Synthesis 2005(4): 588-594  
DOI: 10.1055/s-2005-861801
PAPER
© Georg Thieme Verlag Stuttgart · New York

A New Cysteine-Derived Ligand as Catalyst for the Addition of Diethylzinc to Aldehydes: The Importance of a ‘Free’ Sulfide Site for Enantioselectivity

Antonio L. Braga*a, Elenilson F. Alvesa, Claudio C. Silveiraa, Gilson Zenia, Helmoz R. Appeltb, Ludger A. Wessjohann*c
a Departamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS Brazil
Fax: +55(55)2208031; e-Mail: albraga@quimica.ufsm.br;
b Centro Universitário Franciscano, Rua dos Andradas, 1614 - Centro, 97010-032, Santa Maria, RS Brazil
c Leibniz Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany
Further Information

Publication History

Received 19 June 2004
Publication Date:
09 February 2005 (online)

Abstract

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.

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We thank a referee for pointing out the likely possibility of a transition state with 6-membered S/O-zinc-coordination to ligand 2. This explanation is based on the importance of a free sulfide site and its strength for primary binding. Would thioether-binding be sufficiently stable as sole first coordination point, ligands 5 and 6 should give stereoinduction with opposing signs of rotation. This was not observed.