Rh(I)-Catalyzed Asymmetric Hydrosilylation and Hydroboration/Oxidation Reactions Using Berens Ligand
作者:Carolina S. Marques、Anthony J. Burke
DOI:10.1080/00397910802323106
日期:2008.11.3
ligand 2 was used in a number of Rh(I)-catalyzed asymmetrichydrosilylations of acetophenones under standard conditions, affording the corresponding 1-arylalcohols in ees up to 65%. Some novel Rhcatalysts were generated in situ from the neutral precatalyst [Rh(µ-Cl)(COD)]2 and screened in the catalytic asymmetric hydroboration/oxidation of styrenes, gave enantioselectivities of up to 62%.
Electrophilic amination of catecholboronate esters formed in the asymmetric hydroboration of vinylarenes
作者:Frances I. Knight、John M. Brown、Dario Lazzari、Alfredo Ricci、A. John Blacker
DOI:10.1016/s0040-4020(97)00722-9
日期:1997.8
(S)-(4-Methoxyphenyl)-ethyl-1,3,2-benzodioxaborole, (S)-1-(4-chlorophenyl)ethyl-1,3,2-benzodioxaborole and (S)-1-indanyl-1,3,2-benzodioxaborole. intermediates in the catalytic asymmetric hydroboration of 4-chloro-and 4-methoxystyrene, were isolated as pure oils in 75%, 84% and 49% yield respectively. For the first example, amination with N-chloromagnesio-N-methyl-O-trimethylsilhydroxylamine gave a mixture of (S)-1-(4-methoxyphenyl)-N-methylethylamine in 33% yield, 88% e.e. and (S)-1-(4-methoxyphenyl) ethanol in 31% yield, 86% e.e.. Related results were obtained in the other cases, and the steps of catalytic hydroboration and amination could be combined in a single sequence without isolation of the intermediate. Numerous variants were carried out in the amination procedure with only marginal improvements in chemoselectivity. An investigation of the mechanism was carried out using low temperature heteronuclear NMR on C-13-1-(S)-1-(4-chlorophenyl)ethyl-1,3,2-benzodioxaborole. The dual pathway is a result of an irreversible and unselective initial step. (C) 1997 Elsevier Science Ltd.
Metal promoted asymmetry in the 1,2-diboroethylarene synthesis: diboration versus dihydroboration
作者:Jesús Ramı́rez、Anna M. Segarra、Elena Fernández
DOI:10.1016/j.tetasy.2005.02.016
日期:2005.4
Metal catalysed addition of diboranes to vinylarenes produces the desired 1,2-bis(boronate)ester and mono(boronate)esters as by-products. Their relative rate is a sensitive function between the nature of the catalytic system and the electronic effects of the substrate, that influences the mechanistic steps of the catalytic cycle. However, asymmetry is only induced as moderate enantiomeric excess values, providing an enantioface differentiation, between the bis- and mono(boronate)esters. Alternatively, the method based on the catalytic asymmetric dihydroboration/oxidation of alkynes as diphenyl acetylene can provide 1,2-diphenyl-1,2-ethanediol (hydrobenzoin) with a selectivity of 68% mainly as the erythro isomer. (c) 2005 Elsevier Ltd. All rights reserved.