TRANSANNULAR HYDROGEN ABSTRACTION IN SMALL RING CARBOCYCLIC SYSTEMS

On irradiation cyclopentyl phenyl ketone has been found to rearrange principally to 1-phenyl-5-hexen-1-one. Further irradiation affords 1-phenyl-4-hexen-1-one, 1-phenyl-1-hydroxy-2-vinylcyclobutane, 1-phenyl-3-cyclohexen-1-ol, and acetophenone. The photochemical transformations encountered in this work appear to be best described by a transannular hydrogen abstraction followed by subsequent reactions of the diradical thus formed. The photorearrangement was shown to proceed by way of a triplet n-π* state. In striking contrast to the extraordinarily slow and inefficient reaction in the cyclobutyl system, cyclopentyl phenyl ketone was found to rearrange with high quantum efficiency and of the order of three powers of ten faster.