DFT study of the mechanism and origin of enantioselectivity in chiral BINOL-phosphoric acid catalyzed transfer hydrogenation of ketimine and α-imino ester using benzothiazoline.
J Org Chem
; 78(8): 3731-6, 2013 Apr 19.
Article
in En
| MEDLINE
| ID: mdl-23521654
ABSTRACT
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.
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Main subject:
Phosphoric Acids
/
Biphenyl Compounds
/
Benzothiazoles
/
Imines
/
Nitriles
Language:
En
Journal:
J Org Chem
Year:
2013
Document type:
Article
Affiliation country:
Japan