Olefins, photosensitized dimerization

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 ... 

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). ... 

The antidepressant protriptyline (116) causes skin photosensitization in man. Jones and Sharpies irradiated an aqueous solution of the hydrochloride with a medium-pressure mercury lamp for 16 h and separated the products by preparative TLC. First formed was the epoxide (117) which photohydrated to the diol (118). Also isolated was the enol (119) [84], Earlier, Gasparro and Kochevar had shown that only the hydrochloride was photodegraded under nitrogen in water or ethanol. Three products were isolated and all lysed erythrocytes, but the structure of only one was suggested. This was a cyclobutyl dimer as shown by its mass spectrum and its photolysis back to protriptyline by light of 254 nm. Presumably, a [2 + 2] cycloaddition of the olefine bonds had occurred [85]. 

Hammond and collaborators as part of their extensive studies on photosensitized reactions. Examples include (a) cis-trans isomerization of olefins sensitized by biacetyl 42,64,65,176 2,3-pentanedione 64>65>, benzil 64,65,89)> a. and /3-naphil 64>, and phenanthrenequinone 30-65> (b) diene and anthracene dimerizations sensitized by biacetyl 9>45>94>, 2,3-pentanedione 4,162)j camphorquinone 04>, benzil 84,162) and / -naphthil 94>, and (c) the quadricyclene-norbomadiene interconversion sensitized 66> by benzil. 

While in both cases, the methyl groups are situated within the tapered calix[4]arene sections of the dimeric capsule and the olefinic ends are in close proximity to each other, the two ethenyl groups can assume two different orientations relative to each other. Both orientations react on irradiation with UV light, but the two products are different. Fifty-five percent of the reaction mixture yields a dimer that is the result of a [2- -2] photoaddition, while the remaining 45% are accounted for by a dimer derived from a radical process, which typically takes place in the presence of a suitable photosensitizer. This sensitizing role is fulfilled by the host itself. The product ratio reflects the ratio of the two relative orientations that the guests have with respect to each other. 

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