Electrochemical behaviour of hydrazonates in protic and aprotic medium

Article abstract of DOI:10.1081/SCC-120002006

Cathodic reduction of several hydrazonates of a typical structure (Scheme 1) has been investigated by cyclic voltammetry and controlled potential electrolysis at mercury electrode in aprotic solvent. Quantitative results for preparative electrolysis run i

Full text of DOI:10.1081/SCC-120002006

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ELECTROCHEMICAL BEHAVIOUR OF HYDRAZONATES IN
PROTIC AND APROTIC MEDIUM
T. Saied a , M. L. Benkhoud a & K. Boujlel b
a Campus Universitaire , Laboratoire d'Electrochimie, Faculté des Sciences de Tunis,
1060 Le Belvedere, Tunis, Tunisia
b Campus Universitaire , Laboratoire d'Electrochimie, Faculté des Sciences de Tunis,
1060 Le Belvedere, Tunis, Tunisia
Published online: 16 Aug 2006.
To cite this article: T. Saied , M. L. Benkhoud & K. Boujlel (2002) ELECTROCHEMICAL BEHAVIOUR OF HYDRAZONATES IN
PROTIC AND APROTIC MEDIUM, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 32:2, 225-233, DOI: 10.1081/SCC-120002006
To link to this article: http://dx.doi.org/10.1081/SCC-120002006
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SYNTHETIC COMMUNICATIONS, 32(2), 225–233 (2002)
ELECTROCHEMICAL BEHAVIOUR
OF HYDRAZONATES IN PROTIC
AND APROTIC MEDIUM
T. Saied, M. L. Benkhoud, and K. Boujlel*
Laboratoire d’Electrochimie,
Faculte des Sciences de Tunis, Campus Universitaire,
1060 Le Belvedere, Tunis, Tunisia
ABSTRACT
Cathodic reduction of several hydrazonates of a typical struc-
ture (Scheme 1) has been investigated by cyclic voltammetry
and controlled potential electrolysis at mercury electrode in
aprotic solvent. Quantitative results for preparative electroly-
sis run in the presence of a protic donor show the occurrence
of reductive cleavage of the median N–N bond leading to the
formation of imines and iminoethers in good yields.
INTRODUCTION
The electrochemical reduction of compounds with carbon–nitrogen
double bond of the type [R1(R2)C¼N-Y] has gained considerable interest
over the last years. The reduction of imines (Y : H),1,2 oximes (Y : OH),3,4
azines (Y : N¼CRR0),5,6 hydrazones (Y : NRR0),7,8. . . depends on their
*Corresponding author.
225
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

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SAIED, BENKHOUD, AND BOUJLEL
structure and the experimental conditions.9,10 Cleavage of N–Y bond
and saturation of the carbon–nitrogen double bond are the expected
reactions.
The present work deals with the electrochemical behavior of com-
pounds 1 (cf Scheme 1 for the investigated derivatives) in aprotic medium
using a stationary mercury electrode. The effect of acid addition on their
reduction and oxidation characteristics is also reported.
Scheme 1.
The electrochemical reduction mechanism and the structure of the
final products are investigated.
RESULTS AND DISCUSSION
Typical cyclic voltammograms for reduction of compound 1b is shown
in Figure 1 and peak potentials from cyclic voltammetry measurements are
summarized in Table 1.
All the hydrazonates 1 are reduced via two one-electron processes on
mercury electrodes at a sweep rate of 0,1 V/s, except 1b which shows a third
reduction peak located at a very high negative potential (Ep ¼ ꢀ 2,27 V vs
Ag/AgI/Iꢀ 0,1 M) and ascribed to the reduction of the pyridinyl group.
On the reverse scan, an anodic peak was observed assigned to the
oxidation of the anion radical outlet of the first electron transfer. The
cathodic peak potential Epc and the current fraction Ipc/v0,5 remain constant
(Figure 2) with the increase of the scan rate in the range from 50 to 500 mV/s.

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ELECTROCHEMICAL BEHAVIOR OF HYDRAZONATES
227
Figure 1. Cyclic voltammetry of hydrazonate 1b in DMF-Bu4NClO4 0,1 M.
Hanging mercury electrode. Reference electrode Ag/AgI/Iꢀ 0,1 M. Sweep rate:
v ¼ 50 mV/s. Substrate concentration: c ¼ 10ꢀ3 mol/l.
Table 1. Cyclic Voltammetry of Some Hydrazonates 1 in DMF-Bu4NClO4 0,1 M.
Hanging Mercury Electrode. Reference Electrode Ag/AgI/Iꢀ 0,1 M. Sweep Rate:
v ¼ 50 mV/s. Substrate Concentration: c ¼ 10ꢀ3 mol/l
First Step
Second Step
Third Step
Compounds
Ep (V)
Ip (mA)
Ep (V)
Ip (mA)
Ep (V)
Ip (mA)
1a
1b
1c
1d
ꢀ1,87
ꢀ1,47
ꢀ1,40
ꢀ1,68
10,60
5,50
2,10
ꢀ2,34
ꢀ2,03
ꢀ1,77
ꢀ2,09
4,10
5,10
3,60
2,10
–
–
6,20
–
ꢀ2,27
–
–
10,80
–
On the other hand, the potential difference between Epc and Epa is about
(59/n) mV. These criteria indicate that the first electrode process of hydra-
zonates 1 on mercury is reversible.
Nevertheless, the ratio Ipa/Ipc is less than one regardless of the scan
rate (Table 2). So the first electron transfer may be coupled to a fast chemi-
cal reaction which is very likely a protonation reaction by the tetrabutyl
ammonium cation via an Hofmann elimination11 or by residual proton
impurities.
When adding a proton donor as phenol, the current of the first reduc-
tion peak decreases slightly and a more anodic peak appears (Figure 3)
ascribed to the reduction of the protonated chemical entitiy. Thus, a partial

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SAIED, BENKHOUD, AND BOUJLEL
Figure 2. Influence of the sweep rate on the reduction current value for hydrazo-
nates 1a and 1b in DMF-Bu4NClO4 0,1 M. Hanging mercury electrode. Reference
electrode Ag/AgI/Iꢀ 0,1 M. Substrate concentration: c ¼ 10ꢀ3 mol/l. -A: substrate 1a;
-B: substrate 1b.
Table 2. Ratio Ipa/Ipc of Some Hydrazonates 1 in DMF-Bu4NClO4 0,1 M at
Different Sweep Rates. Reference Electrode Ag/AgI/Iꢀ 0,1 M. Hanging Mercury
Electrode. Substrate Concentration: c ¼ 10ꢀ3 mol/l
Ipa/Ipc Values
Scan Rate
Compounds
50 mV/s
100 mV/s
200 mV/s
500 mV/s
1a
1b
1c
1d
0,63
0,74
0,81
0,72
0,63
0,76
0,68
0,65
0,63
0,75
0,72
0,675
0,63
0,78
0,71
0,65
protonation of the starting material 1 cannot be excluded. The protonation
reaction may be reasonably fast because of the basic nature of hydrazonates
1. For a large excess of phenol the cyclic voltammogram exhibits only one
reduction peak located at a more positive potential.
The controlled potential electrolysis performed on mercury pool elec-
trode in DMF solution using tetrabutyl ammonium perchlorate 0,1 M,
shows that the electrochemical behavior of hydrazonate 1 differs depending
on whether the starting material is reduced in presence or in absence of a
proton donor.

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ELECTROCHEMICAL BEHAVIOR OF HYDRAZONATES
229
Figure 3. Cyclic voltammetry of hydrazonate 1a in DMF-Bu4NClO4 0,1 M. after
addition of phenol (a ¼ [phenol]/[hydrazonate]). Hanging mercury electrode.
Reference electrode Ag/AgI/Iꢀ 0,1 M. Sweep rate: V ¼ 50 mV/s. a: a ¼ 0; b: a ¼ 0,4;
c: a ¼ 0,6; d: a ¼ 1,2.
In aprotic medium, the electrolysis of 1 at applied potential, corre-
sponding to the value of Epc, leads to the saturation of carbon–nitrogen
double bond. The hydrazonates derived of benzylhydrazine 2 are the reduc-
tion products obtained in a good yield. After purification, their ir and nmr
spectra are in good agreement with the assigned structure.
Thus the cathodic reduction process of 1 may be summarized as
follows (Scheme 2).
Scheme 2.

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SAIED, BENKHOUD, AND BOUJLEL
Table 3. Macroelectrolyses of Some Hydrazonates 1. DMF-Bu4NClO4 0,1 M.
Cathode: Mercury Pool (s ¼ 12 cm2). Reference Electrode Ag/AgI/Iꢀ 0,1 M
Aprotic Medium
Protic Medium
Fixed
Electrolysis Consumption Products Electrolysis Consumption Products
Electricity
Fixed
Electricity
Compounds Potential (V)
(F/mole)
Yield (%) Potential (V)
(F/mole)
Yield (%)
1a
1b
1c
1d
ꢀ1,85
ꢀ1,45
ꢀ1,75
ꢀ1,65
2,15
2,11
2,12
2,2
2a (59)
2b (49)
2c (63)
2d (59)
ꢀ1,8
ꢀ1,37
1,68
3,21
3,09
3,12
2,98
3a (71)
3b (69)
3c (65)
3a (53)
ꢀ1,59
After the first charge transfer, the protonation of the radical anion
leads to a neutral radical whose charge is more favorable for the second
electron transfer to occur. Therefore, a second charge transfer occurred
leading, after protonation, to the saturation of carbon nitrogen double
bond. The reduction of 1 occurs according to an ECEC mechanism. In
previous studies12 we have reported that hydrazonates derived from hydra-
zines with similar structures to the final products are not reducible in aprotic
medium. These results are in agreement with the electricity consumption for
all the compounds (Table 3).
In the presence of a ready source of proton (phenol for example),
coulometric measurements prove a consumption of 3 F/mole for all the
investigated substrates 1 and the reaction leads up to the cleavage of the
nitrogen–nitrogen bond.
The cathodic reduction process of 1 displays a rather complex mechan-
ism were a CECE mechanism is most dominant.
The protonation of the nitrogen nearest to the oxygen moiety in the
neutral radical (Scheme 3a) or in the substrate (Scheme 3b) may be consid-
ered as the kinetic determining step to cleave the nitrogen–nitrogen bond.
Whatever the chemical route and the protonated form, the final prod-
ucts are the same. In the electrolysis conditions, imines 3 are not reduced,
Scheme 3a.

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ELECTROCHEMICAL BEHAVIOR OF HYDRAZONATES
231
Scheme 3b.
whereas in the post-electrolysis work-up iminoether 4 is hydrolysed.13 This
may explain why only the imines 3 are isolated.
EXPERIMENTAL SECTION
Materials and Reagent
The solvent, N,N-dimethyl formamide (DMF) was carefully dried
and distilled under vacuum. Dried pure quality tetrabutylammonium per-
chlorate (TBAP) was used without further purification.
All hydrazonates 1 were prepared according to the described
method.13
1a: Rdt: 60%; Eb0,1mm Hg: 120ꢁC; ir: nC¼N ¼ 1632; nmr (1H): 1.1
(t, 3H); 1.9 (s, 3H); 2.1 (s, 3H); 4 (q, 2H); 7.1 (mu, 5H).
1b: Rdt: 69%; Eb0,1mm Hg: 130ꢁC; ir: nC¼N ¼ 1633; nmr (1H): 1.2
(t, 3H); 2 (s, 3H); 2.2 (s, 3H); 4.1 (q, 2H); 8 (mu, 4H).
1c: Rdt: 49%; Eb0,1mm Hg: 110ꢁC; ir: nC¼N ¼ 1632; nmr (1H): 1.1
(t, 3H); 2.20 (s, 3H); 2.23 (s, 3H); 4 (q, 2H); 7 (mu, 3H).
1d: Rdt: 63%; Eb0,1mm Hg: 110ꢁC; ir: nC¼N ¼ 1635; nmr (1H): 1.2
(t, 3H); 2.1 (s, 3H); 4.4 (q, 2H); 7 (mu, 5H); 7.17 (s, 1H).
Apparatus and Procedures
The IR spectra (CHCl3) were recorded on a Perkin-Elmer 1000 PC
spectrophotometer and the 1H NMR spectra were recorded on Brucker

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SAIED, BENKHOUD, AND BOUJLEL
AC 300 MHz spectrometer using CDCl3 containing tetramethylsilane as an
internal standard.
The cyclic voltammetry measurements were performed using a PAR
scanning potentiostat model 362. The cell was a three-electrode type
with hanging mercury drop as a working electrode, the counter electrode
was a platinium wire and Ag/Ag/I/Iꢀ system was used as a reference
electrode.
Preparative electrolyses were carried out in a conventional H-cell with
a medium porosity sintered glass separator between the anolyte and the
catholyte. Mercury was used as a cathode and Pt as an anode. The poten-
tiostat and coulometer were Tacussel radiometer PJT 35-2 and IG5-N
respectively.
In a typical preparative electrolysis hydrazonate 1 (0.5 g) was reduced
in the conditions described in Table 3 under nitrogen flow and magnetic
stirring. At the end of electrolysis, the catholyte was poured into 200 ml
of water.
The resulting mixture was extracted three times with 50 ml ether. The
etheral phase was washed by small amounts of water and dried over magne-
sium sulfate. The ether was removed and the residue was purified by column
chromatography on silica gel 60 (0,05–0,2 mm) using ethyl acetate/
petrol ether (v : v ¼ 3 : 7) as eluent. Pure compounds 2 were identified by
IR and NMR spectra.
2a: ir: nC¼N ¼ 1633; nN-H ¼ 3520. nmr (1H): 1.2 (t, 3H); 1.5 (d, 3H); 2.1
(s, 3H); 4.1 (q, 2H); 4.8 (q, 1H); 7.3 (mu, 5H); 7.8 (ma;1H).
2b: ir: nC¼N ¼ 1634; nN-H ¼ 3522. nmr (1H): 1.2 (t, 3H); 1.6 (d, 3H); 2
(s, 3H); 4 (q, 2H); 4.5 (q, 1H); 5 (ma; 1H); 7.5 (mu, 4H).
2c: ir: nC¼N ¼ 1632; nN-H ¼ 3447. nmr (1H): 1.2 (t, 3H); 1.5 (d, 3H); 2.1
(s, 3H); 4.2 (q, 2H); 5 (q, 1H); 7 (mu, 3H); 7.8 (ma;1H).
2d: ir: nC¼N ¼ 1633; nN-H ¼ 3520. nmr (1H): 1.2 (t, 3H); 1.8 (d, 3H); 4.5
(q, 2H); 5.3 (q, 1H); 7 (mu, 5H); 7.5 (ma;1H); 7.8 (s, 1H).
For electrolysis run in protic medium, phenol (m ¼ 2 g) was added
to the catholyte in the begninning of the electrolysis. After electrolysis,
the catholyte was separated and a Na2CO3 saturated solution was
added. Electrolysis product was extracted with diethylic ether. The products
of electrolysis 3 were identified by IR and NMR spectra.
3a: ir: nC¼N ¼ 1673; nN-H ¼ 3351. nmr (1H): 2.2 (s, 3H); 7.6 (mu, 5H);
9.3 (ma, 1H).
3b: ir: nC¼N ¼ 1681; nN-H ¼ 3348. nmr (1H): 2.2 (s, 3H); 8 (mu, 4H);
9.79 (ma, 1H).
3c: ir: nC¼N ¼ 1673; nN-H ¼ 3351. nmr (1H): 2.23 (s, 3H); 7.26 (mu, 3H);
8.8 (ma, 1H).

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ELECTROCHEMICAL BEHAVIOR OF HYDRAZONATES
ACKNOWLEDGMENT
233
Authors wish to thank SERST (Lab-CH02) for financial support and
assistance.
REFERENCES
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1971, 1918.
4. Vagina, G.A.; Troepol’skaya, T.V.; Kitaev, Yu. P. Izv. Akad. Nauk
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5. Fohlendorff, R.; Lund, H. Acta Chim. Scand. 1988, B42, 52.
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13. Chihaoui, M.; Baccar, B.C.R. Acad. Sciences. Paris 1978, 287, 69.
Received in the UK May 2, 2000

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