| In the course of studying the flash vacuum pyrolysis (FVP) of tetralin, workers in the Trahanovsky group have observed evidence of the secondary pyrolysis of o-allyltoluene. Further studies on the FVP of o-allyltoluene yielded evidence consistent with a rearrangement involving diradicals formed by an uncommon thermally induced intramolecular hydrogen-atom transfer. To investigate the scope of this rearrangement, we have studied the FVP's of several related alkylaryl olefins including o-allyltoluene-α,α,α- d3, (E/Z)-o-(1-propenyl)toluene, o-(1-methylallyl)toluene, o-allylethylbenzene, o-allylcumene, o-(3-butenyl)toluene, o-(4-butenyl)toluene, and 2-methylstyrene. Parallel studies on the phenol and aniline analogs of o-allyltoluene, were also performed. Additional studies on several cyclic and heterocyclic analogs of o-allyholuene, including 3-(o-tolyl)propene oxide, o-cyclopropylmethyltoluene, and o-cyclohexyltoluene were also performed.; Generally, FVP's of o-allyholuene, o-allyltoluene-α,α,α- d3, o-(1-methylallyl)toluene, o-allylethylbenzene, o-allylcumene, o-allylphenol, o-allylaniline, o-(ethenyloxy)toluene, and o-(1-methylallyl)phenol all yield products whose formation is consistent with the existence of diradical intermediates produced by intramolecular hydrogen-atom transfer. The main reaction paths of the diradicals thus generated appear to be either coupling to form the respective five-membered carbocyclic or heterocyclic fused-ring systems or intramolecular disproportionation to form either starting material or the respective ortho substituted 1-propenylbenzene. No conclusive evidence was observed for the thermally induced intramolecular hydrogen-atom transfer in the epoxide, cyclopropyl, and cyclohexyl systems studied. However, some of the results showed unexpected selectivity for isomerization products, such as the selective isomerization of epoxides to aldehydes and the importance of the methyl position in the cyclohexyl systems. |
