Thermolysis - Thermal Conversion

Thermal conversion of diazirines to linear diazo compounds was postulated occasionally and proved by indirect methods. The existence of a diazo compound isomeric to diazirine (197) was proved spectroscopically on short thermolysis in DMSO (76JA6416). An intermediate diazoalkane was trapped by reaction with acetic acid, yielding the ester (198) (77JCS(P2)1214). 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

Thermolysis of the alkylzinc 2-(amidomethyl)pyridines induces an oxidative C—C coupling reaction at the methanide carbon atom. The products 213 ° and 214 (Figure 100) were structurally characterized by X-ray crystallography. These dimers are isostructural with 113, which was obtained from the thermal conversion of the diethylzinc complex of Af-tcrt-butyl-2-pyridylcarbaldimine. Details (Figure 52) are given above (Section in.D). 

Photolysis and thermolysis of 4-aryl-l,2,4-triazoline-3,5-diones (70, R = aryl) have been studied by Wamhoff and Wald (77CB1699). Photolysis produces the same products as were found in FVP. The thermal conversion of 70, R = Ph, into the corresponding s-triazolo[l,2-a]-s-triazole derivative, which takes place below the decomposition temperature of the educt, is assumed to proceed via a radical chain reaction (Scheme 8). 

The synthesis of a number of pyrrole derivatives has been effected by the thermal conversion of 3-vinyl-2H-aziiines to butadienylnitrenes followed by 1,5-electrocyclization (75JA4682). For example, the thermal transformations observed on thermolysis of 2H-azirine 28 were rationalized in terms of an equilibration of the 2H-azirine with a butadienylnitrene, which subsequently rearranged to the final products. The rearrangement of 28 to pyrrole 30 was envisaged as occurring by an electrocyclic reaction followed by a 1,5-sigma tropic ethoxycarbonyl shift and subsequent tautomerization... 

In recent decades much interest has been generated in the reclamation of rubber wastes, mainly used tyres, by thermal decomposition. As tyres contain significant amounts of other components, in addition to rubber, the thermal conversion of tyres is a more complex process than the thermolysis of rubber alone. 

A number of thermal and photochemical transformations of the 1,2-diazine system is noteworthy. Thermolysis isomerizes pyridazines into pyrimidines, as demonstrated for a series of perfluoro- or perfluoroalkyl-substituted derivatives (in analogy to pyridine, cf p. 363). Pyridazine itself is converted into pyrimidine at 300 °C a sequence of valence isomerizations via diazabenzvalenes (simplified as 3 and 4) is thought to be responsible for this thermal conversion. 

For example, 2-arylazirines 13 can rearrange to indoles 14 when heated in xylene (Scheme 6.6) and the direct thermal conversion of alkyl 2-azidocinnamates into indoles has been exploited as a route to indole-2-carboxylic acid esters. The temperature has to be controlled when these vinyl azides were decomposed in solution in order to isolate 2H-azirines. ° Dilute solutions are also usually preferable in order to avoid bimolecular reactions. Photolysis is sometimes preferable to thermolysis for the generation of thermally unstable azirines. ... 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

The thermolysis of cyclopropane, cyclobutane and their derivatives has received considerable attention. The thermal rearrangement of cylcopropane to propene is a clean, first-order process.79 Information concerning the course of the reaction was provided by a study of the thermal isomerization of cis- and mmr-1,2-dideuteriocyclopropane (18).80 The process occurs significantly faster than conversion to propene, suggesting a propane-1,3-diyl 19 as an intermediate. 

An example of 2,4,6-triphenylpyrylium-3-olate (65 R = R = R = Ph, R = H) reacting as a 1,3-dipole was first provided by Suld and Price who obtained a maleic anhydride adduct (C25HigO5). Subsequently, an extensive study of the cycloadditions of this species has been published by Potts, Elliott, and Sorm. With acetylenic dipolarophiles, compound 65 (R = R = R = Ph, R = H) gives 1 1 adducts that have the general structure 74 and that isomerize to 6-benzoyl-2,4-cyclohexadienones (76) upon thermolysis. This thermal rearrangement (74 -> 76) has been interpreted in terms of an intermediate ketene 75. The 2,3-double bond of adduct 74 (R = Ph) is reduced by catalytic hydrogenation. Potential synthetic value of these cycloadducts (74) is demonstrated by the conversion of compound 74 (R = Ph) to l,2,3,4,6-pentaphenylcyclohepta-I,3,5-triene (79 R= Ph) via the alcohol 78 (Scheme 1). ... 

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