A mild and efficient method for the oxidation of benzylic alcohols by two-phase electrolysis

Article abstract of DOI:10.1016/j.tetlet.2007.03.124

Electrochemical oxidation of benzylic and substituted benzylic alcohols by two-phase electrolysis yields the corresponding aldehydes as products. The reaction was carried out in a single compartment cell with platinum electrodes at room temperature in chloroform using an aqueous sodium bromide solution (25%) containing a catalytic amount of HBr. The two-phase electrolysis resulted in high yields (74-96%) of benzaldehyde from primary alcohols and secondary alcohols were oxidized to the corresponding ketone but only in low yields under these conditions.

Full text of DOI:10.1016/j.tetlet.2007.03.124

Tetrahedron Letters 48 (2007) 3681–3684
A mild and efficient method for the oxidation of benzylic
alcohols by two-phase electrolysis
Thasan Raju,* Sankar Manivasagan, Balachandran Revathy,
Kumarasamy Kulangiappar and Arunachalam Muthukumaran
Electroorganic Division, Central Electrochemical Research Institute, Karaikudi 630 006, India
Received 5 January 2007; revised 14 March 2007; accepted 22 March 2007
Available online 25 March 2007
Abstract—Electrochemical oxidation of benzylic and substituted benzylic alcohols by two-phase electrolysis yields the correspond-
ing aldehydes as products. The reaction was carried out in a single compartment cell with platinum electrodes at room temperature
in chloroform using an aqueous sodium bromide solution (25%) containing a catalytic amount of HBr. The two-phase electrolysis
resulted in high yields (74–96%) of benzaldehyde from primary alcohols and secondary alcohols were oxidized to the corresponding
ketone but only in low yields under these conditions.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The oxidation of alcohols is an important transforma-
tion in laboratory and industrial synthetic chemistry,
as the corresponding carbonyl compounds can be used
as important and versatile intermediates for the synthe-
sis of fine chemicals.1–3 Numerous oxidizing reagents
such as chromium(VI) oxide, permanganates, ruthe-
nium(VIII) oxide and dichromates have been employed
to accomplish these transformations. Most of these
reagents are, however, required in stoichiometric quan-
tities which results in the production of large amounts
of environmental waste.4–6 The oxidation of alcohols
with molecular oxygen using transition metal salts of
V,7 Co,8 Cu,9 Mo,10 Ru,11 Rh,12 Pd13 and polyoxometa-
lates14 as catalysts has also been reported. Most of these
systems catalyze the oxidation of primary as well as sec-
ondary alcohols to give the corresponding aldehydes
and ketones. The selective oxidation of primary alcohols
to aldehydes is crucial for the synthesis of fine chemicals
such as fragrances or food additives.15
dized alcohols with electro-generated hypobromite as
an emulsion using a quaternary ammonium salt as the
phase-transfer agent. As the electrolysis was conducted
in an emulsion phase, the voltage was high due to the
poor electrical conductivity of the organic solvent and
ultimately the process was not economic. Recently,
nitroxyl radicals, that is, N-oxyl compound-mediated
electrooxidation of alcohols, has been reported.17–19 In
these processes, costly reagents were used as mediators
along with the catalyst.
Two-phase electrolysis has a distinct advantage over
conventional homogeneous electrolysis in practical elec-
troorganic synthesis. In homogeneous systems, lower
selectivity is observed due to over-oxidation of the sub-
strate on the surface of the electrode leading to a mix-
ture of products. In two-phase electrolysis systems, the
reactive species formed by electrolytic oxidation of a
halide ion in the aqueous phase, can be taken continu-
ously into the organic phase, and then reacted with the
substrate selectively to give the products. After comple-
tion of the electrolysis, separation and evaporation of
the organic layer affords the product.
In recent years, electroorganic synthesis has attracted
attention as an environmentally friendly process, be-
cause electrons are inherently environmentally friendly
reagents compared with conventional oxidizing and
reducing reagents. Earlier Tomov and Jansson16 oxi-
Presently, only a few reports on two-phase electrolysis
are available for electroorganic syntheses of fine chemi-
cals. Recently, we reported that a two-phase electrolytic
system can be used readily to convert alkyl aromatic
compounds to monobromo derivatives in quantitative
yields.20 We have further studied its application for the
Keywords: Benzyl alcohol; Oxidation; Two-phase electrolysis.
*
Corresponding author. Tel.: +91 04565 22772/227555; fax: +91
04565 227779; e-mail: rajuorganic@yahoo.co.in
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.124

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T. Raju et al. / Tetrahedron Letters 48 (2007) 3681–3684
oxidation of alcohols and herein report a mild and effi-
cient method for the selective oxidation of alcohols to
the corresponding aldehydes using electrochemically
generated hypobromous acid.
Hypobromous acid is an inexpensive reagent and can be
generated in situ by electrochemical oxidation of bro-
mide ions in the presence of HBr. The two-phase elec-
trolysis system consists of a 25% NaBr solution
containing a catalytic amount of HBr (5%) as the aque-
ous phase and chloroform containing the alcohol as the
organic phase. Using this system, benzylic and substi-
tuted benzylic alcohols were oxidized to the correspond-
CHO
CH2OH
Pt/Pt, NaBr/CHCl3
r.t, 30 mA/cm2
ing aldehydes at room temperature in
a single
compartment cell in high yields (Scheme 1).21 The yields
of the products are listed in Table 1. Under these condi-
tions, the formation of the benzaldehyde hydrate and its
interaction with electro-generated hypobromite is slow
R
R
Scheme 1. Electrochemical oxidation of benzylic alcohols by two-
phase electrolysis.
Table 1. Electrochemical oxidation of alcohols using a two-phase electrolysis system
Entry
Reactant
Product
Yielda (%)
Charge passed (F/mol)
5.5
Current efficiency (%)
37
CH2OH
CHO
1
96
CHO
CH2OH
2
3
94
90
5.0
5.5
36
33
Cl
Cl
CH2OH
CHO
Cl
Cl
CH2OH
CH2OH
CHO
4
5
6
95
6.0
4.0
2.0
36
44
74
Br
Br
CHO
85
Me
MeO
Me
MeO
CH2OH
CH2OH
CHO
74b
CHO
7
8
96
80
3.0
4.0
71
40
CH2OH
CH3
CHO
CH3
CH2OH
CHO
9
0
6
—
O2N
O2N
10
11
CH2(CH2)6CH2OH
CH2(CH2)4CH2OH
CH3(CH2)6CHO
CH2(CH2)4CHO
22c
13c
6
6
8
4
H
O
12
13
11
30
30
6
6
6
4
10
10
CH3
OH
CH3
H
O
CH3
OH
CH3
Cl
Cl
Br
H
O
14
CH3
OH
CH3
Br
a Isolated yield.
b 7% of 3-bromo-p-anisaldehyde was obtained along with 15% of starting material.
c Analyzed by GC, isolated as bisulfate adduct.

T. Raju et al. / Tetrahedron Letters 48 (2007) 3681–3684
3683
enough on the electrolysis time scale to minimize oxida-
tion to the corresponding acid. We observed smooth
oxidation of benzylic alcohols substituted with elec-
tron-donating groups such as –CH3, –OCH3, –t-butyl
(entries 5–7) with 2–4 F/mol of current, while alcohols
substituted with electron-withdrawing groups (entries
2–4) required 4–6 F/mol. The highly deactivating NO2-
substituted benzyl alcohol did not react at all. Primary
aliphatic alcohols were allowed to react under these con-
ditions but only low yields of the corresponding alde-
hydes were obtained (entries 9 and 10).The secondary
alcohol, 1-phenylethanol, was less reactive affording
the corresponding ketone in only 11% yield. This study
reveals that this method can be applied to the selective
oxidation of benzylic alcohols in the presence of second-
ary hydroxyl groups.
In conclusion, this electrochemical method for the oxi-
dation of benzylic alcohols to the corresponding benzal-
dehydes in excellent yields using in situ prepared
hypobromous acid via a two-phase electrolysis consti-
tutes a novel and an efficient alternative procedure to
traditional oxidation. Easy separation of the product,
a simple work-up, room temperature reaction condi-
tions, and the reuse of the electrolyte are advantages
of this two-phase electrolysis procedure.
Acknowledgements
The authors thank Professor A. K. Shukla, Director,
Central Electrochemical Research Institute, Karaikudi
630 006, India and Shri. V. M. Shanmugam for
Providing NMR analysis and facilities.
A possible mechanism for the oxidation, based on a lit-
erature report, is outlined in Scheme 2.22 As the electrol-
ysis proceeds, the bromide ion is oxidized at the anode
to bromine which, on hydrolysis, results in the forma-
tion of hypobromous acid and HBr. The unstable hypo-
bromous acid forms Br+ due to its ionic nature which
subsequently oxidizes the alcohol to the corresponding
aldehyde, (Scheme 2).
References and notes
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Organic Compounds; Academic Press: London, 1981.
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hyde as a model reaction, various electrode materials
were studied to determine the effectiveness of the elec-
trode for the oxidation of bromide ions (Table 2). Even
though the other electrodes did not perform as well as
Pt, commercially available graphite works quite well
affording an yield of 89% benzaldehyde along with 8%
of recovered benzyl alcohol.
8. Iwahama, T.; Yoshino, Y.; Keitoku, T.; Sakaguchi, S.;
Ishii, Y. J. Org. Chem. 2000, 65, 6502–6507.
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2Br -
Br2 + H2O
-2e-
H+
Br2
HOBr + HBr
Br+ + H2O
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HOBr
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Br
OH
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R
H
R
H
-HBr
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H
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H
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Br
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Scheme 2.
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1980, 10, 583–586.
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chem. Commun. 2005, 7, 312–316.
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Table 2. Efficiency of the electrode for the oxidation
Entry
Electrode
Benzaldehyde
yield (%)
Anode
Cathode
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21. Representative procedure for the electrochemical oxidation
of benzylic alcohol: A solution of 4-methylbenzyl alcohol
(Table 1, entry 5) (1.22 g, 10 mmol) in 25 ml of chloroform
was taken in a beaker-type undivided cell. To the above
1
2
3
Platinum
Graphite
Dimensionally
stable anode (DSA)
Graphite
Platinum
Platinum
Platinum
96
92
90
4
5
Stainless steel
Graphite
85
89
Graphite
Current density = 30 mA/cm2, charge passed = 5.5 F/mol, tempera-
ture = rt, agitation rate = 40–80 rpm.

3684
T. Raju et al. / Tetrahedron Letters 48 (2007) 3681–3684
solution, a 25% aqueous sodium bromide solution (50 ml)
using a UV detector at a wavelength of 254 nm. After
completion of electrolysis, the lower organic phase was
separated, washed with sodium thiosulfate solution fol-
lowed by water (2 · 25 ml), dried over anhydrous Na2SO4
and the solvent was removed under reduced pressure. The
HPLC spectrum of the crude mixture showed the presence
of 88% of 4-methylbenzaldehyde along with 10% of the
starting material and some minor impurities. The crude
product was passed through a column of silica gel (60–
120 mesh) and eluted with a mixture of ethyl acetate/
n-hexane (1:9) to afford the pure product (1.0 g; 85%).
22. Koo, B. S.; Lee, C. K.; Lee, K. J. Synth. Commun. 2002,
32, 2115–2123.
containing 5 ml of HBr (46% solution, 30 mmol) was
added. Two platinum electrodes each of 10 cm2 area were
placed in the upper layer of the aqueous phase. The
organic phase alone was stirred with a magnetic stirrer at a
rate of 40 rpm in such a way that the organic layer did not
touch the electrodes. The electrolysis was conducted
galvanostatically at a current density of 30 mA/cm2 until
the quantity of charge indicated in Table 1 was passed.
The extent electrolysis was monitored by HPLC using a
Shimpack ODS-18 column (120 · 4.5 mm) as the station-
ary phase. The eluent consisted of methanol/water (70:30)
at a flow rate of 1 ml/min. The samples were analyzed