Polymer-supported, carbon dioxide-protected n-heterocyclic carbenes: Synthesis and application in organo- and organometallic catalysis

Article abstract of DOI:10.1002/adsc.200900658

The synthesis of a resin-supported, carbon dioxide-protected N-heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin-bound carbon dioxide-protected NHC-based catalyst was prepared via ring-opening

Full text of DOI:10.1002/adsc.200900658

FULL PAPERS
DOI: 10.1002/adsc.200900658
Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic
Carbenes: Synthesis and Application in Organo- and
Organometallic Catalysis
Gajanan Manohar Pawara and Michael R. Buchmeisera,b,*
a
Leibniz-Institut fꢀr Oberflꢁchenmodifizierung, Permoserstr. 15, 04318 Leipzig, Germany
Fax : (+49)-(0)341-235-2584; phone: (+49)-(0)341-235-2229
Institut fꢀr Polymerchemie, Lehrstuhl fꢀr Makromolekulare Stoffe und Faserchemie, Universitꢁt Stuttgart,
b
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax : (+49)-(0)711-685-64050; (+49)-(0)711-685-64075; e-mail: michael.buchmeiser@ipoc.uni-stuttgart.de
Received: September 22, 2009; Revised: February 11, 2010; Published online: March 12, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900658.
Abstract: The synthesis of a resin-supported, carbon sulting Rh-, Ir-, and Pd-loaded resins were success-
dioxide-protected N-heterocyclic carbene (NHC) fully used in the polymerization of phenylacetylene,
and its use in organocatalysis and organometallic cat- in the hydrogen transfer reaction to benzaldehyde, as
alysis are described. The resin-bound carbon diox- well as in Heck-type coupling reactions. In the latter
ide-protected NHC-based catalyst was prepared via reaction, TONs up to 100,000 were achieved. M1, as
ring-opening
1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethano-
naphthalene (DMNH6) with 3-(bicyclo[2.2.1]hept-5- pyrimidin-2-ylidene]2 (Pd-1) and IrBr[1-(norborn-5-
en-2-ylmethyl)-1-(2-propyl)-3,4,5,6-tetrahydropyrimi- ene-2-ylmethyl)-3-(2-Pr)-3,4,5,6-tetrahydropyrimidin-
din-1-ium-2-carboxylate (M1), using the well-defined 2-ylidine](COD) (Ir-1) were used as homogeneous
Schrock catalyst Mo[N-2,6-(2-Pr)2-C6H3] analogues and their reactivity in the above-men-
(CHCMe2Ph)(OCMe3)2 and was used for a series of tioned reactions was compared with that of the sup-
metathesis
copolymerization
of a non-supported analogue of poly-M1-b-DMNH6, as
well as the complexes PdCl2A[1,3-bis(2-Pr)tetrahydro-
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
organocatalytic reactions, i.e., for the trimerization ported catalytic systems. In all reactions investigated,
reaction of isocyanates, as well as for the cyanosilyla- the TONs achieved with the supported systems were
tion of carbonyl compounds. In the latter reaction, very similar to the ones obtained with the unsupport-
turn-over numbers (TON) up to 5000 were achieved. ed, homogeneous ones, the turn-over frequencies
In addition, the polymer-supported, carbon dioxide- (TOFs), however, were lower by up to a factor of
protected N-heterocyclic carbene served as an excel- three.
lent progenitor for various polymer-supported metal
complexes. It was loaded with a series of rhodium(I), Keywords: Heck reaction; heterogeneous catalysis;
iridium(I), and palladium(II) precursors and the re- metathesis; organocatalysis; polymerization
Introduction
are often more economical and environmentally
friendly. Within that context, NHCs have been report-
The chemistry of N-heterocyclic carbenes (NHCs) has ed to be active organocatalysts for a number of reac-
attracted attention because of the rapid development tions, such as transesterifications/amidations,[22] the
of general synthetic methods and wide applications in living ring-opening polymerization of cyclic esters,[23]
the field of molecular catalysis. The close analogy of the trimerization of isocyanates,[7] as well as for ben-
NHCs to trialkylphosphanes and their excellent s-do- zoin condensation,[24] sila-Stetter,[25] trifluoromethyla-
nating properties make them ligands for transition tion,[26] and cyanosilylation reactions.[27]
metals[1–7] and potent nucleophilic organic catalysts
It is worth noting that the manipulation of the free
for a wide variety of organic transformations.[8–21] carbenes is often difficult due to their highly reactive
Metal-free catalyzed processes are interesting alterna- nature as well as air and moisture sensitivity. Recent-
tives to classical organic transformations since they ly, the use of CO2 adducts of NHCs attracted much
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attention in organocatalysis as well as in synthesis of
ACHTUNGTREN[NUNG 2.2.1]hept-5-en-2-ylmethyl-1-(2-propyl)-3,4,5,6-tetra-
the corresponding NHC-metal complexes.[6,7,28] In this hydropyrimidin-1-ium bromide (3) was accessible by
article, we describe the synthesis and application of a the reaction of 1 with 2 in 90% yield (Scheme 1).
polymer-supported, CO2-protected N-heterocyclic car- Compound 3 was finally converted into the corre-
bene. It was prepared via ring-opening metathesis co- sponding CO2-protected monomer M1 by treatment
polymerization of 1,4,4a,5,8,8a-hexahydro-1,4,5,8- with potassium tert-butoxide followed by the reaction
exo,endo-dimethanonaphthalene
[2.2.1]hept-5-en-2-ylmethyl)-1-(2-propyl)-3,4,5,6-tetra-
hydropyrimidin-1-ium-2-carboxylate, using the well-
defined Schrock catalyst Mo[N-2,6-(2-Pr)2-C6H3] Homopolymerization
(CHCMe2Ph)(OCMe3)2. The resin-bound, NHC-
with
3-(bicyclo- with dry CO2.
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
based catalyst was then used for a series of organoca- In order to determine the ring-opening metathesis
talytic reactions, namely, for the trimerization reac- polymerization (ROMP) propensity of M1 with the
tion of isocyanates, as well as for the cyanosilylation Schrock
initiator
MoACHTGNURTEN[NUG N-2,6-ACHTNUGRTEN(NNUG 2-Pr)2-C6H3]
of carbonyl compounds. Heating of polymer-support- (CHCMe2Ph)
N
ACHTUNGTRNE(NUNG OCMe3)2, the corresponding homopo-
ed CO2-protected, N-heterocyclic carbene in presence lymerization was carried out in CHCl3 at room tem-
of Rh(I), Ir(I), and Pd(II) precursors resulted in the perature. The homopolymer poly(M1) was isolated in
release of CO2 and formation of the polymer-support- 95% yield (Scheme 2). The number average molecu-
1
ed Rh(I), Ir(I), and Pd(II) catalysts. These were used lar weight (Mn) as determined by H NMR-based end
for series of catalytic reactions such as polymerization group analysis was 4100 gmolÀ1 [Mn(calc)=5500 gmolÀ1]
of phenylacetylene, hydrogen transfer to benzalde- and the polydispersity index (PDI) as determined by
hyde, and Heck coupling reactions, respectively.
GPC of poly(M1) was PDI=1.26. In addition, the
polymerization kinetics of M1 were determined (Fig-
ure S1, Supporting Information). As can be seen, the
polymerization of M1 followed 1st order kinetics, sug-
gesting a living polymerization setup. Within 20 min a
degree of polymerization (DP) of 12 could be realiz-
ed.
Results and Discussion
Synthesis of Monomer M1
The synthesis of CO2-protected N-heterocyclic car-
bene-based monomer is outlined in Scheme 1. 1-(2-
Propyl)-1,4,5,6-tetrahydropyrimidine(1) was synthe- Synthesis of Polymer-Supported CO2-Protected N-
sized via reaction of N-(2-propyl)propane-1,3-diamine Heterocyclic Carbenes
with 1,1-dimethoxy-N,N-dimethylmethanamine.[29] 5-
(Bromomethyl)bicycloACTHNUGRTNEUNG[2.2.1]hept-2-ene (2) was syn- The conventional protocol for the preparation of
thesized from cyclopentadiene and allyl bromide.[30] polymer-supported ligands usually entails the surface
The corresponding NHC precursor 3-bicyclo- modification of commercially available polymer sup-
Scheme 1. Synthesis of monomer M1 and Ir-1 and structure of Pd-1.
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Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes
Scheme 2. Polymerization of monomer M1.
ports, for example, polystyrene-divinylbenzene (PS- 1,4,5,8-exo-endo-dimethanonaphthalene
DMNH6
DVB) or chloromethylated PS-DVB (Merrifield poly- (Scheme 3). Monomer M1 was polymerized in a living
mer).[31] These synthetic routes, however, are charac- manner by the Schrock catalyst Mo
[N-2,6-(2-Pr)2-
terized by some disadvantages. To achieve a maxi- C6H3](CHCMe2Ph)(OCMe3)2 (vide supra). In
A
ACHTUNGTRENNUNG
A
R
a
mum derivatization capacity (usually expressed in second step, the living polymer chains were cross-
mmol of functional group/g of resin), porous materials linked using DMNH6. Based on the kinetic measure-
with high surface areas have to be chosen. As a major ments, a time span of 20 min was chosen that ensured
part of the specific surface area (s) results from small for a complete polymerization of M1 prior to the ad-
pores, large amounts of the desired ligand are located dition of the cross-linker. As a consequence of the
inside the particle. This leads to a diffusion-controlled polymerization order, the linear polymer chains bear-
reaction during catalysis which usually significantly ing the CO2-protected NHC groups form tentacles
reduces the overall reaction rate constant.[32] In addi- which are attached to the surface of the cross-linked
tion, the synthetic protocol consists of at least two or carrier. The polymeric resin that was obtained had a
three steps. Since it is difficult to achieve each step in mean particle diameter of 40Æ10 mm. The particles
a quantitative way, ill-defined supports are obtained.
consisted of agglomerates of smaller particles 5–
Some years ago, we described a powerful tool for 10 mm in diameter. The cross-linked material showed
the preparation of functionalized polymeric supports significant swelling propensity, which amounted to 20
via ROMP.[33–42] Following this concept, we prepared and 30% in THF and CH2Cl2, respectively. Used in
polymer-supported, CO2-protected N-heterocyclic car- heterogeneous catalysis, the resin is best used in
benes from the CO2-protected, NHC based monomer stirred reactors and can easily be removed by filtra-
M1 and a cross-linker, i.e., 1,4,4a,5,8,8a-hexahydro- tion.
Scheme 3. Synthesis of a polymer-supported, CO2-protected NHC-containing resin.
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Scheme 4. Synthesis of resin-bound Rh, Ir and Pd catalysts.
Synthesis of Rh-, Ir-, and Pd-Loaded Resins
es. Upon heating, CO2 evolves and the free NHC is
formed which then catalyzes the cyanosilylation[27] of
Rh-, Ir-, and Pd-loaded resins were synthesized by the carbonyl compounds .
reaction of [Rh(COD)Cl]2, [Ir(COD)Cl]2 and PdCl2 To examine the catalytic performance of the poly-
A
ACHTUNGTRENNUNG
with the polymer-supported, CO2-protected N-hetero- mer-supported, CO2-protected NHC for cyanosilyla-
cyclic carbene (Scheme 4). The Rh, Ir and Pd con- tion, reactions between various carbonyl compounds
tents were determined by ICP-OES after dissolving including aldehydes and ketones with TMSCN were
samples in aqua regia and were found to be 22 mg/g carried out in THF at 658C. Product quantification
(0.21 mmol/g), 46 mg/g (0.24 mmol/g) and 85 mg/g was accomplished by GC-MS. A summary of the re-
(0.80 mmol/g) polymer, respectively.
sults is given in Table 1. As can be seen, the catalyst
showed excellent reactivity in these reactions. Thus,
using a catalyst loading of 0.1–0.02 mol% of NHC
with respect to the corresponding carbonyl com-
pound, turn-over numbers (TONs) in the range of
Cyanosilylation of Carbonyl Compounds
The cyanosilylation of carbonyl compounds is an effi- 1000–5000 were achieved. Yields were in the 95–
cient method for the synthesis of cyanohydrins 100% range within 20–90 min. All cyanohydrins were
(Scheme 5), which can be readily converted into other isolated with high purity after simple filtration. In
useful compounds such as a-hydroxy acids, a-hydroxy case the reactions were run with the soluble (mono-
aldehydes, a-amino alcohols and 1,2-diols.[43,44] It is meric) analogue, i.e., with M1, similar TONs were ob-
known that Lewis acids catalyze the cyanosilylation of tained, however, reactions were faster by up to a
aldehydes via activation of the aldeydes,[45,46] while factor of three.
Lewis bases catalyze the reaction via activation of the
reagent, i.e., TMS-CN.[47,48] Here, we used the poly-
mer-supported, CO2-protected NHC for these purpos- Trimerization of Isocyanates
The polymer-supported, CO2-protected N-heterocyclic
carbenes were also used in the trimerization of isocya-
nates. For this purpose, a variety of aromatic isocya-
nates were used (Scheme 6). A summary of the re-
sults is given in Table 2. The CO2-protected, NHC-
containing resin showed excellent catalytic perfor-
mance allowing for the synthesis of the corresponding
Scheme 5. Cyanosilylation of carbonyl compounds catalyzed
by a polymer-supported, CO2-protected NHC catalyst.
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Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes
Table 1. Results of the cyanosilylation of carbonyl compounds catalyzed by the polymer-supported, CO2-protected NHC cat-
alyst.
Poly(M1-b-DMNH6)
M1
Entry Carbonyl compound
Product
Time Conversion TON/
Time Conversion TON/
[min] [%][b]
TOF
[min] [%][b]
TOF
[hÀ1]
[hÀ1]
5000/
15000
4800/
19200
1
2
3
20
20
60
100[c]
100[c]
99[c]
15
–
96[c]
5000
4950
–
–
–
–
–
5000/
15000
4850/
19400
4
20
100[c]
15
97[c]
5
6
30
40
100[c]
98[c]
5000
4900
–
–
–
–
–
–
1000/
1500
7
40
100[a]
30
99[a]
990/1980
8
90
90
60
40
90
100[a]
100[a]
100[a]
100[a]
100[a]
1000/670 30
1000/670 30
96
99[a]
–
960/1920
9
990/1980
10
11
1000
1000
–
–
–
–
–
12
1000/670 30
99[a]
990/1980
[a]
0.1 mol% of CO2-protected NHC with respect to the carbonyl compounds. Reactions were run at 658C temperature.
GC-MS conversions.
0.02 mol% of CO2-protected NHC with respect to the carbonyl compounds.
[b]
[c]
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plished by simply dissolving the reaction mixture in
CH2Cl2, followed by filtration and washing. No by-
1
products were observed by H and 13C NMR. Again,
similar TONs were obtained in cases where the reac-
tions were run with the soluble (monomeric) ana-
logue, i.e., with M1.
Scheme 6. Trimerization of isocyanates catalyzed by the
polymer-supported, CO2-protected NHC catalyst.
Heck Reactions
isocyanurates. Using a catalyst loading of 1.2 mol% of
NHC with respect to the isocyanate, pure products The Heck reaction of aryl halides with terminal ole-
were isolated in the range of 75–92% within 60– fins is among the most important Pd-catalyzed trans-
90 min. Purification of the isocyanurates was accom- formations in organic synthesis and a favorite test re-
Table 2. Results of the trimerization of isocyanates catalyzed by the polymer-supported, CO2-protected NHC catalyst.
Poly(M1-b-DMNH6)
Time [min]
Yield [%][b]
M1
Entry
Ar in Ar-NCO
phenyl
Product
Time [min]
Yield [%][b]
1
60
90
92[a]
20
98[a]
2
3
2-F-C6H4
85[a]
–
–
4-F-C6H4
60
83[a]
60
85[a]
4
3-Me-C6H4
60
87[a]
–
–
5
4-Me-C6H4
60
91[a]
–
–
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Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes
Table 2. (Continued)
Poly(M1-b-DMNH6)
Time [min]
Yield [%][b]
M1
Entry
Ar in Ar-NCO
4-Cl-C6H4
Product
Time [min]
Yield [%][b]
6
90
75[c]
90
72[c]
7
4-MeO-C6H4
90
78[c]
90
65[c]
[a]
1.2 mol% of CO2-protected NHC with respect to the isocyanate were used. Reactions were run at 658C.
Isolated yields.
1.3 mol% of CO2-protected NHC with respect to the isocyanate were used.
[b]
[c]
action for new palladium pre-catalysts (Scheme 7).[49] catalyst showed excellent reactivity. Thus, using a cat-
For checking the efficiency of the resin-bound Pd- alyst loading of 0.1 mol% of Ir with respect to benzal-
NHC catalysts, we carried out a series of test reac- dehyde, a TON of 980 as well as a TOF of 250 hÀ1
tions of styrene and butyl acrylate with various aryl were achieved within four hours. Iridium leaching was
bromides in DMF at 1508C. A summary of the results determined by means of ICP-OES and was found to
is given in Table 3. As can be seen, the Pd-loaded be <50 ppm, which translates into an Ir-leaching of
resin showed excellent reactivity for these kinds of re- <3%. For comparison, the same amount of Ir present
actions and turn-over numbers (TONs) up to 100,000 in a soluble catalyst, i.e., in Ir-1, gave a TON of 800
were achieved without any optimization of the reac- and a TOF of 200 hÀ1.
tion conditions. Pd-leaching to the products was
checked by means of ICP-OES and was found to be
<0.1 ppm, that is, below the limit of detection. For Polymerization of Phenylacetylene
purposes of comparison, reactions were run in the
presence of Pd-1 (Scheme 1). Similar TONs were ob- The use of the Rh-loaded heterogeneous catalyst also
tained, however, the reaction proceeded faster with allowed for the quantitative polymerization of phenyl-
Pd-1 by up to a factor of three.
acetylene to form poly(phenylacetylene) (PPA,
Scheme 9). For this purpose, the Rh-loaded resin was
added to the solution of phenylacetylene in CHCl3
and stirred for 2 hour. Then, the reaction mixture was
passed through a small bed of celite to remove the
Hydrogen Transfer to Benzaldehyde
The catalytic efficiency of the Ir-loaded resin was catalyst and polymer was precipitated by the addition
studied in the hydrogen transfer reaction to benzalde- of pentane. The number average molecular weights
hyde (Scheme 8). Benzadehyde, K2CO3 were mixed (Mn) and polydispersity indices (PDI) observed for
with 2-propanol and the reaction mixture was re- PPA were 8400 gmolÀ1 [Mn(calc)=10200 gmolÀ1], PDI=
1
fluxed in the presence of the Ir-loaded resin. Product 2.45. According to the H and 13C NMR spectra, a cis-
conversion was monitored by GC-MS. The Ir-loaded transoidal PPA was obtained. Rh-leaching to the
Scheme 7. Heck coupling reactions catalyzed by a resin-bound Pd catalyst.
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Table 3. Results of the Heck type reaction catalyzed by resin-bound Pd catalyst.[a]
Resin-Bound Pd Caatalyst
Pd-1
Entry CH2=
Ar-Br
Pd
Time Conversion
TON/TOF Time Conversion
TON/TOF
[hÀ1]
[%][b]
[hÀ1]
[h]
[%][b]
À
CH R
[mol%] [h]
1
2
styrene
styrene
bromobenzene
4-Br-acetophenone
0.02
0.01
0.002
36
20
14
45
2260
–
5
–
100
–
100
100[d]
100
100
87
10000
50000/10000
10000[c]
50000
3
4
5
styrene
styrene
4-Br-nitrobenzene
4-Br-benzonitrile
0.01
0.001
0.001
20
24
4
10000
–
7
2
–
94
100
–
87000/3630
100000/
25000
94000/13400
100000/
50000
butyl ac- 4-Br-acetophenone
rylate
100
6
butyl ac- 4-Br-benzonitrile
rylate
0.001
0.01
4
100
100000/
25000
1800
2
98
98000/49000
7
8
styrene
styrene
4-Br-toluene
1-bromo-4-methoxy- 0.01
benzene
27
29
18
17
–
–
–
–
–
–
1700
[a]
[b]
[c]
Reactions were run at 1508C in DMF.
Conversion was determined by GC-MS.
Reuse of catalyst.
Scheme 8. Hydrogen transfer reaction to benzaldehyde catalyzed by a resin-bound Ir catalyst.
CO2 protected N-heterocyclic carbene. These metal-
loaded resins were found to be excellent catalysts for
Heck coupling reactions, the hydrogen transfer reac-
tion to benzaldehyde, and for the polymerization of
phenylacetylene. However, it has to be stated that
there was significant metal leaching from the support-
ed Ir and in particular from the supported Rh cata-
lyst.
Scheme 9. Polymerization of phenylacetylene catalyzed by a
resin-bound Rh catalyst.
products was determined by means of ICP-OES and
was found to be 715 ppm, which translates into a rho-
dium leaching of 4.5%.
Experimental Section
General Methods
Conclusions
All manipulations were performed under an N2 atmosphere
in a glove box (MBraun LabMaster 130, MBraun, Garching,
Germany) or by standard Schlenk techniques unless stated
In conclusion, a polymer-supported, CO2-protected
N-heterocyclic carbene has been successfully synthe- otherwise. Purchased starting materials were used without
any further purification. Pentane, diethyl ether, toluene,
CH2Cl2 and tetrahydrofuran (THF) were dried using a sol-
vent drying system (SPS, MBraun, Garching, Germany).
Benzene, n-hexane and dimethoxyethane (DME) were
dried and distilled from Na/benzophenone ketyl under
argon. NMR spectra were recorded on a Bruker Avence 600
(600.25 MHz for proton and 150.93 MHz for carbon) and
Bruker Avence 250 (250.13 MHz for proton and 62.90 MHz
for carbon) spectrometer, respectively, at room temperature
unless stated otherwise. Proton and carbon spectra were ref-
sized via ring-opening metathesis copolymerization of
a CO2-protected NHC containing monomer (M1) and
the cross-linker DMNH6. The resin-bound NHC cata-
lyst was found to be a highly efficient catalyst in the
cyanosilylation of carbonyl compounds allowing for
TONs of 1000–5000 within 20–90 min. The resin-
bound NHC catalyst was also successfully used in the
cyclotrimerization of isocyanates. Rh-, Ir-, and Pd-
loaded resins were synthesized via immobilization of
Rh, Ir, and Pd precursors to the polymer-supported, erenced to the internal solvent resonance and are reported
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Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes
in ppm relative to tetramethylsilane. Deuterated NMR sol-
vents (C6D6 and THF-d8) were dried and distilled from Na/
benzophenone ketyl, dichloromethane-d2 and chloroform-d1
were dried and distilled from CaH2. Phenyl isocyanate, 4-
methoxyphenyl isocyanate, 2-fluorophenyl isocyanate, p-
toluyl isocyanate, o-toluyl isocyanate, 4-chlorophenyl isocya-
nate and 4-fluorophenyl isocyanate were purchased from
Aldrich (Germany), dried over P2O5, and degassed prior to
20.74 mmol) was dissolved in THF (8.0 mL) and added to
this suspension. The reaction mixture was stirred for 14 h
and then filtered through celite to remove residual salts. The
filtrate was transferred to a 250-mL Schlenk flask and re-
moved from the glove box. CO2 was bubbled through the
solution at 08C for 60 min, during this time a white precipi-
tate formed. The solvent was removed under vacuum, and
the residue was washed with diethyl ether and dried; yield:
3.3 g (75%). 1H NMR (CDCl3): d=0.64 (m, 1H, NBE),
1.12–1.56 (m, 8H, CH3 +NBE), 1.91 and 2.05 (m, 3H,
NBE), 2.45–2.93 (m, 3H, CH2 +NBE), 3.02–3.64 (m, 6H,
CH2), 4.45 (sept, 1H, CH), 6.00 and 6.19 (m, 2H, NBE);
13C NMR (CDCl3): d=19.4 (d), 19.8 (d), 20.4 (d), 30.0, 30.5,
36.6, 37.7, 38.0, 38.4, 42.3 (d), 44.0, 44.5, 45.0, 45.4, 49.7 (d),
54.2, 56.2, 58.5, 59.2, 131.6 (d), 138.7 (d), 159.1, 161.6. FT-IR
use. [RhACHTUNGTRENNUNG(COD)Cl]2, [IrAHCTUNGTERN(NUNG COD)Cl]2, PdCl2 were purchased
from Aldrich. Molecular weights and polydispersity indexes
(PDIs) of poly(M1) were determined by gel permeation
chromatography (GPC) on three consecutive 4.6ꢃ300 mm
Waters Styragel HR4 columns in N,N-dimethylformamide
(DMF) at 308C using an isocratic pump (model 1515), an
1717 autosampler and a 2415 refractive index detector (all
Waters Co, USA). The flow rate was set to 1 mLminÀ1.
˜
(ATR mode): n=2958, 2867, 1656 (s), 1596 (s), 1459, 1327
Narrow poly
G
(s), 1212 (w), 1149 (w), 1090 (w), 1046 (w), 921(w), 862 (w),
761 (w), 722 cmÀ1 (m); ESI-MS: m/z=277.19, calcd. for
C16H24N2O2 (M+H)+: m/z=276.18.
5,500,000 gmolÀ1 (Easi Vial-red, yellow and green) were
purchased from Polymer Labs. FT-IR spectra were recorded
on a Bruker Vector22 spectrometer using ATR technology.
GC-MS investigations were carried out on a Shimadzu
GCMS-QP5050 with an AOC-20i autosampler, using an
SPB fused silica (Rxi-5MS) column (30 mꢃ0.25 mmꢃ
0.25 mm film thickness) and on a Shimadzu GCMS-QP2010S
equipped with an AOC-20i autosampler using a SPB fused
silica (Rxi-5MS) column (30 mꢃ0.25 mmꢃ0.25 mm film
thickness), respectively. The injection temperature was
1508C, the initial column temperature was set to 708C and
then increased to 1208C within 7.3 min. The column flow
Homopolymerization of M1
M1 (100 mg, 0.36 mmol) and Mo
ACHTUTGNRNE[NUG N-2,6-CAHTUNGTREN(NNGU 2-Pr)2-C6H3]
A
ACHTUNGTRENNUNG
rately dissolved in CHCl3 (2 mL each). The monomer solu-
tion was added to the catalyst solution and the resulting
mixture was vigorously stirred for 2 h. Then the polymeri-
zation was terminated by the addition of benzaldehyde (10
mol equiv. with respect to catalyst). The polymer was then
precipitated by addition of diethyl ether, filtered off and
was 1.12 mLminÀ1. Mo
(OCMe3)2,[50]
dried; yield of Poly(M1): 87 mg (87%). Mn(found)
=
1550 gmolÀ1 (PDI=1.26), Mn(calc) =5500 gmolÀ1. Mn =
4100 gmolÀ1 via 1H NMR end group analysis; 1H NMR
(CDCl3): d=0.80–1.68 (br m, CH3 +NBE), 2.06 (br s,
NHC), 2.43 and 2.66 (br s, CH2 +NBE), 3.29 (br m, CH2),
4.36 (br s, CH), 5.32 (br s, NBE); 13C NMR (CDCl3): d=
19.6, 20.5, 22.6, 34.4, 37.1, 38.6, 42.3, 43.4, 44.6, 125.1, 126.3,
128.6, 129.7, 132.2, 133.5, 135.3, 154.1, 159.5, 161.8; FT-IR
1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphtha-
lene (M2),[51] 1-(2-propyl)-1,4,5,6-tetrahydropyrimidine[29]
and
PdCl2ACHTUNGTRENNUNG(1,3-bisACHTUGNTERN(NUNG 2-Pr)tetrahydropyrimidin-2-ylidene)2
were synthesized according to the literature methods.
3-BicycloACHTUNGTRENNUNG[2.2.1]hept-5-en-2-ylmethyl)-1-(2-propyl)-
3,4,5,6-tetrahydropyrimidin-1-ium Bromide
˜
(ATR mode): n=3390 (w), 2973 (w), 2935 (w), 2866 (w),
1653 (s), 1604 (s), 1451 (w), 1326 (m), 1214 (w), 1118 (w),
1-(2-Propyl)-1,4,5,6-tetrahydropyrimidine (5.0 g, 39.62) was
dissolved in a minimum amount of 1,4 dioxane, and 5-
756 , cmÀ1 (w).
(bromomethyl)bicycloACHTUNRGTNE[NUG 2.2.1]hept-2-ene (7.4 g, 39.62 mmol)
Kinetics
was added slowly to the solution. The reaction mixture was
stirred for 4 h at 1008C. The solvent was removed under
vacuum and pure product was obtained as a white colored
solid; yield: 11.2 g (90%). 1H NMR (CDCl3): d=0.57 and
1.24 (m, 2H, NBE), 1.30 (d, 6H, CH3), 1.41 and 1.86 (m,
2H, NBE), 2.1 (m, 2H, CH2), 2.43 and 2.73 (m, 3H, NBE),
3.20–3.50 (m, 6H, CH2), 4.36 (sep, 1H, CH), 6.00 and 6.19
(m, 2H, NBE), 9.16 and 9.58 (s, 1H, CH); 13C NMR
(CDCl3): d=19.5, 20.7 (d), 30.3, 38.3 (d), 42.6, 44.6 (d), 49.9,
M1 (70 mg, 0.25 mmol) and Mo
G
ACHTUTGNRNE[NUG N-2,6-CAHTUNGTREN(NNGU 2-Pr)2-C6H3)
ACHTUNGTRENNUNG
arately dissolved in CDCl3 (3 mL each). The monomer solu-
tion was added to the catalyst solution and the resulting
mixture was stirred at room temperature. Aliquots (0.5 mL)
were taken at fixed times, quenched with benzaldehyde (10
mol equiv. with respect to catalyst) and product conversion
was monitored by H NMR at defined time intervals. Proton
signals of the monomer and those of the double bonds of
the polymer double were used for calculations.
1
˜
56.5, 59.5, 131.8, 139.1, 152.6. FT-IR (ATR mode): n=2961
(w), 1673 (s), 1437 (m), 1375 (w), 1324 (m), 1252 (w), 1210
(w), 1131 (m), 1004, 825 (w), 738 cmÀ1 (m); ESI-MS: m/z=
313.14, calcd. for C15H25BrN2 (M+H)+: 312.12.
Preparation of the CO2-Protected NHC-Containing
Resin
3-(Bicyclo
ACHTUNGTRENNUNG[2.2.1]hept-5-en-2-ylmethyl)-1-(2-propyl)-
Mo
N
(2-Pr)2-C6H3]
G
(OCMe3)2
(10 mg,
3,4,5,6-tetrahydropyrimidin-1-ium 2-Carboxylate (M1)
18.0.10À3 mmol) was added to a stirred solution of M1
(100 mg, 0.36 mmol) in 5 mL of CHCl3. After 20 min,
DMNH-6 (115 mg, 0.72 mmol) was added to the solution
and stirring was continued for 2 h. Finally, benzaldehyde (10
3-BicycloACHTUNGTRENNUNG[2.2.1]hept-5-en-2-ylmethyl-1-(2-propyl)-3,4,5,6-tet-
rahydropyrimidin-1-ium bromide (5.0, 15.95 mmol) was sus-
pended in 130 mL of THF. Potassium tert-butoxide (2.3 g,
Adv. Synth. Catal. 2010, 352, 917 – 928
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
925

Gajanan Manohar Pawar and Michael R. Buchmeiser
FULL PAPERS
mol equiv. with respect to the catalyst) was added in order
to ensure quantitative termination of the reaction. After 1 h,
the polymer was filtered off, washed with CH2Cl2 and dieth-
yl ether, and dried under vacuum; yield: 180 mg (85%). FT-
The reaction mixture was stirred overnight at 658C, after
this time TLC showed no further conversion. The solvent
was removed under vacuum and the product was purified by
column chromatography (silica gel) using CH2Cl2:ethanol
(250:4) as a mobile phase. The product eluted as a yellow
band in the second fraction. The product fractions were
pooled and evaporated to dryness to furnish a yellow solid;
yield: 0.176 g (45%). Yellow crystals were obtained by layer-
ing pentane over a dilute solution of Ir-1 in CH2Cl2 at
˜
IR (ATR mode): n=3384 (w), 2927 (s), 2863 (m), 1669 (s),
1604 (s), 1449 (w), 1325 (s), 1213 (w), 966 (s), 730 cmÀ1 (s);
elemental anal. (%) calcd. for the resin assuming 100% in-
corporation of M1: C 77.88, H 9.15, N 6.05; found: C 72.82,
H 8.51, N 3.47. This corresponds to 57.3% monomer incor-
poration.
1
À308C. H NMR (CDCl3): d=0.95 (m, 1H, NBE), 1.18–1.28
(m, 8H, CH3 +NBE), 1.42 (m, 2H, NBE), 1.52–2.12 (m,
7H, COD+NBE), 2.18 (m, 2H, CH2), 2.40–2.92 (m, 4H,
NBE+COD), 3.01, 3.09 and 3.31 (m, 5H, NCH2 +NBE),
3.50, 3.84, 4.26, 4.36, 4.57 and 4.75 (m, 4H, COD), 5.81
(hept, 1H, NCH, J=6.0 Hz) 6.02, 6.10, 6.19, and 6.25 (m,
2H, NBE); 13C NMR (CDCl3): d=19.5, 20.4, 21.4, 28.0,
30.8, 31.4, 37.8, 42.3,45.7, 49.9, 52.2, 56.5, 61.9, 62.8, 80.7,
131.5, 132.3, 137.8, 138.4, 199.5 (C-Ir); FT-IR (ATR mode):
Synthesis of a Resin-Bound Pd Catalyst
The CO2-protected NHC-containing resin (100 mg) was sus-
pended in 5 mL of THF, then H2PdCl4 (35 mg, 0.19 mmol)[32]
was added. The mixture was heated to 708C. After 3 h of
stirring, the reaction mixture was cooled to room tempera-
ture. The mixture was filtered off and washed with CH2Cl2
and diethyl ether and dried. The Pd content was determined
by ICP-OES after dissolving a 23.7 mg sample in aqua regia
and found to be 31.1 mggÀ1(0.29 mmolgÀ1). In view of the
amount of NHC present in the polymer (1.23 mmolgÀ1), this
corresponds to a Pd loading of 24% assuming the formation
of a mono-NHC-Pd complex.
˜
n=3055 (w), 2959 (s), 2924 (s), 2869 (s), 2832 (m), 2532,
1507 (s), 1448 (m), 1368 (w), 1335 (m), 1307 (s), 1207 (m),
1160 (m), 1080 (w), 1005 (w), 753 (s), 718 cmÀ1 (m); ESI-
MS: m/z=533.25, calcd. for C23H36BrIrN2 (M-BrÀ): 613.177.
Typical Cyanosilylation Procedure
In a 10-mL Schlenk tube, the CO2-protected NHC-contain-
ing resin (0.10 mol% of NHC with respect to the carbonyl
compound), acetophenone (1.2 g, 10.00 mmol) and trime-
thylsilyl cyanide (1.5 g, 15.00 mmol) were dissolved in 5 mL
of THF. The reaction mixture was heated to 658C. Product
conversion was monitored by GC-MS at defined time inter-
vals. Compounds were identified by GC-MS. Educt and
product peaks were used for calculation.
Synthesis of a Resin-Bound Ir Catalyst
The CO2-protected NHC-containing resin (100 mg) was sus-
pended in 5 mL of THF, then [IrACTHNUTRGNEG(NU COD)Cl]2 (27 mg,
0.04 mmol) was added. The mixture was heated to 708C.
After 3 h of stirring, the reaction mixture was cooled to
room temperature. The mixture was filtered off and washed
with CH2Cl2 and diethyl ether and dried. The Ir content was
determined by ICP-OES after dissolving a 17.9 mg sample
in aqua regia and found to be 45.5 mggÀ1(0.23 mmolgÀ1). In
view of the amount of NHC present in the polymer
(1.23 mmolgÀ1), this corresponds to an Ir loading of 19% as-
suming the formation of a mono-NHC-Ir complex.
Typical Procedure for the Trimerization of
Isocyanates
In
a 10-mL Schlenk tube, phenylisocyanate (120 mg,
1.0 mmol) and the CO2-protected NHC-containing resin
(1.20 mol% NHC with respect to the isocyanate) were
added. The reaction mixture was heated to 658C for 1 hour.
After the reaction was completed, CH2Cl2 (5 mL) was
added, the slurry was stirred and then filtered through a
short silica column. The solvent was evaporated under
vacuum and a white solid was obtained. The crude product
was washed with pentane and dried; yield: 98 mg (82%).
1,3,5-Tris(2-fluorophenyl)-1,3,5-triazinane-2,4,6-trione:
Synthesis of a Resin-Bound Rh Catalyst
The CO2-protected, NHC-containing resin (100 mg) was sus-
pended in 5 mL of THF, then [RhACTHNUTRGNEG(NU COD)Cl]2 (15 mg,
0.03 mmol) was added. The mixture was heated to 708C.
After 3 h of stirring, the reaction mixture was cooled to
room temperature. The mixture was filtered off and washed
with CH2Cl2 and diethyl ether and dried. The Rh content
was determined by ICP-OES after dissolving a 13.2 mg
1H NMR (CDCl3): d=7.06, 7.25, 7.46 and 8.10 (m, Ph);
13C NMR (CDCl3): d=116.6 (d, J=180 Hz), 124.9, 130.5 (d,
J=330 Hz), 131.8, 147.3, 157.3, 159.0; FTIR (ATR mode):
sample
in
aqua
regia
and
found
to
be
21.6 mggÀ1(0.21 mmolgÀ1). In view of the amount of NHC
present in the polymer (1.23 mmolgÀ1), this corresponds to
an Ir loading of 17% assuming the formation of a mono-
NHC-Rh complex.
˜
n=1712 (s), 1595 (w), 1496 (s), 1411 (s), 1255 (m), 1209 (m),
1102 (w), 1029 (w), 904 (w), 853 (w), 745 cmÀ1 (s); ESI-MS:
m/z=412.1, calcd. for C21H12F3N3O3 (M+H)+: 411.083.
IrBr(3-bicyclo
propyl)-3,4,5,6-tetrahydropyrimidin-1-ium)COD (Ir-1)
ACHTUNGTRENNUNG[2.2.1]hept-5-en-2-ylmethyl)-1-(2-
Typical Procedure for a Heck Coupling Reaction
[Ir
(COD)Cl]2 (214 mg, 0.31 mmol) was dissolved in THF
In a 25-mL Schlenk tube, the resin-bound Pd-catalyst
(0.01 mol% of Pd with respect to styrene), 4-bromoaceto-
phenone (5.2 g, 26.12 mmol), styrene (2.7 g, 25.96 mmol),
tributylamine (9.6 g, 51.79 mmol) and tert-butylbenzene
(800 mg, internal standard) were dissolved on 10 mL of
DMF. The reaction mixture was heated to 1508C. Product
(5 mL), and lithium tert-butoxide (128 mg, 1.59 mmol) was
added under vigorous stirring. The mixture was stirred for a
further 30 min at room temperature, then 3-bicyclo-
ACHTUNGTRENNUNG
926
asc.wiley-vch.de
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 917 – 928

Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes
conversion was monitored by GC-MS at defined time inter-
vals. Compounds were identified by GC-MS.
[12] K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan, J.
Am. Chem. Soc. 2000, 122, 4243.
[13] W. S. Jen, J. J. M. Wiener, D. W. C. MacMillan, J. Am.
Chem. Soc. 2000, 122, 9874.
[14] N. A. Paras, D. W. C. MacMillan, J. Am. Chem. Soc.
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[15] G. C. Fu, Acc. Chem. Res. 2000, 33, 412.
[16] S. Arai, S. Bellemin-Laponnaz, G. C. Fu, Angew. Chem.
2001, 113, 240; Angew. Chem. Int. Ed. 2001, 40, 234.
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Chem. Soc. 2000, 122, 9874.
Hydrogen Transfer to Benzaldehyde
The resin-bound Ir-catalyst (0.10 mol% of Ir with respect to
aldehyde), K2CO3 (130 mg, 0.94 mmol) and benzaldehyde
(200 mg, 1.88 mmol) were added to the 10 mL of 2-propa-
nol. The reaction mixture was heated under reflux. Aliquots
(0.5 mL) were taken at fixed times, quenched in n-hexane
(10 mL) and filtered through a short path column of SiO2.
The filtrate was subject to GC-MS analysis. Product identifi-
cation and conversion was monitored by GC-MS.
[18] F. Nederberg, E. F. Connor, T. Glauser, J. L. Hedrick,
Chem. Commun. 2001, 2066.
[19] B. List, R. A. Lerner, C. A. Barbas, J. Am. Chem. Soc.
2000, 122, 2395.
Polymerization of Phenylacetylene
[20] M. Myers, E. Connor, G. Nyce, T. Glauser, A. Mock,
J. L. Hedrick, J. Polym. Sci. Part A: Polym. Chem.
2002, 40, 844.
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Hedrick, Angew. Chem. 2001, 113, 2784; Angew. Chem.
Int. Ed. 2001, 40, 2712.
The resin-bound Rh-catalyst (1.00 mol% of Rh with respect
to phenylacetylene) was added to a solution of phenylacety-
lene (102 mg, 1.0 mmol) in 4 mL of CHCl3. The reaction
mixture was stirred at room temperature for 2 h. The poly-
mer was then precipitated by the addition of pentane, fil-
tered off and dried. Yield of Poly(phenylacetylene): 67 mg
(67%). Mn(found) =8400 gmolÀ1 (PDI=2.45), Mn(calc)
=
[22] G. A. Grasa, T. Gveli, R. Singh, S. P. Nolan, J. Org.
22400 gmolÀ1 1H NMR (CDCl3): d=5.85 (br s, 1H, CH),
;
Chem. 2003, 68, 2812.
6.65 and 6.96 (br s, 5H, Ph); 13C NMR (CDCl3): d=126.8,
[23] G. W. Nyce, T. Glauser, E. F. Connor, A. Mçck, R. M.
Waymouth, J. L. Hedrick, J. Am. Chem. Soc. 2003, 125,
3046.
127.7, 127.9, 131.9, 139.4, 143.0.
[24] A. Miyashita, Y. Suzuki, M. Kobayashi, N. Kuriyama,
T. Higashino, Heterocycles 1996, 43, 509.
[25] A. E. Mattson, A. R. Bharadwaj, A. M. Zuhl, K. A.
Acknowledgements
Scheidt, J. Org. Chem. 2006, 71, 5715.
[26] J. J. Song, Z. Tan, J. T. Reeves, F. Gallou, N. K. Yee,
C. H. Senanayake, Org. Lett. 2005, 7, 2193.
[27] J. J. Song, F. Gallou, J. T. Reeves, Z. Tan, N. K. Yee,
C. H. Senanayake, J. Org. Chem. 2006, 71, 1273; Y.
Suzuki, A. Bakar, K Muramatsu, M. Sato, Tetrahedron
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[29] Y. S. Vygodskii, A. S. Shaplov, E. I. Lozinskaya, O. A.
Filippov, E. S. Shubina, R. Bandari, M. R. Buchmeiser,
Macromolecules 2006, 39, 7821.
This work was supported by the Federal Government of Ger-
many and the Freistaat Sachsen.
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