Rhodium acetate dimer immobilized in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid: A novel and recyclable catalytic system for the cyclopropanation of alkenes

Article abstract of DOI:10.1002/adsc.200303136

Alkenes undergo smooth cyclopropanation with ethyl diazoacetate using a catalytic amount of rhodium acetate dimer, Rh₂(OAc)₄, immobilized in the air- and moisture-stable 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid, [

Full text of DOI:10.1002/adsc.200303136

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Rhodium Acetate Dimer Immobilized in 1-Butyl-3-
methylimidazolium Hexafluorophosphate Ionic Liquid:a Novel
and Recyclable Catalytic System for the Cyclopropanation of
Alkenes
J. S. Yadav,* B. V. S. Reddy, P. Narayana Reddy
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Fax: ( ‡ 91)-40-27160512, e-mail: yadav@iict.ap.nic.in
Received: August 13, 2003; Revised: December 20, 2003; Accepted: December 22, 2003
insertion reactions show moderate to high trans-selec-
tivity.[7,8] In recent reports, iron Lewis acids have been
Abstract: Alkenes undergo smooth cyclopropana-
tion with ethyl diazoacetate using a catalytic amount
found to give cis-cyclopropanes predominantly.[9]
Recently, ionic liquids have gained recognition as
environmentally benign alternatives to more volatile
organic solvents.[10] They possess many interesting
properties such as wide liquid range, negligible vapor
pressure, high thermal stability and good solvating
ability for a wide range of substrates and catalysts.
They are particularly promising as solvents for the
immobilization of transition metal catalysts, Lewis acids
and enzymes.[11] The hallmark of such ionic liquids is the
ability to alter their properties as desired by manipulat-
ing their structure with respect to the choice of organic
cation, anion or side-chain attached to organic cation
(Figure 1).
of rhodium acetate dimer, Rh2(OAc)4, immobilized
in the air- and moisture-stable 1-butyl-3-methylimi-
dazolium
hexafluorophosphate
ionic
liquid,
[bmim]PF6, to afford cyclopropanecarboxylates in
excellent yields with high trans-selectivity. The
recovery of the catalyst is facilitated by the hydro-
phobic nature of [bmim]PF6. The recovered ionic
liquid containing Rh2(OAc)4 can be reused for three
to five subsequent runs with only a gradual decrease
in activity.
Keywords: alkenes; carbene insertion; cyclopropa-
nation; cyclopropanecarboxylic esters; diazo com-
pounds; ionic liquids (ILs)
Their non-volatile nature can reduce the emission of
toxic organic compounds and facilitate the separation of
products and/or catalysts from the reaction solvents.
Owing to the high polarity and ability to solubilize both
organic and inorganic compounds, ionic liquids can
Metal-catalyzed cyclopropanation remains of great enhance reaction rates and selectivities compared to
interest because of its versatile applications in synthetic conventional solvents. As a result of their green
organic chemistry.[1] The cyclopropane ring is a core credentials and potential to enhance rates and selectiv-
structure in a number of biologically active com- ities, ionic liquids are finding increasing applications in
pounds.[2] Consequently, numerous methods have been organic synthesis.[12]
developed for the construction of the cyclopropanes.[3]
In view of the emerging importance of ionic liquids as
In particular, metal-catalyzed cyclopropanation of al- green solvents, we herein report the use of ionic liquids
kenes with ethyl diazoacetate is one of the most simple as recyclable reaction medium for the cyclopropanation
and straightforward approaches for the preparation of of alkenes with ethyl diazoacetate using a catalytic
cyclopropanes.[4] Initially, copper complexes were wide- amount of Rh2(OAc)4 under mild conditions
ly used as catalysts for the cyclopropanation of alkenes (Scheme 1).
with diazo compounds.[5] Subsequently, rhodium car-
Treatment of styrene with ethyl diazoacetate in the
boxylates have been reported to be highly effective presence of 1 mol % of Rh(OAc)4 in 3 mL of [bmim]PF6
catalysts for cyclopropanation.[6] Most of these copper-, ionic liquid resulted in the formation of ethyl 2-phenyl-
cobalt-, rhodium- and ruthenium-catalyzed carbene 1-cyclopropanecarboxylate in 88% yield (entry 3a,
Figure 1.
Adv. Synth. Catal. 2004, 346, 53 56
DOI:10.1002/adsc.200303136
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53

J. S. Yadav et al.
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Scheme 1.
ionic liquid, we have carried out ICP mass analysis. Only
a trace amount of Rh2(OAc)4 (0.001%) was leached out
of the ionic liquid, which did not appreciably effect the
reaction rates. Furthermore, we have also performed the
cyclopropanation reactions in the absence of ionic
liquids. Low conversions and moderate selectivity
Scheme 2.
Table 1). The product was obtained as a mixture of 3 (approximately 1:1 ratio of cis/trans isomers) were
trans- and 4 cis-isomers, favoring trans-diastereomer 3. observed in the absence of ionic liquid and the
The diastereomers 3 and 4 could be easily separated by comparative results are presented in Table 1. The
column chromatography and were characterized by recovery and reuse of the catalyst is especially simple
comparison of their NMR spectra with those of authen- in ionic liquids compared to organic solvents. The
tic samples.[6b] Both electron-rich and electron-deficient chemoselectivity for cyclopropane formation over car-
styrene derivatives afforded cyclopropanecarboxylates bene dimerization was achieved by the use of an excess
in high yields. In all cases, the reaction proceeds of alkene and slow addition of the carbene source, e.g.,
smoothly at room temperature with high trans-selectiv- ethyl diazoacetate (EDA) to the reaction mixture. No
ity. a- and b-substituted styrene derivatives such as a- formation of side products such as diethyl fumarate or
methylstyrene and b-methylstyrene also gave the cor- diethyl maleate was observed when the reaction was
responding cyclopropanecarboxylates in excellent carried out using ionic liquid [bmim]PF6. A wide range
yields with high trans-selectivity (entries 3k and 3 l, of alkenes including electron-rich, electron-deficient
Table 1). In general, the yield for an electron-rich olefin styrene derivatives and few acyclic and cyclic olefins
was higher than for electron-poor olefins. The treatment underwent smooth cyclopropanation with ethyl diazo-
of cyclohexene with ethyl diazoacetate afforded ethyl acetate under similar reaction conditions. In most cases,
bicyclo[4.1.0]heptane-7-carboxylate in 85% yield the products were obtained in high yields with higher
(Scheme 2).
trans-selectivity. The combination of 5 mol % of
In case of cyclohexene, the product was obtained as a Cu(OTf)2 with [bmim]PF6 also gave similar results.
mixture of endo- and exo-isomers, favoring the endo- The scope and generality of this process is illustrated
isomer. No allylic insertion product was observed in the with respect to various olefins and ethyl diazoacetate
reaction of cyclohexene with ethyl diazoacetate. Com- and the results are summarized in Table 1.
pared to conventional solvents, enhanced reaction rates,
In summary, we describe a simple and efficient
improved yields and higher trans-selectivity were ob- protocol for the cyclopropanation of olefins with ethyl
served in ionic solvents. For example, treatment of diazoacetate using Rh(OAc)4 immobilized in air- and
styrene with ethyl diazoacetate in the presence of moisture-stable [bmim]PF6. The simple experimental
1 mol % of Rh2(OAc)4 in [bmim]PF6 at room temper- and product isolation procedures combined with ease of
ature for 6.0 h afforded the corresponding cyclopropane recovery and reuse of this novel reaction media are
in 88% yield in a 9:1 trans:cis ratio whereas the same expected to contribute to the development of green
reaction in CH2Cl2 after 9.0 h gave the products in a 72% strategy for the synthesis of cyclopropanes. In addition
yield as a mixture of trans- and cis- isomers in a 4:1 ratio. to its simplicity and milder reaction conditions, this
In the absence of catalyst, no reaction was observed method provides high yields of products with greater
when a mixture of alkene and EDA was stirred in trans-selectivity which makes it a useful and attractive
[bmim]PF6 at room temperature for 8 12 h. The major strategy for the preparation of trans-cyclopropanecar-
advantage of the use of ionic liquids is that they can be boxylates.
easily recovered and recycled in subsequent reactions.
Since the products were fairly soluble in ionic phase,
they could be easily separated by simple extraction with
diethyl ether. The remaining ionic liquid was further
washed with ether and reused in three to five successive
runs with only a gradual decrease in activity. For
instance, styrene and ethyl diazoacetate in the presence
of 1 mol % of Rh2(OAc)4 in [bmim]PF6 afforded 88%,
85%, 81%, 75% and 72% yields over five cycles. To
Experimental Section
Melting points were recorded on a B¸chi R-535 apparatus and
are uncorrected. IR spectra were recorded on a Perkin-Elmer
FT-IR 240-c spectrophotometer using KBr optics. H NMR
spectra were recorded on a Gemini-200 spectrometer in CDCl3
using TMS as internal standard. Mass spectra were recorded on
1
determine the quantity of the catalyst leaching out from a Finnigan MAT 1020 mass spectrometer operating at 70 eV.
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Adv. Synth. Catal. 2004, 346, 53 56

Rh2OAc4 Immobilized in 1-Butyl-3-methylimidazolium PF6 Ionic Liquid
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diethyl ether (3 Â 10 mL). The combined organic extracts were
dried over anhydrous Na2SO4, concentrated under vacuum and
purified by column chromatography on silica gel (Merck, 100
200 mesh ethyl acetate-hexane, 1:9) to afford pure the
cyclopropanecarboxylate. The cyclopropanes thus obtained
were identified by comparison of their NMR, IR, mass spectra
and physical data with literature values. The spectral data of all
General Procedure
To a stirred solution of alkene (3 mmol) and 1 mol % of
Rh2(OAc)4 or 5 mol % of Cu(OTf)2 in [bmim]PF6 (3 mL),
ethyl diazoacetate (1 mmol) was added slowly in a dropwise
manner. The resulting mixture was stirred at 278C for the
appropriate time (Table 1). After completion of the reaction,
as indicated by TLC, the reaction mixture was extracted with
7]
the products were identical with those of authentic samples.[5
Adv. Synth. Catal. 2004, 346, 53 56
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J. S. Yadav et al.
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trans-Ethyl 2-phenylcyclopropane-1-carboxylate (3a): Liq-
uid, IR (KBr): n ˆ 2985, 2931, 2865, 1720, 1605, 1458, 1369,
1257, 1153, 1070, 1045, 935, 780, 697 cmÀ1; 1H NMR (200 MHz,
CDCl3): d ˆ 1.02 (t, 3H, J ˆ 6.9 Hz), 1.25 (ddd, 1H, J ˆ 4.4, 6.4,
8.4 Hz), 1.53 (ddd, 1H, J ˆ 4.4, 5.2, 9.2 Hz), 1.85 (ddd, 1H, J ˆ
4.2, 5.2, 8.4 Hz), 2.45 (ddd, 1H, J ˆ 4.2, 6.4, 9.2 Hz), 3.99 4.05
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‡
(m, 2H), 7.05 7.32 (m, 5H); EIMS: m/z ˆ 190 (M ), 162, 141,
115, 91, 43.
cis-Ethyl 2-phenylcyclopropane-1-carboxylate (4a): Liquid;
IR (KBr): n ˆ 3059, 2982, 2933, 1728, 1607, 1454, 1381, 1265,
1086, 961, 795, 694 cmÀ1; 1H NMR (200 MHz, CDCl3): d ˆ 0.98
(t, 3H, J ˆ 7.1 Hz), 1.33 (ddd, 1H, J ˆ 5.1, 7.8, 8.7 Hz), 1.70
(ddd, 1H, J ˆ 5.1, 5.6, 7.4 Hz), 2.05 (ddd, 1H, J ˆ 5.6, 7.8,
9.3 Hz), 2.59 (ddd, 1H, J ˆ 7.4, 8.7, 9.3 Hz), 3.90 (q, 2H, J ˆ
‡
7.1 Hz), 7.19 7.28 (m, 5H); EIMS: m/z ˆ 190 (M ), 163, 91, 55;
anal. calcd. for C15H14O2 (190.241): C 75.76, H 7.42; found: C
75.63, H 7.58.
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