Photochemistry of Epoxides

We are looking at the photochemistry of aryl substituted epoxides in which have substituents at the β position to look at migrations in the 1,3-diradical system as well as the effects of stereochemistry on these photoreactions. For example, trans-2-methyl-3-phenyl oxirane undergoes photolysis to generate benzyl methyl ketone, the corresponding cis isomer and the 2,2-dimethl-3-phenyloxirane are relatively unreactive.

Thus, it seems as if there are geometric restraints on the reaction such that the p orbitals of the aromatic ring should be aligned with the carbon-oxygen sigma bond of the oxirane for energy transfer from the aromatic ring to the oxirane ring which leads to oening to the 1,3-diradical..

If these hold true, then once the 1,3-diradical is formed, then it possible for alkyl group migration to occur. Thus, formation of isopropylidene fluorene oxide allows us to examine the possible pathways that the initially formed 1,3-diradical can undergo.

In this case we observe a free radical 1,2-alyl shift althought he low quntum yields (~0.04) indicate an efficient ring closutre of the 1,3-diradical back to starting material.