Furans electrochemical oxidation

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%. 

The Clauson-Kaas pyrrole synthesis was adapted to a soluble polyglycerol (PG) support <060L403>. Electrochemical oxidation of furan 33 in the presence of methanol followed by hydrogenation gave 2,5-dimethoxytetrahydrofuran 34. Cyclocondensation with primary arylamines gave A-arylpyrroles 35. Removal from the PG support was then accomplished by treatment of 35 with LiOH which gave 2-pyrrolepropanoic acids 36. 

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... 

Anodic addition to an electron-rich heteroaromatic compound is used to transform furan to 2,5-dimethoxy-2,5-dihydrofuran, a valuable synthetic intermediate. Again, an indirect electrochemical process occurs. The bromide ion as redox catalyst is electrochemically oxidized to give bromine, which then acts as chemical oxidant for furan [7] ... 

Another example of a photoredox molecular switch is based on a ferrocene-ruthenium trisbipyridyl conjugate, in which the luminescent form 4 switches to the non-luminescent form 5 upon electrochemical oxidation (Figure 2/bottom)171. Biological systems exploit the interplay of redox and molecular recognition to regulate a wide variety of processes and transformations. In an attempt to mimic such redox systems, Deans et al. have reported a three-component, two-pole molecular switch, in which noncovalent molecular recognition can be controlled electrochemically. x Willner et al. have reported on their research activities in developing novel means to achieve reversible photostimulation of the activities of biomaterials (see Chapter 6).[91 Recently, we have shown that it is possible to switch the luminescence in benzodi-furan quinone 6 electrochemically. 101 The reduction in THF of the quinone moiety... 

A laboratory synthesis for vanillin is based on this procedure. 2-Hydroxytetrahydro-furan, an intermediate for cytostatics, was produced from the corresponding carboxylic acid by electrochemical oxidation 253 ... 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

As shown below, polyphenol-based benzo[A furans were biosynthetically made from catechols and 1,3-dicarbonyl compounds in the presence of laccase <07T10958 07TL5073>. A similar type of benzo[A]furan was also obtained via electrochemical oxidation of catechols and methyl acetoacetate <07T3894>. 

Annulation of furans via electrochemical oxidation at the anode has become an important process for the synthesis of complex polycycles, and was covered in a review <2000T9527>. Furans tethered at the 3-position to electron-rich alkenes, enol ethers, or vinyl sulfides were converted to [6,5] and [7,5]-fused ring systems <1996JOC1578, 2002OL3763, 2004JOG3726, 2005JA8034>, as illustrated in Scheme 20. Analysis of crude reaction mixtures and side... 

Otherwise, lithium amides of secondary amines undergo anodic dimerization to form hydrazines in moderate yields [12]. Hydrazines are also generated, if secondary amines are electrochemically oxidized in the presence of an alkali hydroxide [27,28]. This reaction is mainly effective if the coupling takes place intramolecularly to give cyclic hydrazine derivatives [28]. If lithium amides of secondary amines are anodically oxidized in tetrahydro-furan (THF) solution in the presence of the free amine, 2-aminotetrahydrofurans are formed in reasonable yields. In contrast, the respective aminomagnesium bromides only gives A, A -coupling products [29]. 

The bromine/methanol (18.1.1.4) oxidation of furans to give 2,5-dialkoxy-2,5-dihydrofurans and the cycloaddition of singlet oxygen (18.7) are discussed elsewhere in this chapter. 2,5-Dialkoxy-2,5-dihydrofurans can also be obtained by electrochemical oxidation in alcohol solvents or conveniently by oxidation with magnesium monoperoxyphthalate in methanol. Reaction of furan with lead(IV) carboxylates produces... 

The electrochemical oxidation of 2-furyl-2-thienylmethane in methanol resulted in loss of aromaticity of the furan ring and gave 70 (R = 2-thienyl). The reaction took a different course with 2,2 -dithienylmethane, which oxidized at the methylene group to give methoxybis(2-thienyl)methane (71) and di-2-thienyl ketone.106... 

Rethrolones and Related Compounds.—Shono and his co-workers have extended their studies of the synthesis of cyclopentenones via the electrochemical oxidation of substituted furans, and effected a synthesis of 2-alkylrethrolones (Scheme 17), and also of 2-hydroxy-3-methylcyclopent-2-enone (Scheme 18), which is an important flavour compound. 

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. 

Chemical or electrochemical oxidation of numerous resonance-stabihsed aromatic molecules, such as pyrrole (9), thiophene (10), aniline (11), furan (12), carbazole (13), azulene (14) and indole (15), produces electronically conducting polymers (2,17-21,53-55) (see Electrically Active Polymers). 

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

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

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 oxidative reaction of furan with bromine in methanol solution or an electrochemical process using sodium bromide produces 2,5-dimethoxy-2,5-dihydrofuran (19), which is a cyclic acetal of maleic dialdehyde. The double bond in (19) can be easily hydrogenated to produce the corresponding succindialdehyde derivative. Both products find application in photography and as embalming materials, as well as other uses. 

---