Some aspects of the photoreduction of carbonyl compounds.

The absorption of light by the carbonyl group has been reviewed as has been the different ways by which an excited carbonyl group can lose its excess energy. The photoreduction of carbonyl compounds by alcohols has also been reviewed. The photoreduction of carbonyl compounds by tertiary alkylarylamines has been studied in detail and the evidence suggests the reaction proceeds via an initial interaction of the non-bonding electrons of the amine with excited carbonyl forming a charge transfer complex. With amines of low ionisation potential, the complex can dissociate into radical ions if these can be stabilised by solvation. The quenching of the fluorescence of biacetyl, fluorenone and anthracene by triphenylamine, triphenylphosphine and phenylsulphide occurs by a similar mechanism. The photoreduction of fluorenone and benzophenone in ethanolic and aqueous amine solutions gives rise to the radical anion of the ketone, and the differing fate of the radical anion in both cases is discussed. Photoreduction of the carbonyl group by tin alkyls has been studied and the greater activity of tri-n-butylstannane as a hydrogen donor is thought to be due to the weak tin-hydrogen bond. The photoreduction of ?-? unsaturated ketones by amines gave addition products but use of tri-n- butylstannane gave varying amounts of the saturated ketone.