Electrochemical production of novel products from 2,3-dimethylhydroquinone in the presence of some β-diketones

Article abstract of DOI:10.1007/s00706-009-0114-x

Electrochemical oxidation of 2,3-dimethylhydroquinone has been studied in the presence of β-diketones as nucleophiles in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the derivatives of 2,3-dimeth

Full text of DOI:10.1007/s00706-009-0114-x

Monatsh Chem (2009) 140:645–649
DOI 10.1007/s00706-009-0114-x
ORIGINAL PAPER
Electrochemical production of novel products
from 2,3-dimethylhydroquinone in the presence
of some b-diketones
S. Makarem Æ A. R. Fakhari Æ Ali A. Mohammadi
Received: 7 September 2008 / Accepted: 5 December 2008 / Published online: 24 March 2009
Ó Springer-Verlag 2009
Abstract Electrochemical oxidation of 2,3-dimethylhy-
droquinone has been studied in the presence of b-diketones
as nucleophiles in aqueous solution using cyclic voltam-
metry and controlled-potential coulometry. The results
indicate that the derivatives of 2,3-dimethylhydroquinone
participate in a Michael addition reaction to form the
corresponding benzofuran derivatives. The electrochemical
synthesis has been successfully performed at pH 7.0 and
E = 0.1 V versus an Ag|AgCl|KCl (3 M) in an undivided
cell in good yield and purity.
compounds used in other fields of chemistry and in agri-
culture [5].
As we know, one of the prime principles of green
chemistry is to develop an alternative reaction medium,
and synthesis in water by an electrochemical method is the
basis for the development of many cleaner chemical
technologies. With regard to electrochemical oxidation of
hydroquinones in the presence of different nucleophiles,
the main purpose of previous studies was to investigate
mechanistic aspects [7–16]. It has been clearly shown that
the electrooxidation of hydroquinones with electron–with-
drawing groups in aqueous solution leads to corresponding
benzoquinones with consumption of approximately two
electrons per molecule in each step of the oxidation
[17–20]. For this reason, we report the electrochemical
oxidation of 2,3-dimethylhydroquinone in the presence of
b-diketones as nucleophiles.
Keywords Cyclic voltammetry Á Electrochemical
synthesis Á ECEC mechanism Á ECECE mechanism Á
2,3-Dimethylhydroquinone
Introduction
The importance of heterocyclic compounds can not be
overstated; approximately one half of all known organic
compounds contain a heterocyclic ring. These compounds,
either natural products or designed molecules, are respon-
sible for an amazing variety of biological processes and they
feature in most pharmaceuticals [1]. Benzofuran derivatives
are an important class of heterocyclic compounds that are
known to possess important biological properties. Applica-
tions of substituted benzofurans include use as fluorescence
sensors [2], oxidants [3], antioxidants [4], and brightening
agents [5], and they are the basic skeleton of numerous
compounds possessing cardiovascular activity [6] and of
Results and discussion
The cyclic voltammogram of 2 mM 2,3-dimethylhydro-
quinone (1) in a mixture of water–acetonitrile (80:20)
containing 0.20 M phosphate (KH2PO4/K2HPO4) solution
at pH 7.0 shows one anodic peak (A1) and a corresponding
cathodic peak (C1), which correspond to the transformation
of 1 to p-benzoquinone (2) and vice versa, within a quasi-
reversible two-electron process (Fig. 1, curve a). A peak
current ratio (ICP1/IPA1) of nearly unity, particularly during
repetitive recycling of the potential, can be considered as a
criterion for the stability of p-benzoquinone produced at
the surface of electrode under these experimental condi-
tions. In other words, any possible hydroxylation [21–24]
or dimerization [25] reactions are too slow to be observed
on the time scale of the cyclic voltammetry. The oxidation
S. Makarem Á A. R. Fakhari (&) Á A. A. Mohammadi
Department of Chemistry, Faculty of Sciences,
Shahid Beheshti University, G. C., Evin, Tehran, Iran
e-mail: a-zavareh@sbu.ac.ir
123

646
S. Makarem et al.
of the electrode process. In this figure, curve c is the vol-
tammogram of 2 mM 3a alone in the buffer solution.
It is seen that, in proportion to the augmentation of the
potential sweep rate, the height of the C1 peak increases
(Fig. 2, curves a–d). A similar situation was observed when
the concentration ratio of 3a to 1 was decreased. The
current function for the A1 peak, IAP 1/m1/2, changes with an
increase in scan rate (Fig. 2, curve i). On the other hand, a
plot of the peak current ratio (ICP1/IPA1) against the scan rate
for a mixture of 1 and 3a, revealed an increase in the height
of the cathodic peak (C1) at higher scan rates (Fig. 2, curve
j) and such behavior is adopted as indicative of an ECEC
mechanism [23–25].
I/µA
A•
20
10
0
b
a
c
-10
C•
Controlled-potential coulometry was performed in an
aqueous solution containing 0.25 mmol 1 and 0.25 mmol
b-diketones 3a–3c at 0.10 V versus an Ag|AgCl|KCl (3 M)
electrode. The electrolysis progress was monitored by the
use of cyclic voltammetry (Fig. 3).
-20
-0.4
0
0.4
0.8
E/V versus Ag|AgCl|KCl (3 M)
Fig. 1 Cyclic voltammograms of 2 mM 2,3-dimethylhydroquinone
(1) in the absence a and presence b of 2 mM 4-hydroxy-1-methyl-
2(1H)-quinolone (3a); (c) cyclic voltammogram of 2 mM 3a at a
glassy carbon electrode (1.8 mm diameter) in a mixture of water–
acetonitrile (80:20) containing 0.20 M phosphate (KH2PO4/K2HPO4)
solution of pH 7.0. Scan rate: 100 mV s-1; t = 25 1 °C
It is shown that, in proportion to the advancement of
coulometry, the anodic peak (A1) decreases and disappears
when the charge consumption becomes approximately 4e-
per molecule of 1 in the presence of 3a and 3b and 6e- in
the presence of 3c. These observations allowed us to pro-
pose the pathway shown in Scheme 1 for electrochemical
oxidation of 1 in the presence of 3a–3c. According to our
results, it appears that the 1,4-Michael addition reaction of
the anion enolate of 3a–3c with 2 (Eq. 2 in Scheme 1) is
much faster than other secondary reactions, presumably
leading to the intermediates 4a–4c. The oxidation of 4a–4c
is easier than the oxidation of the parent starting molecule
1 by virtue of the presence of an electron-donating group
[26] and it can be oxidized at the electrode surface (Eq. 3
in Scheme 1) or by chemical reaction (Eq. 4 in Scheme 1).
of 1 in the presence of 4-hydroxy-1-methyl-2(1H)-quino-
lone (3a) as a nucleophile was studied in some detail.
Figure 1, curve b shows the cyclic voltammogram obtained
for a 2 mM solution of 1 in the presence of 2 mM 3a. The
voltammogram exhibits one anodic peak at 0.08 V versus
an Ag|AgCl|KCl (3 M) electrode and the cathodic coun-
terpart of the anodic peak (C1) decreases. The anodic and
cathodic peaks shift in the presence of 3a (Fig. 1, curve b),
because of formation of a thin film of product at the surface
of the electrode, inhibiting to some extent the performance
Fig. 2 Typical voltammograms
I/µA
of 2 mM 2,3-dimethylhydro-
60
quinone in the presence of
2 mM 4-hydroxy-1-methyl-
1/2
IPC1/IPA1
IPA1
/
0.9
0.8
0.7
0.6
0.5
2.40
2.30
2.20
2.10
2.00
A•
2(1H)-quinolone at a glassy
carbon electrode (1.8 mm
40
20
d
c
b
a
j
diameter) and at various scan
rates in a mixture of water–
acetonitrile (80:20) containing
0.20 M phosphate (KH2PO4/
K2HPO4) solution at pH 7.0.
Scan rates from a to d are: 50,
100, 200, and 400 mV s-1. (i)
Variation of the peak current
function (IAP 1/m1/2) versus scan
rate. (j) Variation of the peak
current ratio (ICP1/IPA1) versus
scan rate; t = 25 1 °C
0
i
-20
-40
-60
C•
0
100
200
300
/mVs−1
400
500
-0.4
0.1
0.6
1.1
E/V versus Ag|AgCl|KCl (3 M)
123

Electrochemical producing of novel products from 2,3-dimethylhydroquinone
647
Fig. 3 (a) Cyclic
I/µA
voltammograms of 0.25 mmol
2,3-dimethylhydroquinone (1)
in the presence of 0.25 mmol
4-hydroxy-1-methyl-2(1H)-
quinolone (3a) in a mixture of
water–acetonitrile (80:20)
containing 0.20 M phosphate
(KH2PO4/K2HPO4) solution at
pH 7.0 at a glassy carbon
29.80
24.80
19.80
14.80
9.80
I/µA
A
30
25
20
15
10
5
y = -0.296x + 27.537
R2= 0.997
a
B
e
electrode (1.8 mm diameter)
during controlled-potential
coulometry at 0.10 V versus an
Ag|AgCl|KCl (3 M) electrode,
after consumption of: (a) 6.33,
(b) 17, (c) 25, (d) 32, and (e) 45
C electricity. (b) Variation of
peak current (IAP 1) versus charge
consumed. Scan rate
0
0
20
40
60
80
100
4.80
Q/C
-0.20
-0.2
-0.1
0
0.1
0.2
0.3
0.4
100 mV s-1; t = 25 1 °C
E/V versus Ag/AgCl/KCl (3 M)
Thus, the apparent number of electrons transferred incre-
ases from the limits of n = 2 for 6a and 6b to n = 4 electrons
per molecule for 6c. Whereas 6a and 6b are insoluble in the
mixture of water–acetonitrile, 6c is soluble and can then be
further oxidized to form 7c. The best results where achieved
by working at a potential of 0.1 V versus (Ag|AgCl|KCl
(3 M)) and purifying the products by washing with distilled
water. The yields obtained were then 89 (6a), 90 (6b), and
95% (7c).
gauze (2 9 2 cm2) constituted the CE. The WE potentials
were measured versus Ag|AgCl|KCl (3 M) as reference
electrode (all electrodes were obtained from Azar Electrod,
Urmia, Iran). NMR spectra were recorded on a Bruker
DRX-300 Avance instrument. IR spectra were recorded on
a Bruker IFS-66 FT-IR spectrophotometer. MS spectra were
obtained using a QP-1100EX Shimadzu GC-MS (EI at
70 eV). Melting points of the product were obtained using
an electrothermal melting point model 9200.
In conclusion, the results of this work show that 2,3-
dimethylhydroquinone is oxidized in water to its respective
benzoquinone. The benzoquinone is then attacked by the
enolate anion of b-diketones to form benzofuran derivatives
6a, 6b, and 7c. The overall reaction mechanism for the
anodic oxidation of 2,3-dimethylhydroquinone in the pre-
sence of nucleophiles is presented in Scheme 1. According
to our results, it seems that the 1,4-Michael addition of these
nucleophiles to p-benzoquinone leads to the formation of
new benzofuran derivatives as final product, in good yield
and purity.
Electrochemical synthesis of 6a, 6b, and 7c
In a typical procedure, 100 cm3 mixture of water–aceto-
nitrile (80:20) containing 0.20 M phosphate (KH2PO4/
K2HPO4) as a buffer (pH 7.0) and supporting electrolyte
was pre-electrolyzed at the chosen potential (Table 1), in
an undivided cell; subsequently, 2 mmol 2,3-dimethylhy-
droquinone (1) and 2 mmol b-diketone were added to the
cell. The electrolysis was terminated when the decay of the
current became more than 95%. The process was inter-
rupted several times during the electrolysis and the carbon
anode was washed in acetone in order to reactivate it. At
the end of electrolysis, the precipitated solid was collected
by filtration and washed with distilled water. The products
Experimental
1
were then characterized by using IR, H NMR, 13C NMR,
Cyclic voltammetry (CV) experiments were performed
using a Metrohm computerized voltammetric analyzer
model 747 VA stand. Controlled-potential coulometry and
preparative electrolysis were performed using an SAMA
potentiostat/galvanostat (Isfahan, Iran). The working elec-
trode (WE) used in the voltammetry experiments was a
glassy carbon disc (1.8 mm diameter) and a platinum wire
(2 mm diameter and 2 cm length) was the counter electrode
(CE). The WE used in controlled-potential coulometry and
macroscale electrolysis was an assembly of three carbon
rods (8 mm diameter and 6 cm length), and a large platinum
and MS.
7,10-Dihydroxy-5,8,9-trimethylbenzofuro[3,2-c]
quinolin-6(5H)-one (6a, C18H15NO4)
Mp [ 250 °C; IR (KBr): m = 3531(OH), 3077, 2900, 1650
1
(C=O), 1565, 1503 cm-1; H NMR (DMSO-d6): d = 2.06
(s, 3H, CH3), 2.11 (s, 3H, CH3), 3.37 (s, 3H, CH3), 7.19–
7.90 (m, 4H, ArH), 8.97 (s, 1H, OH), 10.61 (s, 1H, OH)
ppm; 13C NMR (DMSO-d6): d = 12.8, 29.5, 40.8, 105.4,
115.1, 116.4, 122.2, 124.1, 131.9, 139.3, 140.9, 142.0,
123

648
S. Makarem et al.
Scheme 1
OH
O
O
CH3
CH3
CH3
CH3
-2e
(1)
-2H
OH
2
1
OH
O-
H
O
O
3a-c
3a-c
OH
OH
CH3
CH3
O-
O
O-
CH3
CH3
(2)
(3)
R1
R1
R2
O
R2
O
O
3a-3c
2
4a-4c
OH
O
CH3
CH3
CH3
O-
O-
-2e,-2H
R1
CH3
R1
OH
O
O
R2
O
R2
4a-4c
5a-5c
O
OH
OH
O
CH3
CH3
CH3
(4)
O-
R2
CH3
CH3
O-
CH3
CH3
R1
R1
CH3
O
O
5a-5c
OH
OH
R2
O
O
1
2
4a-4c
O
OH
CH3
CH3
CH3
O-
O
(5)
H
R1
CH3
R1
R2
O
OH
R2
O
O
5a-5c
6a-6c
O
O
OH
CH3
CH3
CH3
CH3
O
(6)
O
-2e,-2H
S
S
OH
6c
O
F3C
O
F3C
7c
O
O
O
S
O
O
N
O
F3C
3c
3b
3a
123

Electrochemical producing of novel products from 2,3-dimethylhydroquinone
649
Yield (%)
89
Table 1 Electroanalytical and
preparative data
Product
Applied potential (V) vs
(Ag|AgCl|KCl (3 M))
Purification
OH
0.1
Washing with distilled water
CH3
CH3
O
N
OH
OH
O
O
0.1
0.1
Washing with distilled water
Washing with distilled water
90
95
CH3
CH3
O
OH
O
O
F3C
CH3
S
O
CH3
O
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149.8, 151.1, 158.3, 159.8, 161.8 and 185.1 ppm; MS: m/z
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1-(4,7-Dihydroxy-5,6-dimethyl-2-phenylbenzofuran-3-
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Mp [ 250 °C; IR (KBr): m = 3451(OH), 3079, 2922, 1633
1
(C=O), 1596, 1476 cm-1; H NMR (DMSO-d6): d = 1.90
(s, 3H, CH3), 1.98 (s, 3H, CH3), 2.13 (s, 3H, CH3), 6,58 (s,
1H, OH), 7.48–7.97 (m, 5H, ArH), 9.25 (s, 1H, OH) ppm;
13C NMR (DMSO-d6): d = 12.3, 13.5, 20.6, 114.6, 122.6,
125.4, 128.7, 129.1, 130.0, 130.1, 133.6, 136.8, 140.4,
152.8, 169.4 and 197.6 ppm; MS: m/z (%) = 296 (M?,
20), 256 (75), 238 (10), 176 (100), 151 (62.5), 105 (100),
77 (75), 43 (75).
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5,6-dimethyl-2-(thiophen-2-yl)-3-(2,2,2-trifluoroace-
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1677(C=O), 1589, 1449 cm-1 1H NMR (DMSO-d6):
;
d = 1.790 (s, 3H, CH3); 1.92 (s, 3H, CH3); 7.03–7.82
(m, 3H, ArH) ppm; 13C NMR (DMSO-d6): d = 12.4, 13.4,
122.8, 125.8, 128.3, 129.1, 132.6, 133.2, 139.3, 143.4,
146.1, 180.8, 182.4 and 189.1 ppm; 19F NMR (DMSO-d6):
d = -73.44 ppm; MS: m/z (%) = 354 (M?, 10), 285 (25),
269 (15), 203 (100), 176 (20), 111 (45), 33 (10).
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Acknowledgments We gratefully acknowledge financial support
from the Research Council of Shahid Beheshti University.
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