Conformational control of photochemical behavior. Competitive α cleavage and γ-hydrogen abstraction of alkyl phenyl ketones

The photochemical α cleavage and γ-hydrogen abstraction reactions of several acyclic and cycloalkyl phenyl ketones have been investigated. Photochemical reactions of cyclohexyl phenyl ketones are more rapid than ring inversion. As a consequence, product compositions are determined by ground state conformational populations. In contrast, pseudorotation in cyclopentyl phenyl ketones and bond rotations in acyclic ketones are more rapid than photochemical reactions. The Curtin-Hammett principle applies to the reactions of these ketones. Product compositions are determined by relative rates of the competitive primary photoprocesses rather that conformational populations. For cyclohexyl phenyl ketone isoenergetic energy transfer from the long-lived equatorial excited state to the highly reactive axial conformer is observed to occur with a rate constant of 1.2 × 10⁸ M^-1 sec^-1.