Ionic-liquid-influenced expeditious and stereoselective synthesis of olefins

Article abstract of DOI:10.1080/00397910701544794

1-Butyl-3-methylimidazolium chloroaluminate, [bmim]Cl·.AlCl3 (molar fraction, N=0.67), ionic liquid has been used in combination with metallic Zn for the reductive coupling of carbonyl compounds to synthesize symmetrical olefins, with the Z-isomer formed predominantly. The ionic liquid played a dual role of Lewis acid catalyst and solvent. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701544794

Synthetic Communicationsw, 37: 3111–3117, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701544794
Ionic-Liquid-Influenced Expeditious and
Stereoselective Synthesis of Olefins
Sachin V. Bhilare, Nitin B. Darvatkar, Amol R. Deorukhkar,
Meghana S. Rasalkar, and Manikrao M. Salunkhe
Department of Chemistry, Institute of Science, Mumbai, India
.
Abstract: 1-Butyl-3-methylimidazolium chloroaluminate, [bmim]Cl .AlCl3 (molar
fraction, N ¼ 0.67), ionic liquid has been used in combination with metallic Zn for
the reductive coupling of carbonyl compounds to synthesize symmetrical olefins,
with the Z-isomer formed predominantly. The ionic liquid played a dual role of
Lewis acid catalyst and solvent.
Keywords: carbonyl compound, ionic liquid, olefins, reductive coupling,
stereoselectivity
The alkene derivatives are obtained by reductive coupling of carbonyl
compounds involving TiCl4/Zn[1] and TiCl3/Mg.[2] Among other methods,
Sato et al.[3] have used the Lewis acidic AlBr3 with metals such as Zn, Al,
and Mg for the same purpose; however, the yields are quite low. The utility
of metallic Zn has proved beneficial in the reductive coupling of carbonyl
compounds catalyzed by Lewis acids.
The utility of ionic liquids in catalysis and as neoteric solvents for various
synthetic processes is now well recognized by chemists all over the world.[4,5]
The chloroaluminates are an important class of ionic liquids. The Lewis
acidity of such ionic liquids is a function of molar fraction of AlCl3. For the
past few years, we have exploited the potential of chloroaluminate ionic
liquids in various reactions of synthetic importance.[6] In this study, we inves-
tigated chloroaluminate ionic liquids for the reductive coupling of carbonyl
compounds.
Received in India January 24, 2007
Address correspondence to Manikrao M. Salunkhe, Department of Chemistry,
Institute of Science, 15, Madam Cama Road, Mumbai 400 032, India. E-mail:
mmsalunkhe@hotmail.com
3111

3112
S. V. Bhilare et al.
.
Scheme 1. Synthesis of symmetrical olefins in [bmim]Cl AlCl3, 0.50 , N ꢀ 0.67.
To begin with, we investigated the reaction in detail by choosing
4-hydroxy benzaldehyde as the model substrate. The effect of variable
Lewis
acidity
of
1-butyl-3-methylimidazolium
chloroaluminate,
.
[bmim]Cl AlCl3 ionic liquid, on the course of the reaction was explored by
changing the apparent molar fraction (N) of AlCl3. No reaction was
observed in basic and neutral ionic liquid where N ꢀ 0.5, even after 8 h.
We observed that as the Lewis acidity of the ionic liquid was increased
from N ¼ 0.5 to N ¼ 0.67, the percentage conversion increased accordingly
(Table 1). The highest percentage conversion was obtained within 15 min
when N ¼ 0.67. The ionic liquid corresponding to N ¼ 0.67 was hence
employed for subsequent study.
Under optimal conditions, we synthesized a variety of olefins by reductive
coupling of different aldehydes and ketones. The reaction affords olefins in
good yields within a short reaction time under ambient conditions, avoiding
tedious procedures and cumbersome workup. The present system works
well for both aromatic aldehydes and ketones as indicated in Table 2.
One salient feature of the present methodology is that the reaction offers
high stereoselection, as one of the isomers is formed preferentially. We
initially studied the reductive coupling of substituted benzaldehydes
wherein Z-olefins were obtained as the major products. The stereochemistry
1
of the olefins was confirmed by comparing their H NMR spectra with that
of olefins synthesized by standard protocols.[7] Analyses of the respective
.
Table 1. Effect of the Lewis acidity of [bmim]Cl AlCl3,
0.50 , N ꢀ 0.67, on the extent of conversion of reductive coup-
ling of 4-hydroxy benzaldehyde
Molar fraction of AlCl3 in
the ionic liquid (N)
Yield (%)
0.54
0.58
0.63
0.67
12
37
58
85

.
Table 2. Synthesis of symmetrical olefins in [bmim]Cl AlCl3, N ¼ 0.67, ionic liquid and Zn
Entry
Substrate
Time (min)
Olefin
Yield (%)a
81
Z:Eb
1
15
88:12
2
30
79
91:09
3
4
15
15
83
85
93:07
100:00
5
6
15
15
80
72
85:15
95:05
(continued)

Table 2. Continued
Entry
7
Substrate
Time (min)
15
Olefin
Yield (%)a
79
Z:Eb
87:13
8
30
75
97:03
9
30
30
82
78
85:15
10
—
aIsolated yield of mixture of both E- and Z-isomer.
b
1
The ratio of E:Z isomers was determined by GC-MS or H NMR.

Stereoselective Synthesis of Olefins
3115
1H NMR spectra indicated that the major olefins obtained by this protocol
were of Z-configuration.
To optimize the molar stoichiometry of the ionic liquid, we increased the
.
amount of ionic liquid [bmim]Cl AlCl3, N ¼ 0.67, progressively from 0.25 to
1 molar equivalent with respect to the substrate. The maximum yield of the
product was obtained when the molar ratio of the ionic liquid to that of the
substrate was 1:1. No appreciable increase in the yield could be noticed on
further increase in the amount of ionic liquid. In addition, we have found
that the reaction does not occur in the absence of zinc.
To enhance the synthetic utility of the present protocol, we selected
different substituted aromatic aldehydes. In all the cases, the same trend
was observed wherein the aldehyde gave Z olefin as the main product.
The methodology was also successfully extended to ketones; however,
the reaction times were slightly prolonged (Table 2). As can be seen from
the table, the ketones also preferentially gave the Z-isomer. With all the
aldehydes and ketones, the formation of pinacol was not observed.
In conclusion, we have shown chloroaluminate ionic liquid as an efficient
catalyst for the synthesis of symmetrical olefins via reductive coupling of both
aryl aldehydes and ketones. The protocol proves to be expeditious and highly
stereoselective and provides good yields of olefins under ambient conditions.
EXPERIMENTAL
Ionic liquids were prepared by the procedures given in the literature.[8] The
aldehydes were purchased from SRL and SD Fine Chemicals and were used
1
as received. The product was characterized using H NMR and GC-MS.
1H NMR was recorded on Jeol (300-MHz) instrument using CDCl3 as
solvent. Gas chromatography-mass spectroscopy (GC-MS) was recorded on
a Schimadzu QP 2010 spectrophotometer equipped with a dp5 column. All
other chemicals and reagents were of analytical grade and used as obtained.
Typical Procedure
To a well-stirred mixture of substrate (1 mmol) and the ionic liquid 1-butyl-3-
.
methylimidazolium chloroaluminate [bmim]Cl AlCl3 (1 mmol), Zn dust
(5 mmol) was added, and the stirring continued at room temperature for the
specified period of time as indicated in Table 2. All additions were carried
out under an inert atmosphere. The reaction was quenched with 6 molar
HCl under cold conditions. The resultant solution was extracted with
methylene dichloride (3 ꢁ 10 ml). The combined organic extracts were
dried over anhydrous Na2SO4 and concentrated under reduced pressure. The
products were purified by silica-gel column chromatography and were charac-
terized by IR, NMR, GC-MS, and physical constants. GC analysis of crude

3116
S. V. Bhilare et al.
products showed two peaks at different retention times, which corresponded to
GC profiles of cis and trans isomers of authentic olefines.
Representative Spectral Data
1
Z-(4,40-dihydroxy) Diphenyl ethene: Colorless oil. H NMR (300 MHz,
CDCl3): d 6.34 (2H, S, -CH55CH-), 6.70–6.73 (4H, dd, ArH), 6.96–6.99
(4H, dd, ArH).
1
Z-1,2-Diphenyl ethene: Colorless oil. H NMR (300 MHz, CDCl3): d 6.12
(2H, S, -CH55CH-), 6.20–6.39 (10H, M, ArH).
Z-2,3-Diphenyl 2-butene: Colorless oil. 1H NMR (300 MHz, CDCl3): d 2.16
(6H, S, -CH3), 6.94–7.09 (10H, M, ArH).
1
Z-(4,40-dimethoxy) Diphenyl 2-butene: Colorless oil. H NMR (300 MHz,
CDCl3): d 2.14 (6H, S, -CH3), 3.88 (6H, S, -OMe), 6.79–6.82 (4H, dd,
ArH), 7.07–7.10 (4H, dd, ArH).
ACKNOWLEDGMENT
The authors gratefully acknowledge G. K. Trivedi for his valuable suggestions
and the Department of Science and Technology for financial assistance. S. V. B.
and N. B. D. are thankful to the Council of Scientific and Industrial Research,
New Delhi, for the awards of junior research fellowships.
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