Mechanistic aspects of the formation of aldehydes and nitriles in photosensitized reactions of aldoxime ethers

The photooxidation of a series of aldoxime ethers was studied by laser flash photolysis and steady-state (product studies) methods. Nanosecond laser flash photolysis studies have shown that chloranil (CA)-sensitized reactions of the O-methyl (1), O-ethyl (2), O-benzyl (3), and O-tert-butyl (4) benzaldehyde oximes result in the formation of the corresponding radical cations. In polar non-nucleophilic solvents such as acetonitrile, there are several follow-up pathways available depending on the structure of the aldoxime ether and the energetics of the reaction pathway. When the free energy of electron transfer (ΔGET) becomes endothermic, syn−anti isomerization is the dominant pathway. This isomerization pathway is a result of triplet energy transfer from CA to the aldoxime ether. For substrates with α-protons (aldoxime ethers 1−3), the follow-up reactions involve deprotonation at the α-position followed by β-scission to form the benziminyl radical (and an aldehyde). The benziminyl radical reacts to give ben...