Racemization versus retention of chiral information during the formation of silicon and tin complexes with chiral Schiff base ligands

Abstract

Chiral Schiff base ligands with O,N,O′-coordination ability have been prepared with amino acid esters from the chiral pool. Unfortunately the chiral information is lost during the formation of complexes with these chiral ligands with silicon tetrachloride. This was demonstrated with the help of two X-ray structures which show different reaction products depending on chosen reaction conditions. The reaction of N-(salicylidene)-d-phenylglycine methylester (1a) with SiCl4 in presence of triethylamine leads to planarization of the amino acid ester and formation of a ketene acetal 2a. The reaction of N-(salicylidene)-l-valinmethylester (1b) with SiCl4 without triethylamine leads to racemization of the amino acid ester and formation of the complex 2b. A mechanism for the formation of both products has been proposed. It is possible to transfer the chiral information present in the ligand system into the complex by using dichlorodimethylstannane for the complex formation. Dimethyltin[N-(salicylidene)-d-phenylglycine methylester] (3a) crystallizes in the chiral space group P212121 with one diasteromer in the asymmetric unit. 1H NMR analysis of 3a with the help of a lanthanide shift reagent proves the presence of only one diasteromer in the bulk material.