Thermolysis Homolysis

There are some general features of a free radical reaction. Free radical reactions take three distinct, identifiable steps. The first is formation of the free radical that can happen by enzyme catalysis, homolysis, thermolysis, radiation, light induction, combustion and pyrolysis, or other means. The second step, called propagation, is the heart of a free radical reaction. In this step, free radicals are repeatedly regenerated and can react with neutral molecules to produce new free radicals. If there is no intervention, two free radicals can react to form a neutral molecule and the reaction is terminated, which represents the third step in the general reaction scheme. Because of this repetitive nature of the reaction, free radical reactions are called chain reactions and are often represented as a cyclic process [Nagendrappa (27A78)]. 

Oxirane on thermolysis or photolysis suffers C—O homolysis to give a plethora of products (Scheme 2). Substituted oxiranes behave similarly on thermolysis although some C—C cleavage is observed (Scheme 3). Cyclopentene and cyclohexene oxides undergo only C—O cleavage (Scheme 4). 

Bis(aryl)cobalt(II) compounds have been prepared by reaction of R MgX (where R = C6H6 Cl n = 2-4) with Co(PR3)2Cl2.203 They undergo both thermally and oxidatively induced decomposition, with the corresponding biphenyl a product. The reactions of alkyl-cobalt complexes have been reviewed recently, and include thermolysis, photolysis, oxidation, and reduction reactions.25 Homolysis of the Co—C bond is a feature of reactions. 

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The most characteristic reactions for these compounds are thermolysis and decomposition under the action of both acids and nucleophilic reagents. 1,2,4-Dioxazolidines 15 are thermally stable only below 100 °C. The thermolysis of compound 15a in solution at 130 °C afforded benzophenone and benzamide in quantitative yields. The formation of these products is consistent with a radical mechanism involving initial 0-0 homolysis (Scheme 21). The base-catalyzed decomposition of compound 15a afforded the same products. Similar results were observed for the decomposition of compounds 15b and 15c. The interaction of dioxazolidine 15a with triphenylphosphine resulted in deoxygenation of the system with the concomitant formation of benzophenone and its imine with aniline <1995J(P1)41>. 

The contrast between thermal and photochemical reactions of vinyl cyclobutane-dione 36 is of interest60>. Photolysis of 36 proceeded quantitatively with bisdecarbon-ylation to tetrachlorodiene 37 while thermolysis (70°) of 36 resulted in quantitative isomerization to isomeric diketone 39 (the photochemical precursor of 36). Similar effects have been observed with related compounds. The most reasonable mechanism for isomerization 36 - 39 is homolysis to biradical 38 which can either revert to 36 or... 

Hay, B. P., and Finke, R. G., 1987, Thermolysis of the C06C bond in adenosylcorrins. 3. quantification of the axial base effect in adenosylcobalamin by the synthesis and thermolysis of axial base-free adenosylcobinamideoinsights into the energetics of enzyme-assisted cobalt carbon bond homolysis. J. Am. Chem. Soc. 109 801298018. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

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