Electrochemical reduction nitriles

Electrolytic Reductions. Both nitro compounds and nitriles can be reduced electrochemically. One advantage of electrochemical reduction is the cleanness of the operation. Since there are a minimum of by-products, both waste disposal and purification of the product are greatiy simplified. However, unless very cheap electricity is available, these processes are generally too expensive to compete with the traditional chemical methods. 

In general, however, the electrochemical reduction of nitriles offers no significant advantages over traditional chemical methods and has not been widely used. 

One development involves the use of vitamin B 2 to cataly2e chemical, in addition to biochemical processes. Vitamin B 2 derivatives and B 2 model compounds (41,42) cataly2e the electrochemical reduction of alkyl haUdes and formation of C—C bonds (43,44), as well as the 2inc—acetic acid-promoted reduction of nitriles (45), alpha, beta-unsaturated nitriles (46), alpha, beta-unsaturated carbonyl derivatives and esters (47,48), and olefins (49). It is assumed that these reactions proceed through intermediates containing a Co—C bond which is then reductively cleaved. 

Various other reducing methods are employed for the conversion of (3-nitro alcohols to amino alcohols, namely, electrochemical reduction.107 The selective electrohydrogenation of ni-troaliphatic and nitroaromatic groups in molecules containing other groups that are easy to hydrogenate (triple bond, nitrile, C-I) are carried out in methanol-water solutions at Devarda copper and Raney cobalt electrodes (Eq. 6.55).107... 

In the cathodic reduction of activated olefins, chlorosilanes also act as trapping agents of anionic intermediates. Nishiguchi and coworkers described the electrochemical reduction of a,/ -unsaturated esters, nitriles, and ketones in the presence of Me3SiCl using a reactive metal anode (Mg, Zn, Al) in an undivided cell to afford the silylated compounds [78]. This reaction provides a valuable method for the introduction of a silyl group into activated olefins. 

Thiophenecarbaldehydes add smoothly to a,f3-unsaturated ketones and nitriles under cyanide ion catalysis to form y-diketones (366) and y-ketonitriles (367) respectively (76CB534). The 2,5-dicarbaldehyde gives the bis-adduct (368). The aldehydes undergo normal reduction to the hydroxymethylthiophenes by sodium borohydride. However, electrochemical reduction of the 2,5-dialdehyde on a mercury electrode at pH 1-3 gives the bimolecular reduction product (369) as a mixture of meso- and ( )-forms in the ratio 7 3. Reduction with zinc and acetic acid gives only the meso -form of (369) (75CR(C)(280)165>. 

The electrochemical reduction of CO yields squaric acid. The use of nitriles as solvents allows to increase the current efficiencies substantially 530> ... 

However, the electrochemical reduction requires a large excess of acids, which produce waste salts during work up and product isolation. This is certainly one of the reasons why the process has so far not competed successfully with the industrial syntheses based on allyl chloride. A few examples of nitrile reduction, which can also be carried... 

On the other hand, for the reduction of heterocyclic nitriles, the electrochemical reduction can compete with catalytic methods. For example, Reilly 562) produces amino-methylpyridines by electrochemical reduction of the corresponding nitriles ... 

For the first reduction the IR shifts point to a porphyrin-centred electron transfer. This is supported by further spectroscopy on the anion radical complexes [(Por)Ru(CO)(L)]. The observed EPR lines are narrow, unstructured, with g values around 2. The UV-Vis-NIR spectra of the radical anions are characterised by redshifted Soret bands of reduced intensity, a weak structured band system around 600 nm and weak broad absorptions around 800 or 900 nm (see Figure 4.15). Further support comes from resonance Raman investigations on [(OEP)Ru(CO)(THF)] for which the observed Raman bands fit perfectly to those of the [(OEP)VO] radical anion. There is some evidence that if the spectroelectrochemistry is not carried out in very aprotic and unpolar solvents or traces of water are present, the radical anionic complexes are readily transformed. This has been investigated for the [(OEP)Ru(CO)(L)] system, where the use of solvents like MeOH or nitriles for the electrochemical reduction leads to altered species with unreduced porphyrin ligands (see Figure 4.15)." ... 

Electrochemical reduction of [Ru(NH3)e] and related halide complexes, e.g. [Ru(NH3)5C1], produces Ru products hydrolysis of the chloride ligands then occurs. Reduction potentials of [Ru(NH3)sL] and [Ru(NH3)4L2] (L = py-X or RCN) have been measured by cyclic voltammetry. Nitriles have more positive E[ values than py and unsaturated R groups give the largest values electron-withdrawing X substituents increase Ef. Co-ordination of the N-bound 4-pyridine car-boxaldehyde slows the acid-catalysed hydration. 

Finally, the local etching of nickel has recently been demonstrated by the same group (30). The electrochemical reduction of nitrile [Eq. (3)] at a Pt tip close to a thin Ni layer that had been deposited on gold caused the local etching of the nickel and the exposure of the gold underneath. The... 

Another example of an electrochemical reduction of a,j5-unsaturated nitriles was reported later by Simonet and coworkers. ( )- and (Z)-2-alkoxy-3-phenylpropenenitriles, PhCH=C(OR)CN, where R = Me, Et, r-Bu, Ph or PhCHi, showed two reduction waves in the cyclic voltammetry experiments made in DMF at a Hg cathode. The first wave was reversible for R = Me, Et and Ph whereas the second, of much higher current, was irreversible. Presence of increasing amounts of phenol more than doubled the current of the first wave with a simultaneous decrease of the second. Cathodic behaviour of E and Z stereoisomers was very similar. Preparative electrolyses performed in DMF, DMF/H2O or MeCN/HiO are shown in Table 16. Two pathways may be followed during reduction, both leading to a saturated compound but one involving a C—O bond cleavage (Scheme 29). 

Ni(0) complexes of TPPTS have been employed as catalysts for the hydrocyanation of dienes and unsaturated nitriles. Product linearity and catalyst lifetimes can be improved if the catalysis is performed in a xylene/water biphasic system by using TPPTS as co-catalyst [33]. The Ni(0)/TPPTS complexes employed may be obtained by electrochemical reduction of Ni(CN)2 in water in presence of TPPTS [34]. 

Alkoxy- or /3-amino-a,/3-unsaturated ketones, esters, or nitriles undergo electrochemical reductive coupling to give moderate yields of symmetrical 1,4-diketo-, dicarboxy-, or dicyanobuta-1,3-dienes. ... 

This type of reaction has been investigated in most detail for the electrochemical reduction of unsaturated nitriles [19-23]. 

In spite of the fact that the above mechanism for formation of organometallic compounds during electrolysis of nitriles appears most probable to us, it is impossible not to mention another possible explanationfor this reaction. In electrochemical reduction of acrylonitrile organometallic compounds have been obtained from tin, sulfur, selenium, and tellurium, which are themselves reduced electrochemically. 

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