Catalysis, asymmetric enantioselective

Due to many impressive advances in metal-catalyzed transformations, both asymmetric and non-asymmetric, several efforts have been directed towards designing total synthesis routes that very heavily depend on various catalytic methods. These total syntheses benefit from the economic efficiency and environmental consciousness that are two of the inherent attributes of catalytic reactions. The total synthesis of wodeshiol 133 by Corey, discussed above (Scheme 19) is one such example. Two additional catalysis-based enantioselective total syntheses are briefly discussed below. In both efforts, all centers of asymmetiy are attained by a catalytic enantioselective method, and the synthesis is completed through the use of several other catalytic reactions. 

Chirality plays a central role in the chemical, biological, pharmaceutical and material sciences. Owing to the recent advances in asymmetric catalysis, catalytic enantioselective synthesis has become one of the most efficient methods for the preparation of enantiomer-ically enriched compounds. An increased amount of enantiomerically enriched product can be obtained from an asymmetric reaction using a small amount of an asymmetric catalyst. Studies on the enantioselective addition of dialkylzincs to aldehydes have attracted increasing interest. After the chiral amino alcohols were developed, highly enantioselective and reproducible —C bond forming reactions have become possible. 

Although several enzymes can enantioselectively catalyze the hydrocyanation of R2C=0 and R2C=NR bonds [7], (asymmetric) hydrocyanation of C=C double bonds has no precedents in biology. In homogeneous catalysis asymmetric hydrocyanation is still underdeveloped, as is apparent from the relatively few reports in the literature. In the following paragraphs a short overview will be given divided into the two major substrate classes investigated, cyclic (di)enes and vinylarenes. 

Only partial solutions have been provided thus far to many of the most important transformations amenable to asymmetric catalysis. For example, no generally effective methods exist yet for enantioselective epoxidation or aziridination of terminal olefins, or for hydroxylation of C-H bonds of any type. Despite the enormous advances in asymmetric hydrogenation catalysis, highly enantioselective reduction of dialkyl ketones remains elusive [9]. And as far as asymmetric C-C bond-forming reactions are concerned, the list of successful systems is certainly shorter than the list of reactions waiting to be developed. 

Keywords Catalysis Asymmetric dihydroxylation QM/MM Computational chemistry Enantioselectivity... 

Pye, P.J., Rossen, K., Reamer, R.A. et al. (1997) A new planar chiral bisphosphine ligand for asymmetric catalysis highly enantioselective hydrogenations under mild conditions. J. Am. 

Mukherjee S, List B (2007) Chiral Counteranions in Asymmetric Transition-Metal Catalysis Highly Enantioselective PdlBr nsted Acid-Catalyzed Direct a-Allylation of Aldehydes. J... 

A phosphine sulfonamide derived from L-threonine promotes aza-Morita-Baylis-Hillman (aza-MBH) reactions of sulfinylimines in up to 96% yield and 97% ee. A review describes the synthesis of chiral amines under mild conditions via catalytic asymmetric aza-MBH reactions. Proline/DABCO (l,4-diazabicyclo[2.2.2]octane) co-catalysis of enantioselective aza-MBH reactions gives good to high yields and up to 99%... 

Subjects LCSH Enantioselective catalysis. Catalysis. Asymmetric... 

A.L. Casalnuovo et al. - Ligand Electronic Effects in Asymmetric Catalysis Enhanced Enantioselectivity in the Asymmetric Hydrocyanation of Vinylarenes,... 

Rueping, M., Antonchick, A. P, Sugiono, E., Grenader, K. (2009). Asymmetric Brpnsted acid catalysis catalytic enantioselective synthesis of highly biologically active dihydro-quinazoUnones. Angewandte Chemie International Edition, 48, 908-910. 

McGilvra, J.D., Gondi, V.B., and Rawal, V.H. (2007) Asymmetric proton catalysis, in Enantioselective Organocatalysis (ed. P.I. Dalko), Wiley-VCH Verlag GmbH, Weinheim, pp. 189-254. 

Rueping M, Theissmann T, Raja S, Bats JW. Asymmetric counterion pair catalysis an enantioselective Brpnsted acid-catalyzed protonation. Adv. Synth. Catal. 2008 350 1001-1006. 

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic... 

Design of chiral catalysis and asymmetric autocatalysis for diphenyl-(l-methyl-pyrrolidin-2-yl) methanol-catalyzed enantioselective additions of organozinc reagents 97YGK994. 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

Early work on the use of chiral phase-transfer catalysis in asymmetric Darzens reactions was conducted independently by the groups of Wynberg [38] and Co-lonna [39], but the observed asymmetric induction was low. More recently Toke s group has used catalytic chiral aza crown ethers in Darzens reactions [40-42], but again only low to moderate enantioselectivities resulted. 

The most useful of the insertion processes is the intramolecular reactions that occur with high selectivity for the formation of five-membered ring products. The electrophilic nature of the process is suggested by C-H bond reactivity in competitive experiments (3°>20 >1°) [76, 77]. Asymmetric catalysis with Rh2(MPPIM)4 has been used to prepare a wide variety of lignans that include (-)-enterolactone (3) [8], as well as (R)-(-)-baclofen (2) [7],2-deoxyxylolactone (31) [80,81],and (S)-(+)-imperanane (32) [82].Enantioselectivities are 91-96%... 

Optically active polymers are potentially very useful in areas such as asymmetric catalysis, nonlinear optics, polarized photo and electroluminescence, and enantioselective separation and sensing.26 Transition metal coupling polymerization has also been applied to the synthesis of these polymers.27 For example, from the Ni(II)-catalyzed polymerization, a regioregular head-to-tail polymer 32 was obtained (Scheme 9.17).28 This polymer is optically active because of the optically active chiral side chains. 

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