CBS catalyst
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The CBS catalyst or Corey-Bakshi-Shibata catalyst is an asymmetric catalyst derived from proline. It finds many uses in organic reactions such as the CBS reduction, Diels-Alder reactions and [3+2] cycloadditions. Proline is a chiral and natural occurring compound and readily and cheaply available. It transfers its stereocenter to the catalyst which in turn is able to drive an organic reaction enantioselectively to one of several possible enantiomers. This selectivity is due to steric strain in the transition state that develops for one enantiomer but not for the other.
Organic synthesis
The general outline for the organic synthesis of a CBS catalyst is shown below. The first leg of the reaction sequence starts from the azeotropic dehydration of a boric acid (1) such as one based on toluene to a boroxine (2). This boroxine reacts with the proline derivative (3b) to the basic oxazaborolidine CBS catalyst (4). Proline derivative 3b is prepared in a separate leg from a Grignard reaction with Grignard reagent 3c and proline ester 3b. A lewis acid superacid salt (6) can be obtained with the aid of triflic acid (5). Many other such catalysts exist with different derivatives of these reactants.
References
- E. J. Corey, S. Shibata, R. K. Bakshi, J. Org, Chem., 1988, 53, 2861-2863
- Asymmetric Diels-Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine E. J. Corey, Takanori Shibata, and Thomas W. Lee J. Am. Chem. Soc., 124 (15), 3808 -3809, 2002 [Abstract]
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