Corey-Bakshi-Shibata catalyst

Ac = Acetyl acac = Acetylacetonate bda = Benzylidene-acetone BINOL = l,l -bi-2-naphthol Bn = Benzyl brsm = Yield based on recovered starting material Bu = Bntyl CAN = Ceric anunonium nitrate CBS = Corey/Bakshi/Shibata catalyst [(+) or (—)-(S)-2-methyl-oxazaborolidine] COD = Cyclo-l,5-octadiene COT = Cyclooctatetraene Cp = Cyclopentadienyl Cp = Penta-methylcyclopentadienyl Cy = Cyclohexyl DCC = Dicy-clohexylcarbodiimde DMF = Ai,A-dimethylformainide DMPU = l,3-dunethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidin-one DMSO = Dimethylsnlfoxide dppe = Diphenylphosp-hinoethane dr = Diastereomer ratio dppm = Diphenylphos-phinomethane E = Electrophile ee = Enantiomeric excess EHMO = Extended htickel molecular orbital Et =... 

CBS the Corey-Bakshi-Shibata catalyst dvds l,3-divinyl-l,l,3,3-... 

A series of / -hydroxyamide derivatives of salicylic acid and chiral amino alcohols were synthesized to be used as the most effective catalyst for the enantioselective reduction of a prochiral ketone. Different substituted prochiral ketones are reduced to the corresponding secondary alcohols in yields up to 99% and enantiomeric excess up to 86%. The mechanism for this type of catalyst can be explained by considering the reaction mechanism for the Corey-Bakshi-Shibata catalyst. ... 

CBS (Corey-Bakshi-Shibata) catalyst 966 N-Cbz imines 866 centrifugal separation 802 ceric ammonium nitrate (CAN) 72, 1167, 1175... 

One popular method that has been apphed to industrial processes for the enantio-selective reduction of prochiral ketones, leading to the corresponding optically active secondary alcohols, is based on the use of chiral 1,3,2-oxazaborolidines. The original catalyst and reagent system [diphenyl prolinol/methane boronic acid (R)] is known as the Corey-Bakshi-Shibata reagent. Numerous examples... 

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-... 

Corey-Bakshi-Shibata reduction Enantioselective reduction of ketones with BH3 using oxazaborolidines as catalysts. 100... 

The Corey-Bakshi-Shibata reduction (CBS reduction) is a highly enantioselective method for arylketones, diaryl ketones, and dialkylketones. In addition, cyclic a,p-unsaturated ketones, acyclic a,p-unsaturated ketones, and a,p-ynones are reduced enantioselectively in a 1,2-fashion. The high enantioselective nature of this reduction relies on the chiral oxazaborolidine catalyst, shown in the reaction scheme, in the presence of borane or a dialkylborane. Reviews (a) Singh, V. K. Synthesis 1992, 605-617. (b) Deloux, L. Srebnik M. Chem. Rev. 1993,93,163-1. (c) Corey, E. J. Helal, C. J. Angew. Chem. Int. Ed. 1998, 37. 1986-2012. 

The Corey-Bakshi-Shibata (CBS) reduction1 employs the use of borane in conjunction with a chiral oxazaborolidine catalyst to conduct enantioselective reductions of ketones. 

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