Application of S,N-Chelating Chiral Zinc Bis(aminoarenethiolates) as New Precursor Catalysts in the Enantioselective Addition of Dialkylzincs to Aldehydes

Abstract

A new enantiomerically pure S,N-chelated zinc bis(aminoarenethiolate), ( R,R )-Zn(SC 6 H 4 C(Me)HNMe 2 -2) 2 (( R,R )- 3b ), has been synthesized by the reaction of the ( R )-trimethylsilyl aminoarenethiolate species ( R )- 2b with ZnCl 2 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(CH 2 ) 4 or N(CH 2 ) 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, 1 H and 13 C 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 η 2 bonding of the aldehyde substrate is discussed. The solid-state structure of the zinc bis(aminoarenethiolate) ( R,R )-Zn(SC 6 H 4 C(Me)HNMe 2 -2) 2 is reported.