Norrish Type II Reaction

Abstract

The light initiated intramolecular abstraction of a δ‐hydrogen of an excited ketone or aldehyde to generate a 1,4‐biradical, from which different products can form, including alkene, enol, and cyclobutanol and this reaction is generally known as the Norrish type II reaction. This reaction is also called as the Yang reaction. The study finds that this reaction is influenced by both environmental and structural factors. In addition, it is found that both the excited singlet (S1) and triplet (T1) state of a dialkyl ketone can undergo such reaction, whereas an aryl alkyl thioketone proceeds from the excited triplet state (T1), and the reaction of an ester primarily involves the excited triplet (T1). For aliphatic or benzylic hydrogen abstraction, ketones with a lowest π‐π * triplet state are much less reactive than those with the lowest n‐π * triplet state. The mechanism of the reaction for an aryl ketone and special ketones has been discussed. This reaction has been applied in organic synthesis, especially for the generation of free radicals.