Conformational control of photoreactivity: Three α-mesityl ketones that undergo efficient radical cleavage

Abstract

α-Mesitylisobutyropohenone, 1,2-dimesitylethanone, and 2-phenyl- 1,2-dimesitylethanone all undergo only one photoreaction in solution, α-cleavage to acyl radicals. Quantum yields are all over 0.30 and triplet rate constants are ≥5 x 10 8 s −1. In each case, bond rotations are so slow that reaction occurs from the preferred ground state geometries, which hold the molecules in conformations ideal for cleavage. The large cleavage rate constants reflect relief of steric strain as well as ideal orientation of π and σ orbitals.