A Ranking of Diylophilic Reactivities of Olefins toward the Singlet and Triplet Forms of the Trimethylenemethane Biradical, 2-Isopropylidenecyclopenta-1,3-diyl

Article abstract of DOI:10.1021/ja00527a037

The relative rates of cycloaddition of olefins with the trimethylenemethane (TMM) derivative 2-isopropylidenecyclopenta-1,3-diyl (1) can be determined by direct competition experiments.The gross relative activities in turn can be dissected into singlet and triplet relative reactivities by two techniques.The first involves the determination of the competition ratio for entrapment of the singlet TMM by a given olefin vs. intersystem crossing (isc) to the triplet, combined with the assumption that the rate of isc is independent of the olefin.The second uses the concentration of a "marker" product from the limiting singlet product distribution (obtained from oxygen-saturated runs) and the limiting spin-equilibrated (mostly triplet) product distribution as a guide to the fraction of singlet-derived and triplet-derived products from a given olefin under the conditions of competition.The relative reactivities toward the singlet follow: maleic anhydride, 235; maleonitrile, 180; fumaronitrile, 160; dimethyl fumarate, 67; acrylonitrile, 4.5; methyl acrylate, 0.9; dimethyl maleate, 1.0.The relative triplet reactivities for the last four olefins are respectively 59, 35, 12, and (1.0).The results are consistent with the idea of a concerted cycloaddition of the singlet and a nonconcerted one of the triplet.