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

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.