Photosensitized dimerization

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

The photosensitized dimerization of cis- and /ra/w-piperylene has been found to lead to at least 15 products, six of which are cyclobutanes.(9>... 

Three products have been obtained from the photosensitized dimerization of cyclopentadiene,<9,aa) in which the diene is constrained in the cis form ... 

Indene undergoes photosensitized dimerization to produce (25) in high yield(38> ... 

Many derivatives of maleic and fumaric acid are known to undergo photosensitized dimerization.<121> Some derivatives of maleic acid are as follows ... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

The photosensitized dimerization of isoprene is considerably more complex than that of butadiene, yielding cyclobutanes (14)—(16) as well as four dimers of noncyclobutane types<9 11,18) ... 

The major products from the photosensitized dimerization of isoprene predicted. The ratio of diene dimers vary as the sensitizer is varied or not ... 

Diene triplets can also be made to add to perhaloolefins,266 although with less ease than to ground-state dienes. Again the products are those expected from the most stable biradical intermediates, and the product ratios differ from those obtained in thermal additions. No photosensitized dimerization of the haloolefins occurs. 

Table 13.1 Composition of Products from Photosensitized Dimerization of Butadiene...

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

Figure 21. Reaction products of the photosensitized dimerization reactions of arylalkenes in NaX compared with solution studies.

Synthetic application of Paterno-Bilchi reaction of simple dienes with carbonyl compounds is rare. While seemingly an extension of the photocycloaddition of olefins and carbonyl compounds, the reaction between dienes and carbonyls is often complicated by the fact that triplet excited states of carbonyl compounds are quenched by dienes, although the formation of oxetanes can be observed during these reactions. Recall also that the photosensitized dimerization of diene triplet excited states is also a well known reaction vide infra), these two observations would seem to naturally limit the synthetic potential of this process. Kubota and coworkers found that irradiation of propanal in the presence of 1,3-cyclohexadiene produced oxetanes 164a and 164b in a 4 1 ratio (Scheme 37). ... 

Room temperature phosphorescence which can occur in mixed organic media is observed in synthetic enzyme model surfactant systems.The effect of pH on the p-aminobenzoic acid photosensitized dimerization of free thymine base shows that the triplet state is involved in this reaction. 8 The nature of the... 

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