molecules Review
Recent Advances in Catalyzed Sequential Reactions and the Potential Use of Tetrapyrrolic Macrocycles as Catalysts Everton Henrique Santos † , Charles Carvalho † , Carolina Machado Terzi † and Shirley Nakagaki * Laboratório de Bioinorgânica e Catálise, Departamento de Química, Centro Politécnico, Universidade Federal do Paraná (UFPR), Curitiba, Paraná 81531-990, Brazil;
[email protected] (E.H.S.);
[email protected] (C.C.);
[email protected] (C.M.T.) * Correspondence:
[email protected], Tel.: +55-41-3361-3186 † These authors contributed equally to this work. Received: 14 September 2018; Accepted: 26 October 2018; Published: 28 October 2018
Abstract: Complexes of porphyrins and of other similar tetrapyrrolic macrocycles are extensively explored as catalysts for different chemical processes, and the development of solid catalysts for heterogeneous processes using molecules with the ability to act as multifunctional catalysts in one-pot reactions is increasing and can lead to the wider use of this class of molecules as catalysts. This mini review focuses on the application of this class of complexes as catalysts in a variety of sequential one-pot reactions. Keywords: tetrapyrrolic macrocycles; porphyrin; catalysis; tandem; cascade reaction; oxidation
1. Introduction Catalysis is applicable to all chemical processes and contributes enormously to sustainable chemistry, enabling different chemical transformations, and reducing the need to use stoichiometric reactions for the manufacture of fine chemicals and pharmaceuticals. This is in concordance with the ninth principle of green chemistry [1], and is considered to be one of the most important tools to implement it, since catalytic systems often require lower amounts of reactants and external energy, in addition to fewer steps, playing an important role in environmental sustainability [2–4]. Catalysts can be classified as homo- and heterogeneous and enzymatic catalysts [5]. The most explored type of reaction system is homogeneous, since it often exhibits higher yield and selectivity. In terms of scaling up research and development of products, however, homogeneous catalytic processes have a considerable disadvantage: the difficulties of recovering the catalyst for reuse, since it is in the same phase as the other reactants and products, making it an arduous process that is usually not cost-effective [6]. At the same time, in homogeneous catalysis, the catalytic sites in the catalyst species are frequently unprotected, enabling undesirable secondary reactions to happen, such as deactivation, poisoning, side reactions, dimerization, etc. [7]. In this context, many efforts were made by researchers to propose heterogeneous processes to overcome these problems. Heterogeneous catalytic systems facilitate catalyst recovery via simple processes, such as filtration [6], decantation [8], and magnetic recovery [9] (if the solid is a magnetic particle), among others. There are many reports of the preparation of solid catalysts via their immobilization onto inert and usually inorganic matrices, such as layered double hydroxides [10], mesoporous silica [11,12], titanium oxides [8], zeolites [13], magnetic particles [14], and many more [15–18], in order to recover the catalyst for reuse. In addition, catalyst immobilization on inert matrices can produce unusual activities and synergism between the interacting species [19].
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Regarding the chemical approaches investigated, most catalytic systems are still based on homoand heterogeneous single-step reactions [6], and the recovery and purification of catalysts for reuse is still expensive and time-demanding, making them unsuitable for industrial use. Based on this, research is expanding on reactions that involve multiple steps. The main inspiration for the investigation of multifunctional processes is natural systems. In nature, there are many molecules that can act as catalysts in multistep reactions in mild conditions with high yields in particular enzymes. In enzyme systems, there are specific activated sites for specific reactions [20]. Molecules 2018, 23, x FOR PEER REVIEW 2 of 20 Taking the cytochrome P450 enzyme family as an example, they are present in all life forms and Regarding the chemical approaches investigated, most catalytic systems are still based on are responsible many reactions, such thethehydroxylation of saturated hydrogen carbon, homo-for andcatalyzing heterogeneous single-step reactions [6],as and recovery and purification of catalysts for epoxidation of double bonds,and O-dealkylation, N-dealkylation, othersuse. [21–25]. reuse is still expensive time-demanding,and making them unsuitableamong for industrial Based on this, research is expandingand on reactions steps. Mimicking the structure activitythat ofinvolve such multiple enzymes can be an important tool to propose The main inspiration for the investigation of multifunctional processes is natural systems. In synthetic molecules capable of multistep or sequential reactions, such as that which occurs in the nature, there are many molecules that can act as catalysts in multistep reactions in mild conditions generationwith of dienes via enzymatic reaction initiated by theactivated oxidation high yields in particular oxidation. enzymes. In This enzyme systems,isthere are specific sitesof forphenols to generate ortho-quinones, which in turn undergo hydroxylation–oxidation combined with a Diels–Alder specific reactions [20]. Taking the cytochrome P450 enzyme family as an example, they are present in all life forms and reaction to yield the desired bicyclic cycloaddition product [26]. are responsible for catalyzing many reactions, such as the hydroxylation of saturated hydrogen In these natural systems, the reactions occur without separation of intermediates, which are carbon, epoxidation of double bonds, O-dealkylation, and N-dealkylation, among others [21–25]. simultaneouslyMimicking present in same and environment withenzymes other reactants. This kindtool of naturally thethe structure activity of such can be an important to propose occurring reaction insynthetic biological systems can of bemultistep compared, in roughreactions, terms, to theaschemical classified as molecules capable or sequential such that whichreactions occurs in the generation ofhigh dienes via enzymatic oxidation. reaction is initiated by the oxidation of phenols to one-pot, presenting catalyst selectivity andThis high yields in natural environments. generate ortho-quinones, which in turn undergo hydroxylation–oxidation combined with a Synthetic metalloporphyrins and other macrocycles (tetrapyrrolic or not) were described as Diels–Alder reaction to yield the desired bicyclic cycloaddition product [26]. excellent cytochrome to promote selectiveofand efficient family of reactions, In these P450 naturalbiomimetic systems, the catalysts reactions occur without a separation intermediates, which are such as olefin epoxidation andinalkane (linear and cyclic) hydroxylation [27–32]. simultaneously present the same environment with other reactants. This kind of Despite naturally the many occurring in biological can be compared, in rough terms, to the chemical reactions reports, there are reaction few examples ofsystems the design and preparation of efficient catalytic systems able classified as one-pot, presenting high catalyst selectivity and high yields in natural environments. to promote sequential one-pot reactions involving catalyst species based on metalloporphyrins. Synthetic metalloporphyrins and other macrocycles (tetrapyrrolic or not) were described as The challenge is bigger if theP450 catalyst systemcatalysts is heterogeneous recyclable. excellent cytochrome biomimetic to promote aand selective and efficient family of In thisreactions, mini review, progress of this fieldand is reviewed, focusing [27–32]. on some representative such asthe olefin epoxidation andcatalytic alkane (linear cyclic) hydroxylation Despite manyuse reports, thereversatile are few examples thecatalysts—macrocycle design and preparation of efficient catalyticbased systemson pyrrole examples the of the of this familyofof complexes to promote sequential one-pot reactions involving catalyst species based on metalloporphyrins. (Section 4).able Before this, some definitions and terms necessary to understand tetrapyrrolic macrocycle The challenge is bigger if the catalyst system is heterogeneous and recyclable. systems are introduced 2). progress The authors necessary to introduce some definitions In this mini (Section review, the of this found catalyticit field is reviewed, focusing on some because different concepts wereoffound similar sequential reactions [33]. In Section 3, some representative examples the usefor of this versatile family of catalysts—macrocycle complexes based examples pyrrole (Section 4). Before this, some definitions and terms to understand tetrapyrrolic involving on non-tetrapyrrolic catalytic systems are used fornecessary the purpose of clarifying the one-pot macrocycle systems are introduced (Section 2). The authors found it necessary to introduce some sequential catalytic process. 2.
definitions because different concepts were found for similar sequential reactions [33]. In Section 3, some examples involving non-tetrapyrrolic catalytic systems are used for the purpose of clarifying Biological and Some Synthetic Tetrapyrrolic Macrocycle Systems the one-pot sequential catalytic process.
Since the 1960s, after the discovery of a hemeprotein able to catalyze the selective hydroxylation 2. Biological and Some Synthetic Tetrapyrrolic Macrocycle Systems of substrates, such as drugs and steroid hormones [34], biological enzyme systems were explored Since the 1960s, after the discovery of a hemeprotein able to catalyze the selective hydroxylation as inspiration by chemists to develop catalytic system models, able to perform in vitro the same of substrates, such as drugs and steroid hormones [34], biological enzyme systems were explored as catalytic reactions with highsystem selectivity efficiency mild conditions. inspiration observed by chemists in to vivo, develop catalytic models, and able to perform inunder vitro the same catalytic reactions observedas in being vivo, with high selectivity and efficiency under mild conditions. This complex This hemeprotein was identified composed of a metalloenzyme based on the iron(II) hemeprotein identified as beingIX composed a metalloenzyme based on the iron(II) complex of of enzymes of a porphyrin calledwas protoporphyrin (Figureof1), which acts as a cofactor of a family a porphyrin called protoporphyrin IX (Figure 1), which acts as a cofactor of a family of enzymes known as known cytochrome P450. as cytochrome P450.
Figure 1. Schematic representationof of iron(II) iron(II) complex of protoporphyrin IX. Figure 1. Schematic representation complex of protoporphyrin IX.
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The cytochrome P450 enzyme family plays a vital role in most living organisms, performing a cytochrome P450 enzyme family playsfrom a vital role in most living organisms, performing a hugeThe diversity of biological functions, ranging reactions involved in biosynthesis The cytochrome P450 enzyme family plays a vitalcatalyzing role in most living organisms, performing a huge diversity of biological functions, ranging from catalyzing reactions involved in biosynthesis metabolic pathways to the biodegradation of from endogenous and thein metabolism huge diversity of biological functions, ranging catalyzingcompounds reactions involved biosynthesis of metabolic pathways to the biodegradation of endogenous compounds and the metabolism of xenobiotic xenobiotic compounds [35–37]. metabolic pathways to the biodegradation of endogenous compounds and the metabolism of compounds [35–37]. The protoporphyrin IX complex is structurally composed of an aromatic macrocyclic ring xenobiotic compounds [35–37]. The protoporphyrin IX complex is structurally composed of an aromatic macrocyclic ring consisting four pyrrole IX units linked four methinic bridges. macrocycle, known The of protoporphyrin complex is by structurally composed of an This aromatic macrocyclic ring as consisting ofhas four pyrrole unitsunits linked by four methinic bridges. Thisofmacrocycle, knownmore as porphyrin, porphyrin, cavity whose diameter isbyabout Å , capable accommodating than consisting of afour pyrrole linked four 4.0 methinic bridges. This macrocycle, known as 50 has a cavity whose diameter is about 4.0 Å, capable of accommodating more than 50 metal ions, metal ions, leading to the formation of stable complexes named metalloporphyrins [35,38]. porphyrin, has a cavity whose diameter is about 4.0 Å , capable of accommodating more than 50 leading to the formation of formation stable complexes metalloporphyrins [35,38]. metal ions, to the of stablenamed complexes named metalloporphyrins [35,38]. Efforts toleading build catalytic system models based on the reactivity of the cytochrome P450 enzyme Efforts buildcatalytic catalytic systemmodels models reactivity P450 enzyme Efforts build system based on on the the reactivity thecytochrome cytochrome P450 enzyme family led totoato profound knowledge of the mechanism and nature ofofthe these catalytic species. One of family led to a profound knowledge of the mechanism and nature of these catalytic species. One family led to a profound knowledge of the mechanism and nature of these catalytic species. One of the first catalyst models able to mimic the cytochrome P450 catalytic activity was developedofby the first catalyst models able to mimic the cytochrome P450 catalytic activity was developed by the firstand catalyst modelsinable towhere mimicthey the cytochrome catalytic activity developed by Groves coworkers 1979, showed the P450 catalytic activity of thewas synthetic iron(III) Groves coworkersinin1979, 1979,where where ([Fe(TPP)]Cl), they showed showed the catalytic activity ofofthe iron(III) Groves and coworkers they the catalytic activity thesynthetic synthetic iron(III) complex ofand meso-tetraphenylporphyrin which was able to catalyze the hydroxylation complex meso-tetraphenylporphyrin ([Fe(TPP)]Cl), which able the hydroxylation complex ofof meso-tetraphenylporphyrin ([Fe(TPP)]Cl), which was ableto tocatalyze catalyze the hydroxylation and epoxidation of hydrocarbons with iodosylbenzene as anwas oxygen source under mild conditions and epoxidation of hydrocarbons with iodosylbenzene as an oxygen source under mild conditions and [39]. [39].epoxidation of hydrocarbons with iodosylbenzene as an oxygen source under mild conditions [39]. There are compounds with different variations in the structure of tetrapyrrolic macrocycles that There are compounds with different variations in the structure of tetrapyrrolic macrocycles that give rise behavior in in vitro with similar structures in vivo. They are rise to todistinct distinctcompounds compoundsand andchemical chemical behavior vitro with similar structures in vivo. They give rise to distinct compounds and chemical behavior in vitro with similar structures in vivo. They known as chlorin whenwhen one of theofpyrrolic units isunits reduced, or bacteriochlorin when two of the pyrrolic are known as chlorin one the pyrrolic is reduced, or bacteriochlorin when two of the are known as chlorin when one of the pyrrolic units is reduced, or bacteriochlorin when two of the rings at opposite positionspositions are reduced, as illustrated in Figurein 2 [40]. pyrrolic rings at opposite are reduced, as illustrated Figure 2 [40]. pyrrolic rings at opposite positions are reduced, as illustrated in Figure 2 [40].
Figure 2.2.Structures Structures of porphyrin, chlorin, and bacteriochlorin bacteriochlorin from left right. Figure and bacteriochlorinfrom fromleft lefttoto to right. Figure2. Structuresof ofporphyrin, porphyrin, chlorin, chlorin, and right.
In nature, substituted chlorin of chlorophylls (Figure 3), nature, substituted chlorinmacrocycles macrocyclesare arean an important component ofof chlorophylls (Figure In In nature, substituted chlorin macrocycles animportant importantcomponent component chlorophylls (Figure where they form complexes via coordination with magnesium ions and participate in physiological 3), where they form complexes via coordination with magnesium ions and participate in 3), where they form complexes via coordination with magnesium ions and participate in functions of plants through photosynthesis, employing solar energysolar to convert carbon dioxide and physiological functions plants through photosynthesis, photosynthesis, employing energy toto convert carbon physiological functions ofofplants through employing solar energy convert carbon water into carbohydrates and dioxygenand as byproducts dioxide and waterinto intocarbohydrates carbohydrates and dioxygen as byproducts dioxide and water dioxygen as[40,41]. byproducts[40,41]. [40,41].
Figure structuresof ofchlorophyll chlorophylla a(left) (left)and and chlorophyll b (right). Figure3.3.Schematic Schematicrepresentation representation of the structures chlorophyll b (right).
Figure 3. Schematic representation of the structures of chlorophyll a (left) and chlorophyll b (right).
The asbiomimetic biomimeticcatalysts catalysts based Thesuccess successofofstudies studiesusing using synthetic synthetic metalloporphyrins metalloporphyrins as based onon cytochrome P450 systems prompted investigation of other macrocyclic systems as catalysts. In cytochrome P450 systems prompted investigation of other macrocyclic systems as catalysts. In this The success of studies using synthetic metalloporphyrins as biomimetic catalysts basedthis on way, other purely synthetic porphyrinoid complexes are being explored as potential catalysts of way, other purely synthetic porphyrinoid complexes are being explored as potential catalysts of cytochrome P450 systems prompted investigation of other macrocyclic systems as catalysts. In this way, other purely synthetic porphyrinoid complexes are being explored as potential catalysts of
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oxidation ForFor example, the macrocycle systemsystem named phthalocyanine exhibits structural oxidation reactions reactions[42]. [42]. example, the macrocycle named phthalocyanine exhibits analogy with porphyrin complexes. The different ring structures are obtained through the usethe of structural analogy with porphyrin complexes. The different ring structures are obtained through distinct reactants: pyrroles pyrroles and aldehydes for porphyrins, and phthalic acids, phthalic anhydrides, use of distinct reactants: and aldehydes for porphyrins, and phthalic acids, phthalic or phthalonitriles for phthalocyanines (Figure 4a). The main advantage of this class of compounds is anhydrides, or phthalonitriles for phthalocyanines (Figure 4a). The main advantage of this class of the synthetic method employed to obtain them, which allows good yields in just one reaction step. compounds is the synthetic method employed to obtain them, which allows good yields in just one The procedure is based on a cyclotetramerization reaction among the precursors in precursors the presence reaction step. The procedure is based on a cyclotetramerization reaction among the inof thea metal salt, which acts as a template for the macrocycle formation [42]. presence of a metal salt, which acts as a template for the macrocycle formation [42].
Figure 4. 4. Structural Structural representation representation of phthalocyanine (a) Figure of phthalocyanine (a) and and aa corrole corrole (b). (b).
This class of of compounds compoundswas wasalready alreadyused usedto to remove sulfur from petroleum fractions, This class remove sulfur from petroleum fractions, by by catalyzing the oxidation reaction of aromatic thiols, which are the main organic sulfur contaminant catalyzing the oxidation reaction of aromatic thiols, which are the main organic sulfur contaminant in in petroleum petroleum derivatives derivatives [43]. [43]. Another porphyrinoidmacrocycle macrocycleintensively intensively studied in recent years is corrole the corrole family. Another porphyrinoid studied in recent years is the family. This This synthetic compound has structural features similar to porphyrin, but has one fewer methinic synthetic compound has structural features similar to porphyrin, but has one fewer methinic bridge bridge in relation to porphyrin (Figure 4b) [44]. common synthesis methodfor forthe thepreparation preparation of in relation to porphyrin (Figure 4b) [44]. TheThe common synthesis method of meso-substituted corroles is based on a one-pot procedure involving two distinct steps. The first first step step meso-substituted corroles is based on a one-pot procedure involving two distinct steps. The involves the aldehyde aldehyde and involves aa condensation condensation reaction reaction between between the and the the pyrrole pyrrole to to produce produce oligo oligo condensate condensate tetrapyrranes. In the second step, oxidative ring closure occurs [45]. tetrapyrranes. In the second step, oxidative ring closure occurs [45]. Like of metallic ions, leading leading to to the the Like porphyrins, porphyrins, corroles corroles can can form form complexes complexes with with a a wide wide range range of metallic ions, formation of metallocorroles [44]. Due to the structural similarity, the chemical properties of corroles formation of metallocorroles [44]. Due to the structural similarity, the chemical properties of corroles and metallocorroles are aspects, similar similar to to those those of of porphyrins porphyrins and and metalloporphyrins, metalloporphyrins, and metallocorroles are also, also, in in many many aspects, making this class of compound useful in different fields of catalysis. making this class of compound useful in different fields of catalysis. Nozaki Nozaki et et al. al. [46] [46] demonstrated demonstrated the the use use of of manganese–corrole manganese–corrole complexes complexes acting acting as as efficient efficient catalysts in the polymerization reaction of epoxides, leading to the formation of polyethers, catalysts in the polymerization reaction of epoxides, leading to the formation of polyethers, polycarbonates, and polyesters in the polycarbonates, and polyesters in the presence presence of of aa co-catalyst. co-catalyst. Moreover, Moreover, another another study study examined examined other properties of these compounds, such as for photodynamic therapy, where they other properties of these compounds, such as for photodynamic therapy, where they act act as as sensitizers [47]. sensitizers [47]. All macrocycles are known to have atoparticular characteristic All these theseporphyrinoid-related porphyrinoid-related macrocycles are known have aspectral particular spectral due to the high electronic conjugation of their structure. They possess strong absorption characteristic due to the high electronic conjugation of their structure. They possessranging strong from 300 nm to 800 nm electromagnetic spectra, depending the macrocycle structure. absorption ranging from 300innm to 800 nm in electromagnetic spectra,on depending on the macrocycle These particular absorption frequently to characterize these compounds. structure. These patterns particularof patterns of are absorption are used frequently used to characterize these For instance, porphyrins and chlorins possess intense absorption in the region of 400–450 nm due compounds. For instance, porphyrins and chlorins possess intense absorption in the region of a400–450 Soret band, as well as 500–700 originating Q bands.from Other macrocycles like nm due a Soret band, asnm well as 500–700from nm the originating therelated Q bands. Other related phthalocyanines the most intense band between 550band and 800 nm [40]. to their macrocycles like present phthalocyanines presentabsorption the most intense absorption between 550 Due and 800 nm intense absorption ranging in the ultraviolet-visible (UV-Vis) region, this class of compounds [40]. Due to their intense absorption ranging in the ultraviolet-visible (UV-Vis) region, this classcan of be used as photosensitizers and can be applied in different fields, such as for photocatalysis [48] or compounds can be used as photosensitizers and can be applied in different fields, such as for photodynamic therapy [47,49,50]. photocatalysis [48] or photodynamic therapy [47,49,50]. 3. One-Pot Sequential Reactions The term “one pot” derives from reactions observed in biological systems in cases that involve a process in which all reactants, as well as catalysts, are present in the reaction medium from the
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55 of reactions observed in biological systems in cases that involve a of 20 20 process in which all reactants, as well as catalysts, are present in the reaction medium from the outset. outset. these due selectivity, both do with each other, no outset. Inreactions, these reactions, reactions, due to to catalyst catalyst selectivity, both species species do not not interfere interfere with eachno other, no In theseIn due to catalyst selectivity, both species do not interfere with each other, matter matter how many mechanisms compose the process. In this sense, there is only one necessary step matter howmechanisms many mechanisms theIn process. In this sense, there only onestep necessary step how many composecompose the process. this sense, there is only one is necessary for isolation for purification and making it environmentally and for isolation isolation and and purification of of intermediates andit catalysts, catalysts, making it economically environmentally and and purification of intermediates andintermediates catalysts, making environmentally and attractive economically attractive due to lower cost and time and the reduced need for reactants [4,33,51–53]. economically attractive due to lower cost and time and the reduced need for reactants [4,33,51–53]. due to lower cost and time and the reduced need for reactants [4,33,51–53]. In contrast contrast to one-pot sequential sequential reactions, there are that are In to one-pot reactions, there are sequential sequential reactions reactions that are out out of of the the one-pot scope, reaction described described by Dias and and coworkers coworkers in which which they synthetized coworkers in which they they synthetized synthetized one-pot scope, such such as as the the reaction by Dias porphyrin-based magnetic nanocomposites applied sequential olefin epoxidation, followed porphyrin-basedmagnetic magneticnanocomposites nanocompositestoto tobebe beapplied applied in sequential olefin epoxidation, followed porphyrin-based inin sequential olefin epoxidation, followed by by CO 2 cycloaddition to epoxides, leading to the desired cyclic carbonates, as shown in Figure by 2CO 2 cycloaddition epoxides, leadingtotothe thedesired desiredcyclic cycliccarbonates, carbonates, as as shown shown in Figure 5. 5. CO cycloaddition to to epoxides, leading Between another Between the the two two steps, steps, the the solvent solvent was was evaporated evaporated and and the the mixture mixture was was transferred transferred to to another another recipient containing This kind of reaction, reaction, for for example, example, recipient containing aa second second catalyst catalyst to to continue continue the the next next step. step. This This kind kind of leads to 70% yield [54]. 70% yield yield [54]. [54]. leads to 70%
Figure Figure 5. 5. Schematic Schematic representation representation of of aa sequential sequential reaction reaction involving involving two two catalytic catalytic species species based based on on metalloporphyrins, one of manganese (Cat A) and metalloporphyrins, one of manganese (Cat A) and the other of chromium (Cat B), where PPNCl metalloporphyrins, one of manganese (Cat A) and the other of chromium (Cat B), where PPNCl is is [(Ph P)222N]Cl N]Cl[54]. [54]. [(Ph33P) P) N]Cl [54].
In the one-pot context, sequential reactions encompass catalytic reactions with single orsingle multiple In In the the one-pot one-pot context, context, sequential sequential reactions reactions encompass encompass catalytic catalytic reactions reactions with with single or or mechanisms for all steps, using one (multifunctional or not) or more than one catalyst species, but, multiple multiple mechanisms mechanisms for for all all steps, steps, using using one one (multifunctional (multifunctional or or not) not) or or more more than than one one catalyst catalyst as said before, all from the outset. species, species, but, but, as as said said before, before, all all from from the the outset. outset. In the context of one-pot reactions, there there are different different kinds of sequential reactions reactions based on on the In the context of one-pot reactions, In the context of one-pot reactions, there are are different kinds kinds of of sequential sequential reactions based based on the the possibility of single or multiple mechanisms with with one or or more catalysts and catalytic cycles. Hence, Hence, possibility possibility of of single single or or multiple multiple mechanisms mechanisms with one one or more more catalysts catalysts and and catalytic catalytic cycles. cycles. Hence, there are two major kinds of one-pot sequential reactions: domino (or (or the closely related cascade) and there there are are two two major major kinds kinds of of one-pot one-pot sequential sequential reactions: reactions: domino domino (or the the closely closely related related cascade) cascade) tandem reactions [33,55,56]. and and tandem tandem reactions reactions [33,55,56]. [33,55,56]. Those involving one kind of mechanism and catalyst one catalystusually are usually domino called domino or Those Those involving involving one one kind kind of of mechanism mechanism and and one one catalyst are are usually called called domino or or cascade cascade cascade [1,33,57], in which there is no extra reactant addition during the reaction. [1,33,57], [1,33,57], in in which which there there is is no no extra extra reactant reactant addition addition during during the the reaction. reaction. The term “domino” is also subdivided into two different kinds of reactions: intraintra- and The term “domino” is also subdivided into two different The term “domino” is also subdivided into two different kinds kinds of of reactions: reactions: intra- and and intermolecular. The first first one designates designates reactions where where the substrate substrate undergoes multiple multiple molecular intermolecular. intermolecular. The The first one one designates reactions reactions where the the substrate undergoes undergoes multiple molecular molecular transformations caused by by a single catalyst, catalyst, giving birth birth to various various (n) transition transition states that that lead to to a transformations transformations caused caused by aa single single catalyst, giving giving birth to to various (n) (n) transition states states that lead lead to aa product. The second one designates reactions involving multiples steps with one catalytic mechanism product. product. The The second second one one designates designates reactions reactions involving involving multiples multiples steps steps with with one one catalytic catalytic acting repeatedly, generating intermediates that are stable and can be isolated (in intermolecular mechanism mechanism acting acting repeatedly, repeatedly, generating generating intermediates intermediates that that are are stable stable and and can can be be isolated isolated (in (in reactions, it is also possible foralso molecules tofor undergo multiple transformations). intermolecular reactions, it is possible molecules to undergo multiple transformations). intermolecular reactions, it is also possible for molecules to undergo multiple transformations). In this sense, the term domino reaction is applied to reactions where thethe mechanism triggered by In In this this sense, sense, the the term term domino domino reaction reaction is is applied applied to to reactions reactions where where the mechanism mechanism triggered triggered abysingle catalyst action repeats twice, as shown in Figure 6. by aa single single catalyst catalyst action action repeats repeats twice, twice, as as shown shown in in Figure Figure 6. 6.
Figure Figure 6. 6. Schematic Schematic representation representation of of domino domino and and cascade cascade catalyzed catalyzed reactions. reactions.
Following Following the the same same logic, logic, the the term term “cascade” “cascade” is is almost almost synonymous synonymous with with domino; domino; however, however, it it is reserved for reactions containing three or more single catalytic mechanism steps [58–60], such as is reserved for reactions containing three or more single catalytic mechanism steps [58–60], such as that that schematized schematized in in Figure Figure 6. 6. To To exemplify exemplify the the domino domino reaction reaction (which (which can can be be extend extend to to the the cascade cascade reaction), reaction), in in 2018, 2018, Kumar Kumar and Satyanarayana [61] reported the obtainment of fluorenones, which have meaningful interest and Satyanarayana [61] reported the obtainment of fluorenones, which have meaningful interest in in
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6 of term “cascade” is almost synonymous with domino; however, it 20 is reserved for reactions containing three or more single catalytic mechanism steps [58–60], such as that the biomedical and material fields, from 2-bromobenzaldehyde (Figure 7, 1a) and phenylboronic schematized in Figure 6. acid To (Figure 7, 2a), using palladium as (which a catalyst tert-butyl hydroperoxide as an oxidant. exemplify the domino reaction can and be extend to the cascade reaction), in 2018, Following domino one-pot concept, the after the catalyst’s action to form thehave Suzuki product (Figure Kumar andthe Satyanarayana [61] reported obtainment of fluorenones, which meaningful interest Molecules 2018, 23, x FOR PEER REVIEW 6 of 20 7, 3aa), the molecules undergo intramolecular transformations (Figure 7, steps A–C) to generate the in the biomedical and material fields, from 2-bromobenzaldehyde (Figure 7, 1a) and phenylboronic desired product (Figure 7, 4aa), with 86% yield in the best condition studied. The authors also the biomedical andpalladium material fields, 2-bromobenzaldehyde (Figure 7, 1a) as andanphenylboronic acid (Figure 7, 2a), using as a from catalyst and tert-butyl hydroperoxide oxidant. Following proposed the mechanism represented in Figure 7 [61]. acid (Figure 7, 2a), using palladium as a catalyst and tert-butyl hydroperoxide as an (Figure oxidant. 7, 3aa), the domino one-pot concept, after the catalyst’s action to form the Suzuki product Following the domino one-pot concept, after the catalyst’s action to form the Suzuki product (Figure the molecules undergo intramolecular transformations (Figure 7, steps A–C) to generate the desired 7, 3aa), the molecules undergo intramolecular transformations (Figure 7, steps A–C) to generate the product (Figure 7, 4aa), with 86% yield in the best condition studied. The authors also proposed the desired product (Figure 7, 4aa), with 86% yield in the best condition studied. The authors also mechanism represented in Figure 7 [61]. in Figure 7 [61]. proposed the mechanism represented
Figure 7. Catalytic cycle to obtain fluorenones from 2-bromobenzaldehyde (1a) Catalytic cycle fluorenones from 2-bromobenzaldehyde (1a)and andphenylboronic phenylboronic acid Figure 7. Catalytic cycleto toobtain obtain fluorenones from 2-bromobenzaldehyde (1a) and phenylboronic acid (2a) using palladium as a catalyst and tert-butyl hydroperoxide as an oxidant, following thedomino (2a) using palladium as a catalyst and tert-butyl hydroperoxide as an oxidant, following the acid (2a) using palladium as a catalyst and tert-butyl hydroperoxide as an oxidant, following the one-pot concept (Kumar and Satyanarayana; Palladium Catalysis: One-Pot Synthesis of one-potdomino concept Satyanarayana; Palladium Catalysis: Synthesis of Synthesis Fluorenones. domino one-pot (Kumar conceptand (Kumar and Satyanarayana; PalladiumOne-Pot Catalysis: One-Pot of Fluorenones. Chemistry Select. 2018.3.7867-7870. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Chemistry Select. 2018.3.7867-7870. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced Fluorenones. Chemistry Select. 2018.3.7867-7870. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission) [61]. with permission) Reproduced with [61]. permission) [61].
On the other hand, reactions in which two or more distinct mechanisms act on the substrate are
Oncalled the other other hand, reactions in which two or or more distinct mechanisms act on on thetandem, substrate are are tandem reactions. They in arewhich subdivided intomore threedistinct classes: orthogonal tandem, auto On the hand, reactions two mechanisms act the substrate and assisted tandem They [4,33,51,55,56]. called tandem tandem reactions. They are are subdivided subdivided into into three three classes: classes: orthogonal orthogonal tandem, tandem, auto auto tandem, tandem, called reactions. Orthogonal tandem refers to reactions with multiple catalysts working together with mutual and assisted tandem [4,33,51,55,56]. and assisted tandem [4,33,51,55,56]. independence, in which one catalyst does not react before a specific product is formed. In other Orthogonal tandem refers refers to to reactions reactions with with multiple catalysts working working together together with with mutual Orthogonal tandem multiple catalysts mutual words, it also can be seen as “one after another” (Figure 8) [33,55]. independence, in which one catalyst does not react before a specific product is formed. In other independence, in which one catalyst does not react before a specific product is formed. Inwords, other it also can be seen as seen “oneas after another” (Figure (Figure 8) [33,55]. words, it also can be “one after another” 8) [33,55].
Figure 8. Schematic representation of an orthogonal tandem catalytic reaction.
As an example of an orthogonal tandem catalytic reaction, in 2003, Nishibayashi et al. [62] reported the reaction between 1-phenyl-2-propyn-1-ol and a ketone to obtain a specific furan, Figure 8. Schematic representation of an orthogonal tandem catalytic reaction. Figure Schematic representation depending on the ketone used. In this process, they used two catalysts, an Ru complex and PtCl2
As an example of an orthogonal tandem catalytic reaction, in 2003, Nishibayashi et al. [62] reported the reaction between 1-phenyl-2-propyn-1-ol and a ketone to obtain a specific furan, depending on the ketone used. In this process, they used two catalysts, an Ru complex and PtCl2
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As an example of an orthogonal tandem catalytic reaction, in 2003, Nishibayashi et al. [62] reported and a ketone to obtain a specific furan, depending on 77 of 20 ofthe 20 ketone used. In this process, they used two catalysts, an Ru complex and PtCl2 (Figure 9). Both catalysts (Figure 9). Both catalysts were simultaneously present the medium, catalyst were simultaneously present in the reaction medium, andinthe Ptreaction catalyst acted onlyand afterthe thePt formation acted only after thespecies formation of an intermediate species via catalytic of an intermediate via catalytic action of the Ru complex [62]. action of the Ru complex [62]. the reaction 1-phenyl-2-propyn-1-ol Molecules 2018, 23, REVIEW Molecules 2018, between 23, xx FOR FOR PEER PEER REVIEW
Figure Schematic representation of the orthogonal catalytic reaction Figure 9. 9. Schematic Schematic representation representation of of the orthogonal orthogonal tandem tandem catalytic catalytic reaction reaction using using two two different different catalysts, Cat A (ruthenium complex) and Cat B (Pt compound), to obtain furan compounds. The catalysts, Cat Cat A A (ruthenium (ruthenium complex) complex) and and Cat Cat B B (Pt (Pt compound), compound), to to obtain obtain furan furan compounds. compounds. The The scheme is based on Reference where =η Me5MeS and ==MeS scheme is on [62], where Cp η 55Me CH 33S. 5 -C555and scheme is based based on Reference Reference [62],[62], where Cp ==Cp η55-C -C Me and MeS CH= S.CH3 S.
Auto tandem tandem is is aa term term reserved reserved for for reactions reactions with with aa single single multifunctional multifunctional catalyst. catalyst. This This catalyst catalyst Auto has two two or or more more activated activated catalytic catalytic sites sites in in its its structure; structure; thus, thus, it it can can cause cause different different catalytic catalytic reactions reactions has in the substrate, as exemplified in Figure 10 [33]. in the substrate, as exemplified in Figure 10 [33].
Figure Figure 10. 10. Schematic Schematic representation representation of the auto of the auto tandem tandem catalytic catalytic reaction. reaction.
catalyst prepared prepared with gold Liu et al. demonstrated the al. [63] [63] demonstrated the application application of aa multifunctional multifunctional catalyst hydrotalcite (Au/HT) in an auto tandem reaction for nanoparticles immobilized on zinc–aluminum (Au/HT) in an auto tandem zinc–aluminum hydrotalcite condensation products. TheThe catalytic reaction comprised two steps: the preparation preparationofofaldehyde–amine aldehyde–amine condensation products. catalytic reaction comprised two the first occurred via proton from benzyl Figure 11), forming steps: the first occurred via abstraction proton abstraction fromalcohol benzyl(Ralcohol 11C-OH, Figure 11),aldehyde, forming 1 C-OH,(R promoted by basic catalyst sites in hydrotalcite (Au/HT = (Au/HT Cat), to react with 1-hexylamine (H2 N-R2 ); aldehyde, promoted by basic catalyst sites in hydrotalcite = Cat), to react with 1-hexylamine the22N-R second depended on the gold (Cat) acting as acting Lewis as acid sitesacid to activate N-H (H 22); the second depended onnanoparticles the gold nanoparticles (Cat) Lewis sites to the activate bondN-H in the amine to condense with the formed leading to theleading desiredtocondensation the bond in the amine toitcondense it with aldehyde, the formed aldehyde, the desired product (imine), as summarized the mechanism depicted in Figure 11 [63]. condensation product (imine), asinsummarized in the mechanism depicted in Figure 11 [63].
Figure 11. 11. Schematicrepresentation representation ofthe the autotandem tandem catalytic reaction using an Au compound Figure Figure 11. Schematic Schematic representation of of the auto auto tandem catalytic catalytic reaction reaction using using an an Au Au compound compound as as as catalyst (Cat = Au/zinc–aluminum hydrotalcite (HT)) to obtain aldehyde–amine condensation catalyst catalyst (Cat (Cat == Au/zinc–aluminum Au/zinc–aluminum hydrotalcite hydrotalcite (HT)) (HT)) to to obtain obtain aldehyde–amine aldehyde–amine condensation condensation products [63]. products products [63]. [63].
When there is a need to add a reactant to trigger the activity of a second catalytic site in the same catalyst or to change the catalyst to propitiate a second catalytic function, starting a second
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of 20 When there is a need to add a reactant to trigger the activity of a second catalytic site in the 8same Molecules 2018, 23, x FOR PEER REVIEW 8 of 20 catalyst or to change the catalyst to propitiate a second catalytic function, starting a second reaction reaction step, the process is called assisted tandem. This kind of reaction only involves the use of a step, the process is called assisted tandem. This kind of reaction only involves the use of a single step, the process is called assisted tandem. This singlereaction multifunctional catalyst, as illustrated in Figure 12kind [33].of reaction only involves the use of a multifunctional catalyst, as illustrated in Figure 12 [33].12 [33]. single multifunctional catalyst, as illustrated in Figure
Figure Schematic representation of of an an assisted catalytic reaction. Figure 12. 12. Schematic representation assistedtandem tandem catalytic reaction. Schematic representation
Itimportant is important to highlight thatthe thesecond second stage of activity not not occur before toto highlight thatthat the second stagestage of catalytic activity doesdoes not occur before addition It is isimportant highlight ofcatalytic catalytic activity does occur before addition of the triggering agent. To contextualize this, in 2002, Thadani and Rawal reported an of the triggering agent. To contextualize this, in 2002, this, Thadani and Rawal reported an assisted tandem addition of the triggering agent. To contextualize in 2002, Thadani and Rawal reported an assisted tandem reaction associated with bromo- or chloroallylation followed by Suzuki reaction associated with bromoor chloroallylation followed by Suzuki cross-coupling reaction to assisted tandem reaction associated with bromo- or chloroallylation followed by Suzuki cross-coupling reaction to obtain tetra-substituted alkenes (Figure 13). In the first step, PdII was II II obtain tetra-substituted alkenes (Figure 13). In the first step, Pd was applied as catalyst and,Pd after its cross-coupling reactionand, to obtain (Figure 13). In the first applied as catalyst after itstetra-substituted conclusion, in the alkenes same flask, tert-butylphosphine wasstep, added to was II to Pd0 , used as the conclusion, in the same flask, tert-butylphosphine was added to reduce the Pd applied as catalyst and, after its conclusion, in the same flask, tert-butylphosphine was added to II 0 reduce the Pd to Pd , used as the catalytic species for the following step [64]. II 0 catalytic species for the following step [64]. reduce the Pd to Pd , used as the catalytic species for the following step [64].
Figure 13. Schematic representation of the assisted tandem catalytic reaction using a Pd compound as a catalyst and tert-butylphosphine (PtBu3) as a trigger reactant to obtain tetra-substituted alkenes, Figure 13. of the the assisted reaction using using aa Pd where R1 to R4 are representation different substituents [64]. Figure 13. Schematic Schematic representation of assisted tandem tandem catalytic catalytic reaction Pd compound compound
as a a catalyst (PtBu33)) as as aa trigger trigger reactant reactant to to obtain obtain tetra-substituted tetra-substituted alkenes, alkenes, as catalyst and and tert-butylphosphine tert-butylphosphine (PtBu 4. Uses of Tetrapyrrolic Macrocycles in Sequential Reactions where R to R are different substituents [64]. where R11 to R44 are different substituents [64]. In 2008, Jiang and coworkers reported the use of a ruthenium porphyrin applied to the 4. Uses of Tetrapyrrolic Macrocycles in Sequential Reactions 4. Uses of Tetrapyrrolic Macrocyclescatalysis in Sequential Reactions homogeneous aerobic-oxidation of terminal alkenes to aldehydes in a tandem epoxidation–isomerization approach [65]. The objective was an easier approach to achieve In 2008, Jiang and coworkers reported the use ofto acreate ruthenium porphyrin applied to In 2008, Jiang and coworkers reported the use of a ruthenium porphyrin applied to the selectivity for aldehydes in metal-catalyzed oxidation reactions of 1-alkenes that use dioxygen the homogeneous aerobic-oxidation catalysis of terminal alkenes to aldehydes in aas the tandem homogeneous aerobic-oxidation of since terminal alkenes to aldehydes in a tandem sole terminal oxidant under mildcatalysis conditions, the was process commonly also produces epoxidation–isomerization approach [65]. The objective to create an easier approachmethyl to achieve epoxidation–isomerization The14). objective was to create different an easieroxidation approach to achieve ketones instead of only approach aldehydes [65]. (Figure The authors reaction selectivity for aldehydes in metal-catalyzed oxidation reactionsstudied of 1-alkenes that use dioxygen as the selectivity for aldehydes in metal-catalyzed oxidation reactions 1-alkenes dioxygen conditions with ruthenium porphyrins, changing the solvent andof the additives,that anduse observed betteras the sole terminal oxidantIVunder mild conditions, since the process commonly also produces methyl ketones yields using [Ru (TMP)Cl ] (TMPconditions, = tetramesithylporphyrin, Figure 14, 1a) as aalso catalyst in CDCl 3 sole terminal oxidant under 2mild since the process commonly produces methyl instead of only aldehydes (Figure 14). The authors studied different oxidation reaction conditions 1 withinstead additionofof only aqueous NaHCO3.(Figure A closer14). examination of thatstudied reaction different over time oxidation using H-NMR ketones aldehydes The authors reaction with ruthenium porphyrins, changing thepresence solvent and the additives, and observed better yields using spectroscopy revealed porphyrins, that, in the of solvent NaHCO 3, [RuIV(TMP)Cl2] was oxidized to conditions with ruthenium changing the and the additives, and observed better IV(TMP)O [RuIV[Ru (TMP)Cl = tetramesithylporphyrin, Figure 14, 1a) as areaction. catalystThe in CDCl with addition 2], which led to the tandem epoxidation–isomerization species 2 ] (TMP 3 generated IV yields using [Ru (TMP)Cl2] (TMP = tetramesithylporphyrin, Figure 14, 1a) 1as a catalyst in CDCl3 of aqueous NaHCO examination thathowever, reactionthe over time using H-NMR in situ was responsible for the epoxidation of step; active intermediate for thespectroscopy epoxide 3 . A closer with addition of aqueous NaHCO3. A closer IV examination of that reaction over time using 1H-NMR IV (TMP)O IV(TMP)O isomerization step was unclear. They3 ,also theoxidized catalyst to [Ru 2] presented revealed that, in the presence of NaHCO [Ru concluded (TMP)Cl2that ] was [Ru ], which led 2 spectroscopy revealed that, in the presence of NaHCO3, [RuIV(TMP)Cl2] was oxidized to better reaction yield when generated inreaction. situ. Furthermore, they developedinasitu new, recyclable to the tandem epoxidation–isomerization The species generated was responsible [RuIV(TMP)O 2], which led to the tandem epoxidation–isomerizationIVreaction. The species generated ruthenium porphyrin (based porphyrin 1b, Figurefor 14),the [Ruepoxide (TMTTP)O,], which followed for the epoxidation step;catalyst however, theonactive intermediate isomerization step was in situ was responsible for the step; however, the 90%, active intermediate for the five epoxide the same tandem pathway andepoxidation whose reaction yields were above even after being recycled IV unclear. They also concluded that the catalyst [Ru (TMP)O2 ] presented better reaction yield when IV isomerization step was that unclear. They also concluded that the catalyst [Ruwith (TMP)O 2] presented times. In conclusion, work is an example of how assisted tandem reactions porphyrins as generated in situ. Furthermore, they developed a new, recyclable ruthenium porphyrin catalyst (based canyield be useful to achieve the desired selectivity. The authors successfully synthesized new bettercatalysts reaction when generated in situ. Furthermore, they developed a new, arecyclable on porphyrin 1b, recyclable Figure 14), [RuIV (TMTTP)O,], followed the same tandem and whose efficient and catalyst, used one ofwhich the 1b, first reported sequential tandempathway reactions ruthenium porphyrin catalyst (based oninporphyrin Figure 14), [RuIV(TMTTP)O,], whichbased followed reaction were above 90%, even after being recycled five times. In conclusion, that work is an on ayields porphyrin macrocycle. the same tandem pathway and whose reaction yields were above 90%, even after being recycled five
example of how assisted tandem reactions with porphyrins as catalysts can be useful to achieve the times. In conclusion, that work is an example of how assisted tandem reactions with porphyrins as catalysts can be useful to achieve the desired selectivity. The authors successfully synthesized a new efficient and recyclable catalyst, used in one of the first reported sequential tandem reactions based on a porphyrin macrocycle.
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desired selectivity. The authors successfully synthesized a new efficient and recyclable catalyst, used in oneMolecules ofMolecules the 2018, first reported sequential 2018, 23, x FOR PEER REVIEW tandem reactions based on a porphyrin macrocycle. 9 of920 23, x FOR PEER REVIEW of 20
Figure 14. Schematic representation a tandem reaction catalyzed Ru-porphyrin,adapted adapted from Figure 14. Schematic representation of aoftandem reaction catalyzed byby anan Ru-porphyrin, Figure 14. Schematic representation of a tandem reaction catalyzed by an Ru-porphyrin, adapted from Reference [65]. Reference [65]. from Reference [65]. In 2010, Mbuvi coworkersverified verified the the potential potential ofof[Fe(TPP)Cl] as as a catalyst in a in tandem In 2010, Mbuvi andand coworkers [Fe(TPP)Cl] a catalyst a tandem In 2010, Mbuvi N-H and insertion coworkers of [Fe(TPP)Cl] as cyclization, a catalyst in a tandem reaction involving in verified carbenes the frompotential diazo esters and its further reporting reaction involving N-H insertion in carbenes from diazo esters and its further cyclization, reporting a reaction N-H insertion in carbenes from diazo esters and cyclization, reporting a new involving one-pot approach for synthesizing heterocyclic compounds viaits thefurther reaction of amines and the new one-pot approach for synthesizing heterocyclic compounds via the reaction of amines and the diazo reagents (Figure for 15) synthesizing [66]. As expected, the results showed that, using ethylenediamine andthe a new one-pot approach heterocyclic compounds via the reaction of amines and diazoethyl reagents (Figureas15) [66]. Assource, expected, the results acted showed that, using ethylenediamine and diazoacetate the [Fe(TPP)Cl] as athat, highly efficient catalyst for the diazo reagents (Figure the 15) carbene [66]. As expected, the results showed using ethylenediamine and ethylsyntheses diazoacetate as the carbene source, [Fe(TPP)Cl] acted as a highly efficient catalyst for the of 2-piperazinone, with yieldsthe above 80%. Since theseas products important for their ethyl diazoacetate as the carbene source, the [Fe(TPP)Cl] acted a highlyare efficient catalyst for the syntheses of 2-piperazinone, with yields above 80%. Since these products are important for their therapeutic properties, that study is a good example howthese a known synthesis can be improved syntheses of 2-piperazinone, with yields above 80%. of Since products are important for their therapeutic that study is aisgood example of how a authors known synthesis canthat be improved using a properties, porphyrin catalyst in a one-pot tandem reaction. demonstrated the tandemusing therapeutic properties, that study a good example ofThe how a known synthesis can be improved N-H insertion/cyclization process is an easy one-pot procedure for the syntheses of a porphyrin catalyst in a one-pot tandem reaction. The authors demonstrated that the tandem N-H using a porphyrin catalyst in a one-pot tandem reaction. The authors demonstrated that the novel tandem piperazinones and their substituted analogs. Other examples can be found in a recent review insertion/cyclization process is an easy one-pot procedure for the syntheses of novel piperazinones N-H insertion/cyclization process is an easy one-pot procedure for the syntheses of novel concerning carbeneanalogs. transfer reactions catalyzed can by dyes of the metalloporphyrin group [67]. and their substituted Other examples be found in a recent review concerning carbene
piperazinones and their substituted analogs. Other examples can be found in a recent review
transfer reactions catalyzed dyes of catalyzed the metalloporphyrin [67]. concerning carbene transferbyreactions by dyes of thegroup metalloporphyrin group [67].
Figure 15. Schematic representation of a tandem insertion/cyclization reaction [66].
Figure 15.15.Schematic representation aa tandem insertion/cyclization reaction Figure Schematic representation ofthe tandem insertion/cyclization reaction In 2015, Beyzavi and coworkers reportedof preparation and characterization of[66]. a [66]. robust metal organic framework (MOF) consisting of FeIII porphyrin, and an Hf6 node decorated with iron ions. To In 2015, Beyzavi coworkers reported the preparation andcharacterization characterization a robust metal In 2015, Beyzavi and coworkers reported the =preparation and ofof a robust metal synthetize it, theyand used [Fe(TCPP)Cl] (TCPP 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin), organic framework (MOF) consisting of Fe FeIIIIIIporphyrin, porphyrin, and node decorated iron ions. HfOCl 2∙8H2O, benzoic acid, N,N-diethylformamide, and iron ions, in a with solidwith withions. twoTo organic framework (MOF) consisting of and an an Hf 6Hf node decorated iron 6resulting To synthetize synthetize it, they used [Fe(TCPP)Cl] (TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin), additionalit, Fethey atoms coordinated to the Hf 6 node of the framework, to give the overall formula used [Fe(TCPP)Cl] (TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin), C962 H Cl 42Fe Hf 6N8O32 acid, [68]. resulting solid MOF, named Fe@Hf-2, applied catalyst in two a two HfOCl N,N-diethylformamide, andiron ironions, ions,was resulting in solid with ·8H HfOCl 264 ∙8H O,4benzoic benzoic acid,The N,N-diethylformamide, and resulting inas a aa solid with 2 O, tandem reaction comprising styrene oxidation by Fe(III)porphyrin, followed by epoxide ring additionalFeFeatoms atomscoordinated coordinated to to the the Hf Hf66 node node of additional of the the framework, framework,totogive givethe theoverall overallformula formula Hf ions of the structured MOF,named followed by the di-alcohol protection reaction 96opening H 64 Cl 4Feby 4Hf Hf 6Nnodes/iron 8O 32 [68]. The resulting solid MOF, Fe@Hf-2, was applied as a catalyst in in a a C96CH Cl Fe O [68]. The resulting solid MOF, named Fe@Hf-2, was applied as a catalyst 64 4 4 6 8 32 using the reactant TMSN3 (azidotrimethylsilane), resulting in the protected azidohydrin product, tandem reaction comprising styrene oxidation by Fe(III)porphyrin, followed by epoxide ring tandem reaction comprising styrene oxidation by Fe(III)porphyrin, followed by epoxide ring opening which by is an precursor alcohols (Figure 16). by Thethe Fe@Hf-2 solidprotection showed catalytic Hfimportant nodes/iron of to theα-amino structured MOF, followed di-alcohol byopening Hf nodes/iron ions of theions structured MOF, followed by the di-alcohol protection reactionreaction using the activity in the epoxidation reaction with regioselectivity. This work is an example of the application using the reactant TMSN 3 (azidotrimethylsilane), resulting in the protected azidohydrin product, reactant TMSN resulting in the azidohydrin product, 3 (azidotrimethylsilane), of iron porphyrins in sequential reactions, as depicted byprotected the orthogonal tandem catalysis inwhich Figure is an which isprecursor an important precursor to α-amino alcohols (Figure 16). The Fe@Hf-2 solid showed catalytic important to α-amino alcohols (Figure 16). The Fe@Hf-2 solid showed catalytic activity 8, where the first step is catalyzed by Fe(III) porphyrin, and, after the first product’s formation, Hfin the activity in the epoxidation reaction with regioselectivity. This work is an example of the application epoxidation reaction with regioselectivity. This work is anstep, example of the of ironproduct porphyrins nodes/iron ions are responsible for the second catalytic yielding theapplication protected alcohol of iron porphyrins in sequential reactions, as depicted by the orthogonal tandem catalysis in Figure in sequential reactions, as depicted by the orthogonalthe tandem catalysis in Figure 8, where the first using TMSN 3. The authors successfully demonstrated applicability of tandem reactions and also 8, where the first step is catalyzed by Fe(III) porphyrin, and, after the first product’s formation, Hf thecatalyzed reusabilityby of Fe(III) heterogeneous catalysts styrene epoxidation utilizing molecular oxygen as a ions step is porphyrin, and,from after the first product’s formation, Hf nodes/iron nodes/iron ions areand responsible for the second catalytic step, yielding the protected alcohol product terminal oxidant epoxidecatalytic ring opening. are responsible for the second step, yielding the protected alcohol product using TMSN .
using TMSN3. The authors successfully demonstrated the applicability of tandem reactions and also 3
The authors successfully demonstrated the applicability of tandem reactions and also the reusability of the reusability of heterogeneous catalysts from styrene epoxidation utilizing molecular oxygen as a heterogeneous catalysts from styrene epoxidation utilizing molecular oxygen as a terminal oxidant terminal oxidant and epoxide ring opening. and epoxide ring opening.
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Figure 16. Schematic representationofofa atandem tandem reaction reaction (olefin di-alcohol formation, and and Figure 16. Schematic representation (olefinoxidation, oxidation, di-alcohol formation, protection with TMSN 3) using catalytic metalreaction organic framework (MOF) solid Fe@Hf-2 [68].[68].and protection with TMSN ) using thethe catalytic metal organic framework (MOF) solid Fe@Hf-2 Figure 16. Schematic of a tandem (olefin oxidation, di-alcohol formation, 3representation protection with TMSN3) using the catalytic metal organic framework (MOF) solid Fe@Hf-2 [68]. In 2018, Maljutenko and coworkers reported the use of metalloporphyrins as catalysts in In 2018, Maljutenko and coworkers reported the use of metalloporphyrins as catalysts in aerobic aerobic cascade oxidation of cyclopentane-1,2-dione at room temperature, for 24 h at normal air cascadeInoxidation of cyclopentane-1,2-dione at roomthe temperature, for 24 h at normal air pressure, 2018, Maljutenko and coworkers reported use of metalloporphyrins as catalysts in pressure, to produce the oxidation products: hydroxy diacids, ketoacids, and diketoacids (Figure 17) air aerobic cascade oxidation of cyclopentane-1,2-dione at room temperature, for 24 h at normal to produce the oxidation products: hydroxy diacids, ketoacids, and diketoacids (Figure 17) [59]. [59]. To accomplish this, they studied two different porphyrin structures, octaethylporphyrin (OEP) to produce thestudied oxidation products: diacids, ketoacids, and diketoacids (Figure To pressure, accomplish this, they two differenthydroxy porphyrin structures, octaethylporphyrin (OEP)17) and and meso-tetraphenylporphyrin (TPP), metalated with Mn, Fe, and Co, generating six distinct [59]. To accomplish this, they studied two different porphyrin structures, octaethylporphyrin (OEP) meso-tetraphenylporphyrin (TPP), metalated with Mn, Fe, and Co, generating six distinct catalysts. catalysts. The Mn complexes showed at most 3% unreacted substrates, while Fe and Co catalysts meso-tetraphenylporphyrin (TPP), metalated with Mn, Fe, and Fe Co,and generating six distinct Theand Mn complexes at most 3%catalyst unreacted substrates, Co (product catalysts showed showed between showed 20 and 49%. The best to obtain ketoacidswhile was [Mn(OEP)Cl] yield catalysts. The Mn complexes showed at most 3% unreacted substrates, while Fe and Co catalysts between 20 and 49%. The best catalyst to obtain ketoacids was [Mn(OEP)Cl] (product yield 67%) 67%) using 5 mol% of catalyst. They also observed that [Co(TPP)Cl] was the most selective catalystusing showed betweengeneration 20 andalso 49%. The best catalyst to(ketoacid: obtainwas ketoacids wasselective [Mn(OEP)Cl] yield 5 mol% of catalyst. They observed that [Co(TPP)Cl] thediacid: most diketoacid in diketoacid with 48% conversion 24%; 8%). Evencatalyst with(product 1 in mol% of 67%) using 5 mol% catalyst. They also observed that [Co(TPP)Cl] was the1most selective catalyst [Mn(TPP)Cl], they of observed 55% hydroxy diacid but with selectivity (ketoacid: 42%; generation with 48% conversion (ketoacid: 24%;conversion, diacid: 8%). Evenlow with mol% of [Mn(TPP)Cl], in diketoacid: diketoacid 0%). generation with 48% conversion (ketoacid:the 24%; diacid: 8%). Even presented with 1 mol% Under the same experimental reaction only of they observed 55% hydroxy diacid conversion,conditions, but with lownon-catalyzed selectivity (ketoacid: 42%; diketoacid: [Mn(TPP)Cl], they observed 55% hydroxy diacid conversion, but with low selectivity (ketoacid: 42%; 19% yield selectivity in conditions, aerobic oxidation. In an effort toreaction improvepresented the selectivity the 0%). Under thewith samenoexperimental the non-catalyzed only to 19% yield diketoacid: 0%). Under the same experimental conditions, the non-catalyzed reaction presented only desired hydroxy diacid product, they investigated the best solvent to use in the reaction. The use of with no selectivity in aerobic oxidation. In an effort to improve the selectivity to the desired hydroxy 19% yield with1 no selectivity oxidation. improved In an effort improveto the selectivity to the [Mn(TPP)Cl] mol% in tolueneinataerobic room temperature the to selectivity hydroxy acid, with diacid product, they investigated the best solvent to use in the reaction. The use of [Mn(TPP)Cl] desired hydroxy diacid product, investigated the an best solvent to use in the reaction. use of yield up to 76%. In this context,they the authors reported environmentally friendly cascadeThe aerobic 1 mol% in toluene at room temperature improved the selectivity toThis hydroxy acid, with yield up to 76%. reaction involving in temperature homogeneousimproved catalysis. the work is interesting [Mn(TPP)Cl] 1 mol%metalloporphyrin in toluene at room selectivity to hydroxysince acid,the with In this context, the authors reported an environmentally friendly cascade aerobic reaction involving authors (in reported this case, an theenvironmentally best one was [Mn(TPP)Cl]) can promote yield up toproposed 76%. In that this metalloporphyrins context, the authors friendly cascade aerobic metalloporphyrin in homogeneous catalysis. This work is interesting since that the cascade oxidation of cyclopentane-1,2-diones to obtain desiredThis products (Figure 17), proposed similar reaction involving metalloporphyrin in homogeneous catalysis. workthe is authors interesting sincetothe metalloporphyrins (in this case, best6.one(in was the cascade oxidation the cascade reaction shown inthe Figure authors proposed that metalloporphyrins this[Mn(TPP)Cl]) case, the bestcan onepromote was [Mn(TPP)Cl]) can promote of
cyclopentane-1,2-diones to cyclopentane-1,2-diones obtain desired productsto (Figure similar to the cascade reaction shown the cascade oxidation of obtain17), desired products (Figure 17), similar to in Figure 6. reaction shown in Figure 6. the cascade
Figure 17. Schematic representation of the cascade catalytic reaction using Mn, Fe, or Co metalloporphyrin as a catalyst (catalyst N) in aerobic conditions to obtain the respective products, where R1 = H and R2 = Bn, Ph, Me, CH2CH2OBn, CH2CH2OH, CH2CH2NHBoc, CH2COOtBu, Cy, or Et. 17. [59]. Schematic representation of the cascade catalytic reaction using Mn, Fe, or Co Figure Figure 17. Schematic representation of the cascade catalytic reaction using Mn, Fe, or Co
metalloporphyrin conditionsto toobtain obtainthe therespective respective products, metalloporphyrinasasa acatalyst catalyst(catalyst (catalyst N) N) in in aerobic aerobic conditions products, where R = H and R = Bn, Ph, Me, CH CH OBn, CH CH OH, CH CH NHBoc, CH COOtBu, where1 R1 = H and 2R2 = Bn, Ph, Me, CH22CH22OBn, CH2CH CH2CH CH2COOtBu, Cy, orCy, 2 2OH, 2 2 2NHBoc, 2 2 or Et. [59].
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In 2012, Hajimohammadi and colleagues colleagues described described the use of Sb porphyrin as catalyst in a bio-inspired homogeneous pH-dependent cascade photoredox reaction to oxidize alcohol toalcohol aldehyde bio-inspired homogeneous pH-dependent cascade photoredox reaction to oxidize to + and then to carboxylic acid, as represented in Figure 18 They usedThey [Sb(TPP)(OH) aldehyde and then to carboxylic acid, as represented in [69]. Figure 18 [69]. used [Sb(TPP)(OH) 2]+ in 2 ] in a solvent mixture water–acetonitrile with the substrate benzyl alcohol, the presence O2 . Theofcatalytic a solventofmixture of water–acetonitrile with the substrate benzylinalcohol, in the of presence O 2. The mixture was irradiated with visible light (
