Polymer-supported N-heterocyclic carbene-rhodium complex catalyst for the addition of arylboronic acids to aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2006.02.021

A novel polymer-supported N-heterocylic carbene (NHC)-rhodium complex was prepared from chloromethyl polystyrene (CM PS) resin using a simple procedure. This polymer-supported NHC-rhodium complex was used as a catalyst for the addition of arylboronic acids to aldehydes affording arylmethanols in excellent yields.

Full text of DOI:10.1016/j.jorganchem.2006.02.021

Journal of Organometallic Chemistry 691 (2006) 3391–3396
www.elsevier.com/locate/jorganchem
Polymer-supported N-heterocyclic carbene–rhodium complex
catalyst for the addition of arylboronic acids to aldehydes
*
Chun Yan, Xiaoming Zeng, Weifeng Zhang, Meiming Luo
Key Laboratory of Green Chemistry and Technology of Ministry of Education at Sichuan University, College of Chemistry,
Sichuan University, No. 29 Wangjiang Road, Chengdu, Sichuan 610064, PR China
Received 17 September 2005; received in revised form 16 February 2006; accepted 16 February 2006
Available online 6 March 2006
Abstract
A novel polymer-supported N-heterocylic carbene (NHC)–rhodium complex was prepared from chloromethyl polystyrene (CM PS)
resin using a simple procedure. This polymer-supported NHC–rhodium complex was used as a catalyst for the addition of arylboronic
acids to aldehydes affording arylmethanols in excellent yields.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: N-Heterocylic carbene–rhodium complex; Polymeric support; Heterogeneous catalyst; Rhodium; Arylation
1. Introduction
tolerance towards polar substituents in the substrates and
benefit from the stability and ready accessibility of the non-
toxic boron derivatives.
Diarylmethanols are important intermediates for the
synthesis of biologically and pharmaceutically active sub-
stances [1–3]. The addition of organometallic reagents to
aldehydes has been one of the general methods for the syn-
thesis of diarylmethanols. Of these reagents, organolithium
and organomagnesium compounds are most frequently
used for this purpose, but tolerate only a few electrophilic
groups on themselves [4–11]. Examples of using other
functionalized organometallic species [12–19] such as
organocopper, organochromium, organotin, especially
organozinc, have been described. However, these organo-
metallic reagents are usually toxic and sensitive to air and
moisture. The progress that has been achieved by recent
publications describing the addition of arylboronic acid
derivatives to aldehydes in the presence of catalytic
amounts of Rh (I) and phosphine, nitrogen [20–23], espe-
cially NHC ligands [24–29] deserve particular mention.
These methods present high efficiency with a reasonable
Due to their versatile processing capabilities, ease of
product/catalyst separation, immobilized catalysts offer
many advantages for industrial applications. Although
NHCs as ligands for transition metal complexes have
found many successes in homogeneous catalysis [30,31],
including addition of arylboronic acid derivatives to alde-
hydes, heterogenizing NHC–metal complexes still remain
practically useful. The high dissociation energies of
metal–carbon bonds in NHC–metal complexes make
NHCs better ligands for heterogenous systems than others,
e.g., phosphine ligands. With these concerns, a few kinds of
immobilized NHC–metal complexes have been developed.
Polymer-supported 1,3-disubstituted-imidazoline-2-ylidene–
Pd complexes have been applied in Suzuki cross-coupling
[32–35] and Heck reactions [36]. Supported ruthenium
complexes of both 1,3-disubstituted-imidazoline-2-ylidene
and 1,3-disubstituted-imidazolidine-2-ylidene have been
used in ring-closing metathesis [30]. Immobilized rhodium
¨
complexes of NHCs are scarce. Ozdemir et al. reported a
*
silica-supported rhodium complex of 1,3-disubstituted-imi-
dazolidine-2-ylidene for the addition of phenylboronic acid
Corresponding author. Tel.: +86 28 80831949; fax: +86 28 85462021.
E-mail address: luom2@yahoo.com.cn (M. Luo).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.02.021

3392
C. Yan et al. / Journal of Organometallic Chemistry 691 (2006) 3391–3396
to aldehydes [37]. Weberskirch described the immobil-
ization of a 1,3-disubstituted-imidazoline-2-ylidene–Rh
complex to a water-soluble, amphiphilic block copolymer
for hydroformylation of alkene [38]. We report here the
preparation of a novel polymer-supported 1,3-disubsti-
tuted-imidazoline-2-ylidene–Rh complex from chloro-
methyl polystyrene, and its application in the
Rh-catalyzed addition of arylboronic acids to aldehydes.
2% and might be attributed to the release of physisorbed
water. Thermal degradation of 2 occurred at about
260 ꢁC. The loading level of the immobilized rhodium
was measured to be 0.18 mmol/g by inductively coupled
plasma-atomic emission spectrometry (ICP-AES). This
means that 29.0% of the imidazolium groups in the poly-
meric support (1) participated in the formation of Rh com-
plex. The low loading of Rh implies that the majority of the
complex formed between the Rh and imidazolium groups
may be located within the surface layer of the polymeric
support.
2. Results and discussion
The imidazolium-loaded polymeric support (1) for Rh
catalyst was prepared by treating N-(2,4,6-trimethylphe-
nyl)imidazole with CM PS resin using a simple method
(Fig. 1). Thus, a mixture of CM PS resin and an excess
amount of (2,4,6-trimethylphenyl)imidazole was agitated
in NMP at 80 ꢁC for 48 h. The loading of imidazolium
was determined to be 0.61 mmol/g by means of the nitro-
gen content obtained from elementary analysis. Since the
activity of the polymer-supported catalyst is influenced
by the swelling properties of the support in solvent, the
swelling volume of imidazolium-loaded polymeric support
(1) was measured (Table 1). Obviously, 1 has the largest
swelling volume in dioxane.
The catalytic activity of the polymer-supported NHC–
Rh complex (2) was investigated for the addition of phen-
ylboronic acid to p-chlorobenzaldehyde (Table 2). To opti-
mize reaction conditions, we surveyed several reaction
variables, such as reaction temperature, time and solvent.
As shown in Table 2, excellent yield (95%, entry 12, Table
2) was successfully obtained with lower catalyst loading
(0.35 mol%). Higher temperature (90 ꢁC, entry 5, Table 2)
and longer reaction time were generally favorable to the
reaction (48 h, entry 7, Table 2). The reaction in a co-sol-
vent consisting of 1,4-dioxane and water showed better
results (entries 2 versus 8 and 9, Table 2). Furthermore,
with decrease of the amount of water, the yield of the reac-
tion increased greatly from 39.2% to 95% (entries 2, 10–12,
Table 2), presumably due to the best swelling properties of
the imidazolium-loaded polymeric support (1) in 1,4-diox-
ane (Table 1). However, poor result obtained when the
reaction was preformed in 1,4-dioxane alone (entry 13,
Table 2).
The complex
2
formed after the mixture of
[Rh(COD)Cl]2 and the resulting resin (1) was shaken in
THF for 24 h at room temperature in the presence of
KOtBu, which played a role as a base for the deprotonation
of the imidazolium group. FT-IR spectrum of 2 is obvi-
ously different from that of CM PS resin. An absorption
at 3398 cmꢀ1 indicates the presence of physisorbed H2O.
In addition, there are NCN stretching bands at
1545 cmꢀ1 and an imide ring stretching vibration at
1379 cmꢀ1. Thermogravimetry analysis (TGA) showed
the weight loss on heating from 40 to 150 ꢁC is less than
An assessment of the recycling potential of the polymer-
supported NHC–Rh catalyst was made, using phenylbo-
ronic acid and p-chlorobenzaldehyde as substrates. We
found that important factor affecting the stability and
activity of the polymer-supported NHC–Rh catalyst 2
Fig. 1. Synthesis of imidazolium-loaded polymeric support (1) and polymer-supported NHC–Rh complex (2).

C. Yan et al. / Journal of Organometallic Chemistry 691 (2006) 3391–3396
3393
Table 1
Swelling volume in solvent (mL/g polymeric support) of the imidazolium-loaded polymeric support (1)
Dry
1.8
Water
2.2
DME
3.8
1,4-Dioxane
4.5
THF
4.0
MeOH
2.4
Table 2
Addition of phenylboronic acid to 4-chlorobenzaldehyde catalyzed by the NHC–Rh complex (2)a
OH
Rh complex
B(OH)2
CHO
(2)
+
KOt Bu
Cl
Cl
Entry
Solvent (v:v)
Temperature (ꢁC)
Time (h)
Yield (%)b
1c
2
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
1,4-Dioxane/H2O (4:1)
DME/H2O (4:1)
80
80
80
70
90
80
80
80
80
80
80
80
80
80
80
24
24
24
24
24
12
48
24
24
24
24
24
24
24
24
70
88
89
77
89
62
89
71
33
40
92
95
63
82
30
3d
4
5
6
7
8
9
THF/H2O (4:1)
10
11
12
13
14e
15f
a
1,4-Dioxane/H2O (2:1)
1,4-Dioxane/H2O (10:1)
1,4-Dioxane/H2O (100:1)
1,4-Dioxane
1,4-Dioxane/H2O (100:1)
1,4-Dioxane/H2O (100:1)
All of reactions were carried out with phenylboronic acid (1 mmol), p-chlorobenzaldehyde (0.5 mmol), KOtBu (0.5 mmol), and catalyst 2 (0.35 mol%
Rh).
b
c
d
e
f
Isolated yields were based on aldehydes.
Catalyst 2 (0.18 mol% Rh).
Catalyst 2 (1.06 mol% Rh).
[Rh(COD)Cl]2 (0.18 mol% based on aldehyde) and 10 mg imidazolium-loaded polymeric support (1) were added instead of 2.
[Rh(COD)Cl]2 (0.18 mol% based on aldehyde) was added instead of 2.
was the variety of solvent, the ratio of 1,4-dioxane to water
(Table 3). High yield of secondary alcohol was obtained for
the first run when the volume ratio of 1,4-dioxane to water
was 100:1. However, the yield reduced to only about 25%
in the second run, suggesting the catalyst is unstable in this
solvent. With the increasing of water content in the mixture
solvent, the catalyst stability increased, but catalytic activ-
ity decreased markedly. The balance between activity and
stability reached when the volume ratio of 1,4-dioxane to
water was 4:1. After simple filtration and washing, the
recovered polymer-supported NHC–Rh catalyst was
reused 6 times under the same reaction conditions as for
the first run and still remained good activity.
Table 4 summarizes the addition of other arylboronic
acids to various aldehydes catalyzed by polymer-supported
NHC–Rh complex (2). The results reveal the wide scope of
this method that is compatible with nitro, cyano, chloro
and ether groups in aldehydes. Aldehydes with an elec-
tron-withdrawing group such as chloro, nitro, cyano
reacted with arylboronic acids cleanly in excellent yields.
Table 3
Recycling of 2 in the addition of phenylboronic acid to p-chlorobenzaldehydea
Entry
Dioxane/H2O (v/v)
Yield (%)
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
1
2
3
4b
5
a
100:1
10:1
4:1
4:1
2:1
95
92
88
89
36
24.5
55
83
88
38
–
–
–
–
–
85
–
–
–
–
83
–
35.8
80
88
35
19.3
77
86
33
All of reactions were carried out with phenylboronic acid (5.0 mmol), p-chlorobenzaldehyde (2.5 mmol), KOtBu (2.5 mmol), catalyst 2 (0.35 mol% Rh)
at 80 ꢁC.
b
Catalyst 2 (1.0 mol% Rh) was used.

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C. Yan et al. / Journal of Organometallic Chemistry 691 (2006) 3391–3396
Table 4
Rh-catalyzed addition of arylboronic acids to aldehydesa
Entry
1
Boronic acid
C6H5B(OH)2
Aldehyde
Time (h)
16
Product
OH
Yield (%)b
84
C6H5CHO
2
3
p-Cl-C6H4CHO
p-NO2-C6H4CHO
p-CN-C6H4CHO
24
10
89
91
OH
OH
Cl
NO2
4
5
6
94
83
OH
OH
CN
40
OHC
OHC
O
O
O
O
OH
6
7
30
24
85
92
OCH2Ph
OCH2Ph
Cl
p-Me-C6H4B(OH)2
p-Cl-C6H4CHO
OH
H3C
OH
8
9
p-NO2-C6H4CHO
p-CN-C6H4CHO
10
6
93
95
86
95
H3C
H3C
NO2
CN
OH
10
11
a
36
6
OH
OH
OHC
O
O
O
O
H3C
p-MeO-C6H4B(OH)2
p-Cl-C6H4CHO
Cl
MeO
All of reactions were carried out with phenylboronic acid (1.0 mmol), aldehyde (0.5 mmol), Rh–NHC complex (2) (1.0 mol% Rh based on aldehyde),
KOtBu (0.5 mmol) in 1,4-dioxane/water (4:1, v:v), 80 ꢁC. All reactions were monitored by TLC.
b
Isolated yields were based on aldehyde.
Electron-donating group in aldehyde decreased the yield
compared with the electron-withdrawing one (entries 5, 6
versus 2, 3, 4, Table 4).
in the range 500–4000 cmꢀ1 on P-E FT-IR-16RC spec-
trometer. NMR spectra were recorded on a Varian
INOVA 400 MHz or Bruker AC-E 200 MHz or Bruker
Avance 600 MHz NMR spectrometer. Mass spectra exper-
iments were conducted on Bruker Daltonics Data Analysis
3.2. Thermogravimetry analysis (TGA) and differential
scanning calorimetry (DSC) were performed with Netzsch
Sta 449C thermal analyzer. Nitrogen content was deter-
mined on Carlo Erba 1106 analyzer. Rhodium content
was determined by inductively coupled plasma-atomic
emission spectrometry (ICP-AES) on IRIS Adv.
3. Experimental
3.1. General
Manipulations were performed using standard Schlenk
techniques under an Ar atmosphere and with previously
dried solvents. FT-IR spectra were recorded as KBr pellets

C. Yan et al. / Journal of Organometallic Chemistry 691 (2006) 3391–3396
3395
3.2. Procedure for the preparation of imidazolium-loaded
polymeric support (1)
reaction mixture was filtered, the polymeric support was
washed vigorously with ether (10 mL · 3), and then, the fil-
trate and the washing solution were combined. The solvents
were evaporated and the residue was purified by flash chro-
matography to give the corresponding alcohol.
Imidazolium-loaded polymeric support (1) was prepared
from commercially available CM PS resin cross-linked with
1% DVB (100–200 mesh). A mixture of CM PS resin (1.0 g,
2.3 mmol Cl/g polymeric support) and (2,4,6-trimethylphe-
nyl)imidazole (2.0 g, 10.7 mmol) in NMP (120 mL) was
slowly agitated at 80 ꢁC for 48 h. After cooling to room tem-
perature, the reaction mixture was filtered, andthe polymeric
support was washed with NMP (20 mL · 3), 0.1 N aq. HCl
(20 mL · 3), and methanol (20 mL · 3) followed by drying
under reduced pressure to give 1. The loading of the imidazo-
lium was determined to be 0.61 mmol/g by means of the
nitrogen content obtained from elementary analysis.
4-(Phenylmethoxyl)diphenylmethanol: white solid, mp:
1
56–57 ꢁC. H NMR (CDCl3, 400 MHz): d 7.44–7.25 (m,
12H, phenyl-CH), 6.96–6.93 (m, 2H, phenyl-CH), 5.81 (s,
1H, CHOH), 5.06 (s, 2H, OCH2), 2.20 (s, br, 1H, OH).
13C NMR (CDCl3, 100 MHz): d 158.2, 144.0, 136.9,
136.4, 128.5, 128.4, 127.9, 127.4, 127.3, 126.4, 114.8, 75.7,
70.0. HRMS (ESI): m/z calcd. for C20H18O2 [M+Na]+:
313.3515. Found 313.1199.
1,3-Benzodioxol-5-yl-40-methyldiphenylmethanol: white
solid, mp: 48–49 ꢁC. 1H NMR (CDCl3, 400 MHz): d
7.25–7.20 (m, 2H, phenyl-CH), 7.16–7.10 (m, 2H, phenyl-
CH), 6.83–6.80 (m, 2H, phenyl-CH), 6.79–6.73 (m, 1H,
phenyl-CH), 5.90 (m, 2H, OCH2O), 5.70 (s, 1H, CHOH),
2.32(s, 3H, CH3) 2.22 (s, br, 1H, OH). 13C NMR (CDCl3,
100 MHz) d 147.6, 146.7, 140.9, 138.2, 137.0, 129.0, 126.2,
119.8, 107.9, 107.1, 100.8, 75.6, 21.0. MS (ESI): m/z 265.1
[M+Na]+.
Other addition products, diphenylmethanol [11,39],
4-chlorodiphenylmethanol [39], 4-nitrodiphenylmethanol
[40], 4-cyanodiphenylmethanol [11,41], 1,3-benzodioxol-5-yl-
phenylmethanol [42], 4-chloro-40-methyldiphenylmethanol
[41], 4-methyl-40-nitrodiphenylmethanol [43], 4-cyano-40-
methyldiphenylmethanol [41], 4-chloro-40-methoxyldi-
phenylmethanol [44,45] have been identified by comparing
their 1H NMR spectra and melting points with that
reported in the literature.
3.3. Procedure for measuring the swelling volumes of the
imidazolium-loaded polymeric support (1)
The swelling volumes of the imidazolium-loaded poly-
meric support in various solvents were measured in a frit-
ted column (ID 1.0 cm, length 10 cm). The polymeric
support (1.0 g) was swollen in a solvent at room tempera-
ture for 4 h, and then washed with a 10-fold volume of each
solvent. After filtering out the solvent, the volume of the
polymeric support was measured.
3.4. Preparation of the polymer-supported NHC–Rh
complex (2)
[Rh(COD)Cl]2 (30 mg 0.12 mmol) was added to a
Schlenk tube along with 3 equivalents of KOtBu. The ves-
sel was evacuated and flushed with Ar three times. THF
(30 mL) was syringed in and stirred at room temperature
for 2 h before the imidazolium-loaded polymeric support
(1) (100 mg, 0.61 mmol/g) was added. The mixture was stir-
red at room temperature for 24 h. After filtration, the poly-
meric support was washed vigorously with THF
(10 mL · 5) and CH2Cl2 (10 mL · 5) to give 2. To measure
the amount of Rh loaded on the polymeric support, the
polymeric support (10 mg) was treated with a mixture
(5 mL) of hydrochloric acid and nitric acid (1:1, v/v) at
room temperature for 4 h. After the orange-colored solu-
tion was filtered, the polymeric support was washed with
distilled water (2.5 mL · 2), and then, the filtrate and the
washing solution were combined to determine the amount
of Rh by ICP-AES. The loading level of immobilized rho-
dium was measured to be 0.18 mmol/g.
4. Conclusion
In summary, a novel polymer-supported NHC–Rh com-
plex was prepared using a simple procedure. The polymer-
supported NHC–Rh complex showed high catalytic activ-
ity in the addition of arylboronic acids to aldehydes giving
diarylmethanols in satisfactory yields.
Acknowledgment
This work was financially supported by the National
Natural Science Foundation of PR China (Project No:
20372049).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2006.02.021.
3.5. Rhodium-catalyzed addition of arylboronic acids to
aldehydes
Arylboronic acid (1 mmol), KOtBu (0.5 mmol), aldehyde
(0.5 mmol), catalyst (2) (Rh 5 lmol based on aldehyde) and
1,4-dioxane (5 mL) were introduced into a Schenk tube and
then water (1.25 mL) was added. The resulting mixture was
heated at 80 ꢁC. After cooling to room temperature, the
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