Aromatic carbonyl compounds, triplet formation

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyl compounds.130 This result suggests that the reactive excited state is a singlet for aliphatics and a triplet for aromatics. With aromatic aldehydes and ketones, the regio-selecitivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates by bond formation between oxygen and the alkene. 

Further evidence for singlet 1,4-diradical intermediates has been obtained in the case of photocycloaddition of cyclopropanecarbaldehyde with ( )-penta-1,3-diene, in which the formation of the cyclopropyl-oxetane (PS) dominated. This is in contrast to the case of aromatic carbonyl compounds and cyclopropylethylene in which the formation of considerable amounts of ring-opened products and ox-epins, formed by cyclopropylcarbinyl-allylcarbinyl rearrangement, implicates the intermediacy of a triplet 1,4-diradical. ... 

Initial formation of radicals by photoinduced hydrogen attraction from the substrate is not in dispute and, as noted in Section 1, hydrogen abstraction from alcohols, ethers and hydrocarbons is effected with high quantum efficiencies by aromatic carbonyl compounds in which the lowest lying triplet level is of n—n type, e.g. benzophenone, anthra-quinone (42 -44). 

Another example concerning the reduction of carbonyl compounds also relates to the salt effect theme. Shaefer and Peters (1980), Simon Peters (1981,1982,1983,1984), Rudzki et al. (1985), and Goodman and Peters (1986) described photoreductions of aromatic ketones by amines. In this case, the addition of excess NaC104 results in considerable retardation, even prevention, of final product formation. The two fundamental steps in this photoreduction consist of rapid electron transfer from the amine to the photoactivated ketone (in its triplet state), followed by the slow transfer of proton from the amine cation radical to the carbonyl anion radical ... 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

Wilson and Halpern report on exciplex formation between a triplet alkanone and alkylbenzene, and thus reaffirm the role of exciplexes in the quenching of triplet carbonyl compounds by oleflnic and aromatic hydrocarbons. 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

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