Electrochemical reduction dimerization

Electrochemical reductive dimerization and Claisen rearrangement of allyl acrylate [128]. 

Detty published the first example of the titled approach in his pioneering work on teluropyrans (88MI1). The hexafluorophosphate 76 was reduced with diisobutyl aluminium hydride (DIBAL-H) to a 93 7 mixture of isomeric teluropyrans 77 and 78 accompanied by traces (ca. 1%) of the dimeric product 80. The latter was also obtained after the electrochemical reduction of 76 via radicals 79 or by a modification of the reduction with DIBAL-H (Scheme 5). 

Chiang and coworkers synthesized a dimer of compound 26 in which two diiron subunits are linked by two azadithiolate ligands as a model of the active site for the [FeFeJ-hydrogenase [203]. Protonation of 26 afforded the p-hydride complex [26-2H 2H ] via the initially protonated spieces [26-2H ] (Scheme 62). These three complexes were also characterized by the X-ray diffraction analyses. H2-generation was observed by electrochemical reduction of protons catalyzed by 26 in the presence of HBF4 as a proton source. It was experimentally ascertained that [26-2H 2H ] was converted into 26 by four irreversible reduction steps in the absence of HBF4. 

The aqueous solution chemistry of Ir in its higher oxidation states III, IV, and V has been explored by Sykes et al.41,48 Chemical and electrochemical oxidation of Ir(H20)6]3+ gives a brown-green Irv product, which undergoes chemical and electrochemical reduction to a blue and a purple IrIV complex. 170 NMR studies are consistent with double- and single-bridged dimeric structures, with likely formulas [(H20)4Ir(/i-0H)2Ir(H20)4]6+ for the blue complex and [(H20)5Ir(/r-0)Ir(H20)5]6+ for the purple one. 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

In a subsequent study of this type (Durand, Bencosme, Collman Anson, 1983), dimers of type (143) were investigated as potential redox catalysts for the four-electron reduction of dioxygen to water (via peroxide). The Co(ii)/Co(ii) dimer is an effective catalyst for the electrochemical reduction of dioxygen once again the dioxygen binds... 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

The electrochemical reduction of aryllead triacetates was smdied by Chobert and Devaud82, as a re-examination of some previous work83 to detect the role of intermediates such as [ArPb(OAc)2]V The reductions were carried out by polarography in acetic acid or acidic alcohol solutions and show three diffusion controlled waves. The first step involves a single electron transfer to produce a radical anion which dimerizes, arylates the electrode or hydrolyzes to phenol ... 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

In contrast to the direct reduction as described above, the indirect electrochemical reduction of perfluoroalkyl halides is a versatile and novel method for generating perfluoroalkyl radicals selectively. Saveant et al. have demonstrated many successful examples. Using terephthalonitrile as a mediator, the indirect reduction of CF3Br in the presence of styrene leads to the dimer of the radical adduct obtained by the attack of CF on styrene. On the other hand, in the presence of butyl vinyl ether, the mediator reacts with the radical adduct obtained by the attack of CF3. on the olefin (Scheme 3.4) [14]. 

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... 

The electrochemical activation of the catalyst must be possible at potentials less negative than — 0.9 V vs SCE since at more negative potentials the direct electrochemical reduction of NAD(P)+ will lead to NAD dimer formation. 

The electrochemical reduction of W-acyliminium ion pool 2 gave rise to the formation of the corresponding homo-coupling product 13 (Scheme 8).23 Presumably, a radical intermediate 14 was generated by one electron reduction of 2 and homo-coupling of the radical led to the formation of the dimer 13. However, a mechanism involving two-electron reduction to give anion 15 followed by the reaction with cation 2 cannot be ruled out. 

Dimeric products, whose structures have been confirmed by X-ray crystallography [79BSB113 80AX(B)1418], have also been isolated from the electrochemical reduction of N,N -disubstituted 6-phenyldihydrodiazepi-nium salts [79BSB113 81JCS(P2)801]. In these cases formation of pyrol-... 

M (CO)6 complexes all undergo irreversible electrochemical reduction in nonaqueous electrolytes at peak potentials close to —2.7 V versus SCE in tetrahydrofu-ran (THF) containing [NBu4][Bp4]. The product of the reductions are the din-uclear dianions [M2(CO)io] although under some conditions polynuclear products can also been obtained, Sch. 3 [2]. It was initially proposed that the primary step involved a single-electron transfer with fast CO loss and subsequent dimerization of the 17-electron radical anion [M(C0)5] [34]. A subsequent study showed that a common intermediate detected on the voltammetric timescale was the 18-electron species [M(CO)5] and that the overall one-electron process observed in preparative electrolysis arises by attack of the dianion on the parent material in the bulk solution, Sch. 2 [35]. 

A simplified picture of the electrochemical reduction of NAD+ (260) to NADH (262) and other products is illustrated (Scheme 185) (75JA5083). The first reduction of NAD+ to the radical (261) is pH independent, and this free radical rapidly dimerizes to (263) or (264). At more negative potential, NAD+ is reduced to a dihydropyridine by one-electron reduction of (261), but the dimer is not reduced at this potential. 

With primary halides, dimers (R—R) are formed predominantly, while with tertiary halides, the disproportionation products (RH, R(—H)) prevail. Both alkyl nickel(III) complexes, formed by electrochemical reduction of the nickel(II) complex in presence of alkyl halides, are able to undergo insertion reactions with added activated olefins. Thus, Michael adducts are the final products. The Ni(salen)-complex yields the Michael products via the radical pathway regenerating the original Ni(II)-complex and hence the reaction is catalytic. In contrast to that, the Ni(III)-complex formed after insertion of the activated olefin into the alkyl-nickel bond of the [RNi" X(teta)] -complex is relatively stable. Thus, further reduction leads to the Michael products and an electroinactive Ni"(teta)-species. 

Electro-generated and regenerated bis(bipyridine)rhodium(I) complexes were able to catalyze the selective non-enzymatically coupled electrochemical generation of NADH from NAD . The direct cathodic reduction even at very negative working potentials leads to the formation of large amounts of enzymatically inactive NAD dimers, while the indirect electrochemical reduction via the rhodium complex acting as... 

One-electron reduction of pyrylium salts, with dissolving metals or electrochemically, gives dimers (e.g. 382) via pyranyl radicals (80AHC(27)46). 

Electrochemical reduction of 1-vinylazoles in acetonitrile or DMF is also a one-electron process in which the generated radical anions dimerize (86CHE253). 

Covitz in the same company followed the study and clearly established the presence of -xylylene as an intermediate by means of the electrochemical reduction of a,a -dibromo-/>-xylene (29). Half-wave potentials were observed at — 0.80 and —1.72 V vs. SCE. in polarography. A dimer, [2,2]-paracyclophane, was also obtained as a minor product of electrolysis as well as polymer. 

Stoichiometric Reactions. In 1974, Touboul reported amazing selectivity in the controlled potential electrochemical reduction of the enone (Scheme 36) (63). While the dimerization of the enantiomerically pure enone gives solely the cis,threo,cis diol, the racemic compound behaves similarly to produce the racemic dimer with the same relative configuration and no other possible diastereomers. A radical anion intermediate... 

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). 

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