Catalytic asymmetric deprotonation of a phosphine borane: comparison of two-ligand and one-ligand catalysis

Abstract The catalytic asymmetric deprotonation of tert -butyldimethylphosphine borane using s -BuLi or n -BuLi and sub-stoichiometric amounts of (−)-sparteine under one-ligand and two-ligand manifolds has been investigated. Using s -BuLi, slightly higher enantioselectivity was obtained using two-ligand catalysis (use of sub-stoichiometric (−)-sparteine in the presence of a stoichiometric amount of a second achiral ligand) compared to one-ligand catalysis (use of sub-stoichiometric (−)-sparteine only). With n -BuLi, two-ligand catalysis using LiDMAE (DMAE = dimethylaminoethanol) as the stoichiometric ligand was the only method for obtaining good yield and enantioselectivity. In this case, one-ligand catalysis failed as the (−)-sparteine was not turned over.