[reaction: see text] Photoirradiation of benzene solutions containing 1-diarylvinylidene-2,2,3,3-tetramethylcyclopropanes (2a--d) afforded rearranged products 1,2,3-butatrienes (3a-d) in good to high yields. Photorearrangement from 2,2,3-trimethyl and 2,2- and 2,3-dimethyl derivatives 2e--g also proceeded, but the rates of the rearrangement were lower than those of 2a--d. A singlet mechanism is proposed for this photorearrangement, where alkyl migration occurs from 1,3-biradical intermediates generated via the homolysis of the C1-C2 bond. Generation of diarylvinylidene carbenes from 1,3-biradicals might be competitive with the formation of 3.
1-Alkenyl-2-(2′,2′-diphenylethenylidene)cyclopropanes thermally rearranged to 4-(diphenylethenylidene)cyclopent-1-enes at relatively low temperature (≤373 K). Measurement of activation parameters revealed that the reactivity and mode of the reaction largely depend on structures of starting materials and products.
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