PH-Responsive N-heterocyclic carbene copper(i) complexes: Syntheses and recoverable applications in the carboxylation of arylboronic esters and benzoxazole with carbon dioxide

Article abstract of DOI:10.1039/c3gc36830k

A pH-controlled monophasic/biphasic switchable system has been developed as a green and novel strategy for homogeneous catalyst recycling, which has been successfully applied to the Cu-NHC-catalyzed carboxylation of organoboronic esters and benzoxazole with carbon dioxide. Additionally, the present strategy could also be extended to the Ag-NHC-catalyzed carboxylation of terminal alkyne. The tertiary amine-functionalized catalysts could be used for at least four times with a slight loss of activity.

Full text of DOI:10.1039/c3gc36830k

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pH-Responsive N-heterocyclic carbene copper(I)
complexes: syntheses and recoverable applications in
the carboxylation of arylboronic esters and
benzoxazole with carbon dioxide†
Cite this: Green Chem., 2013, 15, 635
Received 17th November 2012,
Accepted 9th January 2013
DOI: 10.1039/c3gc36830k
www.rsc.org/greenchem
Wenlong Wang,a,c Guodong Zhang,a,c Rui Lang,a,c Chungu Xiaa and Fuwei Li*a,b
A pH-controlled monophasic/biphasic switchable system has been aqueous biphasic system and ionic liquid/organic solvent
developed as a green and novel strategy for homogeneous cata- system.5 Although the introduction of a second phase ensures
lyst recycling, which has been successfully applied to the Cu-NHC- efficient catalyst separation and recycling, it induces mass
catalyzed carboxylation of organoboronic esters and benzoxazole transfer limitations that sometimes reduce the reaction rate.
with carbon dioxide. Additionally, the present strategy could also
An interesting solution to overcome these above limitations
be extended to the Ag-NHC-catalyzed carboxylation of terminal is to carry out the reaction in a single phase and to transfer the
alkyne. The tertiary amine-functionalized catalysts could be used catalyst or products into the second phase once the reaction is
for at least four times with a slight loss of activity.
completed, which could not only keep the typical advantages
of homogeneous catalysis, but also achieve the separation of
catalyst from reaction mixture. Xi et al. elaborated phase-trans-
fer catalysis for easy catalyst recycling.6 Gladysz, Jin and Behr
and coworkers developed a temperature-controlled reaction
system to recycle the catalysts respectively.7 Cole-Hamilton
et al. reported their CO2 phase-switchable strategy for the recy-
cling of rhodium catalyst from the hydroformylation reaction
mixture.8 Wang and coworkers designed a light-controlled
phase-tagged Hoveyda–Grubbs catalyst for metathesis reac-
tions.9 Therefore, such a kind of green and efficient catalyst
recycling protocol, viz. catalysis in homogeneous conditions
and separation in different phases, has great potential and is
also highly desirable to be developed further in more sustain-
able catalysis.
N-Heterocyclic carbene (NHC) transition metal complexes
have attracted considerable attention owing to their high stabi-
lity and outstanding application in homogeneous catalysis.10
However, most of them are complicated to synthesize through
multiple steps and difficult to be reused after the fresh reac-
tion. As part of our continuing studies on the synthesis of
recyclable NHC-transition metal complexes and their catalytic
applications,11 we herein describe a pH-controlled monophasic/
biphasic system switching process for catalyst recycling and
its application in Cu-NHC-catalyzed carboxylation of organo-
boronic esters and benzoxazole.
Introduction
Homogeneous catalysis provides a number of advantages over
heterogeneous catalysis, such as high efficiency and selectivity.
However, most homogeneous catalysts have not been widely
spread, especially for precious organometallic catalysts, due to
their intricate recycling problems and thus the metal contami-
nation of the products as well.1
A number of methods have been developed to recycle
homogeneous catalysts, the main one being to heterogenize
the homogeneous catalyst, wherein insoluble catalyst is
anchored to various kinds of supports, such as silica-based
materials,2 polymers3 and magnetic nanoparticles.4 Although
the separation of the catalyst from reaction mixture is realized
by simple filtration, their catalytic reactivity is reduced signifi-
cantly in most cases due to the insolubility of the supported
catalysts. Another one is to develop a biphasic reaction
system1a wherein catalyst and reaction mixture (substrate and
product) dissolve in different phases, such as the well-known
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China.
E-mail: fuweili@licp.cas.cn; Fax: (+86)-931-4968129
bSuzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
Suzhou, 215123, China
cGraduate University of Chinese Academy of Sciences, Beijing, China
†Electronic supplementary information (ESI) available: Experimental details and
characterization data for new compounds. CCDC 866894. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3gc36830k
As shown in Scheme 1, this Cu(I)-catalyzed carboxylation of
organoboronic esters was first reported by Hou et al. and it is
proved that the bulky IPr-NHC is an unique assisting ligand to
achieve high reactivity.12 From the viewpoints of synthetic and
green chemistry, this reaction is very interesting owing to its
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Scheme 1 Cu-NHC catalyzed carboxylation of organoboronic esters.
potential applications in production of functionalized car-
boxylic acids from the easily synthesized organoboronic esters
and the facile utilization of CO2 under mild conditions.13
Undoubtedly, achieving the recycling of the difficult to obtain
IPr-Cu catalyst will make this process more competitive and
sustainable. In view of the chemical environment of such reac-
tion, which proceeds under basic conditions and should be
acidified by a protic acid to generate the desired carboxylic
acid after the carboxylation, therefore, it could be speculated
to realize the catalyst recycling by taking advantage of the
base–acid condition variation. Based on this idea, we herein
synthesize a variety of amine-functionalized (SIPr)CuX catalysts
(6) and they are homogeneously soluble in the basic carboxyla-
tion system; upon addition of a protic acid after the reaction is
complete, the tertiary amine function of 6 will be protonated
to a tertiary ammonium salt, which will make the resultant
ionically-tagged NHC-Cu catalyst more polar and precipitate
from the reaction mixture, thus the product and catalyst could
be separated easily by simple centrifugation. To our delight,
the synthesized NHC-Cu(I) catalyst could be efficiently used at
least four times with a 90% isolated yield of the desired car-
boxylic acid in the last run.
Scheme 2 Syntheses of morpholine-functionalized Cu(I)-NHC catalysts (6).
Reagents and conditions: (i) CH2O, morpholine, EtOH, H2O, reflux; (ii) HCOOH,
glyoxal, MeOH, r.t.; (iii) LiAlH4, THF, 0 °C, r.t.; (iv) NH4Cl, HC(OEt)3, 120 °C;
(v) CuCl or CuBr·SMe2 or CuI, t-BuOK, THF, r.t., 24 h.
Fig. 1 Crystal structure of 6c (ellipsoids are drawn at 50% probability level).
(see ESI†). Crystals of complex 6c suitable for X-ray crystal struc-
tural determination were obtained by diffusion of diethyl ether
to its dichloromethane solution and were analyzed to show the
desired structural motif (Fig. 1, CCDC number 866894).
The illustration of our pH-controlled monophasic/biphasic
system switching for homogeneous catalyst recycling in the
model reaction of Cu-NHC-catalyzed carboxylation of organo-
boronic esters is shown in Scheme 3. The carboxylation was
Results and discussions
The synthetic route of the amine-functionalized (SIPr)CuX cata- carried out in homogeneous conditions since 6 is soluble in
the reaction mixture and the product was in its carboxylate
form when the reaction finished; once HCl diethyl ether was
lysts (6a, 6b and 6c) investigated in this work are shown in
Scheme 2. Morpholine-functionalized [SIPr]·HCl (5) was syn-
thesized according to the previously reported procedure of added as an acid source to acidify RCOO− to RCOOH, the mor-
Plenio et al. and us.11,14 Following Nolan and coworker’s
pholine-functionalized Cu(I)-NHC catalyst was protonated to
its polar ammonium salt-tagged form and precipitated from
methodology,15 morpholine-functionalized [(SIPr)CuCl] could
be prepared in 86% yield in THF by the reaction of CuCl with the non-polar diethyl ether solution; therefore, the catalytic
system switches from monophasic to biphasic conditions (see
photograph in Scheme 3) and the insoluble Cu catalyst could
in situ-generated free NHC from its imidazolium salt and
1.0 equivalent of a strong base (t-BuOK). These synthetic con-
ditions could be successfully extended to the synthesis of 6c in be separated by the method of centrifugation. Catalyst re-
cycling was accomplished by introduction of fresh substrates
and base, and the latter could regenerate the protonated cata-
a yield of 84% with CuI as the copper source. However, reac-
tion of the carbene precursor 5 with CuBr in the presence of
t-BuOK was found to be particularly inefficient, due to the sig- lyst to its neutral form that is soluble again in the reaction
system. It was important to note that HCl diethyl ether was a
perfect acid and solvent source in this catalyst recycling system
nificant formation of unidentified products. In this case, 81%
yield of 6b could be achieved using CuBr·SMe2 as metal source
under the same reaction conditions as for 6a and 6c. The due to its anhydrous nature, non-polar character and excellent
Cu-NHC coordination was established by the 13C NMR shift at
solubility of product.
The 1H NMR evidence along with the corresponding experi-
about 203 ppm, assignable to the 2C-imidazolidin-2-ylidene
carbon, and absence of the 1H resonance of NCHN proton mental phenomena for the protonation process of
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Scheme 3 The illustration of pH-controlled monophasic/biphasic system
switching for homogeneous catalyst recycling.
morpholine-functionalized Cu(I)-NHC catalyst could further
prove the phase-switching phenomena. To keep the final ter-
tiary ammonium-tagged 6a soluble in NMR solvent for ideal
measurement and shield the contamination of water, CD3OD
and dry HCl gas were selected as the solvent and proton label,
respectively. As shown in Fig. 2, it can be seen in the top
photographs that 6a was insoluble in CD3OD solvent because
of its nature of low polarity, however, it became rapidly soluble
in CD3OD after HCl(g) was passed through the tube due to the
protonation process, and a new peak at 9.06 ppm assignable to
the proton bonded to the tertiary amine group of the morpho-
line function introduced by the protonation process was clearly
observed, indicating the current series of amine-functionalized
Cu(I)-NHC catalysts is very sensitive to the pH variation that can
further successfully induce the phase transformation.
Fig. 2 1H NMR spectrum of morpholine-functionalized Cu(I)-NHC catalyst after
the process of protonation and photographs of corresponding experimental
phenomena.
Table 1 Catalyst screening in Cu(I)-NHC-catalyzed carboxylation of organo-
boronic estersa
With these catalysts in hand, we then tested their catalytic
activities in the model carboxylation of 4-fluorophenyl boronic
esters with an atmosphere pressure of carbon dioxide in the
presence of t-BuOK as a base, as presented in Table 1. 6b gave
the highest isolated yield (96%) of the desired aryl carboxylic
acid under the same conditions. Subsequently, we examined
the scope of the present catalytic system (Table 2). Carboxyla-
tion of organoboronic esters with electron-withdrawing groups
regardless of their substitution position proceeded efficiently,
affording 83–96% yield of corresponding products (8a–8g).
The functional vinyl group could be well tolerated. Similarly,
substrates with electron-donating substituents also worked
rapidly. These excellent activities suggested the current amine-
modified (IPr)CuBr catalyst maintained the advantages of
homogeneous carboxylation.
Entry
Catalyst
Yieldb (%)
1
2
3
6a
6b
6c
84
96
87
a Reaction conditions:
7 (1.0 mmol), CO2 (1.0 atm), t-BuOK
(1.05 mmol), catalyst (2.0 mol%). b Isolated yield.
carboxylation of 4-fluorophenyl boronic ester, indicating that
present strategy can not only keep the high reactivity of homo-
genous reaction, but also achieve convenient separation and
recycling of catalyst.
To broaden this conception, 6a-catalyzed direct carboxyla-
tion of benzoxazole with CO2 to yield heterocyclic carboxylic
ester was also investigated.16 As revealed in Table 3, the
present catalyst demonstrated good activity under the similar
reaction conditions in the carboxylation of aryl organoboronic
esters. The recycling strategy also worked well and catalyst 6a
could be used for four times with a slight loss of activity.
Following the isolation procedure, the present homo-
geneous carboxylation catalyst could be precipitated from the
reaction mixture and facilely separated for the next run. As
demonstrated in Fig. 3, catalyst 6b could be used for at least
four times with only a slight loss of activity in the model
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a
Table 2 6b-catalyzed carboxylation of organoboronic esters with CO2
Scheme 4 Synthesis of Ag(I)-NHC complex (9).
Entry
R
Yieldb (%)
1
2
3
4
5
6
7
8
4-FC6H4
4-ClC6H4
4-BrC6H4
4-IC6H4
4-F3CC6H4
3-NO2C6H4
4- NO2C6H4
4-PhC6H4
4-(CH2vCH)C6H4
4-MeOC6H4
4-tBuC6H4
8a (96)
8b (90)
8c (94)
8d (86)
8e (90)
8f (83)
8g (87)
8h (91)
8i (88)
8j (94)
8k (84)
a
Table 4 9-catalyzed carboxylation of terminal alkyne with CO2
Entry
R
Yieldb (%)
9
10
11
1
2
3
4
5
6
7
8
C6H5 (fresh)
C6H5 (2nd run)
C6H5 (3rd run)
C6H5 (4th run)
4-MeC6H4
4-tBuC6H4
4-MeOC6H4
13a (47)
13a (46)
13a (44)
13a (42)
13b (43)
13c (44)
13d (42)
13e (40)
a Reaction conditions: 7 (1.0 mmol), CO2 (1 atm), t-BuOK (1.05 mmol),
catalyst (2.0 mol%). b Isolated yield.
4-CH3CH2C6H4
a Reaction conditions: 12 (1.0 mmol), Cs2CO3 (1.2 mmol), catalyst
(10.0 mol%), DMF (2 mL). b Isolated yield.
stably recycled in the carboxylation reactions of terminal
alkynes with CO2 to generate the corresponding alkynyl car-
boxylic acid for three times, although the yield of this trans-
formation is moderate (Table 4).18
Conclusions
In summary, based on the idea of “catalysis in homogeneous
conditions and separation in different phases”, we have syn-
thesized pH-responsive N-heterocyclic carbene copper(I) com-
plexes and developed a pH-controlled monophasic/biphasic
system switching process as a novel and efficient strategy for
homogeneous catalyst recycling, which has been successfully
applied to the Cu-NHC-catalyzed carboxylation of organoboro-
nic esters and benzoxazole with CO2. Additionally, such strat-
egy could also be extended to Ag-NHC-catalyzed carboxylation
of terminal alkynes with CO2 to yield alkynyl carboxylic acid
and the catalyst could be used for four times. All these appli-
cations indicate that this novel recycling process could be a
powerful complement to other biphasic (or multiphase) separa-
tion systems, and could possibly stimulate new ideas for the
future design of recycling methods.
Fig. 3 Recyclability test of 6b in the model carboxylation.
a
Table 3 6a-catalyzed carboxylation of benzoxazole with CO2
Entry
R
Yieldb (%)
1
2
3
4
5
6
5-Ph (fresh)
5-Ph (2nd run)
5-Ph (3rd run)
5-Ph (4th run)
5-Me
11a (85)
11a (84)
11a (83)
11a (79)
11b (76)
11c (52)
5-Cl
a Reaction conditions: 10 (1.0 mmol), KOtBu (1.1 mmol), THF (5 mL),
DMF (2 mL). b Isolated yield.
Experimental section
General information
In addition, morpholine-functionalized N-heterocyclic
carbene silver complex [Ag(I)-NHC] was conveniently syn-
thesized in high yields according to our and other reported
procedures (Scheme 4; for detailed procedures, see ESI†).17
Based on the same recycling procedure, Ag-NHC catalyst was
All the chemicals and solvents were used as received without
further purification except THF and diethyl ether, which were
dried by distillation over sodium powder and freshly distilled
prior to use. HCl diethyl ether solution was prepared by
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bubbling HCl gas through diethyl ether solution. 1,3-Bis(2,6-
diisopropyl-4-(morpholinomethyl)phenyl)-4,5-dihydro-1H-
Acknowledgements
imidazol-3-ium chloride (5) was synthesized according to the We are grateful to the National Natural Science Foundation of
reported procedure.11,14 NMR spectra were recorded using a China (21002106 and 21133011) and the Chinese Academy of
1
Bruker Avance TM spectrometer operating at 400 MHz for H Science for financial support.
and 100 MHz for 13C. Chemical shifts are given in ppm relative
to TMS or to residual solvent proton resonances. High resolu-
tion mass spectra (HRMS) were obtained on
a Bruker
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