The solution and solid state photochemistry of some substituted acetophenones

A series of α-methyl-α-cycloalkyl- and α-cycloalkyl-para-substituted acetophenones have been synthesized. These compounds were found to react photochemically in the solid state to give cyclization and cleavage products as expected for a Norrish Type II reaction. It has been found that α-methyl substitution of α-cycloalkyl-para-carboxyacetophenones (cyclohexyl and cyclooctyl) results in changing the hydrogen abstraction transition state from boatlike to chairlike. This result is based on the assumption that the abstraction geometry is similar to the geometry of the ground state ketone as determined by X-ray crystallography. The differences in the ratios of cyclization to cleavage in the solid state as compared to solution wer found to be no greater than 10% for most of the ketones studied. This suggests that formation of cyclization and cleavage products are topochemically allowed in the solid state. Alternatively, the reaction may not be under topochemical control, however, the high melting points of the compounds studied tend to support the first possibility. α-Methylation of α-cyclopentyl-para-carboxyacetophenone decreased the percent cleavage from 100% to 45%. This result is likely due to a change in the geometry of the starting ketone which occurs upon α-methylation. Solid state photolysis of α-cycloalkyl-para-substituted acetophenones (cyclooctyl and cycloheptyl) results in almost exclusive formation of the trans-cyclobutanol. This result has been attributed to a restriction of the motions required for cis-cyclobutanol formation in the solid state.Science, Faculty ofChemistry, Department ofGraduat