Efficient oxidation of benzylic alcohols with [hydroxy(tosyloxy)iodo] benzene under microwave irradiation

Article abstract of DOI:10.1016/j.tetlet.2004.04.132

An efficient method for the oxidation of benzylic alcohols with [hydroxy(tosyloxy)iodo]benzene under solvent-free microwave irradiation conditions is described.

Full text of DOI:10.1016/j.tetlet.2004.04.132

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4939–4941
Efficient oxidation of benzylic alcohols with
[hydroxy(tosyloxy)iodo]benzene under microwave irradiation
Jong Chan Lee,* Ji Young Lee and Seung Jun Lee
Department of Chemistry, Chung-Ang University, Seoul 156-756, South Korea
Received 10 March 2004; revised 29 March 2004; accepted 16 April 2004
Available online 14 May 2004
Abstract—An efficient method for the oxidation of benzylic alcohols with [hydroxy(tosyloxy)iodo]benzene under solvent-free
microwave irradiation conditions is described.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The oxidation of benzylic alcohols to carbonyl com-
pounds is a fundamental transformation in organic
synthesis and numerous methods utilizing various re-
agents have been reported.1;2 However, the majority of
known methods commonly suffer from one or more
disadvantages such as difficulty in manipulation, long
reaction times, and utilization of toxic reagents. There-
fore, it is still desirable to develop a new efficient oxidant
with properties of high stability, low toxicity, and ready
availability for the benzylic oxidations. Hypervalent
iodine reagents have received a great deal of attention
due to their versatility in oxidation processes.3;4 In
general, pentavalent iodine reagents such as Dess–
Martin periodinane5 and o-iodoxybenzoic acid6 have
been widely used for efficient oxidation of alcohols to
the carbonyl compounds. However, reported methods
for the oxidation of alcohols mediated by trivalent
iodine reagents have been quite limited. Examples of
these methods include oxidation of alcohols with iod-
osobenzene7 and iodobenzene diacetate.8 Furthermore,
to the best of our knowledge, there has been no example
of utilization of the [hydroxy(tosyloxy)iodo]benzene
(HTIB, Koser’s reagent) for the oxidation of primary
benzylic alcohols into the corresponding aldehydes.
Among other hypervalent iodine reagents, HTIB rep-
resents as one of the most versatile reagents and its use
in a variety of organic transformations are well docu-
mented.9;10
Recently, microwave induced synthetic technique has
received considerable attention as a useful tool to
accelerate a wide variety of organic transformations.11
In particular, the microwave promoted reactions under
solvent-free reaction conditions provided many advan-
tages such as eco-friendlier conditions and very rapid
reaction, experimental simplicity, and high yields.12
In connection with our interest in application of hyper-
valent iodine(III) sulfonates to microwave promoted
organic transformations,13 we considered of interest to
study the effect of HTIB in oxidation of primary benzylic
alcohols. We now wish to report our finding, which
enables facile and efficient oxidation of primary benzylic
alcohols to the corresponding aldehydes. Treatment of
neat primary benzylic alcohol (1.0 mmol) with HTIB
(1.2 mmol) under microwave irradiation using a house-
hold microwave oven for 40–160 s provided the corre-
sponding aldehydes in high yields (Scheme 1).14 A range
of primary benzylic alcohols were investigated using
present conditions and results are summarized in Table 1.
R = H
HTIB, MWI
ArCHO
ArCH(OH)R
ArCOCH(OTs)R'
R = Me, Et
Keywords: Alcohols; Aldehydes; Hypervalent elements; Microwave
irradiation; Oxidations.
R' = H, Me
HTIB, MWI
* Corresponding author. Tel.: +82-2-820-5202; fax: +82-2-825-4736;
e-mail: jclee@cau.ac.kr
Scheme 1.
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.132

4940
J. C. Lee et al. / Tetrahedron Letters 45 (2004) 4939–4941
Table 1. Microwave-assisted oxidation of benzylic alcohols with HTIB
10 s intervals. The reaction mixture was extracted with
dichloromethane (2 · 25 mL) and washed with water
(40 mL). The dichloromethane layer was separated and
dried over MgSO4. After evaporation of the solvent, the
residue was purified by flash column chromatography
(SiO2, ethyl acetate–hexane ¼ 1:4) to afford pure
a-tosyloxyacetophenone.
Entry Substrate
Product
Yield
(%)a
1
PhCH2OH
PhCHO
98
90
93
94
82
2
4-MeC6H4CH2OH
4-ClC6H4CH2OH
4-NO2C6H4CH2OH
2-NO2C6H4CH2OH
3,5-(NO2)2C6H3CH2OH
Ph2CHOH
4-MeC6H4CHO
4-ClC6H4CHO
4-NO2C6H4CHO
2-NO2C6H4CHO
3
4
5
6
3,5-(NO2)2C6H3CHO 78
In summary, microwave promoted reactions of benzylic
alcohols with HTIB provides a rapid and convenient
way to prepare corresponding carbonyl compounds in
solvent-free conditions. The advantages of the present
method in conjunction with eco-friendly conditions and
fast reaction rates should make this protocol a valuable
alternative to the other reported methods.
7
Ph2CO
86
90
85
88
8
PhCH(OH)CH2Ph
C6H5CH(OH)COOMe
C6H5CH(OH)COOEt
PhCOCH2Ph
C6H5COCOOMe
C6H5COCOOEt
9
10
11
12
13
14
15
2-Cl-C6H4CH(OH)COOMe 2-Cl-C6H4COCOOMe 90
PhCH(CH3)OH
PhCOCH2OTs
75b
4-BrC6H4CH(CH3)OH
4-ClC6H4CH(CH3)OH
PhCH(Et)OH
4-BrC6H4COCH2OTs 72b
4-ClC6H4COCH2OTs 78b
PhCOCH(Me)OTs
67b
a Isolated yields.
bObtained by use of 2.5 equiv of HTIB.
Acknowledgements
All of the reactions completed very cleanly without any
noticeable side products. The present method could
equally work with electron rich and very electron defi-
cient benzylic alcohols (entries 2 and 6).
We thank the KOSEF (R01-1999-00036) for financial
support of this work.
a-Keto esters received much attention due to their useful
biological properties and many efforts have been made
for the oxidation of a-hydroxy esters to the corres-
ponding a-keto esters.15;16 Attempts to oxidise a series of
a-hydroxy under the present reaction conditions were
successful in obtaining high yields of a-keto esters
(entries 9–11). Application of present method for the
oxidation of primary aliphatic alcohols gave very low
conversion and product mixtures are obtained in cases
of oxidation of allylic alcohols.
References and notes
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After established optimal conditions for oxidations of
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product mixtures, use of excess amounts of HTIB
(2.5 equiv) with microwave irradiation for extended
reaction times (4–5 min) smoothly furnished a-tosyloxy
ketones in high yields (entries 12–15). The yields of
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(tosyloxy)iodo]styrenes or iodosobenzene/PTSA.17–19
a-Tosyloxyketones are very useful intermediates in
organic synthesis.10 Conventionally, a-tosyloxy ketones
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method for the direct preparation of a-tosyloxy ketones
from secondary benzyl alcohols. Typical experimental
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HTIB (0.981 g, 2.5 mmol) were mixed and placed in a
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neat mixture of benzyl alcohol (0.108 g, 1.0 mmol) and
HTIB (0.471 g, 1.2 mmol) were mixed and placed in a
50 mL glass tube. The reaction mixture was inserted in an
alumina bath inside a household microwave oven and
irradiated (700 W) three times for a period of 20 s with 10 s
intervals. The reaction mixture was extracted with dichloro-
methane (2 · 25 mL) and washed with water (40 mL). The
dichloromethane layer was separated and dried over
MgSO4. After evaporation of the solvent, the residue
was purified by flash column chromatography (SiO2, ethyl
acetate–hexane ¼ 1:2) to afford pure benzaldehyde.

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4941
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