Generation of 2,3-Benzobicyclo<2.2.1)hept-2-ene-5,7-diyl Diradicals via Denitrogenation of Appropriate Azoalkanes and Their Mechanistic Implications in the Di-?-methane Rearrangement of Benzonorbornadienes

Article abstract of DOI:10.1021/jo00334a005

The azoalkanes 16a-c, prepared from the bicycloalkadienes 5a-c via dipolar cycloaddition with 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) and subsequent oxidative hydrolysis, were submitted to thermal and direct and ketone-sensitized photochemical denitrogenation, with the intention of entering the diradical manifold postulated for the di-?-methane rearrangement.It is shown that in each case denitrogenation leads to the expected 2,3-benzobicyclo<2.2.1>hept-2-ene-5,7-diyl diradical 7.Diradical 7 mostly cyclizes into the corresponding tricycloalkene 8, and only a very small fraction (<5percent) suffers retro-di-?-methane rearrangement into the bicycloalkadiene 5 via the cyclopropyl dicarbinyl radical 6.Stabilizing substituents such as benzhydryl and spirocyclopropyl groups in a position α to the rearranging radical site in 7 discourage retro-di-?-methane reaction.