AN MCSCF STUDY OF THE EFFECT OF SUBSTITUENTS AND SOLVENT ON THE [2-BUTYLCYANOKETENE TO PHENYLETHENE(2]CYCLOADDITION OF TERT)

The effect of solvent and substituents on the [2 + 2]cycloaddition of tert-butylcyanoketene to phenylethene (styrene) was studied using a CA S-MCSCF method with a four orbital four electron active space in an ST O-3G basis. The most favoured reaction path proceeds via a biradical i ntermediate in which the steric interactions between the three substit uents are minimised, an arrangement which nevertheless results in a fi nal cyclobutanone product in which the Bu(t) and the Ph groups adopt t he 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 prefere nce. The results agree quantitatively with previous semi-empirical res ults, although the interpretation differs.