Furans electrochemical reduction

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

Raney nickel reduction of 2-benzyl-5-ethylselenophene (71) yields 1-phenylheptane (72), a conversion analogous to the much used reductive desulfurization of thiophenes (73JGU871). The electrochemical reduction of selenophene-2-carboxylic acid gives a mixture of dimeric products the major product is compound (73). This is in contrast to the 2,5-dihydro derivatives obtained by electrochemical reduction of thiophene and furan carboxylic acids (82CS( 19)95). Wolff-Kishner reduction of 2-selenienyl 2 -thienyl ketone gives, in addition to the expected methylene derivative, 2-(pentenyl)thiophene (72ZOB1780). 

Electron spin resonance (ESR) spectroscopy reveals that an increase in the electron-donating ability of substituents in the 2-position of the furan ring produces an increase in the spin density at the 3-position and a decrease at the 4-position the coupling constants in the ESR spectra of anion radicals generated by electrochemical reduction of 2-substituted 5-nitrofurans equate to o values.295... 

Anion-radicals generated from oxazole and isoxazole in an argon matrix were shown, like their counterpart from furan, to have undergone ring scission. Consistent with this, various attempts to reduce isoxazoles polarographically in both protic and aprotic media have also resulted in the loss of ring integrity, and indeed the reduction process has always involved two electrons. However, electrochemical reduction of... 

The electrochemical reduction of 3,6-diphenylphthalic anhydride in the presence of TMS-Cl yields benzo[c]furan (107), which dimerizes immediately to give (108) (Scheme 83) <8lTL3497, 84AG596, 84AG(E)622>. 

One electron electrochemical reduction of 2-NO2 chalcogenophenes becomes progressively easier when going from oxygen to selenium <83MI 2l3-07>. Selenophene-3-carboxylic acid is electro-chemically reducible to its 2,3-dihydroderivative but the corresponding furan is not reduced under the same conditions <83IZV944>. 

The electrochemical properties of 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[l,2-A4,5- ]difuran 72 have been investigated by cyclic voltammetry in the search for new materials with enhanced conducting properties <2000JOC2577>. Upon single-electron reduction the central quinonoid ring gains the aromatic stability of benzene, whereas the fused heterocyclic part loses the aromaticity of the furan nucleus. 

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... 

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. 

Both 2- and 3-nitrothiophenes are reduced by tin and hydrochloric acid to the corresponding aminothiophenes. Reduction of 2,5-dibromo-3,4-dinitrothiophene gives 3,4-diaminothiophene as a stable crystalline solid. 2-Acetamido-furans are prepared by the reduction of 2-nitrofurans in the presence of acetic anhydride. 2-Substituted 5-nitrofurans can be reduced to the S-aminofurans by an electrochemical method. Although catalytic reduction gives 2-aminofurans only in low yields, they can be trapped using ethyl ethoxymethylenecyanoacetate or ethoxymethylenemalononitrile. Benzofuranone 412 and not 2-aminobenzofuran is obtained from tin and hydrochloric acid treatment of 2-nitrobenzo[ ] furan 411. 

A biomimetic route for the synthesis of dihydrobenzo[b]furan heterocycles was investigated via a reagent-based electrochemical transformation of 2-(2 -hydroxyethyl)-quinine precursors. It was believed that a putative oxonium ion was generated by treatment of the quinone with PPTS, followed by reduction with dihydroquinone (DHQ) to generate a radical species which underwent either further reduction by DHQ to give the product or a direct dimerization <01JOC4965>. 

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