Solid State Oxidation of Benzoins on Alumina-Supported Copper(II) Sulfate under Microwave Irradiation

Article abstract of DOI:10.1039/a709148f

Benzoins are rapidly oxidised to benzils in high yields by the solid reagent system copper(II) sulfate-alumina, under the influence of microwaves.

Full text of DOI:10.1039/a709148f

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324 J. CHEM. RESEARCH (S), 1998
Solid State Oxidation of Benzoins on
Alumina-Supported Copper(II) Sulfate under
J. Chem. Research (S),
1998, 324±325$
Microwave Irradiation$
Rajender S. Varma,* Dalip Kumar and Rajender Dahiya
Department of Chemistry and Texas Regional Institute for Environmental Studies (TRIES),
Sam Houston State University, Huntsville, TX 77341-2117, USA
Benzoins are rapidly oxidised to benzils in high yields by the solid reagent system copper(II) sulfate±alumina, under the
influence of microwaves.
The oxidative transformation of benzoins to benzils
has been accomplished by a variety of reagents namely
nitric acid,1 Fehling's solution,1 thallium(III) nitrate
(TTN),2,3 ytterbium(III) nitrate,4 clayfen5 and ammonium
chlorochromate±alumina.6 Besides extended reaction times,
most of these processes su€er from drawbacks such as the
use of corrosive acids and toxic metallic compounds that
generate undesirable waste materials. Consequently, there
is a need for the development of a manipulatively easy
and environmentally benign solvent-free protocol for the
oxidation of benzoins. In this context, organic reactions
on solid supports7,8 and those assisted by microwaves,8±10
especially under solvent-free conditions,8,9 have attracted
attention recently because of their enhanced selectivity,
milder reaction conditions and associated ease of manipu-
lation. Since only the polar reactants adsorbed on the
surface of the solid support absorb microwaves, a variety of
reagents supported on such surfaces can be utilised for the
enhancement of organic reactions using an unmodi®ed
microwave oven.
diketones in high yields within 2±3 min of microwave
irradiation. These solvent-free reactions are conducted by
mixing benzoins thoroughly with the catalyst followed
by irradiation in a domestic microwave oven. The optimum
ratio of substrate to reagent is found to be (1:0.85,
mole:mole) that ensures complete conversion of benzoins
into benzils. Interestingly, the oxidative protocol is e€ective
only for a-hydroxyketones; other secondary alcohols, includ-
ing 1,2-diphenylethane-1,2-diol, are not oxidised under these
conditions.
Experimental
Melting points are uncorrected. A Sears Kenmore household
microwave oven operating at 2450 MHz was used at its full power
of 900 W for all the experiments. The products were identi®ed by
comparison of the mp, IR and NMR spectra of the products with
authentic samples.
Preparation of the Reagent, CuSO4±Al2O3.ÐNeutral alumina
(25 g) was added to a well stirred solution of copper sulfate penta-
hydrate (3.9 g) in distilled water (50 ml) vacuum. The resulting sky
blue powder was stored in a tightly closed bottle and used without
prior activation. The reagent retains its activity for an extended
period; the original batch is still active 3 years after its preparation.
General Oxidation Procedure.ÐBenzoin (1 mmol) and CuSO4±
Al2O3 (1.5 g, 0.85 mmol of CuSO4 Á5H2O) were mixed thoroughly
on a vortex mixer. The reaction mixture contained in glass tubes
was placed in an alumina bath (heat sink) inside the microwave
oven and irradiated for a speci®ed time. On completion of the
reaction, followed by TLC examination (hexane-ethyl acetate, 9:1),
the product was extracted into methylene chloride (3 Â 10 ml). The
solvent was removed under reduced pressure and the residue crystal-
lised from an appropriate solvent to a€ord nearly quantitative yields
of benzils.
We have an ongoing program to develop solvent-free
synthetic protocols that are accelerated by ultrasound or
microwave irradiation.8
A recent report on the utility
of copper salts for the oxidation of hydroquinones and
coupling of naphthols11 prompts us to report our results on
the facile oxidation of benzoins to benzils that proceeds
expeditiously using copper(II) sulfate impregnated on
alumina. A wide variety of benzoins, symmetrical as well
as unsymmetrical (Table 1), undergo rapid oxidation with
this solid reagent system, CuSO4±Al2O3, to a€ord vicinal
That the e€ect may not be purely thermal12 is borne out by the
fact that for a similar product yield the reaction could be completed
in 1.5 h (entry 1, Table 1) at the same bulk temperature of 120 8C
using an alternate mode of heating (oil-bath); the temperature of
the reaction mixture inside the alumina bath reached 0120 8C after
Table 1 Oxidation of benzoins on CuSO4±Al2O3 under microwave irradiation
mp ( 8C)
Entry
R1
R2
t/min
Yield(%)a
found
reported
1
2
3
4
5
6
Ph
Ph
Ph
Ph
2.0
3.0
2.5
3.5
2.0
2.5
96
81
85
82
92
86
92±93
30±31
62
194±196
102±104
131±133
94±963
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-MeC6H4
p-MeOC6H4
29±305
62±635
p-ClC6H4
p-MeC6H4
p-MeOC6H4
195±1973
101±1043
132±1343
7
2.5
82
161±163
162±1646
aRefers to pure isolated products obtained.
*To receive any correspondence (e-mail: chm_rsv@shsu.edu).
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
2 min of irradiation in a microwave oven operating at full power of
900 W.
In conclusion, this solvent-free oxidation of benzoins using
CuSO4-`doped' alumina is
a simple and high-yielding protocol

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J. CHEM. RESEARCH (S), 1998 325
which avoids the drastic conditions that are typical of conventional
oxidation reactions. Yet another environmentally benign feature is
that the alumina support employed can be recovered and reused
after washing o€ the products; the usual activation of the support
by heating at elevated temperatures under vacuum for extended
periods is not required.
8 R. S. Varma, M. Varma and A. K. Chatterjee, J. Chem. Soc.,
Perkin Trans. 1, 1993, 999; R. S. Varma, A. K. Chatterjee and
M. Varma, Tetrahedron Lett., 1993, 34, 3207; R. S. Varma, J. B.
Lamture and M. Varma, Tetrahedron Lett., 1993, 34, 3029; R. S.
Varma, A. K. Chatterjee and M. Varma, Tetrahedron Lett.,
1993, 34, 4603; R. S. Varma and R. K. Saini, Tetrahedron Lett.,
1997, 38, 2623; R. S. Varma and R. Dahiya, Tetrahedron Lett.,
1997, 38, 2043; R. S. Varma and H. M. Meshram, Tetrahedron
Lett., 1997, 38, 5427, 7973; R. S. Varma, R. Dahiya and
S. Kumar, Tetrahedron Lett., 1997, 38, 2039, 5131; R. S. Varma
and R. K. Saini, Synlett, 1997, 857; R. S. Varma, R. K. Saini
and H. M. Meshram, Tetrahedron Lett., 1997, 38, 6525; R. S.
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1997, 38, 4337; 1998, 39, 1481.
We are grateful to the Texas Advanced Research
Program (ARP) in chemistry (Grant No. 003606-023) and
Texas Regional Institute for Environmental Studies
(TRIES) for ®nancial support.
Received, 22nd December 1997; Accepted, 9th February 1998
Paper E/7/09148F
9 D. Villemin and A. Benalloum, Synth. Commun., 1991, 21, 1, 63;
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