Synthesis of organozinc enolates of N,N-disubstituted glycine esters. Crystal structure of [EtZnOC(OMe)=C(H)N(t-Bu)Me]4

Abstract

Pure ethylzinc @a-amino ester enolates have been prepared from N, N-disubstituted glycine esters and N-(ethylzinc)diisopropylamine. An X-ray diffraction study of EtZnOC(OMe)@?C(H)N(t-Bu)Me (2b) has shown it to be a tetrameric species, in which the four crystallographically independent zinc enolate units are interconnected via covalent Zn@?O@?Zn bridges. As a result of intramolecular Zn@?N coordination, all the enolate moieties have the Z-configuration. }1{H NMR spectroscopy and molecular weight determinations in the case of 2b indicate that in benzene the tetramer is in equilibrium with a dimeric species. In the dimer the Zn@?N coordination is either weak, giving rise to a rapid on-off movement of the nitrogen, or totally absent.