Experimental evidence of formation of imines in the course of reduction of hydrazones

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Abstract

The reduction of hydrazones is generally suggested to proceed through a reductive cleavage of the nitrogen–nitrogen bond followed by a reduction of the carbon–nitrogen bond. This sequence of reduction processes is here supported for fluorenone (V) and benzophenone (VI) hydrazones as well as by a comparison of the reduction of fluorenone and benzophenone hydrazonium ions (I,III) with corresponding imines (II,IV). Another proof of the presence of imines as intermediates is the splitting of four-electron waves of hydrazones V and VI and hydrazonium ions I and VIII into two waves at pH < 2. This has been interpreted as due to differences in slopes dE1/2/dpH and pKa-values of protonated hydrazine derivatives on one side and corresponding imines on the other. In this pH-range imines formed in reductions of VI and VIII are reduced in a single two-electron wave, those of I and V in two one-electron steps. Fluorenone imine (II) is sufficiently stable to allow recording of time-independent current–voltage curves between pH 6 and 11. In this pH-range the imine (II) is reduced in two one-electron steps. Benzophenone imine (IV) has been found stable between pH 4.6 and 12. At pH 4.6–8 the reduction of the imine IV takes place in a single two-electron step, at pH 8–12 in two one-electron steps. Final proof of the initial cleavage of the N–N bond is presented by comparison with the reduction of nitrones.

Introduction

The reduction of most studied hydrazones of aromatic aldehydes and ketones occurs in a single four-electron 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.

(a) The hydrogenation of the azomethine group (1) would yield a hydrazine derivative, which would have to be reduced in the second step (2). The overall reaction steps would be:ArC(R)NNH2+2H++2eArCH(R)NHNH2ArCH(R)NHNH2+2H++2eArCH(R)NH2+NH3

Initially, this interpretation seemed attractive, because it is known that reductions of the conjugated azomethine bonds are relatively easy, whereas the reductive cleavage of N–N bonds – as long as they are not activated – needs a considerably higher activation energy. The problem with this interpretation, as pointed out by Lund [1], is the potential of the step (2). In the hydrazine derivative ArCH(R)–NHNH2 the reductive cleavage of the unactivated N–N bond needs a considerable energy. This is manifested by negative potentials of reduction of such compounds [4]. Hence, if the reduction of hydrazones would follow the sequence (1), (2), such process would be manifested by two two-electron steps, the potentials of which would differ by more than about 0.5 V. Such behavior has never been observed for any benzaldehyde, acetophenone or benzophenone hydrazone studied.

(b) For the second alternative it was assumed that the first two-electron process results in reductive cleavage of the N–N bond (3) as first proposed by Lund [1]. This process is followed by a two-electron hydrogenation of the imine (4), electrogenerated in the first reduction step (3):ArC(R)NNH2+2H++2eE3ArC(R)NH+NH3ArC(R)NH+2H++2eE4ArCH(R)NH2

In order to result in a single four-electron step, the potential of the reduction of the imine (E4) must be either more positive than the potential of the first step (E3), or E4 must differ by less than about 50 mV from the potential E3. As imines are known [5] to be easily reducible, the above assumption seemed reasonable.

Nevertheless, a direct experimental evidence for the sequence (3), (4) has not been available. This is at least partly due to the fact that the majority of imines, anticipated as intermediates in electroreduction of hydrazones, was not stable enough for experimental verification.

In this contribution the experimental evidence, proving that imines are formed as reactive intermediates, is based on comparison of current–voltage curves obtained with fluorenone and benzophenone N,N,N-trialkylhydrazones I and III with those obtained with corresponding imines II and IV. The comparison is limited to the pH-range in which the imines do not undergo either an acid or a base catalyzed hydrolysis during the duration of the experiment. 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.

Recent investigations [3] indicated that N,N,N-trimethylhydrazonium ions are reduced in a protonated form CNH+NR3+, 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 CNH+NH3+, which is formed in a heterogeneous process at the electrode surface. In both cases the cleavage of the N–N bond is facilitated by the fact that NH3 and NR3 are good leaving groups. The similarity of structure of CNH+NR3+ and CNH+NH3+ is reflected by a similarity of the type of the reduction process. Introduction of the alkyl groups thus does not affect the reduction mechanism and the difference in inductive effects of NH3+ and NR3+ is only reflected by differences in E1/2.

The extension of the conclusions based on comparison of the reduction of imines with those of N,N,N-trialkylhydrazonium ions, is facilitated by the similarity in the structure of the electroactive species.

Section snippets

Instrumentation

Current–voltage curves were recorded by using Sargent Model 4001 Polarograph and IBM EC/225 Voltammetric Analyzer combined with IBM 7424 MT X-Y-T Recorder as well as capillary electrodes with characteristics of outflow velocity of mercury m = 2.5 mg/s, drop-time t1 = 3.0 s at height of mercury column h = 64 cm [7]. A two-electrode electrolytic cell is used with a S.C.E. separated by a liquid junction (Kalousek cell).

Chemicals

The compounds studied, fluorenone N,N,N-trimethyl hydrazonium perchlorate (I),

Results and discussion

Three types of experimental evidence are offered as a proof of formation of imines as reactive intermediates in the electroreduction of hydrazone derivatives: (a) comparison of iE curves of hydrazones and related hydrazonium ions with those of stable imines; (b) observation of separation of two two-electron waves at pH < 3; (c) comparison with some other azomethine compounds.

Conclusion

Evidence presented here, based on comparison of current–voltage curves of hydrazone derivatives with those of corresponding imines, confirmed the originally [1] proposed initial cleavage of the N–N bond (3), followed by reduction of the resulting imine (4). Proposed sequence of electron and proton transfers was further supported by separation of two two-electron or one two-electron and two one-electron reduction steps at pH < 5.

For this interpretation most significant was the possibility to

Acknowledgement

M.S. Baymak acknowledges generous financial support by the Marmara University, Istanbul, Turkey.

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