Diprotonated hydrazones and oximes as reactive intermediates in electrochemical reductions

Baymak, Melek Sirin; Celik, Hayati; Ludvík, J.; Lund, Henning; Zuman, P.

doi:10.1016/j.tetlet.2004.04.163

Cited by 11 publications

(10 citation statements)

References 6 publications

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“…Presence of two adjacent positively charged nitrogen atoms in hydrazine derivatives has been assumed to be present in the reactive species in benzidine rearrangement [16] and in some reactions in the gas phase [17], but to our best knowledge our experimental evidence offers a first proof of existence of diprotonated hydrazonium ions or protonated N-trialkylhydrazonium ions, briefly mentioned in our preliminary report [18].…”

Section: Resultsmentioning

confidence: 79%

Reduction of diprotonated form of aryl hydrazones

Baymak

Celik

Lund

et al. 2006

Journal of Electroanalytical Chemistry

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Section: Resultsmentioning

confidence: 79%

Reduction of diprotonated form of aryl hydrazones

Baymak

Celik

Lund

et al. 2006

Journal of Electroanalytical Chemistry

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“…Further support of the initial reduction of the N-O bond in the diprotonated species is offered by the comparison with electroreduction of hydrazones and hydrazonium ions. For these compounds it has been recently proved 21,22 that they follow similar reduction pattern, where the reduction is initiated by attack on a species bearing two positive charges on adjacent heteroatoms.…”

Section: Resultsmentioning

confidence: 99%

Electroreduction of Aromatic Oximes: Diprotonation, Adsorption, Imine Formation, and Substituent Effects

et al. 2006

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Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.

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“…The reduction of most studied hydrazones of aromatic aldehydes and ketones occurs in a single fourelectron step [1,2], in which the diprotonated form [3] is reduced to an amine. Such conversion can be initiated: (a) either by the hydrogenation of the azomethine group conjugated with the aromatic ring or (b) by the cleavage of the N-N bond.…”

Section: Introductionmentioning

confidence: 99%

“…The formation of the imine as an intermediate is further supported by a comparison with the reduction of nitrones [6] and by the separation of two two-electron waves observed for some hydrazones at pH < 3. [3] indicated that N,N,Ntrimethylhydrazonium ions are reduced in a protonated form C@NH þ NR þ 3 , formed in the vicinity of the surface of the mercury electrode. The behavior of these alkylated compounds strongly resembles that of non-alkylated hydrazones, which are reduced in a diprotonated form C@NH þ NH þ 3 , which is formed in a heterogeneous process at the electrode surface.…”

Section: Introductionmentioning

confidence: 99%