Hydrogen Abstraction by Ketones

INTRAMOLECULAR HYDROGEN ABSTRACTION BY KETONES (TYPE H CLEAVAGE)... 

Hydrogen abstraction by ketone sensitizers forms the basis of an elegant alkylation procedure for ethers, amides, lactones, amino acids, and peptides developed by Elad and his group.94 Terminal olefins, for example, can be added to y-butyrolactone (15) in good yield.95... 

A second example from Salem s work which illustrates the role of symmetry is the photochemical hydrogen abstraction by ketones, termed the Norrish type II reaction (Norrish, 1937). A typical case is illustrated in (69). 

Fig. 15 State correlation diagram for hydrogen abstraction by ketones. (Adapted from Salem, 1974)...

No attention is given to the mechanistic importance of a reaction rather, an attempt has been made to concentrate on reactions that have an actual (potential) synthetic role. This is not always an obvious selection, because photochemistry has not been sufficiently used in such syntheses, and mechanistic studies are not necessarily a reliable guide towards this aim. As an example, hydrogen abstraction by ketones (Scheme 1.3), which probably is the most thoroughly studied photochemical reaction, is not mechanistically discussed. Neither is presented the resultant photoreduction of ketones (Scheme 1.3, path a), because this will hardly ever become a sensible synthetic alternative for the reduction. However, other reactions arising from the same primary photoprocess, namely bimolecular reduction (path b) and... 

Since a-hydroxyradicals are the primary photoproducts in any hydrogen abstraction by ketones, their strong reducing power should be noted. The photo-reduction of benzophenone in secondary alcohols provided the first example of the reducing capabilities of hydroxyradicals 116>. 

Further evidence to support for this mechanism comes from the work of Salem [118]. Salem examined a number of photochemical reactions with particular attention to potential energy surface symmetry for both ground and excited states. One such reaction considered was hydrogen abstraction by ketones ... 

The results of ab initio calculations for hydrogen abstraction by ketones according to... 

Another barrier on the T, surface is observed for hydrogen abstraction by ketones whose state is of lower energy than the (n,. r ) state. 

Photochemical hydrogen abstraction by ketones in the solid state has drawn much attention (5-11,20-24). As a result of detailed studies on the tetrahydronaphthoquinone system (5,6), the following geometrical requirements for the reaction have been... 

Enthalpies for Hydrogen Abstraction by Ketones and Azoalkanes for Different Hydrogen Donors... 

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