Mechanism of thermal rearrangement

Because citric acid is considered as relatively cheap and abundant material, it was catalytically dehydrated to aconitic acid in the 120-150 °C temperature range by Umbdenstock and Bruin [61]. Aconitic acid can be readily decaiboxylated to a mixture of isomeric itaconic acids (itaconic, citraconic and mesaconic acids). These acids and their esters are nsed to produce alkyl resins and plasticizers. The mechanism of thermal rearrangement of citraconic acid to itaconic acid in aqueous solution was in a great detail investigated by Sakai [62]. In some cases, the applied catalyst caused excessive pyrolysis of citric acid and in the dehydration and decarboxylation reactions acetone dicarboxylic acid (P-ketoglutaric acid) was initially formed and from it acetone. The catalytic pyrolysis of citric acid monohydrate heated up to 140 °C to obtain itaconic and citraconic acids was reported by Askew and Tawn [63],... 

Chambers RD, Musgrave WKR, Sargent CR (1981) Polyhalogenoheterocychc compounds. Part 33. Mechanism of thermal rearrangements of perfluoropyridazine and perfluoroalkylpyridazines. J Chem Soc Perkin Trans 1 1071-1077... 

Figure 4.12 Mechanism of thermal rearrangement (a) hydroxyl-containing poly-imide and (b) thermally rearranged polybenzoxazole (TR-PBO).

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

In a related rhodium- or palladium-catalyzed rearrangement of 7-oxaquadricyclane 1, which follows a different mechanism, a third type of regioisomer 2 was formed,52,64 which was different from the product of thermal rearrangement 3.49 51... 

The (/ -complexes all undergo thermal rearrangement to /2, /4-bicyclo[4.2.1]nona-2,4,7-triene tricarbonyl complexes. The possible mechanism of the rearrangement of the molybdenum complex has been thoroughly investigated by deuterium labeling and kinetic studies228. 

The formation of anils 129 occurs in accordance with the generally accepted mechanism for thermal rearrangements of aliphatic azides to imines which involves nitrenes 127 as the intermediate (74JA480). Ni-trenes may undergo cyclization to aziridines 105 [cf. Section I1I,C,1 (Scheme 6) and Section lII,C,4,b(ii)] which are converted into ben-... 

The polymers of rubber plastics have unsaturated hydrocarbon chain structure, since they are polymerized from alkadienes. The general formula of poly(l,3-butadiene) or butadiene rubber (BR) and polyisoprene or natural rubber (NR) is drawn in Scheme 12.5, where X is hydrogen in BR and methyl group in synthetic polyisoprene or NR. The free radical mechanism of thermal decomposition starts by homolytic scission of the alkyl C-C bonds. Two primary macroradicals (4 and 5) are formed for which the rearrangement... 

The reaction behaviour for the thermal decomposition of the chlorosulphites in solution parallels that of the chloroformates very closely. Thus, Boozer and Lewis " found that when 2-pentyl chlorosulphite is thermally decomposed in dioxane solution at 62 °C both 2-chloropentane and 2-pentene are formed. The relative amounts of the substitution and elimination reactions in this instance are 51 % and 49 % respectively. With allylic chlorosulphites, substitution is accompanied by rearrangement . Boozer and Lewis - have carried out configurational checks on the mechanism of thermal decomposition in solution and deuterium isotope studies have also been made . As with the chloroformates, the experimental evidence points to transition states of a polar nature . Gas phase decompositions of chlorosulphites have been studied by Rieche and Nau-mann and Geiseler et a/. - and radical-chain mechanisms have been proposed . 

The rate of thermal rearrangement of 3-aryl-2,2-dimethylmethylene-cyclopropanes has been studied as a function of aryl substiments. Some of the data are given below. Examine the rate data for correlation with the Hammett a-substituent constants. What conclusion do you draw about the mechanism ... 

The mechanism of thermal degradation of plastics proceeds through a radical chain reaction pathway with hydrogen transfer steps. In secondary reactions, branched products were only formed as a result of the interaction between two radicals without any rearrangement reactions [48]. As a consequence, thermal cracking of polyolefins leads toward a broad distribution of hydrocarbons up to waxy products. More than 500 °C temperatures are needed to receive more oily products. In contrast, catalytic cracking takes place at lower temperatures and leads to the formation of smaller branched hydrocarbons. This catalytic cracking can potentially lower the costs and increase the yields of valuable products. 

Unsaturated polymers, particularly of dienic monomers, undergo a number of interesting thermal rearrangements under non-pyrolytic conditions. Golub has studied these reactions in some detail and has recently reviewed the subject. In the past two years accounts of thermal rearrangements of l,2-poly(hexa-l,4-diene)s and l,2-poly(/ra 5-penta-l,3-diene) have appeared. The former polymers have a predominantly 1,8-diene structure and cyclize mainly by a [2 + 2] or type II mechanism accompanied by a small amount of a type III reaction (Scheme 24). The latter is more important in the trans-, A- than in the cir-1,4-isomer. The first unambiguous example of a type III reaction was provided by the polymer of penta-1,3-diene. Scheme 25 shows a macromolecular... 

Oishi and his co-workers have reported that aromatic thiono-esters (408), in the presence of boron trifluoride ethyl-etherate, rearrange to the corresponding thiolo-esters (409). The disappearance of the thiono-ester took place in accordance with first-order kinetics, and electron-donating para-substituents were found to accelerate the conversion, which was characterized by the Hammett p value of -1.38. The mechanism of the rearrangement was considered to involve the intermediate (410), in agreement with the results of cross-over experiments. The thermal rearrangement of thioaroyl thiocyanates (411) to thioaroyl isothiocyanates (412) was also found to be influenced by substituents. Thiono-ester-thiolo-ester... 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

Another example of a reaction in which the stereochemistry of the process provides some valuable information about the mechanism is the thermal rearrangement of 1,5-dienes and substituted analogs ... 

Deuterium-labeling and mass spectrometry prove that the mechanism of the thermal O to N rearrangement of 4-alkoxypyridines to N-alkyl-4-pyridones is intermolecular (88CS347). 

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