Fate of the intermediate diradicals in the caldera:: Stereochemistry of thermal stereomutations, (2+2) cycloreversions, and (2+4) ring-enlargements of cis- and trans-1-cyano-2-(E and Z)-propenyl-cis-3,4-dideuteriocyclobutanes

This paper addresses the decades-old problem of gaining a measure of intellectual control over the fate of the diradical intermediate in not-obviously-concerted thermal rearrangements. It focuses mainly on the stereochemistry of the thermal rearrangement of cis- and trans-1-cyano-2-trans-propenylcyclobutane to the related ring-enlarged products, 4-cyano-3-methylcyclohexenes. The complete stereochemical profile is revealed by the incorporation of a pair of cis deuterons to serve as a stereochemical lighthouse. The striking result (besides providing a further example of the inapplicability of the orbital symmetry rules of Woodward and Hoffmann to not-obviously-concerted reactions) is the predominance of the same stereoisomer regardless whether starting from the cis or trans educt. This preference is rationalized by a simple conceptual scheme based on two premises of the behavior of the diradical as intermediate: removal of the diradical from the caldera of rotationally labile conformations occurs whenever the two radical centers come within bonding distance in an appropriate orientation of orbitals; relative internal rotational rates are in the order, cyanomethyl faster than methallyl, faster than internal rotation about the backbone.