Thermal rearrangements of di- and triphenyl-substituted benzocyclobutenes and corresponding o-quinodimethanes.
In: European Journal of Organic Chemistry, (1999) ; Nr. 3, S. 551-563
Zeitschriftenaufsatz / Fach: Chemie
7,8-Dimethoxy-7,8-diphenyl-, 7,8-dimethyl-7,8-diphenyl-, 7-methoxy-7,8,8-triphenyl-, 7-methyl-7,8,8-triphenyl-, 7-isocyano-7,8,8-triphenyl-, and 7,7,8-triphenylbenzocyclobutane (1c-1h, resp.) are amenable to a variety of thermal rearrangements following initial electrocyclic ring-opening to the corresponding 7,8-diphenyl-7,8-methoxy-, 7,8-diphenyl-7,8-methyl-, 7,8,8-triphenyl-7-methoxy-, 7,8,8-triphenyl-7-isocyano-, 7,8,8-triphenyl-o-quinodimethane (2c-h, resp.). Meso-1c (crystal structure data provided) was found to undergo a facile meso/rac isomerization at room temp., indicating that other processes such as a symmetry-forbidden disrotatory ring-opening or a stepwise reaction compete with the symmetry-allowed conrotatory process. An est. 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 calcns., 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 derivs. at temps. between 20 and 80 Deg. 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 derivs. 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 derivs.