Photochemical behavior, conjugated

Evidence for the formation of oxabicyclobutane (6) in the irradiation (S) was negative (72JA1193). The striking difference in photochemical behavior of enones such as (5) and the analogous conjugated dienes, which readily form bicyclobutanes, is attributed to the substantially greater endothermicity of the isomerization in the oxa series. 

By comparing the photochemical behavior of conjugated diolefinic monomers in the crystalline state and in solution, a crystal matrix effect on four-center type photopolymerization has been revealed. It has been concluded that high molecular weigth linear polymers are produced photochemically from these monomers only by way of a crystal-lattice controlled mechanism. 

The photochemical behavior of anthranils in acid solution is more complex. In general, mixtures of 3- and 5-substituted 2-aminoacylbenzenes are formed by conjugate addition of the acid to the initially formed resonance-stabilized nitrenium ion as outlined in Scheme 21 for 5-substitution (generally the major product).225,226... 

The amount of experimental data now available In the vitamin D field complemented by experience from simple trlenes makes It worthwhile to consider such features using the material presented In this chapter as a lead. In the next chapter the discussion will be taken up again, now on the basis of the different photochemical behavior of the two Isomeric conjugated trlenes, vitamin D and prevltamln D. 

Caine, D., Brake, P.F., DeBardelen, J.F., Jr., and Dawson, J.B., Tlie influence of electron-withdrawing substituents on the photochemical behavior of bicyclic 6/6-fused cross-conjugated cyclohexadi-enones,/. Org. Chem., 38,967,1973. 

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

Smaragdyrin-azobenzene conjugates of the type 80 have been crystal structure determined and studied with respect to their photochemical and electrochemical behavior. It could be concluded that the relatively electron-withdrawing azobenzene moiety is responsible for a corresponding energy transfer to the smaragdyrin 7t-system <2007EJ0191>. 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

Several reviews of the photochemistry of the conjugated dienes and trienes have been published in recent years. This review is limited to quantitative studies on the photochemical primary processes in systems for which data are available. It is hoped that by gathering such data together, useful correlations in the behavior of linear and cyclic, conjugated dienes and trienes may be observed. 

Chemical behavior of oxepin-2,7-diones is, in general, normal of anhydrides of double unsaturated conjugated carboxylic acids. Thus, methanolysis of 4-methyloxepin-2,7-dione leads to a mixture of isomeric monomethyl (2Z,4Z)-3-methyl-2,4-hexadienedioates <82JOCi2l2>. 3,6-Dimethyloxepin-2,7-dione undergoes photochemical ring closure to the respective bicyclic anhydride (Equation (4))... 

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