Benzothiazoline is an efficient reducing agent for the chiral BINOL-phosphoric acid catalyzed enantioselective transfer hydrogenation of ketimines and α-imino esters to afford the corresponding amines with high enantioselectivities. DFT studies (M05-2X/6-31G*//ONIOM(B3LYP/6-31G*:HF/3-21G)) revealed the reaction mechanism and the origin of the high enantioselectivity in the present BINOL-phosphoric acid catalyzed transfer hydrogenation of ketimines and α-imino esters using benzothiazoline. The reaction mechanism is similar to that reported in the asymmetric transfer hydrogenation of ketimines using Hantzsch ester. Phosphoric acid simultaneously activates ketimine (α-imino ester) and benzothiazoline to form cyclic transition structures. The high enantioselectivity is attributed to the steric interaction between the substituents at the 3,3′-positions of BINOL-phosphoric acid and substrates. In contrast to the C2-symmetrical Hantzsch ester, the readily tunable 2-aryl substituent of unsymmetrical benzothiazoline plays a significant role in the steric interaction, influencing the asymmetric induction. This feature is responsible for the advantage of benzothiazoline over Hantzsch ester.

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 α-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.