Structural studies of the chiral lithium amides [{PhC(H)Me}(2)NLi] and [PhCH2{PhC(H)Me}NLi center dot THF] derived from alpha-methylbenzylamine

Reaction of (R,R')- or (S,S')-bis(1-phenylethyl)amine with (BuLi)-Bu-n in h exane solution gave the chiral amide [{PhC(H)Me}(2)NLi] 1. Complex 1 crysta llises with approximate D-3 symmetry as a ring trimer (1(3)) from hexane so lution, as determined by X-ray crystallography. In direct contrast to the c rystal structure of the related compound dibenzylamidolithium, [{(PhCH2)(2) NLi}(3)], no significant agostic Li ... C(H) contacts are present in 1(3). Solution H-1 and Li-7 NMR spectra of 1 in d(8)-toluene show the presence of two distinct aggregated species which have been assigned as a trimer and a monomer. The complex [PhCH2{PhC(H)Me}NLi . THF] 2 . THF, was prepared by r eaction of (BuLi)-Bu-n with (R)-N-benzyl-alpha-methylbenzylamine in a hexan e-THF solution and has been determined by X-ray crystallography to adopt a dimeric structure (2(2). 2THF) in the solid state with C-1 symmetry. As in 1(3), no short Li ... C(H) contacts are present in 2(2). 2THF. The lack of Li ... C(H) interactions in both 1(3) and 2(2). 2THF suggests that the rota meric conformations adopted for the benzyl groups in the complexes are gove rned mainly by steric effects. Using ab initio molecular orbital calculatio ns (HF/6-31G*), the minimum energy structure for unsolvated monomeric 1(1) was determined to be a C-2 symmetric molecule, I, where the faces of both p henyl groups are directed towards the metal, maximising the Li-aromatic pi interactions. The related C-2 symmetric molecule with both methyl groups di rected towards the metal is 8.68 kcal mol(-1) less stable than I. Therefore , in the absence of aggregation and external solvation, significant stabili sation is achieved through Li ... C(H) benzyl interactions. The energy barr ier to rotation for one benzyl sidearm for geometry I is 4.76 kcal mol(-1), representing a significant lifetime for this conformer.