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

Abstract

Reaction of (R,R′)- or (S,S′)-bis(1-phenylethyl)amine with nBuLi in hexane solution gave the chiral amide [{PhC(H)Me}2NLi] 1. Complex 1 crystallises with approximate D3 symmetry as a ring trimer (13) from hexane solution, as determined by X-ray crystallography. In direct contrast to the crystal structure of the related compound dibenzylamidolithium, [{(PhCH2)2NLi}3], no significant agostic Li· · ·C(H) contacts are present in 13. Solution 1H and 7Li NMR spectra of 1 in d8-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 reaction of nBuLi with (R)-N-benzyl-α-methylbenzylamine in a hexane–THF solution and has been determined by X-ray crystallography to adopt a dimeric structure (22·2THF) in the solid state with C1 symmetry. As in 13, no short Li· · ·C(H) contacts are present in 22·2THF. The lack of Li· · ·C(H) interactions in both 13 and 22·2THF suggests that the rotameric conformations adopted for the benzyl groups in the complexes are governed mainly by steric effects. Using ab initio molecular orbital calculations (HF/6-31G*), the minimum energy structure for unsolvated monomeric 11 was determined to be a C2 symmetric molecule, I, where the faces of both phenyl groups are directed towards the metal, maximising the Li–aromatic π interactions. The related C2 symmetric molecule with both methyl groups directed towards the metal is 8.68 kcal mol–1 less stable than I. Therefore, in the absence of aggregation and external solvation, significant stabilisation is achieved through Li· · ·C(H) benzyl interactions. The energy barrier to rotation for one benzyl sidearm for geometry I is 4.76 kcal mol–1, representing a significant lifetime for this conformer.