Carboxylic esters, thermal decomposition

As in the pyrolysis of carboxylic esters, thermal decomposition of xanthic esters28,29 (the Tschugaeff reaction) provides unsaturated compounds free from impurities that would be due to isomerization ... 

The selection of this reagent was based partly on the observation [209] that, because various quaternary ammonium carboxylates undergo thermal decomposition to methyl esters, such esters could be formed in situ in good yield by the simple injection of a methanolic solution of the quaternary ammonium carboxylate above 250°C. Thus, the use of the quaternary ammonium hydroxide in the transesterification scheme described above permits the simultaneous conversion of both glycerides and fatty acids to methyl esters. 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

DS Tarbell, EJ Longosz. Thermal decomposition of mixed carboxylic-carbonic anhydrides. Factors affecting ester formation. J Org Chem 24, 774, 1959. 

Bromine-atom atomic resonance absorption spectrometry (ARAS) has been applied to measure the thermal decomposition rate constants of CF3Br in Kr over the temperature range 1222-1624 K. The results were found to be consistent with recently published theory. The formation of cyclopent[a]indene and acenaphthylene from alkyl esters of biphenyl-mono- and -di-carboxylic acids has been observed in flash vacuum pyrolyses at 1000-1100 °C. The kinetics and mechanisms of free-radical generation in the ternary system containing styrene epoxide, / -TsOH, and i-PrOH have been examined in both the presence and absence of O2. ... 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

The inclusion of heat stabilizers is essential to protect the system against thermal decomposition at elevated temperatures during processing. For this purpose, tin carboxylate esters or liquid calcium-zinc stabilizers are preferred. Thio-tin compounds are very effective as heat stabilizers but must be regarded with caution, bearing in mind that they can lead to unpleasant and unacceptable residual odours. Secondary stabilizers that can be used include epox-idized soya bean oil. 

Free amino carboxylic acids and their ester derivatives can be added to fluoroalkylatcd alkynes to give amino acid derivatives. Fluoroalkynes and sodium azide afford, after thermal decomposition, the corresponding aziridincs. ... 

Schmitt et worked out an interesting procedure, applicable on a large scale, for introducing alkylaminoalkyl residues in position 10, via the esters of phenothiazine-lO-carboxylic acid (136). On thermal decomposition, the latter eliminate CO2 and give the deri-... 

Allyldiethylamine behaves similarly, but the yields are low since neither the starting amine nor the products are stable to the reaction conditions. For the efficiency of the cyclopropanation of the allylic systems under discussion, a comparison can be made between the triplet-sensitized photochemical reaction and the process carried out in the presence of copper or rhodium catalysts whereas with allyl halides and allyl ethers, the transition metal catalyzed reaction often produces higher yields (especially if tetraacetatodirhodium is used), the photochemical variant is the method of choice for allyl sulfides. The catalysts react with allyl sulfides (and with allyl selenides and allylamines, for that matter) exclusively via the ylide pathway (see Section 1.2.1.2.4.2.6.3.3. and Houben-Weyl, Vol. E19b, pll30). It should also be noted that the purely thermal decomposition of dimethyl diazomalonate in allyl sulfides produces no cyclopropane, but only the ylide-derived product in high yield.Very few cyclopropanes have been synthesized by photolysis of other diazocarbonyl compounds than a-diazo esters and a-diazo ketones, although this should not be impossible in several cases (e.g. a-diazo aldehydes, a-diazocarboxamides). Irradiation of a-diazo-a-(4-nitrophenyl)acetic acid in a mixture of 2-methylbut-2-ene and methanol gave mainly l-(4-nitrophenyl)-2,2,3-trimethylcyclo-propane-1-carboxylic acid (19, 71%) in addition to some O-H insertion product (10%). ... 

A cyclopropyl to allyl radical rearrangement could also be induced in 20 % yield by the thermal decomposition (in carbon tetrachloride at 32 °C with sonication) of the A-hy-droxypyridine-2-thione ester of l-fluoro-2,2-diphenylcyclopropane-l-carboxylic acid (Barton decarboxylation reaction). ... 

Thermal decomposition of the monocarboxylate esters (79) provides a new route145 for the reduction of carboxylic acids to aldehydes (Scheme 16). 

The reaction is particularly useful for the conversion of sensitive alcohols to the corresponding olefins without the rearrangement of the carbon skeleton9,10 and is analogous to the thermal decomposition of carboxylic esters of alcohols and related derivatives.3... 

There is growing evidence that some decompositions of this type, those that involve decarboxylation, may not be concerted but involve discrete intermediates . For example, thermal decomposition of potassium 2-carboxyphenyl p-toluenesulfonate 63 at 170 °C occurs without weight loss carboxylate bands disappear and ester bands are formed, and di-, tri- and polysalicylides 65 are formed. At higher temperatures (300 C), however, products from 1 (for example, cycloadducts with tetraphenylcyclone present as a trap, or products derived from nucleophilic attack) are formed in low to modest yield. These results require a C6H4CO2 intermediate, most likely benzoxet-2-one 64 (or valence tautomers). Similar results were obtained with polymer-bound diaryliodonium carboxy-lates. ... 

Thermal decomposition of poly(vinyl acetate) results in a loss of acetic acid. The reaction is typical of thermal cleavages of esters. It is facilitated by formation of pseudo six-membered rings as a result of interactions between the p-hydrogens of the alcohol residues and the carboxylic groups ... 

By far, the most often used nucleophiles are malonates, which can be deproto-nated by the aUcoxide formed in the reaction of allyl carbonates or by an added base such as NaH. This standard nucleophile has been applied in all types of aUylations, and many applications are also reported in this monograph. The nucleophihc species can also be generated by 1,4-addition, for example, of alkoxides, generated from carbonates, onto alkylidenemalonates both inter- and intramolecularly [92]. The substitution products can be subjected to a thermal desalkoxycarboxylation or, after hydrolysis, decarboxylation, giving rise to carboxylic esters or acids [93]. Therefore, in combination with this decomposition, malonates can also be used as surrogates for ester enolates [94], which generally cannot be used as nucleophiles in allylations. 

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