Why do fluorines rotate “outside” in conrotatory cyclobutene ring opening?

Abstract

trans-3,4-DiZ-cyclobutene can undergo two different modes of thermal conrotation. In one, the Z groups move “inside” and, in the other, “outside” with respect to the breaking σ bond. When Z is a π donor which is more electronegative than hydrogen, MOVB theory predicts that sigma bonding as dictated by symmetry creates a preference for “outside” Z rotation. In contrast, stereoselection (beyond that which is caused by conventional “steric effects”) is predicted to vanish in the case of disrotatory ring opening of cis-5,6-diZ-1,3-cyclohexadiene. This rule is obtained by assuming that the terminal carbons move in opposite directions on the way to the transition state. A different rule is obtained when a coplanar transition state geometry is assumed. Conrotation in 4 N-electron systems is predicted to have Z rotating inside, while disrotation in 4 N + 2 systems will have Z rotate outside. Such a situation is enforced in cyclopropyl anion and cation, respectively. As in the case of “negative hyperconjugation”, we conclude that the Z lone pairs play no vital role, in contrast to popular opinion. The explanation of “outside” rotation in the cyclobutene and the emerging general predictions are at odds with interpretations and implicit predictions based on “aromaticity” considerations.