Dicarboxylic acids, by oxidation

Oxidation of methylpyridines in 60-80 % sulphuric acid at a lead dioxide anode leads to the pyridinecarboxylic acid [213]. The sulphuric acid concentration is critical and little of the product is formed in dilute sulphuric acid [214]. In these reactions, electron loss from the n-system is driven by concerted cleavage of a carbon-hydrogen bond in the methyl substituent. This leaves a pyridylmethyl radical, which is then further oxidised to the acid, fhe procedure is run on a technical scale in a divided cell to give the pyridinecarboxylic acid in 80 % yields [215]. Oxida-tionof quinoline under the same conditions leads to pyridine-2,3-dicarboxylic acid [214, 216]. 3-HaIoquino ines afford the 5-halopyridine-2,3-dicarboxylic acid [217]. Quinoxaline is converted to pyrazine-2,3-dicarboxylic acid by oxidation at a copper anode in aqueous sodium hydroxide containing potassium permanganate [218]. 

Dicarboxylic Acid by Oxidative Cleavage of Keto-Fatty Acids. .. 85... 

A high content of dicarboxylic acid is the most characteristic feature of the composition of aliphatic components of suberin (231). The generation of dicarboxylic acids by oxidation of endogenous cu-hydroxy fatty acids has been demonstrated in cell-free preparations from the excised epidermis of Vida faba leaves (242). This dehydrogenase activity, which showed a strong preference for NADP, was located in the 100000-g supernatant. Modification of the substrate, o>-hydroxyhex-adecanoic acid, by removal or esterification of the carboxyl or incorporation of a hydroxyl moiety at C-10, rendered it a poor substrate. cu-Oxohexadecanoic acid could be trapped by dinitrophenylhydrazine, indicating that the oj-oxo acid was probably an intermediate in the reaction. Additionally, synthetic co-oxohex-adecanoic acid was converted to Cjg dicarboxylic acid by the enzyme preparation. 

Cautious oxidation dehydrogenates the hemiacetal to form the lactone of an acid (in alkaline solution, the salt of that acid). The lactone may be reduced again to reform the hemiacetal. More vigorous oxidation forms the dicarboxylic acid by oxidizing the terminal CH2OH. 

Warwel, S., Sojka, M., and Rusch, M. Synthesis of Dicarboxylic Acids by Transition-Metal Catalyzed Oxidative Cleavage of Terminal-Unsaturated Fatty Acids. 164, 79-98 (1993). Wexle.r, D., Zink, J. I., and Reber, C. Spectroscopic Manifestations of Potential Surface Coupling Along Normal Coordinates in Transition Metal Complexes. 171,173-204 (1994). Willett, P., see Artymiuk, P. J. 174, 73-104 (1995). 

Warwel, S., Sojka, M., and Rusch, M. Synthesis of Dicarboxylic Acids by Transition-Metal Catalyzed Oxidative Cleavage of Terminal-Unsaturated Fatty Acids. 164, 79-98 (1993). 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

Clearly such a method is of limited preparative value, but an important exception is the oxidation of cyclic secondary alcohols which on oxidation with nitric acid give good yields of dicarboxylic acids by way of the intermediate cyclic ketone, e.g. adipic acid from cyclohexanone, Expt 5.123. 

Decarboxylation can be achieved by heating the acids above their melting points, often in the presence of a copper-chromium oxide catalyst, and it is possible to remove one group at a time from imidazole-4,5-dicarboxylic acid by heating the monoanilide. Studies of the kinetics of decarboxylation of 5-aminoimidazole-4-carboxylic acids implicate a first-order decarboxylation of both the acid and the zwitterion the anion is stable and not... 

Tetramethyl-3-thietanone 1-oxide 400 is obtained in 69% yield by oxidation of the thietane with peracetic acid in benzene. Amides of 3-thietanone-2,4-dicarboxylic acid 1-oxide are said to be obtained by treatment of amides of acetone dicarboxylic acid with thionyl chloride and 3-thietanone-l-oxides may be intermediate in the reaction of ketones with thionyl chloride. The -oxide of 3,3,4,4-tetraphenyl-2-thietanone may be an intermediate in the oxidation of the thietanone by nz-chloroperbenzoic acid, which is discussed in Section XIII.4.E. ... 

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