Dimerization and Cycloaddition Reactions of a Trimethylenemethane Derivative, 2-Isopropylidenecyclopenta-1,3-diyl. Mechanistic Separation of Triplet and Singlet Reactions

Article abstract of DOI:10.1021/ja00521a044

Pyrolysis of 7-isopropylidene-2,3-diazanorbornene (1) at 60 deg C in solution liberates N2 and 2-isopropylidenecyclopenta-1,3-diyl (2).The diyl (2) dimerizes readily, but, in the presence of an olefinic trapping agent, 2 can be intercepted as either a fused adduct, F, a bicyclo<3.3.0>oct-1-ene, or a bridged adduct, B, a 7-isopropylidenenorbornane.The composition of the adduct mixture is dependent on the initial concentration of the trapping olefin, fused adducts being heavily favored at high concentrations.The concentration dependence arises from two forms of the diyl which are generated sequentially in a cascade mechanism.The first species is a singlet that can add both regiospecifically and stereospecifically, whereas the second is the triplet ground state of the diyl.Quantitative treatment of the dilution effect data for the addition of dimethyl fumarate leads to the conclusion that the rate constants for capture of the singlet and intersystem crossing to the triplet are in the ratio 7.7 M^-1.