Isobenzofulvenes. I. The in situ preparation of 8,8-Dimethyl- and 8,8-Diphenylbenzo[b]fulveues and their pericyclic reactions with 2π-Cycloaddends

Abstract

A new route to the generation of the previously undescribed 8,8- dimethyl- and 8,8-diphenyl-benzo[b]fulvene (isobenzofulvenes)? is reported. This method involved the mild thermal fragmentation of 3,6- di(2?-pyridyl)-15-(α,α-disubstituted methylene)-4,5- diazatetracyclo[6,6,1,02,7,09,14]pentadeca-3,5,9,11,13-pentaene (39), which was prepared from the benzyne adduct of the 7-(α,α-disubstituted methylene)bicyclo[2,2,1]-hepta-2,5-diene (36) and 3,6-di(2?-pyridyl)-s- tetrazine (37). This retroversion reaction is one of order [8π+6π] and, based on the limited evidence available, appears to be a non-concerted process. The isobenzofulvenes were readily isolated, in quantitative yield, as [8π+2π] cycloadducts with a variety of electron deficient olefins (tetraenophiles). Mixtures of exo- and endo-adducts were often obtained and their ratios explained in terms of a combination of dipole-dipole, steric, and orbital interactions operative in the respective transition states. Ozonolysis of these adducts yielded products formally derived from cycloadditions to isoindenone, and this represents an excellent route to these compounds. No spectral evidence for the existence of the monomeric isobenzofulvenes could be obtained, although acid-catalysed decomposition of the dimethyl precursor yielded significant proportions of 2-isopropenylindene, which is logically derived from 8,8-dimethylbenzo[b]fulvene. P.m.r. and mass spectral data are reported for all new compounds and briefly discussed where relevant.