Thermolysis of esters

The thermolysis of esters is a much used reaction in organic [47] and organometallic [48] syntheses, generally for the creation of a carbon-carbon double bond. The me- 

This example shows that the GS/MW process can be used to accomplish thermal decomposition which cannot be performed efficiently by use of MW irradiation alone, because of weak MW absorption by the starting compound (30) of probable low dielectric loss. 

The decomposition of a secondary carbamate, 1-methylbenzyl phenylcarbamate (31), was more difficult, and only 60% of styrene (33) was obtained under the same conditions (Tmax = 340 °C). Attempts to decompose a primary carbamate, 1-octyl phenylcarbamate, failed because its sublimation preceded its decomposition [15, 16]. All these reactions have been performed in an open reactor by the above procedure (Sect. 7.2.1). 

No rearrangement was observed for the pure compound 34a, adsorbed or not on KF-A1203, probably owing to its low dielectric loss. By using supports known to convert MW energy into thermal energy, the authors observed a conversion rate of 15 to 90% for 34. The best yields (30 (35e) to 90% (35c)) were obtained on graphite powder. 

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The fact that none of these reports has emphasized the physical aspects of electrophilic substitution in the series reflects the paucity of quantitative studies, and the low reactivity of these compounds in the presence of electrophiles. Few kinetic studies have been reported and the regio-chemical effects of substituents have seldom been quoted in quantitative form. The present chapter brings together those quantitative results that are available, and collates data on substituent effects. One worthwhile field of study would appear to be the application to the azines of Taylor s method involving thermolysis of esters [75JCS(P2)277, 75JCS(P2) 1783]. 

Treatment of TV.A -disubstituted thioureas with phosgene yields 4-imino-l,3-thiazetidine-2-ones (e-g-, 465 466 ). Thermolysis of esters 467 yields the thiazetidinones 468, and a 1,3-thiazetidinone 470 has been suggested as an intermediate in the photolysis of 469. l,3-Thiazetidine-2-ones are obtained by cycloaddition of phenylsulfonyl isocyanate to the thiocarbonyl groups of a thioamide. ... 

Sosnovsky has continued his study of the properties of peroxy-esters with one of the thermolysis of esters of type (68), both neat and in solution. These esters... 

The influence of an alpha methyl substituent is much less pronounced on the rate of gas-phase thermolysis of esters than of halides but it is still quite marked, suggesting a certain amount of C -0 bond heterolysis occurs in the transition state. On the other hand, beta methyl substitution has a slight retarding effect in the primary series but a slightly rate enhancing influence for secondary and tertiary acetates ". Possibly double-bond character is more developed in the transition states in the secondary and tertiary series and the mesomeric influence of beta methyl substituents stabilises the transition state. In the primary series, double-bond character is less developed and a balance between mesomeric stabilisation of this more incipient double bond and unfavourable deactivation of beta hydrogen acidity by the inductive effect of the alkyl substituent may explain its observed influence on the rate coefficient. 

That C -X heterolysis is less developed in the transition state for thermolysis of esters than alkyl halides is supported by recent studies on substituted 1-phenylethyl chlorides . At 608°K a Hammett reaction constant of—1.36 was observed, this being much smaller than that of—4.95 at 318°K observed for the solvolysis of the same substrates in 80% aqueous acetone. Although part of the difference between these latter two values is attributable to the difference in reaction temperature, the results suggest that C -X bond breaking is less developed in pyrolyses than in solvolytic reactions. 

Thermolysis of esters can be catalyzed through H+ 3. These protons may be created from several photo acid generators (PAG) through UV-irradiation (e.g. by photolysis of onium salts ). Due to the changes in solubility such systems work as negative tone photoresists. Furthermore this method can be used to build semiconducting structures in a nonconducting matrix. In this paper we present our first results of these experiments. 

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

Likewise, thermolysis of 4-azidophenyl methyl ketone in methanol yields 5-acetyl-2-methoxy-3//-azepine (60%), compared to only an 8% yield from the photolytic reaction.78 119 The thermolysis of phenyl azide in refluxing cyclohexanol yields no 3H-azepine, only diphenyl-diazene (10%) and aniline (30%).74 In contrast, thermolysis of methyl 2-azidobenzoate in cyclohexanol furnishes a mixture of methyl 2-(cyclohexyloxy)-3//-azepine-3-carboxylate (20 % bp 127°C/0.1 Torr) and methyl 2-aminobenzoate (60%). Thermolysis of the azido ester in methanol under nitrogen in an autoclave at 150 C yields a 7 10 mixture (by 1HNMR spectroscopy) of the amino ester and methyl 2-methoxy-3//-azepine-3-carboxylate, which proved to be difficult to separate, and much tar.74 The acidic medium179 is probably responsible for the failure of methyl 2-azidoberjzoate to yield a 3//-azepine when thermolyzed in 3-methoxyphenol aniline (40%) is the major product.74... 

Thermolysis of the dicarbamic acid silyl ester (CH2 N[SiMe3]C02SiMe3)2) gives the 1,3-diazetane derivative 26 <96JOM93>. Alkylation of the 1,3-dithietane tetraoxide 27 with a,co-dihaloalkanes yields the dispiro compounds 28 (n = 1-4) <95ZOR589>. The first 1,2-dithiete S-oxide 29 is reported <95TL8583>. 

The formation of cyclic sulfinic esters (sultines) from vinyl sulfenes is known , and the trapping of the expected intermediate vinyl sulfene in the thermolysis of thiete dioxide (6fc and 194) has been convincingly achieved . Specifically, thermolysis of thiete dioxide 6b in the presence of norbornenes gave cycloadducts of the Diels-Alder type (i.e. 252b), resulting from the trapping of the vinyl sulfene formed. The accumulated evidence thus supports the proposed mechanism for these thermolytic reactions. 

Mandelate and lactate esters have been found to generate diastereoselectivity in reactions of hydroxy-substituted quinodimethanes generated by thermolysis of benzo-cyclobutenols.88 The reactions are thought to proceed by an exo TS with a crucial hydrogen bond between the hydroxy group and a dienophile carbonyl. The phenyl (or methyl in the case of lactate) group promotes facial selectivity. 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

There is a parallel for oxygen abstraction by a sulphonyl radical either from nitrobenzene or by disproportionation thermolysis of benzenesulphonyldiazomethane (54) in benzene gives, among other products, some sulphonate ester (55) 67>, which has been formulated as follows ... 

Biradicals have also been encountered as intermediates in the Mg reduction of ketones to pinacols (p. 218) and, as radical anions, in the acyloin condensation of esters (p. 218). The thermolysis of cyclopropane (131) to propene (132) at 500° is also believed to involve... 

Thermolysis of 2,5-dihydro-l,3,4-thiadiazole 30 in CgDsCl solution at 20-35 °C gave spirothiiranes 43 and 44, O-hydrogen 0,0,A-ortholactone 45, the thio-A-ester 46, and 0,A,A-ortholactone 47 (Scheme 4) <1997HCA1260>. The ratio of these thermolysis products did not significantly vary between 23 and 35 °G. 

Thermal Analysis. Ito et al. have reported the effect of ester structure on ease of thermolysis and Aal-1 acidolysis of poly(p-vinylbenzoates) (//) and... 

The thermolysis of isopropylidene 2-azacycloalkylidenemalonates (468, R2 = R4 = H, n = 0-2) gave unstable ketenes (1641), which were reacted with alcohols, thiols, and protic amines to afford Z-enamino ester derivatives (1642) in 63-98% yields (81TL963). 

The 774-vinylketene complex (85) could be oxidatively decomplexed with Ce(IV) to afford the lactone (87). Although no reaction was observed with methanol (unlike a postulated chromium analogue16,18 26), treatment with sodium methoxide produced the expected /3, y-unsaturated ester (88). Thermolysis of complex 85 afforded no trace of the naphthol that one would expect33 from a proposed chromium vinylketene complex with the same syn relationship between the phenyl group and the ketene moiety. Instead, only the furan (89.a) was seen. Indeed, upon exhaustive reaction of tricarbon-ylcobalt carbenes (84 and 90) with different alkynes, the furans (89.a-d) were isolated as the exclusive products in moderate to excellent yields. 

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