Chiral Zirconium Catalysts Using Multidentate BINOL Derivatives for Catalytic Enantioselective Mannich-Type Reactions; Ligand Optimization and Approaches to Elucidation of the Catalyst Structure

Catalytic enantioselective Mannich-type reactions of silicon enolates with aldimines were
investigated using chiral zirconium catalysts prepared from Zr(OtBu)4, N-methylimidazole, and newly
designed multidentate BINOL derivatives. These new multidentate BINOL ligands were designed on the
basis of an assumed transition state structure of a chiral zirconium catalyst derived from two molecules of
(R)-6,6‘-Br2−BINOL. Not only tetradentate BINOL 4 but also tridentate BINOL derivatives were found to
be effective, and high enantioselectivities were attained. In a structural study of the most effective zirconium
complex prepared from tridentate ligand 6e, several NMR experiments and DFT calculations were carried
out. Consequently, the structure of an active catalyst and plausible mechanism of asymmetric induction
were elucidated.