Efficient procedure for oxidation of benzylic alcohols to carbonyl compounds by N-bromosuccinimide in ionic liquid

Article abstract of DOI:10.1081/SCC-200064962

Efficient oxidation of various benzylic alcohols to the corresponding carbonyl compounds has been achieved in the presence of NBS and 2,6-lutidine in ionic liquid [bmim]BF₄. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200064962

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Efficient Procedure for
Oxidation of Benzylic Alcohols
to Carbonyl Compounds by
N‐Bromosuccinimide in Ionic
Liquid
Jong Chan Lee a , Ji Young Lee a & Jang Mi Lee a
a Department of Chemistry, Chung‐Ang University,
Seoul, South Korea
Published online: 16 Aug 2006.
To cite this article: Jong Chan Lee , Ji Young Lee & Jang Mi Lee (2005) Efficient
Procedure for Oxidation of Benzylic Alcohols to Carbonyl Compounds by
N‐Bromosuccinimide in Ionic Liquid, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 35:14, 1911-1914,
DOI: 10.1081/SCC-200064962
To link to this article: http://dx.doi.org/10.1081/SCC-200064962
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Synthetic Communicationsw, 35: 1911–1914, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200064962
Efficient Procedure for Oxidation of
Benzylic Alcohols to Carbonyl Compounds
by N-Bromosuccinimide in Ionic Liquid
Jong Chan Lee, Ji Young Lee, and Jang Mi Lee
Department of Chemistry, Chung-Ang University, Seoul, South Korea
Abstract: Efficient oxidation of various benzylic alcohols to the corresponding
carbonyl compounds has been achieved in the presence of NBS and 2,6-lutidine in
ionic liquid [bmim]BF4.
Keywords: Benzylic alcohols, carbonyl, ionic liquid, oxidation
The oxidation of benzylic alcohols to carbonyl compounds is an important
reaction in organic synthesis and numerous methods have been developed
for this conversion. For example, recent reports on this transformation have
dealt with the utilization of nitric acid,[1] DMSO/HBr,[2] [hydroxy(tosyloxy)
iodo]benzene,[3] and iodobenzene diacetate on alumina.[4] A readily available
and stable N-bromosuccinimide (NBS) also has been utilized in the oxidation
of alcohols and results were well documented.[5] Representative methods
include the efficient oxidation of alcohols with a NBS/tetrabutylammonium
iodide system[6] and conversion of 1,2-diols to 1,2-diketones in the presence
of NBS/pyridine.[7] However, these NBS-mediated oxidation methods
always utilized environmentally undesirable organic solvents such as aceto-
nitrile or carbon tetrachloride.
Received in Japan February 4, 2005
Address correspondence to Jong Chan Lee, Department of Chemistry, Chung-Ang
University, Seoul156-756, SouthKorea. Fax:þ82-2-825-4736;E-mail:jclee@cau.ac.kr
1911

1912
J. C. Lee, J. Y. Lee, and J. M. Lee
In the past decade, ionic liquids have been received substantial attention
in organic synthesis because of their environmentally benign nature, high
polarity, and good thermal stability.[8,9] Often shorter reaction times, high
yields, cleaner reaction products, and high selectivities are obtained from
ionic-liquid reaction media. A couple of methods have been reported for
the oxidation of alcohols conducted in ionic liquids.[10,11] As a part of our
program related to development of environmentally benign synthetic method-
ology, we now report oxidation of benzylic alcohols promoted by NBS in an
ionic liquid. As far as we are aware, the NBS-mediated oxidation of alcohols
in an ionic liquid medium has not been reported to date. Treatment of
benzylic alcohols with N-bromosuccinimide (1.2 equiv) and 2,6-lutidine
(1.0 equiv) in 1 mL of 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim]BF4) ionic liquid at 408C for 0.5–3 h readily afforded the corre-
sponding carbonyl derivatives (Scheme 1). When the ionic liquid medium
was changed from (bmim)BF4 to more hydrophobic (bmim)PF6, the
reactions gave less satisfactory results with much lower yields. It also
turned out the use of a base was necessary in the reaction conditions. In
the absence of a base, the reactions provided the desired carbonyl
compounds along with significant amount of benzylic bromide side
products. We explored the effect of the base on the oxidation reactions
utilizing various bases such as triethylamine; pyridine; 2,6-lutidine; DBU;
and potassium carbonate and found that among these bases, 2,6-lutidine
afforded the best result in terms of cleanness and reaction times. The
oxidation of both primary and secondary benzylic alcohols occurred
equally well to provide the corresponding carbonyl compounds in high to
excellent yields in short reaction times. The results are summarized in
Table 1.
The competitive aromatic bromination has not been observed in all of the
cases tested. The benzhydrol and benzoin are smoothly oxidized to the corre-
sponding carbonyl compounds in high yields under present reaction con-
ditions (entries 9 and 10). It is also noteworthy that prolonged reaction
times were required in cases of benzylic alcohols with electron-withdrawing
groups on the aromatic rings. This result can be rationalized by the sluggish
formation of hypoboromite reaction intermediates in cases of the less
reactive electron-deficient benzylic alcohols.
In summary, we have discovered a novel and efficient method for the
oxidation of benzyl alcohols into the corresponding carbonyl compounds
utilizing an NBS/2,6-lutidine system in (bmim)BF4 ionic liquid.
Scheme 1.

Oxidation of Benzylic Alcohols
1913
Table 1. Oxidation of benzylic alcohols with NBS in [bmim]BF4
Time
(h)
Yield
(%)a
Entry
Substrate
Product
1
0.5
0.5
1
98
95
91
93
85
2
3
4
5
2
4
6
7
8
9
1
98
92
93
88
2
1
2.5
10
3
87
aIsolated yield.
EXPERIMENTAL
The 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) was
obtained from Fluka. IR spectra were recorded on a Jasco FT/IR 5300 spec-
trophotometer using KBr optics. 1H NMR spectra were recorded on a Varian
2000 (300-MHz) spectrometer in CDCl3 using TMS as internal standard.
Merck silica gel 60 (230–400 mesh) was used for flash column
chromatography.
General procedure for benzylic oxidation reaction using NBS: To a
stirred solution of benzylic alcohol (1.0 mmol) in (bmim)BF4 (1 mL) was
added N-bromosuccinimide (0.214 g, 1.2 mmol) and 2,6-lutidine (0.107 g,

1914
J. C. Lee, J. Y. Lee, and J. M. Lee
1.0 mmol), and stirring was continued at 408C for the time indicated in the
Table 1. After completion of the reaction, the product was extracted with
dichloromethane (2 ꢀ 25 mL), washed with water (40 mL), and dried over
MgSO4. The solvent was removed in vacuo and the crude mixture was
purified by silica-gel chromatography using ethyl acetate–hexane (1:2) to
give the pure carbonyl compound.
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