Furan electrolytic oxidation

The 2,5-dialkoxy-2,5-dihydrofurans can be obtained by electrolytic oxidation of furan in alcoholic ammonium bromide or by bromine oxidation of furan in the appropriate alcohol. ... 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

Bromine or electrolytic oxidation of furan in alcoholic solution gives the corresponding... 

Bromine or electrolytic oxidation of furan in alcoholic solution gives the corresponding 2,5-dialkoxy-2,5-dihydrofuran 159 (R = Alk). Lead tetraacetate in acetic acid oxidation yields 2,5-diacetoxy-2,5-dihydrofuran 159 (R = Ac). 

When electrolytic oxidation is carried out using the technique of Clauson-Kaas, addition does not occur exclusively in the 2,5-position, but ring cleavage of the furan can also take place. This is illustrated by the electrolytic alkoxylation described by the same authors284 ... 

Methyl esters are produced in the electrolytic oxidation of methanolic solutions of aldehydes in the presence of sodium cyanide at a platinum anode the eight examples that have been studied show yields of 38 to 80%. Carefully purified butyraldehyde reacts with RuHgfPPha) to give butyl butyrate aliphatic and aromatic aldehydes undergo this dimerization process, which may have considerable synthetic potential." The electrolysis of furan-2-carboxylic acid (66) provides an efficient synthesis of 4,4-dimethoxy-esters (67), typically in 77 % yield (Scheme 35). "... 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

The electrochemical oxidation of furans has been exploited since 1952 [170]. The usual electrolyte is ammonium bromide in methanol, at - 5 °C, using an undivided cell with either platinum or graphite as anode and a nickel or stainless steel... 

The direct electrochemical methoxylation of furan derivatives represents another technically relevant alkoxylation process. Anodic treatment of furan (14) in an undivided cell provides 2,5-dimethoxy-2,5-dihydrofuran (15). This particular product represents a twofold protected 1,4-dialdehyde and is commonly used as a C4 building block for the synthesis of N-heterocycles in life and material science. The industrial electroorganic processes employ graphite electrodes and sodium bromide which acts both as supporting electrolyte and mediator [60]. The same electrolysis of 14 can be carried out on BDD electrodes, but no mediator is required The conversion is performed with 8% furan in MeOH, 3% Bu4N+BF4, at 15 °C and 10 A/dm2. When 1.5 F/mol were applied, 15 is obtained in 75% yield with more or less quantitative current efficiency. Treatment with 2.3 F/mol is rendered by 84% chemical yield for 15 and a current efficiency of 84% [61, 62]. In contrast to the mediated process, furan is anodically oxidized in the initial step and subsequently methanol enters the scene (Scheme 7). 

Furan is, as a 7r-electron rich compound, easily oxidized electrolytically. In acetonitrile the oxidation potential of furan is comparable to that of anisole [35]. In methanol the oxidation is an ECEC mechanism in which 2,5-dihydro-2,5-dimethoxyfuran is formed [166] in the Clauson-Kaas reaction ... 

The anodic oxidation of furans in methanol was also carried out without an intentionally added electrolyte (Figure 12.11) [38, 39]. 2,5-Dimethoxy-2,5-dihydro-furan was obtained in 98% yield. The anodic methoxylation and acetoxylation of various organic compounds was also achieved using this system. 

The synthesis of tropine and of its esters has been made practical because succinic dialdehyde has become easily available. Furane, now available commercially, gives on anodic oxidation in methanol with ammonium bromide as electrolyte good yields of 2,5-dimethoxy-2,5-2H-furane (37a). This mixed ketal of maleic dialdehyde could be hydrogenated readily and quantitatively over Raney nickel to 2,5-dimethoxy-4H-furane (37b). The latter as a mixed ketal of succinic dialdehyde undergoes acid hydrolysis easily. Optimum conditions for the condensation of this dialdehyde formed in situ to tropinone have been recorded (38) with yield up to 93% and at a higher rate than described earlier (18). 

The materialization of a functional molecule, by incorporation into a conducting polymer matrix, is achieved in the electrolytic polymerization of pyrrole, thiophene, furan, aniline, etc., in the presence of negatively charged functional molecules. The incorporation of the functional molecules is driven electrostatically, by the positive charges of the partially oxidized conducting polymer matrices, through a doping process, as shown in Fig. 2. 

A soln. of diphenyl disulfide in 0.1 M tetra- -butylammonium bromide (as both mediator and electrolyte) in dichloromethane electrolysed under argon in a divided cell with platinum electrodes at a constant current of 0.02-0.03 A (current density 3 mA/cmO until 10 As of charge consumed, 0.5 eqs. 4-pentenol added, and electrolysis continued until reaction complete by g.c. or t.l.c. 2-(phenylthiomethyl)tetr hydro-furan. Y 72%. The method is inexpensive, less toxic than Se-based methodologies, and generally applicable to 5- and 6-membered O-heterocyclics reaction may also be achieved (preferably) by direct electrolysis with bis(4-methoxyphenyl) disulfide (having a much lower oxidation potential than diphenyl disulfide). F.e. and stereoselectivity, also 2-(aryIthio)lactones from ethylenecarboxylic adds, s. S. Toteberg-Kaulen, E. Steckhan, Tetrahedron 44, 4389-97 (1988). 

Oxidation of furans can be also carried out using a ceramic microflow electrochemical reactor (CEM) using H2SO4 as the supporting electrolyte [30]. Scheme 7.7 shows the oxidative methoxylation of methyl 2-furoate. 

Electrochemical oxidation of fiirans can also been carried out without intentionally added electrolyte using a microflow system. I n this case, an electrochemical thin-layer flow cell, which has a simple geometry with a glassy carbon anode and a platinum cathode directly facing each other at a distance of 80 pm apart is used (Figure 7.9) [65, 66]. 2,5-Dimethoxy-2,5-dihydrofuran is obtained in 98% yield by the oxidation of furan in methanol solvent. Similar electrochemical methoxylation and acetoxylation of various organic molecules can also be carried out using this system. 

Additives have been developed to improve the cathode cyclability performance of lithium batteries (33). Benzene derivatives (biphenyl and o-terphenyl) and heterocyclic compounds (furan, thiophene, N-methylpyrrole and 3,4-ethylenedioxythiophene), which have lower oxidation potentials than those of electrolyte solvents have been tested. The functional electrolytes used are shown in Figure 2.4. 

Oxoalkanals may be prepared via anodic oxidation of 2-substituted furans (Scheme 45). Of particular note in this efficient route is the scale of the electrolytic step, giving an isolated yield of 80% when carried out on 1.5 moles of the furan. 

A stack-type electrochemical flow microreactor having interdigitated band electrodes on a ceramic body was developed (Figure 9.2). The anodic oxidation of furans in methanol using H2SO4 as the supporting electrolyte was carried out, and the effect of the residence time was investigated by mass spectrometry analysis [13]. 

Figure 9.4 Electrochemical flow microreactor system without using intentionally added supporting electrolyte. Electrochemical oxidation of furan.

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