Morpholine adsorbed on silica gel: A novel and mild basic catalyst for the synthesis of α,β-unsaturated nitroalkenes

Article abstract of DOI:10.1007/PL00010317

Syntheses of α,β-unsaturated nitroalkenes have been carried out under mild condition using morpholine adsorbed on silica gel as a novel catalyst.

Full text of DOI:10.1007/PL00010317

Monatshefte f uÈ r Chemie 131, 949±952 (2000)
Morpholine Adsorbed on Silica Gel: A Novel
and Mild Basic Catalyst for the Synthesis of
a,b-Unsaturated Nitroalkenes
Ã
Babasaheb P. Bandgar , Lav S. Uppalla, and Vaibhav S. Sadavarte
Organic Chemistry Research Laboratory, School of Chemical Sciences, Swami Ramanand Teerth
Marathwada University, Nanded-431606, India
Summary. Syntheses of ꢀ,ꢁ-unsaturated nitroalkenes have been carried out under mild condition
using morpholine adsorbed on silica gel as a novel catalyst.
Keywords. Henry reaction; Morpholine adsorbed on silica gel; Base catalyst; ꢀ,ꢁ-Unsaturated
nitroalkenes.
Introduction
Nitroalkenes are considered important because of their biological activities as
insecticidal [1, 2], fungicidal [2±5], bactericidal [6, 7], rodentrepellent [8], and
antitumor agents [9] as well as of other pharmacological values [6±9]. They
have proved to be suitable precursors for a wide variety of target molecules. The
utility of nitroalkenes in organic synthesis is largely due to their easy conversion
into a variety of functionalities [10, 11]. Alternatively they are powerful
dienophiles in Diels-Alder reactions and readily undergo addition reactions
with many different nucleophiles [11]. A few nitroalkenes also occur in nature
[
12].
The most versatile and classical preparation of nitroalkenes involves the Henry
condensation reaction, followed by dehydration of the resultant ꢁ-nitro alcohols
13]. For this purpose several reagents have been used [14±15]. Recently,
[
nitroalkenes have been synthesized using Envirocat EPZG and microwaves under
drastic conditions [16, 17]. The signi®cance of nitroalkenes and the increasing
importance of heterogenous catalysis in organic synthesis prompted us to
investigate the Henry reaction in more detail.
à Corresponding author

950
B. P. Bandgar et al.
In this contribution we report on the synthesis of ꢀ,ꢁ-unsaturated nitroalkenes
using morpholine supported on silica gel as an inexpensive and mild catalyst
Scheme 1).
(
Results and Discussion
The attempt to prepare ꢀ,ꢁ-unsaturated nitroalkenes using either morpholine or
silica gel as a catalyst resulted in very poor yields, even after stirring for 12 h at
room temperature. In contrast, morpholine adsorbed on silica gel acts as a mild and
effective catalyst for the synthesis of various nitroole®ns in good to excellent
yields.
Scheme 1
The (E)-geometry of the newly formed double bond was readily assigned on
1
the basis of H NMR spectra, the vinyl proton of the (E)-isomer appearing at lower
®
eld than the corresponding proton of the (Z)-isomer because of the strong
1
3
anisotropic effect of the nitro group [15]. C NMR spectra of the products
indicated that no contamination with (Z)-isomers occurred [14±15].
In summary, the presented method offers a mild and useful alternative to
existing procedures for the chemoselective synthesis of functionalized conjugated
nitroalkenes.
Exprimental
1
IR spectra were recorded on a Bomem MB 104 FTIR spectrometer, H NMR spectra on a Varian
90 MHz NMR instrument (Varian FT90).
Preparation of the catalyst
3
3
A mixture of 5 g silica gel (60±120 mesh) and 1 cm morpholine in 10 cm acetone was stirred at
room temperature for 1 h. Acetone was removed under vacuum to get a free-¯owing powder of
morpholine adsorbed on silica gel.
General procedure for the synthesis of ꢀ,ꢁ-unsaturated nitroalkenes
A mixture of aldehyde/ketone (5 mmol), nitromethane/nitropropane (6 mmol), and 1.5 g morpholine
3
adsorbed on silica gel in 10 cm acetonitrile was stirred at room temperature for the speci®ed time
Table 1). After completion of the reaction the catalyst was ®ltered off and washed twice with 10 cm3
(
ether. Removal of the solvent under reduced pressure gave a crude product which was further
puri®ed by column chromatography (silica gel 60±120 mesh, petroleum ether:ethylacetate ˆ 4:1).

Synthesis of ꢀ,ꢁ-Unsaturated Nitroalkenes
951
Table 1. ꢀ,ꢁ-Unsaturated nitroalkenes synthesized by Henry's procedure catalyzed by morpholine
a
adsorbed on silica gel
1
2
R
R
R
Reaction
time/h
Yield/%
Melting
ꢀ
Melting
ꢀ
point/ C
point/ C
(
Literature)
H
H
H
H
H
H
0.2
0.5
6
78
75
80
125
104
112
124±126 [20]
104 [20]
111 [20]
H
H
H
H
2
2
68
65
167
168
167±168 [19]
168±169 [19]
H
H
1.5
75
oil
±
H
H
1
74
76
oil
58
±
H
H
H
H
2.5
58±59 [19]
CH3
CH3
H
01
03
0.8
78
80
80
44
63
44±45 [19]
64±65 [19]
120 [20]
118
a
Yields refer to pure, isolated products characterized by physical constants, spectroscopic properties,
and comparison with authentic samples
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[
[
[
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[
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952
B. P. Bandgar et al.: Synthesis of ꢀ,ꢁ-Unsaturated Nitroalkenes
[
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[
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[
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Received February 14, 2000. Accepted March 28, 2000