An MCSCF study of the effect of substituents and solvent on the [2 + 2]cycloaddition of tert-butylcyanoketene to phenylethene

Abstract

The effect of solvent and substituents on the [2 + 2]cycloaddition of tert-butylcyanoketene to phenylethene (styrene) was studied using a CAS-MCSCF method with a four orbital four electron active space in an STO-3G basis. The most favoured reaction path proceeds via a biradical intermediate in which the steric interactions between the three substituents are minimised, an arrangement which nevertheless results in a final cyclobutanone product in which the Bu^t and the Ph groups adopt the less stable cis rather than the more stable trans stereochemistry. A continuum solvation model based on a multipolar expansion within an ellipsoidal cavity suggests that benzene solvent enhances this preference. The results agree quantitatively with previous semi-empirical results, although the interpretation differs.