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 the reaction of the (R)-trimethylsilyl aminoarenethiolate species (R)-2b with ZnCl2 in a 2:1 molar ratio. (R,R)-3b is an efficient catalyst for the addition of dialkylzinc compounds to aliphatic and aromatic aldehydes to give the corresponding secondary alcohols in nearly quantitative yields with optical purities of 69-99% ee under mild reaction conditions. Although excellent selectivities were obtained with this simple ligand containing only one stereogenic (carbon) center, further modifications of the amino substituents were studied. Cyclic N(CH2)(4) or N(CH2)(5) amino-substituted aminoarenethiolate ligands considerably enhanced the reaction rates, resulting in shorter reaction times and higher ee's. The mechanism of these 1,2-addition reactions has the general characteristics as reported by Noyori et al. This conclusion is based 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 process. We present evidence for an interpretation of the binding in the product-forming key intermediate complex in terms of an organozinc cation/anion pair. The possibility that the very efficient transfer of chiral 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 structure of the zinc bis(aminoarenethiolate) (R,R)-Zn(SC6H4C(Me)HNMe2-2)(2) is reported.