246 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 246^247y
Microwave-assisted Syntheses of 1,2-Diketonesy
Alok Kumar Mitra,* Aparna De and Nilay Karchaudhuri
Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road,
Calcutta-700 009, India
A comparative study of the conversion of a number of a-hydroxyketones into 1,2-diketones by three oxidants under
microwave irradiation is reported.
OH
CH
O
1,2-Diketones are important compounds in organic chemistry
and can be utilized for the preparation of a variety of organic
compounds.1 1,2-Diketones can be converted in alkynes,2
a-ketohomoallyl alcohols,3 cyclopentadienones4 etc. In view
of a study in salen5 chemistry, we needed to prepare a
number of 1,2-diketones, from which 1,2-diaryl-1,2-
diaminoethane6 can be prepared.
oxidant
Ar
Ar
Ar
Ar
microwaves
O
O
1
a
2
Entry:
Ar :
b
c
d
e
Cl
MeO
O
1,2-Diketones can be prepared either by cross coupling of
acyltins with acyl halides or by the coupling of acyl chlorides
using hexabutylditin in the presence of palladium catalysts.7
MeO
Scheme 1
1-Naphthyllithium
and
2,6-dimethylphenyllithium
on
carbonylation a¡ord the corresponding 1,2-diketone.8
1,2-Diketones can also be prepared easily by oxidizing
a-hydroxyketones in the presence of iodine,9 copper(II),
10
nitric acid,11 dimethyl sul¢de ditri£ate,12 ammonium
chlorochromate.13
There has been a growing interest in the application of
microwave irradiation in chemical reaction enhancement,14
the salient features being improved reaction rates and the for-
mation of cleaner products. A variety of reactions in MORE15
chemistry have been reported. A rapid synthesis of a number
of 1,2-diketones by oxidizing a-hydroxyketones in the
presence of a copper(II) catalyst (method A), pyridinium
chlorochromate16 (method B) and concentrated nitric acid
(method C) under microwave irradiation have been carried
out by us (Scheme 1). We report herein a comparative study
of methods A, B and C for the conversion of a number
of a-hydroxyketones into 1,2-diketones (Table 1). The results
depicted in Table 1 show that these are the most convenient,
rapid and high-yielding methods for the conversion of
a-hydroxyketones into 1,2-diketones. In the conventional
heating method, utilizing copper(II), concentrated nitric acid
as oxidants, reactions generally require 1.5 h. To the best
of our knowledge this is the ¢rst report of a comparative
study of the oxidation of a-hydroxyketones to 1,2-diketones
by various oxidants under microwave irradiation.
Experimental
Melting points were determined in open capillaries on an electrically
heated metal block. The reactions were carried out in a domestic
microwave oven (BPL BMO 700T, 1200 W) at medium power level
(power level 5). Reactions were carried out in an Erlenmeyer £ask
(25 ml) ¢tted with a funnel (i.d. 5 cm) as a loose top, upon which a
round-bottomed £ask (10 ml) containing ice (7 g) was placed as a con-
denser. General procedures for the syntheses of 1,2-diketones 2a^e
by methods A, B and C are described below. All the products were
characterized by comparing the melting points of the authentic
samples.13;17
Method A.öA mixture of a-hydroxyketone (1.0 mmol), copper(II)
acetate (2.0 mg), ammonium nitrate (1.25 mmol) and 80% (v/v) aque-
ous acetic acid (0.7 ml) was irradiated in a microwave oven for 1^5 min.
It was then cooled, the precipitate was ¢ltered o¡ and washed with cold
water. Then it was dried and recrystallized from ethanol.
Method B.öA mixture of a-hydroxyketone (1.0 mmol) and
pyridinium chlorochromate (1.5 mmol) in chloroform (2.0 ml) was
irradiated in a microwave oven for 1^1.5 min. It was then cooled
and extracted with chloroform ꢀ3 Â 5 ml); removal of the solvent
a¡orded the 1,2-diketone, which was recrystallized from ethanol.
Table 1 The oxidation of a-hydroxyketones to 1,2-diketones under microwave irradiation
Method A
Time/min
Method B
Time/min
Method C
Time/s
1,2-Diketone
Yield(%)
Yield(%)
Yield(%)
2a
2b
2c
2d
2e
1
3
1
5
3
96
91
95
78
88
1
1
1
1.5
1
94
90
90
80
87
42
48
44
60
^
95
89
90
79
^
Method C.öA mixture of a-hydroxyketone (1.0 mmol) and concen-
trated nitric acid (1.06 ml) was irradiated m a microwave oven for
42^60 s. It was then cooled and poured into ice-cold water with stirring.
The solid products were ¢ltered o¡, dried and recrystallized from
ethanol.
* To received any correspondence (e-mail: akmitra@cucc.ernet.in).
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).