Asymmetric oxazaborolidine-catalyzed reduction of prochiral ketones with N-tert-butyl-N-trimethylsilylamine–borane

Recent development and improvement for boron hydride-based catalytic asymmetric reduction of unsymmetrical ketones

“…(R)-1b-catalyzed reduction of aryl methyl ketones and pinacolone using this reagent provided excellent enantioselectivities (93-97% ee). 9 A fluorous borane methyl sulfide prepared by complexation of borane gas on 2-(perfluorooctyl)ethyl methyl sulfide was successfully used for the CBS reduction of acetophenone as reductant. 10 In contrast to BMS, this hydride is found to be an environmental friendly reagent since it is not only odorless and non-pyrophoric solid, but also can be recycled by fluorous extraction (Scheme 4).…”

A convenient enantioselective CBS-reduction of arylketones in flow-microreactor systems

“…Modified protocols have been introduced which, however, use genotoxic alky halides 12 or large amounts of N,N-diethylaniline (DEAN) 13 and N-t-Bu-N-trimethylsilylamine as a stabilizer for the borane. 14 Recently, Sanderson and coworkers developed a multi-step flow synthesis of fluoxetine where the CBS asymmetric reduction of an arylketone was the stereo-determining step. Under optimised conditions, the reduction was accomplished in 10 minutes with 3 equivalents of borane-DEAN, 30% of the catalyst, CH 2 Cl 2 /toluene as the solvent and −7 °C.…”

Influences of Electronic Effects and Anions on the Enantioselectivity in the Oxazaborolidine-Catalyzed Asymmetric Borane Reduction of Ketones

“…Numerous new efficient oxazaborolidines as catalysts have been reported and a plethora of applications have appeared until now. In comparison with the numerous attempts to search for new catalysts and to improve the enantioselectivity, several papers have concentrated on the mechanistic investigation of the catalytic asymmetric reduction reaction, , and some papers have paid attention to the factors which affect the enantioselectivity in the asymmetric reduction, such as the structure, ,, stability 3a,6 (including dimerization) and the amount of the catalyst, 3a,6a, the borane source 8 and the borane amount, 3a,7c the order and rate of the addition of a ketone or borane into the reductive system, 2d,7c the reduction temperature, 7c, solvent, 6a,7c,8c additive, 9g, secondary reduction, 10a, and stabilizer in borane, etc. Although a few papers have considered the influence of the electronic effects of ketones and catalysts on the enantioselectivity, all of the results indicated that no obvious influence has been observed in the asymmetric reduction 5a,6a,13 except for the observation of Corey et al13c Very recently we investigated the temperature-dependent enantioselectivity of the asymmetric reduction and found that the noncatalytic reduction is an important factor for the enantioselectivity in asymmetric reductions 7c.…”

“…The factors governing enantioselectivity in the catalytic asymmetric reaction are usually interpreted in steric terms, affected by the temperature and solvent, etc. 6a,7c,8c, Electronic effects have been reported to control the enantioselectivity recently. In reported studies on the electronic effects on the enantioselectivity, , the underlying reasons are poorly understood in most of the cases.…”