Hydrogen abstraction, ketones cyclic systems

Altnough it does not involve hydrogen abstraction, it is worth noting that some ketones add alcohol on irradiation to give an acetal (4.41). The reaction is similar to the thermal, acid-catalysed formation of acetals, except that it occurs, especially for cyclic ketones, in solutions which do not have any strong acid added. There is an element of irreproducibility in the systems, and it seems that traces of acid in the alcohol initiate the reaction, which can be prevented by adding a small amount of solid potassium carbonate to the alcohol solution. 

Hydrogen abstraction can occur from a position within the ketone molecule, and this generates a biradical that may cyclize by combination of the radical centres. The overall photocydization process is observed for a wide variety of compound types, and it has been used extensively to make cyclic or polycyclic systems, in an unconstrained system a ketone (n,n ) excited state shows a preference tor abstraction from the y-position (4.42), which can be understood on... 

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 ( ). 

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