An efficient electrochemical synthesis of diamino-o-benzoquinone: Mechanistic and kinetic evaluation of the reaction of azide ion with o-benzoquinone

Article abstract of DOI:10.1039/b612224h

An efficient method for the synthesis of diamino-o-benzoquinone based on the Michael reaction of electrochemically generated o-benzoquinone with azide ion is described, as well as an estimation of the homogeneous rate constant (k_obs) of the rea

Full text of DOI:10.1039/b612224h

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An efficient electrochemical synthesis of diamino-o-benzoquinone:
Mechanistic and kinetic evaluation of the reaction of azide ion with
o-benzoquinone{
a
Davood Nematollahi,* Abbas Afkhami, Esmail Tammari, Tahere Shariatmanesh, Mahdi Hesari and
a
b
a
ac
d
Maryam Shojaeifard
Received (in Cambridge, UK) 25th August 2006, Accepted 3rd October 2006
First published as an Advance Article on the web 25th October 2006
DOI: 10.1039/b612224h
An efficient method for the synthesis of diamino-o-benzoqui-
none based on the Michael reaction of electrochemically
generated o-benzoquinone with azide ion is described, as well
as an estimation of the homogeneous rate constant (kobs) of the
reaction of o-benzoquinone with azide ion by the digital-
simulation method.
current ratio (IpC1/IpA1) of nearly unity, which can be considered as
a criterion for the stability of o-benenzoquinone (2), produced at
5
the surface of the electrode under the experimental conditions.
The oxidation of catechol (1) in the presence of azide ion as a
nucleophile was studied in some detail. Fig. 1 (curve b) shows the
cyclic voltammogram obtained for a 1 mM solution of 1 in the
presence of 5 mM azide ion in aqueous solution containing 0.2 M
phosphate buffer (pH 6.5). The voltammogram exhibits one
anodic peak A and four cathodic peaks (‘‘C + C ’’, C and C ).
Amino derivatives of quinones, important building blocks in the
1
synthesis of a variety of natural products, medicinal compounds
1
1
2
3
4
2
with antitumor and antimalarial activities are usually prepared by
These peaks are related to the reduction of intermediates 2, 5ox
(counterpart of 5), 4 and 6 respectively. The second cyclic
voltammogram shows that, parallel to the decrease in current of
1,3
alternative, multistep synthetic pathways.
Also, the ortho-
benzoquinone derivatives have been studied to a lesser extent
than the para-benzoquinone derivatives because they are generally
2b,4
more difficult to prepare in even moderate yields.
A and the shift of its potential in a positive direction, three new
1
anodic peaks (A , A and A ) appear at less positive potentials
4
2
3
We have a long-standing interest in the study of Michael
additions to quinones and their use in the synthesis of compounds
5
with natural patterns and of biological interest. The obvious need
(Fig. 1 (curve c)). These new peaks are related to electro-oxidation
of intermediates 5, 4red and 6red respectively. Intermediates, 4red and
6red are counterparts of 4 and 6 respectively. The positive shift of
for a convenient and high yielding synthetic method for the
preparation of ortho-aminoquinones has prompted us to investi-
gate the electrochemical oxidation of catechol in the presence of
azide ion. We now report a new synthetic strategy involving the
electro-oxidation of catechol to reach 4,5-diamino-o-benzoquinone
in a single step with an environmentally friendly reagentless
method in aqueous solution with high atom economy in ambient
conditions in an undivided cell using a graphite electrode.
Furthermore the observed homogeneous rate constant (kobs) of
the intramolecular reaction of o-benzoquinone with azide ions
has been estimated by the digital simulation of the cyclic
voltammograms.
the A peak in the presence of azide ion is probably due to the
1
formation of a thin film of product at the surface of the electrode,
inhibiting to a certain extent the performance of the electrode
process. Also, it is seen that proportional to the augmentation of
the potential sweep rate, parallel to the increase in height of the C1
2 3 4
the height of C , C and C decreases. A similar situation is
observed when the azide ion to 1 concentration ratio is decreased.
A plot of peak current ratio (IpA1/IpC1) versus logarithm of scan
rate for a mixture of catechol (1) and azide ion, appearing as an
increase in the height of the cathodic peak C at higher scan rates,
1
The cyclic voltammogram of a 1 mM solution of catechol (1) in
aqueous solution containing 0.2 M phosphate buffer (pH 6.5) is
shown in Fig. 1 (curve a). The voltammograms show one anodic
(A ) and a corresponding cathodic peak (C ), which corresponds
1 1
to the transformation of catechol (1) to o-benzoquinone (2) and
vice-versa within a quasi-reversible two-electron process. A peak
a
Faculty of Chemistry, University of Bu-Ali-Sina, Hamadan, 65174,
Iran. E-mail: nemat@basu.ac.ir; Fax: +98-811-8272404
b
Asadabad Center, Region 6, Payame Noor University, Iran
Department of Soil Science, Faculty of Agriculture, University of
c
Bu-Ali-Sina, Hamadan, 65174, Iran
Department of Chemistry, Graduate School, Azad University, Arak,
Iran
d
1
Electronic supplementary information (ESI) available: H NMR,
13
C
Fig. 1 Cyclic voltammograms of 1 mM catechol: (a) in the absence of
azide ion, (b) in the presence of 5 mM of azide ion, the first cycle and (c)
second cycle, at a glassy carbon electrode, in aqueous solution containing
{
NMR, FT-IR, MS of 8a and cyclic voltammograms of the electrooxida-
tion of catechol in the presence of azide at various pH and various scan
rates. See DOI: 10.1039/b612224h
21
0.2 M phosphate buffer (pH 6.5). Scan rate, 50 mV s . t = 25 ¡ 1 uC.
1
62 | Chem. Commun., 2007, 162–164
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Fig. 2 Cyclic voltammograms of 0.15 mM catechol in the presence of
.75 mM of sodium azide, at a glassy carbon electrode during controlled
0
potential coulometry at 0.40 V (versus SCE). (a) to (e) in the course of
coulometry at 0, 30, 80, 200 and 400 min. Other conditions are the same as
reported in Fig. 1.
confirms the reactivity of 2 towards azide ion. On the other hand,
1
/2
the current function for the A
1
peak (IpA1/v ) changes on
increasing the scan rate.
Controlled-potential coulometry was performed in aqueous
solution containing 0.15 mM of 1 and 0.75 mM of azide ion in a
divided cell at 0.40 V (versus SCE). Monitoring of the progress of
the electrolysis was carried out by cyclic voltammetry (Fig. 2).
During coulometry, in parallel with the decrease in height of
Scheme 2
intramolecular oxidation–reduction reactions (eqn (5) and (6))
converts 4 to diamino-o-benzoquinone 6 as the final product (68%
isolated yield).{ Formation of amino-o-benzoquinone (4) or
diamino-o-benzoquinone (6) under intramolecular oxidation–
reduction reaction conditions can be rationalized as shown in
Scheme 2. According to this mechanism, 3b is converted to imino
1 1
anodic peak A and its cathodic counterpart (C ), other anodic
and cathodic peaks appear and the height of them increases. At the
end of the coulometry all of the anodic and cathodic peaks
disappear and only anodic and cathodic peaks A
4
and C
4
remain.
2
intermediate 3d by loss of N . Compound 3d is then readily
The residual A
4
and C
4
peaks are related to the redox reactions of
disappears when the charge consumption
transformed to the stable tautomeric form of 4-amino-o-benzo-
6a
6. The anodic peak A
1
quinone (4).
2
becomes about 4.7 e per molecule of 1. These observations allow
us to propose the pathway in Scheme 1 for the electrochemical
oxidation of 1 in the presence of azide ion.
In order to elucidate more of the reaction mechanism and also
to determination of the optimal conditions for the addition of
azide ion to catechol, we studied the electrochemical oxidation of
catechol (1) in the presence of azide ion at various pHs. The results
According to our results, it seems that the Michael addition
reaction of azide ion to o-benzoquinone (2) (eqn (3)) leads to
intermediate 3. In the next step, azidocatechol 3, via an
indicate an increase in anodic peak current (A ) against a decrease
1
in current of cathodic peaks (C , C and C ) with decreasing pH.
2
3
4
6
Also, in controlled potential coulometry the number of consumed
electrons increases with decreasing pH. These observations allow
us to propose a homogeneous catalytic pathway in Scheme 1,
eqn (2) for the electrochemical oxidation of 1 in the presence of
overall intramolecular oxidation–reduction reaction, converts to
amino-o-benzoquinone 4 (eqn (4)). Next Michael addition and
7
azide ion in acidic medium. .§ This type of reaction, previously
reported by Couladouros and co-workers in the reaction of
6
a
hydrozoic acid with naphthoquinone, is confirmed by other
typical voltammetric tests. This reaction is also responsible for
2
increasing the number of consumed electrons from 2 to 4.7 e per
molecule of 1 (Scheme 1). According to our results, the rate of
formation of the azido adduct 3 increases proportionally with the
increase in pH. In basic solutions, the peak current ratio (IpC1/IpA1
)
of catechol (in the absence of nucleophiles) is significantly less than
unity and dramatically decreases with increasing pH. These can be
related to the coupling of anionic or dianionic forms of catechols
8
with o-benzoquinone (dimerization reaction). The rate of this
reaction is pH dependent and increases with increasing pH.
2 3 4
Therefore, in basic solutions the cathodic peaks C , C and C
have been removed from the cyclic voltammogram of catechol in
the presence of azide ion. Because of the decrease in the rate of
electron transfer reaction between azide ion and o-benzoquinone
(
2) (Scheme 1, eqn (2)) on the one hand and the decrease in the rate
of homogeneous dimerization reaction on the other hand, pH 5.0–
.5 has been selected as a suitable medium for the electrochemical
6
study and synthesis of diamino-o-benzoquinone (6).
Constant-current coulometry was performed in an aqueous
solution containing 0.25 mM of 1 and 1.25 mM of azide ion in an
Scheme 1
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Chem. Commun., 2007, 162–164 | 163

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of experimental and related simulated results (Fig. 4), confirm the
validity of the estimated kobs
.
We would like to thank Dr Rudolph for his free cyclic
voltammogram digital simulation software (Digielch).
Notes and references
{
Synthesis of 6 using controlled potential method: A solution (ca. 80 mL)
of acetate buffer solution (pH 5.0, 0.2 M) containing 2 mM of catechol (1),
and 10 mM of sodium azide was electrolyzed in an undivided cell equipped
with a carbon anode (an assembly of four rods) and a large platinum gauze
as cathode, at 25 uC at 0.40 V vs. SCE. The electrolysis was terminated
when the current decayed to 5% of its original value. At the end of the
electrolysis, the cell was placed overnight at 40 uC. The precipitated solid (6)
(violet crystals) was collected by filtration and washed with water. The
resulting violet crystals (mp . 300 uC) were characterized by IR (KBr)
Fig. 3 (I) Variation of peak current IpA1 versus charge consumed during
constant-current coulometry. (II) Cyclic voltammograms of 0.25 mM
catechol (1) in the presence of 1.25 mM sodium azide during constant-
current coulometry. (III) Cyclic voltammogram of final product. In
aqueous solution containing 0.20 M acetate buffer (pH 5.0). Other
conditions are the same as reported in Fig. 1.
21
(cm ): 3421, 3276, 3063, 1729, 1693, 1594, 1515, 1470, 1343, 1294, 834,
1
7
48, 670; H NMR: d ppm (90 DMSO-d
6
): 5.29 (s, 2H), 7.36 (broad, 4H);
): 98.7, 153.3, 178.8; MS (m/z) (relative
intensity): 138 (34), 110 (19), 94 (11), 83 (46), 68 (46), 41 (100).
1
3
CNMR, d ppm (125 DMSO-d
6
10
2
.
§
The oxidation product of azide ion is N
"
under similar conditions as for the controlled potential method, under a
Synthesis of 6 using the controlled current method: Synthesis performed
2
2
constant current density of 2 mA cm . The quantity of the electricity
passed was determined using the exponential curve and related equation in
Fig. 3.
1
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Fig. 4 (a) The correlation between simulated and experimental results.
(b) Variation of peak current IpA1 versus azide ion concentration.
2
2
2 (a) T. S. Lin, S. P. Xu, L. Y. Zhu, A. Divo and A. Sartorelli, J. Med.
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undivided cell, under current density 2 mA cm . Monitoring of
the progress of the electrolysis was performed by cyclic
voltammetry (Fig. 3). The results are the same as those reported
for the controlled potential coulometry. A characteristic feature of
the electrolysis is the use of low current density: the current
efficiency and yield of product decrease with increasing current
density. These observations can be explained by the occurrence of
some back reactions, such as the reduction of o-benzoquinones 2, 4
or 6 on the cathode and side reactions such as oxidation of the
nucleophile and/or solvent during constant current electrolysis in
an undivided cell. In this work to improve the applicability of the
procedure, the electrochemical synthesis of 6 was also performed
4
C. Kashima, A. Tomotake and Y. Omote, J. Org. Chem., 1987, 52,
616–5621.
5
5 (a) D. Nematollahi and E. Tammari, J. Org. Chem., 2005, 70,
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2006, 1631–1633.
2
2
by means of constant current electrolysis (ca. 2 mA cm )."
21 21
The observed homogeneous rate constant (kobs/M
6
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s ) of the
reaction of o-benzoquinone (2) with azide ion has been estimated
9
by comparison of the simulation results with the experimental
cyclic voltamograms. The simulation was performed based on an
ECE electrochemical mechanism. The procedure is performed
based on achieving the best fit between simulated and experimental
cyclic voltammograms. This method is applied to a variety of scan
rates and nucleophile concentrations. The estimated value for kobs
7 A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley, New
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8
(a) D. Nematollahi, M. Rafiee and A. Samadi-Maybodi, Electrochim.
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9 M. Rudolph, J. Electroanal. Chem., 2002, 529, 97–108. Also, see: http://
www.digielch.de/.
21 21
is 35 ¡ 3 M
s
f b
and k /k is 150 ¡ 10. A correlation of 99.7%
1
0 (a) A. Dalmia, S. Wasmus, R. F. Savinell and C. C. Liu, J. Electrochem.
Soc., 1996, 143, 556–560; (b) P. Bandyopadhyay, B. B. Dhar,
J. Bhattacharyya and S. Mukhopadhyay, Eur. J. Inorg. Chem., 2003,
24, 4308–4312.
between oxidation peak current (IpA1) of two sets of experimental
and related simulated cyclic voltammograms over a variety of scan
rates and azide ion concentrations, and also close correspondence
1
64 | Chem. Commun., 2007, 162–164
This journal is ß The Royal Society of Chemistry 2007