Mechanistic and Exploratory Photochemistry. XV.¹ The Relation of Cyclohexenone to Cyclohexadienone Rearrangements

The photochemical rearrangements of 4a-methyl-4,4a,9,10-tetrahydro-2(3H)phenanthrone and ∆^1,9-10-methyl-2-octalone are described as giving products formally analogous to those arising in cyclohexadienone photolyses. In the former case the product structure was proven by degradation and in the latter by synthesis. The cyclohexenone rearrangement was shown to proceed by way of a triplet excited state. The electronic configuration of this species is discussed. In striking contrast to the extraordinarily rapid (i.e., k_r ≅10⁹ sec^-1) and efficient dienone rearrangement, the cyclohexenones were found to rearrange with very low quantum efficiency and of the order of four powers of ten more slowly. In isopropyl alcohol the pinacol was formed. It was demonstrated that this and the photoketone are formed from the same or very rapidly equilibrated excited states.