DFT Study on Bifunctional Chiral Brønsted Acid-Catalyzed Asymmetric Hydrophosphonylation of Imines

Asymmetric hydrophosphonylation reaction of aldimines with dialkyl phosphites proceeds catalytically by means of a phosphoric acid diester, derived from (R)-BINOL, as a chiral Brønsted acid to afford alpha-amino phosphonates with good to high enantioselectivities (up to 90% ee). The use of the aldimines derived from cinnamaldehyde derivatives and sterically demanding dialkyl phosphites was essential for achieving high enantioselectivity as well as high yield. To elucidate the reaction mechanism and the origin of the high enantioselectivity, DFT calculation (BHandHLYP/6-31G*) was carried out. The reaction proceeds via the nine-membered zwitterionic transition state (TS) with the chiral phosphoric acid, where aldimine and phosphite could be activated by the Brønsted acidic site and Lewis basic site, respectively. The si-facial attacking TS could be less favored by the steric repulsion of 3,3'-aryl groups on the chiral phosphoric acid with the bulky phosphite. When using the aldimine derived from benzaldehyde, the re-facial attacking TS is destabilized to decrease the enantioselectivity in agreement with the experimental results.