Photoisomerization of Bridgehead Monosubstituted Dibenzobarrelenes and Interesting Thermal Isomerization of Their Photoproducts1

Phototransformations of a few bridgehead monosubstituted dibenzobarrelenes, 4a − d, and thermal isomerizations of their primary photoproducts, 7a and 11a − c, are described. Irradiation of 4a in benzene gave a mixture of regioisomeric products, 7a (8b-isomer) and 11a (4b-isomer), in 20 and 70% yields, respectively. In contrast, irradiation of 4b, under identical conditions, yielded only the corresponding 4b-substituted dibenzosemibullvalene, 11b, in 95% yield. Similarly, irradiation of the nitro- (4c) and chloro- (4d) substituted dibenzobarrelenes gave exclusively the corresponding dibenzosemibullvalenes, 11c (81%) and 11d (81%), respectively. The formation of regioselective products in these systems has been attributed to the relative stabilities of the diradical intermediates involved in these transformations. Interestingly, the dibenzosemibullvalene 7a, on refluxing in xylene, gave the corresponding dibenzopentalenofuran 9a (94%). Thermolysis of 11a and 11c in o-dichlorobenzene and, likewise, of 11b in xylene, yielded 94−97% of the corresponding dibenzopentalenofurans 12a, 12c, and 12b, respectively. Activation energies for the thermal isomerizations of 7a to 9a, 11a to 12a, and 11b to 12b have been found to be 21.57, 25.97, and 17.93 kcal/mol, respectively. The structures of 11a, 11c, 11b, and 12b were established unambiguously through X-ray crystallographic analysis, whereas the structures of the different photoproducts, 7a, 11a − d, 12a − c and 9a, have been arrived at on the basis of spectral data and analytical results.