Ketones, photochemistry photoreduction

Earlier reviews on the photochemistry of unsatured ketones and amines are available39,40. The photoreactions of a,/i-unsaturated carbonyl compounds in the presence of amines have been reported to yield 1 1 amine adducts32,33 as well as photoreduction... 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

This review has been concerned with the photochemistry of heterocyclic systems photoreactions which are more correctly designated as reactions of functional groups have, in general, been omitted or not seriously considered. Thus, the formation of pinacols by the photoreduction of such heterocyclic ketones as 3-acetylpyridine 453 and the keto sulfone (422)454 has not been included, nor has cis-trans isomerization been reviewed. 

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... 

Irick Jr., G., and Pacifici, J. G. (1969). Photochemistry of azo compounds. II. Effect of ketonic sensitizers on photoreduction of 4-dimethylamino-4 -nitroazobenzene. Tetrahedron Lett. 2207-2209. 

Hydrogen abstraction (Section 4.9) by excited carbonyl compounds is one of the most fundamental reactions in organic photochemistry. In a classical photoreduction reaction, an excited carbonyl compound, such as a ketone, undergoes hydrogen abstraction from a hydrogen donor [H] to form the ketyl radicals, which subsequently abstract another... 

The ketone group is a useful model because it can be excited selectively in the presence of other groups commonly contained in polymer chains, such as the phenyl rings in polystyrene, and so the locus of excitation is well defined. Furthermore, there is a great deal known about the photochemistry of aromatic and aliphatic ketones, and one can draw on this body of information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits a number of photochemical processes from the same excited state. Thus one has a probe of die effects of the polymer matrix on these processes by determination of the quantum yields. The competing processes include (1) fluorescence (Eq. 26), (2) phosphorescence (Eq. 27), (3) the Norrish type-I reaction (Eq. 28), (4) the Norrish type-II reaction (Eq. 29), (5) photoreduction (Eq. 30), (6) the... 

The reactions of n,n triplet-excited ketones are classics in photochemistry and have contributed much to the definition of molecular organic photochemistry as a field of its own [13]. For example, it took 60 years to realize that it is the triplet state of benzophenone [6] that is responsible for its photoreduction, one of the first described photoreactions [3]. With the further development of molecular organic photochemistry, the dependence of photochemical reactivity on the electronic configuration of excited states has received more detailed attention. Thus, it has been recognized that n,7t -excited states behave more radical-like in their reactions than 7t,7t -excited states [13-25]. The extensively studied hydrogen abstraction reactions of triplet-excited ketones, which mimic the behavior of alkoxyl radicals [15,16,24-29], have been taken as a test case for the characteristic radical-like n,ji reactivity. 

The solution photochemistry of a,/8-unsaturated ketones has received a great deal of attention. It is found that the course of the photoreactions is sensitive to small variations in the structure of the reactant, the concentration of the reactant, and the solvent. The reactions observed thus far involve structural rearrangements which may be accompanied by reaction with the solvent, photoreduction, both inter- and intramolecular, and dimerization. 

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