Researches on the Effect of Spiro-Activation of Small Ring Compounds in Their Ring Cleavage Reactions.

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In continuation of the study on spiro-activation of cyclopropanes, reactions of spiroindans, spiroindenes, spiroheptenes, and spiroheptadienes with TCNE were examined. Although the basic reaction courses were [pi2+pi2] and [pi2+sigma2] cycloaddition as those observed in the reactions of spirofluorene derivatives, varied reactions were observed to occur depending upon substituents on the substrates. One of the most interesting observation was that the reaction of 1, 1-dicyclopropylspiro [2.4] hepta-4, 6-diene with TCNE gave, in addition to the expected Diels-Alder adduct, an imine derivative, which should be produced after extensive rearrangement. Similar transformation was observed to occur also in the reactions of spiroindenes. Evidently, this should be one of the basic reactions to be considered when TONE was allowed to react with vinylcyclopropanes. The other interesting result was that oxidation potential as well as ionization potential of spiroheptanes and spiroheptenes were influenced by the substituents on the cyclopropane ring as expected, but those of spiroheptadienes were not subjected to the substituent effect. The results are of great interest with regard to the structure of five-membered unsaturated cation radicals. In addition to these studies, the stereochemistry of TCNE cycloaddition and the reaction of diphenyl-substituted spirofluorene derivatives were investigated. All results gave important basic knowledge with regard to the spiro-activation of cyclopropanes. Finally, it was found that even 1, 2-dicyclopropylethylene reacted with TCNE in the [pi2+sigma2] manner under certain reaction conditions. In relation to this result, the photochemical reaction of 1, 2-dicyclopropylethylene with chloranil was also examined. The results provided supporting evidences that the reaction involving cyclopropane cleavage might be characteristic to that of cation radical intermediates.