Electrochemical reduction imines

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]. 

Scheme 3 Electrochemical reduction (n-doping) of n-conjugated heteroaromatic polymers. Effect of the imine group...

The electrochemical reduction of aliphatic amides in dilute hydrochloric acid will tolerate other functional group.s active towards the alternative hydride reducing agents. With aliphatic amides the electrochemical step generally leads to the aldehyde because the intermediate imine is rapidly hydrolysed under the conditions... 

Quinone Imines. - These display similar chemical properties to quinones, differing slightly due to the difference in electronegativity between the -C = O and -C=NH functions. Alberti et al. have examined the redox properties of the anti-cancer agent 5//-pyridophenoxazin-5-one. 18 Electrochemical reduction under anaerobic conditions gave the spectrum of the radical anion 02 was trapped in the presence of 02. 

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

The oxidation of phenylhydrazine and 1,2-disubstituted hydrazines to hydrazones and diazenes by CI2C proceeds via formation of unstable azomethine imines.95 The conversion of alcohols into alkyl halides is achieved by reaction with CCI4 (or CBr4) in DMF under electrochemical reduction.96 The reaction of dihalocarbene X2C with DMF to form a Vilsmaier reagent (93) is proposed as the key process. The reaction of simple isocyanates (RNCO) with dimethoxycarbene normally gives hydantoin-type products. In the reaction with vinyhsocyanates such as (94), however, hydroindoles (95) are formed in good yields.97... 

Electrochemical reduction of imines (25 Schiff bases) in acidic media proceeds via the iminium species, i.e. the protonated imine (26) (Scheme 5)29. Since 26 bears a positive charge, it is very easily reduced, so much so that the resulting neutral radical (27) is formed at a potential positive of its reduction potential. The products are therefore derived from 27 rather than the corresponding carbanion (28). This stands in contrast with the electrochemical behavior of imines in neutral media, where 27 is immediately reduced to 2830. Thus, cathodic reduction of /u. s-irnincs of 1,2-diamines (29) in DMF containing methanesulfonic acid affords tetrahydropyrazines (equation 13)31. A similar reaction can... 

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... 

Imine radical anions appear to be substantially more basic than their ketyl anion counterparts. In 1991, Zhan and Hawley reported that Ph2C=NH (generated via the electrochemical reduction of benzophenone imine) was a sufficiently strong base to depro-tonate weak carbon acids whose pK values were as high as 33185. 

When a disubstituted imine (19a-d) is employed instead of the trisubstituted imine, in addition to the homoallylamine (20) also the hydrodimer (21) of the imine, i.e., the dimer of the postulated a-aminobenzyl radical, is isolated (Figure 30) [214, 45]. The observation that the hydrodimer is produced only from the disubstituted and not from the trisubstituted imine parallels the electrochemical reduction which affords hydrodimers from aldimines [215] but not from ketimines [215, 216]. 

Bicyclic oximes also give the corresponding amines with high stereoselectivity, but in a manner different from the bicyclic imine of Eq. (23) the stereochemical feature of the electrochemical reduction is the same as that of the LiAlH4 reduction, but opposite that of the sodium reduction [182],... 

Diastereoselective reaction of diimines with sodium in ether leads to the formation of macrocyclic ethers of the type (32 equation 67) and (33 equation 68) in modest yields, However, there are no reports of examples using the templating effect of low-valent transition metals, as used to good effect for intramolecular carbonyl couplings. Electrochemical reduction of imines has been used to produce a variety of bicyclic structures via cyclization-aromatization (equation 69) or transannular cyclization (equation 70). ... 

Whilst most ring transformations in the synthesis of 1,3-diazepines involve ring-expansion reactions, there are isolated reports of ring contraction. Thus irradiation of the bis-imine (70) affords the diazocine (71) which, after meso-dl transformation, is converted into the bis-l,3-diazepine (72) <95LA657>. The product is identical to that obtained by the electrochemical reduction of (70) <82JOC4452>. 

Electrochemical reduction of bis(imine) (49) in strongly acidic media affords a 30% yield of trans-perhydrodiazocine (36), apparently via intramolecular coupling of an intermediate biradical (Scheme 9) <91JOC3063>. 

A commercially available polystyiyl diphenylphosphine has been rectanmended fpolymer-bound phosphine imine is initially generated, from which the aminonucleoside can be liberated on hydrolysis. Furtho studies have also been rqiorted on the electrochemical reduction of azidoniicleosides. ... 

Phenylketene generated from phenylacetyl chloride with triethylamine gives [2 + 2] imine cycloaddition with catalysis by the N-heterocyclic carbene prepared by partial electrochemical reduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) as solvent gives iraws-p-lactam formation (Eqn (4.92)). The zwitterionic intermediate formed in the reaction is stabiHzed by the highly ionic solvent, which also allows isomerization to form the trans product. The use of such N-heterocyclic carbenes in the catalysis of [2 + 2] cycloadditions of ketenes has been reviewed. ... 

Application of electrochemical reductive allylation of IV-ben lidene-ethanolamine using allyl bromide mediated by the Pb(ii)/Pb(0) redox couple has been reported, although yields are lower than for the simple imine systems. 

Scheme 2 Mechanism of the electrochemical or metal-promoted reductive coupling of imines in an acidic medium...

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

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