Electrooxidative Syntheses

Prior to our own studies in the late 1980 s, the possibility that ultrasound could be deliberately employed to alter the course of a reaction had not really been addressed. It had simply been used to improve reactivity. 

We chose to examine a system where both pathways operate [56] in order to best identify any sonoelectrochemical effect on mechanism [57]. Tab. 6.13 shows product ratios from the electrooxidation of partially-neutralised cydohexanecarboxylate in methanol at platinum, at a current density of 200 mA cm in the presence and absence of ultrasound. 

In the absence of ultrasound, the results show a substantial amount (49 %) of the dimer bicyclohexyl from the one-electron pathway, together with cyclohexylmethyl-ether, cyclohexanol and other products from the two-electron pathway (approx. 30%). The methyl cyclohexanoate ester (17%) can be thought to arise from the acid catalysed chemical esterification of the starting material with the solvent methanol. (As a result of the high current densities needed, (parasitic) discharge of the solvent methanol produces a large quantity of protons around the anode as a competitive reaction [54].) 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane ( 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts 

We also found that in the presence of ultrasound that not only was there a drop in overall cell voltage from 8.3 V to 7.3 V but the reaction approached completion in a shorter timescale despite the apparent switch to the two-electron process. Overall, ultrasound appeared to favour the two-electron mechanism, and the greatest effect of sonication upon product distribution was the substantial enhancement of alkene formation. 

The general mechanism breaks down into a pathway involving one electron per molecule of starting material, giving products from the radical intermediate, e.g., the dimer [R-R] (the actual Kolbe reaction), and a two-electron pathway per 

50 Matschiner, H. Wildner, K. Thiele, W. German Patent DE 4430391 1994 Chem. Abstr. 1996, 124,21091Z. 

51 Gindre, M. Josiane, E. Trench Patent 2724163 1994 Chem. Abstr. 1996,224, 32489j. 

53 Vassiliev, Y.B. Grinberg, V.A. /. Electroanal. Chem. 1991,308,1-16, and papers cited. 

Other procedural benefits in the sonoelectrooxidation of cyclohexanecarboxylate include a drop in overall cell voltage from 8.3 V to 7.3 V needed to maintain the 

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In this section we will focus on electroreductive and electrooxidative synthesis and touch briefly on the electrosynthesis of selected organometallics and electroinitiated chain polymerisation, previously introduced in Chapter 5. 

Among the efforts undertaken to realize original electrooxidative synthesis of ECPs, the attempts to structure the solution environment of the monomer should be recalled. An obvious way, which was... 

Falck JR, Miller LL, Stermitz FR (1974) Electrooxidative synthesis of morphinandienene from 1-benzyltetrahydroisoquinolines. Tetraheefaen 30 931-934... 

Menke EJ, Brown MA, Li Q, Hemminger JC, Penner RM (2006) Bismuth telluride (Bi2Te3) nanowires Synthesis by cyclic electrodeposition/stripping, thinning by electrooxidation, and electrical power generation. Langmuir 22 10564-10574... 

Oiganic synthesis 30 [OS 30] Electrooxidative C-C bond formation of carbamates... 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

Studies on the electrochemical oxidation of silyl-substituted ethers have uncovered a rich variety of synthetic application in recent years. Since acetals, the products of the anodic oxidation in the presence of alcohols, are readily hydrolyzed to carbonyl compounds, silyl-substituted ethers can be utilized as efficient precursors of carbonyl compounds. If we consider the synthetic application of the electrooxidation of silyl-substituted ethers, the first question which must be solved is how we synthesize ethers having a silyl group at the carbon adjacent to the oxygen. We can consider either the formation of the C-C bond (Scheme 15a) or the formation of the C-O bond (Scheme 15b). The formation of the C Si bond is also effective, but this method does not seem to be useful from a view point of organic synthesis because the required starting materials are carbonyl compounds. 

Synthesis of Poly(phenylene Oxides) by Electrooxidative Polymerization of Phenols... 

Another useful route to alkaloids involves the electrochemical oxidation of lactams (145) bearing functionality on nitrogen that can be used to intramolec-ularly capture an intermediate acyl im-minium ion (146). The concept is portrayed in Scheme 33 and is highlighted by the synthesis of alkaloids lupinine (150) and epilupinine (151) shown in Scheme 34 [60]. Thus, the electrooxidation of lactam (147) provided a 71% yield of ether (148). Subsequent treatment with titanium tetrachloride affected cyclization and afforded the [4.4.0] bicyclic adduct (149). Krapcho decarbomethoxylation followed by hydride reduction of both the... 

Yoshida J, Takada K, Ishichi Y, Isoe S (1995) Electrooxidative cyclization using group 14 metals. In Torii, S (ed) Novel trends in electroorganic synthesis, Kodansha, Tokyo, pg 295... 

The EC technique is a general and versatile synthesis method for the preparation of high-quality single crystals involving molecular ions (Batail et al, 1998). The method requires electroactive species, neutral or charged, leading their electrooxidation (or reduction) to stable radicals. If soluble, the generated radical species may diffuse into solution, but under suitable conditions of concentration, solvent, temperature and current density, they will crystallize on the electrode. The choice... 

The electrooxidation procedure has been used in the preparation of a key intermediate for the fi /-helminthosporal synthesis 93). Aromatic steroids have been functionalized by means of anodic substitution introducing nucleophiles in the benzylic position 94a) or at the aromatic nucleus 94b). 

Further investigations on the field of oxidative bond cleavage even made single bonds accessible. Thus, biaryls 10 and 11 were similarly obtained by electrooxidation of 9,10-dihydrophenanthrene. Moreover, the cleaving reaction of benzylic carbons was also exploited in the synthesis p-tert-butylbenzaldehyde dimethyl acetale (3) starting from l,2-di-(p-ferf-butylphenyl)ethane (4, 1,2-DPTE) (Fig. 5.8) (Zollinger et al. 2004b). 

The same combination of supercritical fluid extraction (SFE) and mediated electrooxidation (MEO) has an application potential for synthesis of organic compounds. In an industrial scale one limiting factor for the efficiency of the production of organic compounds is the amount of by-products. With the novel integrated method the selectivity of the desired reaction can be influenced and so the space-time-yield enhanced. 

In our institute a novel method is under development to integrate mediated electrooxidation and supercritical fluid extraction for synthesis of organic compounds. Figure 2 shows the principle of this combination... 

A novel combination of two non-polluting engineering steps has been demonstrated mediated electrooxidation (MEO) and supercritical fluid extraction (SFE). The combination has an application potential for waste destruction and organic synthesis. A small apparatus has been constructed for each application. 

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