Oximes cathodic reduction

Electrochemical oxidation of aldoximes using halide ions as mediators afforded the corresponding nitrile oxides in the anode compartment, which were simultaneously reduced to nitriles by cathodic reduction (equation 15). Sodium chloride affords the best result among the supporting electrolytes (Cl > Br > 1 > C104 > TsO ). Accordingly, the electrochemical reaction of oximes carried out in the presence of dipolephiles yielded isooxazolines (equation 16). 

Cathodic reduction of camphor and norcamphor oxime 144a appears to be cases where the stereochemistry is controlled by attack by the electrode from the least hindered side of the molecule ... 

Electrolytic reduction in sulphuric acid with a lead cathode, reduction with sodium hydrosulphite or hydrazine [28], and reduction of the oxime [1], have all been reported to give codeine, but the sodium hydrosulphito and hydrazine reductions have been reinvestigated by Findlay and Small [3], who obtained only complex transformation products and suggest that Hill s starting material [28] (inuoh more... 

Cathodic reduction consists of electron supply from the cathode, followed by acquisition of protons by anionically charged substrates. Thus, ready accessibility of the reductive electron transfer processes leads to a variety of reactions, including reductive abstraction of halogens [13, 14], reduction of alcohols to alkanes [15], reduction of imines, oximes, and hydrazones to the corresponding amines [16], and conversion of nitro groups to the corresponding nitroso, hydroxy amino, and amino groups [17]. In... 

Scheme 6 Cathodic reduction of cyclopropylphe-nyfketone oxime in high yields...

Reductive cross-dimerization has been established with ketones and 0-meth-oximes upon reduction in isopropanol with a Sn cathode as a convenient route to yS-amino alcohols, diastereoselectivities of up to 85 15 were obtained. A chiral ligand was obtained this way from the coupling of (-) - menthone with O-methyl acetaldoxime. Similarly, ketones could be coupled to hydrazones and nitrones. Also, intramolecular couplings were achieved with good yields and diastereoselectivity (Fig. 56) [308]. 

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

The asymmetric reduction of C=N double bonds in prochiral oximes afforded a maximum of 18% ee [380, 384, 385]. Prochiral azomethines were reduced to the corresponding 1,2-diamines and secondary amines using 36 optically active supporting electrolytes. The dimers were optically inactive, while the monomers showed low optical inductions (<11% ee). The effect of electrolyte, substrate concentration, temperature, pH, and cathode potential on the induction was studied. It was proposed that the enantioselectivity... 

Intermolecular coupling between ketones and 0-methyl oximes, hydrazones and nitrones is achieved on reduction at a tin cathode in isopropanol [105]. It is not clear which of the reacting species accepts the initial electron in these processes. The reaction with 0-methyloximes, followed by catalytic reduction of the first formed O-methylhydroxylamine, is a convenient synthetic route to 2-amino-alcohols. 

Preparative scale reduction of oximes at a mercury or lead cathode in acid solution has been used in the conversion of the carbonyl function to amine. Originally, 30-50% sulphuric acid was used as solvent [195] but ethanol with dilute hydrochloric acid is usually satisfactory. Aliphatic and aromatic oximes give amines in 64-86% yields [196]. Aromatic ketoximes are also reducible in alkaline solution and acetophenone oxime has been converted to 1-phenylethylamine in a tri-potassium orthophosphate solution [197], The reduction of oximes in acid solution is tolerant of many other substituents as indicated by a number of examples [198, 199, 200. Phenylglyoxa monoxime in acid solution is however reduced at both the carbonyl and the oxime centres by sodium amalgam to yield 2-amino-1-phenylethanol [201]... 

The reduction of aliphatic nitrocompounds in acid solution proceeds in two steps. First the nitrosocompound is formed. A low steady state concen ation of 2-methyl-2-nitrosopropane has been detected during the reduction of 2-methyl-2-nitropropane [13]. At the cathode potential necessary to attach the first electron to a nitro group, the nitroso intermediate undergoes further reduction to the hydroxyla-mine. When the nitrocompound has one a-hydrogen substituent, tautomerism of the nitroso intermediate to an oxime is in competition with further reduction. Both temperature and proton availability affect the rate of this isomerisation. Reduction of aliphatic nitrocompounds to the hydroxylamine is usually carried out in acid solution at 0-5° C to minimise oxime formation [14, 15], The hydroxylamine is stable towards further reduction in acid solution. Oximes in acid solution are reduced... 

Electrochemical reductions of camphor oxime (R = Me) and norcamphor oxime (R = H) at a mercury cathode proceed with a high degree of diastereoselectivity (equation 3) . The products are in fact of opposite stereochemistry to those formed in dissolving metal (sodium-ethanol) reductions of the oximes. 

It is known that electrochemical reduction of oximes in protic media occurs in two steps the N—O bond is first reduced to form an imine and the latter is then reduced to afford a primary amine1,29. Tallec has shown that the amine from oxime 33 can be trapped intramolecularly (equation 16)35. Interestingly, the SS diastereomer predominates the chiral pyrrolidine ring derivative serves to control the stereochemistry of formation of the new benzylic chiral center. Electrochemical reductive cross-coupling of O-methyl oximes with carbonyl compounds in isopropanol at a tin cathode affords adducts (equation 17) which can be reduced further to 2-amino alcohols36. In this fashion, menthone could... 

Fig. 5.13 Electrochemical oxime reduction. Reagents and conditions divided cell, BDD/Nb cathode, 1% NaOCH3 in anhydrous MeOH, 40°C, 3.4 Adm-2,4 h, yield = 96% (by GC)...

Although only a few imines are stable in aqueous solution, reduction in cold 50% sulfuric acid at a lead cathode may give the amine [15,16]. In this connection it can be mentioned that 2,4-dihydroxybenzophenone oxime on reduction on the first wave in acid solution gives an imine stable enough to be isolated [17]. Schiff bases of some other benzophenone derivatives are stable in acid solution their reduction is a straightforward saturation of the azomethine group [18,19]. 

Among the azomethine derivatives of hydroxylamine, the ordinary oximes are the most important, and many polarographic and electrolytic investigations of oximes have been made [1,85,86]. Most oximes are reducible in acid solution, and only few in alkaline solution polarographic data prove that it is the protonated oxime that is the reducible species in acid and neutral solutions. Classic reductions in sulfuric acid at lead cathodes or controlled potential reductions in acid solution at mercury cathodes yield the amine in a four-electron reduction. As the hydroxylamine corresponding to the oxime is not reducible under the conditions employed, it has been suggested [1] that the reduction proceeds as in ... 

The literature describes mainly reductions in aqueous media using electrodes with high hydrogen overvoltage (mercury pool cathode, lead, graphite). In this review, we chiefly describe the possibility of hydrogenation of derivatives bearing nitro, nitroso, azo, and azoxy functions, as well as oximes and imines, ketones, and aldehydes. 

With the syn and anti isomers of 5-nitrofuraldoxime Kemula and Zawadowska [81] only found differences in neutral medium, where the syn isomer gave an additional cathodic wave. In alkaline solution of the anti isomer an additional wave was observed, which was attributed to interaction between the reduction products from the nitro and oxime groups. 

---