Aromatic carbonyl compounds, triplet

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

The oxygen-perturbed singlet-triplet spectra of aromatic carbonyl compounds were investigated by Warwick and Wells (Fig. 23). Transitions to states were enhanced by the perturbing agent while transitions to (n,7t ) states remained unaffected. It should be mentioned, however, that Evans also observed an oxygen-perturbed increase of the intensity of the Tnn So transitions in pyrazine and acridine. 

Warwick. D. A., Wells, C. H. J. Perturbation of singlet-triplet transitions in aromatic carbonyl compounds. Spectrochim. Acta 24A, 589 (1968). 

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. 

Aromatic carbonyl compounds are good sensitizers of triplet state because of small singlet-triplet splitting. On the other hand, aromatic hydrocarbons and olefines have convenient singlet-triplet levels to act as good triplet quenchers or acceptors. 

Schenck and co-workers [43] reported also the reaction of some aromatic carbonyl compounds (benzophenone, benzaldehyde) with benzofuran (Scheme 3.13). They showed that when high triplet energy compounds were used, dimers of... 

Abe, M., Kawakami, T., Ohata, S., Nozaki, K., and Nojima, M. (2004) Mechanism of Stereo- and regioselectivity in the Paterno-Buchi reaction of furan derivatives with aromatic carbonyl compounds Importance of the conformational distribution in the intermediary triplet 1,4-diradicals. Journal of the American Chemical Society, 126 (9), 2838—2846. 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

In case of Paterno-Buchi reaction of cycloalkenes with prostereogenic aromatic carbonyl compounds (which show rapid ISC to the triplet excited carbonyl) less clear results were obtained. The reaction of cyclohexene with benzaldehyde was reported in the literature [51] and the spectral data of the main product 56 (35%) described as consistent with the assignment of exo-stereochemistry (Sch. 35) [110]. 

Zero-Field Splitting Parameters of Some Aromatic Carbonyl Compounds in their Phosphorescent Triplet State9... 

Several years ago it was found that many aromatic carbonyl compounds are photoreduced by aliphatic and aromatic amines (Cohen et al., 1973 Davidson, 1975). It was suggested that these reactions proceed by electron transfer from the amine to the triplet carbonyl compound. Evidence in support of this mechanism was that the ability of amines to quench the phosphorescence of carbonyl compounds, and to react with them, correlated with the ionisation potential of the amines. On the basis of this evidence the reaction scheme in... 

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. ... 

Undoubtedly the most important distinction between (n, Jt ) and (n, n ) excited states of aromatic carbonyl compounds is the much greater efficiency of intersystem crossing (i.e. Si Ti conversion) in the former cases. This follows because, as already noted, triplet states are the longest lived and most useful intermediates in organic photochemistry. Two factors contribute to the high probability of intersystem crossing between (n, n ) states as compared to that between (jr, n ) states ... 

Whether the charge transfer state is lower in energy than the corresponding (n,n ) triplet state depends largely on the solvent polarity, and since such compounds have not yet found practical use in polymerization processes they will not be considered further. Whenever the degree of conjugation in aromatic carbonyl compounds is more... 

Whereas aromatic carbonyl compounds having lowest lying (rt, 3t ) triplet excited states are not photorcduced by hydrogen donors such as alcohols and ethers, they are readily photoreduced by electron donors such as arnin and sulphur compounds (5). The electron transfer processes may be generalisoi as follows ... 

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). 

Table 4. Reduction potentials and triplet energies of aromatic carbonyl compounds/...

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