Photolytic cleavage rearrangement

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

At higher temperatures than those usually employed, flash vacuum pyrolysis (640°C) of the doubly ortho-blocked benzotriazole 315 (R = H) gave 1,4-dimethylcarbazole (30%), 1-methy(carbazole (35%) and 1,8-dimethylcar-bazole, each product thought to arise by rearrangement of intermediate 316 (R = Photolytic cleavage of 315 (R = Me) giving 1,2,3,4-tetramethyl-... 

A mechanism involving Norrish type I cleavage followed by radical cyclization has been proposed to explain this photolytically induced rearrangement.An alternative possibility might involve bonding from the carbonyl carbon to the 6-carbon followed by alkoxy radical expulsion and ring closure. 

Other approaches to fused -lactams have been described. An attempted transannular cyclization of the 1,4-thiazepine sulphone (155 f) to the penicillin ring system failed the products obtained were those resulting from ring cleavage and elimination." A photolytic Wolff rearrangement of... 

Cyclopropylmethyl radicals have been generated by thermolytic, photolytic or electron transfer reactions of a wide variety of precursor molecules. The parent cyclopropylmethyl radical 1 contains approximately 27.5 kcal mol ring strain and therefore homolytic cleavage of a C -C bond (j -scission) to give the but-3-enyl radical 2 is exothermic, and occurs very rapidly, even though the rearranged radical is primary. 

Oxidation of solutions of benzodiazepines in benzene or acetic acid with a high-pressure mercury arc in oxygen also causes oxidative ring contraction to give quinoxalines.49 Since hydrolytic cleavage is ruled out in this case a photolytic norcaradiene-type rearrangement of the intermediate ketone to a quinoxaline was suggested (Scheme 5). 

The reverse isomerization, viz. I —> II, occurs under photolytic conditions when I is embedded in a polymer film. A pseudovotation involving Ru-Ru bond cleavage is believed to bring about the rearrangement. The apparent conflict between the thermal and photolytic processes is probably a result of heterolytic and homolytic Ru-Ru bond fission brought about by the different initiation techniques. 

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