A Research on the Chemical Behavior of Cation-Radicals of Olefins

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In a course of studies on the chemical behavior of cation-radical intermediates, the following results were obtained.[1] The treatment of 1,1-dicyclopropylethylene (1) with a small amount of tris-(4-bromophenyl)amminium salt produced 1,1-dicyclopropyl-2-(1-cyclopropylvinyl)-cyclopentane. This dimerization appears to be the product of the reaction of 1 <+!・> with 1 at an early stage of the investigation, but later it was found to be the product of proton-catalyzed dimerization. In fact, cyano-aromatics-sensitized photochemical reaction of 1 was the reaction involving 1 <+!・> , which yielded 1,1,2,2-tetracyclopropylcyclobutane and 1,1,4-tricyclopropyl-1,3-heptadiene.[2] Sensitized-photochemical reaction of 1-cyclopropyl-1,3-butadiene (2) produced more than 5 dimers. The dimers were mainly [2+2]cycloadducts whereas a [4+2] cycloadduct remained to be a minor component (27%). This finding was contradictory to the known fact that dienes usually produced mainly [4+2]cycloadducts when the cation-radical initiated reaction was employed. Thus, the present results appear to indicate that commonlu accepted interpretation of the cycloaddition mechanism should be rescrutinized.[3] Electrochemical oxidation of 1 produced the products in which the cyclopropyl group was retained.[4] Some methylcyclopropanes activated by a spiro-fused fluorene group (3) were found to be dehydrogenated by DDQ to afford 1,3-dienes. Since this unprecedented transformation was not brought about by chloranil, but photostimulation was effective, and the dehydrogenation took place only in the spiro-activated substrates, the reaction will most probably involve a cation-radical of the spiro-activated cyclopropane.All these results provide important basic knowledges on the chemical behavior of the cation-radicals. The present results also suggest that the cation-radical should be considered as the fifth important reactive intermediate in the field of organic reaction mechanism.