The Formation and the Mass Spectra of Adducts from the Reaction of Some α - Substituted Vinylcyclopropanes with Benzyne

The reactions of benzyne, generated by the thermolysis of benzenediazonium-2-carboxylate, with 1,1-dicyclo- propylethylene (1), 2-cyclopropylpropene (6), and α -cyclopropylstyrene (11) are herein described. Benzyne shows no tendency to add across the vinylcyclopropane systems in 1, 6, and 11. With substrates containing proper allylic hydrogens, the “ene” reaction (6 → 7 → 10) exceeds that of [2 + 2] cycloaddition (6 → 9). When the allylic hydrogens are part of a cyclopropane ring, the [2 + 2] cycloaddition (1 → 2) or the Diels-Alder reaction (11→13 → 14) prevail. In the case of 1, the 1 → 2 conversion was accompanied by a novel “ene” reaction involving the cyclopropane methine hydrogen (1 → 3). The intermediacy of a diradical species of structure 17 was invoked to explain the 1 → 2 and 1 → 3 conversions. Small amounts of 3-cyclopropyl-3-phenyl- isochroman-l-one (12) were isolated from the reaction of 11 with benzyne. Peaks corresponding to ions M — CH₃ and M — C₂H₅, and/or M — C₂H₄, M — C₃H₅, M — C₃H₇, and M — C₄H₈, are prominent in the mass spectra of most of the adducts described herein. The appearance of M — CH3 and M — C₂H₅ ions implies rearrangement of the cyclopropyl group followed by hydrogen migration prior to fragmentation. Ionized 3 and 10 are featured by the loss of either cyclopropyl or benzyl radical to yield the methylenecyclopropane (21 and 30) and phenylvinylcyclopropane (33) cations, respectively.