Photochemical dimerization ketones, mechanism

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

Chemical Quenching. Many photochemical reactions of organic compounds are known to occur via intermolecular interactions of the excited species with reactant molecules in solution. Common examples which may involve this mechanism include the photo-reduction of ketones by hydrogen donor solvents and many photo-dimerizations (227). The scarcity of quenching reactions of metal complexes that have been attributed to chemical processes has already been discussed by Balzani et al. (204). With the increasing interest in the mechanistic aspects of OTM complex photochemistry, however, many more examples should be discovered. 

The silyl diene (90) photochemically adds benzophenone via a SET mechanism to yield the two oxetanes (91, 18X) and (92, SIX) on irradiation at 436 nm in acetonitrile solution. The oxetanes are accompanied by the (242)-dimer of the diene. The reaction appears to be efficient and can be carried out with a variety of diaryl ketones. 

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