Hydrogen abstraction, ketones disproportionation

Dibenzyl ketone, for example, is readily cleaved photolytically. " Similarly, f-butyl ketones undergo a-cleavage quite readily on photolysis in solution. The radicals generated undergo coupling, disproportionation, and hydrogen abstraction reactions. 

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... 

Studies of deuterium isotope effects have proved useful in the determination of the details of excited chemical and physical processes. However, some of the observed isotope effects pose as many questions as they answer. Such is the case for the formation of unsaturated ketones from certain cyclic ketones, e.g. (l).ei The observed primary isotope effect is of the same order as that observed earlier by these authors for similar systems, but is much smaller than literature values for radical disproportionations. One explanation is that the rate of hydrogen abstraction is controlled by the conformation at the starred carbon ( ). 

It is generally true that almost all the hydroxybiradicals formed by internal hydrogen abstraction partially revert to starting ketone by an internal radical disproportionation reaction in which the carbon-centered radical site abstracts the hydrogen from the hydroxyl group. This process often is the major reaction of the intermediate biradical, so that the overall quantum yield of product formation is low even when hydrogen abstraction by the excited state is 100% efficient Should the efficiency of hydrogen abstraction be low because of electronic or conformational problems, then biradical disproportionation lowers quantum efficiency even further. 

A wide variety of carbonyl compounds undergo photoinduced intramolecular hydrogen atom abstraction to form 1-hydroxy-IvJc-biradicals, which then undergo two common competing reactions (1) coupling to produce cycKc alcohols and (2) disproportionation back to ketone or to various enols. The overall process closely parallels the well-known bimolecular photoreduction of ketones, the most common products of which are formed by radical coupling. These intramolecular hydrogen abstraction processes have been widely studied, and several reviews are available. ... 

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. 

Inasmuch as this chapter covers the cyclization of hydroxy-biradicals, it is important to remember that excited-state hydrogen abstraction forms a biradical with its two radical sites initially very close to each other. Unless some of the connecting bonds rotate very rapidly, disproportionation back to ketone would be aU that happens. One would think that rather simple conformational changes are all that is required for cyclization to take place. In order to predict cyclization quantum efficiency, the challenge is to understand the kinetics of those bond rotations as well as other conformational changes that lead to competing disproportionations and cleavage (only 1,4-biradicals). 

A few systematic studies of e-hydrogen abstraction, which generally requires the absence of both y- and 5-hydrogen atoms, have been performed. Carless and Fekarurhobo provided one of the earhest examples in y-allyloxy ketones and aldehydes, which photocychze to roughly equal amounts of diastereomeric tetrahydropyranols. The -allylic radical site can disproportionate at each end to generate both starting ketone and a vinyl ether. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

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