Ligand Anatomy: Probing Remote Substituent Effects in Asymmetric Catalysis through NMR and Kinetic Analysis

A series of structurally related -amino alcohols only differing in the steric bulk of a remote alkoxy group exhibit striking differences in kinetic behavior when used as ligands in the asymmetric diethylzinc addition to benzaldehyde (R = Trityl, much more active). A combination of NMR titration studies and kinetic analysis allows the quantitative decomposition of the remote substituent effect into a lower dimerization constant of the active species and a much faster ethyl addition step.