Norrish Type I Reaction

Abstract

The light‐initiated carbon‐carbon bond cleavage between the α‐carbon and the carbonyl of a ketone or an aldehyde to form an alkyl and an acyl radical pairs, from which a variety of products can be formed, such as alkane, alkene, cyclic acetal, decarbonyl compound, and oxetane and this reaction is referred to as the Norrish type I reaction. This reaction is initiated by a n‐π* excitation followed by an α‐cleavage to form an acyl and an alkyl radical pairs. It has been reported that this reaction is influenced by both the structure of the substrate and the reaction conditions, as evidenced in the different product distributions between the reactions in the gas phase and in the solution phase. The aliphatic cyclic imides and small cyclic ketones up to cycloheptanone undergo only the Norrish type I reaction from both singlet and triplet excited states, whereas acetophenone and benzophenone are often used as the Norrish type I photosensitizers. This reaction in the presence of zeolite and cyclodextrin and in the solid state produces good results. This reaction has been used extensively in organic synthesis and photochemistry.

Photopolymerization Mechanisms at a Spatio-temporally Ultra-confined Light

“…For example, in radical photoinitiating systems the cleavage happens in aromatic carbonyl compounds that are known to undergo homolytic C-C bond scission upon intense light exposure. This is common for the Norrish type 1 PIs [38]. One more case when the free-radicals can be induced is hydrogen atom transfer reaction with an additional co-initiator molecule.…”