Structural studies of the chiral lithium amides ({PhC(H)Me}_2NLi) and [PhCH_2{PhC(H)Me}NLi centre dot THF] derived from #alpha#-methylbenzylamine

摘要

Reaction of (R, R')- or (S, S')-bis(1-phenylthyl) amine with ~nBuLi in hexane solution gave the chiral amide [{PhC(H)Me}_2NLi] 1. Complex 1 crystallises with approximate D_3 symmetry as a ring trimer (1_3) from hexane solution, as determined by X-ray crystallography. In direct contrast to the crystal structure of the related compound dibenzylamidolithium, [{(PhCH_2)_2NLi}_3], no significant agostic Li...C(H) contacts are present in 1_3. Solution ~1H and ~7Li 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 [PhCH_2{PhC(H)Me}NLi centre dot THF] 2 centre dot THF, was prepared by reaction of ~nBuLi with (R)-N-benzyl-#alpha#-methylbenzylamine in a hexane-THF solution and has been determined by X-ray crystallography to adopt a dimeric structure (2_2 centre dot 2THF) in the solid state with C_1 symmetry. As in 1_3, no short Li...C(H) contacts are present in 2_2 centre dot 2THF. The lack of Li...C(H) interactions in both 1_3 and 2_2 centre dot 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 1_1 was determined to be a C_2 symmetric molecule, I, where the faces of both phenyl groups are directed towards the metal, maximising the Li-aromatic #pi# interactions. The related C_2 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.