Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): Applications to the facile preparation of NHC metal complexes and to NHC-organocatalyzed molecular and macromolecular syntheses

Article abstract of DOI:10.1021/ja3005804

Anion metathesis of imidazol(in)ium chlorides with KHCO₃ afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO₃]. In solution, these compounds were found to be in equilibrium with

Full text of DOI:10.1021/ja3005804

Article
pubs.acs.org/JACS
Imidazol(in)ium Hydrogen Carbonates as a Genuine Source of
N-Heterocyclic Carbenes (NHCs): Applications to the Facile
Preparation of NHC Metal Complexes and to NHC-Organocatalyzed
Molecular and Macromolecular Syntheses
Mareva Fevre,†,‡ Julien Pinaud,†,‡ Alexandre Leteneur,†,‡ Yves Gnanou,†,‡ Joan Vignolle,*,†,‡
́
̀
and Daniel Taton*,†,‡
†Centre National de la Recherche Scientifique, Laboratoire de Chimie des Polymer
̀
es Organiques, UMR 5629, 16 avenue
Pey-Berland, F-33607 Pessac cedex, France
‡Universite
́ ̀
de Bordeaux, Laboratoire de Chimie des Polymeres Organiques, UMR 5629, IPB-ENSCBP, F-33607 Pessac cedex, France
Karinne Miqueu§ and Jean-Marc Sotiropoulos§
§Universite
de Pau & des Pays de l’Adour, IPREM, UMR CNRS 5254, 2 Avenue du President P. Angot, 64053 PAU cedex 09, France
́ ́
S
* Supporting Information
ABSTRACT: Anion metathesis of imidazol(in)ium chlorides with KHCO3 afforded an
easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as
[NHC(H)][HCO3]. In solution, these compounds were found to be in equilibrium with
their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene
(NHC)-CO2 adducts. The [NHC(H)][HCO3] salts were next shown to behave as
masked NHCs, allowing for the NHC moiety to be readily transferred to both organic
and organometallic substrates, without the need for dry and oxygen-free conditions. In
addition, such [NHC(H)][HCO3] precursors were successfully investigated as pre-
catalysts in two selected organocatalyzed reactions of molecular chemistry and polymer
synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of D,L-lactide, respectively. The
generation of NHCs from [NHC(H)][HCO3] precursors occurred via the formal loss of H2CO3 via a concerted low energy
pathway, as substantiated by Density Functional Theory (DFT) calculations.
synthesis,7f,g,8,12,16 their preparation generally involves the
INTRODUCTION
■
generation of free NHC. Structural diversity is thus often limited
by the stability of the corresponding carbene and decomposition
of some masked NHCs in solution, in the presence of water, has
been observed. In particular, Rogers et al.17 and Louie et al.11g
have evidenced that the Ccarbene−CO2 bond of NHC−CO2
adducts can hydrolyze in solution, forming stable imidazolium
hydrogen carbonate salts, denoted as [NHC(H)][HCO3]
(eq 1, Scheme 1). Interestingly, this hydrolysis reaction proves
In the past 20 years, stable carbenes, in particular N-hetero-
cyclic carbenes (NHCs), have emerged not only as versatile
ligands for transition metals1 but also as powerful organocatalysts
in molecular chemistry for a variety of transformations2 and, more
recently, in macromolecular chemistry for precision polymer syn-
thesis.3 NHCs are generally prepared by deprotonation of azolium
salts with a strong base.4 Because this method not only necessi-
tates dry and air-free conditions but also provides limited tolerance
to various functionalities, different approaches have been devel-
oped to circumvent these limitations. Apart from the encapsulation
of free NHCs into hydrophobic silicon polymers allowing their
handling in air,5 most efforts have been focused toward the design
of masked NHCs (Figure 1),6 whose thermal activation can in
situ generate the free carbene. Several masked NHCs such as
2-alkoxy,7 2-trichloromethyl,7d,8 2-pentafluorophenyl imidazoli-
dines,8,9 5-alkxytriazolines,7g,10 imidazolium-2-carboxylates
referred to as NHC−CO2 adducts,11 imidazolium-2-thioisocya-
nates,12 and NHC-Ag(I) complexes13 have been developed.
Although most of these compounds have been successfully applied
as NHC-transfer agents for transition metals,7a−d,8,9,11e,14 and
as (pre)catalysts for molecular8,12,15 and macromolecular
−
reversible, suggesting a noninnocent role of the HCO3
counteranion toward the imidazolium cation. This behavior is
reminiscent of that observed for basic anions such as AcO− in
imidazolium-based ionic liquids (ILs), which are capable of
reversibly deprotonating the imidazolium cation, generating the
corresponding NHC.18 By analogy, and from a mechanistic
point of view, we thus hypothesized that, in the case of
[NHC(H)][HCO3] 2, NHC 6 could be reversibly generated
by the formal loss of H2CO3 (eq 3, Scheme 1). It might also be
anticipated that subsequent reaction of the NHC with CO2
Received: January 27, 2012
Published: March 29, 2012
© 2012 American Chemical Society
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EXPERIMENTAL SECTION
■
Materials. Solvents were used without any purification unless
otherwise stated. Benzyl alcohol was purified by fractional distillation
and stored over molecular sieves. D,L-Lactide (99%, Alfa) was recrys-
tallized three times in warm toluene (freshly distilled from poly-
styryllithium prior to use), dried under vacuum, and kept in the glovebox.
Benzaldehyde (99%, Alfa) was purified by fractional distillation. Carbon
disulfide was used as received (99.9%, Aldrich). 1,3-Bis(isopropyl)-
imidazolium chloride (Roth), 1,3-bis(2,4,6-trimethylphenyl)imidazolium
chloride (Strem), 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride
(Aldrich), and KHCO3 (Alfa) were dried at 60 °C for 12 h under
vacuum. [Pd(allyl)Cl]2 and Au(Cl)SMe2 were purchased from Strem
Chemicals Inc. and Aldrich, respectively, and used as received.
Instrumentation. 1H NMR (400 MHz) spectra were recorded on
a Bruker AC-400 spectrometer in appropriate deuterated solvents.
Molar masses were determined by size exclusion chromatography
(SEC) in THF as the eluent (1 mL/min) and with trichlorobenzene as
a flow marker at 25 °C, using both refractometric (RI) and UV
detectors (Varian). Analyses were performed using a three-column set
of TSK gel TOSOH (G4000, G3000, G2000 with pore sizes of 20,
́
75, and 200 Å respectively, connected in series) calibrated with
polystyrene standards. The data for the crystal structure of compound
2a have been collected on a Rigaku MM07 HF rotating anode at the
Cu Kα wavelength. The system featured the Micromax microfocus
X-ray source with the RAPIDII image plate detector combined with
the AFC-Kappa goniometer and the osmic mirrors Varimax HF optics.
The system was driven by the CrystalClear suite which was also used
for the unit cell determination, integration, scaling, and absorption
correction of the raw data (Reference: CrystalClear: An Integrated
Program for the Collection and Processing of Area Detector Data,
Rigaku Corporation, 1997−2002). MALDI-ToF spectrometry was
performed using a Voyager-DE STR (Applied Biosystems) spec-
trometer equipped with a nitrogen laser (337 nm), a delay extraction,
and a reflector. The instrument is equipped with a pulsed N2 laser
(337 nm) and a time-delayed extracted ion source. Spectra were
recorded in the positive-ion mode using the reflectron and with
an accelerating voltage of 20 kV. Samples were dissolved in THF at
10 mg/mL. The IAA matrix (Indole acrylic acid) solution was pre-
pared by dissolving 10 mg in 1 mL of THF. A MeOH solution of
cationization agent (NaI, 10 mg/mL) was also prepared. The solutions
were combined in a 10:1:1 volume ratio of matrix to sample to
cationization agent. A 1−2 μL aliquot of the obtained solution was
deposited onto the sample target and vacuum-dried. Mass spectra
(ESI) were obtained on a QStar Elite mass spectrometer (Applied
Biosystems). The instrument is equipped with an ESI source, and
spectra were recorded in the positive mode. The electrospray needle
was maintained at 4500 V and operated at room temperature. Samples
were introduced by injection through a 10 μL sample loop into a 200
μL/min flow of methanol from the LC pump.
Figure 1. Masked NHC precursors reported in the literature and those
described in this work.
Scheme 1. Reversible Transformation of NHC−CO2
Adducts into [NHC(H)][HCO3] Salts via the Generation of
NHCs
(formed by decomposition of H2CO3) would afford the
corresponding NHC−CO2 adduct 2′ (eq 2, Scheme 1).
To the best of our knowledge, no report has mentioned the
NHC-like behavior of imidazolium hydrogen carbonates in
(macro)molecular chemistry.19,20 We wish to report herein the
facile synthesis of such [NHC(H)][HCO3] salt precursors by a
simple anion metathesis from commercially available imidazo-
lium chlorides, [NHC(H)][Cl], using KHCO3, and their use as
stoichiometric transfer agents of NHCs toward organic and
organometallic substrates. We also show that [NHC(H)][HCO3]
organic salts can serve as precatalysts in (macro)molecular reac-
tions. NHC formation from its [NHC(H)][HCO3] precursor, via
formal loss of H2CO3, is investigated by Density Functional
Theory (DFT) calculations.
Synthesis of 1,3-Bis(isopropyl)imidazolium Hydrogen Car-
bonate [IiPr(H)][HCO3] 2a. A mixture of 1,3-bis(isopropyl)-
imidazolium chloride (1 g, 5.30 mmol) and 1.05 equiv of KHCO3
(550 mg, 5.56 mmol) was dried at 60 °C under vacuum for 12 h. Dry
MeOH (5 mL) was then added at rt, and the resulting suspension was
stirred for 48 h at rt. After filtration of the suspension over Celite to
remove KCl, MeOH was evaporated under vacuum to yield a sticky
solid. Trituration of the solid with acetone and filtration afforded 805
mg of [IiPr(H)][HCO3] 2a as a white powder, upon drying under
vacuum (yield: 71%). Recrystallization of 2a in a MeOH/Et2O mixture
Figure 2. Structure of imidazol(in)ium chloride precursors used in this study.
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1:1.5 ratio, in favor of 2c′. 1H NMR (400 MHz, MeOD): δ = 2.33 (s,
6H, p-CH3Mes), 2.41 (s, 12H, o-CH3Mes), 4.52 (s, 4H, CH2), 7.10 (s,
4H, m-CHMes). The N2CH and HCO3 protons could not be
Scheme 2. Synthesis of Imidazolium Hydrogen Carbonates
[NHC(H)][HCO3] 2a−c via Anion Metathesis of
Imidazolium Chlorides [NHC(H)][Cl] 1a−c with KHCO3
in MeOH
−
observed due to their rapid exchange with the deuterated solvent on
the NMR time scale. 13C NMR (100 MHz, MeOD): δ = 17.9 (o-
CH3Mes), 21.2 (p-CH3Mes), 52.6 (CH2), 131.1 (m-CHMes), 132.1
(p-CqMes), 136.7 (o-CqMes), 142.1 (CipsoMes), 161.5 (N2CH), 162.0
−
(br, HCO3 ). In dmso-d6, the 1H and 13C NMR data obtained for the
major compound 2c′ matched those reported in the literature.14d
Minor compound 2c: 1H NMR (400 MHz, dmso-d6): δ = 2.27 (s, 6H,
p-CH3Mes), 2.33 (s, 12H, o-CH3Mes), 4.44 (s, 4H, CH2), 7.08 (s, 4H,
m-CHMes), 9.06 (s, 1H, N2CH). 13C NMR (100 MHz, dmso-d6): δ =
17.1 (o-CH3Mes), 20.6 (p-CH3Mes), 51.0 (CH2), 129.5 (m-CHMes),
130.9 (CqMes), 135.4 (CqMes), 139.7 (CqMes), 156.3 (N2CH), 160.3
at 5 °C yielded colorless plate-like crystals, suitable for X-ray diffrac-
tion analysis. In MeOD, 2a was the only compound observed, while, in
dmso-d6, 2a equilibrates with IiPr-CO2 2a′ in a 1:3 ratio, in favor of 2a.
1H NMR (400 MHz, MeOD): δ = 1.62 (d, J = 6.8 Hz, 12H, CH3iPr),
4.73 (sept, J = 6.8 Hz, 2H, CHiPr), 7.76 (s, 2H, CHCH). The
−
(HCO3 ).
Synthesis of IMes-CS2 3b. CS2 (10 equiv, 2.7 mmol) was added
to a THF suspension (1.5 mL) of 2b (100 mg, 0.27 mmol) at room
temperature, and the reaction mixture was stirred at 60 °C in a capped
vial for 2 h. After removal of all the volatiles under vacuum, 98 mg of a
−
N2CH and HCO3 protons could not be observed due to their rapid
exchange with the deuterated solvent on the NMR time scale. 13C
1
purple-brownish solid were isolated (yield 96%). Data from H NMR
NMR (100 MHz, MeOD): δ = 23.0 (CH3iPr), 54.6 (CHiPr), 121.9
analysis of 3b in dmso-d6 were found to be in good agreement with
−
(CHCH), 134.4 (br, N2CH), 161.3 (HCO3 ). In dmso-d6, the
those reported in the literature.11f
major compound was 2a: 1H NMR (400 MHz, dmso-d6): δ = 1.48 (d,
J = 6.8 Hz, 12H, CH3iPr), 4.66 (sept, J = 6.8 Hz, 2H, CHiPr), 7.98 (s,
2H, CHCH), 9.71 (s, 1H, N2CH). 13C NMR (100 MHz, dmso-d6):
δ = 23.3 (CH3iPr), 53.1 (CHiPr), 121.5 (CHCH), 135.1 (N2CH),
Synthesis of SIMes-CS2 3c. A similar procedure to that described
for the synthesis of 3b was used, yielding 95 mg of 3c as an orange
1
powder (yield 92%), upon removal of the volatiles. H NMR data of
3c in CDCl3 were found to be in good agreement with those reported
−
1
156.8 (HCO3 ). Minor compound 2a′: H NMR (400 MHz, dmso-
d6): δ = 1.40 (d, J = 6.8 Hz, 12H, CH3iPr), 5.27 (sept, J = 6.8 Hz, 2H,
CHiPr), 7.86 (s, 2H, CHCH). 13C NMR (100 MHz, dmso-d6): δ =
23.3 (CH3iPr), 51.2 (CHiPr), 118.4 (CHCH), 143.4 (N2C), 155.4
(CO2). HRMS (MALDI+): m/z calculated for C9H17N2 [M]+
153.1381, found 153.1384.
in the literature.11f
Synthesis of IiPr-Pd(allyl)Cl 4a. [Pd(allyl)Cl]2 (21.3 mg, 5.8 ×
10−5 mol), compound 2a (29.4 mg, 1.4 × 10−4 mol), and THF (2 mL)
were put in a capped vial (air atmosphere). After 1 d of stirring at rt,
1H NMR analysis in CDCl3 attested the complete disappearance of
signals corresponding to [Pd(allyl)Cl]2. The solution was filtered over
silica to remove residual compound 2a, and THF was finally removed
under vacuum. The resulting pale yellow powder was further washed
with Et2O (yield 87%). 1H NMR (400 MHz, CDCl3): δ = 1.40
(pseudo t, J = 7.3 Hz, 12H, CH3iPr), 2.35 (d, J = 12.1 Hz, 1H, CH2),
3.28 (d, J = 13.6 Hz, 1H, CH2), 3.34 (td, J = 2.0, 7.2 Hz, 1H, CH2),
4.24 (dd, J = 2.0, 7.2 Hz, 1H, CH2), 4.98 (br, 2H, CHiPr), 5.30 (m,
1H, CHallyl), 6.94 (s, 2H, CHCH). 13C NMR (100 MHz, CDCl3):
δ = 23.5 (CH3iPr), 23.6 (CH3iPr), 47.2 (CH2), 52.8 (CHiPr), 73.0
(CH2), 114.6 (CHallyl), 116.9 (CHCH), 177.2 (Ccarbene).
Synthesis of 1,3-Bis(mesityl)imidazolium Hydrogen Carbo-
nate [IMes(H)][HCO3] 2b. A similar procedure to that described for
the preparation of [IiPr(H)][HCO3] 2a was used. 2b was obtained as
a white solid (yield: 76%). In MeOD, 2b was the only compound
1
observed, while, in dry dmso-d6, 2b was only sparingly soluble. H
NMR (400 MHz, MeOD): δ = 2.20 (s, 12H, o-CH3Mes), 2.40 (s, 6H,
p-CH3Mes), 7.20 (s, 4H, m-CHMes), 8.06 (s, 2H, CHCH). The
−
N2CH and HCO3 protons could not be observed due to their rapid
exchange with the deuterated solvent on the NMR time scale. 13C
NMR (100 MHz, MeOD): δ = 17.6, 21.3, 126.5, 131.0, 132.4, 135.8,
139.8, 143.0, 161.5.
Synthesis of SIMes-Pd(allyl)Cl 4b. [Pd(allyl)Cl]2 (5 mg, 1.4 ×
10−5 mol), compound 2b (12.1 mg, 3.3 × 10−5 mol), and THF
(0.5 mL) were put in a capped vial (air atmosphere). After 10 min of
stirring at rt, 1H NMR analysis in CDCl3 attested the complete
disappearance of [Pd(allyl)Cl]2 peaks. The solution was filtered over
silica to remove residual compound 2b, and THF was finally removed
Synthesis of 1,3-Bis(mesityl)imidazolinium hydrogen car-
bonate [SIMes(H)][HCO3] 2c. A similar procedure to that described
for the preparation of [IiPr(H)][HCO3] 2a was used. 2c was obtained
as a white solid (yield: 88%). In MeOD, 2c was the only compound
observed, while, in dmso-d6, 2c equilibrates with SIMes-CO2 2c′ in a
Figure 3. 1H NMR spectra of 2a in CD3OD, DMSO-d6 where a few drops of water were added, dry DMSO-d6 (leading to an equilibrium between
1
compounds 2a and 2a′), and H NMR spectrum of 2a′ in dry DMSO-d6 synthesized by carboxylation of the corresponding free NHC.
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was analyzed by NMR in CDCl3. Spectra were in agreement with
those reported in the literature.22
Scheme 3. Transfer of the NHC from [NHC(H)][HCO3]
Salts 2 to CS2, [Pd(allyl)Cl]2, and Au(SMe2)Cl
Synthesis of SIMes-AuCl 5b. In a similar fashion, 1 h of reaction
at rt between 11.4 mg (3.9 × 10−5 mol) of Au(SMe)2Cl and 17.2 mg
(4.7 × 10−5 mol) of compound 2b in THF followed by the
aforementioned purification procedures led to compound 5b (yield
82%). NMR spectra in CDCl3 were in agreement with those reported
in the literature.23
Synthesis of IMes-AuCl 5c. In a similar fashion, 1 h of reaction at rt
between 11.4 mg (3.9 × 10−5 mol) of Au(SMe)2Cl and 16.7 mg (4.7 ×
10−5 mol) of compound 2c in THF followed by the aforementioned
purification procedures led to compound 5c (yield 89%). NMR spectra
were in agreement with those reported in the literature.23
Benzoin Condensation Reaction. In a typical reaction, 74 mg
(0.2 mmol) of 1,3-bis(mesityl)imidazolinium hydrogen carbonate 2c
(stored in a capped vial under air) and molecular sieves were
introduced into a Schlenk tube. The powder was submitted to 30 min
under vacuum and finally three Ar/vacuum cycles. THF (2 mL),
previously distilled from Na/benzophenone, and then benzaldehyde
(0.2 mL 2 mmol) were added. The Schlenk tube was then transferred
into a 60 °C preset oil bath. Conversion was calculated by 1H NMR in
CDCl3, comparing the integral value of the CHO signal of
benzaldehyde (δ = 10 ppm) with the integral value of the α-hydroxy
CH signal of benzoin (δ = 6 ppm); after 24 h, 88% and 83%
conversion was obtained with compounds 2b and 2c, respectively.
Polymerization of D,L-LA. In a typical polymerization, 3.6 mg
(9.7 × 10−6 mol, 10 mol % relative to the initiator) of 1,3-bis(mesityl)-
imidazolinium hydrogen carbonate 2c (stored in a capped vial under
air) were introduced into a Schlenk tube. The powder was degassed
for 30 min under vacuum followed by three Ar/vacuum cycles, and
then 5 mL of dry THF, previously distilled from Na/benzophenone,
were added. In a glovebox, molecular sieves, 10 μL of benzyl alcohol
(9.7 × 10−5 mol), and 380 mg (2.6 × 10−3 mol, targeted DP = 26) of
D,L-lactide were introduced. After a the mixture was stirred for few
minutes at RT complete homogenization was observed. After 3 h at rt,
the conversion was calculated by 1H NMR in CDCl3 comparing
integral values of the CH-signal of the polymer (broad peak around
δ = 5.1 ppm) and that of the CH-monomer signal (quadruplet at
δ = 5.0 ppm). Poly(D,L-LA) was precipitated in cold MeOH, dried
under vacuum, and analyzed by 1H NMR in CD2Cl2 to calculate
DPNMR (comparing benzyl protons of chain ends, δ = 7.3 ppm with
CH-peak of the polymer, δ = 5.1 ppm). Molar masses and dispersities
were obtained by SEC analysis in THF (RI detector).
Scheme 4. Benzoin Condensation Catalyzed by 10 mol % of
2b,c in THF at 60 °C
Scheme 5. RT Ring-Opening Polymerization of D,L-Lactide
Using Benzyl Alcohol As Initiator and 2c as Catalyst
Precursor
under vacuum. The resulting pale yellow powder was further washed
with Et2O. NMR spectra in CDCl3 were in agreement with those
reported in the literature (yield 95%).21
Synthesis of IMes-Pd(allyl)Cl 4c. A similar procedure to
compound 4b was used: 10 min of reaction between 5 mg (1.4 ×
10−5 mol) of [Pd(allyl)Cl]2 and 11.7 mg (3.3 × 10−5 mol) of
compound 2c in THF followed by the aforementioned purification
procedures led to compound 4c (yield 95%). NMR spectra were in
agreement with those reported in the literature.21b
Synthesis of IiPr-AuCl 5a. Au(SMe)2Cl (11.4 mg, 3.9 × 10−5
mol), compound 2a (9.8 mg, 4.7 × 10−5 mol), and THF (0.7 mL)
were put in a capped vial (air atmosphere). After 1 h of stirring at
50 °C, 1H NMR analysis in CDCl3 attested the consumption of 1 equiv of
compound 2a compared to Au(SMe)2Cl. The solution was filtered
over silica to remove residual compound 2a, and THF was finally
removed under vacuum (yield 95%). The resulting off-white powder
RESULTS AND DISCUSSION
■
The synthesis of imidazolium hydrogen carbonates [NHC-
(H)][HCO3] has been scarcely investigated in the literature.24
They can be prepared by reacting free NHCs with NH4HCO3,
Figure 4. (a) SEC traces of PLA obtained using 2c as NHC-precursor: in red, [LA]/[BnOH]/[2c] = 26/1/0.1; in black, [LA]/[BnOH]/[C] = 68/
1/0.1 (SEC in THF calibrated with PS standards). (b) MALDI-ToF MS spectrum in reflector mode of PLA ([LA]/[BnOH]/[2c] = 26/1/0.1).
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preparation of the free NHC. To avoid the manipulation of
air-sensitive free carbene intermediates, and thus provide an
easy access to [NHC(H)][HCO3] salts, we turned to the anion
metathesis of commercially available imidazolium halide
precursors, [NHC(H)][Cl], using KHCO3. This method has
been employed for the synthesis of molecular,20,24a polymer-
ic24e [NHC(H)][HCO3]-based ILs and for the surface
modification of IL-functionalized gold nanoparticles,24d using
24e
NH4HCO3,24a NaHCO3,20b,24d or KHCO3
as a HCO3
−
source and i-PrOH or water as a reaction medium.26 For this
study, we selected three imidazol(in)ium chlorides (Figure 2)
and resorted to a modified procedure of the aforementioned
literature for anion metathesis.
Thus, reacting 1,3-bis(isopropyl)imidazolium chloride, [IiPr-
(H)][Cl], 1a, with 1.05 equiv of KHCO3 in MeOH for 2 days
led, after workup, to a white powder in 71% yield (Scheme 2).
Analysis of the crude powder by 1H NMR in CD3OD
revealed the presence of one product. As expected, the chemical
shifts of the different protons of the imidazolium backbone
were similar to that of the starting material 1a, thus preclud-
ing its identification. However, in the 13C NMR spectrum, the
−
Figure 5. Energy profile computed at the B3LYP/6-31G** level (free
energies G at 25 °C including ZPE correction in kcal/mol) for the
rearrangement 2→6 + CO2 + H2O, noted 2→6 in the text.
characteristic signals of both the N2CH carbon and the HCO3
quaternary carbon atoms were clearly detected at 134.4 and
161.3 ppm respectively, in agreement with data from the
literature.17,24c Note that the absence of the H NMR signal
1
as reported by Kuhn et al.,25 or by hydrolysis of NHC−CO2
corresponding to the N2CH proton was attributed to the rapid
exchange of this proton with the deuterated solvent, on the
adducts.11g,17,24c Note that both methods involve prior
Figure 6. Geometrical structures and main bond lengths for all the compounds involved in the 2→6 rearrangement (distances in Å).
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NMR time scale.24c The absence of the signal due to the more
−
acidic HCO3 proton is likely due to the same phenomenon.
Crystals of 2a suitable for X-ray diffraction analysis could be
obtained by cooling a solution of 2a in a MeOH/Et2O mixture.
The modest quality of the crystallographic data precludes
a discussion of the geometric parameters, but the connecti-
vity of the complex could be unambiguously established
(see Supporting Information). In particular, the asymmetric
−
unit cell consists of one imidazolium cation and one HCO3
−
anion, the HCO3 anions forming hydrogen-bonded dimers
around crystallographic centers of inversions, consistently with
previous observations for similar structures.17,24a,25
In contrast to CD3OD, 2a was found to coexist with
compound 2a′, in a 3:1 ratio in dry DMSO-d6, as deduced from
1
the presence of a new set of signals in the H NMR spectrum.
Compound 2a′ was eventually identified as being the cor-
1
responding NHC−CO2 adduct, by comparing its H NMR
spectrum with that of an authentic sample prepared by reacting
the free NHC with CO2 (see Figure 3 for a comparison
between 1H NMR spectra of 2a in CD3OD, dry or wet DMSO-
d6 and 2a′ in dry DMSO-d6). Consistent with observations
made by Rogers et al.17,24c with some [NHC(H)][HCO3] ILs,
2a and 2a′ are in equilibrium in DMSO. Interestingly, the
equilibrium can be totally shifted toward the formation of 2a by
adding a few drops of water in DMSO or by letting the sample
stand in air, while, in CD3OD, the equilibrium is totally shifted
toward the formation of 2a.
Figure 7. Energy profile computed at the B3LYP/6-31G** level (free
energies G at 25 °C including ZPE correction in kcal/mol) for the
rearrangement 6 + CO2 + H2O →2′ + H2O, noted 6→2′ in the text.
To generalize the synthesis of imidazolium hydrogen
carbonates from imidazolium chlorides, the anion metathesis
method was applied to precursors featuring aryl substituents on
the nitrogen atoms, namely, 1,3-bis(mesityl)imidazolium chloride
[IMes(H)][Cl], 1b, and its saturated counterpart, namely, 1,
3-bis(mesityl)imidazolinium chloride [SIMes(H)][Cl], 1c.
Subjecting the two latter salts to anion metathesis in dry
MeOH yielded, after the usual workup, 2b and 2c as off-white
and white solids, in 76% and 88% yield, respectively. Given that
2b is sparingly soluble in dry DMSO-d6 and that 2c is in
equilibrium with 2′c in dry DMSO-d6, their characterization was
[NHC(H)][HCO3] 2 can deliver the free carbene in solution,
allowing its transfer to an organic substrate.
To further illustrate their potential as NHC precursors, the
reactivity of [NHC(H)][HCO3] salts 2 toward transition
metals was investigated (Scheme 3). Thus, 1.2 equiv of 2a,b,c
was reacted with 0.5 equiv of [Pd(allyl)Cl]2 in THF. The
corresponding NHC−Pd(II) complexes 4a,b,c were isolated in
excellent yields (>87%), after 24 h of stirring at RT for com-
pound 4a, and 1 h for 4b,c. 4b and 4c were identified by com-
1
performed in MeOD by H and 13C NMR spectroscopy. In
particular, both the HCO3− anion and the N2CH carbon atoms
could be identified by 13C NMR spectroscopy through the
presence of signals at 161.5 and 132.4 ppm, respectively, for 2b,
and at 162.0 and 161.5 ppm, respectively, for 2c. As in the case
of 2a discussed above, shifting the equilibrium toward the quan-
titative formation of the imidazolium hydrogen carbonates could
be achieved by adding a few drops of water. Thus, coexistence in
solution of [NHC(H)][HCO3] and NHC−CO2 strongly depends
on the nature of the solvents and on the presence of water.
We next examined whether [NHC(H)][HCO3] 2 could be
of some practical use as a source of NHCs that either can be
transferred in a stoichiometric fashion to transition metals or
serve as precatalysts in selected (macro)molecular reactions.
For instance, in the presence of CS2, an organic substrate known
to readily react with free NHCs,11f,27 imidazolium hydrogen car-
bonates 2b,c afforded the corresponding highly colored azolium
dithiolate type betaines, denoted as NHC−CS2 adducts 3b,c, in
high yield (92−96%; see Experimental Section). Reactions of 2
with an excess of CS2 were carried out in THF at 50 °C for 1 to
6 h (Scheme 3). The structures of 3b−c NHC−CS2 adducts
21
1
paring their H NMR spectra with those already reported.
Compound 4a was completely characterized by H and 13C
NMR spectroscopy. In particular, its 1H NMR spectrum exhib-
ited five different signals (δ = 2.35, 3.28, 3.34, 4.24, and 5.30 ppm)
corresponding to the allyl fragment, indicating a dissymmetric
structure around the palladium center. In 13C NMR, the char-
acteristic signal of the carbene center was observed at 177.2 ppm,
in agreement with that observed for analogous palladium−NHC
complexes (δ = 175.6−188.5 ppm).21 It is worth mentioning
that these transfer reactions were performed in air with non-
purified solvents. Similarly, transfer of the NHC moiety from 2
to AuCl(SMe2) was successfully accomplished in air, giving
excellent yields of the corresponding NHC−Au complexes 5,
after 1 h of stirring in THF at 50 °C for 5a and RT for 5b,c.
1
1
Excellent agreement between the H NMR spectra of 5a,b,c
synthesized from [NHC(H)][HCO3] 2 and data reported in
the literature was observed.22,23
To test the activity of precursors 2 as organic precatalysts,
benzoin condensation, known as being selectively catalyzed by
NHCs,1,2 was implemented as a model reaction (Scheme 4).
Reactions were performed in THF at 60 °C, in the presence of
10 mol % of precatalyst 2b,c, using molecular sieves to trap the
water released from the [NHC(H)][HCO3]. After the reactions
1
were authenticated by comparing their H NMR spectra with
those reported in the literature.27 Although this reaction involv-
ing CS2 has limited synthetic utility, it clearly establishes that
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Article
Figure 8. Geometrical structures and main bond lengths for the all the compounds involved in the 6→2′ rearrangement (distances in Å).
stirred for 24 h, 88% and 83% conversion was obtained using
2b and 2c, respectively, attesting to the efficient NHC
generation from these compounds (see Experimental Section).
Finally, given the growing attention of NHCs to trigger metal-
free polymerization reactions for precision polymer synthesis,3 the
precursor 2c [SIMes(H)][HCO3] was evaluated as a precatalyst
for the ROP of D,L-lactide (LA). It is noteworthy that this
monomer was polymerized at RT in a THF solution, while
others reported that masked NHCs required heating the reac-
tion mixture (generally >60 °C).7e−g,8 In addition, precatalyst
2c was simply kept in air without any particular precautions.
Benzyl alcohol (BnOH) was used as an initiator ([LA]/[BnOH]/
[2c] = 26/1/0.1 and 68/1/0.1), in the presence of 10 mol % of 2c
relative to BnOH (Scheme 5). Again, 3 Å molecular sieves were
added to the reaction mixture to avoid competitive initiation of
the polymerization by H2O generated from the [SIMes(H)]-
[HCO3] precatalyst. High monomer conversions (87 and 75%)
were achieved in 3 to 6 h under such conditions, the resulting
PLA being characterized by narrow dispersities (<1.07) and
well controlled molar masses (MnSEC = 4000 and 7800 g·mol−1,
measured by SEC in THF calibrated with PS standards; see
Figure 4a), in agreement with the initial [LA]/[BnOH] feed
ratio and the conversion. In the MALDI-ToF mass spectrum
([LA]/[BnOH]/[2c] = 26/1/0.1), only one series of signals
was observed at m/z = 108.14 + 144.13 n + 23 (where 108.14 is
the molar mass of the benzyloxy and H end-groups and 23 the
molar mass of sodium ion), which perfectly matched the expected
molar masses of the targeted BnOH-initiated [SIMes(H)]-
[HCO3]-catalyzed PLA (Figure 4b).
These results as a whole suggest that NHC−H2CO3 2 do
behave as masked NHCs in solution, at room temperature,
regardless of the nature of the substituents on the nitrogen
atoms and the presence or not of an unsaturation at C4 and
C5 of the imidazole backbone. Attempts to spectroscopically
characterize the generated NHCs 6 in solution so far failed, the
dehydrated NHC−CO2 adducts, 2′, being the only product
observed. To gain better insight into the generation of NHCs 6
from [NHC(H)][HCO3] 2, and into the facile transformation
of 2 into 2′ in solution, DFT calculations were performed at the
B3LYP/6-31G** level of theory (see Supporting Information).
The reactions 2→6 were found to be reversible (Scheme 1,
eq 3), regardless of the nature of the substituents on the
nitrogen atoms. Indeed, very small energetic differences
between 2 and 6 were calculated for the three transformations
(ΔG2→6 = (−)0.2 − (−)2.6 kcal/mol). For each reaction, a
transition state TS2→6, connecting directly 2 and 6 and lying at
10.0, 7.5, and 10.2 kcal/mol above 6a, 6b, and 6c, respectively,
could be located on the potential energy surface (Figure 5).
The geometry of the TS (Figure 6) indicates that the process is
concerted for the three transformations and quasi synchronous
for 2b→6b and 2c→6c. In the case of 2a→6a, the
transformation is asynchronous, the breaking of the C5−O2
bond being more advanced than the proton transfer of H1 from
C to O2 in TS6a→2a. The low energy barrier associated with the
reversibility of this transformation agrees well with the NHC
behavior of [NHC(H)][HCO3] 2 observed in solution, at
room temperature. The reactions of CO2 with free NHCs 6
were also investigated computationally (Figure 7). Here
again, the rather small differences calculated for the three
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Journal of the American Chemical Society
Article
transformations (ΔG6→2′ = (−)3.7 − (−)8.0 kcal/mol) indicate
the reversibility of the process, regardless of the nature of the
substituents on the nitrogen atoms. For each reaction, a
transition state TS6→2′ connecting directly 6 and 2′ and lying at
5.6, 6.3, and 2.7 kcal/mol above 6a, 6b, and 6c, respectively,
could be located on the potential energy surface (Figures 7 and
8 for geometrical parameters). The low energy barrier
associated with the reversibility of the reaction accounts for
the facile interconversion of [NHC(H)][HCO3] 2 into NHC−
CO2 2′ through the intermediacy of NHC 6. This is also
consistent with the fact that NHCs were not experimentally
observed; it may be assumed that these intermediate species
react very rapidly with CO2 or (H2O and CO2), yielding the
respective NHC−CO2 and [NHC(H)][HCO3] compounds,
which are in equilibrium.
ACKNOWLEDGMENTS
■
The authors are grateful to CNRS, Reg
Agence Nationale de la Recherche (ANR) - Programme Blanc
(CATAPULT Project) for financial support. Christelle Absalon
́
(Institut des Sciences Moleculaires, UMR 5255, Bordeaux) is
acknowledged for the MALDI-ToF MS and ESI MS experi-
ments. Brice Kauffmann (Chimie et Biologie des Membrames
et des Nanoobjets, UMR 5248, Bordeaux) is warmly acknowl-
edged for X-ray diffraction analysis. Part of the theoretical work
was granted access to HPC resources of Idris under allocation
2012 (i2012080045) made by GENCI (Grand Equipement
National de Calcul Intensif).
́
ion Aquitaine and to the
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■
Anion metathesis of imidazolium chlorides with KHCO3
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of 2a−c and 4a, the molecular
structure, crystal data and structure refinement of 2a, and
Cartesian coordinates of all optimized structures with the
corresponding energies. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
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vignolle@enscbp.fr; taton@enscbp.fr
Notes
The authors declare no competing financial interest.
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