Electrochemical synthesis and mechanestic study of quinone imines exploiting the dual character of N,N-dialkyl-p-phenylenediamines

Article abstract of DOI:10.1021/ol103146k

Two one-pot electrochemical approaches for the synthesis of similar quinone imines via either the nucleophilic character of N,N-dialkyl-pphenylenediamines or electrophilic reactivity of their oxidized forms through Michael-type addition reactions are reported.

Full text of DOI:10.1021/ol103146k

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1928–1931
Electrochemical Synthesis and
Mechanestic Study of Quinone
Imines Exploiting the Dual
Character of N,N-Dialkyl-p-
phenylenediamines
Abbas Maleki and Davood Nematollahi*
Faculty of Chemistry, Bu-Ali-Sina University, Hamedan 65178À38683, Iran
nemat@basu.ac.ir
Received January 26, 2011
ABSTRACT
Two one-pot electrochemical approaches for the synthesis of similar quinone imines via either the nucleophilic character of N,N-dialkyl-p-
phenylenediamines or electrophilic reactivity of their oxidized forms through Michael-type addition reactions are reported.
Synthesis of molecules with desirable functional groups in
the absence of catalysts and drastic conditions continues to
be of great interest. In organic synthesis, electrochemical
electron-transfer-driven reactions have been widely used for
various transformations.1 However, their potential has not
yet been fully utilized. Quinone imines and diimines have
been of long-standing interest in chemistry.2 Quinone imine
dyes are commonly used as redox indicators3,4 as well as
solvent polarity indicators.5 The preparation of simple
quinone imines by chemical oxidation of aniline derivatives
is often complicated by the reactivity of the quinone imine
under the reaction conditions. Thus, the electrochemical
synthesis of stable quinone imines is desirable. In previous
studies, we have shown that N,N-dialkyl-p-phenylenedi-
amines and catechols can be oxidized electrochemically to
corresponding quinone diimines6 and quinones,7 respec-
tively, that can, further, be attacked as Michael acceptors
by a variety of nucleophiles. To the best of our knowledge,
designing two synthesis platforms in order to synthesize a
common pruduct with a specific functional group via two
(6) (a) Nematollahi, D.; Maleki, A. Electrochem. Commun. 2009, 11,
488. (b) Maleki, A.; Nematollahi, D. Electrochem. Commun. 2009, 11,
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(7) (a) Nematollahi, D.; Habibi, D.; Rahmati, M.; Rafiee, M. J. Org.
Chem. 2004, 69, 2637. (b) Nematollahi, D.; Rafiee, M. Green Chem.
2005, 7, 638. (c) Davarani, S. S. H.; Nematollahi, D.; Shamsipur, M.;
Najafi, N. M.; Masoumi, L.; Ramyar, S. J. Org. Chem. 2006, 71, 2139.
(d) Nematollahi, D.; Goodarzi, H. J. Org. Chem. 2002, 67, 5036. (e)
Nematollahi, D.; Amani, A.; Tammari, E. J. Org. Chem. 2007, 72, 3646.
(f) Nematollahi, D.; Workentin, M. S.; Tammari, E. Chem. Commun.
2006, 1631. (g) Nematollahi, D.; Afkhami, A.; Tammari, E.;
Shariatmanesh, T.; Hesari, M.; Shojaeifard, M. Chem. Commun. 2007,
162. (h) Nematollahi, D.; Tammari, E. J. Org. Chem. 2005, 70, 7769.
(i) Nematollahi, D.; Rafiee, M.; Fotouhi, L. J. Iran. Chem. Soc. 2009,
6, 448.
(1) Yoshida, J.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev.
2008, 108, 2265.
(2) Swenton, J. S.; Shih, C.; Chen, C.; Chou, C. J. Org. Chem. 1990,
55, 2019.
(3) Barra, M.; Croll, L. M.; Tan, A.; Tao, W. Dyes Pigm. 2002, 53,
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r
10.1021/ol103146k
Published on Web 03/18/2011
2011 American Chemical Society

In the first strategy, the electrochemical oxidation of 1a was
studied in the presence of 3. Cyclic voltammetry of 1 mM of
1a shows one anodic peak (A1) and its cathodic peak (C1),
which correspond to the transformation of 1a to quinone-
diimine 2a and vice versa, through a quasi-reversible two-
electron process (Figure 1I,a).6,8 Under these experimental
conditions, a peak current ratio (IpC1/IpA1) that is close to 1
can be considered as a criterion for the stability of 2a. In the
presence of 3, the first cycle of the voltammogram of 1a shows
a decrease in the cathodic peak C1 and the appearance of a
new cathodic peak (C0) in the more negative potentials
(Figure 1I,b). In the second cycle, a new anodic peak (A0),
which is the counterpart of C0, appears with an Ep value of
À0.05 V versus SCE. This new peak is related to the electro-
oxidation of intermediate 6a. Furthermore, with increasing
the potential sweep rate and the decrease in the peak height of
C0, the peak current ratio (IpC1/IpA1) increases. An increase in
the ratio of (IpC1/IpA1) and(IpC1/IpC0) with increasing the scan
rate for a mixture of 1a and 3 confirms the reactivity of 2a
toward 3 (see Supporting Information, Figure S1).
On the other hand, the current function for peak A1
(IpA1/v1/2) decreases with increasing the scan rate and such
a behavior is an indication of an ECE mechanism.9
Monitoring the electrolysis progress by cyclic voltammetry
synchronously during controlled-potential coulometry in
an aqueous solution containing 1a and 3 at 0.15 V versus
SCE shows that as the coulometry progresses, the IpA1 and
IpC1 decrease (See Supporting Information, Figure S2).
These peaks (A1 and C1) disappear when the charge
consumption becomes about 4eÀ per molecule of 1a.
Furthermore, time-dependent absorption spectra of the mix-
ture of 1a and 3 were measured during a controlled-potential
coulometry experiment (see Supporting Information, Figure
S3). As the coulometry experiment progresses, absorption peaks
with λmax at 275 and 670 nm that belong to the blue color of the
product 7a appear and their heights gradually increase.
Figure 1. Cyclic voltammograms of (I,a and II,a) 1 mM 1a; (III,a
and IV,a) 1 mM 1b;(I,b) 1mM1a in the presence of 1 mM 3; (II,b) 1
mM 1a in the presence of 1 mM 4; (III,b) 1 mM 1b in the presence of
1mM3;(IV,b) 1mM1b inthepresenceof1mM4;(I,c) 1mM3;and
(II,c) 1 mM 4, at glassy carbon electrode, in aqueous solution conta-
ining 0.2 M phosphate buffer (pH = 8.0). Scan rate: 20 mV sÀ1
.
different pathways where a reagent with either nucleophilic
or electrophilic character is used has not yet been reported.
In the current study, we present two one-pot easy electro-
chemical approaches by which N,N-dialkyl-p-phenylenedi-
amines having nucleophilic or electrophilic reactivities can be
successfully applied to the synthesis of similar quinone
imines. Either the nucleophilic characteristics of N,N-di-
alkyl-p-phenylene diamines or electrophilic reactivity of their
oxidized forms which subsequently engage in the Michael
addition reaction were utilized for the synthesis of similar
quinone imines. The following two electrochemical strategies
based on the dual character of N,N-dialkyl-p-phenylenedi-
amines, electrochemical oxidation of N,N-diethyl-p-phenyle-
nediamine (1a) and N,N-dimethyl-p-phenylenediamine (1b)
in the presence of phenol (3) and 1-naphthol (4) as nucleo-
philes, were studied. In addition, the electrochemical oxida-
tion of 4-tert-butylcatechol (1c) in the presence of 1a and 1b
as nucleophiles was investigated.
The diagnostic criteria of cyclic voltammetry, the con-
sumption of four electrons per molecule of 1a, and the
spectroscopic data of the final product support the qui-
none-imine structure of 7a.
According to our results, the Michael addition reaction of
the anion phenolate 3 to quinone-diimine 2a is faster than
other secondary reactions, leading to the intermediate 6a.
Because the oxidation of intermediate 6a is easier than the
oxidation of 1a, following the chemical reaction the apparent
number of electrons transferred increases from 2 to 4 elec-
trons and quinone-imine 7a is synthesized. The electroche-
mical oxidation of 1a in the presence of 1-naphthol (4) pro-
ceeds in a similar way to that of 3 (see Supporting Informa-
tion, Scheme S1) and produces quinone-imine 9a as the final
product (see Scheme 1, inset). However, its cyclic voltammo-
gram shows an intense increase in the current for peak A1 as
well as a complete disappearance of peak C1 even with high
scan rates (Figure 1II,b). This might be related to the adsorp-
tion of 4 on the surface of the electrode. Under these condi-
tions, the surface concentration of 4 is high and remains
(8) (a) Seymour, E. H.; Lawrence, N. S.; Compton, R. G. Electro-
analysis 2003, 15, 689. (b) Kershaw, J. A.; Nekrassova, O.; Banks, C. E.;
Lawrence, N. S.; Compton, R. G. Anal. Bioanal. Chem. 2004, 379, 707.
(c) Lawrence, N. S.; Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.;
Compton, R. G. Electroanalysis 2000, 12, 1453. (d) Lawrence, N. S.;
Davis, J.; Jiang, L.; Jones, T. G. J.; Davies, S. N.; Compton, R. G.
Electroanalysis 2001, 13, 432. (e) Lawrence, N. S.; Jiang, L.; Jones,
T. G. J.; Compton, R. G. Anal. Chem. 2003, 75, 2054. (f) White, P. C.;
Lawrence, N. S.; Davis, J.; Compton, R. G. Anal. Chim. Acta 2001,
447, 1.
(9) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001; pp 512À515.
Org. Lett., Vol. 13, No. 8, 2011
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Scheme 1. Strategy 1 for the Synthesis of Quinone Imine
Scheme 2. Electrochemical Synthesis of 11b by Strategy 1
essentially unchanged during the voltammetric experiment.
However, 2a is gradually generated at the electrode surface
and reacts with the large amount of 4. In this case, the rate of
reaction of 4 with 2a increases and subsequent oxidation
leads to a fast growth in the current of peak A1. The rapid re-
action of generated 2a with 4causes the surface concentration
of 2a to become effectively zero. Such behavior is a general
treatment of the ECE system with pure kinetic conditions.9
To follow this strategy for the electrochemical synthesis
of quinone imines, electrochemical oxidation of 1b in the
presence of 3 and 4 was studied under the same conditions.
The electrochemical oxidation of 1b in the presence of 3
proceeds similarly to that of 1a (Scheme 1); however, the
electrochemical oxidation of 1b in the presence of 4
proceeds differently.
4-tert-butylcatechol (1c) was studied in the presence of 1a
and 1b as nucleophiles in an aqueous solution containing
0.2 M phosphate buffer (pH = 7.5). Cyclic voltammetry of
1c shows one anodic (A1) and the corresponding cathodic
peak (C1), which correspond, through a quasi-reversible
two-electron process, to the transformation of 1c to related
o-benzoquinone 2c and vice versa (Figure 2, curve a).7 Also
one anodic (A2) and the corresponding cathodic peak (C2)
were observed in the cyclic voltammograms of 1b alone
(Figure 2, curve b). Figure 2 (curve c) shows the cycle
voltammogram of 1c in the presence of 1b. Under these
conditions, IpA1 increases and its cathodic counterpart (C1)
decreases. Also the voltammogram exhibits a new cathodic
peak C0 at more negative potentials. In the second cycle
(curve d), a new peak (A0) appearswithanEp value of À0.14 V
versus SCE. This new peak is related to the oxidation of
intermediate 13b. Furthermore, the height of the peak C1
increases with increasing potential sweep rate (see Support-
ing Information, Figure S4). This indicates the reactivity of
electrochemically generated o-benzoquinone 2c toward 1b.
These observations are indicative of an ECE mech-
anism9 and allow us to propose a mechanism for the
electrooxidation of 1c in the presence of 1a,b (Scheme 3).
Accordingly, the 1,4-addition (Michael) reaction of 1a,b to
the o-benzoquinone 2c leads to the intermediate 13a,b. The
oxidation of 13a,b is easier than the oxidation of 1c due to
the presence of an amine group as an electron-donating
group. Therefore, 1c in the presence of 1a,b after consump-
tion of four electrons converts to quinone-imine 14a,b.
In the case where two substrates are susceptible to oxida-
tion and their oxidation potentials are close to each other, the
Cyclic voltammograms of 1b in the presence of 4 have
been shown in Figure 1IV,b. Controlled-potential coulo-
metry of 1b in an aqueous solution containing 0.2 M
phosphate buffer (pH = 8.0) at 0.15 V versus the SCE
yields an napp value of about 3.
The diagnostic criteria of cyclic voltammetry, the con-
sumption of three electrons per molecule of 1b, and the
spectroscopic data of the isolated product indicate that the
reaction mechanism of the electrooxidation of 1b in the
presence of 4 is ECECE (Scheme 2). According to our
results, it seems that the Michael-type addition reaction of
4 to quinone-diimine 2b leads to the intermediate 8b.
Another oxidation process converts 8b to quinone-diimine
9b, and next Michael addition of 1b to 9b transforms 9b to
intermediate 10b. Because of the presence of an electron-
donating group, the oxidation of this compound (10b) is
easier than the oxidation of 1b. Further oxidation converts
intermediate 10b to the final product 11b.
Based on Scheme 2, in Figure 1IV,b, the anodic peaks of
A1 and A2 pertain to the oxidation of 1band 8b toquinone-
diimines 2b and 9b, respectively. Also the anodic peak A0
and its counterpart (C0) correspond to the oxidation of 10b
to 11b and vice versa. Another less probable pathway for
the formation of 11b is proposed in Supporting Informa-
tion, Scheme S2.
In the second strategy for the electrochemical synthesis
of quinone imines, 1a and 1b act as nucleophiles. Using
cyclic voltammetry, the electrochemical oxidation of
1930
Org. Lett., Vol. 13, No. 8, 2011

Scheme 3. Strategy 2 for the Synthesis of Quinone Imine
one-step process conducted under ambient conditions are
attractive features with respect to green chemistry ideas.
Although one-pot reactions are performed potentios-
tatically on a mmol scale in divided cells, there is little dif-
ficulty in producing larger quantities by using larger cells.
Information for the preparative electrolysis including
the potential, electricity, temperature, and yields is de-
scribed in the Supporting Information.
Figure 2. Cyclic voltammogram of 1 mM (a) 1c, (b) 1b, (c) 1c in
the presence of 1b, and (d) initial part of second cycle of cyclic
voltammogram of 1c in the presence of 1b, at glassy carbon
electrode, in aqueous solution containing 0.2 M phosphate
buffer (pH = 7.5). Scan rate: 50 mV sÀ1; t = 25 ( 1 °C.
choice of an appropriate potential is often crucial for the
success of an electrode reaction. In this case, for the
selective oxidation of 1c, a controlled-potential method
was used. In contrast to one-pot chemical synthesis where
there is no control on the oxidation of a single component,
controlled-potential electrolysis serves as a powerful meth-
od for the synthesis of a desirable product via selective
oxidation of a target reagent. As expected, at the small
applied potential of 0.05 V, the selective electrochemical
oxidation of 1c proceeds preferentially, thereby providing
the desired quinone imine 14a,b.
Acknowledgment. We thank Prof. Wolfgang Schuh-
€
mann at the Ruhr-Universitat Bochum-Germany for the
high resolusion mass spectra. The authors acknowledge
the Bu-Ali Sina University Research Council and the
Center of Excellence in Development of Chemical Meth-
ods (CEDCM) for supporting this work.
Supporting Information Available. Experimental pro-
cedures, some electrochemical investigations, and full
spectroscopic data for all compounds. This material is
available free of charge via the Internet at http://pubs.
acs.org.
From the point of view of green chemistry, the use of an
electrosynthesis method has some important advantages.
Clean synthesis, the use of electricity instead of chemical
reagents, and the achievement of high atom economy via a
Org. Lett., Vol. 13, No. 8, 2011
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