Formal Kinetic Proof of Reversible Unimolecular Transformation to a Biradical as an Obligatory First Step in the Mechanism of Cycloaddition of 5-Isopropylidenebycyclopentane to Olefins

Article abstract of DOI:10.1021/ja00372a018

A study of the kinetics of the cycloadditions of 5-isopropylidenebicyclo<2.1.0>pentane (2b) with acrylonitrile and maleic anhydride shows the operation of a two-step mechanism: unimolecular ring opening of the hydrocarbon to a singlet biradical (1b-S) followed by intermolecular capture of the singlet.The regiospecifity of the cycloaddition thus cannot be explained by a bimolecular hydrocarbon + olefin reaction <_s2_? + _s2_? + _s2_?> but is most simply interpreted as an orbital simmetry effect dependent on the hierarchy of nominally NBMOs of the diyl.The kinetic data permit the assignment of a barrier of about 2.3 kcal/mol to the cyclization of the singlet biradical.