Cyclopentene, photochemistry

Ganem and co-workers have expanded earlier studies on the photochemistry of pyrylium salts to give functionalized cyclopentenes such as 79 and 80 (Equation 2) <2001JA10425, 1973CC410, 1971CG1240, 1980S589>. 

The 8.4 eV photodecomposition of cyclopentene was also studied in the same laboratory. Ethylene, acetylene, and cyclopentadiene are the major products, although a high yield of hydrogen atoms was also observed (94). On the other hand, methylenecyclobutane (d> = 0.04) and bicyclo[2. l.Ojpentane (d> = 0.03) were the only primary products observed in the 184.9 nm photochemistry of n-heptane solution of cyclopentene (95). The formation of 1,4-pentadiene (d> = 0.01) was ascribed to secondary processes. Very recently we have undertaken a systematic study of the 184.9 nm gas phase photochemistry of cyclopentene at pressures from 1 Torr to 6 atm of added propane. Again at low pressure, ethylene (d> = 0.12), acetylene (d> = 0.03), allene (d> = 0.06), and cyclopentadiene (d> = 0.22) are the main products (96). About 80% of the formation of cyclopentadiene involves the elimination of a hydrogen molecule in agreement with the Woodward-Hoffmann allowed 1,4 concerted molecular elimination process (97). Moreover, several isomers are also formed provided... 

Veiy recently we have restudied the 184.9 nm photochemistry of cyclohexene. At high pressure, there is formation of methylenecyclopentane and bicyclo-[3.1.0]hexane. The Stem-Volmer plots for the quantum yields of these isomers appear in Figs. 13 and 14. It is clear iiom these results that these isomers do not involve the same precursor. In the same manner, in the 184.9 nm photochemistry of cyclopentene, the methylenecyclobutane quantum yield increases with an increase in the pressure of added propane. Conversely, we have not seen any formation of bicyclo[2.1.0]pentane. At 213.8 nm (that is at a lower energy), the formation of meftylenecyclobutane and bicyclo[2.1. OJpentane was observed at high pressure of added propane (Fig. 11). Clearly, both products are formed in somewhat different pressure scales (114). Thus, it is also fairly safe to assume that two different precursors are involved in such formations. More particularly, fi-om the Stem-Volmer plots of methylenecyclobutane and ethylene, the lifetime of an excited methylenecyclobutane molecule can be estimated ... 

Coyle has summarized the photochemistry of carboxylic acid derivatives. For arene carboxylic acid esters it has been shown that [2-1-2]-cycloaddition competes with hydrogen abstraction by the excited ester from an allylic position of the alkene. The addition of methyl benzoate 17 to 2-methyl-2-butene gave a 1 1 mixture of the Paterno-Bilchi adduct 18 and the coupling product 19. Less electron-rich alkenes (e.g., cyclopentene) did add preferentially toward the benzene ring of 17 in an ortho- and metacycloaddition manner. Furans could also be added photochemically to methyl benzoate and other aren-ecarboxylic acid esters. The resulting bicyclic oxetanes could be transformed into a series of synthetically valuable products. [2-1-2]-Cycloadducts and/or their cleavage or rearrangement products have also been described for photoreactions of alkenes with diethyl oxalate,benzoic acid, " and carbamates. ... 

P,y-Unsaturated aldehydes are a family of carbonyl compounds that have been considered ODPM unreactive for many years. This conclusion was based on literature precedents that indicated that the normal photochemical reactivity of such compounds, both on direct and sensitized irradiation, was decarbonylation to the corresponding alkene. > f> Thus, compounds 5, 6, 7, and 8 are representative examples of a series of steroidal and cyclic P,y-unsaturated aldehydes, studied by Schaffner et al. about 20 years ago, that undergo decarbonylation on irradiation. Diirr et al. have studied the photochemistry of some aryl-substituted-2-cyclopentene-l-carbaldehydes 9. In this instance, ds-stilbene-type electrocy-clic cyclization and decarbonylation were the only reactions observed. Other authors have also reported the decarbonylation of the acyclic P,y-unsaturated aldehydes 10 and 11 on direct irradiation. 

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