In-situ-generated palladium nanoparticles in novel ionic liquid: an efficient catalytic system for Heck–Matsuda coupling

Article abstract of DOI:10.1007/s11164-017-2888-5

Abstract: A green, convenient, ecological and recyclable method comprising dual functionalized, task-specific, ionic liquid (IL)-triggered, in situ-generated Pd nanoparticles (NPs) and their catalytic application for Heck–Matsuda coupling of olefins is described. Both arenediazonium tetrafluoroborate and silica sulphate salts are coupled with olefins under ligand-free and aerobic conditions at ambient temperature furnishing excellent yields of products. The Ionic liquid used acts as a reducing as well as stabilizing agent for in situ-generated Pd NPs. The formed NPs were characterized by transmission electron microscopy (TEM) analysis, having a size below 50?nm, and exhibited high catalytic activity. The catalytic system can be reused for eight times effectively without any significant loss of activity. The method was found to be highly stereo-specific, giving exclusively the ‘E’ isomer.

Full text of DOI:10.1007/s11164-017-2888-5

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Res Chem Intermed
DOI 10.1007/s11164-017-2888-5
In-situ-generated palladium nanoparticles in novel ionic
liquid: an efficient catalytic system for Heck–Matsuda
coupling
D. S. Gaikwad1 K. A. Undale1 D. B. Patil1
•
•
•
D. M. Pore2 S. N. Korade2 A. A. Kamble3
•
•
Received: 23 October 2016 / Accepted: 2 February 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract A green, convenient, ecological and recyclable method comprising dual
functionalized, task-specific, ionic liquid (IL)-triggered, in situ-generated Pd nanopar-
ticles (NPs) and their catalytic application for Heck–Matsuda coupling of olefins is
described. Both arenediazonium tetrafluoroborate and silica sulphate salts are coupled
with olefins under ligand-free and aerobic conditions at ambient temperature furnishing
excellent yields of products. The Ionic liquid used acts as a reducing as well as stabi-
lizing agent for in situ-generated Pd NPs. The formed NPs were characterized by
transmission electron microscopy (TEM) analysis, having a size below 50 nm, and
exhibited high catalytic activity. The catalytic system can be reused for eight times
effectively without any significant loss of activity. The method was found to be highly
stereo-specific, giving exclusively the ‘E’ isomer.
&
D. S. Gaikwad
dgchemistry@gmail.com
1
2
3
Department of Chemistry, Vivekanand College, Kolhapur, Maharashtra 416003, India
Department of Chemistry, Shivaji University, Kolhapur, Maharashtra 416004, India
Department of Chemistry, Bharatratna Indira Gandhi College of Engineering, Kegaon, Solapur,
Maharashtra 413255, India
123

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D. S. Gaikwad et al.
Graphical Abstract
Keywords Heck–Matsuda Á Stereospecific Á Nanoparticles Á Task-specific IL
Introduction
The attention towards room temperature task-specific Ionic Liquids (TSILs) has
appreciably increased over the last decade as environmental concerns originated
from usual synthetic procedures inspired the researchers to investigate alternatives
to conventional organic solvents [1, 2]. Some unique characteristics of TSILs such
as adjustable physico-chemical properties, negligible vapor pressure and high
polarity merged with hydrophobicity provide good solubility for a wide range of
organic, inorganic and organometallic compounds [3–6]. ILs have been found in
numerous applications not only as environmentally benign reaction media, but also
as catalysts and reagents [7].
Furthermore, the highly polar nature of ILs in biphasic system ensures
immiscibility with many organic solvents or in certain cases, also with aqueous
media, offering unique opportunities for recycling and phase-switching techniques
[8–10]. On the catalysis front, many reactions ranging from transition metal-
mediated processes to Friedel–Crafts reactions, C–C bond-forming condensations,
and cycloadditions are dramatically accelerated in ILs. Because of these particular
features, ILs are very attractive liquid media to carry out a plethora of organic
transformations. In this perspective, several review articles in the field of catalysis
have captured much attention on the utility of this media [11, 12]. The first
experiment in IL catalysis was carried out a way back more than 30 years ago and
thereafter the field has experienced a fabulous growth, especially over the last
decade [13].
The palladium-catalyzed C–C coupling of olefins and aryl or alkenyl halides,
universally referred to as the ‘‘Heck–Mizoroki coupling’’, and has received
immense attention in the past decade because it is a selective method for formation
of new C–C bonds in a single operational step [14, 15]. Nowadays, instead of using
aryl or alkenyl halides, diazonium salts are being widely used as precursors to carry
out coupling reactions including Heck–Matsuda coupling [16–30]. However,
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In-situ-generated palladium nanoparticles in novel ionic…
homogeneous catalysts and non-volatile solvents have been of limited use, mainly
because of the difficulty of separation of products from the reaction mass that in turn
may lead to economical/environmental problems especially in case of expensive or
toxic metal catalysts. Therefore, the development of greener approaches is highly
essential which will offer recovery of solvent as well as catalyst.
Kaufmann et al. [31] for the first time used ILs (tetraalkylammonium and
tetraalkylphosphonium bromides) as reaction media for the palladium-catalyzed
Heck coupling, giving moderate to high yields of product. In such coupling
reactions, the IL is believed to stabilize the palladium catalyst, and no precipitation
of palladium was observed even after complete conversion of the aromatic halide to
the product.
It is well known that transition metal nanoparticles (MNPs) are utilized to carry
out coupling reactions [32–34] due to their superiority over traditional metal
catalytic systems with respect to controllable size, more degrees of freedom and
small surface with active sites accessible to reactants [35, 36].
Although different catalytic systems consisting of other than IL systems and
MNPs are reported in literature [37, 38], a recently combined system of TSILs and a
Pd source (especially palladium NPs) is being extensively employed to efficiently
catalyze coupling reactions [32, 39–41]. The TSILs acts as a versatile component
because they behave as a reducing as well as stabilizing agent for palladium NPs.
Till today, a variety of TSILs have been reported; however, hydroxyl-
functionalized IL possess added advantages over the rest of the ILs. These offer
higher polarities and high dielectric constants resulting in increased hydrophilicity,
increased H bond acidity and basicity with no change in dipolarity (the Kamlet–Taft
scale) [42–45]. It has been observed that hydroxyl-functionalized ILs act as good
reducing agents and lead to formation of highly stable Pd NPs with a narrow size
distribution [46].
Amino acids (AAs) are one of the most versatile classes of natural, bio-
renewable, and non-toxic raw materials. Other additional benefits viz low cost,
biodegradability [47], biological activity [48] induce utilization of AAs as
precursors for designer IL synthesis. Due to these reasons, AA-functionalized ILs
have become popular amongst the scientific community and are being developed as
reagents and catalysts to mediate chemical transformations [49–52].
Though varieties of AAs are available, proline is widely utilized as a precursor
for ILs, as it is cheaper and easily available. Thus, prolinate-functionalized ILs have
become a prime choice of researchers and are being used to synthesize
organometallic catalysts for organic transformation [53–60]. In this context and in
continuation with our earlier work in ILs and MNPs [61, 62], herein we have
synthesized novel dual-functionalized TSILs and explored their catalytic activity for
Heck–Matsuda coupling (Scheme 1).
Initially, attention was focused towards synthesis of TSILs for Heck–Matsuda
coupling reaction which would induce in situ generation of Pd NPs as well as
stabilize them through ligand coordination interactions. As discussed earlier,
hydroxyl functionality possesses reducing properties; we decided to incorporate
hydroxyl functionality in the cationic core. While selecting a suitable precursor as
an IL cation, it was observed in literature that in most of the cases, imidazolium-
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D. S. Gaikwad et al.
80oC
OH
OH
+
N
+
Cl
N
N
24h
N
Cl-
2
1
Tolune
3
KOH
RT,
24h
CHCl3
OH
+
OH
N
+
COOH
N
N
H
N
5
N
OH-
COO-
N
H
H2O, RT, 12h
4
6
Scheme 1 Synthesis of prolinate-functionalized TSIL
based cations lead to room temperature ILs. Hence, for our experiment, the
synthesis of IL was carried out using 1-methyl imidazole as a precursor. In order to
stabilize the Pd NPs, prolinate functionality as an anion was incorporated in the IL
via metathesis reaction. The synthetic route of novel dual-functionalized TSIL
1-hydroxylethyl-1-methylimidazolium prolinate [HEMIM][Pro] is outlined in
Scheme 1. Thus, synthesized IL was equipped with cations having pendant
hydroxyl functionality. This induces the coupling reaction and also acts as a
reducing agent with prolinate functionality as an anion. This, in turn, serves as a
ligand which stabilizes and/or activates the in situ-formed Pd NPs. The formation of
1
IL was confirmed by H and 13C nuclear magnetic resonance (NMR) analysis.
After the synthesis of TSIL, we targeted our attempt towards application of TSIL
for the coupling reaction. In order to discover suitable reaction conditions and
to examine the activity of the catalyst, the coupling of benzenediazonium
tetrafluoroborate salt (1 mmol) and styrene (1 mmol) using PdCl2 (2 mol%) as a
Pd source was used as a model reaction. We used IL as a reaction medium and as a
ligand to stabilize the palladium catalyst and, surprisingly, 89% yield of coupling
product was observed within 1 h. During the progress of the reaction, we had
observed the change in initial yellowish brown color of the reaction mixture to
grayish black, which was supposed to be due to in situ formation of Pd NPs. To
verify this, after completion of the reaction. diethyl ether was added to the reaction
mixture to extract the product, and the IL phase was analyzed by transmission
electron microscopy (TEM) analysis. The TEM analysis confirmed the formation of
Pd NPs having size below 50 nm (Fig. 1).
The size and shape of generated MNPs depends on several factors, such as
reaction conditions (temperature, metal concentration, time, etc.), type of metal
precursor, reducing agent, and the nature of the IL. However, there is evidence
indicating that the relative size of MNPs is directly related to the volume of the IL
(polar and nonpolar sphere). Certainly, metal precursors tend to be dissolved in the
polar IL sphere, and, thus, the size is directly related to the volume of the anion of
the imidazolium-based IL [32, 63–65]. Several groups have reported the use of ILs
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