Fate of Diradicals in the Caldera:  Stereochemistry of Thermal Stereomutation and Ring Enlargement in cis- and trans-1-Cyano-2(E)-propenylcyclopropanes

This study of thermally induced stereomutation and ring enlargement in both (−)-trans-1-cyano-2(E)-propenylcyclopropane [(−)-trans-1] and (+)-cis-1-cyano-2(E)-propenylcyclopropane [(+)-cis-1] to cyclopentenes definitively contraindicates the usefulness of Woodward−Hoffmann rules of orbital symmetry as a theoretical basis for predicting the stereochemistry of the products. From both diastereomers, the same (+)-trans-4-cyano-3-methylcyclopentene [(+)-trans-2] is the major product among the four diastereomeric products, “allowed” and formally the result of a single internal rotation of the cyano-bearing carbon atom from (−)-trans-1, “forbidden” and the result of zero internal rotations from (+)-cis-1. Stereomutation and ring enlargement are discussed in detail in terms of rotational propensity, thermodynamic preference, and the possible role of diradicals in transit and diradicals as intermediates in a caldera.