Photosensitized reactions of oxime ethers: A steady-state and laser flash photolysis study

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that chloranil-sensitized reactions of the oxime ethers result in the formation of the corresponding radical cations. The radical cation species react with nucleophiles such as MeOH by clean second-order kinetics with rate constants of (0.7-1.4) x 10(6) M-1 s(-1). Only a small steric effect is observed in these reactions, which is taken as an indication that the reaction center is not the O-alkyl moiety, but rather somewhere else in the molecule. Product studies in a polar nonnucleophilic solvent (MeCN) revealed that in order for the oxime ether radical cation to react more readily, alpha-protons must be available on the alkyl group. The O-methyl (1), O-ethyl (2), and O-benzyl (3) acetophenone oximes all reacted readily to give acetophenone oxime as the major product (as well as an aldehyde derived from the O-alkyl group), whereas O-tert-butyl acetophenone oxime (4) did not. The product formation can be explained by a mechanism that involves electron transfer followed by proton transfer (alpha to the oxygen) and subsequent beta-cleavage. When using 3 in MeOH, a change in the product formation is observed, the most important difference being the presence of benzyl alcohol rather than benzaldehyde as the major product. On the basis of the data from UP and steady-state experiments, it is suggested that the competing mechanism under these conditions involves electron transfer, followed by a nucleophilic attack on the nitrogen, a MeOH-assisted [1,3]-proton transfer, and subsequent loss of benzyl alcohol. This mechanism is supported by DFT (B3LYP/6-31G*) and AM1 calculations.