Disproportionation ketones, Norrish type

This section deals primarily with the photochemistry of aryl substituents which is initiated by the arene excited state. A novel exception is the photochemistry of the ketones (362) and (363), which is perturbed by the presence of the ground state arene rings. Photolysis of (362) in methanol yields (364) this is presumably derived from Norrish Type I cleavage followed by disproportionation to a ketene which is quenched by methanol. The reaction fails for... 

The Norrish type I reaction of acyclic and cyclic ketones in solution typically results in recombination, decarbonylation and disproportionation (hydrogen abstraction) products.903 For example, irradiation of di-tert-butyl ketone (265) in hexane solution provides nearly exclusively decarbonylation products from both the singlet and triplet states (>90% chemical yield), whereas the carbonyl group-containing products are produced only in traces (Scheme 6.111).922 923... 

Wagner and Liu60 have studied the reaction of the ketone (60) in an endeavour to obtain more accurate values for the lifetime of 1,4-biradicals produced by the Norrish Type II process. In this example the biradical produced, (61), can follow several reaction paths, i.e. oxetan formation, fission, reversion to ground state by disproportionation, and formation of norbornanol (62). The products obtained are illustrated in Scheme 3. From the analysis of the system the authors 59 concluded that cyclization and cleavage of the biradical (61) each occur with a rate constant of 6 x 10 s 1. Coyle and Kingston51 have reported a study of Norrish Type II cleavage in a series of 2-(dimethylamino)ethylbenzoates (63). Since neither penta-1,3-diene nor biacetyl quenches the reaction, although both... 

In most cases, the biradicals formed by intramolecular hydrogen abstraction can both cychze and disproportionate. In the case of Norrish type 11 1,4-biradicals, reversion to starting ketone is the sole result of disproportionation, but biradicals with more connecting atoms between the two radical sites can disproportionate to various enol structures, as is described below. It appears that singlet 1,4-biradicals undergo little if any cyclization or disproportionation. It is not known to what extent larger singlet biradicals partition between cyclization and disproportionation. 

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