Camphorquinone, hydrogen abstraction

An example of a bimolecular photoinitiator that generates free radicals by a hydrogen abstraction process is the combination of camphorquinone with a benzyl alcohol (equation [2.20]) [CRI 08], The carbon-centered free radical, 57, is capable of reducing a diaryliodonium salt in a similar manner to that shown previously in Diagram 2.5. 

Camphorquinone (12), which has been studied in several solvents, gives typical hydrogen atom abstraction reactions. It undergoes photoreduction in isopropyl alcohol39,71,72 and forms mixtures of solvent addition and reduction products when irradiated in methanol,39,71 aldehydes,73 and xylene.23 No transformation is observed in benzene or carbon tetrachloride.74... 

A novel method for generating semidione radicals has recently been reported by Monroe, Weiner, and Hammond 178) who found that the quantum yield for photoreduction of camphorquinone in 2-propanol was markedly enhanced when benzophenone was added and the solution irradiated at wavelength 3660 A where most of the light was absorbed by benzophenone. Instead of benzpinacol formation, the dione underwent photoreduction. Similar enhancement was not observed with jw-methoxy-acetophenone which does not abstract hydrogen from 2-propanol. The conclusion was that the ketyl radical, formed in the efficient H-abstraction reaction of benzophenone, transferred a hydrogen atom to camphorquinone to generate the semidione radical. It was suggested that this phenomenon be called "chemical sensitization . 

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