Efficient oxidation of benzylic alcohols with trichloroisocyanuric acid and ionic liquid in water

Article abstract of DOI:10.1080/00397911.2010.494811

A new environmentally friendly method for oxidation of benzylic alcohols to aldehydes or ketones has been developed using trichloroisocyanuric acid and [bmim]BF4 in water.

Full text of DOI:10.1080/00397911.2010.494811

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Efficient Oxidation of Benzylic Alcohols
with Trichloroisocyanuric Acid and Ionic
Liquid in Water
Jong Chan Lee a , Jimi Kim a , Seung Bae Lee a , Soon-Uk Chang a &
Yeon Jin Jeong a
a Department of Chemistry, Chung-Ang University, Seoul, South
Korea
Version of record first published: 02 May 2011.
To cite this article: Jong Chan Lee , Jimi Kim , Seung Bae Lee , Soon-Uk Chang & Yeon Jin Jeong
(2011): Efficient Oxidation of Benzylic Alcohols with Trichloroisocyanuric Acid and Ionic Liquid in
Water, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 41:13, 1947-1951
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Synthetic Communications1, 41: 1947–1951, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.494811
EFFICIENT OXIDATION OF BENZYLIC ALCOHOLS
WITH TRICHLOROISOCYANURIC ACID AND IONIC
LIQUID IN WATER
Jong Chan Lee, Jimi Kim, Seung Bae Lee, Soon-Uk Chang,
and Yeon Jin Jeong
Department of Chemistry, Chung-Ang University, Seoul, South Korea
GRAPHICAL ABSTRACT
Abstract A new environmentally friendly method for oxidation of benzylic alcohols to
aldehydes or ketones has been developed using trichloroisocyanuric acid and [bmim]BF4
in water.
Keywords Benzylic alcohol; ionic liquid; oxidation; water
Oxidation of benzylic alcohols to the corresponding aldehydes and ketones is one of
the most fundamental transformation in organic chemistry. Several reagents, such as
chromium(VI) species,[1] nitric acid,[2] and transition metal nitrates,[3,4] have been
widely utilized for the oxidation of benzylic alcohols in toxic organic solvents. How-
ever, use of those environmentally demanding oxidants limits their further practical
applications in this ever-increasing eco-conscious world. Hence, the development of
an environmentally friendlier oxidation protocol became of paramount importance
in organic synthesis. In this context, water can be considered as a most ideal and
clean solvent in chemical synthesis because of the many desirable properties such
as low cosr, nontoxicity, large heat capacity, and low compressibility. Therefore,
the development of a new method for the oxidation of benzylic alcohols to the
aldehydes and ketones using transition metal–free oxidizing agents under aqueous
reaction conditions are highly desirable.
Trichloroisocyanuric acid (TCCA) has been known as a powerful, stable, and
readily available chlorination reagent.[5,6] However, there are only a few examples about
its utilization to the oxidation reactions of alcohols. These include the oxidation of alco-
hols to the corresponding carbonyl compounds using the reagent combination of
Received August 5, 2009.
Address correspondence to Jong Chan Lee, Department of Chemistry, Chung-Ang University,
Seoul 156-756, South Korea. E-mail: jclee@cau.ac.kr
1947

1948
J. C. LEE ET AL.
TCCA=2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)=NaBr=NaHCO3 in aqueous
acetone[7] or TCCA=RuCl3=n-Bu4NBr=K2CO3 in aqueous acetonitrile solution.[8] How-
ever, these methods have invariably utilized mixtures of aqueous organic solvents and
strong basic reaction conditions. To the best of our knowledge, TCCA-induced oxi-
dation of alcohols in water without use of organic solvents or bases is unprecedented.
Herein, we report preliminary findings on the metal-free oxidation of benzylic
alcohols using TCCA in water. In an attempt to develop a greener method for the
TCCA-induced oxidation of benzylic alcohols, we chose to investigate the effect of
1-butyl-3-methylimidazolium tetrafluoroborate, [bmim]BF4, ionic liquid. This
approach avoids the need for activators such as TEMPO or RuCl3 as well as the
additional halide ions that have been usually employed in previously reported
TCCA-induced oxidations. Treatment of benzyl alcohol (1.0 mmol) with TCCA
(0.5 mmol) and [bmim]BF4 (0.2 mL) in water (1.0 mL) at room temperature smoothly
gave the desired benzaldehyde in good yield. It
had been noticed that the absence of ionic liquid in this protocol gave only unchanged
starting alcohols. Presumably, the acidic C-2 proton of imidazolium cation in
[bmim]BF4 assists the activation of TCCA.[9,10] The reactions of a various primary
and secondary benzylic alcohols successfully provided oxidized products in good
yields. All of the reactions studied were completed in 1 h, and the results are summar-
ized in Table 1. In cases of primary benzylic alcohols, overoxidation products were
not detected with the present reaction conditions. It is clear from Table 1 that second-
ary benzylic alcohols gave better yields of oxidized products compared to primary
benzylic alcohols. Electron-donating and withdrawing groups had relatively minor
influence on the outcome of the oxidation of substituted benzylic alcohols. Probably,
in this protocol, the oxidation of benzylic alcohols proceeded by the reaction with
hypochlorous acid (HOCl) formed in situ by the reaction of TCCA with ionic liquid.
In summary, the oxidation reaction of benzylic alcohols with TCCA and
[bmim]BF4 in water for 1 h at room temperature gave the corresponding carbonyl
compounds in good yields. The use of pure water as reaction medium, mildness of
reaction conditions, and operational simplicity should make present method an
important alternative to the other methods previously reported.
EXPERIMENTAL
1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) was obtained from
Aldrich. Merck silica gel 60 (230–400 mesh) was used for flash-column chromatography.
General Procedure
TCCA (0.12 g, 0.5 mmol) was added to a solution of a benzylic alcohol
(1.0 mmol), [bmim]BF4 (0.2 mL), and H2O (1.0 mL) at room temperature, and the

OXIDATION OF BENZYLIC ALCOHOLS
1949
Table 1. TCCA-promoted oxidation of benzylic alcohols in water
Entry
1
Substrate
Product
Yield (%)a
90
2
3
88
80
4
5
80
90
90
78
93
94
95
6
7
8
9
10
11
85
aIsolated yield.
mixture was stirred for 1 h. After completion of reaction, the mixture was extracted
with dichloromethane (2 ꢀ 25 mL) and washed with water (25 mL). The combined
organic phases were dried over MgSO4 and evaporated under reduced pressure.
The residue was purified by flash-column chromatography (EtOAc=hexane 1:3) on
silica gel to obtain pure benzaldehyde. Products were confirmed by comparison with
reported spectral data of authentic samples.[11,12]
Selected Data
1
Benzaldehyde (1a). Bp: 177–179 ꢁC; H NMR (300 MHz, CDCl3): d 10.03
(s, 1H), d 7.90–7.88 (d, 2H), d 7.56–7.53 (m, 3H); MS 106 (Mþ).
1
4-Chlorobenzaldehyde (2a). Mp: 45–49 ꢁC; H NMR (300 MHz, CDCl3):
d 9.40 (s, 1H), d 7.97 (m, 2H), d 7.11 (m, 2H); MS 141 (Mþ).

1950
J. C. LEE ET AL.
1
4-Bromobenzaldehyde (3a). Mp: 56–58 ꢁC; H NMR (300 MHz, CDCl3):
d 9.80 (m, 1H), d 7.76 (m, 2H), d 7.64 (m, 2H); MS 185 (Mþ).
1
4-(Trifluoromethyl)benzaldehyde (4a). Bp: 65–67 ꢁC; H NMR (300 MHz,
CDCl3): d 10.09 (s, 1H), d 8.02–7.99 (d, 2H), d 7.82–7.79 (d, 2H); MS 134 (Mþ).
1
Acetophenone (5a). Bp: 64 ꢁC = 12 mmHg; H NMR (300 MHz, CDCl3):
d 8.00 (m, 2H), d 7.50–7.60 (m, 3H), d 2.70 (s, 3H); MS 120 (Mþ).
1
Propiophenone (6a). Bp: 216 ꢁC; H NMR (300 MHz, CDCl3): d 7.97–7.94
(d, 2H), d 7.54–7.51 (m, 3H), d 7.48–7.42 (m, 2H), d 3.03–2.96 (q, 2H), d 1.25–1.19
(m, 3H); MS 134 (Mþ).
4’-Methylacetophenone (7a). Bp: 225 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.82–7.78 (m, 2H), d 7.21–7.17 (m, 2H), d 2.48 (s, 3H), d 2.31 (s, 3H); MS 134 (Mþ).
4’-Fluoroacetophenone (8a). Bp: 195 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.92–7.87 (m, 2H), d 7.07–7.01 (m, 2H), d 2.50 (s, 3H); MS 138 (Mþ).
4’-Chloroacetophenone (9a). Bp: 232 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.90–7.87 (d, 2H), d 7.44–7.41 (d, 2H), d 2.58 (s, 3H); MS 155 (Mþ).
4’-Bromoacetophenone (10a). Mp: 47 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.82–7.79 (d, 2H), d 7.60–7.57 (d, 2H), d 2.57 (s, 3H); MS 178 (Mþ).
Benzophenone (11a). Mp: 47–50 ꢁC; 1H NMR (300 MHz, CDCl3):
d 7.81–7.78 (d, 2H), d 7.60–7.55 (m, 4H), d 7.50–7.44 (m, 4H); MS 182 (Mþ).
ACKNOWLEDGMENT
This research was supported by the Chung-Ang University Research Scholar-
ship Grants in 2011.
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