Photochemical reactions bond cleavage

The N-diazeniumdiolates are quite photosensitive. Studies of various 02-substi-tuted compounds, both alkyl and aryl, revealed a primary photochemical reaction involving cleavage of the N=N bond to yield a nitrosoamine and an O-substituted nitrene which rearranges to a C-nitroso compound (Scheme 3.31), the latter is often isolated as the oxime [224]. 

Scheme 5.11 shows a general blueprint that serves as a useful framework for considering reaction mechanisms that involve biradical intermediates B. The scheme also applies to carbenes (Section 5.4.1) and nitrenes (Section 5.4.2). When formed by thermal or photochemical homolytic bond cleavage from the reactant molecule M, biradicals will initially have the multiplicity of their immediate precursors. Singlet biradicals ( B ) tend to be short-lived because spin-allowed bond formation leading back to the reactant M or to a... 

Penn, J. H. and Owens, W. H., The photochemistry of mixed anhydrides a search for selectivity in photochemically initiated bond-cleavage reactions,/. Am. Chem. Soc., 115, 82, 1993. 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

The formation of 2H-pyrroles (21) and a pyrrole derivative (22) from the reaction of 3-phenyl-2//-azirines and acetylenic esters in the presence of molybdenum hexacarbonyl is intriguing mechanistically (Schemes 24, 25).53 Carbon-nitrogen bond cleavage must occur perhaps via a molybdenum complex (cf. 23 in Scheme 26) but intermediate organometallic species have not yet been isolated.53 Despite the relatively poor yields of 2H-pyrrole products, the process is synthetically valuable since the equivalent uncatalyzed photochemical process produces isomeric 2H-pyrroles from a primary reaction of azirine C—C cleavage54 (Scheme 24). 

The associative, thermochemical, and photochemical properties of PATE polymer will be described and evidence will be provided supporting the thesis of labile sulfur-sulfur bonds. Photolysis of a model compound and a polymer similar to PATE but lacking thiosulfate functional groups has been conducted to provide supportive evidence for the proposed sulfur-sulfur bond cleavage reactions. Finally, a preliminary investigation of the imaging characteristics of the PATE polymer is presented in order to place the photochemical investigation in perspective. 

Photochemical, carbon-sulfur bond cleavage is also observed in compounds containing sulfur in oxidation states higher than that which exists in sulfides and in dialkyl dithioacetals. For example, the irradiation of the sulfoxide 47 in methanol produces109 a 58% yield of galacti-tol (52). Even though homolysis of the carbon-sulfur bond does occur in 47, it is unlikely that 52 results from a simple, carbon-sulfur bond-cleavage, as such a reaction predicts products that were not observed... 

Photosolvolysis of some vinyl iodonium substrates can be compared with thermal solvolysis of the same substrates. Photochemical reaction of (E)- and (Z)-2-phenylprop-l-enyl(phenyl)iodonium tetrafluoroborate (31) in methanol and TFE gave products of heterolytic cleavage of vinylic C-I bond as summarized in Table III (for the product structures see eq 14.17 In contrast to... 

Understand how photolysis produces radicals by bond cleavage and account for the importance of radical species in photochemical chain reactions, stratospheric ozone chemistry and the photochemistry of the polluted troposphere. 

Most of the studies on organo-gallium, indium, and thallium porphyrin complexes have focused on synthesis and properties of the complexes, and rather little attention has been devoted to reaction chemistry. Two areas which have received some attention are the insertion of small molecules (SO2 or CO2) into the metal—carbon bonds and photochemical metal—carbon bond cleavage. The... 

Irradiation of ethyl or isopropyl pyruvate in the vapor phase produces a complex mixture of products which does not fit any simple photochemical mechanism.93 The initial step of the reaction appears to be formation of radicals by a carbon-carbon bond cleavage. 

A second common photochemical reaction of carbonyl compounds is the cleavage of the bond adjacent to the carbonyl group ... 

The degradation can be photochemically induced (a) homolytic or (b) heterolytic cleavage at the weaker bonds. The photolysis of the type (a) may lead to elimination reactions and the type (b) may lead to free radical formation. The point of bond cleavage may not be the seat for light absorption. The energy can migrate from unit to unit until it finds itself at the seat of reaction. 

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