Thermal rearrangements of di- and triphenyl substituted benzocyclobutenes and corresponding o-quinodimethanes

7,8-Dimethoxy-7,8-diphenyl- (Ic), 7,8-dimethyl-7,8-diphenyl- (Id), 7-methox y-7,8,8-triphenyl- (le), 7-methyl-7,8,8-triphenyl- (If), 7-isocyano-;7,8,8- triphenyl- (1g), Emd 7,7,8-triphenylbenzocyclobutene (Ih) are amenable to a variety of thermal rearrangements following initial electrocyclic ring-ope ning to the corresponding 7,8-diphenyl- (2c,d) and 7,8,8-triphenyl-o-quinod imethanes (2e-h). meso-le was found to undergo a facile meso/rac isomerizat ion at room temperature, indicating that other processes such as a symmetry -forbidden disrotatory ring-opening or a stepwise reaction compete with the symmetry-allowed conrotatory process. An estimate of the energy profile of the 1c/2c reaction system was made by kinetic simulation in combination wi th oxygen trapping of the intermediate o-quinodimethanes (2c) and semiempir ical PM3 calculations, and revealed that the barrier for the symmetry-forbi dden pathway is merely about 4 kJmol(-1) higher than that for the symmetry- allowed one. o-Quinodimethanes 2c, 2g, 2e, and 2h underwent further electro cyclic hexatriene-cyclohexadiene ring-closure to give 4a,10-dihydroanthrace ne derivatives at temperatures between 20 and 80 degrees C. The 4a,10-dihyd roanthracenes were further transformed to 9,10-disubstituted anthracenes by elimination of methanol or HCN, as well as to 9,10-substituted 9,10-dihydr oanthracene derivatives. ESR and ENDOR spectroscopic detection of related 9 -anthryl radicals lends support to the view that 9,10-dihydroanthracene pro ducts are formed by a homolytic hydrogen-transfer reaction (retrodisproport ionation). By Way of contrast, the aforementioned transformations play only a minor role in the case of methyl-substituted benzocyclobutenes Id, If as here they are overruled by faster 1,5-H shift reactions of the correspondi ng o-quinodimethanes 2d, 2f, leading to styrene derivatives.