336 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
Selective Conversion of Nitroalcohols into Nitroolefins over
1
997, 336–337†
Zeolite under Heterogeneous Conditions†
a
a
b
M. Anbazhagan, G. Kumaran and M. Sasidharan
a
Division of Organic Synthesis, National Chemical Laboratory, Pune, India
b
Catalysis Division, National Chemical Laboratory, Pune, India
Various zeolites catalyse the formation of aliphatic, aromatic and heteroaromatic nitroolefins from the corresponding
nitroalcohols with high selectivity and yield at optimum reaction temperature.
In recent years zeolites have found new applications as
heterogeneous acid catalysts for various liquid-phase organic
was cooled to room temperature and the catalyst was filtered off.
The solvent was removed under reduced pressure and the crude
product obtained was purified by column chromatography
1
reactions at moderate temperatures. This is primarily due to
[
100–200 mesh size, 10–15% ethyl acetate–light petroleum (bp
5–60 °C) as eluent] to afford (E)-3-methyl-1-nitrobut-1-ene
(Table 1, entry 1) as a liquid. All the reaction products were charac-
the advantages associated in the use of solid acids such as
easy work-up, eco-friendly nature, regeneration, reusability
and their shape-selective nature. Over the last two decades
nitroalkenes have been found to be an important inter-
3
1
13
terised by IR, H NMR, C NMR, bp and mp. By analogous
procedures several other nitroalcohols were converted into
nitroolefins over H-Y zeolite (Scheme 1).
2,3
mediate both in industry and in organic synthesis. For
example, nitroalkenes are commonly used as dienophiles in
4
,5
Diels–Alder reactions and they readily undergo addition to
a wide range of nucleophiles. In addition, several synthetic
transformations of nitroolefins are also known in the litera-
1 OH
R3
R1
R3
R
H-Y, benzene, reflux
8
h
R2
R2
NO2
NO2
6
,7
ture. Usually nitroolefins are prepared by the initial acyla-
tion of the hydroxy group of nitroalcohols followed by elimi-
nation with sodium acetate in a homogeneous non-catalytic
1
2
R , R = alkyl, aryl, heteroaryl, H
R3 = alkyl, H
9
method. Several reagents such as phthalic anhydride, di-
1
0
11
Scheme 1
cyclohexylcarbodiimide, phosphorus pentoxide, pivaloyl
1
2
13
chloride, PPh –CCl –Et N and methanesulfonyl chlor-
ide–Et N are used to effect the dehydration of nitroalcohols
3
4
3
1
4
3
Results and Discussion
to nitroolefins.
Table 1 shows the yield and selectivity of the conversion of
various nitroalcohols into nitroolefins over 10% w/w H-Y
zeolite. The procedure is general, as aliphatic (entries 1–2),
aromatic (entries 3–6) and heteroaromatic (entry 7) nitro-
alcohols were dehydrated to the corresponding nitroolefins.
However, the limitation of this method is that the reaction
fails in the case of primary nitroalcohols. It is pertinent to
mention that, in all cases, the product selectivity was found to
be 100% (for the E-isomer) and no side reactions such as
nitroolefin polymerisation were observed under the reaction
conditions used.
In all the above mentioned cases, the separation of the
products from the reagents makes the procedure laborious.
However, to the best of our knowledge, neither homogene-
ous nor heterogeneous catalytic methods for the dehydration
11,15
of nitroalcohols to nitroolefins have been reported.
Here
we report a catalytic dehydration of nitroalcohols to nitro-
olefins over different zeolites with high conversion and selec-
tivity, at optimum reaction temperature under liquid-phase
conditions.
Experimental
Table 2 shows the effectiveness of various zeolites in the
formation of nitrostyrene from 1-phenyl-2-nitroethanol.
Zeolites with high Br o¨ nsted acid strength such as H-Y
(Si:Al = 2.4, large three dimensional 12-member ring pores
of 7.4 Å with supercages), H-beta (Si:Al = 13, 12-member
ring channels intersecting 6.5Å5.6 and 7.5Å5.7 Å), and
H-mordenite (Si:Al = 5.5, intersecting 8 and 12-member
rings 6.5Å7.0 and 2.6Å5.7 Å) induce more conversion (93,
89 and 86 respectively, Table 2, entries 1–3) than zeolites
with moderate acidity such as ZSM-5 (Si:Al = 60) with three
dimensional intersecting 10-member rings 5.3Å5.6 and
The catalysts ZSM-5 and Beta (with Si:Al ratios of 60:1 and 13:1,
1
6,17
respectively) were prepared according to published procedures
using tetrapropylammonium bromide and tetraethylammonium
hydroxide respectively as templates. The Na-Y and Na-Mordenite
(
Si:Al = 2.4 and 5.5, respectively) were obtained commercially
from United Catalyst India Ltd and Zeolon, respectively. The
above zeolite Na-forms were converted into the H-form by treating
2
3
g of zeolite with 30 ml of ammonium acetate (1 ) at 80 °C for
M
h. This procedure was repeated in order to ensure the complete
ǹ
exchange of Na by the NH
4
ion. The zeolite was then filtered off,
washed thoroughly with deionized water and calcined at 500 °C for
h in a flow of dry air to obtain the zeolite H-forms. The RE-Y
rare earth-Y) was obtained by treating Na-Y with a 10% solution
5
(
5
.1Å5.5 Å (Table 2, entry 4). Further, the slightly lower
of a mixture of rare earth oxide (containing approximately 18 wt%
Pr, 48 wt% Nd, the rest being La and a small percentage of other
rare earth elements) and calcining at 500 °C for 5 h in a flow of dry
air.
reactivity in the case of RE-Y (Rare earths exchanged zeo-
lite-Y which contain more Lewis acid sites) may be due to the
presence of less Br o¨ nsted acid sites, which are required for
this dehydration reaction. The above results suggest that the
reaction is mainly catalysed by Br o¨ nsted acid sites rather than
In a typical reaction, 3-methyl-1-nitrobutan-2-ol (266 mg, 2
mmol) was taken in 6 ml of dry toluene and then 10 wt% of H-Y
zeolite was added and the mixture was azeotropically distilled for
the structure of zeolites in the dehydration of 1-phenyl-
2
8
h. The progress of the reaction was monitored by TLC using 20%
10,11
-nitroethanol in accordance with previous reports.
How-
ethyl acetate–light petroleum (bp 35–60°C). The reaction mixture
ever other solvents seem to exert little influence on the
product formation. Entries 8 and 9 show the conversions
using benzene (82%) and xylene (78%) as solvent. The cata-
lyst was reused five times without loss of activity, after activat-
ing at 400 °C in a flow of air.
In conclusion, we have demonstrated the effective dehy-
dration of nitroalcohols to nitroolefins over zeolite catalysts
with high selectivity and good yield.
*
To receive any correspondence. Present address: Department of
Chemistry, University of St. Andrews, St. Andrews KY16 9ST,
UK.
†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).