Benzyl(trimethylsilyl)amidolithium: Structural studies by ab initio MO and X-ray crystallographic methods, and comparison with the known structures of dibenzyl and bis(trimethylsilyl) analogues

Prompted by its excellent nucleophilic properties in organic synthesis, benzyl(trimethylsilyl)amidolithium, a new commercial lithium amide reagent, has been subjected to a detailed structural study. X-ray crystallographic studies have revealed that in the absence of solvent ligands, it is trimeric with a classical, shield-shaped N3Li3 ring core, the amido substituents of which are organized in a cisoid manner. Solvation by hexamethylphosphoramide causes it to break down to a dimer, having a familiar lozenge-shaped N2Li2 ring core. These structures are compared with previously reported dibenzylamidolithium and bis(trimethylsilyl)amidolithium analogues. Primarily with a view to shedding further light on the role of Li ... benzyl interactions in these structures, ab initio MO geometry optimizations have been carried out on model monomeric fragments. Short Li ... C (benzyl) contacts observed in the crystal structure of benzyl(trimethylsilyl)amidolithium are retained in the theoretical monomer, although their presence appears to be dictated more by steric factors than by electronic deficiencies of the Li-delta+ centre. Copyright (C) 1996 Elsevier Science Lt