Ultrasound promoted C-C bond formation: Heck reaction at ambient conditions in room temperature ionic liquids

Article abstract of DOI:10.1039/b104532f

Heck reaction proceeds at ambient temperature (30 °C) with considerably enhanced reaction rate (1.5-3 h) through the formation of Pd-biscarbene complexes and stabilized clusters of zero-valent Pd nanoparticles in ionic liquids under ultrasonic irradiation.

Full text of DOI:10.1039/b104532f

Ultrasound promoted C–C bond formation: Heck reaction at ambient
conditions in room temperature ionic liquids
Ravindra R. Deshmukh, R. Rajagopal and K. V. Srinivasan*
Organic Chemistry, Technology Division, National Chemical Laboratory, Pune - 411 008, India.
E-mail: kvsri@dalton.ncl.res.in
Received (in Cambridge, UK) 22nd May 2001, Accepted 27th June 2001
First published as an Advance Article on the web 1st August 2001
Heck reaction proceeds at ambient temperature (30 °C) with
considerably enhanced reaction rate (1.5–3 h) through the
formation of Pd–biscarbene complexes and stabilized clus-
ters of zero-valent Pd nanoparticles in ionic liquids under
ultrasonic irradiation.
solvents such as DMF and NMP even in the presence of a ligand
such as PPh . The reaction also did not proceed with the less
reactive aryl chlorides (chlorobenzene, p-nitrochlorobenzene
and 2,4-dinitrochlorobenzene) and aryl bromides (bromo-
benzene) under the sonochemical conditions of the present
work. Obviously, no reaction even in traces could be observed
under ambient conditions in the absence of ultrasound.
3
The palladium catalyzed Heck reaction involving the coupling
of alkenes/alkynes with aryl and vinyl halides is one of the most
Phosphine ligands, ammonium and phosphonium halides and
quarternary ammonium salts have been found to stabilize the
Pd-catalysts probably via formation of zero-valent Pd species
and accelerate the olefination reaction.6 In particular, Pd–
carbene complexes with alkylimidazol-2-ylidenes have been
1
powerful tools in synthetic organic chemistry. The reactions
which are carried out in polar solvents such as DMF and NMP
,7
generally involve long reaction times (8–72 h) at temperatures
2
ranging from 80–140 °C. In recent times, ionic liquids have
4
b
gained prominence as attractive alternatives to volatile organic
reported very recently to be active in the Heck reaction. In the
present work, the formation of such a complex was studied by
3
solvents for catalytic reactions and separation processes. Heck
reaction in ionic liquids, in particular those involving dialkyl-
imidazolium salts, have been reported very recently. However,
subjecting a mixture of Pd(OAc)
2
2
or PdCl
respectively to ultrasound
2
and NaOAc in
4
2
[bbim] + Br and [bbim] + BF
4
even in such media, wherein the ionic liquid anion and cation
can exert a marked effect on the rate of the reaction, the reaction
times involved are 24–72 h at temperatures ranging from
irradiation for 1 h. The complex was extracted into chloroform
from the ionic liquid, the chloroform evaporated and the
resulting crude product purified by column chromatography
[petroleum ether–EtOAc, 1+1]. Indeed, the formation of the
80–100 °C. This communication reports for the first time an
1
ultrasound promoted Heck reaction in ionic liquids at ambient
temperature (30 °C) with considerably enhanced reaction rates
complex A was established and characterized by H-NMR by
the appearance of N–CH vinylic protons at d 6.85 as a multiplet
4
b
(
reaction times 1.5–3 h) than those reported so far.
The sonochemical reactions were carried out in a thermo-
in the complex similar to the value reported by Xiao et al. and
by the conspicuous absence of the N CH protons which appears
2
stated ultrasonic cleaning bath of frequency 50 KHz (Branson
200). Iodobenzene and substituted iodobenzenes were reacted
with various alkenes/phenylacetylene in ionic liquids 1,3-di-n-
as a singlet at d 8.82 in the parent ionic liquid. The complexes
were also characterized by MS which showed the respective
molecular ion peaks. It is highly probable that complex A could
be the immediate precursor for the likely active catalyst which
could lead to a zero-valent Pd-species whose in situ generation
by reduction of the divalent Pd–carbene complex is accelerated
by electron transfer reactions under the sonochemical condi-
tions primarily through the phenomenon of cavitation. It is well
known that sonochemical processes proceed through SET
mechanistic pathway by means of formation and adiabatic
collapse of transient cavitation bubbles.8 It is also highly
probable that the Pd complexes formed in situ will experience
secondary reactions in the liquid phase after the bubble
collapses. Moreover, such Pd carbene complexes have been
shown to be the precursors for active catalysts for the Heck and
5
+
2
butylimidazolium bromide [(bbim) Br ] and 1,3-di-n-butyli-
+
2 5
4 2
] using Pd(OAc)
midazolium tetrafluoroborate [(bbim) BF
as well as PdCl as catalyst and sodium acetate as base under
2
ultrasonic irradiation as shown in Fig. 1.
The products could be easily separated from the catalyst by
extraction with 10% ethyl acetate in petroleum ether leaving
behind the palladium catalyst in the dissolved state in the
immiscible ionic liquid. The catalyst thus recovered as a
solution in the ionic liquid could be reused at least three times
without any loss of activity. Pure products were isolated by
column chromatography.
The results are summarized in Table 1. As is evident, the
ultrasound assisted Heck reaction of the iodobenzenes with
alkenes/alkynes proceeded smoothly at ambient temperature
,9
related C–C bond forming reactions in both molecular solvents
and ionic liquids.1
0–12
Detailed investigation of the mechanistic
(
1
(
30 °C) with complete conversion of iodobenzenes in just
.5–3 h to afford the trans products in excellent isolated yields
73–87%). No reaction under similar sonication conditions was
pathways as regards the formation/decomposition of the Pd–
carbene complex under the sonochemical conditions is in
progress and will form part of a full paper.
observed when the ionic liquid was replaced by molecular
It has long been known that chemical or electrochemical
reduction of transition metal salts in the presence of ammonium
+
2
13
4
salts leads to R N X stabilized colloids. A recent study
points to the involvement of intermediary colloidal Pd-
nanoparticles generated under thermolytic conditions for the
catalysis of the Heck reaction.14 The formation of such Pd-
nanoparticles was investigated in the present work by subjecting
the reaction mixture, after successful Heck reaction of iodo-
+
2
benzene with ethyl acrylate in [bbim] BF
ical conditions, for ‘in situ’ TEM analysis.
4
under sonochem-
TEM analysis was carried out in Transmission Electron
Microscope Model JEOL-1200 EX operated at 100 kV with a
magnification of 200 K. The sample after appropriate dilution
with isopropyl alcohol was directly deposited on carbon film
Fig. 1
1544
Chem. Commun., 2001, 1544–1545
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104532f

View Article Online
+
2
+
a
Table 1 Heck reaction of iodobenzenes with activated alkenes/alkyne under sonication in [bbim] Br /[bbim] BF
4
%
Yieldb
No.
Aryl halide
Olefin/alkyne
Time/h
Product
(Isolated)
1
2
3
.
.
.
Methyl acrylate
Ethyl acrylate
Styrene
Phenylacetylene
Methyl acrylate
Ethyl acrylate
Styrene
Phenylacetylene
Methyl acrylate
Ethyl acrylate
Styrene
2.0
1.5
1.5
2.0
3.0
3.0
3.0
2.0
1.5
1.5
1.5
2.0
Methyl cinnamate
Ethyl cinnamate
Stilbene
Diphenylacetylene
4-Methoxymethyl cinnamate
4-Methoxyethyl cinnamate
4-Methoxystilbene
(4-Methoxyphenyl)phenylacetylene
4-Chloromethyl cinnamate
4-Chloroethyl cinnamate
81
87
82
78
82
79
80
77
79
77
73
78
4-Chlorostilbene
(4-Chlorophenyl)phenyl acetylene
Phenylacetylene
a
2
Reaction conditions: a mixture of iodoarene (2 mmol), alkene/alkyne (2.1 mmol), sodium acetate (0.2 g), Pd(OAc) (0.02 mmol) and ionic liquid (1.5 ml)
b
1
sonicated in an atmosphere of argon. Yields are based on iodobenzenes. All the products were trans and fully characterized by H-NMR, IR, MS and
elemental analysis.
Pd–biscarbene complex as a precursor and its subsequent
sonolytic conversion to a highly stabilized cluster of zero-valent
Pd nanoparticles has been established by NMR/MS and in situ
TEM analyses respectively. Further work is in progress to
elaborate these findings to other C–C bond formations such as
Suzuki and Stille coupling reactions.6
R. R. D. and R. R. thank CSIR, New Delhi for the award of
a Senior Research Fellowship. We thank Dr M. Bhadbhade and
Dr Rajesh Gonnade for help in TEM analysis.
Notes and references
1
(a) H. A. Dieck and R. F. Heck, J. Org. Chem., 1975, 40, 1083; (b) T.
Jeffery, J. Chem. Soc., Chem. Commun., 1984, 1287.
2
(a) J. I. i. Kim, B. A. Patil and R. F. Heck, J. Org. Chem., 1981, 46,
1
1
067; (b) D. D. Bender, I. G. Stakem and R. F. Heck, J. Org. Chem.,
982, 47, 1278.
3
4
(a) T. Welton, Chem. Rev., 1999, 99, 2071; (b) P. Wasserschied and W.
Keim, Angew. Chem., Int. Ed., 2000, 39, 3772.
(a) A. J. Carmichael, M. J. Earle, J. D. Holbrey, P. B. McCormac and
K. R. Seddon, Org. Lett., 1999, 1, 997; (b) L. Xu, W. Chem and J. Xiao,
Organometallics, 2000, 19, 1123.
+
2
5
[bbim] Br was prepared by heating 1-butylimidazole with 1-bromo-
butane at 70 °C for 2 h and distilling off excess 1-bromobutane under
high vacuum to afford a thick brownish yellow liquid having density,
2
3
+
2
+
1
.228 g cm . [bbim] BF
4
was obtained by the metathesis of [bbim]
2
Br with NaBF
4
in water to give a pale yellow free flowing liquid
23
having density, 1.152 g cm . Both the ionic liquids were well
characterized by IR, H and C NMR, and elemental analysis.
1
13
Fig. 2 TEM image of palladium clusters formed in the sonolytic Heck
reaction.
6 (a) S. Brase and A. de Meijere, Metal catalyzed cross coupling
reactions, ed. F. Diederich and P. J. Stang, Wiley-VCH, Chichester,
1
998; (b) J. Tsuji, Palladium Reagents and Catalyst-Innovations in
coated TEM grids forming a thin film of colloidon. The TEM
image (Fig. 2) shows the presence of monodispersed grains
nearly spherical in shape. The average size of the grains
obtained from the TEM picture is about 20 nm. The porous
grains show that they are composed of dispersed particles of
Organic Synthesis, Wiley, Chichester, 1995.
7
(a) T. Jeffery, Tetrahedron Lett., 1985, 26, 2267; (b) C. Amatore, M.
Azzahi and A. J. Juand, J. Am. Chem. Soc., 1991, 113, 8375.
J. L. Luche, Ultrasonics, 1992, 30, 156.
A. Henglein, Advances in Sonochemistry, ed. T. J. Mason, JAI Press,
London and Greenwich, 1993, vol. 3, pp. 17–83.
8
9
0
approximately 1 nm size. The clusters of Pd nanoparticles in
ionic liquids were found to be stable even after storage for a
week since no change in the TEM picture was observed after
this period, the ionic liquid obviously contributing to the
stability. Further work is in progress in investigating the
structural details of the nano-assembly of metallic palladium
and its dispersion in ionic liquids under sonolytic conditions.
In conclusion, the Heck reaction has been performed at
ambient temperature with considerably enhanced reaction rates
by the combined use of ultrasonic irradiation and ionic liquids
as solvent. Under the sonochemical conditions, the formation of
10 W. A. Herrmann and C. Kocher, Angew. Chem., Int. Ed. Engl., 1997, 36,
2163.
1
1
1
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