Catalysis by ionic liquids: Cyclopropyl carbinyl rearrangements catalyzed by [pmim]Br under organic solvent free conditions

Article abstract of DOI:10.1016/j.tetlet.2005.12.003

Aryl substituted cyclopropyl carbinol derivatives undergo stereoselective rearrangements catalyzed by the ionic liquid, 1-methyl-3-pentylimidazolium bromide, under sonication, without any organic solvent, to produce the substituted conjugated all-trans-butadienes.

Full text of DOI:10.1016/j.tetlet.2005.12.003

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 881–884
Catalysis by ionic liquids: cyclopropyl carbinyl
rearrangements catalyzed by [pmim]Br under organic
solvent free conditions
Brindaban C. Ranu,* Subhash Banerjee and Arijit Das
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Received 14 September 2005; revised 21 November 2005; accepted 1 December 2005
Available online 20 December 2005
Abstract—Aryl substituted cyclopropyl carbinol derivatives undergo stereoselective rearrangements catalyzed by the ionic liquid,
1-methyl-3-pentylimidazolium bromide, under sonication, without any organic solvent, to produce the substituted conjugated
all-trans-butadienes.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Recently, room temperature ionic liquids have been the
subject of considerable interest because of their proper-
ties of non-volatility, non-flammability and recycl-
ability, amongst others.1 Ionic liquids have been used
as solvents in chemical transformations;1 however, the
ability of ionic liquids to serve as catalysts2 and
reagents1h,3 has not been explored to any great extent.4,5
We demonstrate here a novel application of a neutral
ionic liquid, [pmim]Br, as an efficient catalyst for the rear-
rangement of cyclopropyl carbinol derivatives leading to
conjugated butadienes (Scheme 1).
the presence of a catalytic amount (20 mol %) of the
ionic liquid, [pmim]Br. The product can be isolated by
extraction with ethyl acetate.
A variety of structurally diverse aryl substituted cyclo-
propyl carbinol derivatives were found to undergo the
rearrangement catalyzed by [pmim]Br to provide
the aryl substituted all trans-conjugated butadienes.
The results are summarized in Table 1. The trans config-
uration of both the double bonds was established from
the couplings of the olefinic protons. All the products
(except two) were crystalline solids and the observed
melting points of these compounds were in good agree-
ment with reported data. Substituents such as Cl, OMe,
dioxymethylene, thiomethyl and benzyl remained un-
affected under the reaction conditions. Sensitive molecules
such as the furan, thiophene and pyridine derivatives
(entries 12, 13 and 14) also survived the reaction.
Although cyclopropyl carbinols have been reported to
undergo rearrangement to conjugated butadienes on
treatment with acid6a or heating under reflux in DMSO
for 5 h,6b to the best of our knowledge, there is no report
of catalysis of this reaction by an ionic liquid.
The experimental procedure for this transformation is
very simple.7 The cyclopropyl carbinol is sonicated in
In general, the reactions were reasonably fast (0.75–
1.75 h) under sonication. The reaction can also be car-
ried out at room temperature with stirring, where the
time required for completion was found to be 3–6 h.
Twenty mole percent of ionic liquid was sufficient to
push the reaction forward and no other solvent was
required. However, in the absence of [pmim]Br, the reac-
tion did not proceed at all under sonication or at room
temperature with stirring. The catalyst could be reused
and recycled for up to five runs without any loss of
activity. The products were obtained in high yields and
purities after short filtration chromatography through
OH
[pmim]Br
R
Ar
Ar
R
Sonication
Scheme 1.
Keywords: Ionic liquid; Catalysis; Rearrangement; Cyclopropyl car-
binol; Butadienes.
*
Corresponding author. Tel.: +91 33 24734971; fax: +91 33
24732805; e-mail: ocbcr@iacs.res.in
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.12.003

882
B. C. Ranu et al. / Tetrahedron Letters 47 (2006) 881–884
Table 1. Rearrangement of cyclopropyl carbinol derivatives catalyzed by [pmim]Br
Entry
Cyclopropyl carbinol
Time (h)
Product
Yield (%)a
Ref.
OH
Ph
Ph
1
2
0.75
1.2
95
90
13
14
OH
Ph
Ph
Cl
Br
Cl
OH
Ph
Ph
Br
3
4
5
1.0
1.0
1.0
90
92
90
15
16
16
Ph
OH
Ph
Me
Me
Me
Ph
OH
Ph
Me
OH
Ph
Ph
OH
Ph
OH
Ph
OH
Ph
6
7
8
9
1.0
95
92
85
95
13
—
—
—
MeO
MeS
AllylO
BnO
MeO
MeS
Ph
1.0
Ph
1.25
0.75
AllylO
Ph
OH
Ph
BnO
O
Ph
10
11
1.2
1.5
85
85
—
16
O
O
OH
Ph
O
Ph
O2N
O2N
O
OH
Ph
12
13
1.5
1.5
80
80
14
14
Ph
O
OH
Ph
Ph
N
N
OH
Ph
14
15
1.5
78
78
14
17
Ph
OH
S
S
Ph
1.75
Ph
OH
Me
Me
16
17
1.75
1.25
75
76
18
14
OH
Me
Me
Me
Me
a Yields refer to those of pure isolated products characterized by spectroscopic data (IR, 1H and 13C NMR) and elemental analysis.

B. C. Ranu et al. / Tetrahedron Letters 47 (2006) 881–884
883
References and notes
Br
11C5
H
N
H
N
Br
δ
O
N
O
H
N
Me
Ar
Me
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C5H11
R
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H2O
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Scheme 2.
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employed are those based on the cross-coupling of alkenyl
metals with haloalkenes in the presence of transition
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The imidazolium cation of the ionic liquid [pmim]Br
may be polarizing the C–O bond of the cyclopropyl car-
binol derivatives thus facilitating cleavage of the cyclo-
propyl moiety to produce the benzylic carbonium ion,
which eventually leads to the diene (Scheme 2).
7. General experimental procedure for the rearrangement
of cyclopropyl carbinol derivatives catalyzed by [pmim]Br.
Representative procedure for 1,4-diphenyl-1,3-butadiene
(entry 1). Phenyl (2-phenylcyclopropyl)-methanol (222 mg,
1 mmol) was sonicated in the presence of a catalytic
amount (47 mg, 20 mol %) of [pmim]Br (prepared by
irradiation of an equivalent mixture of 1-methylimi-
dazole and n-pentyl bromide (neat) in a domestic micro-
wave oven for 2 min, then washing with a small amount of
ether twice and drying under vacuum at 80 ꢁC for 5 h) in an
ultrasonic cleaner (Julabo, Germany) for 45 min (TLC).
The reaction mixture was extracted with ethyl acetate
(3 · 10 mL). Evaporation of the solvent left the crude
product, which was purified by short column chromato-
graphy over silica gel to provide pure 1,4-diphenyl-1,3-
butadiene as a white solid (196 mg, 95%), mp 150–151 ꢁC
(lit. mp 152 ꢁC); IR(KBr) 1560, 1500, 1405 cmÀ1; 1H NMR
(300 MHz, CDCl3) d 6.72 (dd, J = 12.0, 2.9 Hz, 2H), 7.01
(dd, J = 12.0, 2.9 Hz, 2H), 7.27–7.30 (m, 2H), 7.35–7.40
(m, 4H), 7.47–7.50 (m, 4H); 13C NMR (75 MHz, CDCl3) d
126.5 (4C), 127.6 (2C), 128.7 (4C), 129.3 (2C), 132.9 (2C),
137.4 (2C). These values are in good agreement with the
literature.10,13 This procedure was followed for the rear-
rangement of all the cyclopropyl carbinol derivatives listed
in Table 1. All products were characterized by their IR, 1H
NMR and 13C NMR spectroscopic data and by elemental
analysis (for new compounds). The spectroscopic data and
mp for known compounds are in good agreement with
those reported.
As proposed in the mechanism, the imidazolium cation
of the ionic liquid may play a crucial role in directing
the course of the reaction.19 To test this hypothesis,
the reaction was conducted in the presence of different
imidazolium based ionic liquids such as [pmim]BF4
and the reaction was found to proceed with equal effi-
ciency. However, [pmim]Br, being less expensive and
of neutral character was the preferred choice.
In conclusion, the present procedure using an easily
accessible and inexpensive ionic liquid as catalyst for
the rearrangement of cyclopropyl carbinol derivatives
provides a novel protocol for the synthesis of substituted
conjugated all trans-butadiene systems. This procedure
offers marked improvements with regard to operational
simplicity, stereoselectivity (exclusively trans) and high
isolated yields (75–95%) of products, considerably faster
reaction time (0.75–1.75 h) and mild and neutral reac-
tion conditions. This procedure also demonstrates the
potential of a neutral ionic liquid in catalyzing a rear-
rangement process.
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We are pleased to acknowledge the financial support
from CSIR, New Delhi [Grant No. 01(1936)/04], for this
investigation. S.B. is thankful to CSIR for his fellowship.

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