Thermal Rearrangements of Di‐ and Triphenyl‐Substituted Benzocyclobutenes and Corresponding o ‐Quinodimethanes

7,8‐Dimethoxy‐7,8‐diphenyl‐ ( 1c ), 7,8‐dimethyl‐7,8‐diphenyl‐ ( 1d ), 7‐methoxy‐7,8,8‐triphenyl‐ ( 1e ), 7‐methyl‐7,8,8‐triphenyl‐ ( 1f ), 7‐isocyano‐7,8,8‐triphenyl‐ ( 1g ), and 7,7,8‐triphenylbenzocyclobutene ( 1h ) are amenable to a variety of thermal rearrangements following initial electrocyclic ring‐opening to the corresponding 7,8‐diphenyl‐ ( 2c , d ) and 7,8,8‐triphenyl‐ o‐ quinodimethanes ( 2e–h ). meso ‐ 1c was found to undergo a facile meso/rac isomerization 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 with oxygen trapping of the intermediate o‐ quinodimethanes ( 2c ) and semiempirical PM3 calculations, and revealed that the barrier for the symmetry‐forbidden pathway is merely about 4 kJ·mol –1 higher than that for the symmetry‐allowed one. o‐ Quinodimethanes 2c , 2g , 2e , and 2h underwent further electrocyclic hexatriene‐cyclohexadiene ring‐closure to give 4a,10‐dihydroanthracene derivatives at temperatures between 20 and 80 °C. The 4a,10‐dihydroanthracenes were further transformed to 9,10‐disubstituted anthracenes by elimination of methanol or HCN, as well as to 9,10‐substituted 9,10‐dihydroanthracene derivatives. ESR and ENDOR spectroscopic detection of related 9‐anthryl radicals lends support to the view that 9,10‐dihydroanthracene products are formed by a homolytic hydrogen‐transfer reaction (retrodisproportionation). By way of contrast, the aforementioned transformations play only a minor role in the case of methyl‐substituted benzocyclobutenes 1d , 1f as here they are overruled by faster 1,5‐H shift reactions of the corresponding o‐ quinodimethanes 2d , 2f , leading to styrene derivatives.