1,3-Cyclohexadiene, direct irradiation

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-... 

The normally sluggish Diels-Alder cycloaddition between cyclohexa-1,3-diene and various enones and enals can be activated by precoordination of the diene to a jt-basic molybdenum complex TpMo(NO)MeIm(//-cyclohexadiene)].134 The 4-1-2-cycloaddition of cyclohexa-2,4-dienones with electron-deficient 2n -dienophiles produced bridged bicyclo[2.2.2]octenones. Triplet-sensitized irradiation of these bridged bicyclooctenones produced bicyclo[3.3.0]octanoids, whereas direct irradiation yielded bicyclo[4.2.0]octanes.135... 

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). ... 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

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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