Redox-mediated oxidation of alcohols using Cl-/OCl- redox couple in biphasic media

Article abstract of DOI:10.1002/poc.3454

The electrochemical oxidation of benzyl and substituted benzyl alcohols including primary and secondary alcohols and piperonyl alcohol was carried out in an undivided cell using platinum electrodes in a biphasic medium using Cl^-/OCl^-

Full text of DOI:10.1002/poc.3454

Research article
Received: 15 December 2014,
Revised: 17 February 2015,
Accepted: 30 March 2015,
Published online in Wiley Online Library: 8 May 2015
(wileyonlinelibrary.com) DOI: 10.1002/poc.3454
Redox-mediated oxidation of alcohols using
ClÀ/OClÀ redox couple in biphasic media
A. John Boscoa, S. Lawrenceb, C. Christopherc, S. Radhakrishnand,
A. Arul Joseph Rosarioe, S. Rajae and D. Vasudevand*
The electrochemical oxidation of benzyl and substituted benzyl alcohols including primary and secondary alcohols and
piperonyl alcohol was carried out in an undivided cell using platinum electrodes in a biphasic medium using ClÀ/OClÀ as
the redox mediator. In most cases, selective oxidation of alcohols to the corresponding aldehydes or ketones was achieved
with high yields of products by the in situ generated chloronium ion. This method is simple and gives better yields of products
compared with chemical methods of oxidation. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: aldehydes and ketones; electrochemical oxidation; redox-mediated synthesis
organic synthesis.[16] Chemical and environmental advantages
of electrochemical processes catalyzed by mediator oxidation
INTRODUCTION
systems using both organic and inorganic mediators were
demonstrated for the oxidation of various classes of organic
compounds.[17]
Carbonyl compounds, which are the oxidation products of alco-
hols, find a wide range of applications as chemical intermediates,
pharmaceuticals, agricultural chemicals, and flavor and/or fra-
grance materials.[1] The chemical oxidation of alcohols to carbo-
nyl compounds has been well studied. The oxidation has been
achieved using 2,2,6,6-tetramethylpiperidinyloxy, a stable nitro-
xyl radical-based catalysts,[2,3] transition metal (Cu, Co, Ir, Cr, Pd,
Mo, V, Ru, etc.)-based catalysts[4–10] diisopropylaodicarboxylate
catalyzed by nitroxyl radicals,[11] and 2-iodoxylbenzenesulphonic
acid.[12]
In spite of high yields of products obtained in most of these
methods, they often require the use of toxic solvents like toluene
and acetonitrile and stringent experimental conditions like very
high temperature, very low temperature, catalysts, and use of
expensive metals like Ru, Ag, and several toxic metals as most
of the aforementioned materials. Therefore, the search is on for
the design of chemical products and processes (chemical or elec-
trochemical) that reduces or eliminates the use and generation
of hazardous substances.[13]
The oxidation of benzyl alcohol by electrogenerated hypobro-
mite in a two-phase emulsion has been reported using a quater-
nary ammonium salt as a phase transfer agent.[15] Low current
efficiency (CE) was reported in the absence of a phase transfer
catalyst. The oxidation of alcohols was also reported earlier using
exclusively the (ClÀ/ OClÀ) redox mediator without the use of
any phase transfer catalyst giving average yields of products.[18]
The Ce4+/Ce3+ redox system was employed for the quantitative
oxidation of benzyl alcohol to benzaldehyde at a graphite
anode.[19] In biphasic electrolysis in aqueous K2CO3 and ether,
benzylic and allylic alcohols were selectively oxidized to
aldehydes at hydroxide nickel anode. Aldehydes formed when
extracted into the nonpolar organic phase during electrolysis
prevented further oxidation to carboxylic acids.[20]
Oxidation of benzyl alcohol to benzaldehyde using two-phase
electrolysis method has been studied with carbon anode and
stainless steel cathode. High yield of benzaldehyde (95%) with
The heterogeneous electrocatalytic oxidation of benzyl alco-
hol on a three-dimensional nickel foam electrode was investi-
gated in a pulsed flow reactor under homogeneous and
two-phase conditions. The best yields of benzaldehyde are
obtained for a 16% petroleum ether volume ratio and a pH range
of 10–13 of the aqueous phase.[14]
* Correspondence to: D. Vasudevan, Central Electrochemical Research Institute,
Karaikudi 630 006, India.
E-mail: vasudevand@rediffmail.com
Biphasic electrolysis has a distinct advantage over conven-
tional homogeneous electrolysis with respect to easy handling
and mediator recycling.[15] In homogeneous systems, lower selec-
tivity is observed because of overoxidation of the substrate on
the surface of the electrode leading to a mixture of products. In
biphasic electrolysis systems, the reactive species formed by elec-
trolytic oxidation of a halide ion in the aqueous phase, can be
taken continuously into the organic phase, and then reacted with
the substrate selectively to give the products. After completion of
the electrolysis, separation and evaporation of the organic layer
afford the product. Pletcher et al. reported on the advantages of
two-phase electrolysis as an advantageous process in practical
a A. J. Bosco
SRM University, Kattankulathur 603203, India
b S. Lawrence
Sacred Heart College, Tirupattur 635 601, India
c C. Christopher
St Xavier’s College, Palaymkottai, Tirunelveli District, India
d S. Radhakrishnan, D. Vasudevan
Central Electrochemical Research Institute, Karaikudi 630 006, India
e A. A. Joseph Rosario, S. Raja
St Joseph’s College, Tiruchirapalli 620 002, India
J. Phys. Org. Chem. 2015, 28 591–595
Copyright © 2015 John Wiley & Sons, Ltd.

J. BOSCO ET AL.
a high CE (95%) was achieved.[21] The use of BrÀ/OBrÀ and NO3À/NO.3
as redox mediators for the conversion of a few primary and
secondary alcohols has been reported.[22,23]
of NaCl (60 mL) containing 0.36-M HCl served as the upper aqueous
phase. A couple of platinum electrodes (area of 15 cm2) was placed in
the aqueous phase but very close (~0.5 cm) to the interphasial region
of organic phase. The aqueous phase served as the supporting electro-
lyte, and the in situ generated chlorine was the cationic source. The or-
ganic phase was stirred with a magnetic pellet at a slow rate of 30 rpm
in such a way that the organic layer did not touch the electrodes. A cur-
rent density of 30 mA/cm2 was employed (which was the optimized
value based on the best yield obtained as a function of current density)
based on which the current to be passed was calculated. The charge
passed corresponded to 2 F/mol of the substrate for the oxidation of all
the alcohols under study. The reaction was continued till maximum yield
of the product was obtained.
All the alcohols employed in the present work were of AR grade
(Merck, Kenilworth, NJ, USA) and were used as such without further puri-
fication. All the aqueous solutions were prepared using deionized water.
During the progress of the reaction, an aliquot was drawn periodically
from the organic phase and monitored by high-performance liquid chro-
matography (HPLC). On the completion of electrolysis, the lower organic
phase was separated (the aqueous layer was also separated and mixed
with chloroform to remove any alcohol product that might have gone
to the aqueous phase because of partial solubility in some cases), washed
with water (2 × 25 mL), and dried over anhydrous Na2SO4, and finally, the
The present work reports the use of the mediator (ClÀ/OClÀ)
for the conversion of a series of primary and secondary alcohols
to their corresponding aldehydes or ketones as shown in
Scheme 1.
EXPERIMENTAL SECTION
The biphasic electrolysis was performed in an undivided cell using an
electrophoresis power supply. A solution of 10 mmol of the alcohol dis-
solved in 20-mL chloroform served as the lower organic phase. A 12%
Scheme 1. Electrooxidation of alcohols
Table 1. Chloride-mediated selective oxidation of various alcohols in biphasic electrolysis medium (conditions: current density
30 mA/cm2; charge passed 2 F/mol; temperature 30 °C)
S
1
Alcohol
Product
Yield* (%)
94
CE** (%)
94
2
3
96
83
64
33
4
5
6
95
96
96
48
48
64
7
0
0
(Continues)
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J. Phys. Org. Chem. 2015, 28 591–595

REDOX-MEDIATED OXIDATION OF ALCOHOLS USING CIÀ/OCIÀ REDOX COUPLE
Table 1. (Continued)
S
8
Alcohol
Product
Yield* (%)
78
CE** (%)
31
9
70
70
10
11
72
93
29
37
12
19
13
13
14
11
93
7
93
15
78
78
*Isolated yield of product.
**Current efficiency based on Faraday’s law.
solvent was removed by vacuum distillation. After the work-up, the resi-
due was weighed based on which the isolated yield percent was calcu-
lated. The isolated products were characterized by recording their
melting/boiling points, infrared spectroscopy (IR), and nuclear magnetic
resonance (NMR) spectra and comparing them with those of authentic
samples of the aldehyde or ketone reported in literature.
The IR spectra were recorded using a Perkin Elmer Spectrum RX1
(PerkinElmer, Inc., Waltham, MA, USA). The 1H-NMR spectra were re-
corded using a 300-MHz Bruker Avance spectrometer (Bruker Corpora-
tion, Billerica, MA, USA). NMR solutions were prepared using CDCl3, and
Me4Si was used as the internal standard for all NMR measurements. For
the HPLC, a SHIMADZU LC-8A (250 × 4.6 mm; Shimadzu Corporation,
Kyoto, Japan) was used as the stationary phase.
All the reactions were carried out at 30 °C.
RESULTS AND DISCUSSION
The results on the galvanostatic oxidation of alcohols carried out
under the optimized electrolysis conditions in biphasic medium
using redox mediator are summarized in Table 1.
It can be seen from Table 1 that oxidation of primary benzyl
alcohols in general resulted in high yields of the corresponding
benzaldehydes (83–96%) whenever the activating groups were
present on the benzene ring (Table 1, S 1–6). The CE values var-
ied from low to high depending on the alcohol. It is because of
the adsorption of the alcohol on the electrode surface, which
blocked further reaction, and the electrode had to be wiped
frequently that the reaction required extra time and the CE
values went down in some cases. In the case of benzyl alcohol
Silica was used as the adsorbent, and methanol (HPLC grade) was the
eluent used for HPLC studies. The flow rate was 0.2 mL/min. Ultraviolet
(254 nm) detector was used. One microliter of the sample was injected
into the column.
J. Phys. Org. Chem. 2015, 28 591–595
Copyright © 2015 John Wiley & Sons, Ltd.
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J. BOSCO ET AL.
substituted by the highly electron withdrawing nitro group, the
reaction did not proceed at all as expected because of the high
destabilization of the carbocation intermediate that would be
formed during the reaction. In addition to this, the solubility of
the nitro compound in chloroform was too low.
With benzyl alcohols wherein the alcohols are secondary, the
yields of the corresponding ketones were lower as compared
with the yields of aldehydes obtained by the oxidation of pri-
mary benzyl alcohol (S 1, 8, 9). Actually, there is a steady de-
crease in the yield on going from 1 to 8 to 9. This is expected
because of the steric hindrance caused by the alkyl branch for
the approach of the chloronium ion.
high stabilization of the allylic carbocation intermediate that is
formed. However, the oxidation of piperonol (Table 1, S 12)
and furfuryl alcohol (Table 1, S 13) gave low yields of the corre-
sponding aldehydes. The low yield is caused (Table 1, Entry 12, 13)
by the adsorption of the substrates on the electrodes due to
the presence of ethereal oxygen atoms present in the mole-
cule, which may hinder the normal oxidation process. In case
of 1-naphthyl alcohol also, the yield of 1-naphthaldehyde was
94% similar to that observed in the case of benzyl alcohol.
In the case of oxidation of a linear aliphatic secondary alcohol,
the yield of the product ketone yield was 78% (Table 1, S15).
The yields of some of these aldehydes and ketones obtained
as products in the present work are comparable with the values
obtained with the use of BrÀ/OBrÀ as the redox mediator studied
earlier.[22] However, with the
The oxidation of cinnamyl alcohol yielded cinnamaldehyde
product quantitatively (99%) (Table 1, S11). This is due to the
use of NOÀ3 /NO.3 as the redox
mediator studied earlier, the
yields of some of the products
obtained were much lower[23]
than those obtained in the pres-
ent work.
From the aforementioned re-
sults, it can be concluded that
the present method is applica-
ble for the oxidation of all types
of alcohols. The products were
characterized by their IR and
NMR spectra and HPLC data
compared with those of authen-
tic samples. The obtained results
are summarized as Data 1 under
the Supporting information.
Mechanism of oxidation of
alcohols
As the electrolysis proceeds, the
chloride ion is oxidized at the
anode to form chlorine mole-
cule, which upon hydrolysis
results in the formation of
hypochlorous acid and HCl. The
aforementioned formed unsta-
ble hypochlorous acid produced
Cl+ ion in situ, which immediately
oxidized the alcohols in the
interphasial region of the aque-
ous and organic phases to the
corresponding aldehyde. Cl+
ion is unstable, but its genera-
tion is in situ only at the
interphasical region and reacts
instantaneously to give the
products. Such positive halo-
gen species is also suggested
in mechanisms involving aro-
matic halogenations.
The equations are given un-
der Scheme 2 as follows. An
identical mechanism has also
Scheme 2. Mechanism of galvanostatic oxidation of alcohols in biphasic medium
been proposed earlier.[21,24]
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J. Phys. Org. Chem. 2015, 28 591–595

REDOX-MEDIATED OXIDATION OF ALCOHOLS USING CIÀ/OCIÀ REDOX COUPLE
[7] G. An, M. Lim, K. S. Chun, H. Rhee, Synlett. 2007, 95–98.
CONCLUSIONS
[8] S. Maignien, S. Aït-Mohand, J. Muzart, Synlett. 1996, 439–440.
[9] S. Velusamy, T. Punniyamurthy, Org. Lett. 2004, 6, 217–219.
[10] M. Musawir, N. Paul Davey, G. Kelly, I. V. Kozhevnikov, Chem.
Commun. 2003, 1414–1415.
The advantages of the two-phase electrolysis over homoge-
neous electrolysis are emphasized. Improved selectivity is
achieved in the two-phase electrolysis as the reactive species
formed by the electrolytic oxidation of a halide ion in the aque-
ous phase can be taken continuously into the organic phase and
then reacted with the substrate selectively to give the desired
products. Overoxidation of the reactant is avoided.
The present method involving oxidation of primary and
secondary alcohols using the redox mediator to the correspond-
ing aldehydes or ketones is simple and inexpensive and gives
moderate to high yields of products. It is important to note that
primary alcohols are oxidized to the corresponding aldehydes
and overoxidation to the corresponding carboxylic acid does
not result. Also, it avoids the use of toxic chemicals and expen-
sive reagents.
[11] M. Hayashi, M. Shibuay, Y. Iwabuchi, J. Org. Chem. 2012, 77,
3005–3009.
[12] M. Uyanik, M. Akakura, K. Ishihara, J. Am. Chem. Soc. 2009, 131(1),
251–262.
[13] H. J. Park, J. C. Lee, Synlett. 2009, 79–80.
[14] P. T. Anastas, J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, New York, 1998, 30.
[15] R. E. W. Jansson, N. R. Tomov, J. Appl. Electrochem. 1980, 10, 583–586.
[16] S.R. Forsyth, D. Pletcher, Extended abstracts of the 1st International
Symposium on electro organic synthesis. Kurashiki, 1986, 35.
[17] Y. N. Ogibin, M. N. Elinson, G. I. Nikishin, Russ. Chem. Rev. 2009, 78(2),
89–140.
[18] R. Ciriminna, J. Blum, D. Avnir, M. Pagliaro, Chem. Commun. 2000,
1441–1442.
[19] S. Lawrence, C. Christopher, A. John Bosco, K. Kulangiappar, S. Raja,
N. Xavier, Der Chemica Sinica. 2012, 3(1), 255–261.
[20] R. Schneider, H.-J. Schifer, Synthesis. 1989, 10, 742.
[21] C. Christopher, S. Lawrence, A. John Bosco, N. Xavier, S. Raja, Catal.
Sci. Technol. 2012, 2, 824–827.
Acknowledgement
[22] T. Raju, S. Manivasagan, B. Revathy, K. Kulanjiappar, A. Muthukumaran,
Tet. Lett. 2007, 48, 3681–3684.
[23] C. Christopher, S. Lawrence, M. A. Kulandainathan, K. Kulanjiappar,
M. Easu Raja, N. Xavier, S. Raja, Tet. Lett. 2012, 53, 2802–2804.
[24] B. S. Koo, C. K. Lee, K. J. Lee, Synth. Comm. 2002, 32, 2115.
We are thankful to Dr N. Xavier, St Joseph’s College, Tiruchirapalli,
India, for his encouragement and technical assistance.
REFERENCES
[1] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph
SeriesACS, Washington, DC, 1990.
[2] J. M. Hoover, S. S. Stahl, J. Am. Chem. Soc. 2011, 133, 16901–16910.
[3] N. Tamura, T. Aoyama, T. Takido, M. Kodomari, Synlett. 2012, 223,
1397–1401.
[4] N. Jiang, A. J. Ragauskas, J. Org. Chem. 2006, 71, 7087–7090.
[5] R. Kawahara, K. Fujita, R. Yamaguchi, J. Am. Chem. Soc. 2012, 134,
3643–3646.
SUPPORTING INFORMATION
Additional supporting information may be found in the online
version of this article at publisher’s web-site.
[6] S. Zhang, L. Xu, M. L. Trudell, Synthesis. 2005, 1757–1760.
J. Phys. Org. Chem. 2015, 28 591–595
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