Thermolysis and photolysis of 1-substituted triptycenes. Divergent fragmentation pathways of the triptycyl skeleton

While thermolysis of 1-benzyltriptycene 1a in the gas phase at 600 degrees C/10(-4) mmHg did not result in any appreciable decomposition, similar pyrolysis of compounds 1 bearing various substituents (X) at the benzyl methylene (1b-d) underwent smooth decomposition to give 8-benzylbenz[a]aceanthrylene 2. The reaction is explained in terms of homolysis of the C-X bond followed by triptycyl ring cleavage in the resulting benzyl radical 7. Generation of radical 7 in solution, however, did not result in a similar ring-opening reaction. Irradiation of compounds 1, on the other hand, afforded norcaradienes 3 almost exclusively, obviously asa result of di-pi-methane rearrangement. No products resulting from a carbene species proposed to be involved in the di-pi-methane reaction of triptycenes were detected. Thermolysis of compounds 3 gave not only compound 2 but also benz[a]aceanthrylene 4 and benz[a]indeno[1,2,3-cd]azulene 5 whose compositions were found to be sensitive to both the substituent (X) and the thermolysis temperature. Mechanisms of the reactions are discussed in terms of the relationship between the thermal and photochemical fragmentations.