APPLICATION OF S,N-CHELATING CHIRAL ZINC BIS(AMINOARENETHIOLATES) AS NEW PRECURSOR CATALYSTS IN THE ENANTIOSELECTIVE ADDITION OF DIALKYLZINCS TO ALDEHYDES

A new enantiomerically pure S,N-chelated zinc bis(aminoarenethiolate), (R,R)-Zn(SC6H4C(Me)HNMe2-2)(2) ((R,R)-3b), has been synthesized by th e reaction of the (R)-trimethylsilyl aminoarenethiolate species (R)-2b with ZnCl2 in a 2:1 molar ratio. (R,R)-3b is an efficient catalyst fo r the addition of dialkylzinc compounds to aliphatic and aromatic alde hydes to give the corresponding secondary alcohols in nearly quantitat ive yields with optical purities of 69-99% ee under mild reaction cond itions. Although excellent selectivities were obtained with this simpl e ligand containing only one stereogenic (carbon) center, further modi fications of the amino substituents were studied. Cyclic N(CH2)(4) or N(CH2)(5) amino-substituted aminoarenethiolate ligands considerably en hanced the reaction rates, resulting in shorter reaction times and hig her ee's. The mechanism of these 1,2-addition reactions has the genera l characteristics as reported by Noyori et al. This conclusion is base d on the synthesis, isolation, and characterization (X-ray, H-1 and C- 13 NMR) of the enantiopure zinc bis(aminoarenethiolate) and organozinc aminoarenethiolate intermediates and on monitoring of the reaction pr ocess. We present evidence for an interpretation of the binding in the product-forming key intermediate complex in terms of an organozinc ca tion/anion pair. The possibility that the very efficient transfer of c hiral information in this compact complex may be due to a combination of the shortness of the Zn-N coordinate bonds with concomitant eta(2) bonding of the aldehyde substrate is discussed. The solid-state struct ure of the zinc bis(aminoarenethiolate) (R,R)-Zn(SC6H4C(Me)HNMe2-2)(2) is reported.