New chiral sulfides and disulfides were synthesized from readily available and inexpensive cysteine by straightforward methods in order to elucidate the relative importance of the various donor atoms (N, O, S) available in free or alkylated form resulting in covalent or dative bonds to the metal, respectively. Their application in the addition of diethylzinc to aldehydes provides secondary alcohols with up to 99% ee, and S-configuration, when catalytic amounts of disulfide ligands with the ability to form an S-Zn bond were used. In contrast to this, benzyl alcohols with the opposite absolute configuration R could be achieved, albeit with decreased yield and enantioselectivity, by the use of alkylated sulfide ligands.
A new functionalized, chiral disulfide derived from l-cysteine: (R,R)-bis[(3-benzyloxazolan-4-yl)-methane] disulfide as a catalyst in the diethylzinc addition to aldehydes
作者:Antonio L Braga、Helmoz R Appelt、Paulo H Schneider、Claudio C Silveira、Ludger A Wessjohann
DOI:10.1016/s0957-4166(99)00145-7
日期:1999.5
A new, easily accessible, chiral disulfide 3 was prepared from L-cysteine in a short synthetic sequence (Scheme 1) and applied successfully as a highly efficient catalyst for the enantioselective addition of diethyl zinc to aromatic and aliphatic aldehydes to afford the product alcohols in up to more than 99% ee. In contrast to the more common amino alcohols used in similar reactions, catalyst 3 does not have a protic hydrogen in the form of a free hydroxy group. (C) 1999 Elsevier Science Ltd. All rights reserved.