ALTERNATIVE ELECTROCYCLIC PATHWAYS PHOTOLYSIS THERMOLYSIS OF DIMETHYLALLENE DIMERS

The thermal and photochemical isomerizations of 1,2-diisopropylidenecyclobutane and 1-isopropyli-dene-2-methylene-3,3-dimethylcyclobutane are discussed. The results for both compounds can be interpreted in terms of a concerted 1,5-hydrogen shift, followed in the thermal case by electrocyclic opening of the resulting cyclobutene ring to give a cross-conjugated triene. The relative rates of the thermal and photochemical processes suggest that the initial sigmatropic rearrangement proceeds via an antarafacial transition state in the latter case, as predicted by orbital symmetry rules. © 1969, American Chemical Society. All rights reserved.