Chiral Brønsted Acid Catalyzed Enantioselective Mannich-Type Reaction

Mannich-type reaction of ketene silyl acetals with aldimines proceeded catalytically by means
of a phosphoric acid diester, derived from (R)-BINOL, as a chiral Brønsted acid to afford β-amino esters
with good diastereoselectivity in favor of the syn isomer and high enantioselectivity (up to 96% ee). The
highest enantioselectivity was achieved by the phosphoric acid diester bearing 4-nitrophenyl groups on
the 3,3‘-positions of BINOL. The N-2-hydroxyphenyl group of aldimine was found to be essential for the
present Mannich-type reaction. In combination with these experimental investigations, two possible
monocoordination and dicoordination pathways were explored using density functional theory calculations
(BHandHLYP/6-31G*). The present reaction proceeds via a dicoordination pathway through the zwitterionic
and nine-membered cyclic transition state (TS) consisting of the aldimine and the phosphoric acid. The
re-facial selectivity was also well-rationalized theoretically. The nine-membered cyclic structure and aromatic
stacking interaction between the 4-nitrophenyl group and N-aryl group would fix the geometry of aldimine
on the transition state, and the si-facial attacking TS is less favored by the steric hindrance of the 3,3‘-aryl
substituents.