Poly(N-heterocyclic-carbene)s and their CO2 adducts as recyclable polymer-supported organocatalysts for benzoin condensation and transesterification reactions

Article abstract of DOI:10.1021/ma1024285

The synthesis of poly(N-heterocyclic carbene)s, denoted poly(NHC)s, and of their poly(NHC-CO₂) adducts for a use in organocatalysis is described. Poly(NHC)s were readily obtained in a three-step sequence of reactions, involving i) the free-radical polymerization of ionic liquid monomers, that is, 1-vinyl-3-alkylimidazolium-type monomers with bromide (Br^-) as counteranion, followed by ii) anion exchange of Br ^- for bis(trifluoromethanesulfonyl)imide (^-NTf ₂), of the poly(1-vinyl-3-alkylimidazolium bromide) precursors, affording poly(1-vinyl-3-alkylimidazolium bis(trifluoromethanesulfonyl)imide) derivatives, and iii) deprotonation of the latter polymeric ionic liquids with a strong base. Carbon dioxide (CO₂) was found to reversibly react with poly(NHC)s forming relatively air-stable and thermolabile poly(NHC-CO ₂) adducts. Both poly(NHC)s and their poly(NHC-CO₂) adducts were used as polymer-supported organic catalysts and precatalysts, respectively, in transesterification and benzoin condensation reactions under homogeneous conditions. Both types of polymer-supported NHCs were recycled and used several times, but the manipulation of poly(NHC)s-like their molecular NHC analogues-was more complicated owing to their air and moisture sensitivity. In this regard, zwitterionic poly(NHC-CO₂) adducts like their molecular NHC-CO₂ analogues could be easier manipulated than their bare poly(NHC) counterparts, providing good to excellent yields even after several organocatalytic cycles, in particular toward the transesterification reaction.

Full text of DOI:10.1021/ma1024285

ARTICLE
pubs.acs.org/Macromolecules
Poly(N-heterocyclic-carbene)s and their CO2 Adducts as Recyclable
Polymer-Supported Organocatalysts for Benzoin Condensation and
Transesterification Reactions
Julien Pinaud,†,‡ Joan Vignolle,†,‡ Yves Gnanou,*,†,‡ and Daniel Taton*,†,‡
†Centre National de la Recherche Scientifique, Laboratoire de Chimie des Polymꢀeres Organiques, 16 avenue Pey-Berland, F-33607
Pessac Cedex, France
‡Universitꢁe de Bordeaux, Laboratoire de Chimie des Polymꢀeres Organiques, IPB-ENSCBP, F-33607 Pessac Cedex, France
S Supporting Information
b
ABSTRACT: The synthesis of poly(N-heterocyclic carbene)s, denoted poly-
(NHC)s, and of their poly(NHC-CO2) adducts for a use in organocatalysis is
described. Poly(NHC)s were readily obtained in a three-step sequence of reactions,
involving i) the free-radical polymerization of ionic liquid monomers, that is,
1-vinyl-3-alkylimidazolium-type monomers with bromide (Br-) as counteranion,
followed by ii) anion exchange of Br- for bis(trifluoromethanesulfonyl)imide
(-NTf2), of the poly(1-vinyl-3-alkylimidazolium bromide) precursors, affording
poly(1-vinyl-3-alkylimidazolium bis(trifluoromethanesulfonyl)imide) derivatives,
and iii) deprotonation of the latter polymeric ionic liquids with a strong base.
Carbon dioxide (CO2) was foundto reversibly react with poly(NHC)s forming relatively air-stable andthermolabile poly(NHC-CO2)
adducts. Both poly(NHC)s and their poly(NHC-CO2) adducts were used as polymer-supported organic catalysts and precatalysts,
respectively, in transesterification and benzoin condensation reactions under homogeneous conditions. Both types of polymer-
supported NHCs were recycled and used several times, but the manipulation of poly(NHC)s -like their molecular NHC analogues-
was more complicated owing to their air and moisture sensitivity. In this regard, zwitterionic poly(NHC-CO2) adducts like their
molecular NHC-CO2 analogues could be easier manipulated than their bare poly(NHC) counterparts, providing good to excellent
yields even after several organocatalytic cycles, in particular toward the transesterification reaction.
’ INTRODUCTION
facilitate their handling and to in situ generate NHCs without the
need for a strong base.10-12 For instance, the thermal activation
of 2-alkoxy,10 trichloromethyl11 or pentafluorophenyl12 adducts
of NHCs generates free carbenes. However, these methodologies
cannot be applied to generate imidazol-2-ylidenes (unsaturated
NHCs). Another route to thermal-induced formation of NHCs
employs so-called NHC-betaine adducts.13 Of particular interest
are air-stable imidazol(in)ium-2-carboxylates (NHC-CO2 ad-
ducts) whose reversible decarboxylation leads to the formation of
free imidazol(in)-2-ylidenes.14 In contrast to the aforementioned
2-adducts of NHCs, NHC-CO2 adducts are readily formed by
reaction of virtually all types of NHCs with CO2. These zwitter-
ionic adducts have been extensively used as NHC transfer agents
in organometallic chemistry for the synthesis of NHC metal
complexes15 and, more seldom, as latent organic (pre)catalysts in
organic16 and polymer chemistry.17
Since the isolation of the first carbenes in the 1990s,1,2 these
species have received a considerable attention in molecular
chemistry.3-5 Stable singlet carbenes, in particular N-heterocyclic
carbenes (NHCs), have not only become versatile ligands for
transition metals in coordination chemistry,5 but are also used as
true organocatalysts for various organic transformations, such as
benzoin condensation, transesterification, cyanosilylation, Stetter
reactions, etc.4 This is due to their unique electronic and steric
properties that can be finely tuned through variation of their
substituent pattern, offering many possibilities to modulate their
reactivity. Organic catalysts in general also appear as viable alter-
natives to metal-based catalysts for precision polymer synthesis,
providing high rates and selectivities, and tolerance to functional
groups.6 A few research teams have resorted to NHCs to trigger
metal-free polymerization reactions, including chain growth pro-
cesses such as ring-opening7 and group transfer polymerizations,8
as well as step-growth polymerization reactions.9
Whereas significant efforts have been directed toward an easier
handling of NHCs, their recycling has been little explored.
Recent works by Ying et al. have shown that cross-linked
polymer-supported NHCs (poly(NHC)s) can be used as
NHCs are generally prepared by deprotonation of air-stable
imidazol(in)ium salts with a strong base. Although this method
allows one to generate NHCs in situ, it is restricted to base
tolerant substrates. In most applications, it is required to
isolate the NHCs, which is not often trivial given their air and
moisture sensitivity. A few strategies have thus been developed to
Received: October 25, 2010
Revised:
February 10, 2011
Published: March 02, 2011
r
2011 American Chemical Society
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Scheme 1. Formation of N-Heterocyclic Carbenes (NHCs)
and NHC-CO2 Adducts from 1,3-Dialkylimidazolium-Based
Ionic Liquids, And Polymeric Analogues Proposed in This
Work
condensation reactions. It is shown that poly(NHC-CO2)
adducts are easier to handle and can be readily recycled com-
pared with their bare poly(NHC) counterparts.
’ EXPERIMENTAL SECTION
Instrumentation. 1H NMR and 13C NMR spectra were recorded
on a Bruker AC-400 spectrometer in appropriate deuterated solvents. A
Bruker Tensor 27 spectrometer was used for ATR-FTIR analysis. TGA
analyses were performed on a TA Instruments, TGA-Q500, under an
inert atmosphere of argon at a heating rate of 5 °C/min. Molar masses
were determined by size exclusion chromatography (SEC) using a
PL-GPC50 plus Integrated SEC system, equipped with PSS SUPREMA
Max columns connected in series with pore sizes of 30 and 1000
Å respectively, fitted with dual detectors (refractometry and UV) and
a water/formic acid (0.3 M) as the mobile phase at a flow rate of 0.6
mL/min. Pyridine was used as a flow-marker and the calibration was
done using poly(2-vinylpyridine) standards.
Materials. 1-Vinylimidazole (99%), 1-bromobutane (99%), 2-bro-
mopropane, and 2-bromoethylbenzene (99%) were obtained from Alfa
Aesar and used as received. Azobis(2-methylpropionitrile) (AIBN, 99%)
was received from Aldrich and was purified by recrystallization from
methanol. NaH (Aldrich, 60% in mineral oil), t-BuOK (Aldrich),
potassium bis(trimethylsilyl)amide (KHMDS, Aldrich), lithium bis-
(trifluoromethanesulfonyl)imide (LiNTf2, 98%, TCI) were used as
received. Benzyl alcohol (Aldrich, 99%) and benzaldehyde (Aldrich,
99,5%) were distilled prior to use. Vinyl acetate (Aldrich, 99%) was dried
over CaH2 and distilled prior to use. THF was distilled over Na/
benzophenone. Diethyl ether was distilled over polystyryllithium
(PS-Li). CO2 (N-45, Air Liquide) was purified by passing through a
click-on inline “super clean purifier” (SGT) prior to use.
Synthesis of 1-Vinyl-3-isopropylimidazolium Bromide
(ViPrImþBr-). 1-Vinyl-3-isopropylimidazolium bromide was prepared
following the procedure described in the literature.23 Under vigorous
stirring, 9.3 g (0.075 mol; 7.1 mL) of 2-bromopropane were added
dropwise onto 5 g (0.053 mol, 4.8 mL) of 1-vinylimidazole in a 100 mL,
round-bottomed flask. The reaction mixture was then refluxed for 16 h.
Upon cooling to room temperature, a white-yellow powder was formed,
which was extracted with ethyl acetate. Removing of the solvent yielded
ViPrImþBr- as a white solid, which was further dried under vacuum until
constant weight (11.5 g, 100% yield). 1H NMR (CD3OD): δ 9.6 (s, N-
CH-N, 1H), 8.2 (s, N-CHdCH-N, 1H), 8.1 (s, -CHdCH-N,
1H), 7.5 (dd, J = 8.7 and 15.7 Hz, CH2dCH-N, 1H), 6.1 (d, J = 15.7
Hz, HCHdCH-N, 1H), 4.9 (d, J = 8.7 Hz, HCH=CH-N, 1H), 4.9
(sept, J = 6.7 Hz, (CH3)2CH-, 1H), 1.8 (d, J = 6.7 Hz, (CH3)2CH-,
6H). 13C NMR (CD3OD): δ 136.1 (N-CH-N), 130.1 (CHN), 123.5
(CHN), 121.7 (CHN), 110.7 (CH2), 55.9 (CH3)2CH-), 23.7
(CH3)2CH-). IR (ATR): 3059, 3022, 2976, 1648, 1551, 1286, 1266,
1165, 1153, 959, 919, 850, 755.
recyclable active organocatalysts for a variety of reactions, such as
the hydrosilylation of ketones and imines, the cyanosilylation of
aldehydes and ketones and the benzoin condensation. These
poly(NHC)s were obtained via quaternization of bisimidazole
with 2,4,6-tris(bromomethyl)mesitylene forming cross-linked
poly(imidazolium) salts, followed by deprotonation with a
strong base.18 Poly(NHC-CO2) adducts have been even more
scarcely investigated and two synthetic routes involving either
the carboxylation of poly(NHCs) or the polymerization of
NHC-CO2-containing monomers have been reported. Lu
et al. have thus derived linear poly(NHC)s by post modification
of presynthesized poly(p-chloromethylstyrene) with a 1-alkyli-
midazole moiety, followed by deprotonation of the ensuing
poly(imidazolium) salts.19 Corresponding poly(NHC)s were
evaluated to reversibly capture and release CO2 at various
temperatures, but no mention of their use in organocatalysis
has been reported. Buchmeiser et al. have developed cross-linked
poly(NHCs-CO2)-based materials by ring-opening metathesis
polymerization of norbornenyl monomers containing the
NHC-CO2 moiety.20 These cross-linked polymers were shown
to exhibit excellent catalytic activity in the organocatalyzed
cyanosilylation of various carbonyl compounds upon in situ
thermal generation of poly(NHCs). The ability of these poly-
(NHC-CO2) adducts to react as masked poly(NHCs) was also
demonstrated with the preparation of polymer-supported NHC-
based metal complexes.
In this contribution, we describe the synthesis of linear and
soluble polymeric analogues of both NHCs and NHC-CO2
adducts, namely poly(NHCs) and poly(NHC-CO2), and their
use as recyclable polymer-supported organic (pre)catalysts.21 For
this purpose, we first prepared poly(1-vinyl-3-alkylimidazolium
halide)s asprecursors, by analogywith the generation of NHCs by
deprotonation of 1,3-dialkylimidazolium salts with a strong base
(Scheme 1). Poly(1-vinyl-3-alkylimidazolium) derivatives are
part of an emerging class of polymeric materials referred to as
polymeric ionic liquids (PILs).22 PILs combine the physicochem-
ical qualities of ionic liquids, such as ionic conductivity, and
tunable solubility and viscosity through variation of the cation and
anion pairing, with the specific properties of polymers. PILs find
potential applications as supports of catalysts, polymeric surfac-
tants, CO2 absorbing resins, polymer electrolytes, etc.22
Synthesis of 1-Vinyl-3-butylimidazolium bromide (VBuImþ
Br-). 1-Vinyl-3-butylimidazolium bromide was prepared following the
procedures described in the literature.23 Under vigorous stirring, 13.7 g
(0.1 mol, 9.5 mL) of bromobutane were added dropwise to 5 g (0.053
mol, 4.8 mL) of 1-vinylimidazole in a 100-mL, one-necked, round-
bottom flask. The reaction mixture was then refluxed for 24 h and the
resulting brown viscous liquid was allowed to cool down to room
temperature. After washing several times with ethyl acetate, VBuImþBr-
was obtained as a viscous yellowish oil and dried under dynamic vacuum
(10.6 g, 87% yield). The product was then stored under static vacuum.
NMR data were found in accordance with those reported in the
literature.23
Both poly(NHC)s and poly(NHC-CO2) adducts described
here have been used as (pre)catalysts under homogeneous
conditions, for metal-free transesterification and benzoin
Synthesis of 1-Vinyl-3-(1-phenylethyl)imidazolium bromide
(VPhEtImþBr-). 1-Vinyl-3-(1-phenylethyl)imidazolium bromide was
prepared following the procedures described in the literature.23 Under
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vigorous stirring, 2.8 g (0.015 mol, 2 mL) of 1-phenylethylbromide were
added dropwise to 1.04 g (0.011 mol, 1 mL) of 1-vinylimidazole in a 100-
mL, one-necked round-bottom flask. The mixture was refluxed for 20 h and
the resulting brown viscous liquid was allowed to cool down to room
temperature. After washing several times with ethyl acetate and THF,
VPhEtImþBr- was obtained as a viscous brownish oil and dried under
dynamic vacuum (2.6 g, 85% yield). The product was then stored under
static vacuum. 1H NMR (MeOD): δ 9.6 (s, N-CH-N, 1H), 8.0 (s, N-
CHdCH-N, 1H), 7.8 (s, N-CHdCH-N, 1H), 7.3-7.5 (m, Ph, 5H),
7.2 (dd, J = 8.7 and 15.6 Hz, CH2dCH-N, 1H), 5.8 (dd, J = 2.8 and 15.6
Hz, HCHdCH-N, 1H), 5.8 (q, J = 7.0 Hz,-CH-(CH3)Ph, 1H), 5.5
(dd, J = 8.7 and 2.8 Hz, HCHdCH-N, 1H), 2.0 (d, J = 7.0 Hz, -
CH-(CH3)-Ph, 3H). 13C NMR (MeOD): δ 140.7 (Carom), 136.6 (N-
CH-N), 131.3 (CHarom), 131.2 (CHarom), 130.7 (CHN), 128.8 (CHarom),
124.2 (CHN), 121.9 (CHN), 111.0 (CH2), 62.5 (Ph(CH3)CH-), 22.1
(Ph(CH3)CH-). IR (ATR): 3060, 2985, 1651, 1545, 1455, 1159, 1025,
958, 918, 761, 707.
Polymerization of 1-Vinyl-3-alkylimidazolium-TypeMono-
mers. In a typical experiment, a 10 mL Schlenk tube was flame-dried and
charged with 4 mmol of monomer, 0.13 mmol of AIBN and 2 mL of
methanol. The Schlenk tube was subjected to five freeze-thaw cycles and
placed in a thermostatted oil bath previously maintained at 80 °C. The
polymerization reaction was quenched after 3 h by sudden cooling with
liquid nitrogen. The resulting poly(1-vinyl-3-alkylimidazolium) salts 1
were isolated by precipitation in chloroform or acetone. After drying
under vacuum, 1 were obtained as a yellowish powder.
Figure 1. 1H NMR spectrum (THF-d8) of the poly(1-vinyl-3-isopropyl
imidazolium bis(trifluoromethanesulfonyl)imide) 2a and its poly
(N-heterocyclic carbene) derivative 3a.
solution as a white powder. After filtration, the polymers were dried
under vacuum overnight.
Synthesis of Poly(ViPrImþNTf2-) 2a. The anion exchange was carried
out in water (92% yield). 1H NMR (THF-d8, see Figure 1): δ 1.3-1.6
(br, N-CH-(CH3)2, 6H), 2.2-2.9 (br, N-CH-CH2,2H), 4.0-4.6
(br, N-CH-CH2 and N-CH-(CH3)2, 2H), 7.0-8.0 (br, CHdCH,
2H), 8.5-9.0 (br, N-CH-N, 1H). 13C NMR (THF-d8, see Figure S04
of the Supporting Information): δ 21.2 (br, N-CH-(CH3)2), 41.1
(br, N-CH-CH2), 55.4 (br, N-CH-CH2 and N-CH-(CH3)2),
112.6 (br, CHdCH), 119.3 (q, JCF = 333.0 Hz, NTf2-), 134.7 (br, N-
CH-N); IR (ATR): 3139, 2967, 1548, 1343, 1172, 1128, 1051,
791, 740.
Synthesis of Poly(ViPrImþBr-) 1a. ViPrImþBr- (1 g; 4.7 mmol);
AIBN (25 mg; 0.15 mmol). Precipitated in CHCl3. (1 g, Yield 99%);
Mn = 4600 g mol-1 by SEC. PDI = 2.48. 1H NMR (MeOD, see Figure
3
S01 of the Supporting Information): δ 9.3-10.0 (br, N-CH-N, 1H),
7.5-8.3 (br, CHdCH, 2H), 4.5-5.2 (br, N-CH-CH2 and N-
CH-(CH3)2,2H), 2.5-3.3 (br, N-CH-CH2, 2H), 1.4-1.9 (br,
N-CH-(CH3)2, 6H). 13C NMR (CD3OD): δ 136.1 (br, N-CH-
N), 124.3 (br, CHN), 121.9 (br, CHN), 56.2 (br, CHN), 55.3
(CH3)2CH-), 41.9 (br, CH2), 23.2 (CH3)2CH-). IR (ATR): 3056,
3024, 2991, 1649, 1556, 1400, 1288, 1190, 1167, 1154, 1142, 916, 753.
Synthesis of Poly(VPhEtImþBr-) 1b. ViEtBImþBr- (1 g; 3.6 mmol);
AIBN (19 mg; 0.12 mmol). Precipitated in acetone. Not soluble in
Synthesis of Poly(VPhEtImþNTf2-) 2b. The anion exchange was
carried out in methanol. After 2 h of stirring, water was added to the
reaction mixture to precipitate 2b (70% yield). 1H NMR (THF-d8, see
Figure S05 of the Supporting Information): δ 8.8-9.7 (br, N-CH-N,
1H), 7.0-8.0 (br, CHdCH and aromatics, 7H), 5.3-6.0 (br, N-CH-
CH2, 1H), 4.2-5.1 (br, N-CH-(CH3)-Ph, 1H), 2.3-3.3 (N-CH-
CH2, 2H), 1.8-2.1 (br, N-CH-(CH3)-Ph, 3H). 13C NMR (THF-d8,
see Figure S06 of the Supporting Information): δ 137.8 (br, N-CH-
N), 128.2 (br, Ph), 119.9 (q, JCF = 333.0 Hz, NTf2-), 122.6
(br, CHdCH), 61.4 (br, N-CH-CH2), 56.1 (br, N-CH-(CH3)-
Ph), 40.5 (br, N-CH-CH2, 2H), 20.1 (br, N-CH-(CH3)-Ph, 3H);
IR (ATR): 3144, 2970, 1550, 1343, 1179, 1127, 1049, 790, 740, 706.
Synthesis of Poly(VBuImþNTf-) 2c. The anion exchange was carried
out in water (86% yield). 1H NMR (THF-d8): δ 8.6-8.9 (br., N-CH-
N, 1H), 7.2-8.0 (br, CHdCH, 2H), 4.0-4.5 (br, N-CH2-CH2-
CH2-CH3, and N-CH-CH2, 3H), 2.4-3.0 (br, N-CH-CH2, 2H),
1.8-2.1 (br, N-CH2-CH2-CH2-CH3, 2H), 1.4-1.6 (br, N-
CH2-CH2-CH2-CH3, 2H), 1.0-1.2 (br, N-CH2-CH2-CH2-
CH3, 3H). 13C NMR (THF-d8): δ 135.4 (br, N-CH-N), 119.8 (br,
CHdCH), 119.4 (q, JCF = 333.0 Hz, NTf2-), 48.3 (br, N-CH2-
CH2-CH2-CH3, and N-CH-CH2), 32.3 (br, N-CH-CH2), 29.1
(br, N-CH2-CH2-CH2-CH3), 19.8 (br, N-CH2-CH2-CH2-
CH3, 2H), 16.9 (br, N-CH2-CH2-CH2-CH3, 3H). IR (ATR):
3139, 2966, 1545, 1343, 1179, 1127, 1050, 791, 740, 706.
1
water. (1 g, 99% yield). H NMR (DMSO-d6, see Figure S03 of the
Supporting Information): 8.9-9.8 (br, N-CH-N, 1H), 6.8-8.0 (br,
CHdCH and aromatic protons, 7H), 5.1-5.9 (br, N-CH-CH2 and
N-CH-(CH3)-ph, 2H), 3.6-4.0 (br, N-CH-CH2, 2H), 1.6-2.0
(br, N-CH-(CH3)-Benzyl, 3H). 13C NMR (DMSO-d6): δ 139.1
(br, Carom), 135.3 (br, N-CH-N), 129.3 (br, CHarom), 127.5 (br,
CHarom), 123.0 (br, CHN), 120.7 (br, CHN), 59.7 (br, Ph(CH3)CH-),
54.1 (br, CHN), 20.7 (br, Ph(CH3)CH-). IR (ATR): 3030, 2978, 2935,
1544, 1495, 1455, 1151, 758, 703.
Synthesis of Poly(VBuImþBr-) 1c. ViBuImþBr- (1 g; 4.3 mmol);
AIBN(24 mg; 0.14 mmol). Precipitated in acetone. (1 g, Yield 99%);
Mn = 8722 g mol-1 by SEC, PDI = 2.9. 1H NMR (D2O, see Figure S02
3
of the Supporting Information): δ 8.9-9.3 (br, N-CH-N,1H), 7.3-
7.8 (br, CHdCH, 2H), 3.9-4.5 (br, N-CH2-CH2-CH2-CH3, 2H),
4.4-4.8 (br, N-CH-CH2, 1H), 2.4-3.0 (br, N-CH-CH2, 2H),
1.7-2.1 (br, CH3-CH2-CH2-CH2-, 2H), 1.3-1.6 (br, CH3-
CH2-CH2-CH2-, 2H), 0.9-1.2 (br, CH3-CH2-CH2-CH2-,
3H); 13C NMR (D2O): δ 134.7 (br, N-CH-N), 124.2 (br, CHN),
120.3 (br, CHN), 55.6 (br, CHN), 50.3 (CH2N), 40.1 (br, CH2CH),
30.9 (CH2), 19.2 (CH2), 12.9 (CH3); IR (ATR): 3055, 2959, 2933,
2871, 1568, 1549, 1461, 1435, 1161, 1115, 829, 751.
Synthesis of Poly(NHC)s 3. All deprotonations were carried out under
an inert atmosphere, using Schlenk techniques.
3a: A Schlenk tube was charged with 417 mg (1.00 mmol) of
poly(ViIPrImþNTf2-) 2a. After being dried azeotropically with diox-
ane, 2a was suspended in 10 mL of dry THF, and the reaction mixture
was cooled to -80 °C. To this solution was added KHMDS
(2.00 mmol) in 10 mL of THF. The reaction mixture was then allowed
Anion Exchange. We dissolved 4 mmol of poly(1-vinyl-3-alkyli-
midazolium bromide) 1 in water or methanol, and the resulting solution
was added to a stirred solution of (CF3SO2)2NLi (8 mmol), previously
dissolved in water or methanol. After mixing, the corresponding poly-
(vinylimidazolium N-triflate) 2 immediately precipitated out of the
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Figure 3. TGA of polyNHC-CO2 adducts 4a and 4b from 25 to 250
°C.
vacuum, the solvents and the excess of vinyl acetate were evaporated
from the solution, yielding a yellowish liquid, which was analyzed by
1H NMR in CDCl3. The recovered poly(NHCs) 3b,c were then
dissolved in 10 mL of THF and reused for the next run of tran-
sesterification.
Figure 2. 1H NMR and 13C NMR spectrum of poly(NHC-CO2)
adduct 4a in DMSO-d6.
Benzoin Condensation Using Poly(NHC)s 3 As Polymer-
Supported Catalysts. The ratio between the benzaldehyde substrate
and the free poly(NHC) was equal to 10 mol % relatively to the
repeating monomer unit. A Schlenk tube was charged with a solution of
3 (1 mmol in 10 mL of THF) and benzaldehyde (9.9 mmol, 1 mL). After
stirring for 24 h at room temperature, poly(NHCs) 3 were precipitated
by the addition of dry diethyl ether and recovered by filtration under
vacuum. The solvents were evaporated from the filtrate, and the residue
was analyzed by 1H NMR in DMSO-d6. The identity of the product was
confirmed by comparing its 1H NMR spectrum with those of standard
(commercial) samples. The recovered poly(NHCs) 3 were then dis-
solved in 10 mL of THF and reused for the next run of benzoin
condensation.
to warm slowly to room temperature and was stirred for 45 min. The
resulting clear orange solution was directly used for catalysis experiments.
1H NMR (THF-d8, see Figure 2): δ 1.2-1.5 (br, N-CH-(CH3)2, 6H),
1.9-2.4 (br, N-CH-CH2,2H), 3.4-3.8 (br, N-CH-CH2, 1H),
4.3-4.6 (br, N-CH-(CH3)2, 1H), 6.2-6.9 (br, CHdCH, 2H).
3b,c: A Schlenk tube was charged with 1 mmol (431 mg) of
poly(ViBuImþNTf2-) 2c or 479 mg of poly(ViEtBImþNTf2-) 2b.
After being dried azotropically with dioxane, 2b,c was dissolved in 10 mL
of THF, and this solution was added at -80 °C to a suspension of NaH
(2 mmol, 80 mg) and t-BuOK (0.2 mmol, 22 mg) in 10 mL of THF. The
reaction mixture was allowed to warm slowly to room temperature and
was filtered to remove the excess of NaH. The resulting orange solution
was directly used for catalysis experiments. 1H NMR of 3b (THF-d8, see
Figure S07 of the Supporting Information): δ 7.2-8.0 (br, aromatics,
7H), 6.5-7.1 (br, CHdCH, 2H) 5.2-5.7 (br, N-CH-CH2 and
N-CH-(CH3)-Ph, 2H), 1.4-1.9 (br, N-CH-CH2, 2H), 1.0-1.3
(br, N-CH-(CH3)-Ph, 3H).
Synthesis of Poly(NHC-CO2) Adducts 4a,b. A Schlenk tube was
charged with 1 mmol of poly(ViPrImþNTf2-) 2a (417 mg) or poly-
(VPhEtImþNTf2-) 2b (479 mg). After being azeotropically dried with
dioxane, 2a,b were dissolved in 10 mL of THF, and the reaction mixture
was cooled to -80 °C. To this solution was added KHMDS (1.5 mmol).
After 15 min of stirring at -80 °C, the reaction mixture was allowed to
warm to room temperature and was further stirred for 45 min to
generate 3a,b. We then introduced 1 atm of CO2 to the Schlenk tube,
causing the immediate precipitation of 4a,b. The resulting suspension
was stirred for 1 h under 1 atm of CO2. After filtration under vacuum, 4a,
b were recovered as an orange powder. (70-90% yield) These polymers
were directly used for catalysis experiments or kept in a glovebox to
avoid hydration (t1/2 ≈ 3 h).
Transesterification Using Poly(NHC)s 3 As Polymer-Sup-
ported Catalysts. In these organocatalyzed reactions, the ratio
between the benzyl alcohol substrate and the free poly(NHC) was
equal to 10 mol % relatively to the repeating monomer unit.
Transesterification with 3a. A Schlenk tube was charged with a
solution of 3a (1 mmol in 10 mL of THF), benzyl alcohol (9.7 mmol,
1.0 mL), and vinyl acetate (13 mmol, 1.2 mL). The reaction mixture was
stirred for 30 min at room temperature, and 50 mL of dried diethyl ether
was added to precipitate 3a. The reaction mixture was then filtered
under vacuum. The solvents and the excess of vinyl acetate were
evaporated from the filtrate, and the resulting yellowish liquid was
4a. 1H (DMSO-d6, see Figure 2): δ 1.0-1.6 (br., N-CH-(CH3)2,
6H), 2.0-2.8 (br., N-CH-CH2, 2H), 4.6-5.9 (br., N-CH-CH2 and
N-CH-(CH3)2, 2H), 7.2-8.3 (br., CHdCH, 2H). 13C NMR
(DMSO-d6, see Figure 3): δ 21-24 (br., N-CH-(CH3)2), 50-54
(br., N-CH-CH2, N-CH-CH2 and N-CH-(CH3)2), 116-121
(br., CHdCH), 114, 118, 121, and 124 (s, NTf2-), 140-143 (br.,
1
analyzed by H NMR in CDCl3. The conversion of the reaction was
determined by measuring the relative intensities of the CH2 signals at 5.0
and 4.5 ppm corresponding to benzyl acetate and benzyl alcohol,
respectively. The identity of the product was confirmed by comparing
their 1H NMR spectrum with those of standard (commercial) samples.
The recovered poly(NHC) 3a was then reused for the next run of
transesterification.
Transesterification with 3b,c. A Schlenk tube was charged with a
solution of 3b or 3c (1 mmol in 10 mL of THF), benzyl alcohol (9.7
mmol, 1.0 mL), and vinyl acetate (13 mmol, 1.2 mL). The reaction
mixture was stirred for 30 min at room temperature and 3b,c were
precipitated by the addition of dry diethyl ether. After filtration under
(N)2-C-C), 153-156 (br., C-CO2). IR (ATR): νCO 1670 cm-1
.
TGA (see Figure S09 of the Supporting Information): A weight loss of
25% corresponding to the loss of CO2 was observed, in agreement with
the theoretical value of 25% calculated for the full conversion of 3a to 4a.
4a was soluble in polar solvents (DMSO, acetone) and protic solvents
(MeOH, EtOH) but insoluble in THF and CH2Cl2.
4b. 13C NMR (DMSO-d6, see Figure S08 of the Supporting In-
formation): δ 161-163 (br., C-CO2), 136-141 (br., (N)2-C-C),
128-132 (br., aromatics), 114, 118, 121, and 124 (s, NTf2-),
120-128 (br., CHdCH), 61-63 (br., N-CH-CH2), 53-55 (br.,
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Scheme 2. Synthesis of Poly(1-vinyl-3-alkylimidazolium
chloride)s and Anion Exchange
molecular weight measurement.22 Two PVRImþBr- samples
(1a and 1c) were thus analyzed by aqueous SEC, affording
information about their dispersity and their apparent values of
molecular weight (Table 1), although these numbers must be
taken with caution for the reasons mentioned above. Unimodal
molar masses distribution were observed with a dispersity
(D = Mw/Mn) between 2 and 3. Because of the insolubility of
1b in water, analysis of this polymer by aqueous SEC could not be
carried out.
These PVRImþBr- materials were soluble in polar protic
solvents and insoluble in apolar organic solvents (Table 1). As
mentioned above, this solubility could nonetheless be tuned by
changing the counteranion from Br- to Tf2N- (Scheme 2).
Accordingly, the resulting PILs, denoted PVRImþTf2N-, were
soluble in aprotic solvents such as acetone, DMSO, and THF, in
contrast with their counterparts carrying the bromide counter-
anion. For the latter reason, we used PVRImþTf2N- as pre-
cursors to get access to poly(NHC)s, as discussed further.
Following the common synthetic route to NHCs, that is, by
deprotonation of the acidic proton of 1,3-dialkylimidazolium
salts by a strong base,2,3 PVRImþTf2N- was treated by an excess
of NaH (or KHMDS) in THF at -80 °C to generate the desired
poly(NHC)s (Scheme 3). For polymers that were soluble in
THF, NaH was preferred over KHMDS as deprotonating agent
of PVRImþTf2N- precursors, the excess of this base being easily
removed by filtration. For precursors that could be not solubi-
lized in THF (i.e., 2a), KHMDS was used instead.
N-CH-(CH3)-Ph), 40-47 (br., N-CH-CH2), 21-24 (br., N-
CH-(CH3)-Benzyl). IR (ATR): νCO 1670 cm-1. TGA (see Figure
S09 of the Supporting Information): a weight loss of 25% corresponding
to the loss of CO2 was observed, in agreement with the theoretical value of
25% calculated for the full conversion of 3b to 4b. 4b was soluble in polar
solvents (DMSO, acetone) and protic solvents (MeOH, EtOH) but
insoluble in THF and CH2Cl2.
Transesterification and Benzoin Condensation Using
Poly(NHC-CO2) Adducts 4a,b as Polymer-Supported Preca-
talysts. In a typical experiment, a Schlenk tube was charged with 1 mmol
of 4a or 4b and 10 mmol of reactants, suspended in 5 mL of THF.
The flask was then placed in an oil bath at 80 °C for 2 (transesterification)
or 24 h (benzoin condensation) under vigorous stirring. After this
period, the reaction mixture was allowed to cool to room temperature,
and 1 atm of CO2 was added, leading to the precipitation of 4a,b.
After stirring for 15 min, the mixture was filtered under vacuum, and the
1
filtrate was analyzed by H NMR, as described above. The identity
This chemical modification could be monitored by 1H NMR
spectroscopy through the complete disappearance of the signal
corresponding to proton f of PVRImþTf2N-, after treatment
with NaH or KHMDS. A representative example of this depro-
tonation is illustrated in Figure 1, showing the 1H NMR
spectrum of PViPrImþTf2N- 2a. Nevertheless, the 13C NMR
spectra of these poly(NHC)s could not be analyzed with
accuracy because of the extreme broadening of the signals.
Because of their air and moisture sensitivity, the as-prepared
poly(NHC)s 3 were further handled in a dry THF solution, in
particular, for the purpose of organocatalysis.
of the products was confirmed by comparing their 1H NMR spectrum
with those of standard (commercial) samples. The recovered polymer
4a,b was then suspended in THF and reused for the next run of
catalysis.
’ RESULTS AND DISCUSSION
Synthesis of Poly(NHC)s and Their Evaluation in Organo-
catalysis. Among PILs, those based on imidazolium moieties
have been the most investigated.22 The solubility of these PILs,
like their molecular IL analogues (Scheme 1), is mainly influ-
enced by the anion, ranging from a high solubility (e.g., with Br-
as anion) to immiscibility in water (with Tf2N-). We have
recently reported the synthesis and self-assembling properties of
IL-based block copolymers.24 The self-assembly was triggered by
a simple anion exchange, giving rise to either polymeric micelles
or polymeric vesicles in aqueous solutions, depending on the
overall hydrophilic mass fraction of the block copolymers.
To generate poly(NHC)s that could be used as polymer-
supported organocatalysts, we turned to poly(1-vinyl-3-al-
kylimidazolium) (PVRImþ) precursors whose synthesis by
free-radical polymerization of 1-vinyl-3-alkylimidazolium-type
monomers (VRImþ) is well-documented.22,24 A series of
PVRImþ with bromide as counteranion, denoted PVRImþBr-,
and different substituents (R = i-Pr, n-Bu, 1-phenylethyl,
Scheme 2) was thus synthesized in methanol using AIBN as
radical source at 80 °C. PVRImþBr- with R = i-Pr, R = n-Bu, and
R = 1-phenylethyl has already been reported.23 Their character-
ization by 1H NMR spectroscopy confirmed the expected
structure.(See the Experimental Section and ESI.) In particular,
the signal corresponding to the CH proton of the imidazolium
moiety could be clearly identified at 9 to 10 ppm.
The three poly(NHC)s 3a,b,c were then investigated as
polymer-supported organocatalysts in both transesterification
and benzoin condensation. These reactions are known to be
efficiently catalyzed by molecular NHCs.4,9b In a typical transes-
terification reaction, benzyl alcohol (BnOH) and vinyl acetate
(VAc) were added to a THF solution containing the poly(NHC)
3, and the reaction was stirred for 30 min at room temperature
(Scheme 4). Adding dry diethyl ether onto the latter solution led
to the precipitation of the poly(NHC), allowing its easy recovery
by filtration, whereas the transesterified benzyl acetate remained
1
soluble. Analysis by H NMR spectroscopy of the filtrate con-
firmed the formation of the reaction product and allowed the
conversion to be determined (ESI).
As can be seen in Table 2, the three poly(NHC)s tested all
exhibit excellent catalytic activity in the first run, providing
excellent yield of benzyl acetate (85-100%), irrespective of
the nature of the substituent on the imidazol-2-ylidene. After
the first recycling, however, a significant decrease in the yield of
the transesterified product was noted with poly(NHC)s 3a and
3c (R = i-Pr and n-Bu; yield = 14 and 27%, respectively), whereas
a moderate yield (57%) could be achieved with poly(NHC) 3b
(R = PhEt) under the same conditions. This loss in catalytic
activity of the poly(NHC)s after recycling might be ascribed to
their partial deactivation, likely because of their sensitivity to
Characterization of PILs by SEC is always complicated owing
to possible interactions between these peculiar polylectrolytes
and the columns or the solvent, making difficult an accurate
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Table 1. Molecular Characteristics and Solubility Properties of PVRImþ with Br- or Tf2N- as Counter Anion in Different
Solventsa
PViPrImþX- (Mn = 4600 g mol-1; PDI = 2.48)
PVPhEtImþX-
X: Br 1b X: NTf2 2b
PVBuImþX- (Mn = 8700 g mol-1; PDI = 2.90)
3
3
X: Br 1a
X: NTf2 2a
X: Br 1c
X: NTf2 1c
H2O
þ
þ
þ
-
-
-
-
þ
þ
-
-
-
þ
þ
þ
-
-
-
MeOH
EtOH
acetone
THF
-
þ
þ-
-
-
þ-
þ
þ
þ-
þ
þ
þ
a
þ: soluble; -: insoluble; þ-: swollen but not soluble.
Scheme 3. Synthesis of Poly(NHC)s from PVRImþTf2N-
Scheme 5. Poly(NHC)-Catalyzed Benzoin Condensation
Reaction
Scheme 4. Poly(NHC)-Catalyzed Transesterification
Reaction
Table 3. Poly(NHC)-Catalyzed Benzoin Condensation in
THF at Room Temperature for 24 h NMRa
catalyst
run
3a
3b
3c
yield (%)
yield (%)
yield (%)
1
2
3
4
92
20
13
85
72
46
35
28
23
a All reactions were carried out with benzaldehyde (1 equiv) and desired
polyNHC (cat.: 10% mol of active species) in THF at room temperature
for 24 h. Yield in benzoin was determined by 1H.
Table 2. Yield of Benzyl Acetate Obtained in the Poly(NHC)-
Catalyzed Transesterification of Benzyl Alcohol and Vinyl
Acetatea
center, which allows us and to rank them as follows according
to the size of the substituents on the imidazole framework: 3b
(R = 1-phenylethyl) > 3a (R = i-Pr) > 3c (R = n-Bu).
polyNHC catalyst
run
3a Rdi-Pr
3b RdPhEt
3c RdBu
Next, we examined the catalytic potential of these poly(NHC)s
for the so-called benzoin condensation, that is, the self-condensa-
tion of benzaldehyde leading to the formation of aromatic β-keto
alcohol, also called benzoin (Scheme 5). This C-C bond
formation reaction is also known to be selectively catalyzed by
carbenes.4,26 The poly(NHC)-catalyzed benzoin condensation
was carried out in THF for 24 h at room temperature, and results
are shown in Table 3. As in the case of transesterification
reactions, the polymer-supported organocatalyst could be recov-
ered by simple filtration upon the addition of diethyl ether. The
first run led to benzoin in excellent yield (85-92%) with poly-
(NHC)s 3a and 3b as polymeric catalysts, whereas poly(NHC)
3c afforded much lower yield (28%). After the first recycling, only
poly(NHC) 3b retained a fairly good catalytic activity (72%
yield). In contrast, the yield of benzoin significantly decreased for
both poly(NHC)s 3a and 3c, in line with the observation
previously made about the transesterification carried out in the
presence of these polymer-supported catalysts.
yield (%)
yield (%)
yield (%)
1
2
3
4
100
14
100
57
85
27
13
10
12
a Benzyl alcohol (1 equiv), vinyl acetate (1,2 equiv), and polyNHC (cat.:
10% mol of active species) in THF at r.t. and for 30 min. Yield in benzyl
acetate was determined by 1H NMR (ESI).
trace impurities and/or by a dimerization reaction pathway,25 as
discussed further. After the second recycling, no transesterified
product was formed at all with poly(NHC) 3c, suggesting a
complete degradation of the catalyst, whereas the conversion
reached only 10% with poly(NHC)s 3a and 3b. Poly(NHC) 3b
was found to retain a catalytic activity in the fourth run (giving
12% conversion). The catalytic efficiency of poly(NHC)s 3 can
be correlated with the steric hindrance around the carbene
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Scheme 6. Reversible Formation of Poly(NHC-CO2) Ad-
ducts by Carboxylation of Poly(NHC)s
Table 4. Poly(NHC-CO2)-Catalyzed Transesterification of
Benzyl Alcohol and Vinyl Acetate Carried out in THF at 80 °C
for 2 ha
cycle
1
2
3
4
5
6
7
8
yield (%) with 4a
yield (%) with 4b
94
83
51
77
23
74
70
77
76
77
70
a Benzyl alcohol (1 equiv); vinyl acetate (1,2 equiv), and poly(NHC-
CO2)adduct (10% mol of active species) were added to THF at 80 °C
for 2 h. Yield in benzyl acetate was determined by 1H NMR.
(δ = 6.2-6.9 ppm), in agreement with the formation of such
an imidazolium-type structure. The presence of diagnostic
signals at 140 and 160 ppm in the 13C NMR spectrum of 4a,
attributable, respectively, to the quaternary carbon atom of the
imidazolium moiety Cf and to that of the carboxylate group Cg,
unambiguously established the imidazolium-2-carboxylate be-
taine structure (Figure 2).
We could thus demonstrate that these poly(NHC)s efficiently
catalyze both transesterification and benzoin condensation,
although their manipulation was complicated because of their
air and moisture sensitivity, similarly to that of their molecular
NHC counterparts.1-4 This directly impacted the recycling
process as lower yields were obtained upon the first and
subsequent recycling for both catalytic reactions tested. Partial
deactivation of poly(NHC)s might result from the presence of
trace impurities and occurred more readily with a decrease in the
steric hindrance around the carbene center. However, deactiva-
tion might also be due to a dimerization reaction of the poly-
(NHC)s. Such dimerization reactions are likely to occur with
molecular free NHCs, in particular, with NHC compounds
possessing poorly hindered substituents around the carbene
center.25 As mentioned above, the 13C NMR spectra of poly-
(NHC)s described did not provide any clear information about
their structure owing to the broadening of the signals. Therefore,
an experimental proof for the dimerization reaction from our
poly(NHC)s could not be established, with the characteristic
signal, if present, being overlapped with the CH of the imidazole
backbone.
Synthesis of Poly(NHC-CO2) Adducts and Their Evalua-
tion in Organocatalysis. To improve the stability and facilitate
the recycling of poly(NHC)s, we looked into the possibility
of forming air-stable poly(NHC-CO2) adducts and use them
as latent polymer-supported catalysts. Release of the free
poly(NHC)s was expected to occur in situ by thermal activa-
tion of such poly(NHC-CO2) adducts, by analogy to the
generation of NHC from molecular NHC-CO2 adducts. (See
Scheme 1.)13-15 As already mentioned, there are only a few
reports on the synthesis of poly(NHC-CO2) adducts19,20 and,
to the best of our knowledge, their use as polymer-supported
precatalysts in organic reactions has been described only once.20
In the latter case, the organocatalysis occurred under hetero-
geneous conditions due to the cross-linked nature of the poly-
meric support.20
Characterization by TGA of the polymeric materials obtained
after carboxylation showed a weight loss of ∼5% due to CO2
release at ∼150 °C (Scheme 4), in agreement with temperatures
reported for the loss of CO2 in molecular imidazolium carbox-
ylates adducts.14 The TGA results were also consistent with the
expected composition of the two poly(NHC-CO2) adducts 4a,
b. This allowed us to conclude that poly(NHC)s were quantita-
tively transformed into their poly(NHC-CO2) derivatives by
simple carboxylation with 1 atm of CO2. Interestingly, poly-
(NHC-CO2) 4a and 4b decomposed at much higher tempera-
ture (T > 250 °C) compared with molecular NHC-CO2
adducts featuring alkyl substituents on the nitrogen atom.14b
This can be attributed to the higher thermal stability brought by
the polymeric backbone of the poly(NHC-CO2) adducts. It is
also worth mentioning that, unlike their poly(NHC) precursors
3, polymeric adducts 4 are air stable and can be manipulated
much more easily. It must be noted, however, that prolonged
exposure to air leads to their progressive hydration (complete
hydration in about 6 h) as deduced from the change of the
chemical shift of the CO2 moiety in the 13C NMR spectrum of
4a,b.
The potential of such poly(NHC-CO2) adducts as polymer-
supported precatalysts for the transesterification reaction was
next investigated. Similar experimental conditions to those
mentioned for poly(NHC) catalysts were used (Scheme 4),
except that the temperature was raised to 80 °C to in situ
generate free poly(NHC)s under homogeneous conditions.27
Good to excellent yields of benzyl acetate were thus obtained
using 4a (94%) and 4b (83%) for the first run (Table 4). This
indicated that free poly(NHC)s were efficiently generated by
decarboxylation upon heating the poly(NHC-CO2) adducts at
80 °C. The reaction medium was cooled to room temperature
and CO2 was added, which resulted in the immediate precipita-
tion of the thus regenerated poly(NHC-CO2) adducts;
the latter could be filtered off and reused. The as-recovered
polymer-supported precatalysts were indeed subjected to new
runs of organocatalysis. Fairly good conversions (70-77%) were
achieved with 4b over a total of eight runs, attesting to their
efficient recycling with virtually no loss of catalytic activity. In
contrast, yields of transesterificaion with 4a significantly dropped
from the first to the second and third runs (from 94 to 51 to
23%).
Here poly(NHCs-CO2) adducts 4 were easily generated by
carboxylation of freshly prepared poly(NHC)s 3 by flushing the
reaction vessel with 1 atm of CO2, in THF, at room temperature
(Scheme 6).
Immediate precipitation of an orange solid was observed upon
addition of CO2. Analysis by IR spectroscopy of the solid
compound recovered at room temperature showed the presence
of a band at 1670 cm-1, characteristic of the carboxylate group.14
The formation of the poly(imidazolium-2-carboxylate betaine)
adducts 4 was further confirmed by NMR spectroscopy, and a
1
representative example of H and 13C NMR spectra of such
adducts is provided in Figure 2 for compound 4a. In the 1H NMR
spectrum of 4a, the signal corresponding to protons c of
the imidazole backbone (δ = 7.2-8.3 ppm) was shifted down-
field compared with that of the poly(NHC) precursor 3a
Characterization by TGA of 4a recovered after the first and the
second run revealed a weight loss of CO2 of 23% for the first run,
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Table 5. Poly(NHC-CO2)-Catalyzed Benzoin Condensa-
tion in THF at 80 °C for 24 ha
organocatalysts and yet soluble polymers, which could be easily
separated from the reaction mixture. The synthesis of poly-
(NHC)s could be achieved from ionic liquid monomers in a
three-step sequence, involving the free-radical polymerization of
1-vinyl-3-alkylimidazolium bromide monomers, followed by an
exchange of bromide for bis(trifluoromethanesulfonyl)imide
anion, and a last step of deprotonation with a strong base. As
for poly(NHC-CO2) adducts, they could be readily prepared by
carboxylation of the parent poly(NHC)s with 1 atm of CO2. This
carboxylation step is reversible and provides a convenient
method to mask and unmask poly(NHC)s, paving the way
toward a practical use and recovery of these polymer-supported
catalysts. As their molecular analogues, poly(NHC)s are less air-
stable than their zwitterionic poly(NHC-CO2) counterparts, a
premature deactivation being observed in the former case. In the
latter case, excellent yields of organocatalysis could be achieved
for the transesterification reaction, although the proper choice of
the alkyl substituent on the imidazol-2-ylidene appears to be
crucial. Indeed, the poly(NHC-CO2) adduct derived from
poly(1-vinyl-3-phenylethylimidazolium) has proven to be more
efficient than its poly(1-vinyl-3-isopropylimidazolium) homolo-
gue. In the case of benzoin condensation catalyzed by poly-
(NHC-CO2) adducts, the slow kinetic of the reaction and the
relatively high temperature required have not allowed us to
recycle efficiently the polymer catalyst, likely because of its
premature deactivation. Optimization of the reaction conditions
to generate smoothly poly(NHC)s from poly(NHC-CO2)
adducts is underway. We are also currently investigating the
behavior of poly(NHC-CO2) adducts when exposed to air for
prolonged time as well as the catalytic efficiency of corresponding
poly(NHC)s generated by thermolysis as a function of the
substituents of the carbene. Application of such recoverable
polymer-supported NHCs to other organocatalyzed reactions,
including cyanosilylation, Stetter reactions, or even organocata-
lyzed polymerization reactions, for example, group transfer or
ring-opening polymerizations, among others, is also in progress.
cycle
1
2
3
yield (%) with 4a
yield (%) with 4b
71
65
27
35
8
21
a All reactions were carried out with benzaldehyde (1 equiv) and desired
polyNHC (cat.: 10% mol of active species) in THF at 80°C for 24 h.
Yield in benzoin was determined by 1H NMR.
instead of the expected 25%, and of 15% for the second run,
instead of the 23% observed after the first run (ESI). This means
that CO2 was only partially reincorporated, most likely because
of a partial deactivation of the in situ generated poly(NHC).
Poly(NHC-CO2) adducts 4a,b were then investigated as
precatalysts for the benzoin condensation at 80 °C in THF for
24 h (Table 5). Surprisingly, moderate yields of benzoin were
obtained with both 4a and 4b (71 and 65% respectively) during
the first run of catalysis, which contrasted with the excellent
yields reached for the same reaction with poly(NHC)s 3a,b as
polymer-supported catalysts. Interestingly, the removal of
KNTf2 from the catalysts by washing several times 4a,b with
THF allowed us to increase the yield of benzoin to 93% under
otherwise identical conditions. We postulate that KNTf2 might
be able to deactivate indirectly the polymer catalyst by forming
stable poly(NHC-PhCHO-COO-KNTf2) adducts, by ana-
logy to the reaction of molecular NHC-CO2 adduct with
benzaldehyde in the presence of NaBPh4, as described by
Tommasi et al.28 Note that these adducts formally result from
irreversible trapping of the so-called Breslow intermediate by
CO2.
Upon the first recycling, a marked decrease in yield was
observed for both 4a and 4b (27 and 35% respectively), whereas
the conversion reached only 8 and 21% with 4a and 4b,
respectively, in the third run. In the case of 4a,b free of KNTf2,
only 5-10% of benzoin was obtained during the second run,
suggesting that KNTf2 might be involved in the stabilization of
the generated poly(NHC)s, probably by coordination of the
potassium cation to the carbene center. These results suggest that
deactivation of in situ generated poly(NHC)s 3a,b is more likely
to occur because of the long reaction time (24 h) and the
relatively high temperature (80 °C). Decreasing the reaction
time from 24 to 12 h in the presence of 20% mol of KNTf2 free
catalyst still provided a good yield (75%) of benzoin at 80 °C, but
recycling was not possible. The temperature could be lowered to
60 °C, but 24 h of reaction was necessary to obtain 80% yield of
benzoin. Here again the recycling was precluded because of
excessive catalyst deactivation. Attempts to generate poly-
(NHC)s 3a,b by heating 4a,b to 80 °C under vacuum for 2 h
in the absence of substrate, followed by the addition of benzal-
dehyde, resulted in no conversion after 24 h at room tempera-
ture. Optimization of the experimental conditions of the benzoin
condensation reaction catalyzed by poly(NHC-CO2) adducts
4a,b is currently underway.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of
S
b
compounds 2a, 2b, 3b, 4b are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail gnanou@enscbp.fr (Y.G.); taton@enscbp.fr (D.T.).
’ REFERENCES
(1) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand, G. J. Am.
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’ CONCLUSIONS
Poly(N-heterocyclic carbenes)s, referred to as poly(NHC)s,
and their poly(NHC-CO2) adducts can be viewed as polymer-
supported versions of the well-established molecular NHCs and
NHC-CO2 adducts, respectively. Both poly(NHC)s and their
poly(NHC-CO2) adducts have been synthesized and used as
1907
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Macromolecules
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