Modification of (1R,2S)-1,2-Diphenyl-2-aminoethanol for the Highly Enantioselective, Asymmetric Alkylation of N-Diphenylphosphinoyl Arylimines with Dialkylzinc

Experimental studies on the modification of (1R,2S)‐1,2‐diphenyl‐2‐aminoethanol, which is used to promote the alkylation of N‐diphenylphosphinoyl benzalimine with diethylzinc, revealed that N‐monosubstituted amino alcohols exhibited higher enantioselectivities than their N,N‐disubstituted counterparts and imino alcohols. Application of the optimal chiral ligand 3 c to activate the reaction of N‐diphenylphosphinoyl arylimines with diethylzinc and dibutylzinc resulted in excellent enantiomeric selectivities of up to 98 % ee. The origin of the experimentally observed enantioselectivities was revealed by density functional calculations (B3LYP/6‐31G*) on the transition structures of several model reactions. Enantioselectivities of up to 98 % ee can be achieved when N‐monosubstituted amino alcohols are used to promote the alkylation of N‐diphenylphosphinoyl benzalimine with diethylzinc (see scheme). Theoretical calculations on the transition structures of several model reactions shed light on the origin of the experimentally observed enantioselectivities.