Carbonyl triplets, hydrogen transfer reaction

Another common hydrogen transfer reaction of carbonyl triplet is the photoenolization of the c-methylbenzoyl chromophore, illustrated in reaction 3 for the syn conformer of c-methylaceto-phenone (j+). Reaction 3 can act as a very efficient energy sink, and a number of properties of this group led us to believe that this process could be used to reduce photodegradation i.e. the excellent absorption characteristics of the chromophore, the short triplet lifetime and the fact that the disappearance of the carbonyl triplet does not take place at the expense of the formation of another excited state. 

Hydrogen Transfer - Irradiation of valerophenone (29) in aqueous solution has been studied. The reaction follows the same path as that in hydrocarbon solution and yields acetophenone and cyclobutanols. The reaction in water arises from the triplet state. Interestingly, the formation of the cyclobutanols cis. trans ratio is 2.4 1) is more efficient in the aqueous system than in hydrocarbons. Cyclobutanols are also formed on irradiation of the butanoate derivatives (30). Hydrogen abstraction by the triplet excited state carbonyl group occurs from the alkyl groups on C2 of the butanoate chain. 

The reaction enthalpies for the photo-reduction of ketones and azoalkanes in n,ji states by the model hydrogen donors methanol and dimethylamine have been calculated. These data, together with a consideration of excited-state electrophilicity and the antibonding character of transition states, have been used to rationalize the nucleophilic character of singlet n,ji excited azoalkanes and the electrophilic character of correspondingly excited ketones. A theoretical model for the photo-reduction of carbonyl compounds, based on the assumption that hydrogen transfer involves electron transfer followed by proton transfer, has been developed. The model was able to describe the available kinetic data for the photo-reduction of o-benzoquinones and fluorenone in the presence of para substituted iV,iV-dimethylanilines and those for the photo-reduction of p-chloro-anil and 2,6-dichloro-l,4-benzoquinones in the presence of polymethylben-zenes. The photochemical properties of the triplet ji, n state of 5,12-naph-... 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

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