Thermal and Photochemical Transformations

Interest in the mechanism and product distribution of thermal and photochemical transformations of aryl azides led to the isolation of some nitrogen-containing derivatives of heptafulvalene. Based on elemental analysis and spectroscopic data it has been suggested tentatively that the compound isolated following vapor-phase pyrolysis of azidopentafluoro-... 

Reinscheid UM, H Zuilhog, R Muller, J Vervoort (1998) Biological, thermal and photochemical transformation of 2-trifluoromethylphenol. Biodegradation 9 487-499. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

The cyclic isomers of diazoalkanes, diazirines also decompose under the influence of heat and light to give carbenes (Schmitz, 1967 Milligan et al., 1964 Mitsch, 1965 Moss, 1967). They are, however, much less sensitive to acid-catalysed decomposition than their open-chain counterparts. They should therefore represent a useful source of carbenes, since the interpretation of the results of experiments would not be complicated by the necessity of considering the involvement of carbonium ions. In some cases, however, there is evidence that thermal and photochemical transformation of diazirines into the isomeric diazoalkane takes place (Overberger and Anselme, 1963 Amrich and Bell, 1964 see also Hoffmann, 1966), though this has not been confirmed elsewhere (Moore and Pimentel, 1964 Frey and Stevens, 1965). 

Figure 17.3 Thermal and photochemical transformations of iron-sulfur-nitrosyl clusters.

The thermal and photochemical transformations of carboxylic dithiocarbonic anhydrides, e.g. 

Chong, K. C. W., Scheffer, J. R., Thermal and Photochemical Transformation of Conformational Chirality into Configurational Chirality in the Crystalline State, J. Am. Chem. Soc. 2003, 125, 4040 4041. 

Pyridines show a number of thermal and photochemical transformations which are analogous to the valence tautomerization of benzene, i.e. formation of Dewar benzene, prismane and benzvalene. 

A number of thermal and photochemical transformations of the 1,2-diazine system are noteworthy. Thermolysis isomerizes pyridazines into pyrimidines and/or pyrazines, as has been demonstrated for a series of perfluoro and perfluoroalkyl derivatives (see p 288). Pyridazine is converted into pyrimidine at 300°C. A process of valence isomerism is thought to be responsible for the thermal isomerization of pyridazines via diazabenzvalenes (simplified as 3 and 4). 

SCHEME 2.2 Thermal and photochemical transformations of isomeric 3,4-dimethyltyclobutenes. 

Encapsulated a-pyrone molecule of 1 1 hemi-carceplex of the covalent capsule 262 undergoes thermal and photochemical transformations by Scheme 5.54 within its cavity [52]. A key stage of these processes is the formation of the corresponding cage complex with encapsulated cyclo... 

Low-temperature (matrix) photolysis (and also laser flash photolysis), eombined with high-level quantum-chemical calculations, are powerful tools for investigation of reaction mechanisms. A lot of information has been obtained by this method relative to the structure and further thermal and photochemical transformations of arylnitrenes, primary products of aiylazides, their rearranged intermediates (azacycloheptatetraenes). Some recent results on photolysis of azidopyridines and related heterocyclic azides at cryogenic temperatures are discussed below. 

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