Bakshi-Shibata CBS reduction

Enantioselective borane reduction of ketones catalyzed by chiral oxaborolidines. 

Bakshi, R. K. Shibata, S. J. Am. Chem. Soc. 1987, 109, 5551. Elias 1. Corey (1928—) was bom in Metbuen, Massacbusetts. After earning bis Pb.D. at MET from John Sbeeban at age 22, be began bis independent academic career at tbe University of Illinois in 1950 and moved to Harvard University in 1959. Corey won tbe Nobel Prize in Chemistry in 1990 for development of novel methods for the synthesis of complex natural compounds and retrosynthetic analysis. He is still carrying out active research at Harvard. Prof. Corey has been a consultant to Pfizer for more than 50 years. 

---

The pioneering studies by Itsuno [1] and Corey [2] on the development of the asymmetric hydroboration of ketones using oxazaborolidines have made it possible to easily obtain chiral secondary alcohols with excellent optical purity [3]. Scheme 1 shows examples of Corey s (Corey-Bakshi-Shibata) CBS reduction. When oxazaborolidines 1 were used as catalysts (usually 0.01-0.1 equiv), a wide variety of ketones were reduced by borane reagents with consistently high enan-tioselectivity [2]. The sense of enantioselection was predictable. Many important biologically active compounds and functional materials have been synthesized using this versatile reaction [2-4]. 

Numerous theoretical treatments have been carried out to understand the mode of asymmetric induction of the Corey-Bakshi-Shibata (CBS) reduction, more thoroughly.12 Liotta et al. carried out computational studies to identify the transition states for CBS reductions of various ketones13 (Scheme 4.3k).In the asymmetric reduction of acetophenone with the catalyst (R)-28a, four transition states were found. Of the lowest energy is chairlike transition state A, which would lead to formation of the major enantiomer. In transition state A, the phenyl group of acetophenone occupies an equatorial position that is free from any steric interaction, as it is 5.5 A away from one of the two phenyl groups of the diphenylprolinol ring. On the other hand, transition state B, leading to the... 

For aryl ketones the Corey-Bakshi-Shibata (CBS) reduction using oxazaborolidines as catalysts for the boron hydride mediated hydrogenation is particularly useful, with maximum selectivities up to 99 % ee (see Scheme 4) [34]. The excellent review by Corey et al. [35] also shows clearly the power for chemo- and enantioselective reduction of purely aliphatic a,//-enones and -ynones only on the carbonyl group. In the re-... 

Related reactions Corey-Bakshi-Shibata (CBS) reduction, Noyori asymmetric hydrogenation ... 

---