Dimerization preparative scale electrolysis

Although separate determination of the kinetic and thermodynamic parameters of electron transfer to transient radicals is certainly important from a fundamental point of view, the cyclic voltammetric determination of the reduction potentials and dimerization parameters may be useful to devise preparative-scale strategies. In preparative-scale electrolysis (Section 2.3) these parameters are the same as in cyclic voltammetry after replacement in equations (2.39) and (2.40) of Fv/IZT by D/52. For example, a diffusion layer thickness S = 5 x 10-2 cm is equivalent to v = 0.01 V/s. The parameters thus adapted, with no necessity of separating the kinetic and thermodynamic parameters of electron transfer, may thus be used to defined optimized preparative-scale strategies according to the principles defined and illustrated in Section 2.4. 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

The quantitative formation of PhjAsOH in a two-electron oxidation is in accord with reactions 94-97, when it is assumed that on the time scale of preparative electrolysis equilibrium 97 is pulled to the left. The oxidation of Ph3Sb probably follows the same mechanism, but the initially formed PhjSbOH" dimerizes and eliminates a molecule of water" (reaction 98). 

Reduction of 9-substituted anthracenes, (91), leads to radical anions, which, because of the electron-withdrawing substituents, are quite stable with respect to protonation and cleavage in aprotic solvents. In polar aprotic solvents the radical anions exclusively dimerize, and the reaction has been the subject of a number of studies [247-258]. The products are the tail-to-tail dimeric dianions as in Eq. (57), which are fairly stable. In CV the dimer dianions can be detected as a new oxidation peak on the reverse scan at a potential several hundred millivolts anodic relative to the potential of radical anion formation. On preparative or semipreparative scales the dimer dianion has in a single case been detected by H-NMR [249], and oxidative electrolysis of the dimer dianions in most cases restores the starting material. 

The dimerization of half esters to l,n-diesters is also called Brown-Walker electrolysis. Thereby valuable intermediates for the synthesis of medium-sized rings or l,n-difunctionalized compounds can be prepared (Table 2, entry 4). This reaction is also of industrial interest since in this way sebacic acid can be prepared from adipic acid half ester. This process has been scaled-up in Germany, the USSR and Japan, and yields as high as 93% have been reported. Reaction conditions and yields for the coupling of other half esters have also been studied in detail. 1, -Polyethylene- or polydifluoromethylene-dicar-boxylic acids are reported to be formed by electrolysis of azelaic acid or perfluoroglutaric acid. Ketocarboxylic acids can be coupled to 1,4-, 1,6- or 1,14-diketones (Table 2, entries 9 and 10). Aldehydes must be dimerized in the form of their acetals to obtain good yields, as has been shown for (17) and (18). The arrow on (10)-(18) indicates the location of dimerization, along with the yield and reaction conditions. 

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