264 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
1997, 264–265†
Selective Oxidation of Allylic Methyl Groups over a Solid
Support under Microwave Irradiation†
Jasvinder Singh,*a Munisha Sharma,a Govesdhan L. Kada and Balder R. Chhabrab
aDepartment of Chemistry, P.U., Chandigarh-160014, India
bDepartment of Chemistry, P.A.U., Ludhiana-141004, India
SeO2/ButOOH adsorbed on SiO2 is found to be a highly selective reagent for the oxidation of allylic methyl groups to trans-
a,b-unsaturated aldehydes under microwave irradiation.
Selective oxidation of allylic methyl groups of gem-dimethyl
trisubstituted olefins to trans-a,b-unsaturated alcohols and
aldehydes is an important transformation in organic
chemistry. Several reagents are known1 to achieve this goal,
but selenium dioxide has been found to be a promising
reagent. Some difficulties such as the removal of colloidal
selenium and the formation of organoselenium as byproducts
have been circumvented by the Sharpless method2 using cata-
lytic selenium dioxide in combination with tert-butyl hydro-
peroxide, but still it requires some modifications. By making
certain changes in the above method we have been successful
in obtaining trans-a,b-unsaturated alcohols and aldehydes3
from allylic methyl groups and in the selective oxidation4 of
primary allylic alcohols to a,b-unsaturated aldehydes in the
presence of secondary allylic alcohols.
There are a large number of reactions over solid supports
with microwave irradiation reported in the literature.5,6 These
reactions generally proceed with greater selectivity, under
mild, simple and safe reaction conditions than the analogues
homogeneous reactions. Moreover, the reaction rate is
enhanced over a solid support as it takes place in two dimen-
sions rather than the three dimensions using an ordinary
reaction method. In continuation of our earlier work in the
selective iodination7 of benzylic alcohols over a solid support
with microwave irradiation, we achieved the selective oxida-
tion of the allylic methyl groups of gem-dimethyl trisubsti-
tuted olefins to trans-a,b-unsaturated aldehydes only, prob-
ably due to the faster oxidation of allylic alcohols under these
reaction conditions. However, allylic alcohols were formed
when the reaction time was reduced to 2–3 min (checked by
1H NMR). Compounds containing allylic methyl groups,
SeO2 and ButOOH were adsorbed over silica and exposed to
microwave irradiation, resulting in the selective conversion of
the allylic methyl group to a trans-a,b-unsaturated aldehyde
group. In most cases no intermediate a,b-unsaturated alcohol
or any other product was detectable under the given experi-
mental conditions. However, in some cases a very small
amount (ꢀ5%) of the intermediate unsaturated alcohol was
also obtained, depending on the reaction time. A comparison
of the reaction rate with and without microwave irradiation is
presented in Table 1.
Table 1
Entry
Substrate
Reaction timea
Product
Yield (%)
85 (75)
OAc
OAc
1
2
3
4
5
6
10 min (24 h, 80:20)
CHO
a
Br
Br
10 min (18 h, 70:30)
10 min (20 h, 80:20)
10 min (10 h, 70:30)
10 min (24 h, 80:20)
10 min (24 h, 80:20)
75 (60)
81 (60)
77 (60)
70 (75)
68 (60)
CHO
O
b
O
CHO
c
O
O
O
O
CHO
d
OAc
OAc
CHO
O
e
O
OAc
OAc
CHO
f
aThe figures in parenthesis are the reaction times and
alcohol:aldehyde yield without microwave irradiation.3
Experimental
The IR spectra were recorded on a Perkin Elmer model 1430
1
spectrophotometer and H NMR spectra with a Varian EM-360
(90 MHz) NMR spectrometer. Chemical shifts (d) are reported in
ppm with Me4Si as internal standard. Elemental analyses were
carried out using a Perkin-Elmer 2400 elemental analyser. Micro-
wave induced reactions were carried out in a BPL BMO 700T
(640 W) microwave oven.
OH
OH
SeO2, ButOOH, SiO2
General Procedure.sCitronellol (3.2 mmol, 0.500 g) SeO6 (1.6
mmol, 0.176 g) and ButOOH (70%, 4.48 mmol, 0.576 g) were dis-
solved in a small amount of dichloromethane. Silica (1 g) was then
added to form a slurry and excess solvent was evaporated off to
obtain free flowing silica which was then exposed to microwave
irradiation at power level 9 (640 W) for 10 min. Diethyl ether was
then added and the mixture filtered. The filtrate was then washed
(10% KOH, brine) and then dried (anhydrous Na2SO4). The sol-
vent was evaporated to furnish the pure product in 83% yield; dH
9.5 (s, 1 H), 6.5 (t, J 6 Hz, 1 H), 3.7 (t, J 7 Hz, 2 H), 3.3 (brs, 1 H,
D2O exchangeable), 2.4 (m, 2 H), 1.8 (s, 3 H), 1.5 (m, 5 H), 1.0 (d,
J 7.5 Hz, 3 H); vmax/cmꢁ1 3400, 2720, 1700, 1640 (Found: C, 70.38;
H, 10.48. C10H18O2 requires C, 70.54; H, 10.65%).
microwave, 640 W
10 min
CHO
*To receive any correspondence.
†This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).