Hydrogen abstraction, ketones regioselectivity

It has been thought for some time that the interactions of triplet ketones with olefins involve a competition between hydrogen abstraction from, energy transfer to, and cycloaddition to the double bond 126>. Cycloaddition has generally been considered to proceed via a diradical intermediate. At first, the only evidence for a diradical was that the regioselectivity of oxetane formation often — but not always — is such as would be expected from the relative stabilities of the possible diradical intermediates 126>. Moreover, n,n ketone triplets are known to act like alkoxy radicals, which add to olefins, albeit less efficiently than do most other... 

With ketones, a remote regioselective acetamidation is found. This result is mechanistically in accordance with the oxidation of the oxygen atom of the carbonyl group to a radical cation that abstracts a hydrogen from a remote CH bond. The generated radical is then oxidized to a cation, that reacts with acetonitrile to get the acetamide (Fig. 3) [15],... 

There is considerable current interest over the question of what factors govern the a versus p regioselectivity of hydrogen atom abstraction by the carbon-carbon double bonds of photoexcited a,p-unsaturated ketones (12-14). In solution, abstraction by the p-carbon is normally preferred, and this is the regioselectivity followed in six of the eight enones studied in this work. This... 

---