DOI: 10.1002/chem.201406444
Concept
& Asymmetric Synthesis
Vanadium in Asymmetric Synthesis: Emerging Concepts in Catalyst Design and Applications Shinobu Takizawa,[a] Harald Grçger,*[a, b] and Hiroaki Sasai*[a]
Chem. Eur. J. 2015, 21, 8992 – 8997
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Concept Abstract: In recent years vanadium catalysis has been extended to a range of different and even complementary directions in asymmetric synthesis. Inspired by nature’s way to activate both substrate and reagent in many cases, the design of efficient bifunctional and dinuclear vanadium catalysts has been achieved. Furthermore, vanadium catalysis has been an early field in which “hybrid catalysts” have been studied in detail by incorporation of oxovanadium complexes into proteins, thus giving artificial enzymes. In addition, a high compatibility of vanadium with proteins enabled the use of vanadium chemocatalysts for combinations with enzyme catalysis in one-pot, thus leading to dynamic kinetic resolutions. In this contribution, these three concepts of vanadium catalysis opening up new perspectives for asymmetric synthesis are reviewed.
Introduction Vanadium—jointly with, for example, copper, zinc, and iron— represents one of the few metals that serve as a key metal component in a broad range of both man-made as well as “nature-made” catalysts.[1] Man-made-type vanadium catalysts have been increasingly used in recent years as metal-based chemocatalysts in organic synthesis and in the field of asymmetric catalysis an increasing number of examples with chiral vanadium catalysts has been reported.[2] A major field of application has been related to vanadium-catalyzed redox chemistry as well as Lewis acid based chemistry. Many efficient and elegant catalysts have been reported that typically consist of a mononuclear vanadium center metal and a chiral multidentate ligand.[2, 3] What makes vanadium further attractive as a metal catalyst in organic synthesis is—besides exciting catalytic capabilities— its abundant availability in nature as well as the relatively low toxicity compared with other heavy metals.[1, 4] It is often widely unknown that vanadium is a metal that is present in nature to the same extent as zinc (albeit in a more widely distributed form, thus being more difficult to access).[1] Thus, it is not surprising that vanadium has it also made in nature to a “privileged” metal, being present in numerous metalloenzymes. As for vanadium-based metalloenzymes, such biocatalysts have been also applied successfully in organic synthesis already with a major focus on sulfoxidation reactions.[5] Another exciting aspect of vanadium in nature is that vanadium-capturing organisms are capable to concentrate vanadium by means of suitable proteins binding this metal in signifi[a] Dr. S. Takizawa, Prof. Dr. H. Grçger, Prof. Dr. H. Sasai The Institute of Scientific and Industrial Research (ISIR) Osaka University, Mihogaoka, Ibaraki-shi, Osaka 567-0047 (Japan) E-mail: [email protected] [b] Prof. Dr. H. Grçger Faculty of Chemistry, Bielefeld University Universittsstr. 25, 33615 Bielefeld (Germany) E-mail: [email protected] Chem. Eur. J. 2015, 21, 8992 – 8997
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cant amounts. In particular aquatic animal organisms have been reported to accumulate vanadium in concentrations of up to 0.15 m.[1, 6] Such a high concentration of vanadium compounds in living organisms also indicates an excellent compatibility of vanadium with proteins, which makes the option of combinations of vanadium catalysis with enzyme catalysis towards chemoenzymatic one-pot processes promising. Making use of these beneficial properties, in recent years the exciting field of vanadium catalysis has been extended to a range of different and even complementary directions. Inspired by nature’s way to activate both substrate and reagent in many cases, the design of bifunctional and dinuclear vanadium catalysts and their applications in asymmetric catalysis has been reported recently. Furthermore, vanadium catalysis has been an early field in which “hybrid catalysts” have been studied in detail by incorporation of oxovanadium complexes into proteins, thus leading to so-called “artificial enzymes”. In addition, the compatibility of vanadium with proteins enabled the joint use of synthetic, man-made vanadium complexes and enzymes in multistep one-pot transformations, thus leading to highly efficient dynamic kinetic resolution of alcohols. These types of chemoenzymatic processes underline the high potential when combining vanadium catalysis with enzyme catalysis. In the following, these three representative extensions of vanadium catalysis opening up new perspectives for asymmetric synthesis are reviewed.
Chiral Dinuclear and Bifunctional Vanadium Chemocatalysts Asymmetric synthesis with mononuclear vanadium catalysts has been successfully demonstrated by numerous groups identifying an impressive broad range of suitable applications,[2, 3] such as cyanation reactions of aldehydes, imines (Strecker reaction) and nitroalkenes, epoxidation, oxidation of a-hydroxy carboxylates, pinacol coupling, sulfoxidation and oxidative coupling of 2-naphthol and derivatives. For the last process leading to 1,1’-bi-2-naphthol (BINOL), which represents a privileged ligand in asymmetric catalysis, the proof of concept was demonstrated by the Chen[7] and Uang[8] groups. Since initially the enantioselectivity was in a moderate range, improved mononuclear vanadium catalysts have been developed based on different ligand systems. For example, Chen et al. achieved up to 87 % ee when using a ketopinidine-based vanadium catalyst albeit the reaction time was long (15 days).[9] An increased enantioselectivity of up to 98 % ee was achieved by the Luo and Gong groups by utilizing a dinuclear vanadium catalyst bearing a V-O-V linkage at 5 mol %.[10] However, a reaction time of 5 days was required, which could be decreased to 2 days at the expense of enantioselectivity, which was somewhat lower (93 % ee) when operating at a catalyst loading of 10 mol %. Thus, development of a catalyst fulfilling the criteria of both high enantioselectivity and activity, which enables its use at a low catalyst loading (as a key feature also for a later purpose to scale up the process) remained a challenge for a long time. Inspired by the potential of bifunctional catalysts[11] to activate both reagent and substrate, some of us[12] applied this
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Concept concept for designing a vanadium catalyst which has the following properties: 1) capability to bind both binaphthol molecules; 2) flexibility in enabling the binaphthol moieties to reach a favorable position to react with each other and 3) sterically rigid chiral environment to achieve a high asymmetric induction in the oxidative C¢C bond formation. These criteria were fulfilled with the design of a dinuclear vanadium catalyst of type 1 (Scheme 1).[12] Therein, the two oxovanadium moieties
tions which proceed with up to 97 % ee, and the role of air versus molecular oxygen was studied.[12b–d] Notably, the choice of solvent also plays an important role enabling, for example, the access to opposite enantiomers as products when switching from CH2Cl2 to CCl4.[12d] Recently, this methodology has been successfully extended to the enantioselective coupling of polycyclic phenols leading, for example, to the biaryl derivatives 5 and 7 with 85 and 90 % ee, respectively (Scheme 2).[12g] In addition, enantiomeri-
Scheme 1. Asymmetric coupling of 2-naphthol (2) with oxygen in the presence of a chiral dinuclear vanadium complex as a catalyst.
Scheme 2. Asymmetric oxidative coupling of 2-anthracenol (4) and 9-phenanthrol (6) catalysed by a chiral dinuclear vanadium complex.
each bind to a 2-naphthol molecule, required for the reaction, and the C2-symmetric chiral BINOL framework, jointly with a chiral amino acid subunit, leads to the formation of the desired enantiomer with a high enantiopreference. A further key feature is the “steric independency” of the vanadium moieties from each other. Thus, rotation about the C¢C axis of the binaphthol subunits in combination with the free V moieties leads to a favored position for C¢C coupling. Accordingly, by means of this concept both high enantioselectivity exceeding 90 % ee and high activity were achieved, thus underlining the capability of this type of catalyst to bind two molecules of 2naphthol and adjust them in a position for rapid C¢C bond formation due to the steric flexibility of the two oxovanadium moieties.[12] For example, in the presence of 5 mol % of vanadium catalyst 1, already after 24 h the desired (S)-1,1’-bi-2-naphthol (3) was obtained in 76 % yield and with a high enantioselectivity of 91 % ee (Scheme 1).[12c,d] A quantitative yield of 3 accompanied by again a high enantioselectivity of 90 % ee was found when conducting the reaction at 0 8C at a somewhat prolonged reaction time of 72 h.[12c,d] Furthermore, it was possible to gain a detailed insight into the catalyst structure, which was also supported by X-ray analysis.[12b–d] In addition, an extensive catalyst fine tuning was conducted leading to asymmetric 2-naphthol coupling reac-
cally pure forms (> 99 % ee) of 5 and 7 were subsequently obtained after a single recrystallization. Besides oxidative coupling of phenols, this catalytic dinuclear vanadium catalyst of type 1 also turned out to be suitable for Lewis acid based catalytic transformations as demonstrated very recently by some of us for asymmetric Friedel– Crafts-type reactions of imines with 2-naphthols or indoles, which proceed with high enantioselectivities of up to 91 % ee.[12f]
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Artificial Vanadium-Based Enzymes as “Hybrid Catalysts” A complementary approach towards the development of novel oxovanadium catalysts for organic synthesis consists of making use of natural sources, utilizing vanadium-containing metalloenzymes as (bio-)catalysts for applications in non-natural reactions, such as asymmetric sulfoxidation. For this reaction iron-containing heme-type haloperoxidases were reported earlier as suitable catalysts with a broad substrate scope.[13] However, operational stability of these metalloenzymes turned out to be a severe issue due to the sensitivity of heme-type enzymes. Vanadium-containing haloperoxidases represent a promising alternative. Their operational stability has been shown to be significantly higher, which can be explained by the absence
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Concept of a heme functionality. When applying vanadium-type bromoperoxidases from the alga Corallina officinalis, the Andersson group could achieve up to 91 % ee, and Wever et al. reported vanadium bromoperoxidases from different seaweed sources to be suitable obtaining also up to 91 % ee, whereas a vanadium chloroperoxidase from a fungus only gave a racemic product.[14] As a limitation of natural vanadium haloperoxidases, however, one might see in the limited substrate scope, which was explained, for example, with a narrow active site in case of the vanadium peroxidase from Curvularia inaequalis.[15] An exciting novel approach to overcome these limitations by developing “engineered artificial vanadium biocatalysts” with enlarged substrate (and potentially even reaction) scope was disclosed by the Sheldon group.[15] Making use of vanadium’s capability to bind proteins, a range of “hybrid” catalysts were obtained when starting from an inexpensive inorganic vanadium salt and proteins suitable for binding a vanadium moiety. In particular, a combination of VO43¢ as metal component with phytase as protein component turned out to give an efficient “artificial vanadium biocatalyst” for asymmetric synthesis, as demonstrated in sulfoxidation reactions (Scheme 3).
Mechanistically the peroxide is bound side-on to the vanadium ion of the vanadium-phytase “hybrid catalyst”, which then enables a sulfoxidation of a thioether in an asymmetric fashion under regeneration of the artificial enzyme catalyst (as shown in Scheme 4).[15b,c] Although the enantioselectivities achieved so far with these artificial vanadium enzyme catalysts are still in a moderate range, this catalytic concept appears to be promising with respect to both further improvement of catalytic properties for sulfoxidation (in particular enantioselectivity) as well as expansion towards other substrates and reaction types. What makes this approach further attractive from an economic perspective is the use of a cheap protein source, since phytases are known as inexpensive proteins due to their wide use in animal feed applications.
Scheme 3. Asymmetric sulfoxidation in the presence of a vanadium-phytase “hybrid” catalyst.
For example, thioanisol was converted into sulfoxide (S)-9 with > 99 % conversion, > 99 % selectivity for the sulfoxide (compared to sulfone), and an enantioselectivity of 66 % ee. A range of other thioethers was successfully oxidized as well. The preferred use of phytase to form artificial vanadium enzymes was also rationalized by the Sheldon group.[15] Since the active site of phytase (exemplified for the one from Aspergillus ficuum) is structurally closely related to the one of vanadium chloroperoxidase (from Curvularia inaequalis), with comparable amino acid residues in the active site, incorporation, strong binding and even activation of vanadium in such a protein can be expected.[15b,c] In this vanadium–phytase complex, the vanadium center metal is bound through a lysine, two arginine, and two histidine residues (Scheme 4). Notably, replacement of vanadate by other metal oxides, such as molybdate and tungstate, led to a dramatically decreased activity of the resulting metal–protein “hybrid catalyst”, underlining the preferred binding of vanadate by phytase. Chem. Eur. J. 2015, 21, 8992 – 8997
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Scheme 4. Catalytic cycle of asymmetric sulfoxidation in the presence of a phytase with a vanadium complex incorporated therein.
Integration of Vanadium Catalysts in Chemoenzymatic Synthesis A further exciting extension of vanadium catalysis has been reported by Akai et al., who applied a vanadium catalyst in a chemoenzymatic multistep one-pot process.[16] This combination of a chemo- and a biocatalyst consists of a vanadium-catalyzed racemization of allylic alcohols and an enantioselective enzymatic acylation, leading to non-racemizable allylic esters. The resulting dynamic kinetic resolution (DKR) runs with high efficiency in terms of both activity and enantioselectivity. This process complements previously developed dynamic kinetic resolutions of alcohols based on the use of lipases and (in particular) ruthenium or aluminum catalysts.[17, 18] It is worth noting that after the catalyst development, the vanadium catalyst and the enzyme turned out to be highly compatible with each other, thus being able to act in a favorable simultaneous fashion. To start with early developments in this field, a vanadiumcatalyst of type O=V(L)n, for example, O=V(OSiPh3)3, and
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Scheme 5. Chemoenzymatic DKR of allylic alcohols in the presence of the vanadium complex O=V(OSiPh3)3 and a lipase as catalyst components.
a lipase from Candida antarctica B were combined towards DKR processes in organic media, which proceed under formation of esters (R)-12 in up to 99 % yield and with up to 99 % ee (Scheme 5).[16a,d] When using such monomeric vanadium species, however, the catalytic loading remained high with 10 mol %, which— albeit attractive for lab scale applications—is still a large catalytic amount for industrial application. The major breakthrough was made when discovering that an oxovanadium species can be inserted into a mesoporous silica (MPS) matrix (Scheme 6).[16c] Being on the inner surface of MPS, only the substrate can interact with vanadium, whereas the lipase is too large for entering the inner area of MPS. This avoids the previously observed deactivation of the enzyme through interaction with vanadium. Thus, by means of this compartmentation strategy the Akai group succeeded in developing a highly efficient and economical process, which is underlined by both low catalyst loading and high recyclability.[16c,d] Accordingly, the catalyst loading can be lowered to 1 mol % when using this VMPS racemization catalyst, and excellent recyclability over six cycles was demonstrated with yields in the range of 99–100 % and enantioselectivities of 99 % ee in all cases (whereas in the 7th cycle a somewhat lower yield of 85 % was found). Furthermore, leaching of vanadium is very low with less than 0.0003 %.[16c,d] A major further finding was that vanadium is not only suitable to catalyze racemization of allylic alcohols (by forming allyl cation-type species) but further enables racemization of other types of secondary alcohols, such as benzyl alcohols (Scheme 6).[16c,d] Thus, the Akai group succeeded in extending their DKR process towards the transformation of a broad range of secondary alcohols, making it to an efficient and broadly applicable technology platform.[16] Since usually a hydroxy group is a disfavored leaving group, the suitability of vanadium to activate and racemize secondary alcohols also underlines the impressive catalytic capabilities of vanadium. Chem. Eur. J. 2015, 21, 8992 – 8997
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Scheme 6. Substrate scope of the chemoenzymatic DKR using V-MPS as a racemization catalyst.
Summary In summary, vanadium catalysis in asymmetric synthesis has been extended by various groups in different and even complementary directions. Inspired by nature’s way to activate both substrate and reagent, the design of efficient bifunctional and dinuclear vanadium catalysts and their application in C¢C coupling reactions has been achieved. In addition, vanadium catalysis has been successfully “merged” with protein chemistry: on the one hand “hybrid catalysts” by incorporation of oxovanadium complexes therein have been developed and on the other hand vanadium-based chemocatalysis has been combined with enzyme catalysis to give a one-pot process, leading to dynamic kinetic resolution of alcohols. These examples underline the impressive potential of vanadium catalysis in organic synthesis and open up new perspectives for their future use when addressing challenging applications in asymmetric catalysis.
Acknowledgements H.G. thanks Osaka University for a visiting professorship at the Institute of Scientific and Industrial Research (ISIR) of Osaka University. Keywords: asymmetric synthesis · enzyme catalysis · metal catalysis · oxidation · vanadium
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[1] Review: D. Rehder, Angew. Chem. Int. Ed. Engl. 1991, 30, 148 – 167; Angew. Chem. 1991, 103, 152 – 172. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2009, 1667 – 1669; f) S. Takizawa, F. A. Arteaga, Y. Yoshida, J. Kodera, Y. Nagata, H. Sasai, Dalton Trans. 2013, 42, 11787 – 11790; g) S. Takizawa, J. Kodera, Y. Yoshida, M. Sako, S. Breukers, D. Enders, H. Sasai, Tetrahedron 2014, 70, 1786 – 1793; h) Review: S. Takizawa, Chem. Pharm. Bull. 2009, 57, 1179 – 1188. a) S. Colonna, N. Gaggero, L. Casella, G. Carrea, P. Pasta, Tetrahedron: Asymmetry 1992, 3, 95 – 106; b) S. Colonna, N. Gaggero, G. Carrea, P. Pasta, Chem. Commun. 1997, 439 – 440; c) M. P. J. van Deurzen, I. J. Remkes, F. van Rantwijk, R. A. Sheldon, J. Mol. Catal. A 1997, 117, 329 – 337; d) Review: W. Adam, M. Lazarus, C. R. Saha-Mçller, O. Weichold, U. Hoch, D. Hring, P. Schreier, Adv. Biochem. Eng./Biotechnol. 1999, 63, 73 – 108. a) M. Andersson, A. Willetts, S. Allenmark, J. Org. Chem. 1997, 62, 8455 – 8458; b) H. B. ten Brink, A. Tuynman, H. L. Dekker, W. Hemrika, Y. Izumi, T. Oshiro, H. E. Schoemaker, R. Wever, Inorg. Chem. 1998, 37, 6780 – 6784. a) F. van de Velde, L. Kçnemann, F. van Rantwijk, R. A. Sheldon, Chem. Commun. 1998, 1891 – 1892; b) F. van de Velde, L. Kçnemann, F. van Rantwijk, R. A. Sheldon, Biotechnol. Bioeng. 2000, 67, 87 – 96; c) F. van de Velde, I. W. C. E. Arends, R. A. Sheldon, J. Inorg. Biochem. 2000, 80, 81 – 89; d) I. Correia, S. Aksu, P. Adao, J. C. Pessoa, R. A. Sheldon, I. W. C. E. Arends, J. Inorg. Biochem. 2008, 102, 318 – 329; e) Review: F. Van de Velde, I. W. C. E. Arends, R. A. Sheldon, Top. Catal. 2000, 13, 259 – 265. a) S. Akai, K. Tanimoto, Y. Kanao, M. Egi, T. Yamamoto, Y. Kita, Angew. Chem. Int. Ed. 2006, 45, 2592 – 2595; Angew. Chem. 2006, 118, 2654 – 2657; b) S. Akai, R. Hanada, N. Fujiwara, Y. Kita, M. Egi, Org. Lett. 2010, 12, 4900 – 4903; c) M. Egi, K. Sugiyama, M. Saneto, R. Hanada, K. Kato, S. Akai, Angew. Chem. Int. Ed. 2013, 52, 3654 – 3658; Angew. Chem. 2013, 125, 3742 – 3746; d) Review: S. Akai, Chem. Lett. 2014, 43, 746 – 754. Reviews on dynamic kinetic resolutions by combination of chemo- and biocatalysts: a) I. Hussain, J.-E. Bckvall in: Enzyme Catalysis in Organic Synthesis, Vol. 2, 3. ed. (Eds.: K. Drauz, H. Grçger, O. May), Wiley-VCH, Weinheim, 2012, Chap. 43, p. 1777 – 1806; b) M. Ahmed, T. Kelly, A. Ghanem, Tetrahedron 2012, 68, 6781 – 6802; c) P. Hoyos, V. Pace, A. R. Alcntara, Adv. Synth. Catal. 2012, 354, 2585 – 2611; d) C. A. Denard, J. F. Hartwig, H. Zhao, ACS Catal. 2013, 3, 2856 – 2884. Selected examples (pioneer work): a) P. M. Dinh, J. A. Howarth, A. R. Hudnott, J. M. J. Williams, W. Harris, Tetrahedron Lett. 1996, 37, 7623 – 7626; b) A. L. E. Larsson, B. A. Persson, J. E. Bckvall, Angew. Chem. Int. Ed. Engl. 1997, 36, 1211 – 1212; Angew. Chem. 1997, 109, 1256 – 1258; c) B. A. Persson, A. L. E. Larsson, M. Le Ray, J. E. Bckvall, J. Am. Chem. Soc. 1999, 121, 1645 – 1650; d) J. H. Choi, Y. H. Kim, S. H. Nam, S. T. Shin, M. J. Kim, J. Park, Angew. Chem. Int. Ed. 2002, 41, 2373 – 2376; Angew. Chem. 2002, 114, 2479 – 2482; e) A. Berkessel, M. L. Sebastian-Ibarz, T. N. Mìller, Angew. Chem. Int. Ed. 2006, 45, 6567 – 6570; Angew. Chem. 2006, 118, 6717 – 6720.
Published online on March 25, 2015
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Concept
& Asymmetric Synthesis
Vanadium in Asymmetric Synthesis: Emerging Concepts in Catalyst Design and Applications Shinobu Takizawa,[a] Harald Grçger,*[a, b] and Hiroaki Sasai*[a]
Chem. Eur. J. 2015, 21, 8992 – 8997
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Concept Abstract: In recent years vanadium catalysis has been extended to a range of different and even complementary directions in asymmetric synthesis. Inspired by nature’s way to activate both substrate and reagent in many cases, the design of efficient bifunctional and dinuclear vanadium catalysts has been achieved. Furthermore, vanadium catalysis has been an early field in which “hybrid catalysts” have been studied in detail by incorporation of oxovanadium complexes into proteins, thus giving artificial enzymes. In addition, a high compatibility of vanadium with proteins enabled the use of vanadium chemocatalysts for combinations with enzyme catalysis in one-pot, thus leading to dynamic kinetic resolutions. In this contribution, these three concepts of vanadium catalysis opening up new perspectives for asymmetric synthesis are reviewed.
Introduction Vanadium—jointly with, for example, copper, zinc, and iron— represents one of the few metals that serve as a key metal component in a broad range of both man-made as well as “nature-made” catalysts.[1] Man-made-type vanadium catalysts have been increasingly used in recent years as metal-based chemocatalysts in organic synthesis and in the field of asymmetric catalysis an increasing number of examples with chiral vanadium catalysts has been reported.[2] A major field of application has been related to vanadium-catalyzed redox chemistry as well as Lewis acid based chemistry. Many efficient and elegant catalysts have been reported that typically consist of a mononuclear vanadium center metal and a chiral multidentate ligand.[2, 3] What makes vanadium further attractive as a metal catalyst in organic synthesis is—besides exciting catalytic capabilities— its abundant availability in nature as well as the relatively low toxicity compared with other heavy metals.[1, 4] It is often widely unknown that vanadium is a metal that is present in nature to the same extent as zinc (albeit in a more widely distributed form, thus being more difficult to access).[1] Thus, it is not surprising that vanadium has it also made in nature to a “privileged” metal, being present in numerous metalloenzymes. As for vanadium-based metalloenzymes, such biocatalysts have been also applied successfully in organic synthesis already with a major focus on sulfoxidation reactions.[5] Another exciting aspect of vanadium in nature is that vanadium-capturing organisms are capable to concentrate vanadium by means of suitable proteins binding this metal in signifi[a] Dr. S. Takizawa, Prof. Dr. H. Grçger, Prof. Dr. H. Sasai The Institute of Scientific and Industrial Research (ISIR) Osaka University, Mihogaoka, Ibaraki-shi, Osaka 567-0047 (Japan) E-mail: [email protected] [b] Prof. Dr. H. Grçger Faculty of Chemistry, Bielefeld University Universittsstr. 25, 33615 Bielefeld (Germany) E-mail: [email protected] Chem. Eur. J. 2015, 21, 8992 – 8997
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cant amounts. In particular aquatic animal organisms have been reported to accumulate vanadium in concentrations of up to 0.15 m.[1, 6] Such a high concentration of vanadium compounds in living organisms also indicates an excellent compatibility of vanadium with proteins, which makes the option of combinations of vanadium catalysis with enzyme catalysis towards chemoenzymatic one-pot processes promising. Making use of these beneficial properties, in recent years the exciting field of vanadium catalysis has been extended to a range of different and even complementary directions. Inspired by nature’s way to activate both substrate and reagent in many cases, the design of bifunctional and dinuclear vanadium catalysts and their applications in asymmetric catalysis has been reported recently. Furthermore, vanadium catalysis has been an early field in which “hybrid catalysts” have been studied in detail by incorporation of oxovanadium complexes into proteins, thus leading to so-called “artificial enzymes”. In addition, the compatibility of vanadium with proteins enabled the joint use of synthetic, man-made vanadium complexes and enzymes in multistep one-pot transformations, thus leading to highly efficient dynamic kinetic resolution of alcohols. These types of chemoenzymatic processes underline the high potential when combining vanadium catalysis with enzyme catalysis. In the following, these three representative extensions of vanadium catalysis opening up new perspectives for asymmetric synthesis are reviewed.
Chiral Dinuclear and Bifunctional Vanadium Chemocatalysts Asymmetric synthesis with mononuclear vanadium catalysts has been successfully demonstrated by numerous groups identifying an impressive broad range of suitable applications,[2, 3] such as cyanation reactions of aldehydes, imines (Strecker reaction) and nitroalkenes, epoxidation, oxidation of a-hydroxy carboxylates, pinacol coupling, sulfoxidation and oxidative coupling of 2-naphthol and derivatives. For the last process leading to 1,1’-bi-2-naphthol (BINOL), which represents a privileged ligand in asymmetric catalysis, the proof of concept was demonstrated by the Chen[7] and Uang[8] groups. Since initially the enantioselectivity was in a moderate range, improved mononuclear vanadium catalysts have been developed based on different ligand systems. For example, Chen et al. achieved up to 87 % ee when using a ketopinidine-based vanadium catalyst albeit the reaction time was long (15 days).[9] An increased enantioselectivity of up to 98 % ee was achieved by the Luo and Gong groups by utilizing a dinuclear vanadium catalyst bearing a V-O-V linkage at 5 mol %.[10] However, a reaction time of 5 days was required, which could be decreased to 2 days at the expense of enantioselectivity, which was somewhat lower (93 % ee) when operating at a catalyst loading of 10 mol %. Thus, development of a catalyst fulfilling the criteria of both high enantioselectivity and activity, which enables its use at a low catalyst loading (as a key feature also for a later purpose to scale up the process) remained a challenge for a long time. Inspired by the potential of bifunctional catalysts[11] to activate both reagent and substrate, some of us[12] applied this
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Concept concept for designing a vanadium catalyst which has the following properties: 1) capability to bind both binaphthol molecules; 2) flexibility in enabling the binaphthol moieties to reach a favorable position to react with each other and 3) sterically rigid chiral environment to achieve a high asymmetric induction in the oxidative C¢C bond formation. These criteria were fulfilled with the design of a dinuclear vanadium catalyst of type 1 (Scheme 1).[12] Therein, the two oxovanadium moieties
tions which proceed with up to 97 % ee, and the role of air versus molecular oxygen was studied.[12b–d] Notably, the choice of solvent also plays an important role enabling, for example, the access to opposite enantiomers as products when switching from CH2Cl2 to CCl4.[12d] Recently, this methodology has been successfully extended to the enantioselective coupling of polycyclic phenols leading, for example, to the biaryl derivatives 5 and 7 with 85 and 90 % ee, respectively (Scheme 2).[12g] In addition, enantiomeri-
Scheme 1. Asymmetric coupling of 2-naphthol (2) with oxygen in the presence of a chiral dinuclear vanadium complex as a catalyst.
Scheme 2. Asymmetric oxidative coupling of 2-anthracenol (4) and 9-phenanthrol (6) catalysed by a chiral dinuclear vanadium complex.
each bind to a 2-naphthol molecule, required for the reaction, and the C2-symmetric chiral BINOL framework, jointly with a chiral amino acid subunit, leads to the formation of the desired enantiomer with a high enantiopreference. A further key feature is the “steric independency” of the vanadium moieties from each other. Thus, rotation about the C¢C axis of the binaphthol subunits in combination with the free V moieties leads to a favored position for C¢C coupling. Accordingly, by means of this concept both high enantioselectivity exceeding 90 % ee and high activity were achieved, thus underlining the capability of this type of catalyst to bind two molecules of 2naphthol and adjust them in a position for rapid C¢C bond formation due to the steric flexibility of the two oxovanadium moieties.[12] For example, in the presence of 5 mol % of vanadium catalyst 1, already after 24 h the desired (S)-1,1’-bi-2-naphthol (3) was obtained in 76 % yield and with a high enantioselectivity of 91 % ee (Scheme 1).[12c,d] A quantitative yield of 3 accompanied by again a high enantioselectivity of 90 % ee was found when conducting the reaction at 0 8C at a somewhat prolonged reaction time of 72 h.[12c,d] Furthermore, it was possible to gain a detailed insight into the catalyst structure, which was also supported by X-ray analysis.[12b–d] In addition, an extensive catalyst fine tuning was conducted leading to asymmetric 2-naphthol coupling reac-
cally pure forms (> 99 % ee) of 5 and 7 were subsequently obtained after a single recrystallization. Besides oxidative coupling of phenols, this catalytic dinuclear vanadium catalyst of type 1 also turned out to be suitable for Lewis acid based catalytic transformations as demonstrated very recently by some of us for asymmetric Friedel– Crafts-type reactions of imines with 2-naphthols or indoles, which proceed with high enantioselectivities of up to 91 % ee.[12f]
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Artificial Vanadium-Based Enzymes as “Hybrid Catalysts” A complementary approach towards the development of novel oxovanadium catalysts for organic synthesis consists of making use of natural sources, utilizing vanadium-containing metalloenzymes as (bio-)catalysts for applications in non-natural reactions, such as asymmetric sulfoxidation. For this reaction iron-containing heme-type haloperoxidases were reported earlier as suitable catalysts with a broad substrate scope.[13] However, operational stability of these metalloenzymes turned out to be a severe issue due to the sensitivity of heme-type enzymes. Vanadium-containing haloperoxidases represent a promising alternative. Their operational stability has been shown to be significantly higher, which can be explained by the absence
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Concept of a heme functionality. When applying vanadium-type bromoperoxidases from the alga Corallina officinalis, the Andersson group could achieve up to 91 % ee, and Wever et al. reported vanadium bromoperoxidases from different seaweed sources to be suitable obtaining also up to 91 % ee, whereas a vanadium chloroperoxidase from a fungus only gave a racemic product.[14] As a limitation of natural vanadium haloperoxidases, however, one might see in the limited substrate scope, which was explained, for example, with a narrow active site in case of the vanadium peroxidase from Curvularia inaequalis.[15] An exciting novel approach to overcome these limitations by developing “engineered artificial vanadium biocatalysts” with enlarged substrate (and potentially even reaction) scope was disclosed by the Sheldon group.[15] Making use of vanadium’s capability to bind proteins, a range of “hybrid” catalysts were obtained when starting from an inexpensive inorganic vanadium salt and proteins suitable for binding a vanadium moiety. In particular, a combination of VO43¢ as metal component with phytase as protein component turned out to give an efficient “artificial vanadium biocatalyst” for asymmetric synthesis, as demonstrated in sulfoxidation reactions (Scheme 3).
Mechanistically the peroxide is bound side-on to the vanadium ion of the vanadium-phytase “hybrid catalyst”, which then enables a sulfoxidation of a thioether in an asymmetric fashion under regeneration of the artificial enzyme catalyst (as shown in Scheme 4).[15b,c] Although the enantioselectivities achieved so far with these artificial vanadium enzyme catalysts are still in a moderate range, this catalytic concept appears to be promising with respect to both further improvement of catalytic properties for sulfoxidation (in particular enantioselectivity) as well as expansion towards other substrates and reaction types. What makes this approach further attractive from an economic perspective is the use of a cheap protein source, since phytases are known as inexpensive proteins due to their wide use in animal feed applications.
Scheme 3. Asymmetric sulfoxidation in the presence of a vanadium-phytase “hybrid” catalyst.
For example, thioanisol was converted into sulfoxide (S)-9 with > 99 % conversion, > 99 % selectivity for the sulfoxide (compared to sulfone), and an enantioselectivity of 66 % ee. A range of other thioethers was successfully oxidized as well. The preferred use of phytase to form artificial vanadium enzymes was also rationalized by the Sheldon group.[15] Since the active site of phytase (exemplified for the one from Aspergillus ficuum) is structurally closely related to the one of vanadium chloroperoxidase (from Curvularia inaequalis), with comparable amino acid residues in the active site, incorporation, strong binding and even activation of vanadium in such a protein can be expected.[15b,c] In this vanadium–phytase complex, the vanadium center metal is bound through a lysine, two arginine, and two histidine residues (Scheme 4). Notably, replacement of vanadate by other metal oxides, such as molybdate and tungstate, led to a dramatically decreased activity of the resulting metal–protein “hybrid catalyst”, underlining the preferred binding of vanadate by phytase. Chem. Eur. J. 2015, 21, 8992 – 8997
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Scheme 4. Catalytic cycle of asymmetric sulfoxidation in the presence of a phytase with a vanadium complex incorporated therein.
Integration of Vanadium Catalysts in Chemoenzymatic Synthesis A further exciting extension of vanadium catalysis has been reported by Akai et al., who applied a vanadium catalyst in a chemoenzymatic multistep one-pot process.[16] This combination of a chemo- and a biocatalyst consists of a vanadium-catalyzed racemization of allylic alcohols and an enantioselective enzymatic acylation, leading to non-racemizable allylic esters. The resulting dynamic kinetic resolution (DKR) runs with high efficiency in terms of both activity and enantioselectivity. This process complements previously developed dynamic kinetic resolutions of alcohols based on the use of lipases and (in particular) ruthenium or aluminum catalysts.[17, 18] It is worth noting that after the catalyst development, the vanadium catalyst and the enzyme turned out to be highly compatible with each other, thus being able to act in a favorable simultaneous fashion. To start with early developments in this field, a vanadiumcatalyst of type O=V(L)n, for example, O=V(OSiPh3)3, and
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Concept
Scheme 5. Chemoenzymatic DKR of allylic alcohols in the presence of the vanadium complex O=V(OSiPh3)3 and a lipase as catalyst components.
a lipase from Candida antarctica B were combined towards DKR processes in organic media, which proceed under formation of esters (R)-12 in up to 99 % yield and with up to 99 % ee (Scheme 5).[16a,d] When using such monomeric vanadium species, however, the catalytic loading remained high with 10 mol %, which— albeit attractive for lab scale applications—is still a large catalytic amount for industrial application. The major breakthrough was made when discovering that an oxovanadium species can be inserted into a mesoporous silica (MPS) matrix (Scheme 6).[16c] Being on the inner surface of MPS, only the substrate can interact with vanadium, whereas the lipase is too large for entering the inner area of MPS. This avoids the previously observed deactivation of the enzyme through interaction with vanadium. Thus, by means of this compartmentation strategy the Akai group succeeded in developing a highly efficient and economical process, which is underlined by both low catalyst loading and high recyclability.[16c,d] Accordingly, the catalyst loading can be lowered to 1 mol % when using this VMPS racemization catalyst, and excellent recyclability over six cycles was demonstrated with yields in the range of 99–100 % and enantioselectivities of 99 % ee in all cases (whereas in the 7th cycle a somewhat lower yield of 85 % was found). Furthermore, leaching of vanadium is very low with less than 0.0003 %.[16c,d] A major further finding was that vanadium is not only suitable to catalyze racemization of allylic alcohols (by forming allyl cation-type species) but further enables racemization of other types of secondary alcohols, such as benzyl alcohols (Scheme 6).[16c,d] Thus, the Akai group succeeded in extending their DKR process towards the transformation of a broad range of secondary alcohols, making it to an efficient and broadly applicable technology platform.[16] Since usually a hydroxy group is a disfavored leaving group, the suitability of vanadium to activate and racemize secondary alcohols also underlines the impressive catalytic capabilities of vanadium. Chem. Eur. J. 2015, 21, 8992 – 8997
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Scheme 6. Substrate scope of the chemoenzymatic DKR using V-MPS as a racemization catalyst.
Summary In summary, vanadium catalysis in asymmetric synthesis has been extended by various groups in different and even complementary directions. Inspired by nature’s way to activate both substrate and reagent, the design of efficient bifunctional and dinuclear vanadium catalysts and their application in C¢C coupling reactions has been achieved. In addition, vanadium catalysis has been successfully “merged” with protein chemistry: on the one hand “hybrid catalysts” by incorporation of oxovanadium complexes therein have been developed and on the other hand vanadium-based chemocatalysis has been combined with enzyme catalysis to give a one-pot process, leading to dynamic kinetic resolution of alcohols. These examples underline the impressive potential of vanadium catalysis in organic synthesis and open up new perspectives for their future use when addressing challenging applications in asymmetric catalysis.
Acknowledgements H.G. thanks Osaka University for a visiting professorship at the Institute of Scientific and Industrial Research (ISIR) of Osaka University. Keywords: asymmetric synthesis · enzyme catalysis · metal catalysis · oxidation · vanadium
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