Mechanisms and stereoselectivities of phosphine-catalyzed domino reaction of α-benzyl allenoate with 5-phenylmethylene thiazolone: a computational investigation

Abstract

A computational examination of the phosphine-catalyzed domino reaction of α-benzyl allenoate with 5-phenylmethylene thiazolone has been carried out at the SMD(toluene)M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level. Various possible reaction pathways have been located and compared. Computations show that the preferred mechanism consists of four reaction steps: (I) the nucleophilic attack of phosphine catalyst to α-benzyl allenoate and generation of dienolate intermediate; (II) the nucleophilic conjugate addition to 5-phenylmethylene thiazolone; (III) the intramolecular cyclization, and (IV) the liberation of catalyst. The first step is the rate-determining step with an overall free energy barrier of 29.4 kcal/mol, and the second step is found to be the stereoselectivity-determining step. The predicted high enantioselectivities are in good agreement with the experimental observations. The present study should be useful for understanding this kind of reaction in the future.