A simple procedure for the polymer-supported N-heterocyclic carbene-rhodium complex via click chemistry: A recyclable catalyst for the addition of arylboronic acids to aldehydes

Article abstract of DOI:10.1039/c1cc14898b

A novel polymer-supported carbene-rhodium complex was prepared using a simple procedure via click chemistry. This polymer-supported N-heterocyclic carbene-rhodium complex was characterized and used as a catalyst for the addition of arylboronic acids to aldehydes in good to excellent yields.

Full text of DOI:10.1039/c1cc14898b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 12319–12321
www.rsc.org/chemcomm
COMMUNICATION
A simple procedure for the polymer-supported N-heterocyclic
carbene–rhodium complex via click chemistry: a recyclable catalyst for
the addition of arylboronic acids to aldehydesw
Ying He and Chun Cai*
Received 8th August 2011, Accepted 10th October 2011
DOI: 10.1039/c1cc14898b
6
and addition of arylboronic acid to aldehydes, there are only
A novel polymer-supported carbene–rhodium complex was
prepared using a simple procedure via click chemistry. This
polymer-supported N-heterocyclic carbene–rhodium complex
was characterized and used as a catalyst for the addition of
arylboronic acids to aldehydes in good to excellent yields.
7
few reports on using organic polymers as supports in this area.
8
Copper-catalyzed azide–alkyne cycloaddition reaction,
which was coined as ‘‘click chemistry’’, has been proved to
be a powerful tool for conjugation between appropriately
functionalized partners of azides and alkynes. The process is
modular, wide in scope, high yielding, and requires only simple
reaction and purification conditions. Taking advantage of the
outstanding reaction profile, the chemical orthogonality of
the formation conditions not requiring activating reagents and
the stability toward strong bases, we tried to develop a new
Transition metal catalysis has been recognized as a powerful
synthetic tool and a major area of interest in multiple organic
1
transformations for academic and industrial processes. The
design of transition metal catalysts has to take into account
several basic characteristics including high efficiencies and
selectivities as well as economic and environmental considerations.
In recent years, a diverse range of ligand frameworks have
been widely studied and their corresponding metal complexes
have also been used in homogeneous systems. N-Heterocyclic
carbenes (NHCs), to some extent, are some of the most
‘‘click resin’’ incorporating a 1,2,3-triazole moiety for the
fusion of linker functionality and the resin backbone.
In continuation of our studies in developing new functional
supported catalysts, herein we present the preparation of a
novel polymer-supported carbene–rhodium complex through
copper-mediated 1,3-dipolar cycloaddition between azides and
alkynes (Scheme 1), and its application in the synthesis of
diaryl-methanols which are important intermediates for the
2
important ligands in coordination chemistry.
Since the isolation of the first free carbene by Arduengo
3
et al. in 1991, N-heterocyclic carbenes (NHCs) have become
9
synthesis of biologically and pharmaceutically active substances.
an important class of ligands in organometallic chemistry
due to their excellent s-donor properties, ease of synthesis,
and variable steric bulk. NHC-based catalysts feature robust
carbon–metal bonds that provide high thermal stability and
low dissociation rates, and consequently better resistance
against oxidation or leaching phenomena, making the use of
The construction of our novel ‘‘click chemistry resins’’ started
À1
from a Merrifield resin (1.0 mmol g ) where nucleophilic
substitution reaction with a large excess of NaN afforded the
3
respective azidomethyl polystyrene 2, and the product could be
isolated and purified by simple filtration and washed with
1
methanol and dichloromethane. The azidation reaction was
0
4
ligand excess unnecessary.
Over the years, great efforts in catalysis research have been
devoted to the introduction and application of effective hetero-
geneous catalysts due to their versatile processing capabilities,
ease of product separation, catalyst separation and reusability.
With the concerns above, a few kinds of immobilized NHC–
metal complexes have been widely studied. Lee et al. have
reported the polymer-supported N-heterocyclic carbene–
palladium complexes for the Suzuki cross-coupling reaction
5
successfully. Although, immobilized NHC–Rh complexes
have been widely studied as catalysts in hydroformylation
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing 210094, China. E-mail: c.cai@mail.njust.edu.cn;
Fax: +86-25-84315030; Tel: +86-25-84315514
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data, images and the copies of NMR for the
products. See DOI: 10.1039/c1cc14898b
Scheme 1 Synthesis of polymer-supported NHC–Rh complex 4.
Reaction conditions: (a) NaN
3
, DMSO, 80 1C, 48 h; (b) 3-methyl-
1
-propargylimidazolium bromide, CuI, DIPEA, DMSO/THF, 35 1C;
t
2
(c) [Rh(COD)Cl] , KO Bu, THF, rt, 24 h.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12319–12321 12319

View Article Online
which demonstrates that the particles of support 3 had a good
mechanical stability during the immobilization step. However,
the surface morphology of these two samples is different. It
shows that the surface of 3 was very slick, but roughness on
the surface of 4 was observed which implied the presence of the
Rh complex (ESIw).
In order to evaluate the activity of our catalyst, a set of
additions of phenylboronic acid to p-nitrobenzaldehyde under
several conditions were carried out (Table 1). Initially, the
influence of solvent was explored using 2 mol% (Rh content)
of the catalyst at 80 1C (Table 1, entries 1–4). The co-solvent
consisting of 1,4-dioxane/water (6 : 1) showed the best result
and a yield of 96% was obtained (Table 1, entry 2). Next,
reaction temperature, time, and catalyst loading were also
investigated. As can be seen in Table 1, lower temperature,
shorter reaction time and lower catalyst loading all led to a
decrease in the yields (Table 1, entries 5–7).
Fig. 1 IR spectra of Merrifield resin 1, azidomethyl polystyrene 2,
imidazolium-loaded polymeric support 3, and polymer-supported
NHC–Rh complex 4.
To demonstrate the scope of the reaction, a series of
aldehydes and arylboronic acids were used for the reaction.
The results are summarized in Table 2. As can be seen from
Table 2, 75–84% yields of the products were obtained for the
addition of arylboronic acid to benzaldehyde (compounds
1–3). Then, various aromatic aldehydes were also investigated.
As expected, aldehydes with an electron-withdrawing group
such as halogeno and nitro reacted with arylboronic acids
smoothly in high yields (compounds 6–12). However, only
moderate yields were obtained when aldehydes with an electron-
donating group at the para-position or ortho-position were
employed for this addition (compounds 4 and 5).
verified using infrared spectroscopy (IR) by the appearance of
À1
the N
3
band at 2096 cm (Fig. 1). Meanwhile, the loading of
À1
N
3
group was determined to be 0.79 mmol g by means of the
nitrogen content calculated by elemental analysis.
Reaction of an excess of 3-methyl-1-propargylimidazolium
bromide using CuI and N,N-diisopropylethylamine (DIPEA)
as a catalyst system led to complete conversion of all azide
groups. Infrared spectroscopy shows a strongly absorbing
À1
n(N ) = 2096 cm band which disappears on formation of
3
the 1,2,3-triazole linker during the imidazolium grafting reaction.
The imidazolium graft is visible from appearance of a new
À1
band at 1571 cm which is present in the ionic liquid starting
Recycling studies were then performed under the optimal
reaction conditions using p-chlorobenzaldehyde and p-methoxy-
benzeneboronic acid as a model reaction. As expected, the
catalyst could be easily recovered by filtration and showed
good reusability with a slight decrease in its activity after
several runs (ESIw). An SEM observation of the recovered
catalyst after five runs was also made, and there was no
obvious change in the morphology and size in comparison
with the fresh catalyst (ESIw). Furthermore, after separation
of the catalyst, it was shown that less than 0.8% of the total
amount of the original rhodium species was lost into solution
material (Fig. 1).
The polymer-supported NHC–Rh complex formed after
2
the mixture of [Rh(COD)Cl] and the resin 3 was stirred in
t
THF for 24 h at room temperature in the presence of KO Bu,
which played a role as a base for the deprotonation of the
imidazolium group. In the IR spectra of 3, the intensities of
the bands corresponding to the imidazolium units (1161 and
À1
6
21 cm ) were sharply reduced after reaction with the
rhodium complex, suggesting the formation of the NHC–Rh
complex (Fig. 1).
Thermogravimetry analysis (TGA) and differential scanning
calorimetry (DSC) analysis of 4 were carried out to investigate
the stability of the polymer-supported NHC–Rh complex
Table 1 Optimization of the addition of phenylboronic acid to
p-nitrobenzaldehyde
a
(
ESIw). The weight loss of 4 on heating from 40 to 150 1C is
less than 2% and might be attributed to the release of
physisorbed water and loosely bound volatiles on the surface.
Thermal degradation of 4 occurred after 250 1C which implied
the stability. Meanwhile, the DSC curve shows that decom-
position of the organic structure mainly occurred in one step
from 350 to 460 1C, which is related to the main weight loss.
The peak in the DSC curve shows that the fastest loss of the
catalyst occurred at 425 1C. All these suggest that the supported
catalysts was very stable and could be used within a broad
temperature scale. The loading level of the immobilized rhodium
was measured to be 2.02% by inductively coupled plasma (ICP).
Scanning electron micrograph (SEM) was also recorded to
understand the surface of the support and catalyst (ESIw). The
particle size of the polymer-supported NHC–Rh complex 4 is
similar to that of imidazolium-loaded polymeric support 3,
Catalyst
loading/
Entry mol% Solvent
b
Time/ Yield
(%)
Temperature/1C h
1
2
3
4
5
6
7
2
2
2
2
2
2
1
Dioxane/H
Dioxane/H
Dioxane/H
2
2
2
O (4 : 1) 80
O (6 : 1) 80
O (10 : 1) 80
80
O (6 : 1) 60
O (6 : 1) 80
O (6 : 1) 80
6
6
6
6
6
3
6
91
96
95
82
78
89
82
DME/H
2
O (6 : 1)
Dioxane/H
Dioxane/H
Dioxane/H
2
2
2
a
Reaction conditions: phenylboronic acid (1.0 mmol), p-nitrobenz-
t
aldehyde (0.5 mmol), KO Bu (0.5 mmol), catalyst in 1,4-dioxane/water
b
2 ml, v/v) for a certain time. Isolated yield.
(
1
2320 Chem. Commun., 2011, 47, 12319–12321
This journal is c The Royal Society of Chemistry 2011

View Article Online
Table 2 Polymer-supported NHC–Rh complex catalyzed the addi-
tion of arylboronic acids to aldehydes
The excellent catalytic performance and easy preparation of
the catalyst make it a useful alternative to other heterogeneous
rhodium catalysts.
a
Notes and references
1
(a) F. M. Dautzenberg and P. J. Angevine, Catal. Today, 2004,
3–95, 2; (b) H. U. Blaser, A. Indolese, F. Naud, U. Nettekoven
and A. Schnyder, Adv. Synth. Catal., 2004, 346, 1583;
(c) W. A. Herrmann, K. Ofele, D. von Preysing and
S. K. Schneider, J. Organomet. Chem., 2003, 687, 229.
9
¨
2
(a) D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand,
Chem. Rev., 2000, 100, 39; (b) V. Lavallo, Y. Canac, A. DeHope,
B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2005,
4
T. Scherg, M. Muhlhofer, E. Herdtweck and W. A. Herrmann,
4, 7236; (c) G. D. Frey, C. F. Rentzsch, D. von Preysing,
¨
J. Organomet. Chem., 2006, 691, 5725; (d) J. W. Faller and
P. P. Fontaine, J. Organomet. Chem., 2006, 691, 5810;
(
e) V. Lavallo, Y. Canac, C. Prasang, B. Donnadieu and
G. Bertrand, Angew. Chem., Int. Ed., 2005, 44, 5705;
f) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290.
(
3
4
A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem.
Soc., 1991, 113, 361.
(a) S. Dı
´
ez-Gonza
009, 109, 3612; (b) N. Marion and S. P. Nolan, Acc. Chem. Res.,
008, 41, 1440; (c) F. E. Hahn and M. C. Jahnke, Angew. Chem.,
´
lez, N. Marion and S. P. Nolan, Chem. Rev.,
2
2
Int. Ed., 2008, 47, 3122; (d) E. A. B. Kantchev, C. J. O’Brien and
M. G. Organ, Angew. Chem., Int. Ed., 2007, 46, 2768; (e) E. Peris
and R. H. Crabtree, Coord. Chem. Rev., 2004, 248, 2239;
(
f) E. Peris, J. A. Loch, J. Mata and R. H. Crabtree, Chem.
Commun., 2001, 201; (g) J. Huang, E. D. Stevens and
S. P. Nolan, Organometallics, 2000, 19, 1194; (h) J. Schwarz,
a
Reaction conditions: arylboronic acid (1.0 mmol), aldehyde
t
0.5 mmol), KO Bu (0.5 mmol), catalyst (2 mol%), 1,4-dioxane/water
(
(
V. P. W. Bo
W. Hieringer and G. Raudaschl-Sieber, Chem.–Eur. J., 2000,
, 1773.
(a) J. W. Byun and Y. S. Lee, Tetrahedron Lett., 2004, 45, 1837;
b) J. H. Kim, J. W. Kim, M. Shokouhimehr and Y. S. Lee, J. Org.
¨
hm, M. G. Gardiner, M. Grosche, W. A. Herrmann,
2 ml, 6/1, v/v), 80 1C, 6 h.
6
5
6
during the course of reaction by ICP analysis. The slight
decrease of the rate may be due to the leaching level of the
catalyst and the small amount of catalyst lost during the
process of manipulation.
(
Chem., 2005, 70, 6714.
(a) Y. Yang, C. Deng and Y. Yuan, J. Catal., 2005, 232, 108;
(
b) C. P. Mehnert, R. A. Cook, N. C. Dispenziere and
M. Afeworki, J. Am. Chem. Soc., 2002, 124, 12932;
c) U. Hintermair, G. Zhao, C. C. Santini, M. J. Muldoon and
A hot-filtration experiment was run to investigate whether
the reaction proceeded in a heterogeneous or homogeneous
fashion. The catalyst was then removed by filtration after 1 h
(
¨
D. J. Cole-Hamilton, Chem. Commun., 2007, 1462; (d) I. Ozdemir,
N. Gurbuz, T. Seckin and B. Cetinkaya, Appl. Organomet. Chem.,
005, 19, 633.
¨
¨
¸
¸
2
(
yield 58%) at the reaction temperature. The filtrate was
7
(a) M. T. Zarka, M. Bortenschlanger, K. Wurst, O. Nuyken and
R. Weberskirch, Organometallics, 2004, 23, 4817; (b) C. Yan,
X. M. Zeng, W. F. Zhang and M. M. Luo, J. Organomet. Chem.,
t
treated with fresh KO Bu under the same reaction conditions
and heated for an additional 5 h (yield 67%). The reaction
continued, although the conversion did not reach the level
obtained in the normal manner, which means that at least a
part of the catalytic activity of catalyst was assigned to a
homogeneous way.
2
006, 691, 3391; (c) W. Gila, K. Boczon
J. J. Zio"kowskia, E. Garcia-Verdugob, S. V. Luisb and V. Sans,
J. Mol. Catal. A: Chem., 2009, 309, 131.
´
a, A. M. Trzeciaka,
´
8 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
Ed., 2001, 40, 2004.
(a) K. Meguro, M. Aizawa, T. Sohda, Y. Kawamatsu and
A. Nagaoka, Chem. Pharm. Bull., 1985, 33, 3787; (b) F. Tada,
K. Tanaka and K. Koshiro, Tetrahedron: Asymmetry, 1991, 2, 873;
9
In summary, we have successfully prepared a novel polymer-
supported N-heterocyclic carbene–rhodium complex via
click chemistry. The complex could catalyze the addition of
arylboronic acids to aldehydes efficiently. Meanwhile, the
catalyst could be easily recovered by simple filtration and
reused several times without a significant loss in its activity.
(
c) M. Botta, V. Summa, F. Corelli, G. DiPietro and P. Lombardi,
Tetrahedron: Asymmetry, 1996, 7, 1263.
0 (a) S. Loeber, P. Rodriguez-Loaiza and P. Gmeiner, Org. Lett.,
003, 5, 1753; (b) S. Arseniyadis, A. Wagner and C. Mioskowski,
Tetrahedron Lett., 2002, 43, 9717.
1
2
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12319–12321 12321