Novel Cycloaddition Reactions of Nonvertical Radical Cations

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In order to gain insight into the structure and reactivity of the above novel radical cations, nitrones (1)-oxaziridines (2), stable phosphorus (3) and sulfur ylides (4), and hydroxyacetophenones (5) were chosen as the substrates and photo and thermal single electron-transfer (SET) reactions were studied. The results are summarized as follows :1. Compound 1 and 2 were interconvertable each other depending on the substituent on the aryl group under photosensitized electron-transfer (PET) conditions.2. Ring-opening of 2 were proceeded in a stereoselective manner under PET conditions. It is also applicable to the convenient preparation of sterically hindered 1.3. Although stable 3 are employed for the synthesis in a very limited scope, SET nature of such ylides are focused to enhance the reactivity. The reaction of fluorene substituted 3 with prenyloxybenzene diazonium salt gave dihydrobenzofuran derivative. The reaction proceeded via the SET-radical couplingionic termination sequence. Additionally, (cyclo)addition reactions of 3 with electron-deficient alkenes and alkynes were studied to aim at enhancing the reactivity by charge-transfer nature. Unusual temperature-dependence of NMR of fluorenylideneethylidene substituted 3 was also studied in comparison with the analogously substituted 4.4. Photoreactivity of 5 which usually is unreactive upon photoirradiation were enhanced in the presence of aluminum halides. In one case, bromination of the neighboring alkyl group occurred in anti-Markownikoff manner. In another, remarkable structural conversion into oxabicycloheptenone occurred under PET conditions. The reaction seemed to be proceeded by the SET and the following addition of oxygen.