Electrooxidation electrolysis

Other recovery methods have been used (10). These include leaching ores and concentrates using sodium sulfide [1313-82-2] and sodium hydroxide [1310-73-2] and subsequentiy precipitating with aluminum [7429-90-3], or by electrolysis (11). In another process, the mercury in the ore is dissolved by a sodium hypochlorite [7681-52-9] solution, the mercury-laden solution is then passed through activated carbon [7440-44-0] to absorb the mercury, and the activated carbon heated to produce mercury metal. Mercury can be extracted from cinnabar by electrooxidation (12,13). 

An electrooxidation process was developed by Asahi Chemical Industry ia Japan, and was also piloted by BASF ia Germany. It produces high purity sebacic acid from readily available adipic acid. The process consists of 3 steps. Adipic acid is partially esterified to the monomethyl adipate. Electrolysis of the potassium salt of monomethyl adipate ia a mixture of methanol and water gives dimethyl sebacate. The last step is the hydrolysis of dimethyl sebacate to sebacic acid. Overall yields are reported to be about 85% (65). 

Stereoselective conversion of a thiane 57 to the corresponding trans-thiane-1-oxide 58 was achieved by bromonium ion mediated electrooxidation while a preferential formation of the cis-sulphoxide 58 was observed under acidic electrolysis (equation 38). 

Heteroatom Oxidation, Dehydrogenation Electrooxidative kinetic resolution of rac alcohols mediated with a catalytic amount of an optically active A-oxyl was performed in an undivided cell at constant current conditions. A high enantiomeric purity for the recovered alcohol was found, which could be increased by electrolysis at lower temperatures. The optically active A-oxyl was recovered and used repeatedly without change in efficiency and selectivity [368]. Cyclovoltammetry with the A-oxyl (GR, 7S, 10/f)-4-oxo-2,2,7-trimethyl-10-isopropyl-l-azaspiro[5.5]undecane-A-oxyl as catalyst showed for rac-1-phenylethanol a highly enhanced catalytic... 

Formation of the P—N bond has been observed when the cross-coupling of dialkylphosphites (59) with amines (60) proceeds by an iodo cation [I]+-promoted electrooxidation, affording N-substituted dialkylphosphor-amidates (61) (Scheme 22) [76]. Lack of alkali iodide in the electrolysis media results in the formation of only a trace of (61), indicating that the iodide plays an important role in the P—N bond-forming reaction. In contrast, usage of sodium bromide or sodium chloride brings about inferior results since the current drops to zero before the crosscoupling reaction is completed. 

Success in indirect electrooxidation with a metal redox carrier depends on the choice of a metal ion (1) that is best suited for the desired functionalization (2) that is soluble in the electrolysis media in both the high and the low oxidation states (3) that is expected to undergo electrooxidative regeneration with high current efficiency as well as to react with the substrate in a high yield (4) that can be readily separated from the products ... 

It has been known that the electrolysis in an MeCN- NaClO system generates an acid The hydrogen has to originate from the solvent. A mechanism for hydrogen abstraction from acetonitrile by the electrooxidatively generated radical 104- to produce perchloric acid has been proposed, but no evidence for the succinonitrile formation appeared (Eq. (5)). The detection of the 104- radical by the aid of HSR was tried But it was found to be difficult to differentiate between the perchlorate radical and the radical from chlorine dioxide The electrolysis in a CH Clj—... 

A synthetic method of introducing a methoxy group into the alpha position of alpha-amino acid derivatives and a/p/ia-amino-fern-lactams has been exploited by employing an indirect electrooxidation process 23). For example, the electrolysis of the lactam 33a in a MeOH—NaCl—(Pt) system yields the methoxylated lactam 34a in 92% yield. The indirect methoxylation of fern-lactams proreeds successfully without cleavage of the azetidinone ring (Scheme 2-11). 

Debenzylation of Ar-benzyl-6e/a-lactams 41 has been achieved by electrooxidative methoxylation of 41 at the benzylic position followed by hydrolysis with p-toluene-sulfonic acid in acetone 27>. For example, the electrolysis of A-benzyl-3-methylene-6e/fl-lactam 41 (R = OMe) in an MeOH E NClC —fPt) system in an undivided cell forms iV-methoxybenzyl-3-methylene-6efa-lactam 42 (R = MeO) in 54% yield (Scheme 2-14). The debenzylation of 42 is carried out on treatment with p-toluene-sulfonic acid in aqueous acetone to give 3-methylene-6e/a-lactam 43 in 50% yield. 

The electrooxidation of alpha- and hefa-pinenes in an AcOH—Et4NOTs—(C) system affords ring-opened products53 . For example, the electrolysis of alpha-pinene 17 gives carveols 18 and p-menth-6-ene-2,8-diol derivatives 19 (Scheme 3-6). 

The electrooxidation of the enol acetate 23 of isopinocamphone generally leads to three different types of products 7-carvone 7, 8-acetoxy-p-menth-6-en-2-one 24, and 2-acetoxy-2,6,6-trimethylbicyclo[3.1.1]heptan-3-one. The product distribution is largely dependent on the combination of solvent and electrolyte. It is found that the enol acetate 23 leads to /-carvone 7 selectively, if electrolysis is performed in a CH2C12/ AcOH(8/l)—Et4NOTs—(C) system (Scheme 3-8)55>. 

Electrooxidative ring opening of polycyclic terpenoids 25 has been investigated in an AcOH—EtjN—(C) system in an undivided cell. The electrolysis of tricyclene 25a at 15 °C yields exo-2,2-dimethyl-3-methylenebicyclo[2.2.1]heptan-5-ol 26a, Nojigiku alcohol, in 76% yield (Scheme 3-9)S6). The results reported on the electrooxidative cleavage of terpenoids are summarized in Table 3.2. 

Electrooxidation of n/p/ia-dihydroionol in an MeOH—LiCIO —(C) system at 1.25 V (SCE) can lead to the corresponding methoxylated products in 57% yield, whereas the electrolysis in an MeCN-l,6-Lutidine—(Pt) system at 1.6 V (Ag/Ag+) resulted in edulan derivatives in ca. 23% yields. The electrolysis of ieto-dihydroionol in either an MeOH—LiC104—(C) or an MeCN—H20—(Pt) system gives spiro compounds including 6-methoxy- and 6-hydroxydihydrotheaspiranes and theaSpirane in 21-23% yields77). 

The characteristics of the electrooxidation of fluorosulfate anions in the electrolysis of a potassium fluorosulfate solution in fluorosulfonic acid have been investigated. The formation of oxide layers on platinum and the modification of glassy carbon with fluorosulfate groups during anodic polarization in fluorosulfonic acid are studied. The reactions of fiuoroolefin fluorosulfation are considered and a mechanism is suggested223. Trifluoromethylation of carbonyl compounds can be achieved using bromo-trifluoromethane and a sacrificial electrode in solvents such as DMF/pyridine, and DMF/TMEDA, as seen in equation 126224. 

Table 2.4 Results of electrooxidation of a dyeing bath obtained after 40 min of electrolysis applying a current of 2 A dm 2 using various anodes (Szpyrkowicz et al. 2000)...

Electrolysis — (Greek . ..lysis - splitting) Decomposition of a material by application of an electrical voltage resulting in a flow of electric current associated with electroreduction at a cathode and electrooxidation at an anode. 

The formation of grafted films on carbon electrode surfaces is attained by electrooxidation of arylacetates under Kolbe electrolysis conditions [64]. 

Fig. 14.23 Microtiter plate anodic electrooxidation from lb to form 3ba in the presence of CH3OH, c(lb) = 4mM, c(lu) = 50mM, c(CH3OH) = 2M, electrolysis potential E = +0.4 V vs. id fc+ (a) steady-state microdisk electrode (d = 25 pm) cyclic voltammetry during electrolysis, v = 0.02 V s 1, times after start of electrolysis indicated, (b) current development during electrolysis, (c) steady-state voltammograms before (1) and after 900 s of electrolysis with (3) and without (2) mixing by convection. (Figure reprinted from Markle et al.72). Copyright Elsevier Ltd. (2005)...

Empirical rules have been elaborated to account for competition between these pathways, depending on electrolysis conditions [180], The Coventry group chose to examine a system almost at balance where both pathways operate [183] in order to best identify any sonoelectrochemical effect on mechanism [184], Table 4 shows product ratios (by glc) from the electrooxidation of partially neutralized cyclohex-anecarboxylate in methanol at platinum, at a current density of 200 mAmp cm-2. The first column shows a substantial amount (49%) of the dimer bicyclohexyl from the one-electron pathway, together with cyclohexylmethylether, cyclohexanol, and other products from the two-electron pathway (totaling -30%). The methyl cyclo-hexanoate ester (17%) is considered to arise from acid-catalyzed chemical esterification of the starting material with methanol solvent, due to the quantity of protons produced around the anode since at the high current densities needed, the parasitic... 

The Coventry group has also examined the behavior ofp-chlorophenyl acetate electrooxidation under ultrasound [187]. This substrate is known to markedly favor the two-electron mechanism [180], showing that the choice of reaction pathway is more dependent on substrate nature than upon manipulation of electrolysis parameters. A further feature of this system is the appreciable yield of p-chlorobenzalde-hyde-derived products. This is shown in Table 6 where it can be seen that sonication produces little change in relative product ratio, although there is an increase in total yield after ether extraction. Thus 46% by weight of mixed product is obtained (unreacted acid is not recovered by this procedure) with ultrasound, but only 23% by weight from the silent reaction. This represents increased reaction efficiency since the same quantity of charge was passed in each case. 

It may be that ultrasonic enhancement of mass transport sweeps the intermediate radicals that have escaped the electrode back to the electrode surface where they are further oxidized, although this would depend upon radical lifetimes. Direct enhancement of the second electron transfer to give the cation while species remain in first contact with the electrode would be complicated by the decarboxylation step after the first electron transfer, and also by an observation of weak electrochemiluminescence (ECL) from the electrolysis cell in phenylacetate electrooxidation (see Section 5.2) [215]. This is enhanced by ultrasound, as are a number of other ECL systems. This suggests that at least a proportion of the reaction pathway involves benzyl radicals which escape the electrode, although the sonoelectrochemilumines-cence reaction conditions of low carboxylate concentration, low current density, and presence of electrolyte salt are different to those for the preparative electrolyses. 

---