Poly(ionic liquid)s based on imidazolium hydrogen carbonate monomer units as recyclable polymer-supported N-heterocyclic carbenes: Use in organocatalysis

Article abstract of DOI:10.1002/pola.26869

Synthesis of novel poly(ionic liquid)s, namely, poly(1-vinyl-3- alkylimidazolium hydrogen carbonate)s, denoted as poly([NHC(H)][HCO ₃])s or PVRImHCO₃, where R is an alkyl group (R = ethyl, butyl, phenylethyl, dodecyl), is described. Two distinct synthetic routes were explored. The first method is based on the free-radical polymerization (FRP) of 1-vinyl-3-alkylimidazolium monomers featuring a hydrogen carbonate counter anion (HCO₃^-), denoted as VRImHCO₃. The latter monomers were readily synthesized by alkylation of 1-vinylimidazole (VIm), followed by direct anion exchange of 1-vinyl-3-alkylimidazolium bromide monomers (VRImBr), using potassium hydrogen carbonate (KHCO₃) in methanol at room temperature. Alternatively, the same anion exchange method could be applied onto FRP-derived poly(1-vinyl-3-alkylimidazolium bromide) precursors (PVRImBr). All PVRImHCO₃ salts proved air stable and could be manipulated without any particular precautions. They could serve as polymer-supported precatalysts to generate polymer-supported N-heterocyclic carbenes, referred to as poly(NHC)s, formally by a loss of "H₂CO₃" (H₂O +CO₂) in solution. This was demonstrated through selected organocatalyzed reactions of molecular chemistry, known as being efficiently mediated by molecular NHC catalysts, including benzoin condensation, transesterification and cyanosilylation of aldehyde. Of particular interest, recycling of the polymer-supported precatalysts was possible by re-carboxylation of in situ generated poly(NHC)s. Organocatalyzed reactions could be performed with excellent yields, even after five catalytic cycles.

Full text of DOI:10.1002/pola.26869

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Poly(ionic liquid)s Based on Imidazolium Hydrogen Carbonate
Monomer Units as Recyclable Polymer-Supported N-Heterocyclic
Carbenes: Use in Organocatalysis
1
,2,3
1,2
1,2
1,2
1,2
Paul Coupillaud,
Julien Pinaud, Nicolas Guidolin, Joan Vignolle, Mar eꢀ va F e` vre,
1
,2
3
1,2
Ellen Veaudecrenne, David Mecerreyes, Daniel Taton
1
Centre National de la Recherche Scientifique, Laboratoire de Chimie des Polym e` res Organiques, 16 avenue Pey-Berland,
F-33607 Pessac cedex, France
2
Universit eꢀ de Bordeaux, Laboratoire de Chimie des Polym e` res Organiques, IPB-ENSCBP, F-33607 Pessac cedex, France
3
POLYMAT Institute for Polymer Materials, University of the Basque Country UPV/EHU, Joxe Mari Korta Center,
Avda. Tolosa 72, 20018 Donostia-san Sebastian, Spain
Correspondence to: D. Taton (E-mail taton@enscbp.fr)
Received 5 May 2013; accepted 23 June 2013; published online 30 July 2013
DOI: 10.1002/pola.26869
ABSTRACT: Synthesis of novel poly(ionic liquid)s, namely,
precatalysts to generate polymer-supported N-heterocyclic car-
benes, referred to as poly(NHC)s, formally by a loss of “H CO
(H 1CO in solution. This was demonstrated through
poly(1-vinyl-3-alkylimidazolium hydrogen carbonate)s, denoted
2
3
”
as poly([NHC(H)][HCO
3
])s or PVRImHCO
3
, where R is an alkyl
2
O
2
)
group (R 5 ethyl, butyl, phenylethyl, dodecyl), is described.
Two distinct synthetic routes were explored. The first method
is based on the free-radical polymerization (FRP) of 1-vinyl-3-
selected organocatalyzed reactions of molecular chemistry,
known as being efficiently mediated by molecular NHC cata-
lysts, including benzoin condensation, transesterification and
cyanosilylation of aldehyde. Of particular interest, recycling of
the polymer-supported precatalysts was possible by re-
carboxylation of in situ generated poly(NHC)s. Organocatalyzed
reactions could be performed with excellent yields, even after
alkylimidazolium monomers featuring a hydrogen carbonate
2
counter anion (HCO
3
), denoted as VRImHCO
3
.
The latter
monomers were readily synthesized by alkylation of 1-
vinylimidazole (VIm), followed by direct anion exchange of 1-
vinyl-3-alkylimidazolium bromide monomers (VRImBr), using
five catalytic cycles. V
2013 Wiley Periodicals, Inc. J. Polym.
C
potassium hydrogen carbonate (KHCO
3
) in methanol at room
Sci., Part A: Polym. Chem. 2013, 51, 4530–4540
temperature. Alternatively, the same anion exchange method
could be applied onto FRP-derived poly(1-vinyl-3-alkylimidazo-
lium bromide) precursors (PVRImBr). All PVRImHCO
3
salts
KEYWORDS: N-heterocyclic carbenes; organocatalysis; poly(ionic
liquid)s; polymer-supported catalysts; radical polymerization;
catalysis; synthesis
proved air stable and could be manipulated without any
particular precautions. They could serve as polymer-supported
INTRODUCTION In the last two decades, N-heterocyclic car-
instance, as precatalysts in molecular and macromolecular
reactions.
5
,9
benes (NHCs) have been employed not only as unique
1
–3
ligands for transition metals,
but also as true organocata-
lysts in molecular chemistry for various molecular transfor-
For instance, NHCs can be masked in the form of NHC-Ag(I)
4
–6
15,16
17–20
21
mations.
More recently, NHCs have also served as
complexes
fluorophenyl,
or 2-alkoxy,
trichloromethyl,
carboxylic acid,
penta-
2
1
22
23,24
organocatalysts or building blocks in metal-free polymer syn-
isothiocyanate,
and
7
–9
25,26
thesis.
Steric and/or electronic properties of NHCs can be
NHC-CO
carboxylates usually result from the carboxylation of free
carbenes with CO . Hence, the intermediacy of air- and
2
adducts.
The latter zwitterionic azol(in)ium-2-
finely tuned through variation of their substituent pattern,
which allows modulating their overall reactivity (e.g., nucleo-
2
1
0,11
25,27–30
philicity and/or basicity) toward various substrates.
However, their poor stability when exposed to air makes
moisture-sensitive species is required in most cases.
Therefore, a need still exists to develop air-stable NHC pre-
cursors that could be of practical usage for the purpose of
1
2–14
NHCs difficult to handle.
To circumvent this limitation,
3
1–42
various masked NHCs have proved of practical use, for
organocatalysis or organometallic chemistry.
Additional Supporting Information may be found in the online version of this article.
2013 Wiley Periodicals, Inc.
V
C
4
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In addition, progressive hydrolysis of NHC-CO2 adducts,
forming imidazol(in)ium hydrogen carbonates, denoted as
both refractometric (RI) and UV detectors (Varian). Analyses
were performed using a PL-GPC50 plus integrated GPC Sys-
tem equipped with PSS SUPREMA Max columns with pore
sizes of 30 and 1000 Å, respectively (connected in series),
fitted with dual detectors (refractometry and UV). Calibration
curve was done using poly(2-vinylpyridine) as polymer
standards.
4
1,43
[
NHC(H)][HCO ], has been reported.
However, we have
3
shown that these salts can serve as a genuine source of
NHCs under very mild conditions, formally by a loss of
4
1,44
“
H CO ” (H O 1CO ).
Synthesis of [NHC(H)][HCO ] pre-
3
2
3
2
2
cursors can be readily achieved in one pot, by anion
exchange from commercial imidazol(in)ium halides
Materials
(
3
[NHC(H)][X]; X 5 Br or Cl), in the presence of KHCO in
1-Vinylimidazole
(99%),
1-bromobutane
(99%),
1-
methanol. Their NHC-like behavior was then experimentally
evidenced in model (macro)molecular reactions, and NHC
formation was supported by density functional theory (DFT)
bromoethane (99%), 2-bromoethylbenzene (99%), and 1-
bromododecane (99%) were obtained from Alfa Aesar and
used as received. Azobis(2-methylpropionitrile) (AIBN, 99%)
was received from Aldrich and was purified by recrystalliza-
tion from methanol. Dimethylaminopropionitrile (DMAPN,
98%) and potassium persulfate (KPS, 99%) were purchased
4
1,45
calculations.
We now wish to report polymeric versions of these imidazolium
hydrogen carbonates, referred to as poly([NHC(H)][HCO ])s, and
their use in organocatalysis. Catalyst recycling is obviously of
3
0
from Aldrich and used as received. 4,4 -Azobis(4-cyanovaleric
4
6–51
0
prime importance in catalyzed processes,
including molecu-
acid) (V-501, 99%) and 2,2 -azobis(4-methoxy-2,4-dimethyl
52
lar reactions by an organocatalytic pathway. In a previous con-
tribution, we described the synthesis of polymeric analogs of
valeronitrile) (V-70, 99%) were purchased from Wako chemi-
cals used as received. Benzyl alcohol (Aldrich, 99%) and
benzaldehyde (Aldrich, 99.5%) were distilled before use.
Vinyl acetate (Aldrich, 99%) was dried over CaH2 and dis-
tilled before use. Trimethylsilylcyanide (TMSCN, 98%, ABCR)
both NHCs and NHC-CO adducts, denoted as poly(NHC)s and
2
3
5
2
poly(NHC-CO ) adducts. When used as polymer-supported
precatalysts for metal-free transesterification and benzoin con-
densation reactions, poly(NHC-CO ) adducts proved easier to
was used as received. Potassium bicarbonate (KHCO 99.7%,
Aldrich) was dried at 50 C for 12 h under vacuum before
2
3
ꢀ
handle and could be more easily recycled, than their “bare” pol-
y(NHC) counterparts. One limitation of this approach, however,
was the need for the prior synthesis of related air sensitive pol-
use. Tetrahydrofuran (THF) was distilled over Na/benzophe-
none. Dimethyl sulfoxide (DMSO 99.5%, Fischer) and ethyl
acetate (99.7%, Aldrich) were used without further purifica-
tion. MeOH was distilled over Na metallic before use. CO2
(N-45, Air Liquide) was purified by passing through a click-
on inline “super clean purifier” (SGT) before use.
53
y(NHC) intermediates. We now provide an easy synthetic
access to air-stable and recyclable poly([NHC(H)][HCO ])s for
3
facile organocatalysis. These salt precursors were thus readily
synthesized following two distinct routes that did not require
the prior formation of poly(NHC)s. These particular poly(ionic
54–60
Synthesis of 1-Vinyl-3-ethylimidazolium Bromide
VEtImBr (1a)
liquid)s (PILs)
were subsequently used as a source of pol-
y(NHC)s, by analogy with their molecular versions, for selected
organocatalyzed molecular reactions. Recarboxylation at the
completion of each tested reaction allowed us to recycle the
1
-Vinyl-3-ethylimidazolium bromide ([NHC(H)][Br]) 1a was
3
5,61–63
prepared following a procedure already described.
The monomer was recovered as a white solid (100% yield).
poly([NHC(H)][HCO ]) precursors (Scheme 1), while maintain-
3
6
1
1
-Vinyl-3-butylimidazolium bromide VBuImBr (1b) , 1-
ing excellent yields, even after five catalytic cycles.
vinyl-3-(1-phenylethyl)-imidazolium bromide VEtPhImBr
3
5
(1c),
and 1-vinyl-3-dodecylimidazolium bromide VDo-
EXPERIMENTAL
6
2
deImBr (1d) were prepared following a similar procedure.
Instrumentation
H NMR and C NMR spectra were recorded on a Bruker
NMR data were in accordance with those reported in the
literature.
1
13
35
AC-400 spectrometer in appropriate deuterated solvents.
Molar masses were determined by size exclusion chromatog-
Synthesis of 1-Vinyl-3-ethylimidazolium Hydrogen
carbonate VEtImHCO (2a)
3
raphy (SEC) in H O/formic acid (0.3 M) as the eluent (0.6
2
ꢀ
A mixture of 1-vinyl-3-ethylimidazolium bromide, 1a, (1 g,
mL/min) and with pyridine as a flow marker at 25 C, using
4
.92 mmol) and 1.05 equiv of KHCO (518 mg, 5.18 mmol)
3
ꢀ
was dried at 60 C under vacuum for 12 h. Dry methanol (5
mL) was then added, and the resulting suspension was
stirred for 24 h at rt. After filtration of the suspension over
Celite, methanol was evaporated under vacuum to yield a
sticky solid. After trituration of the solid with acetone and
filtration, 2a was obtained as a yellowish powder and dried
under dynamic vacuum (yield: 83%). The product was then
stored under static vacuum.
SCHEME 1 Reversible generation of poly(NHC)s 5 from poly
(
3 2
[NHC(H)][HCO ]) salts 4 and poly(NHC-CO ) adducts 6. [Color
1
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
H NMR (CD OD): d 1.56 (d, CH ACH A, 3H), 4.39 (q, CH A
3
3
2
3
CH A, 2H), 5.43 (dd, HCH@CHAN, 1H), 5.92 (dd, HCH@CHAN,
2
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FIGURE 1 H NMR spectrum of 1-vinyl-3-butylimidazolium hydrogen carbonate 2b in CD
3
OD. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
1
H), 7.26 (dd, CH @CHAN, 1H), 7.78 (s, NACH@CHAN, 1H),
(DMSO-d ): d 19.36 (CH ACHA), 58.9 (CHA(CH )(C H )),
2
6
3
3
6 5
13
8.00 (s, NACH@CHAN, 1H). C NMR (CD OD): d 15.41
107.7
(CH @CHAN),
118.8
(NACH@CHAN),
120.8
3
2
(
CH ACH A), 52.8 (CH ACH ), 109.9 (CH @CHAN), 120.8
(NACH@CHAN), 126.0 and 128.0 (CHAC H ), 133.5
3
2
2
3
2
6 5
(
NACH@CHAN), 124.1 (NACH@CHAN), 129.9 (CH @CHAN),
(CH @CHAN), 138.1 (NACHAN), 158.3 (HCO ).
2 3
2
136.1 (NACHAN), 161.4 (HCO3).
Synthesis of 1-Vinyl-3-dodecylimidazolium Hydrogen
Synthesis of 1-Vinyl-3-butylimidazolium Hydrogen
Carbonate VDodeImHCO (2d)
3
Carbonate VBuImHCO (2b)
3
2d was prepared following a similar procedure to that used
2b was prepared following a similar procedure to that used
for the synthesis of 2a.
for the synthesis of 2a.
1
3 3 2 2
H NMR (CD OD): d 0.7 (t, CH (ACH A)10CH , 3H), 1.2–1.3 (m,
1
H NMR (CD OD, Fig. 1): d 0.98 (t, CH ACH ACH ACH A,
3
3
2
2
2
CH (ACH A) CH ACH , 18H), 1.7 (q, CH (ACH A) CH ACH ,
3
2
9
2
2
3
2
9
2
2
3
H), 1.32 (s, CH ACH ACH ACH A, 2H), 1.90 (t,
3 2 2 2
2
H), 4.1 (t, CH (ACH A) CH ACH , 2H), 5.4 (dd, HCH@CHAN,
3 2 9 2 2
CH ACH ACH ACH A, 2H), 4.27 (t,NACH ACH ACH ACH ,
3
2
2
2
2
2
2
3
1
H), 5.92 (dd, HCH@CHAN, 1H), 7.26 (dd, CH @CHAN, 1H),
2
2
1
1
3
1
H), 5.4 and 4.9 (d, NACHACH , 2H), 7.25 (dd, NACH@CH ,
13
2
2
7.78 (s, NACH@CHAN, 1H), 8.00 (s, NACH@CHAN, 1H).
C
13
H), 7.7 and 8.0 (d, CH@CH, 2H). C NMR (CD OD, Fig. 2): d
3
NMR (CD OD): d 15.1 (CH ACH ACH (ACH A) CH ACH A),
3
3
2
2
2
7
2
2
3.57 (CH
7.72 (CH
09.9 (CH @CHAN),
3
ACH
3
ACH
2
ACH
2
ACH
2
A), 20.28 (CH
A), 50.73 (CH
120.8 (NACH@CHAN),
3
ACH
3
ACH
2
ACH
2
ACH
2
ACH
ACH
124.1
2
A),
2
A),
2
4.9 (CH
CH (ACH A) CH ACH A), 31.2 (CH ACH ACH (ACH A)
7
3 2 2 2 7 2 2 3 2
ACH ACH (ACH A) CH ACH A), 28.2 (CH ACH A
2
ACH
2
ACH
2
2
2
2
7
2
2
3
2
2
2
2
CH ACH A), 34.7 (CH (ACH A) CH ACH A), 54.2 (CH A
2
2
3
2
9
2
2
3
(
NACH@CHAN), 129.9 (CH @CHAN), 135.1 (NACHAN),
2
CH ACH ACH A), 110.5 (CH @CHAN), 122.8 (NACH@CHAN),
2
2
2
2
161.0 (HCO3).
1
24.1 (NACH@CHAN), 130.9 (CH @CHAN), 137.3 (NACHAN),
2
161.4 (HCO3).
Synthesis of 1-Vinyl-3-(1-phenylethyl)imidazolium
Hydrogen Carbonate VEtPhImHCO (2c)
3
Synthesis of PVBuImBr (3b) by Polymerization of
VBuImBr (1b)
2c was prepared following a similar procedure as that used
for the synthesis of 2a.
In a typical experiment, a 10-mL Schlenk tube was flame
dried and charged with 1 g (4.3 mmol) of 1b, 12.7 mg (0.08
mmol) of AIBN and 4 mL of methanol. The Schlenk tube was
subjected to five freeze-thaw cycles and placed in a thermo-
stated oil bath previously maintained at 80 C. The reaction
was quenched after 3 h by sudden cooling with liquid
1
H NMR (CD OD): d 1.89 (d, CH ACHA, 3H), 5.42 (dd,
3
3
3 6 5
HCH@CHAN, 1H), 5.83 (m, CHA(CH )(C H ), 1H), 6.00 (dd,
HCH@CHAN, 1H), 7.29 (dd, CH @CHAN, 1H), 7.43 (m,
CHAC H , 5H), 8.00 (s, NACH@CHAN, 1H), 8.25 (s,
NACH@CHAN, 1H), 9.84 (s, NACHAN, 1H).
2
ꢀ
6
5
1
3
C NMR
4
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1
3
FIGURE 2 C NMR spectrum of 1-vinyl-3-butylimidazolium hydrogen carbonate 2b in CD
3
OD. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
nitrogen. The resulting poly(1-vinyl-3-butylimidazolium) bro-
mide (3b) was isolated by precipitation in acetone. After
drying under vacuum, 3b was obtained as a yellowish pow-
mation Fig. S1) : d 14.7 (CH ACH A), 40.9 (H CACHAN),
3 2 2
58.3 (CH ACH ),120.3 and 122.0 (NACH@CHAN), 136.8
2 3
(NACHAN), 160.9 (HCO3).
21
der (yield 95%). M 5 25,000 g mol (D 5 1.45) by SEC in
n
H O/acid formic (0.3 M) (see Fig. 5).
2
Synthesis of PVBuImHCO (4b, R 5 butyl) by
3
Polymerization of VBuImHCO3
4b was prepared following a similar procedure to that used
1
H NMR (CD OD, see Supporting Information Fig. S7): d 8.9–
3
2
1
9
.3 (br, NACHAN, 1H), 7.3–7.8 (br, CH@CH, 2H), 3.9–4.5 (br,
NACH ACH ACH ACH , 2H), 4.4–4.8 (br, NACHACH , 1H),
.4–3.0 (br, NACHACH2, 2H), 1.7–2.1 (br, CH A
for 4a (yield: 98%). M
n
5 26,200 g mol (D 5 1.78) by SEC
2
2
2
3
2
in H
O/formic acid (0.3 M) (see also Fig. 5).
2
2
3
1
CH ACH ACH A, 2H), 1.3–1.6 (br, CH ACH ACH ACH A,
2
PVPhEtImBr (3c), and PVDodeImBr (3d) were obtained fol-
lowing a similar procedure.
H
NMR (CD
ACH A, 3H), 1.2–1.4 (br, CH
1.6–1.9 (br, CH ACH ACH ACH
CH
br,NACH
3
OD, see Fig. 3): 0.8–1 (br, CH
3
ACH
A, 2H),
A, 2H), 2.1–2.4 (br,
ACHAN, 1H), 4.2–4.7
, 2H), 7.5–8.0 (br, CH@CH, 2H),
A
2
2
2
2
3
2
2
2
H), 0.9–1.2 (br, CH ACH ACH ACH A, 3H). PVEtImBr (3a),
CH
2
2
3
ACH ACH ACH
2 2 2
3
2
2
2
3
2
2
2
ACHAN, 1H), 3.7–4.1 (br, CH
ACH ACH ACH
2
2
(
2
2
2
3
1
3
Synthesis of PVEtImHCO
3
(4a, R 5 ethyl) by
9.5–10 (br, NACHAN, 1H). C NMR (CD OD, see Fig. 4): d
3
Polymerization of VEtImHCO3
13.5 (CH ACH ACH ACH A), 20.28 (CH ACH ACH ACH A),
3
2
2
2
3
2
2
2
In a typical experiment, a Schlenk tube was flame-dried and
charged with 1 g (5.43 mmol) of 2a, 24.3 mg (0.09 mmol) of
KPS and 10 mL of DMSO. The Schlenk tube was subjected to
five freeze-thaw cycles before adding 10.1 mL (0.09 mmol) of
DMAPN under argon flow. The reaction mixture was stirred
for 24 h at room temperature. The obtained polymer was
precipitated in acetone to remove residual monomer, fil-
trated, and dried under vacuum. 4a was recovered as a yel-
32.0 (CH ACH ACH ACH A), 42.2 (CH ACHAN), 50.7
3 2 2 2 2
(CH ACH ACH ACH A), 54.2 (CH ACHAN), 120.8–122.3
3
2
2
2
2
(NACH@CHAN), 137.1 (NACHAN), 159.0 (HCO3).
Synthesis of PVPhEtImHCO (4c, R5 1-phenylethyl) by
3
Polymerization of VPhEtImHCO3
4c was prepared following a similar procedure to that used
for 4a (yield 95%).
2
1
lowish powder (yield: 85%). M 5 29,000 g mol (D 5 2.3)
n
1
H NMR (CD OD, see Supporting Information Fig. S4) : 1.7–
by SEC in H O/formic acid (0.3 M) (see also Fig. 5).
3
2
2
.1 (br, CH ACHA, D3H), 2.3–3.3 (br, H CACHAN, 2H), 4.6–
3
2
1
H NMR (CD
3
OD, see Supporting Information Fig. S2): 1.4–
2 3 6 5
5.1 (br, CH ACHAN, 1H), 5.3–5.6 (br, CHA (CH )(C H ), 1H),
1
4
.7 (CH ACH , 3H), 2.4–2.7 (H CACHAN, 2H), 4.1–
7.2–7.7 (br, CHAC H , NACH@CHAN, NACH@CHAN, 7H).
C NMR (CD OD, see Supporting Information Fig. S3): 22.3
3
(CH ACHA), 41.4 (CH ACHAN), 60.2 (CHA(CH )(C H )),
3 2 3 6 5
3
2
2
6 5
1
3
.4(CH ACH , 2H), 4.4–4.7 (H CACHAN, 1H), 7.4–8.0
2
3
2
1
3
(
NACH@CHAN, 2H). C NMR (CD OD, see Supporting Infor-
3
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6
FIGURE 3 H NMR spectrum of poly(1-vinyl-3-butylimidazolium hydrogen carbonate) salt 4b in DMSO-d . [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
6
1.9 (CH ACHAN), 121.5 (NACH@CHAN), 128.7–136.3
(ACH A) CH ACH , 18H), 1.7–2.0 (br, CH (ACH A)
2 9 2 2 3 2 9
2
(
CHAC H ), 139.8 (NACHAN), 161.4 (HCO ).
CH ACH , 2H), 2.22.5 (br, CH ACHAN, 2H), 4.1–4.3 (br,
5
5
3
2 2 2
CH (ACH A) CH ACH , 2H), 4.3–4.5 (br, CH ACHAN, 1H),
3
2
9
2
2
2
1
3
7
.5–8.0 (br, CH@CH, 2H). C NMR (CD OD, see Supporting
3
Synthesis of PVDodeImHCO
3
(4d, R 5 dodecyl) by
Information Fig. S5):
d
15.1 (CH ACH ACH (ACH A)
3
2
2
2
7
Polymerization of VDodeImHCO3
d was prepared following a similar procedure to that used
for 4a (yield: 68%).
CH ACH A), 24.9 (CH
2
2
3
2 2 2 7 2 2
ACH ACH (ACH A) CH ACH A),
4
2
8.2 (CH ACH ACH (ACH A) CH ACH A), 31.2 (CH ACH A
3 2 2 2 7 2 2 3 2
CH (ACH A) CH ACH A), 34.7 (CH (ACH A) CH ACH A),
2
2
7
2
2
3
2
9
2
2
1
H NMR (CD OD, see Supporting Information Fig. S6) d 0.7–
44.2 (CH
124.2 (NACH@CHAN), 137.3 (NACHAN), 162.2 (HCO
3
ACHAN), 54.2 (CH
2
ACH ACH ACH A), 122.8–
3 2 2 2
3
0
.8 (br, CH (ACH A) CH , 3H), 1.2–1.1–1.4 (br, CH3
).
3
2
10
2
1
3
6
FIGURE 4 C NMR spectrum of poly(1-vinyl-3-butylimidazolium hydrogen carbonate) salt 4b in DMSO-d . [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
4
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FIGURE 5 Aqueous SEC traces of PVBuImBr 3b, PVEtImHCO
a (entry 3, Table 1) and PVBuImHCO 4b (entry 4, Table 1)
3
FIGURE 6 Results of the transesterification of benzyl alcohol (1
ꢀ
4
3
equiv) and vinyl acetate (1.2 equiv) in THF at 80 C for 2 h in
obtained by free radical polymerization in DMSO at rt. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
3
the presence of poly([NHC(H)][HCO ]) 4 (see also Scheme 2).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
Synthesis of PVRImHCO via Anion Exchange of PVRImBr
Poly(1-vinyl-3-alkylimidazolium bromide) (4 mmol) 3 were
dissolved in dry methanol, and the resulting solution was
allowed to stir for 1 h under vacuum and the flask was then
subjected to three Ar/vacuum cycles. A 5 mL portion of THF,
0.5 mL (5 mmol) of benzyl alcohol, and 0.55 mL (6 mmol) of
vinyl acetate were added. The reaction mixture was stirred
for 2 h at 80 C. Note that a suspension was obtained under
these conditions owing to the nonsolubility of precursor 4a–
c. After the reaction mixture was allowed cooling to rt, 1
atm of CO was added, to favor the formation of poly(1-
2
vinyl-3-alkylimidazolium carboxylate) 6 from the corre-
sponding poly(NHC) 5 (see Scheme 1). After stirring for 30
3
added to a stirred solution of KHCO (1.05 equiv), in metha-
3
ꢀ
nol. The resulting suspension was stirred for 24 h at rt. After
filtration over Celite, methanol was evaporated under vac-
uum to yield a sticky solid. After trituration of the solid with
acetone and filtration, poly(1-vinyl-3-alkylimidazolium hydro-
gen carbonate) 4 was obtained as a yellowish powder and
dried under dynamic vacuum (yield 95%). NMR data were in
accordance with 4b (see Figs. 3 and 4).
min, the mixture was filtered under vacuum, and the filtrate
1
was analyzed by H NMR in CDCl . Conversion in benzyl ace-
3
1
Catalytic Tests Using PVRImHCO Salt Precursors
tate was calculated by H NMR in CDCl
3
(Supporting Infor-
3
All catalytic tests were carried out under a dry and inert
atmosphere, at rt and at 80 C, using Schlenk equipments.
mation Fig. S8) by comparing the integral value of the
ACH A benzyl alcohol signal (d 4.5 ppm) to that of the
ꢀ
2
ACH A benzyl acetate signal (d 5 ppm). The recovered poly-
2
mer was suspended in THF and reused for a next run of
Transesterification
catalysis.
In a typical experiment (see Fig. 6), precursor 4 (0.5 mmol)
was introduced in a Schlenk tube. The solid compound was
Cyanosilylation
In a typical experiment (see Fig. 7), a Schlenk tube was
charged with 4 (0.25 mmol), and the solid compound was
allowed to stir for 1 h under vacuum and the flask was then
subjected to three Ar/vacuum cycles. A 5 mL portion of THF
followed by 0.5 mL (5 mmol) of benzaldehyde and 0.75 mL
of TMSCN (6 mmol) were added. The rest of the procedure
was identical to that described above for the transesterifica-
tion reaction. Conversion in a-trimethylsilyloxy-phenylaceto-
1
nitrile was determined by H NMR in CDCl3 (Supporting
Information Fig. S9) by comparing the integral value of the
aldehyde signal of benzaldehyde (d 10 ppm) to that of the
ACHA cyanide product (d 5.5 ppm).
3
SCHEME 2 Synthesis of PVRImHCO 4a–d by anion exchange
of VRImBr 1a–d, followed by FRP of VRImHCO
3
2a–d (path a)
Benzoin Condensation
In a typical experiment (see Fig. 8), 4 (0.5 mmol) and molec-
ular sieves were introduced into a Schlenk tube. The solid
0
and synthesis of PVRImHCO
3
4b by FRP of VRImBr 1b fol-
lowed by anion exchange exchange of PVRImBr 3b (path b).
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ꢀ
FIGURE 7 Results of the cyanosilylation of benzaldehyde and trimethylsilyl cyanide performed in THF at 80 C for 1 h in the pres-
3
ence of poly([NHC(H)][HCO ]) 4 (see also Scheme 2). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
mixture was allowed to stir for 1 h under vacuum and the
flask was then subjected to three Ar/vacuum cycles. A 5 mL
portion of THF and then 0.5 mL (5 mmol) of benzaldehyde
were added. The reaction mixture was stirred for 24 h at 80
Figures 1 and 2 show the NMR spectra of the VBuImHCO3
1
2b compound. Analysis by H NMR in CD OD did not allow
3
stating on the quantitative anion exchange, from bromide to
hydrogen carbonate (Fig. 1); chemical shifts of protons of
the imidazolium backbone were similar to that of the start-
ꢀ
C. The rest of the procedure was identical to that dscribed
above for the transesterification reaction. Benzoin conversion
was determined by H NMR in CDCl3 (Supporting Informa-
tion Fig. S10) by comparing the integral value of the alde-
hyde signal of benzaldehyde (d 10 ppm) with that of the
ACHA benzoin signal (d 6 ppm).
ing material 1b. In contrast, the characteristic signals of both
1
2
the N CH carbon and the HCO
were observed by C NMR at 136.0 and 161.1 ppm, respec-
quaternary carbon atoms
2
3
1
3
1
tively (Fig. 2). All other signals observed both on the H and
1
3
C NMR spectra were consistent with data reported on
4
1
molecular imidazolium hydrogen carbonate homologs.
Synthesis of PILs by free-radical polymerization (FRP) of 1-
RESULTS AND DISCUSSION
5
4–60
vinyl-3-alkylimidazolium is well-documented.
FRP of
Synthesis of Poly(1-vinyl-3-alkylimidazolium hydrogen
carbonate)s, 4
Poly(1-vinyl-3-alkyl imidazolium hydrogen carbonate)s 4
VRImHCO monomers 2a–d was investigated under different
conditions, as summarized in Table 1. For instance, attempts
3
to polymerize 2b using either azobis(2-methylpropionitrile)
synthesized in this work, denoted as poly(VRImHCO )s or
0
3
(AIBN) or 4,4 -azobis(4-cyanovaleric acid) (V-501) as a radi-
poly([NHC(H)][HCO ])s, where R is an alkyl substituent
ꢀ
3
cal source at 80 C were not successful, neither in DMSO nor
(
R 5 ethyl, butyl, phenylethyl, dodecyl), represent novel pol-
in MeOH as solvent (entry 1). We hypothesized that a too
5
4–60
y(ionic liquid)s (PILs).
As depicted in Scheme 2, synthe-
ꢀ
high temperature (80 C) could lead to the degradation of
sis of these specific PILs could be achieved following two
distinct routes that did not require the prior formation of
poly(NHC) precursors, in contrast to our previous work.
VRImHCO3 monomers 2. 2,2’-Azobis(4-methoxy-2,4-dimethyl
valeronitrile) (V-70) was then employed as initiator at 40 C,
ꢀ
3
5
The first synthetic method is based on the free-radical poly-
merization (FRP) of 1-vinyl-3-alkylimidazolium monomers
2
featuring a hydrogen carbonate (HCO
3
) counter anion,
denoted as VRImHCO (Scheme 2, path a). The latter mono-
3
mers were readily synthesized by alkylation of 1-
vinylimidazole (VIm), followed by direct anion exchange of
1-vinyl-3-alkylimidazolium bromide (VRImBr) monomers,
using potassium hydrogen carbonate (KHCO ) in methanol,
3
at room temperature. This method could be generalized to
VRImBr monomers carrying various alkyl substituents in 3-
position. Four different VRImBr monomers 1a–d were thus
synthesized by quaternization of VIm, using various alkyl (1-
phenylethyl, n-butyl, ethyl and n-dodecyl) bromides, follow-
3
5,61–63
ing a well-established procedure.
The four corre-
sponding VRImHCO3 monomers 2a–d (R5 ethyl, n-butyl, 1-
phenylethyl and n-dodecyl) were next obtained by anion
exchange on the basis of our recent reports on molecular
FIGURE 8 Results of the benzoin condensation from benzalde-
ꢀ
hyde performed in THF at 80 C for 24 h in the presence of
poly([NHC(H)][HCO ]) 4 (see also Scheme 2). [Color figure can
3
4
1,44
imidazolium precursors.
The novel VRImHCO3 mono-
be viewed in the online issue, which is available at
1
13
mers were characterized by H and C NMR spectroscopy.
wileyonlinelibrary.com.]
4
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TABLE 1 Free-Radical Polymerization of VRImHCO
3
Monomers 2
a
ꢀ
b
21 c
)
c
Entry
Monomer
Initiator
T ( C)
Solvent
Time (h)
Conv. (%)
Mn (g mol
D
1
2
3
4
5
6
7
2b
2b
2a
2b
2b
2c
2d
AIBN or V-501
V-70
80
DMSO or MeOH
DMSO
144
72
18
18
18
18
18
0
–
–
40
30
85
98
40
95
68
–
–
KPS/DMAPN
KPS/DMAPN
KPS/DMAPN
KPS/DMAPN
KPS/DMAPN
R.T.
R.T.
R.T.
R.T.
R.T.
DMSO
29,000
26,200
–
2.30
1.78
–
DMSO
MeOH
d
d
DMSO
–
–
d
d
DMSO
–
–
a
c
A [monomer]/[initiator] ration equal to 60:1 was used.
Conversion was calculated by H NMR from the disappearance of the
Molecular weight and dispersity were determined by SEC in water/for-
mic acid (0.3 M) calibrated with poly(2-vinyl pyridine).
b
1
d
signal at 7.2 ppm and appearance of signal at 0.8 ppm corresponding
to the polymer.
Molecular weight ant dispersity were not determined due to the insol-
ubility of corresponding polymer in aqueous solvent used for SEC.
which allowed the polymerization to proceed, albeit with a
low conversion after a reaction time of 72 h (entry 2). In
contrast, the redox initiating system utilizing potassium per-
sulfate (KPS) and dimethylaminopropionitrile (DMAPN) at
room temperature (rt) enabled quantitative conversion of
monomer 2b in DMSO, after 18 h (entry 4), whereas a low
conversion was noted in MeOH, likely owing to the poor solu-
bility of KPS in the latter solvent (entry 5). All monomers 2a–
d were thus efficiently polymerized in DMSO at rt in the pres-
to FRP-derived poly(1-vinyl-3-alkylimidazolium bromide)s,
PVRImBr 3 (Scheme 1, path b). The FRP of VRImBr mono-
3
5,54–56,65,66
mers 1 was already reported.
For instance,
PVBuImBr 3b was synthesized in methanol using AIBN as a
ꢀ
1
radical source at 80 C. Characterization of 3b by H NMR
spectroscopy confirmed the expected structure (Supporting
0
Information Fig. S7). Corresponding PVBuImHCO3 4b was
obtained by anion exchange of 3b with KHCO in methanol at
3
rt for 24 h (see Scheme 2). A similar yellowish powder to 4b
1
13
ence of KPS/DMAPN, providing polymers, PVRImHCO 4a–d,
(path a) was obtained, with identical H and C NMR spectra.
3
in near quantitative yields (Table 1). Although the possibility
of anion exchange between KPS and PVRImHCO cannot be
3
ruled out, this exchange reaction could not be evidenced.
Thus, synthesis of novel 1-vinyl-3-alkyl imidazolium-based
PILs with hydrogen carbonate as counter-anion could be
readily achieved, following two distinct routes both relying
1
2
2
Characterization of these polymers by H NMR spectroscopy
on a simple and direct anion exchange, from Br to HCO3
.
confirmed the expected structure (Experimental and ESI). In
particular, the signal corresponding to the CH proton of the
imidazolium moiety could be clearly identified at 9–10 ppm.
In addition, signals due to the vinylic protons of monomers
at 5.4–6 ppm completely vanished, while protons of mono-
Use of Poly(1-vinyl-3-alkyl imidazolium hydrogen
carbonate)s 4 in Organocatalysis
The potential of poly(vinylimidazolium hydrogen carbonate)
0
salts 4a–d and 4b (see Scheme 2) as polymer-supported
mer units CHACH were observed at 4–4.3 ppm (Figs. 3 and
precatalysts was then explored. Here we provide our prelimi-
nary results regarding their catalytic potential, without a real
investigation into the optimization of the catalytic efficiency.
It is well established, indeed, that polymer-supported cata-
lysts with optimized catalytic properties requires a specific
design and a systematic and logical approach, from the pre-
cise understanding of the mechanism of elementary catalytic
2
4
and Supporting Information Figs. S1 and S6). While all sig-
nals were broadened after polymerization, that correspond-
2
ing to the HCO3 carbon atom at 160.0 ppm was found as
1
3
sharp as in the C NMR spectrum of monomer 2 (Fig. 4).
Characterization of PILs by size exclusion chromatography
(
SEC) is often challenging owing to interactions of these pol-
67
reactions. Various parameters can be manipulated, includ-
6
4
ylectrolytes with SEC columns.
Very recently, however,
ing the nature of the polymer support, the nature of the
linker, catalyst density along the polymer support, and the
nature of the connectivity of the catalyst to the support.
Matyjaszewski et al. reported a “universal” method to ana-
lyze well-defined PILs based on imidazolium-based with bis(-
2
trifluoromethanesulfonyl)imide ( NTf
2
) counter-anions, by
2
0
employing THF as eluent in presence of the same NTf2
anion. In the present work, analysis of some polymers
Here, polymers 4a–d and 4b were expected to in situ gener-
ate poly(NHC)s 5, as illustrated in Scheme 1, by analogy
3
5
0
(PVBuImBr 3b, PVEtImHCO3 4a and PVBuImHCO3 4b) by
with their molecular versions. In this regard, 4a–d and 4b
aqueous SEC showed a unimodal molecular weight distribu-
tion in each case (Fig. 5), with a dispersity between 1.4 and
are also denoted as poly[NHC(H)][HCO ]s.
3
Three different organocatalyzed reactions of molecular chem-
2.3. Due to the nonsolubility of 4c and 4d in water, analysis
3
5,68–70
istry were implemented, namely, transesterification,
of these polymers by aqueous SEC could not be carried out.
3
5,36,70,71
benzoin condensation
and cyanosilylation of alde-
7
2,73
An alternative synthetic route to PVRImHCO 4 consisted in
hydes, known as being efficiently catalyzed by both
molecular NHCs and ([NHC(H)][HCO ]) salt precursors.
3
4
4
applying the same anion exchange method described above
3
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(
Scheme 2), showed the same catalytic efficiency for this trans-
esterification reaction, indicating that the structure of both
polymers was very similar. The poly([NHC(H)][HCO ]) precur-
3
sor 4d allowed reaching a complete conversion in benzyl ace-
tate after the first run (Fig. 6). However, 4d (or 6d, Scheme 1)
could not be recovered by re-carboxylation due to its solubility
in THF, unlike 4a–c (or 6a–c). The catalytic efficiency of the
latter precursors however decreased progressively with the
number of cycles, which might be explained by uncomplete re-
incorporation of CO and/or of partial deactivation of in situ
2
SCHEME 3 Use of poly([NHC(H)][HCO
3
])
4
as polymer-
generated poly(NHC)s 5a–c.
supported precatalysts of transesterification between vinyl ace-
tate and benzyl alcohol (see also Scheme 2).
Precursors 4a–d were then investigated as polymer-
supported precatalysts for the cyanosilylation of benzalde-
hyde with trimethylsilyl cyanide (TMSCN), another NHC-
Similar experimental conditions were used for all organoca-
7
2,73
catalyzed molecular reaction.
Only one report previously
talytic tests: reagents and polymer salt precursors 4 were
ꢀ
described the use of some polymer-supported version of
NHC precursors for this reaction (1 mol % of catalyst, rt, 10
mixed in THF and stirred at 80 C for 1, 2, or 24 h, depend-
ing on the implemented reaction. It is worth pointing out,
however, that 4 were not soluble under such conditions,
organocatalyzed reactions taking place heterogeneously,
except in the case of precursor 4d carrying the dodecyl
group on the imidazolium ring. Moreover, access to the NHC
catalytic sites—generated from [NHC(H)][HCO3] units—
might be reduced because of the close vicinity of NHCs to
the polymer backbone (short linker) and the steric hindrance
brought by the alkyl group on the imidazole ring. It is there-
7
4
min).
with TMSCN catalyzed by 5%mol. of 4a–d in THF for 1 h
Scheme 4) are presented in Figure 7.
Results of the cyanosilylation of benzaldehyde
(
0
Excellent yields were obtained with 4a–c and 4b up to five
catalytic cycles, meaning that all polymer-supported NHC
precursors could be readily recycled after carboxylation,
without any significant loss of catalytic activity (Fig. 7). For
the same reason mentioned above, the THF soluble precur-
sor 4d/6d could not be recycled, though also providing a
quantitative conversion after the first run of organocatalysis.
fore likely that the in situ generation poly(NHC)s from poly[-
0
NHC(H)][HCO
3
] 4a–d and 4b was not complete. This also
explained why a catalytic amount of up to 10%mol. of these
polymer-supported precatalysts was employed, while molec-
Then, we examined the catalytic potential of poly([NHC(H)]
ular versions, [NHC(H)][HCO ], required only 0.1–1% mol at
room temperature for the same reactions.
3
[HCO ])s 4 for the benzoin condensation, a NHC-catalyzed
3
4
4
self-condensation of benzaldehyde forming a b-keto alcohol
3
5,36,70,71
called benzoin.
2
Reactions were carried out in THF for
Addition of CO , at rt, at the completion of each tested reaction,
2
ꢀ
4 h at 80 C, in the presence of 10%mol. of 4 (Scheme 5).
allowed retrieving polymer-supported precatalysts, presumably
co-existing in the form of a mixture of poly[NHC(H)][HCO ] 4
3
The first catalytic cycle led to benzoin in rather good yield
(62–79%) with 4a–b and 4d. In contrast, poor yield (28%)
in benzoin was obtained with 4c after the first run of cataly-
and poly[NHC-CO ] adducts 6 (see Scheme 1). The as-
0
2
recovered polymers could thus be filtered off and recycled.
sis. This might be ascribed to the poorer solubility of poly([-
In
BnOH) and vinyl acetate (VAc) were added to a THF solu-
tion containing poly[NHC(H)][HCO ] salts 4, and the reaction
was stirred for 2 h (Scheme 3). Analysis by H NMR spec-
troscopy of the filtrate confirmed the formation of benzyl
acetate and the conversion could thus be determined.
a typical transesterification reaction, benzyl alcohol
NHC(H)][HCO ]) 4c in the reaction mixture compared to the
3
(
other precursors, the corresponding poly(NHC) being more
hardly generated or prematurly deactivated. The slow kinetic
3
1
All poly([NHC(H)][HCO ]) salts exhibited an excellent cata-
3
lytic activity at the first run, providing excellent conversion
of benzyl acetate (83–100%), irrespective of the nature of
the alkyl substituent on the imidazolium ring (Fig. 6). This
indicated that free poly(NHC)s 5 were efficiently generated
ꢀ
at 80 C, at least partially, from the poly([NHC(H)][HCO ])
3
salt precursors, formally by loss of H CO . In contrast, the
2
3
same precursors did not show any catalytic activity at rt.
Fairly good conversions (63–98%) were achieved with 4a–c
0
and 4b over 3 runs of organocatalysis, attesting to an efficient
SCHEME 4 Use of poly([NHC(H)][HCO ])
4
as polymer-
3
recycling with no significant loss of catalytic activity. Precur-
sors 4b and 4b , obtained from path a and path b, respectively
supported precatalysts of the cyanosilylation of benzaldehyde
with trimethylsilyl cyanide (see also Scheme 2).
0
4
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ACKNOWLEDGMENTS
The authors are grateful to the French Ministry of Education
and Research for financial support of PC and to CNRS and
R ꢀe gion Aquitaine for financial support of MF. The P2M RNP pro-
gram from ESF is also gratefully acknowledged.
REFERENCES AND NOTES
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3
])
4
as polymer-
1
09, 3612–3676.
supported precatalysts of the benzoin condensation of benzal-
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polymer precatalyst. A significant decrease of the yield was
also noted in the second catalytic cycle both for 4b and 4c
6
J. Moore, T. Rovis, In Asymmetric Organocatalysis; B. List,
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(
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improving the generation of free poly(NHC)s 5 from 4, as
conversions were close to those obtained after 24 h. Overall,
results of the benzoin condensation utilizing polymer precur-
sors 4 are consistent with our previous observations when
poly(NHC-CO ) adduct homologues were employed: the
2
reaction is rather slow, yields drop after a few cycles, and
polymer-supported precatalysts can be hardly recycled.
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CONCLUSIONS
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009, 253, 862–892.
Novel poly(ionic liquid)s, namely, poly(1-vinyl-3-alkyl imida-
1
3 M. K. Denk, J. M. Rodezno, S. Gupta, A. J. Lough, J. Orga-
zolium hydrogen carbonate)s, poly([NHC(H)][HCO ]), can be
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nomet. Chem. 2001, 617–618, 242–253.
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readily accessed and serve as air stable polymer-supported
precatalysts for organocatalyzed molecular reactions. The in
situ generation of related polymer-supported NHCs, pol-
1
1
y(NHC)s, occurs by a loss of H CO3 (H O 1CO ). Re-
2
2
2
carboxylation of poly(NHC)s allows easily recycling polymer
precursors and reusing them for several organocatalytic
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