Amphiphilic ionic palladium complexes for aqueous-organic biphasic Sonogashira reactions under aerobic and CuI-free conditions

Article abstract of DOI:10.1016/j.catcom.2013.05.005

The ionic Pd(II)-complexes of 2 (ammonium-[1-(2-hydroxyethyl)-3- methylimidazolium] bis[3-(diphenylphosphino)benzenesulfonate]- dichloropalladium(II) ([(NH₄)(Hemim)][PdCl₂(TPPMS) ₂])) and 3 (bis[1-n-butyl-3-methylimidazolium] bis[3- (diphenylphosphino)benzenesulfonate]-dichloropalladium(II) ([Bmim] ₂[PdCl₂(TPPMS)₂])) were synthesized and fully characterized. The single crystal X-ray diffraction analyses show that 2 and 3 are composed of the imidazolium-based cations and [PdCl₂(TPPMS) ₂]^{2 -} anions. The properties of such imidazolium-based Pd-complexes of 2 and 3, in terms of the aqueous solubilities and the catalytic behaviors in water, could be dramatically varied. When 2 and 3 were applied as the precatalysts for the Sonogashira coupling of iodobenzene with phenylacetylene under aerobic and CuI-free conditions, the much higher yields of 1,2-diphenylethyne were obtained due to their amphiphilicity. The wide generality of 2 was available for aqueous-organic biphasic Sonogashira reactions.

Full text of DOI:10.1016/j.catcom.2013.05.005

Catalysis Communications 40 (2013) 23–26
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Catalysis Communications
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Short Communication
Amphiphilic ionic palladium complexes for aqueous–organic biphasic
☆
Sonogashira reactions under aerobic and CuI-free conditions
⁎
Xuezhu Wang, Jing Zhang, Yongyong Wang, Ye Liu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2013
Received in revised form 3 May 2013
Accepted 4 May 2013
Available online 10 May 2013
The ionic Pd(II)-complexes of
(diphenylphosphino)benzenesulfonate]-dichloropalladium(II) ([(NH4)(Hemim)][PdCl2(TPPMS)2])) and
2
(ammonium-[1-(2-hydroxyethyl)-3-methylimidazolium] bis[3-
3 (bis
[1-n-butyl-3-methylimidazolium] bis[3-(diphenylphosphino)benzenesulfonate]-dichloropalladium(II) ([Bmim]2
[PdCl2(TPPMS)2])) were synthesized and fully characterized. The single crystal X-ray diffraction analyses show
that 2 and 3 are composed of the imidazolium-based cations and [PdCl2(TPPMS)2]2− anions. The properties of
such imidazolium-based Pd-complexes of 2 and 3, in terms of the aqueous solubilities and the catalytic behaviors
in water, could be dramatically varied. When 2 and 3 were applied as the precatalysts for the Sonogashira coupling
of iodobenzene with phenylacetylene under aerobic and CuI-free conditions, the much higher yields of 1,2-
diphenylethyne were obtained due to their amphiphilicity. The wide generality of 2 was available for aqueous–
organic biphasic Sonogashira reactions.
Keywords:
Sonogashira reaction
Water-soluble palladium complexes
Ionic complexes
Biphasic catalysis
© 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction
of the ligands, and the sulfonated phosphines are generally the most im-
portant ones in AOC [20–22]. Typically, under aqueous–organic biphasic
Palladium-catalyzed Sonogashira reaction is a powerful tool for the
formation of Csp2\Csp bond, and the most important method for the
synthesis of internal acetylenes [1]. In a typical Sonogashira procedure,
the reaction is conducted using a phosphine-ligated palladium complex
with a catalytic amount of copper(I) iodide as a co-catalyst in the pres-
ence of base under inert atmosphere [1,2]. However, the in situ formed
copper(I) acetylides with moisture- and air-sensitivity induce unwanted
homocoupling products of terminal alkynes through Glaser reaction [2–4].
In the last few decades, a variety of efficient palladium catalysts
containing hindered phosphines [5–9], palladacycles [10,11] and
N-heterocylic carbene (NHC) [12–15] have been developed for the
copper-free Sonogashira reactions, which allow the manipulation of
the Sonogashira reaction with low catalyst loadings under copper-free
conditions and without exclusion of air and moisture. The absence of
copper also makes the Sonogashira reactions possibly be performed in
water medium [14–16].
condition in AOC, the addition of a phase-transfer catalyst, such as sur-
factant [23,24] or cyclodextrin [25,26], is required in order to eliminate
the serious mass transfer limitation observed with hydrophobic sub-
strates. The use of amphiphilic phosphines has recently been found
to be a promising alternative to suppress the mass transfer factor be-
cause the amphiphilic phosphines incorporate the surface-active
property and the coordinating ability to the metal into the same
compound [27–29]. Unfortunately, the syntheses of amphiphilic
phosphines are often time-consuming, laborious and expensive.
The activities in the functionalization of ionic liquids in our group high-
light us to replace the typical counter-cation of Na+ or NH4+ in TPPMS
by the imidazolium-based cation, which can provide amphiphilicity
via incorporating lipophilic organic moieties. On the basis of this con-
ception, herein, the amphiphilic TPPMS-ligated Pd complexes of 2 and
3 were synthesized (see Supplementary Information) and fully charac-
terized (Scheme 1), which were further studied as the precatalysts for
aqueous–organic biphasic Sonogashira reactions under aerobic and
CuI-free conditions.
Aqueous organometallic catalysis (AOC) by water-soluble phos-
phine complexes has been developed into a most successful way for
product isolation and/or catalyst recycling [17–19]. The solubility of
the organometallic catalysts in water is determined by the solubility
2. Results and discussion
2.1. Synthesis and characterization of 1, 2 and 3
☆
This is an open-access article distributed under the terms of the Creative Commons
Attribution-NonCommercial-No Derivative Works License, which permits non-commercial
use, distribution, and reproduction in any medium, provided the original author and source
are credited.
At room temperature, 2 and 3 were synthesized through exchanging
the cation of NH4+ in 1 [30] by the imidazolium-based cations, which
were fully characterized by 1H NMR, 31P NMR, CHN-elemental analysis,
and single-crystal X-ray analysis. The molecular structures of 2 and 3
⁎
Corresponding author. Tel.: +86 2162232078.
E-mail address: yliu@chem.ecnu.edu.cn (Y. Liu).
1566-7367/$ – see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.05.005

24
X. Wang et al. / Catalysis Communications 40 (2013) 23–26
R'
R'
1, 2, or 3 / base
located in trans-position in the corresponding Pd-complex-anion with
R
+
X
R
high symmetry, which is counteracted by two [Bmim]+ (1-n-butyl-3-
methylimidazolium). The 1H NMR and 31P NMR spectra further support-
ed the assigned structures of 2 and 3. The 31P NMR spectra show the
different resonances at 37.6 ppm for 2 and 25.1 ppm for 3, due to the
influence of the different cations on the chemical environment of
[PdCl2(TPPMS)2]2− via the varied electrostatic interaction, and the ad-
ditional hydrogen bond interaction in 2.
H2O
(X= I or Br)
-O3S
SO3
-O3S
-
N
+
N
NH4
PPh2
PPh2
PPh2
N+
N+
N+
N
Cl Pd Cl
PPh2
Cl Pd Cl
Cl Pd Cl
PPh2
The solubilities of 1, 2, and 3 in water were determined by the equilib-
rium method at 20 °C. It was found that, with the same Pd-complex
anion, the different composition of the counter-cations could dramatically
affect the solubility of the corresponding ionic compounds. When one of
NH4+ in 1 was replaced by [Hemim]+, the solubility of the resultant com-
pound of 2 was increased from 36.9 g/L to 52.9 g/L, due to the presence of
\OH in the cation which developed the strong hydrogen bond interac-
tion with water. While two of NH4+ in 1 were replace by [Bmim]+, the sol-
ubility of the resultant compound 3 was decreased to 3.4 g/L, due to the
presence of the more lipophilic group of [Bmim]+. On the other hand,
besides hydrophilicity derived from [PdCl2(TPPMS)2]2−, 2 and 3 were
featured with lipophilicity because of the presence of the alkyl- and
imidazolium-moieties. Hence, 2 and 3 are featured with more amphiphil-
ic character in comparison to 1, which is purely hydrophilic due to the
present of NH4+ as the counter-cations.
PPh2
HO
+
+
-
NH4
NH4
-
-
SO3
SO3
SO3
1
2
3
Scheme 1. The ionic Pd-complexes of 1, 2, and 3 as the precatalysts for aqueous–organic
biphasic Sonogashira reactions.
were depicted in Fig. 1, which were both composed of the imidazolium-
based cations and the Pd-complex anions. The Pd-complex anion
possesses the square-planar geometry which is similar to that in
PdCl2(PPh3)2 [31]. The Pd center, lying at the center of inversion, is coor-
dinated by two chloride ions and two mono-sulfonated phosphine li-
gands. The Pd–P distances observed in 2 (2.33–2.34 Å) and 3 (2.32 Å)
are in the classical range, compared to those observed in the reported
phosphine-ligated Pd(II) complexes with structural similarity [31,32].
As for 2, two benzenesulfonate groups located in the Pd-complex-
anion ([PdCl2(TPPMS)2]2−) are located in cis-position due to the strong
2.2. Catalytic performance of 1–3 for aqueous–organic biphasic Sonogashira
reaction
hydrogen bond interactions between the sulfonate groups with NH4+
.
The coupling of phenylacetylene with iodobenzene was studied as a
model reaction to investigate the catalytic performance of 1–3 in water
under CuI-free conditions (Table 1). Through screening the bases, piper-
idine as a strong base and effective acid-scavenger could expedite
the reaction rate more rapidly than the others such as K2CO3, Cs2CO3,
The presence of such strong hydrogen bond interaction led to the
slightly distorted square-planar [PdCl2(TPPMS)2]2−, which is counter-
acted by one NH4+ and one [Hemim]+ (1-(2-hydroxyethyl)-3-
methylimidazolium). Whereas in 3, two benzenesulfonate groups are
Fig. 1. The single crystal structures of 2 and 3 [the selected bond distances (Å) and bond angles (°): 2 Pd1-P1 2.3253(12), Pd1-P1A 2.3434(12), P1-Pd1-P1A 177.51(5), Cl1-Pd1-Cl1A 175.96(7),
Cl1-Pd1-P1 91.06(5), Cl1-Pd1-P1A 89.86(4); 3 Pd1-P1 2.3326(18), Pd1-P1A 2.3326(17), P1-Pd1-P1A 180.0, Cl1-Pd1-Cl1A 180.0, Cl1-Pd1-P1 91.64(7) Cl1-Pd1-P1A 88.36(7)].

X. Wang et al. / Catalysis Communications 40 (2013) 23–26
25
Table 1
(1-bromo-4-methoxybenzene and 1-bromo-4-methylbenzene) were
converted to the corresponding coupling products in moderate yields
under the same conditions (Entries 16 and 17). The coupling of chloro-
benzene with phenylacetylene led to the yield of 38% under the relatively
harsh conditions (110 °C, 5 h). Anyway, the activated chlorobenzene
with the electron-deficient substituent like \NO2 or \CHO corresponded
to the poor yields unexpectedly (Entries 19 and 20). When the other ter-
minal acetylenes of 2-ethynylpyridine with the conjugated effect and
but-3-yn-1-ol possessing hydroxyl substituent were used to couple with
iodobenzene, the acceptable yields of the products were obtained
(Entry 21: 78%; Entry 22: 92%). As for hex-1-yne without any other sub-
stituent, the low yield of 34% was found (Entry 23).
The aqueous–organic biphasic Sonogashira reaction of phenylacetylene with iodobenzene
catalyzed by 1–3.a
Entry
Precatalyst
Solvent
Conversion (%)b
Yield (%)b
1
2
1
2
3
2
H2O
H2O
H2O
CH3CN
72
100
97
72
100
97
3
4c
40
40
a
Precatalyst 0.2 mol% (2 μmol), iodobenzene 1.0 mmol, phenylacetylene 1.5 mmol,
piperidine 1.5 mmol, solvent 4 mL, temperature 70 °C, time 1.5 h.
b
GC analysis (with 100% selectivity to 1,2-diphenylethyne).
Homogeneous system.
c
pyridine, Et3N, Et2NH, (i-Pr)2NH, and n-Bu2NH. Under the optimized
condition (1.5 h, 70 °C, piperidine as the base), the complete con-
version of iodobenzene into the cross-coupling product of 1,2-
diphenylethyne was obtained over 2 with the best aqueous solubil-
ity (Entry 2). In comparison with 2, the lowest yield of 72% was observed
over 1 (Entry 1), and the competitive yield up to 97% was obtained over 3
(Entry 3) although the aqueous solubility of 3 (3.4 g/L) was much lower
than those of 2 (52.9 g/L) and 1 (36.9 g/L). Actually, as precatalysts,
the catalytic nature of 1–3 was basically the same, which was all de-
rived from the anion of [PdCl2(TPPMS)2]2−. The different yields of
1,2-diphenylethyne over 1, 2, and 3 implied that the corresponding
counter-cations of 1, 2, and 3 could affect aqueous–organic biphasic
coupling reaction definitely. In comparison to 1 with NH4+ as the
counter-cations possessing the hydrophilic character, 2 and 3 were
feathered with the amphiphilicity due to the presence of the
imidazolium-based organic cations. As a result, the amphiphilic 2
and 3 not only behaved as the precatalysts for the Sonogashira reaction,
but also acted as the quaternary ammonium surfactants to suppress the
mass transfer limitation in the biphasic system. On the other hand, the
timely removal of the byproduct of piperidine⋅HI salt from the organic
phase into the aqueous one was the additional driving force to promote
the coupling reaction with the expedited reaction rate, leading to the
higher yield of 1,2-diphenylethyne in the biphasic system than that in
the homogeneous catalysis (Entries 1–3 vs 4).
AOC was to allow the facile separation workup and/or catalyst
recycling. Upon completion, the lipophilic product of 1,2-diphenylethyne
was obtained simply through decantation of the upper organic phase
without contamination by the by-product of piperidine⋅HI salt, which
was completely stripped into the aqueous phase. Anyway, the deactivated
Pd-black was precipitated from the aqueous phase badly and the lower
aqueous phase became colorless and limpid upon the 1st-run. Resultant-
ly, the recycling use of the left aqueous phase containing the precipitated
Pd-black for the coupling of ArI with phenylacetylene was unsuccessful
with the yield of 1,2-diphenylethyne less than 5% in the 2nd run, which
suggested that 2 suffers from serious deactivation probably due to the
fragmentation by water [33].
The generality of 2 as the precatalyst for aqueous–organic biphasic
Sonogashira reactions, in which a wide range of the substrates with differ-
ent electronic and steric effects were investigated (see S. Table 2 in Sup-
plementary Information). In order to obtain the acceptable yields for the
coupling products, the concentration of 2 was increased up to 5 mol%.
When the aryl iodides were used to couple with phenylacetylene over 2
under the mild conditions, generally the cross-coupling products were
obtained in excellent yields without obvious discrimination for the elec-
tronic and steric properties of the substituents (Entries 1–12). Reason-
ably, while the substituent was located at para-position, the best
coupling yields were found due to the less steric hindrance (Entries
3, 6, 9, and 12). As for the aryl bromides with relatively low reactiv-
ity, the activated substrates with electron-deficient character, such
as 4-bromobenzaldehyde and 4-bromonitrobenzene, coupled with
phenylacetylene in better yields than non-activated bromobenzene
when the reaction temperature was raised to 110 °C (Entries 13 vs
14 and 15). The deactivated substrates with electron-rich character
3. Conclusions
The ionic Pd(II)-complexes of 2 and 3 were synthesized through ex-
changing the cation of NH4+ in 1 by the imidazolium-based organic moi-
eties. The difference of the counter-cations could dramatically change
the properties of the corresponding Pd-complexes of 1–3, in terms of
the aqueous solubilities and the catalytic behaviors in water, respective-
ly. 2 and 3 were featured with the amphiphilic character due to the
dually presence of the lipophilic imidazolium-based cation and the hy-
drophilic anion of [PdCl2(TPPMS)2]2−. When 2 and 3 were applied as
the precatalysts for the coupling of iodobenzene with phenylacetylene
in water under aerobic and CuI-free conditions, the much higher yields
of 1,2-diphenylethyne were obtained due to their amphiphilicity. The
wide generality of 2 was observed for aqueous–organic biphasic
Sonogashira couplings of terminal acetylenes with aryl halides.
Acknowledgments
This work was financially supported by the National Natural Science
Foundation of China (Nos. 21273077 and 21076083).
Appendix A. Supplementary data
CCDC-928534 and CCDC-928535 contain the supplementary crystal-
lographic data of 2 and 3 for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif. The Supplementary Information associ-
ated with this article can be found online at http://dx.doi.org/10.1016/j.
catcom.2013.05.005.
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