LETTER
79
Oxidation of Benzylic Alcohols with Urea–Hydrogen Peroxide and Catalytic
Magnesium Bromide
O
xidation of Benz
e
ylic
A
lcoho
e
ls Joong Park, Jong Chan Lee*
Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea
Fax +82(2)8254763; E-mail: jclee@cau.ac.kr
Received 8 August 2008
best of our knowledge, there is only one example reported
to date for the oxidation of benzylic alcohols to aldehydes
or ketones using UHP in the presence of Amberlite IRA-
120 acidic resin.16 However, this method gave the oxi-
dized carbonyl compounds only as minor products in very
poor yields ranging from 17% to 32% and primarily pro-
duced phenols as major products. We report here the first
general utilization of UHP in the oxidation of a series of
substituted benzylic alcohols.
Abstract: Urea–hydrogen peroxide in the presence of a catalytic
amount of magnesium bromide has been utilized efficiently for ox-
idation of primary and secondary benzylic alcohols into the corre-
sponding aromatic aldehydes and ketones.
Key words: alcohols, hydrogen peroxide, ionic liquid, magnesium,
urea
Oxidation of alcohols into the corresponding aldehydes
and ketones is one of the most fundamental transforma-
tions in organic synthesis. Usually the oxidations of ben-
zylic alcohols have been carried out using oxidants such
as chromium(VI) trioxide,1 nitric acid,2 dimethyl sulfox-
ide/HBr,3 and hypervalent iodine compounds.4 In recent
years, replacement of toxic oxidants in organic reactions
has become of high priority in environmentally benign
chemistry. Among other reagents, hydrogen peroxide is a
cheap and easily available oxidizing reagent, and it is con-
sidered as the most desirable oxidant in terms of environ-
mental considerations. Various methods for the hydrogen
peroxide promoted oxidation of benzylic alcohols have
been reported. Representative examples include the oxi-
dation of benzylic alcohols with hydrogen peroxide in
combination with activators such as sodium tungstate5 or
strongly acidic HBr.6 However the unstable nature of liq-
uid hydrogen peroxide limits its applications in practical
oxidation reactions. In recent years, urea–hydrogen per-
oxide (UHP) has emerged as a safer substitute for unstable
concentrated hydrogen peroxide due to its particular prop-
erties, such as high stability, high hydrogen peroxide con-
tent as well as ease of accessibility.7 Urea–hydrogen
peroxide has been utilized for a variety of organic reac-
tions which include epoxidation of olefins,8 conversion of
sulfide to sulfones,9 oxidation of aromatic aldehydes to
benzoic acids,10 and oxidative halogenation of arenes.11
However, due to the high stability of UHP the oxidation
reactions were performed in the presence of strong activa-
tor such as hexafluoro-2-propanol,12 methyl trioxorheni-
um (MTO),13 and formic acid. In addition, these reactions
usually required the use of large molar excess amount of
UHP. Furthermore, in some instant, unconventional mi-
crowave irradiation and solvent-free techniques were also
required.14,15 Although vast array of UHP-promoted trans-
formations have been reported as described above, to the
In the past decade, ionic liquids have attracted much atten-
tion because of their excellent solvent properties and de-
signer solvent features as well as environmentally friendly
nature. Ionic liquid is attractive greener alternative to tox-
ic volatile organic solvents.
As part of our ongoing efforts to develop greener oxida-
tion method for alcohols, we studied oxidation of benzylic
alcohols using UHP in the presence of magnesium bro-
mide in ionic liquid. The experimental procedure is very
straightforward. The reaction of benzylic alcohols with
UHP (1.2 equiv) in the presence of magnesium bromide
(0.1 equiv) at 60 °C for 2–3 hours in 1-butyl-3-methyl-
imidazolium tetrafluoroborate, [bmim]BF4, provided the
corresponding aldehydes or ketones as the sole observable
products in all cases. A wide range of structurally diverse
primary and secondary benzylic alcohols successfully un-
derwent oxidation reactions by this procedure to provide
the corresponding carbonyl compounds. The results are
summarized in Table 1. In all cases, the use of 0.1 equiv-
alent of magnesium bromide was sufficient to achieve
high yields of the desired oxidation products. In the ab-
sence of magnesium bromide, no oxidation reactions were
observed. Recycling of the same ionic liquid medium over
five reaction runs gave comparable yields. The electron-
donating and electron-withdrawing substituent on the ar-
omatic ring did not profoundly affect the efficiency of the
reactions. But in the case of strong electron-withdrawing
groups such as nitro group somewhat lower yields were
obtained (entry 7). Primary benzylic alcohols did not un-
dergo overoxidation under the present reaction condi-
tions. The acid-sensitive methoxy and ester functionalities
(entries 6, 18, and 19) were retained, showing the mild-
ness of the present reaction conditions. The oxidation of
diphenylmethanol, benzoin, and a-hydoxy esters smooth-
ly gave benzophenone, benzyl, and a-keto esters, respec-
tively (entries 16–19). However, application of present
method to aliphatic alcohols provided complex product
mixtures. Although the mechanism is unclear, this reac-
SYNLETT 2009, No. 1, pp 0079–0080
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Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1087391; Art ID: U08308ST
© Georg Thieme Verlag Stuttgart · New York