Indirect electrochemical oxidation of substituted polycyclic aromatic hydrocarbons to corresponding para-quinones with potassium bromide in water-chloroform medium

Article abstract of DOI:10.1016/j.tetlet.2014.08.121

Indirect electrochemical synthesis of quinone derivatives of a series of substituted anthracene and naphthalene by the electrolysis of aqueous solution of potassium bromide (3.0 M) using Pt anode at constant current density (40 mA/cm²) has been carried out. These reactions resulted in good to excellent yields of the corresponding para-quinones as confirmed by physical and spectral data.

Full text of DOI:10.1016/j.tetlet.2014.08.121

Accepted Manuscript
Indirect electrochemical oxidation of substituted polycyclic aromatic hydrocar-
bons to corresponding para-quinones with potassium bromide in water-chloro-
form medium
Palani Natarajan, Vinuta Devi Vagicherla, Muthana Thevar Vijayan
PII:
S0040-4039(14)01474-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.08.121
TETL 45088
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
14 July 2014
26 August 2014
28 August 2014
Please cite this article as: Natarajan, P., Vagicherla, V.D., Vijayan, M.T., Indirect electrochemical oxidation of
substituted polycyclic aromatic hydrocarbons to corresponding para-quinones with potassium bromide in water-
chloroform medium, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.08.121
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Indirect electrochemical oxidation of substituted polycyclic aromatic
hydrocarbons to corresponding para-quinones with potassium bromide in
water-chloroform medium
Palani Natarajan,* Vinuta Devi Vagicherla and Muthana Thevar Vijayan
Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi -
6
30006, Tamil Nadu, India.
E-mail: pnatarajan@cecri.res.in
1

Abstract
Indirect electrochemical synthesis of quinone derivatives of a series of substituted anthracene
and naphthalene by the electrolysis of aqueous solution of potassium bromide (3.0 M) using
2
Pt anode at the constant current density (40 mA/cm ) has been carried out. These reactions
resulted in good to excellent yields of corresponding para-quinones as confirmed by physical
and spectral data.
2

1
The synthesis of quinone derivatives has received much attention because it is the starting
2
point in the manufacture of a wide variety of industrially valuable products such as dyes,
pigments, DNA intercalation agents, pharmaceutically active compounds, and other newer
materials. As a result, intense synthetic efforts have been devoted to the development of
1
,2
3
methods that would yield quinones. In this context, recently we reported a mild and one-
pot conversion of deactivated polycyclic aromatic hydrocarbons into corresponding para-
quinones by hypobromous acid (HOBr) generated in situ from the aqueous N-
bromosuccinimide (Scheme 1). Although this method carries several advantages including
metal-free, shorter reaction time and insensitive to the moisture and oxygen, there are some
limitations, i.e., the reactions occurred at elevated temperature (75-80 °C) and the byproduct,
3
succinimide, need to be removed after reaction to obtain the pure product. Thus, we decided
to develop a practical and environmentally friendly procedure for the preparation of quinones
4
using HOBr.
5
Electrochemical synthesis has been realized as a promising alternative route to a
6
conventional chemical method. Furthermore, indirect electrochemical preparations (also
called mediated electrochemical syntheses) in comparison with direct electrochemical
methods have, in general, important advantages such as low consumption of electricity,
avoiding passivation of the electrode surface, require less posterior purification, and even
5
,6
electrochemically inactive and/or unstable substrates can be used for the reaction. The
concepts of indirect electrochemical synthesis have frequently been used in the preparations
7
8
8,9
10
11
11
of alkyl-, benzyl- and arene halides, benzoxazoles, epoxides, halohydrins, as well as
1
2
13
for the degradation of hydrocarbon- and cyanide pollutants. Curiously, electrochemical
mediated approaches have not being extended to the production of quinones particularly from
polycyclic aromatic hydrocarbons substituted with either −CHO or −COOH or −Br or
5
-9
OH and so on. This omission is probably due to the low reactivity of substituted
−
CH
2
3

1
4
aromatic hydrocarbons while comparing with unsubstituted analogues. The interest in the
substituted polycyclic aromatic hydrocarbons (SPAHS) was spurred by the fact that they are
available readily in pure form, stable under different reaction conditions, and can be readily
2
,15
converted to the desired compounds in the later stages of multistep organic syntheses.
Herein, we describe a novel and efficient protocol for the electrochemical synthesis of 9,10-
anthraquinone (AQ) and 1,4-naphthoquinone (NQ), respectively, from their mono- and
2
disubstituted hydrocarbons by HOBr in situ generated by hydrolysis of bromine (Br )
produced from aqueous potassium bromide (KBr) through electrolysis (Scheme 1). Reaction
conditions such as current density, electrolyte concentration and the medium pH were
optimized using 9-anthraldehyde (1A) as the model substrate. To the best of our knowledge,
synthesis of para-quinones from SPAHS has not been investigated in detail.
Scheme 1. Synthetic routes to para-quinones
4

To make sure the formation of HOBr, under the experimental conditions used here, first we
electrolyzed the aqueous solution of KBr in the absence of hydrocarbon substrate. Figure 1
shows the typical voltammograms obtained from aqueous solution of KBr (1.0 mM)
3
containing potassium nitrate (KNO , 1.0 M) as supporting electrolyte at a scan rate of 50
mV/s. One may notice that the reversible profile with anodic and cathodic peak potentials are
at 1.02 V vs. SCE and 0.83 V vs. SCE, respectively, corresponding to the two electrons
−
transformation of bromide (Br ) to Br
6
2
(eq. 1) and vice versa. This electrochemically
generated Br can react with either water or hydroxide ions produced at cathode to form
2
16
HOBr (eqs. 2 and 5). It was demonstrated by UV-vis spectroscopy. As can be seen in
Figure 1, the aqueous solution of KBr did not show any absorption in the spectra. Whereas
the spectra measured immediately after electrolysis (< 1 min) display intense peak
1
6
2
characteristic for Br at 396 nm. This band gradually decreased with solution aging and
concomitantly a new peak at 266 nm emerged. Eventually the peak at 396 nm disappeared
completely, whereas the peak at 266 nm attains a maximum. These combined observations
indicate that the Br
2 2
generated by the electrolysis reacts immediately with H O yielding
16
expected HOBr, which was reported to absorb at 266 nm. HOBr is a weak acid
4
dissociation constant, pKa, is 7.54 at 25 °C) and is recognized to dissociate to form H and
+
(
−
BrO (hypobromite ion) at ambient conditions. The formation of BrO is evidenced from the
−
16
−
absorption maximum at 331 nm, cf. Figure 1. Thus, oxidizing agents HOBr/BrO can be
generated readily by the hydrolysis of Br produced by electrolysis of aqueous solution of
KBr under the experimental conditions employed.
2
5

−
−
2
Br
Br
HOBr
Br
2
+ 2e
eq. 1 (at anode)
eq. 2
2
+ H
2
O
HOBr + HBr
−
+
BrO + H
2OH + H
HOBr
eq. 3
−
−
2
H
2
O + 2e
−
+ OH
2
eq. 4 (at cathode)
eq. 5
Br
2
0
0
0
0
0
0
.0010
8
aqueous KBr
before electrolysis
after electrolysis
min aged solution
3 min aged solution
.0008
.0006
.0004
.0002
.0000
2
66 nm of HOBr
6
4
2
0
2
331 nm of BrO-
1
2
3
5 min aged solution
0 min aged solution
0 min aged solution
3
96 nm of Br2
-
-
-
-
0.0002
0.0004
0.0006
0.0008
-
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
240
300
360
420
480
540
600
Potential (V)
Wavelength (nm)
Figure 1. In left are shown the typical cyclic voltammograms of aqueous solution of KBr
1.0 mM) containing KNO (1.0 M) as supporting electrolyte at a scan rate of 50 mV/s, and
the absorption profiles of aqueous solution of KBr measured before and after the electrolysis
are depicted in right. Notice that the absorption peak at 396 nm corresponding to Br
(
3
2
disappeared with time and new peaks appeared at 266 nm and 331 nm representing to HOBr
−
and BrO , respectively.
The results obtained by cyclic voltammetry (Figure 1) led us to attempt the reaction between
−
A (a representative example) and HOBr/BrO generated in situ from electrochemical
1
1
7,18
oxidation of aqueous KBr. Accordingly, the preparative-scale two-phase electrolysis
was
carried out in an undivided electrochemical cell supplied with two platinum sheets (Pt, 1.0 ×
2
.5 cm ) as an anode and a cathode. The upper phase, aqueous KBr solution (30 mL, 3.0 M),
1
−
acted as a supporting electrolyte as well as Br source, whereas, the substrate (0.5 mmol)
dissolved in chloroform (CHCl
3
, 30 mL) behaved as the lower phase. The aqueous phase
2
alone was electrolyzed galvanostatically at a current density of 20 mA/cm using a DC power
18
source. In view of the fact that HOBr and the substrate (1A) react in a 4:1 mole ratio to
3
yield quinones, the electrolysis time was estimated to provide 8 F/mol charge consumption.
The electrolysis was stopped after a charge of 8 F/mol was passed and the reaction mixture
6

was examined by thin layer chromatography using 2% ethyl acetate in n-hexane. As we
expected, the product did form, however, some unchanged 1A persisted as well (Figure 2).
After silica gel column chromatography, the product and the starting material were isolated in
pure state in yields of 38% and 61%, respectively. Comparing the product’s melting point,
1
elemental analysis and H NMR data (Figure 2) to those given in the literature, this
3,19
compound was identified as 9,10-anthraquinone (AQ).
♦
*
♦
♦
* ♦
♦
♦
1
Figure 2. H NMR (CDCl
3
, δ = 7.26 ppm) spectra of 1A recorded after charge of 8 F/mol
(
top) and 18 F/mol (bottom) were passed. Notice that more than 60% of unchanged starting
material remained when the electrolysis was stopped after passing the theoretical quantity (8
F/mol) of charge. Signals arose from the starting compound and the product were indicated in
symbols ♦ and *, respectively in the top figure.
As the isolated yield of AQ was unsatisfactory (38%), we increased the quantity of charge
more than theoretical amount, i.e. 8 F/mol. The amount of unchanged substrate (1A) in the
reaction mixture decreased to trace amounts by prolongation of the electrolysis time (Figure
2
). Thus, the consumption of 18 F/mol charge gave the product in 94 % yield (isolated).
Further prolongation of the electrolysis time yielded several new side products, which could
not be isolated and identified. Hence, 18 F/mol was chosen as optimum value of charge to be
passed in the preparative work. In aqueous solutions oxygen evolution (1.23 V vs. SCE) is
5
evolution potential (1.02 V vs. SCE, cf. Figure 1). In addition, some Br
closer to the Br
2
2
7

may have escaped from the electrochemical cell into the air without being consumed. So the
conversion efficiency was found to be below 40% when the electrolysis was stopped after
just passing the theoretical quantity of charge.
Effect of current density on electrochemical oxidation of 1A
In order to shorten the electrolysis time, the oxidation reaction of 1A was conducted with
2
2
increased current densities from 20 mA/cm to 135 mA/cm . Results were summarized in
Table 1. While compared to lower current densities, the rate of oxidation of 1A is maximal in
increased current densities. Nevertheless, at very high current densities the product yield got
5
decreased due to the formation of unknown side products. So 40 mA/cm was chosen as an
2
optimum current density for the oxidation of SPAHS presented.
Table 1. Study on the effect of current density on the oxidation of 1A to AQ.
a
b
c
entry
current density time
(min)
product yield
(%)
elemental analysis
(found for C14 , %)
2
mA/cm )
(
8 2
H O
1
2
3
4
5
6
20
25
468
351
234
141
94
95
94
96
89
92
85
C, 80.75; H, 3.87
C, 80.74; H, 3.87
C, 80.75; H, 3.86
C, 80.74; H, 3.87
C, 80.76; H, 3.87
C, 80.75; H, 3.87
40
65
100
135
70
All reactions were performed at identical experimental conditions except that the current
2
density was varied from 20-135 mA/cm . Aplab power supply was used as a direct current
a
b
source for the electrolysis.
recrystallization using n-hexane-ethyl acetate mixture. Analytical calculated for C14
0.76%; H, 3.87%.
Yield was determined from the product isolated by
c
H
8
O
2
: C,
8
Effect of electrolyte concentration on electrochemical oxidation of 1A
−
The rate of oxidation of Br has been known to depend on the electrolyte concentration.
5,6
Thus, we studied the oxidation reaction of 1A in various concentrations of KBr at constant
8

2
current density (40 mA/cm ). In the presence of 0.5 M, 1.0 M, 1.5 M, 2.0 M, 2.5 M, 3.0 M,
3
4
.5 M and 4.0 M KBr, yields of AQ obtained after passing 18 F/mol charge were about 15%,
0%, 55%, 73%, 78%, 93%, 96%, and 95%, respectively. The increasing yield of AQ with
−
the amount of KBr indicates that the Br ions play an important role in the reaction through
5
controlling the conductivity of the medium. Although yields of the product (AQ) at higher
concentrations of KBr were found to be slightly high, an electrolyte concentration of 3.0 M
2
was selected as the operating concentration owing to vigorous evolution of Br /HBr as well
−
20
as the known toxic effects of the Br excess to humans and the environment. It is worthy to
mention that the electrochemical experiments conducted utilizing sodium bromide as the
electrolyte gave completely the same results as those performed using KBr.
Effect of pH on electrochemical oxidation of 1A
As mentioned already, HOBr is the major oxidant produced in the hydrolysis of Br
2
formed
by oxidation of aqueous KBr (Figure 1). Nevertheless, HOBr is a weak acid (pKa, is 7.54 at
+
5 °C) and is known to dissociate into H and BrO in the solution. The concentrations of
−
16
2
−
HOBr and BrO depend on the pH of the medium. For example, when the medium pH value
−
is between 6.5-8 both HOBr and BrO can be found in water, whereas when pH is higher than
−
the pKa, BrO whose nucleophilicity is greater than that of the HOBr accounted for larger
proportion in the medium. Thus, the effect of different pH on the oxidation of 1A to AQ was
studied and the results were summarized in Table 2.
To examine the effect of pH on the oxidation reaction of 1A, the reaction mixture was
adjusted to the desired pH for each experiment using solutions of either aqueous sulphuric
acid or aqueous sodium hydroxide. As shown in Table 2, the rate of oxidation of 1A to AQ at
basic pH was found to be a greater extent while compared to acidic solutions. In other words
the reaction rate decreased gradually with the decreasing of pH from neutral to highly acidic
9

range, whereas the addition of base to the reaction mixture led to noticeable improvements in
the yield of AQ (Table 2). We concluded from the UV-vis measurements that in highly acidic
2
environment the hydrolysis of Br is strongly inhibited. As a result the yields of the AQ were
found to be minimum at moderate acidic pH and practically no oxidation could be achieved
in very high acidic mediums (pH ≤ 2). Therefore, pH 9 was taken an optimum value for this
process as product decomposition was detected at more alkaline media.
Table 2. Study on the effect of pH on the oxidation of 1A to AQ.
a
b
c
entry
medium pH
product yield (%)
elemental analysis
8 2
found for C14H O , %)
(
1
2
3
4
5
6
2
4
<5
20
38
76
91
92
C, 80.74; H, 3.88
C, 80.77; H, 3.86
C, 80.76; H, 3.87
C, 80.75; H, 3.88
C, 80.76; H, 3.86
C, 80.77; H, 3.88
6
8
10
12
All reactions were performed at identical experimental conditions except that the medium pH
a
was varied from 2 to 12. The desired pH for each experiment was adjusted by using
b
solutions of either aqueous sulphuric acid or aqueous sodium hydroxide. Yield was
determined from the product isolated by recrystallization using n-hexane-ethyl acetate
O
c
mixture. Analytical calculated for C14
H
8
2
: C, 80.76%; H, 3.87%.
Overall, the optimized conditions for a perfect electrochemical oxidation of 1A to AQ, a
representative example, are as follows. To a solution of 1A (0.5 mmol) in 30 ml of CHCl in
an undivided cell was added aqueous solution of KBr (3.0 M, 30 ml). With the resultant two-
3
1
7,18
phase solution,
the aqueous phase alone was electrolyzed galvanostatically at a constant
2
current density of 40 mA/cm until the 18 F/mol charge was passed, cf. Supporting
information (SI). The temperature of the electrochemical cell contents was maintained at
1
0

2
5±2 °C throughout the electrolysis. After completion of the electrolysis, the lower organic
1
8
phase was separated and purified.
Having established the optimum conditions for the electrochemical oxidation reaction, the
methodology was extended to a series of other SPAHS whose structures are shown in Table
3
. Without any surprise various mono- and disubstituted derivatives of anthracene (2A-8A)
and naphthalene (9N-13N) underwent an electrochemical oxidation smoothly under the
optimized conditions yielding the expected AQ and NQ, respectively, in good to excellent
yields. In contrast to the 1-naphthyl derivatives (9N-13N), the 2-methylnaphthalene (14N)
and 2-methoxynaphthalene (15N) were prone to undergo complete oxidation under the
condition studied. They yielded mixture of products after prolonged electrolysis. Therefore,
the electrochemical method for the oxidation of 9- and 9,10-disubstituted anthracenes and α-
substituted naphthalenes to corresponding para-quinones were developed using two-phase
1
electrolysis (SI). We feel that the present finding may have several advantages including
8
low cost of production, ease of operation, simple work-up procedure and no waste handling
1
-4
problems when compared to classical chemical syntheses.
1
1

Table 3. The electrochemical synthesis of AQ and NQ from various SPAHS in a water-
CHCl medium.
3
a
b, c
c
c, d
s.
substrate
product
yield
melting
elemental analysis
no.
(%)
point (°C) (found for AQ & NQ,
%)
1
1A, R = CHO; R = H
2
1
2
3
4
5
6
7
8
AQ
AQ
AQ
AQ
AQ
AQ
AQ
AQ
94
284-286
284-286
283-286
283-286
283-286
284-287
285-286
284-286
C, 80.75; H, 3.87
C, 80.76; H, 3.86
C, 80.76; H, 3.87
C, 80.75; H, 3.86
C, 80.76; H, 3.86
C, 80.76; H, 3.87
C, 80.75; H, 3.87
C, 80.76; H, 3.87
1
2
2A, R = COOH; R = H
91
96
96
84
92
95
94
1
3A, R = OCH
2
; R = H
3
1
4A, R = Br; R = H
2
1
5A, R = CH
2
OH; R = H
2
; R = H
2
1
6A, R = CH
3
1
7A, R = CHO; R = Br
2
1
8A, R = CHO; R = OCH
2
3
1
9N, R = CHO; R = H
2
9
NQ
NQ
87
92
74
66
93
61
123-126
124-126
124-126
125-126
125-126
−
C, 75.93; H, 3.83
C, 75.94; H, 3.81
C, 75.93; H, 3.83
C, 75.94; H, 3.82
C, 75.94; H, 3.82
−
1
10N, R = COOH; R = H
2
1
1
1
1
0
1
2
3
1
11N, R = Br; R = H
2
NQ
1
12N, R = CH
2
; R = H
3
NQ
1
13N, R = Br; R = OH
2
NQ
1
4
multiple
products
OCH3
15
multiple
products
87
−
−
1
5N
a
2
All reactions were performed at constant current density (40 mA/cm ) using aqueous KBr
−
3.0 M, pH = 9) as a supporting electrolyte and Br source. Yields were determined from
b
(
c
products isolated by either recrystallization or column chromatography.
compounds was established by elemental analysis and melting point determinations.
Analytical calculated for AQ (C14 ): C, 80.76%; H, 3.87% and for NQ (C10 ): C,
5.94%; H, 3.82%.
Purity of
d
8
H O
2
6 2
H O
7
A plausible reaction mechanism for the oxidation of SPAHS into para-quinones has been
3
reported by us recently. The reaction starts by the electrochemical oxidation of Br ions at
−
−
in the aqueous form of HOBr/BrO and HBr (eqs. 2 and 5
the anode yielding molecular Br
2
−
and Figure 3). The initial nucleophilic attack to the para-position of aromatic unit by BrO
1
2

results in a corresponding phenolic intermediate, which is expected to undergo ipso-attack by
−
second molecule of BrO followed by eliminations will yield the final product (Figure 3). The
appearance of only para-quinone as the product, among potential two quinones (ortho- and
para isomers) for naphthalene compounds may be rationalized in terms of the resonance
2
1
structures of the intermediate. For the para attacked intermediate seven resonance structures
can be described, of which four maintain an aromatic ring. On the other hand for the ortho
attacked, the intermediate has only six resonance structures and only two of these are
aromatic. Of course, the effect of the electronic and the steric factors of the intermediate
generated in the reaction as well contributed in the stabilizations of the 1,4-naphthoquinone
22
over 1,2-isomer.
O
O
CH3/CH2OH/CHO
n
n
n = -1 or 0
electrolysis
refs. 5-9
-CO2
Br2
-
Br
O
BrO H
O
O
O
H
O
Br
O
Br
O
Br
Br
Br
O
O
aq. KBr
Pt anode
aq. KBr
Pt anode
-HBr
n
n
n
n
O
O
O
-HBr
Br
Br
BrO H
Figure 3. The proposed reaction mechanism for the electrochemical oxidation of SPAHS into
the corresponding para-quinones.
In summary, the electrochemical method involving the oxidation of SPAHS into the
corresponding para-quinone by two-phase electrolysis constitutes a novel and efficient
alternative method over classical chemical methodologies. The yields obtained are
comparable with those published for the oxidation of hydrocarbons using different oxidizing
4
,21
agents.
1
3

Acknowledgements
P.N. gratefully acknowledges the financial support of the Department of Science &
Technology (DST), India through INSPIRE Faculty Fellowship [IFA12-CH-62]. V. D. V. is
grateful to CSIR-CECRI for financial support. We are indebted to Dr. Vijayamohanan K.
Pillai and Dr. D. JayaKumar, CSIR-CECRI, for infrastructure facilities.
Supplementary data
Supplementary data (a photograph of typical electrochemical apparatus and general aspects)
associated with this article can be found, in the online version, at
http://dx.doi.org/00.0000/j.tetlet.0000.00.0000.
1
4

References and Footnotes
(
1) (a) Finley, K. T. In the Chemistry of the Functional Groups. The Chemistry of the
Quinonoid Compounds, Patai, S. Ed., John Wiley & Sons, New York, 1974, Vol. 2, pp
1047–1072. (b) Moore, H. W.; Decker, O. H. W. Chem. Rev. 1986, 86, 821–830. (c)
Osman, F. H.; El-Samahy, F. A. Chem. Rev. 2002, 102, 629–678.
(
2) (a) Fujii, I.; Ebizuka, Y. Chem. Rev. 1997, 97, 2511−2523. (b) Corma, A.; Iborra, S.;
Velty, A. Chem. Rev. 2007, 107, 2411–2502. (c) Masters, K-S.; Bräse, S. Chem. Rev.
2012, 112, 3717–3776. (d) Schneider, H-J.; Agrawal, P.; Yatsimirsky, A. K. Chem.
Soc. Rev. 2013, 42, 6777–6800.
(
(
(
3) Natarajan, P.; Vagicherla, V. D.; Vijayan, M. T. Tetrahedron Lett. 2014, 55, 3511–
3515.
4) Mundy, B. P.; Ellerd, M. G. Name Reactions and Reagents in Organic Synthesis, John
Wiley & Sons, New York, 1988.
5) (a) Shono, T. Electroorganic Chemistry as a New Tool in Organic Synthesis, Springer,
Berlin, 1984. (b) Grimshaw, J. Electrochemical Reactions and Mechanisms in Organic
Chemistry, Elsevier, Amsterdam, 2000. (c) Lund, H.; Hammerich O. Organic
Electrochemistry, Marcel Dekker, Inc., New York, 2001. (d) Dawood, K. M.;
Fuchigami, T. J. Org. Chem. 2004, 69, 5302−5306. (e) Cao, Y.; Hidaka, A.; Tajima, T.;
Fuchigami, T. J. Org. Chem. 2005, 70, 9614−9617. (f) Nematollahi, D.; Habibi, D.;
Rahmati, M.; Rafiee, M. J. Org. Chem. 2004, 69, 2637−2640. (g) Esmaili, R.;
Nematollahi, D. J. Org. Chem. 2013, 78, 5018−5021. (h) Zeng, C-C.; Liu, F-J.; Ping,
D-W.; Hu, L-M.; Cai, Y-L.; Zhong, R-G. J. Org. Chem. 2009, 74, 6386–6389. (i)
Gallardo, I.; Guirado, G.; Marquet, J. J. Org. Chem. 2003, 68, 631−633.
1
5

(
(
(
(
6) (a) Steckhan, E. Top. Curr. Chem. 1987, 142, 1−69. (b) Lincot, D.; Froment, M.;
Cachet, H. Advances in Electrochemical Science and Engineering, Alkire, R. C.; Kolb,
D. M. Eds., Wiley-VCH, Weinheim, 1999.
7) (a) Tsuchida, K.; Kochi, T.; Kakiuchi, F. Asian J. Org. Chem. 2013, 2, 935–937. (b)
Kreh, R. P.; Spotnitz, R. M.; Lundquist, J. T. J. Org. Chem. 1989, 54, 1526–1531. (c)
Ohkubo, K.; Fukuzumi, S. Org. Lett. 2000, 2, 3647–3650.
8) (a) Gourcy, J.; Simonet, J. S.; Jaccaud, M. Electrochim. Acta 1979, 24, 1039−1046. (b)
Raju, T.; Kulangiappar, K.; Anbu Kulandainathan, M.; Shankar, G. K.; Muthukumaran,
A. Electrochim. Acta 2005, 51, 356–360.
9) (a) Ungureanu, E. M.; Razus, A. C.; Birzan, L.; Buica, G.; Cretu, M.; Cristian, E.
Electrochim. Acta 2006, 52, 794–803. (b) Forsyth, S. R.; Pletcher, D. J. Appl.
Electrochem. 1987, 17, 905−913. (c) Raju, T.; Kulangiappar, K.; Anbu Kulandainathan,
M.; Uma, U.; Malini, R.; Muthukumaran, A. Tetrahedron Lett. 2006, 47, 4581–4584.
(
(
10) Li, B. W-C.; Zeng, C-C.; Hu, L-M.; Tian, H-Y.; Little, R. D. Adv. Synth. Catal. 2013,
355, 2884–2890.
11) (a) Nematollahi, D.; Akaberi, N. Molecules 2001, 6, 639–646. (b) Torii, S.; Uneyama,
K.; Tanaka, H.; Yamanaka, T.; Yasuda, T.; Ono, M.; Kohmoto, Y. J. Org. Chem. 1981,
46, 3312–3315.
(
(
12) (a) Feng, Y. J.; Li, X. Y. Water Res. 2003, 37, 2399−2407. (b) Zhang, F.; Feng, C.; Li,
W.; Cui, J. Int. J. Electrochem. Sci. 2014, 9, 943–954.
13) Felix-Navarro, R. M.; Lin, S. W.; Violante-Delgadillo, V.; Zizumbo-Lopez, A.; Perez-
Sicairos, S. J. Mex. Chem. Soc. 2011, 55, 51–56.
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(
(
14) (a) Danheiser, R. L.; Brisbois, R. G.; Kowalczyk, J. J.; Miller, R. F. J. Am. Chem. Soc.
1990, 112, 3093–3100. (b) Figueira-Duarte, T. M.; Müllen, K. Chem. Rev. 2011, 111,
7260–7314.
15) (a) Allen, C. F. H. Chem. Rev. 1959, 59, 983−1030. (b) Yates, K. Acc. Chem. Res.
971, 4, 136−144.
1
(
(
16) Takahashi, M.; Nakamura, K.; Jin, J. Electroanalysis 2008, 20, 2205–2211.
17) Two-phase electrolysis is one where the electrolyte and the substrate are present in
different phases. When electrolysis occurs, electrolytically generated species travels
from aqueous phase to organic phase and attacks the substrate; See: Christopher, C.;
Lawrence, S.; John Bosco, A.; Xavier, N.; Raja, S. Catal. Sci. Technol. 2012, 2, 824–
827.
(
18) Cyclic voltammetry of the aqueous solution of KBr: Cyclic voltammograms were
obtained in a Pyrex electrochemical cell containing a standard three-electrode setup
2
i.e., a 1 cm platinum working electrode, a platinum counter electrode, and a potassium
(
chloride saturated calomel reference electrode) connected to a potentiostat. Before
analysis, the working electrode was polished with a emery paper and then rinsed with
3
water followed by acetone and subsequently dried with air. KNO was used as the
supporting electrolyte. Potentials were measured at a scan rate of 50 mV/s. The
potential range investigated was chosen such as to avoid the hydrogen or oxygen
evolution in the aqueous electrolytic solution. Aqueous solution contained the KBr at
3
1.0 mM and the supporting electrolyte was KNO (1.0 M). Three electrodes were
placed and the potential was swept to obtain a voltammograms.
General procedure for electrochemical oxidation: To an undivided electrochemical
3
cell the respective substrate (0.5 mmol) and CHCl (30 mL) was added. Thirty millilitre
1
7

of aqueous solution of KBr (3.0 M) was then added over the above solution. The
electrolytic cell was equipped with a magnetic stirrer in order to keep the CHCl
3
2
solution well mixed. Two Pt electrodes (1.0 × 1.5 cm ) were placed vertically having an
inter-electrode gap of 4 cm in the aqueous phase without touching the organic phase,
but very close to the interphase of the two-phases (SI). All electrolyses were performed
2
at a constant current density (40 mA/cm ) at room temperature. The electrolysis was
stopped after passage of a charge of 18 F/mol. Subsequently, the organic layer was first
collected and then the aqueous solution was extracted with two portions of
dichloromethane (2 × 15 mL). The combined extracts were washed with a solution of
NaHSO
NaHCO
MgSO , the solvent was removed by rotary evaporation under reduced pressure and the
residue was subjected to either recrystallization using n-hexane-ethyl acetate mixture
90:10, v/v) or column chromatographed over SiO . Yields and physical data are given
in Tables 1-3.
3
(10%, 20 mL) to remove residual amounts of Br
2
, then with saturated
3
(10 mL), brine (10 mL) and H O (30 mL). After dried over anhydrous
2
4
(
2
(
19) Wang, W.; Zhang, W.-J.; Wang, L.; Quah, C. K.; Fun, H.-K.; Xu, J.-H.; Zhang, Y. J.
Org. Chem. 2013, 78, 6211−6222.
(
(
20) Cowman, G. A.; Singer, P. C. Environ. Sci. Technol. 1996, 30, 16−24.
21) Clayden, J.; Greeves, N.; Warren, S.; Wolters, S. Organic Chemistry, Oxford
University Press, Oxford, U.K., 2006.
(
22) (a) Sad, J. M.; Morey, J.; Rubido, C. J. Org. Chem. 1986, 51, 4471−4473. (b) Bailey, P.
S.; Bath, S. S.; Dobinson, F.; Garcia-Sharp, F. J.; Johnson, C. D. J. Org. Chem. 1964,
29, 697−702. (c) Spitzer, U. A.; Lee, D. G. J. Org. Chem. 1974, 39, 2468−2469.
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Indirect electrochemical oxidation of substituted polycyclic aromatic
hydrocarbons to corresponding para-quinones with potassium bromide in
water-chloroform medium
Palani Natarajan,* Vinuta Devi Vagicherla and Muthana Thevar Vijayan
Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi -
6
30006, Tamil Nadu, India.
E-mail: pnatarajan@cecri.res.in
Abstract
Indirect electrochemical synthesis of quinone derivatives of a series of substituted anthracene
and naphthalene by the electrolysis of aqueous solution of potassium bromide (3.0 M) using
2
Pt anode at the constant current density (40 mA/cm ) has been carried out. These reactions
resulted in good to excellent yields of corresponding para-quinones as confirmed by physical
and spectral data.
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