Vicinal diacids, decarboxylation

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

With vicinal diacids, the occurrence of mono- or bis-decarboxylation could be controlled by light flux. At low light flux, the primary product is monodecarboxylation, as is consistent with ideas described earlier for photoelectrochemical current control, Eq. (28)... 

A case in point involves the electrochemical oxidation of vicinal diacids. A standard synthetic method for the preparation of carbon-carbon double bonds occurs by the bis-decarboxylation of such diacids. Even relatively strained, synthetically inaccessible double bonds have been introduced in this way, e.g., eqn 6. 

This current-control feature can be illustrated in the electrochemical decarboxylation of vicinal diacids. These reagents have long been used as protecting groups for double bonds, since electrochemical deprotection by two-electron oxidation causes bis-decarboxylation and production of a C=C double bond [60]. In contrast, when platinized titanium dioxide is irradiated in the presence of one such vicinal diacid (cyclohexenedicarboxylic acid), the major reaction pathway leads to monodecarboxylation, rather than to benzene formation (Eq. 12). 

As we saw earlier [60, 145] with vicinal diacids, mono- or bis-decarboxylation occur, depending on incident light flux (Eq. 12). At low light flux, the primary product is that of monodecarboxylation. Multiple carboxylic acids such as ethylene diaminetetracarboxylic acid also suffer decarboxylation under photocatalytic oxidation conditions [146]. 

Decarboxylation.—Direct decarboxylation of hindered geminal diesters has been achieved using amine bases (DBN Dabco). This attractive one step process generally gives good yields and avoids the use of both acidic and aqueous conditions. An improved method for the oxidative decarboxylation of vicinal diacids to olefins employs copper(i) oxide and 2,2 -dipyridyl in quinoline temperatures of 180 °C are however required. Excellent yields have been achieved in the decarboxylation of bridgehead carboxylic acids by sequential bromination and photochemical reduction in the presence of tri-n-butyltin hydride. 

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