1,3-Cyclohexadiene, direct irradiation products

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Unsubstituted cyclic dienes, such as 1,4-cyclohexadiene, which contain both double bonds in the same ring, possess very low extinction coefficients at standard wavelengths. The reaction can be carried out by mercury sensitization" or under direct irradiation at 185 nm (equation 5). The next higher homolog, i.e. 1,4-cycloheptadiene, can be generated in the photolysis of bicyclo[4.1.0]hept-3-ene (5) and further transformed into the expected di-ir-methane product (6) at 185 nm (equation 6). ... 

Products. Photolysis of 1,3-cyclohexadiene gives rise to 1,3,5-hexatriene of unknown stereochemistry. During irradiation in the condensed phase initial product is the m-triene which subsequently isomerizes photochemically. The other primary processes which seem to originate directly in 1,3-cydohexadiene are dehydrogenation ... 

Diastereomer-differentiating intramolecular [2+2] photocycloaddition has been reported for the diastereomeric 2,6-dienones (105) and (106). Both direct and sensitised irradiation of (105) yields (107) whereas (108) is formed from (106). Reinvestigation, at -78 °C, of the photocycloadditions of 1-cyanonaphthalene (1-CN) and 2-cyanonaphthalene (2-CN) with 1,3-cyclohexadiene has suggested that the primary major singlet-derived products are the corresponding exo-[4+4]adducts, (109) and (110) respectively, and the s>>/i-[2+2]adducts, (111) and (112) respectively. Consideration of primary and secondary orbital interactions has been used to interpret these observations. ... 

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