Imidazoles anodic oxidation

Anodic oxidation of o-amino substituted aromatic Schilf bases (38 and 40) to imidazole derivatives 39 and 41 were carried out in CH3CN-O.I mol/1 Et4C104 solution with addition of pyridine as a base, using controlled potential electrolysis and a divided cell [72] (Scheme 22). 

The indirect anodic oxidation of ketones 42 in ammonia - containing methanol using iodide as a mediator afforded 2,5-dihydro-IH-imidazols 44 via oxidation of the intermediate ketimine 43 to AT-iodo imine followed by elimination of HI to afford the nitrenium ion, which subsequently reacts with ketimine 43 to give the product 44 [73] (Scheme 23). 

Anodic oxidation of JV,iV-disubstituted trifluoroethanimidamide 45 in dry and in aqueous acetonitrile gave the imidazole 46 and quinoneimine 47 as the reaction products (Scheme 24). The constant current electrolysis on a glassy carbon anode and a platinum cathode was performed in an undivided cell [74]. 

The electrochemical generation of a nitrilimine provides an entrance to a wide range of heterocyclic systems via anodic oxidation of aldehyde hydra-zones. The same reaction was used for annelation of various heterocyclic systems,86 e.g., substituted pyridines, quinolines, isoquinolines, indoles, imidazoles, benzimidazoles, and benzotriazoles. 

The anodic oxidation of 2,4,5-triarylimidazole was studied in aprotic solvents.291-295 The 2,4,5-triarylimidazole anions undergo a one-electron oxidation, forming dimeric bis-(2,4,5-triarylimidazolyls).294 The isomeric bis imidazolyls consist of imidazole and isoimidazole systems. The dimerization is a result of a nucleophilic attack of 2,4,5-triarylimidazole anions on the electrochemically generated 2,4,5-triarylimidazolium cations. 

Imidazole carboxylic acids are readily converted into hydrazides,436 acid halides,437 amides,437-439 and esters,439-440 and they may be reduced to alcohols with lithium aluminum hydride,441 and to aldehydes by controlled potential reduction.442 Anodic oxidation of l-methylimidazole-5-acetic acid (94) using cooled platinum electrodes yields l,2-bis(l-methylimidazol-5-yl)ethane (95).443... 

Anodic oxidation of substituted hydrazones may induce ring closure. Oxidation of /7flrfl-substituted phenylhydrazones of 2-oxophenylacetonitrile yields derivatives of 1-phenyl-3-cyano-l/7-indazoles [73] p-nitrobenzylidene-o-phenylenediamine is oxidized in MeCN-LiC104 to 2-(pnitrophenyl)benzimidazole [74], chalchone phenylhydrazone in MeCN-C5H5N-LiC104 to 1,3,5-triphenylpyrazol [74], and benzylidene 2-pyridylhydra-zone (XXIV) to 3-phenyl-j -triazolo[4,3-a]pyridine (XXV) oxazoles and imidazoles may be prepared similarly [74] ... 

A remarkable approach was reported in 2004 by Simormeaux and coworkers [53]. Manganese complexes of spirobifluorenyl-substituted porphyrins were elec-tropolymerized by anodic oxidation and the resulting poly(9,9 -spirobifluorene manganese porphyrin) films were shown to be efficient epoxidation catalysts in the presence of imidazole. The polymers were tested in the epoxidation of cyclooctene and styrene using PhIO or PhI(OAc)2 as oxidants. Epoxide yield reached 95% in the case of cyclooctene and 77% in the case of styrene. The electrosynthesized polymers could be recovered by filtration and reused up to eight times without loss of activity and selectivity. 

To protect copper films on electrical boards, the copper surface can be covered by a polymer film obtained by anodic oxidation of a corrosion inhibitor, such as amine or imidazole monomers. By electro-oxidation not only vinyl monomers, but also compounds with other functionalities, such as phenols, acrolein, benzonitrile, etc., can be polymerized. ... 

A third oxidation method is the anodic oxidation of the imidazole in dimethylformamide or acetonitrile containing a supporting electrolyte such as alkali metal chlorate. 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

The electrolysis of asymmetric ketones 43 led to the formation of isomers and stereoisomers. Kinetic measurements for the formation of ketimine 43 in saturated ammoniacal methanol indicated that at least 12 h of the reaction time were required to reach the equilibrium in which approximately 40% of 42 was converted into the ketimine 43. However, the electrolysis was completed within 2.5 h and the products 44 were isolated in 50-76% yields. It seems that the sluggish equilibrium gives a significant concentration of ketimine 43 which is oxidized by the 1 generated at the anode, and the equilibrium is shifted towards formation of the product 44. 2,5-Dihydro-IH-imidazols of type 44, which were unsubstituted on nitrogen, are rare compounds. They can be hydrolyzed with hydrochloric acid to afford the corresponding a-amino ketones as versatile synthetic intermediates for a wide variety of heterocyclic compounds, that are otherwise difficult to prepare. 

For both compounds, a complete electrolysis at 0.48 V/SCE in the presence of imidazole corresponded to a total charge circa 2 F/mol. Curves c and c, recorded after the electrolyses, no longer displayed the first two-electron oxidation wave (as seen in curves b and b recorded before the electrolyses) but only the more anodic reversible one-electron oxidation ascribed to the OQ. Finally, the bielectronic nature of the first wave was quantitatively confirmed at the timescale of preparative electrolyses. 

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