Dimerization Electrochemical methods

For enzymatic reductions with NAD(P)H-dependent enzymes, the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, in all cases leads to varying amounts of enzymatically inactive NAD-dimers generated due to the one-electron transfer reaction. One rather complex attempt to circumvent this problem is the combination of the NAD+ reduction by electrogenerated and regenerated potassium amalgam with the electrochemical reoxidation of the enzymatically inactive species, mainly NAD dimers, back to NAD+ [51]. If one-electron... 

Phenols (1.8 V vs Ag/Ag+) [62], in particular phenolate ion (0.55 V vs Ag/Ag+) [63], are easily oxidized by the electrochemical method to give cationic intermediates, which react with nucleophilic solvents such as MeOH, MeCN, or H2O or with the phenol itself, yielding dimers. 

A third important reaction of aromatic radical-cations is carbon-carbon bond formation with a further aromatic substrate. This reaction is limited to the oxidation in acetonitrile of substrates with electrondonating substituents. Radical-cations from benzene, naphthalene and anthracene form a-complexes but do not form a a-bonded reaction intermediate. Tlie dimerization reaction has been investigated both by pulse-radiolysis [22] in water and by electrochemical methods [27] in acetoni-... 

One of the most reliable electrochemical methods for the synthesis of alkaloidal skeletons is the oxidative coupling of phenolic and related compounds. A typical early example is the oxidative coupling of corypalline 1 to its dimer 2 ... 

Electrochemical methods were used to obtain kinetic information concerning the cation-radical dimerization of anisole (and related compounds). Two mechanisms were consistent with data A radical-radical coupling (RRC) mechanism and a radical-substrate coupling (RSC) mechanism (Fig. 42) [198]. 

Two sesquiterpenes, y- and 5-acoradiene (118 and 119), were synthesized efficiently using an electrochemical method as a key step. The readily available 4-substituted phenol 120 was submitted to constant current electrolysis in 2 1 MeOH-THF to afford three spiro compounds (121, 122 and 123) in 43% yield (relative ratio 121/122/123 = 1 2 1). All of them were readily converted to both 118 and 119. However, the use of only THF as a solvent provided the corresponding dimer 124 in 80% yield 5 (Scheme 22). 

This involves the dimerization of acrylonitrile with hydrogenation. This techniquewas developed by an electrochemical method in the 1960s by Monsanto and commercialized as the EHD (Electro Hydro Dimerization) process. Other companies, manufacturers of nylon-6,6, have proposed a number of variants, including Asahi, Du Pont. Hakon, 1CI and UCB (Union Chimiqae Beige). 

The base (149) has been oxidized by electrochemical methods to the dibenzo-quinolizinium salt (150) the methoperchlorate, under similar conditions, is oxidized to a 2 2 dimer. A bisphenethylisoquinoline, jolantinine (151), has been isolated from Merendera jolantae its structure has been assigned on the basis of spectroscopic evidence. 

Electrochemical methods are preferred in analysis of phenols and halogenated organics since often there is no need for extensive separation. However direct determination on noble metal electrodes is not favored due to high over-potentials. Electrochemical oxidation of phenols readily occurs on unmodified electrodes, but oxidation results in the formation of dimers which poison the electrodes, decreasing the oxidation currents. In order to improve sensitivity and selectivity, chemically modified electrodes are employed. In this regard M-N4 complexes have shown remarkable catalytic activity towards the detection of phenols and other species when either employed as homogeneous catalysts or when adsorbed to electrodes. 

Compared with chemical oxidation polymerization, electrochemical polymerization is performed at an electrode (conductive substrate) using the positive potential [97-104]. Whereas the powder forms are obtained by chemical polymerization, the electrochemical method leads to films deposited on the anode. When a positive potential is apphed at the electrode, pyrrole monomer such as a heterocychc compound is oxidized to form a delocalized radical cation, which includes the possible resonance forms. Radical-radical coupling reaction produces the dimerization of the monomer radicals at the a-position. Removal of 2H+ ions consequently forms the neutral dimer. Next step is chain propagation which includes the oxidation of the neutral dimer to form the dimer radical. The resultant radical can react with other monomer or dimer and this radical coupling and the electrochemical oxidation processes repeat in order to extend the polymer chain. The final step involves the termination of chain growth and the resultant PPy film is formed on the anodic electrode. 

Polythiophene and its derivatives can be polymerized by chemical or electrochemical techniques. In this study, the electrochemical method was utilized.The mechanism is a cationic radical polymerization 11). The polymerization pathway can be summarized in the following steps 1) oxidation of the monomer to form a radical cation, 2) dimerization of the radical cations, 3) loss of proton to yield a neutral dimer, 4) oxidation of dimer to form a radical cation, 5) reaction of dimer radical cation with another radical cation, 6) repeat of the this study, are 3-methylthiophene, tetrabutylammonium tetrafluoroborate (TBATFB), as the supporting electrolyte. The organic solvent was acetonitrile. The resulting polymer was the first conducting polymer family found to be stable in air and water in both their doped or undoped state. 

The relatively low standard potential of the Ns /Ni redox couple in aqueous solution (+1.33 0.01 V vs. NHE) is even more reduced in organic solvents, and therefore the generation of azidyl radicals by electrochemical methods is perfectly feasible. The multigram scale dimerization of styrene represents an early synthetic application of the electrochemical process (Scheme 8.15). " However, the scope of this reaction is so far limited since other substrates give poor yields and/or significant amounts of by-products. 

Enolates derived from monoalkylated malonates can be dimerized by electrochemical methods unfortunately the yields decrease with increase in the size of the alkyl substituent. Cerium(iv) salts can also be used for this type of coupling, but only with a limited number of polyesters. Treatment of malonate... 

Typical aryl-aryl coupling reactions that utilize catalytic amounts of Ni(II) salts are outlined in Scheme 12.24. Zinc metal was used to reduce Ni(II) to the cataly tically active Ni(0) species in the homocoupling of dimethoxypyridine 108 and the efficient synthesis of dimer 109 (Eq. 12.24-1) [88]. Alternatively, nickel salts can also efficiently be reduced by means of electrochemical methods. As shown in Equation 12.24-2, Troupel and coworkers were able to recover Ni(0) on a steel electrode and isolated the coupling products in excellent yield [89]. Comparable studies were carried out with slightly more complex substrates and confirmed that... 

The principal difficulty in the electrochemical method for generation of radical ions is the very short life of most of the radical ions formed as the initial products of the electrode reaction as a rule they are immediately subject to further chemical changes such as protonation, disproportionation, dimerization, oxidation, etc., or... 

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