 2 equiv of alkynes gives [2+2+2] cycloaddition products 1,2-benzo-o-carboranes, in very comparable yields with those of stoichiometric reactions. In addition, mono alkyne insertion species [{[2-C(nBu)@C(o-C5H4N)-1, 2-C2B10H10]Ni}2(l-Cl)][Li(THF)4] is isolated and structurally confirmed from the reaction of n-butyl-2-pyridinylacetylene. The presence of pyridinyl unit can stabilize the nickelacyclopentene complex by the coordination of nitrogen to Ni atom, thus preventing further insertion of the second equiv of alkyne (Scheme 9).30 This work represents the first example of metalcatalyzed reaction of carboryne with unsaturated molecules. The reactivity of metallacycles is dominated by the nature of corresponding transition metals. Nickelacyclopentenes incorporating a carboranyl unit are very reactive toward alkynes, resulting in no selectivity between two different alkynes in the reaction with Ni–carboryne. On the other hand, the corresponding zirconacyclopentenes incorporating a carboranyl unit are thermally very stable and chemically inert toward unsaturated organic molecules such as alkenes and alkynes.27 To this end, transmetalation from
R Cp2 Zr
n
Bu R
H H
1) 2 nBuLi; 2) I 2
Cl Ni N
n
Cp2Zr
N [Li(THF)4]
Cl
3) NiCl2(PPh3)2
Ni
Bu
Li
OEt2 OEt2
R
R = Aryl Cu(OTf)2
or toluene Cp2 Zr
R
N n
Bu
Scheme 9. Stable alkyne-insertion intermediate.
R = Alkyl
Scheme 12. Synthesis of carborane-fused zirconacyclopentanes and cyclobutanes.
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Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 R1
PPh3
R1
Ni PPh3
R2 Zr Cp2
Ph H β
Ph β'
Ph Ni
Ni
H
Ph3P
PPh3 β-H elimination
H
H "ene-reaction-type"
Scheme 13. Nickel-mediated coupling of o-carboryne with alkenes.
zirconacycles to nickel allows the insertion of the second alkyne, making chemoselective [2+2+2] cycloaddition of o-carboryne with two different alkynes possible.31 Also these benzocarboranes can be prepared in similar yields from one-pot reaction of Cp2Zr (l-Cl)(l-C2B10H10)Li(OEt2)2 with alkyne, followed by treatment with another type of alkyne in the presence of NiCl2(PMe3)2. In addition, using a catalytic amount of nickel in the presence of 3 equiv of FeCl3 allows the reaction with activated alkynes such as MeO2CC„CCO2Me (DMAD), which often homo-cyclotrimerizes in the presence of Ni(0) prior to the insertion into the Ni–C bond (Scheme 10).31 This catalytic reaction represents an important advance in the development of zirconacycle-based synthetic methodologies. The key transmetalation intermediate nickelacyclopentene 1,2-[(dppe)NiC(Ph)@C(Ph)]-1,2-C2B10H10 is isolated and structurally characterized from the reaction of 1,2-[Cp2 ZrC(Ph)@C(Ph)]-1,2-C2B10H10 with 1 equiv of NiCl2(dppe) (Scheme 11). The use of diphenylacetylene and dppe ligand can stabilize this complex by preventing from b-H elimination.31 Reaction with alkenes In a similar fashion, reaction of zirconocene–carboryne precursor Cp2Zr(l-Cl)(l-C2B10H10)Li(OEt2)2 with terminal alkenes RCH@CH2 in refluxing toluene gives zirconacyclopentanes 1, 2-[Cp2ZrCH(R)CH2]-1,2-C2B10H10 or 1,2-[Cp2ZrCH2CH(R)]-1, 2-C2B10H10 in good to high isolated yields with high regioselectivity (Scheme 12).32 The results show that electron-withdrawing aryl substituents go to the a position, whereas the electron-donating alkyl substituents prefer the b position. The resultant zirconacycles incorporating a carboranyl unit are also thermally stable and chemically inert toward unsaturated organic molecules. However, transmetalation of zirconacycle to Cu(II) leads to the formation of carborane-fused cyclobutanes (Scheme 12).28 The reaction between (g2-C2B10H10)Ni(PPh3)2 and alkenes is very different from that with alkynes. ‘Heck-type’ and ‘ene-reaction-type’
PPh3 Ni PPh3
R1 R3
R2
R1 = 2-Py, CO 2Me R2, R3 = alkyl, aryl
R1 Ni
δ+
δ−
R4 R3
R1
of products can be exclusively obtained for styrenes and aliphatic alkenes, respectively (Scheme 13).33 b-H elimination prior to the insertion of the second molecule of alkene followed by reductive elimination affords alkenylcarboranes. The selectivity is determined by the cyclic and acyclic b-H elimination rate in the nickelacyclopentane intermediates. The labeling experiments support the proposed b-H elimination mechanism: treatment of (g2-C2B10H10)Ni(PPh3)2 with styrene-d3 affords 1-[DC@CD(Ph)]-2-D-1,2-C2B10H10. Furthermore, (g2-C2B10H10)Ni(PPh3)2 does not react with anthracene, furan, or thiophene, whereas these 4p systems react readily with in situ generated carboryne to give [4+2] cycloaddition products.17 This result suggests that carboryne and Ni–carboryne should undergo different reaction pathways. Above b-H elimination reaction of the nickelacyclopentane can be suppressed by intramolecular coordination of a heteroatom, which would lead to the formation of stable nickelacyclopentanes. The stabilized five-membered nickelacycle intermediates can react readily with alkynes. As activated alkenes are much more reactive than alkynes toward Ni–carboryne, the reaction can be carried out in one-pot to give three-component [2+2+2] cycloaddition products, dihydrobenzocarboranes, with excellent control over the chemo- and regioselectivity (Scheme 14).34 This work offers a direct route to the synthesis of dihydrobenzo-o-carborane derivatives from simple molecules. In order to achieve high chemoselectivity between unactivated alkenes and alkynes, zirconacyclopentanes incorporating a carboranyl unit, prepared by treatment of Cp2Zr(l-Cl)(l-C2B10H10) Li(OEt2)2 with 1 equiv of 1-hexene or styrene, are treated with a variety of alkynes in the presence of 1 equiv of NiCl2, affording dihydrobenzocarboranes (Scheme 15).35 Symmetrical alkynes give the single products in very good isolated yields. The regioselectivity in the reaction of unsymmetrical alkynes is dependent upon both the polarity of alkynes and relative bulkiness of two substituents. Similar to zirconacyclopentenes incorporating a carboranyl unit, the transmetalation species nickelacyclopentane 1,2-[Ni(dppe) CH2CH(Bun)]-1,2-C2B10H10 can also be isolated from the reaction of 1,2-Cp2Zr[CH2CH(Bun)]-1,2-C2B10H10 with 1 equiv of NiCl2 (dppe). Its reaction with 5-decyne in THF at 110 °C gives the corresponding dihydrobenzocarborane in >90% yield. This offers an example to control the chemoselectivity among different alkenes and alkynes for assembling complex molecular architectures. In the above reaction of nickelacyclopentane with alkynes, when RC„CTMS is used as substrates, unprecedented dihydrofulvenocarboranes are isolated together with the expected dihydrobenzocarboranes (Scheme 16).36 The results show that the ligand has a major influence on product formation with triphenylphosphine
R2 R3
R1
R1 R2
R3 R4
H
Ph
"Heck-type"
R4
Scheme 15. Zr/Ni-co-mediated [2+2+2] cycloaddition of o-carboryne with unactivated alkenes and alkynes.
H
β'-H elimination
Ph
R2
+
R1 = H, R2 = Ph; R1 = nBu, R2 = H R3, R4 = alkyl, aryl
Ph H
R3
R1 R2
NiCl2
R1 R3
Zr Cp2
R2 Ni
R1 NiCl2(PMe3)2
1
R3
Scheme 14. Nickel-mediated [2+2+2] cycloaddition of o-carboryne with activated alkenes and alkynes.
R = H, alkyl R2 = TMS, aryl
R2
TMS R2
TMS
Scheme 16. Zr/Ni-co-mediated [2+2+1] cycloaddition of o-carboryne with unactivated alkenes and trimethylsilylalkynes.
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Z. Qiu / Tetrahedron Letters 56 (2015) 963–971
offering the excellent selectivity for dihydrobenzocarboranes and trimethylphosphine giving the best selectivity for dihydrofulvenocarboranes. The E/Z selectivity (configuration of alkenes) is dominated by the relative size of R/TMS. A DFT calculation reveals that an unprecedented reaction mechanism involving alkyne insertion, g1-alkenyl to g2-alkenyl rearrangement, and TMS migrations is found to be responsible for the favorable [2+2+1] cycloaddition.37
I I
H Pd(PPh3 )2 Cl2 CuI
I I
H
H H
I
TMS
R
H R R
PdCl2 (PPh3 )2
or
TMS
TMSCCMgBr I
or
H
R
R
R
R
R
H
TMS
H
I
H
I
H
H
PdCl2 (PPh3 )2 TMS
MgBr
TMS
TMS
H H RO OR
n
Scheme 20. Pd-catalyzed coupling for synthesis of photoluminescent material.
Further hydroboration/oxidation of the terminal double bonds can lead to a quadruped-shaped structure which might serve as a versatile dendritic precursor (Scheme 18).
OH H
HO
H
H H
NMO OsO 4
10 mol% Pd(PPh3 )4 H
PhI(OAc)2 OHC OsO 4
H
H 1) TBAF/THF 2) tBuOK
HOH 2 CH 2C
H H H
H H H H
TBA
H H
H MeCOSH AIBN AcSH2 CH2 C 1) BH3 /THF 2) NaBO3
3) M(acac) 2
HO2 C +
MgBr
M
EtOH
Scheme 19. Pd-catalyzed coupling reaction with alkynyl Grignard reagent.
One of the commonly used cage boron functionalization methods is the coupling reaction of iodocarboranes with Grignard reagents catalyzed by Pd or Ni complexes.38 B-alkyl-, aryl-, alkenyl-, and alkynyl-substituted o-carboranes are synthesized for potential applications in materials and pharmacophores.39 Listed below are selected examples. DFT calculations show that the HOMO–LUMO energy gaps for X-benzene and 3-X-o-C2B10H11 decrease in the order F > Cl > Br > I for both benzene and o-carborane systems; and carboranes have larger HOMO–LUMO gaps. Thus the reactivity of 3-X-o-carboranes should increase in the order F < Cl < Br < I.40 The methyl groups introduced to o-carborane cage via Pd-catalyzed coupling of B-iodocarborane and MeMgBr are found to be electron-withdrawing in boron clusters.41 It is reported that 9-vinyl-o-carborane can be prepared via a palladium-catalyzed cross-coupling of iodocarborane with vinylmagnesium bromide. Further transformations of its side chain yield bromination, epoxidation, dihydroxylation, oxidative cleavage, radical sulfur addition, and hydroboration/oxidation products (Scheme 17).42 Phenyl and allyl Grignard reagents are found to undergo the cross-coupling reaction with tetraiodinated o-carborane in the presence of Pd(II) and Cu(I). Both di- and tetra-B-substituted carboranes are obtained, respectively, due to the steric effect.43
THF
R KOH
or
R
R = H, Me
H
R
R
I
H
1) BH3 2) H 2O 2 NaOH
Scheme 18. Pd-catalyzed derivatization of 8,9,10,12-tetraiodo-o-carborane.
Cage B–I bond activation
mCPBA
H
H
Generally, electrophilic substitution at cage BH vertices and capitation reaction of nido-C2B9H2 11 with boron halides are two common synthetic methods for direct cage boron derivatization;7 and the selective synthesis of B-substituted derivatives prove to be rather difficult. To this end, transition metal promoted boron functionalization methodologies have been developed via the activation of either B–I or B–H bond of o-carboranes mediated/catalyzed by transition metals.
O
H
RMgCl
H
HO HO HO HO
Cage boron functionalization of o-carboranes
I
I Ph Ph I
H
M = Co; Ni
Scheme 17. Synthesis and transformation of 9-vinyl-o-carborane.
969
Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 H I H
Ar
H
ArB(OH) 2, K2CO3 Pd(PPh3 )4
H
CO 2H
R
Scheme 21. Pd-catalyzed coupling of 3-iodo-o-carborane with boronic acids.
[Cp*IrCl2 ]2 Ar
Ar
Ar
H
Cu(OAc)2 AgOAc
R
R
Cp*Ir(OAc) 2(DMSO) R2
R3
4 R2
R
DMSO R3
R2
H 1) nBuLi, Tol. R1 2) 5 mol % Pd(PPh3)4 I 5 mol % Ni(cod)
R1 R
R
R
R1 = H, Ph, alkyl R2, R3 = alkyl, aryl
R = Me Et
Ph Et
Ph
R1 Me [Pd]
Li Me [Pd]
Et Ph(Et)
Ni
Et(Ph)
Me Pd0 Et
Me [Ni]
I
Ph Ph
Ph
Et
[Ni] Me Ph
Et
Et
Scheme 22. Pd/Ni-co-catalyzed [2+2+2] cycloaddition of 1,3-o-carboryne with alkynes.
9,12-(HC„C)2-o-carborane and 9-(HC„C)-o-carborane are also prepared via Pd-catalyzed Kumada-type cross-coupling reactions of the corresponding iodinated carboranes with Me3SiC„CMgBr, followed by desilylation (Scheme 19).44 Polymers consisting of alternating 9,12-disubstituted o-carborane and p-phenylene-ethynylene units are prepared via Pd-catalyzed coupling reaction, which display intense blue photoluminescence (Scheme 20).45 p-Conjugated substituent at 9- and/or 12-positions in o-carborane is electrically independent, and both the HOMO and the LUMO levels slightly increase, whereas LUMO of the p-conjugated substituent at 1- and/or 2-positions in o-carborane decrease due to r⁄–p⁄ conjugation.46 Apart from Grignard reagents, aryl boronic acids can also be used in Pd-catalyzed coupling reaction with 3-iodo-o-carborane via a Suzuzki–Miyaura process to produce 3-aryl-o-carborane derivatives bearing easily transformable functional groups, including CH3O–, NO2–, CHO–, and CN– (Scheme 21).47 Transition metal-catalyzed cage C,B-functionalization through 1,3-o-carboryne48 (Chart 1) was developed in 2010.49 Unlike 1, 2-o-carboryne, the 1,3-o-carboryne precursor, 1-Li-2-Me-3-I-1, 2-C2B10H9, is very thermally stable, and does not generate 1,3-ocarboryne under thermal condition. In the presence of Pd catalyst, however, an oxidative addition of the cage B–I in 1-Li-2-Me3-I-1,2-C2B10H9 on Pd(0) proceeds, followed by a subsequent elimination of LiI to afford the target intermediate Pd-1,3-dehydro-o-carborane. Therefore, Pd(PPh3)4 can catalyze the [2+2+2]
H H
[Cp*IrCl2 ]2 CH 3 CCH
selective
hydroboration
of
diarylacetylene
with
Ph
Et Ni0
Me
Scheme 24. Ir-catalyzed 1-carboxyl-o-carborane.
R1
+ Ph
I
Li
O
R3 2
2
LiI
O
R1
+ R3
2 3
*CpIr
cycloaddition reaction of 1,3-o-carboryne precursor with alkynes, affording 1,3-benzo-o-carboranes (Scheme 22). Addition of Ni(cod)2 can significantly accelerate the above reaction. Mechanistic study suggests that a transmetalation process between Pd and Ni occurs to afford a more reactive Ni-1,3-dehydro-o-carborane intermediate. The relatively higher activity of the Ni species can be ascribed to the weaker Ni–B bond over the Pd–B one or the Ni–B bonding pair is more nucleophilic.50 And relatively higher activity of the M–B than M–C bond in Ni-1,3-dehydro-o-carborane intermediate is concluded on the basis of the regioisomers obtained in the electronically controlled regio-selective insertion of unsymmetrical alkyne PhC„CEt (Scheme 22).49 This work offers a new methodology for cage C,B-functionalization of carboranes and demonstrates that metal-1,3-o-carboryne can be viewed as a new kind of boron nucleophile. Cage B–H bond activation and functionalization Although transition metal promoted cage B–H activation is known as a new and efficient method to produce M–B bond in carborane systems,51 direct coupling of a cage B–H with functional groups is still rather rare. The first example of catalytic cage B–H activation and functionalization was reported in 1988. This reaction can be viewed as a hydroboration of terminal alkyne with o-carborane (Scheme 23).52 [Cp*IrCl2]2 can catalyze the reaction of o-carborane with propyne at 100 °C to produce 3-(trans-1-propenyl)-o-carborane in 40% GC yield, in which the oxidative addition of Ir(I) on cage B(3,6)–H bond is the key step. Very recently, [Cp*IrCl2]2-catalyzed selective hydroboration of diarylacetylenes with 1-CO2H-o-C2B10H11 has been achieved, leading to the preparation of a new class of 4-[ArCH@C(Ar)]-o-C2B10H11 (Scheme 24).53 The isolation of the key intermediate, five-membered iridacycle, suggests that selective electrophilic substitution of Ir(III) at B(4)-H unit initiates the reaction, in which the traceless carboxyl directing group plays a crucial role for the excellent regioselectivity. Pd(II)-catalyzed direct selective fluorination of o-carboranes using F+ reagent has recently been reported to give 8,9,10,12-tetrafluoro-o-carboranes (Scheme 25).54 The high regioselectivity in electrophilic palladation at B(8,9,10,12) positions can be attributed
R
Pd(MeCN) 4(BF4) 2
H R
H
Proton Sponge CH3
Scheme 23. Ir-catalyzed hydroboration of terminal alkyne with o-carborane.
F F F F
R R
N F OTf-
Scheme 25. Pd-catalyzed selective fluorination of o-carboranes.
970
Z. Qiu / Tetrahedron Letters 56 (2015) 963–971 *CpRh
S
CO2 Me
RhCp*
MeO2 C
S
S
S CO2 Me
CO2 Me CO 2Me
S RhCp* CO2 Me
S
S
S
S
RhCp*
S
H
Rh Cp* S
RhCp*
CO 2Me
CO 2Me
S
Scheme 26. Reaction of Cp*Rh(S2C2B10H10) with acetylene methyl carboxylate.
R(O)C
C(O)R Cp Co S S
C(O)R
2
C(O)R
S
C(O)R
S
+
C(O)R
C(O)R C(O)R
R(O)C CpCo S
S
S S
Co
+ S
other hand, selective stepwise substitution of o-carborane cage at B(3,6) positions can be achieved by taking advantage of the slightly reduced reactivity of the iridium complexes IrCp*(E2C2(B10H10) (E = S or Se) compared with their rhodium analogues.56 Reactivity of a series of mononuclear 16-electron half-sandwich complexes Cp#M(E2C2B10H10) (Cp# = Cp, Cp⁄; M = Co, Rh, Ir; E = S, Se) possessing an o-carborane-1,2-dichalcogenolate ligand toward electron-deficient alkynes and diazoacetates has been systematically investigated, generating novel compounds that result from B–H activation and further chemical transformations.57 It is noted that the coordinated cyclopentadienyl moiety can further participate in Diels–Alder reaction to give B(3)-norbornyl carboranes (Scheme 27).58 Furthermore, in the presence of HC„CCO2Me and another unsaturated molecule, both metal-promoted B–H and C–H activation occur, leading to selective B-functionalization of carborane by a metal-bound g5-Cp group through the cooperation of cobalt and two organic compounds in a one-pot reaction (Scheme 28).59 During the study of Zr/Ni co-mediated three-component [2+2+1] cross-cyclotrimerization of carboryne, alkene, and trimethylsilylacetylene, it is discovered that when bromophenyltrimethylsilylacetylene is used as the alkyne component, unprecedented C,C,B-substituted carborane-fused tricyclic compound is obtained. This may result from a three-component [2+2+1] crosscyclotrimerization of carboryne, alkene, and alkyne, followed by nickel(0) mediated cage B–C(sp2) coupling via direct B–H activation (Scheme 29).60
S O
Conclusions and outlook
R
Scheme 27. Synthesis of B-norbornyl carboranes via B–H activation.
MeO2 C
Cp Co S S
S S Co
CO 2Me
S S
Scheme 28. Co-mediated B–H activation of o-carborane incorporated with a C–H activation of cyclopentadiene.
to unique electronic structure of o-carborane cage. The F+ species functions as a fluorinating reagent as well as oxidant. This serves as a new methodology for the generation of a series of polyfluorocarboranes, which does not require special solvents and reaction vessels. For stoichiometrical process, it is reported that the 16e rhodium complex Cp*Rh(S2C2B10H10) reacts with acetylene methyl carboxylate to provide a straightforward route to B(3,6)-disubstituted ocarboranes.55 The Z configuration at the C@C bonds in the product rules out the possibility of direct 1,2-hydroboration. The M–S bond alkyne insertion enables the rhodium atom to approach the sites of B(3,6)–H with the result of B–H activation (Scheme 26). On the R1
Acknowledgments Financial supports from the National Natural Sciences Foundation of China (No. 21372245), the Shanghai Pujiang Program (No. 13PJ1410800), and Shanghai Science and Technology Committee (No. 13ZR1465000) are gratefully acknowledged.
R1 1) NiCl2(PMe3)2
Zr Cp2
Transition metal promoted functionalization of o-carboranes can be achieved at either cage C or B sites with high efficiency. Cu can mediate the coupling reaction between C-lithio-o-carborane and sp2/sp3 carbon halides. Zr and Ni can mediate or catalyze the reaction of carboryne with electrophiles in a diverse array, depending upon the electronic configurations of the metal center. On the other hand, in addition to Pd-catalyzed coupling reaction between B-iodo-o-carboranes and Grignard regents, direct functionalization via B–H bond activation can proceed in the presence of transition metals such as Pd, Ir, Rh, and Co. By taking advantage of the unique electronic feature of different transition metals, B(3,6) and B(8,9,10,12)-substituted o-carboranes can be prepared, respectively. These methodologies offer new routes for the functionalization of carboranes that cannot be achieved by other means. Compared to the very rich literature concerning the chemistry of benzene, studies of its three-dimensional relative, o-carborane, remain a very young research area. Transition metal catalyzed selective cage B–H bond activation is largely unexplored, and will be a challenging innovation for functionalization of carboranes. New synthetic methods would be expected with both high efficiency and accurate selectivity control among different cage B-reaction sites.
R2
2) TMS TMS
References and notes
Br
Cs 2 CO 3
R2
Scheme 29. Synthesis of C,C,B-substituted carborane-fused tricyclics.
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