Water-soluble gold(I)-NHC complexes of sulfonated IMes and SIMes and their catalytic activity in hydration of alkynes

Article abstract of DOI:10.1016/j.molcata.2011.03.009

The water-soluble carbene ligand precursors sIMesH⁺Cl ^- and sSIMesH⁺Cl^- were synthesized in high yields by direct sulfonation of IMesH⁺Cl^- (1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride) and SIMesH ⁺Cl^- (1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride). Gold(I)-N-heterocyclic carbene complexes [AuCl(sIMes)] and [AuCl(sSIMes)] were prepared by carbene transfer from the zwitterionic [Ag(sIMes)₂] and [Ag(sSIMes)₂] to [AuCl(tht)] (tht = tetrahydrothiophene). In methanol-water mixtures or in neat water, the new gold(I)-NHC complexes showed high catalytic activity in Markovnikov type hydration of terminal alkynes (up to a turnover frequency 1990 h^-1; ethynyltoluene, 0.1 mol% catalyst) but were markedly less active in case of internal alkynes (TOF = 3.6 h^-1; diphenylethyne, 1 mol% catalyst). These new Au(I)-NHC catalysts do not require acid co-catalysts or activation by Ag(I)-additives.

Full text of DOI:10.1016/j.molcata.2011.03.009

Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Editor’s Choice paper
Water-soluble gold(I)–NHC complexes of sulfonated IMes and SIMes and their
catalytic activity in hydration of alkynes
a
b
a
a,b,∗
Csilla Enik o˝ Czégéni , Gábor Papp , Ágnes Kathó , Ferenc Joó
a
Institute of Physical Chemistry, University of Debrecen, P.O. Box 7, H-4010 Debrecen, Hungary
Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences, P.O. Box 7, H-4010 Debrecen, Hungary
b
a r t i c l e i n f o
a b s t r a c t
+
−
+
−
Article history:
The water-soluble carbene ligand precursors sIMesH Cl and sSIMesH Cl were synthesized in
Received 21 January 2011
Received in revised form 16 March 2011
Accepted 16 March 2011
Available online 9 April 2011
+
−
high yields by direct sulfonation of IMesH Cl (1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride)
and SIMesH Cl (1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride). Gold(I)-N-heterocyclic carbene
complexes [AuCl(sIMes)] and [AuCl(sSIMes)] were prepared by carbene transfer from the zwitterionic
+
−
[
Ag(sIMes)2] and [Ag(sSIMes)2] to [AuCl(tht)] (tht = tetrahydrothiophene). In methanol–water mixtures
or in neat water, the new gold(I)–NHC complexes showed high catalytic activity in Markovnikov type
Keywords:
Alkynes
Gold(I)–NHC complexes
Hydration
IMes
−1
hydration of terminal alkynes (up to a turnover frequency 1990 h ; ethynyltoluene, 0.1 mol% catalyst)
−1
but were markedly less active in case of internal alkynes (TOF = 3.6 h ; diphenylethyne, 1 mol% catalyst).
These new Au(I)-NHC catalysts do not require acid co-catalysts or activation by Ag(I)-additives.
© 2011 Elsevier B.V. All rights reserved.
SIMes
Water-soluble catalysts
1
. Introduction
Aqueous organometallic catalysis allows substantial advances
catalyzed reactions have been developed. By now hydration of
alkynes became a valuable tool in organic synthesis [6,7].
An important development in homogeneous catalysis is the use
of well-defined, stable N-heterocyclic carbene (NHC) complexes
of transition metals for a wide variety of chemical transforma-
tions [11–13] owing to the relative ease of their synthesis, to
the large flexibility of ligand structure modifications and to the
high stabilities of their metal complexes. Taking into account
the very successful use of water-soluble phosphine complexes in
organometallic catalysis [1–4], it is reasonable to expect, that the
catalytic chemistry by water-soluble N-heterocyclic carbene com-
plexes might be similarly transferred to purely aqueous or biphasic
systems by using suitable ligands.
Convenient precursors to carbene ligands are the corresponding
imidazolium salts from which the NHC-s can be obtained by depro-
tonation [14]. A fairly large number of properly 1,3-disubstituted
imidazolium salts are known (incorporating sulfonate, carboxylate
or ammonium groups) which – in principle – allow the synthe-
sis of water soluble transition metal–NHC complexes [15–17].
However, free N-heterocyclic carbenes are highly basic [18], so
much that their synthesis in water is not possible. Neverthe-
less, Taube and co-workers observed the spontaneous formation
in aqueous solution of several C-2-bound imidazole species
towards making chemical processes greener not only by leading
to increased reaction rates and selectivities but also by replac-
ing organic solvents by an environmentally benign one [1–4].
Water has many excellent solvent properties (amply discussed in
the literature), however, less attention is focussed on H O as a
reagent. Transition-metal catalyzed reactions such as the telomer-
ization of dienes with water [5], hydration of alkynes [6,7], alkenes
2
[
7] and nitriles [8] afford valuable products. All these processes
require water-soluble or at least water-tolerant catalysts. These
can be simple salts, such as RhCl3 [9] in hydration of acetylene
or more elaborate catalysts, in most cases with tertiary phosphine
ligands carrying sulfonate [10] or other hydrophilic solubilizing
groups. A recent example is the use of a Ru(II)–arene complex with
i
t
Ph PC H -2 or 3-CH NHR (R = Pr or Bu ) ligand for the very effi-
2
6
4
2
cient hydration of nitriles [8] under mild conditions. In most cases,
hydration of alkynes proceeds according to Markovnikov’s rule and
yields methyl ketones. This reaction was traditionally carried out
using strongly acidic solutions of Hg(II)-salts, however, this is a
noxious procedure and more efficient transition metal complex-
[
[
19] in the reaction of N-protonated 4,5-dimethylimidazole and
2+
Ru(NH )5(H O)] . High thermodynamic stability of late transi-
∗ Corresponding author. Tel.: +36 52 512900x22382; fax: +36 52 512915.
3
2
tion metal–NHC complexes is also shown by the findings that
such complexes can be prepared in partly or fully aqueous
E-mail addresses: nagycsi@yahoo.com (C.E. Czégéni), combi1@yahoo.com
(
G. Papp), kathoagnes@yahoo.com (Á. Kathó), joo.ferenc@science.unideb.hu (F. Joó).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.009

2
C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
solutions at elevated temperatures. A case in point is the
activity in hydration of a series of alkynes [46,55,56]. Recently Leyva
and Corma reported [58] that cationic [Au(PR )]NTf (PR = tertiary
reaction of 1,3-dibenzylimidazolium bromide and [{RhCl(cod)} ]
2
3
2
3
(
cod = 1,5-cyclooctadiene) in aqueous K CO solution afford-
phosphine, NTf = bis(trifluoromethanesulfonyl)imidate) catalysts
2
3
2
ing [RhCl(dbim)(cod)] (dbim = 1,3-dibenzylimidazol-2-ylidene) in
quantitative yield [20]. Similarly, the simplest synthesis of Ag(I)-
and Cu(I)–NHC complexes involves refluxing of suspensions of
Ag O [21] or Cu O [22] together with an equivalent amount of
the appropriate imidazolium salt in water. Several cationic NHC-
complexes showed appreciable water-solubility and stability in
aqueous solutions [23–26].
exhibited high activity without the need for acid co-catalysts. Also
recently, Nolan et al. disclosed, that [AuCl(IPr)] was an extremely
active catalyst for alkyne hydration (ppm catalyst loadings) in com-
bination with AgSbF6 in the absence of acids [59]. Interestingly,
under the same conditions [AuCl(IMes)] was completely inactive.
Protonation of [Au(OH)(IPr)] with a Brønsted acid yielded an active,
silver-free catalyst for alkyne hydration [60].
2
2
The first attempts to use water-soluble transition metal–NHC
complexes for organometallic catalysis were made by Herrmann
et al. who synthesized the 1-methyl-3(butyl-4-sulfonate) betain
and applied its in situ formed Rh(I)-complex for hydroformyla-
tion of various olefins [27]. C¸ etinkaya and co-workers obtained
Ru(II)- and Rh(I)–NHC complexes with –NMe2 functionalized
imidazolin-2-ylidene ligands, protonation of which resulted in
stable, water-soluble complexes. The latter were found active
catalysts for the synthesis of 2,3-dimethylfuran by cyclization
of (Z)-3-methylpent-2-en-4-yn-1-ol in aqueous-organic bipha-
sic systems [28]. Hydroformylation of 1-octene under aqueous
biphasic conditions was studied by Weberkirsch et al. using
a Rh(I)–NHC complex covalently attached to an amphiphilic
block copolymer [29]. We have reported that redox isomer-
In the course of our studies on catalysis by transition
metal-NHC complexes in water, we synthesized several ␻-
sulfoalkylimidazolium salts and used them for the synthesis
of water-soluble Ag(I)–NHC and Au(I)–NHC complexes. Such
Au(I)–NHC complexes were successfully applied for hydration of
terminal alkynes [61]. In continuation of these studies we devel-
oped an independent procedure for the synthesis of disulfonated
+
−
1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride (sIMesH Cl ,
1a) and 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride
+
−
(sSIMesH Cl , 1b). This procedure is reported here together with
the syntheses of the respective Ag(I)–NHC and Au(I)–NHC com-
plexes. Hydration of various alkynes in aqueous solvents with
[AuCl(sIMes)] (3a) and [AuCl(sSIMes)] (3b) as catalysts was studied
in detail and the results are also given below.
ization of allylic alcohols in neat water was catalyzed by
6
[
RuCl L(␩ -p-cymene)] (L = 1-butyl-3-methylimidazol-2-ylidene,
2. Experimental
2
p-cymene = p-isopropyltoluene) [24,25]. For the same reaction
Peris and co-workers used cationic Cp-functionalized NHC com-
plexes of Ru(II) [30], as well as an arene–Ru(II) catalyst [31]
having a 1-methyl-3(propyl-3-sulfonate)imidazol-2-ylidene lig-
and. Ru–NHC complexes play an extremely important role in
catalysis of olefin metathesis; this reaction can also be run in
aqueous systems with various water-soluble NHC complexes as
catalysts [32–35].
All experiments were carried out in deaerated solvents under an
oxygen-free atmosphere (Ar or N ) using standard Schlenk tech-
2
niques. The chemicals used in this work were purchased from
Sigma–Aldrich, Fluka, Molar Chemicals and Spektrum 3D and
were used without further purification. Fuming sulfuric acid was
obtained from Merck. Ag O was prepared by the reaction of AgNO3
2
and NaOH. [AuCl(tht)] was prepared by the method described in
the literature [62]. Doubly distilled water was used throughout.
Reaction mixtures were analyzed by gas chromatography
(HP5890 Series II; Chrompack WCOT Fused Silica 30 m × 32 mm CP
WAX52CB; FID; carrier gas: argon). The products were identified by
comparison to known compounds. 1H and C NMR spectra were
recorded on a Bruker Avance 360 MHz spectrometer and referenced
to 3-(trimethylsilyl)propanesulfonic acid Na-salt (TSPSA). ESI mass
data were collected on a BRUKER BioTOF II ESI-TOF spectrometer.
IR spectra were recorded on a Perkin Elmer Instruments Spectrum
One FT-IR spectrometer equipped with a Universal ATR Sampling
Accessory.
Palladium(II)–NHC complex-catalyzed C–C coupling reac-
tions (Heck-, Suzuki-Miyaura-, Sonogashira-couplings) have been
actively investigated [36–40] using water-soluble Pd-complexes
with various N-heterocyclic carbene ligands. Plenio and co-
workers studied Suzuki-Miyaura- and Sonogashira-couplings in
aqueous solvents applying in situ formed catalysts obtained
from Na PdCl and disulfonated 1,3-diarylimidazolium or 1,3-
13
2
4
diarylimidazolinium salts [41,42]. In particular, the NHC ligands
included disulfonated IMes, SIMes and IPr (IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene). These compounds were
prepared starting from the appropriate 3-sulfonated alkylaniline
via the respective diimine or diamine followed by ring-closure.
Alternatively, the same products could be obtained by direct sul-
fonation of the 1,3-diarylimidazolium or –imidazolinium salts with
chlorosulfonic acid followed by hydrolysis with aqueous NaOH.
For a long period, complexes of gold did not attract much inter-
est in homogeneous organometallic catalysis due to their generally
conceived inactivity in such processes. However, this situation has
drastically changed in the last two decades and now gold has a
prominent place among the most valued homogeneous catalysts
2
.1. Sulfonation of 1,3-bis(2,4,6-trimethylphenyl)imidazolium
chloride
To a mixture of 4 mL 30% fuming sulfuric acid (oleum) and 1 mL
◦
cc. H SO4 cooled to 0 C in an ice bath was added 1,3-bis(2,4,6-
2
trimethylphenyl)imidazolium chloride (1.00 g, 2.93 mmol) in small
portions. Addition of the total amount of the imidazolium salt took
at least 20 min. The mixture was then allowed to warm to room
temperature and the content of the flask was carefully added to a
100 mL flask containing crushed ice (36 g). The reaction mixture
was again cooled in ice bath and neutralized to pH 7 with 50%
NaOH. The water was removed in vacuum. The white solid residue
was extracted at room temperature with dry methanol (40 mL), fil-
tered and the remaining Na SO was treated again with 2 × 40 mL
[
12,43–45]. It is illustrative to mention, that although the complex
formation of gold(I) with a sulfonated tertiary phosphine, mtppms
diphenylphosphinobenzene-m-sulfonate) were studied in aque-
(
ous solution as early as 1970 [46], the first paper on the catalytic
activity of a water-soluble gold(I)–tertiary phosphine complex was
published not before 2007 [47]. Since gold shows high affinity for
alkynes, its use for catalytic hydration has been studied by sev-
eral groups [48–57]. For effective catalysis, in most cases a strong
acid co-catalyst is needed. Laguna et al. studied the water soluble
2
4
methanol at room temperature. The combined extracts were evap-
+
−
orated to dryness to give sIMesH Cl (1a) as white solid. Yield:
1.16 g, 72%.
1
[
AuCl(PR )] complexes with PR = mtppms, mtppds and mtppts, i.e.
H NMR (360 MHz, H O saturated with NaCl, D O capillary),
3
3
2
2
mono-, di- and trisulfonated triphenylphoshines and established,
that in the presence of sulfuric acid they all showed high catalytic
ı [ppm]: 8.91 (s, 1H, CHim); 7.56 (s, 2H, CH); 6.97(s, 2H, ArH);
2.26 (s, 6H, CH ); 1.98 (s, 3H, CH ) 1.99 (s, 3H, CH ); 1.77 (s, 3H,
3
3
3

C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
3
1
3
1
CH ); 1.78 (s, 3H, CH ). C{ H} NMR (90 MHz, D O), all singlets,
[AuCl(tht)] (170 mg, 0.528 mmol) (tht = tetrahidrothiophene) in
acetone (18 mL) was added and the mixture was stirred at room
temperature in the dark for 72 h. The solution was filtered through
Celite and the solvent was removed under vacuum to afford the
crude product. The product was washed three times with cold ace-
tone (3 × 10 mL) and dried under vacuum to afford [AuCl(sIMes)]
as white powder. Yield: 340 mg, 84%.
3
3
2
ı [ppm]: 140.3 (HCim-N); 139. 8 (C -N); 137.9 (C -SO Na);
37.1 (Caryl-Me); 133.9 (Caryl-Me); 132.9 (Caryl-H); 132.4 (Caryl
Me); 125.1 (CHim); 22.5 (CH ); 16.8 (CH ); 15.8 (CH ). ESI-MS: m/z
observed 509.079, calcd. for C21H23N Na O S [M–Cl] 509.078
see Fig. S1 in Supplementary Material). IR (ATR) ꢀ(SO ) = 1183,
049 cm
Caution: Fuming sulfuric acid is strongly corrosive and reacts
violently with water. Protective clothing, glasses and suitable
gloves should be worn and all manipulations should be done in
a well ventilated fume-hood.
aryl
aryl
3
1
-
3
3
3
+
2
2
6 2
(
1
3
−
1
.
1H NMR (360 MHz, MeOH, D2O capillary), ı [ppm]: 7.51 (s,
2H, CH); 7.19(s, 2H, ArH); 2.69 (s, 6H, CH3); 2.40 (s, 6H, CH3)
2.12 (s, 6H, CH3). 13C{ H} NMR (90 MHz, MeOH, D2O capillary),
all singlets, ı [ppm]: 172.8 (NCim-N); 142.1 (Caryl-N); 139.8 (C-
1
SO Na); 137.3 (Caryl-Me); 137.1 (Caryl-Me); 135.4 (Caryl-H); 132.8
3
2
.2. Sulfonation of 1,3-bis(2,4,6-trimethylphenyl)imidazolinium
(Caryl-Me); 124.3 (CHim); 23.2 (CH3); 17.7 (CH3); 16.5 (CH3). ESI-
2−
chloride
MS: m/z observed 347.018, calcd. for C21H22N2O6S2AuCl [M-2Na]
−
1
3
47.014 (see Fig. S3). IR (ATR) ꢀ(SO ) = 1185, 1055 cm
.
3
+
−
+
−
sSIMesH Cl (1b) was prepared from SIMesH Cl (1.00 g,
2
.93 mmol) using the same procedure as for the preparation of
2.6. Synthesis of [AuCl(sSIMes)], 3b
+
−
sIMesH Cl (1a). White powder, yield 1.30 g, 81%.
1
H NMR: (360 MHz, H O saturated with NaCl, D O capillary), ı
This compound was prepared using 200 mg (0.18 mmol) 2b
applying the same procedure as for the synthesis of [AuCl(sIMes)].
White powder, yield 223 mg, 83%.
2
2
[
ppm]: 8.88 (s, 1H, CHim); 7.33 (s, 2H, ArH); 4.59 (s, 4H, CH ); 2.80
2
(
s, 6H, CH ); 2.75 (s, 6H, CH ); 2.55 (s, 6H, CH ).
3
3
3
13
1
1H NMR: (360 MHz, H2O saturated with NaCl, D2O capillary), ı
[ppm]: 7.06 (s, 2H, ArH); 4.12 (s, 4H, CH2); 2.68 (s, 6H, CH3); 2.64
(s, 6H, CH3); 2.33 (s, 6H, CH3). 13C{ H} NMR (90 MHz, CD3OD),
all singlets, ı [ppm]: 194.9 (NCim-N); 142.3 (Caryl-N); 138.6 (C-
SO3Na); 137.9 (Caryl-Me); 136.9 (Caryl-Me); 136.2 (Caryl-H); 132.8
(Caryl-Me); 51.1 (CH2im); 22.8 (CH3); 17.7 (CH3); 16.1 (CH3). ESI-
C{ H} NMR (90 MHz, D O), all singlets, ı [ppm]: 160.9 (HCim-
2
N); 142.4 (C -N); 139.5 (C-SO Na); 136.5 (C -Me); 135.2
aryl
3
aryl
1
(
(
Caryl-Me); 132.6 (Caryl-H); 132.1 (Caryl-Me); 51.3 (CH2im); 22.3
CH ); 16.7 (CH ); 15.6(CH ). ESI-MS: m/z observed 511.090, calcd.
3
3
3
+
for C21H25N Na O S [M–Cl] 511.094 (see Fig. S1). IR (ATR)
2
2
6 2
−1
ꢀ
(SO ) = 1186, 1066 cm
.
3
2−
MS: m/z observed 348.021, calcd. for C21H24N O S AuCl [M-2Na]
348.022 (see Fig. S3). IR (ATR) ꢀ(SO3) = 1181, 1072 cm
2
6 2
−
1
2.3. Synthesis of [Ag(sIMes) ], 2a
.
2
+
−
In a round-bottom flask, a mixture of sIMesH Cl (465 mg,
2.7. Typical procedure for the hydration of phenylacetylene with
complexes 3a or 3b
0
.85 mmol) and Ag O (192 mg, 0.83 mmol) in dry methanol (25 mL)
2
was heated to reflux and stirred for 16 h at this temperature and an
additional 36 h at room temperature in the dark. The solution was
filtered through Celite, and the solvent was removed under vacuum
A Schlenk flask equipped with a magnetic stirring bar and a
reflux condenser was charged with methanol (2.5 mL), water or
0.2 M aqueous H2SO4 (2.5 mL) and phenylacetylene (5.0 mmol).
Catalyst 3a or 3b (4.0 mg, 0.005 mmol) was added and the reaction
mixture was stirred at reflux temperature for 3 h (reaction mix-
to afford [Ag(sIMes) ] as a white solid. Yield 437 mg, 93%.
2
1
H NMR: (360 MHz, H O saturated with NaCl, D O capillary), ı
2
2
[
6
ppm]: 7.48 (s, 2H, CH); 7.14 (s, 2H, ArH); 2.92 (s, 6H, CH ); 2.29 (s,
3
13
1
◦
H, CH ); 1.79 (s, 6H, CH ). C{ H} NMR (90 MHz, H O saturated
tures in 50–50 v% MeOH–H2O solvent refluxed at 74 C). Samples
3
3
2
1
with NaCl, D O capillary), ı [ppm]: 182.8 (dd,
J
= 205.9 Hz,
(150 ␮L) were periodically withdrawn and added to 1 mL water
which then was extracted with 1 mL chloroform or toluene. The
organic phase was dried over MgSO4 and analyzed by gas chro-
matography.
2
C109Ag
1
JC107Ag = 178.4 Hz) (NCim-N); 140.3 (s, Caryl-N); 139.2 (s, C-SO Na);
3
1
37.7 (s, Caryl-Me); 137.3 (s, Caryl-Me); 136.5 (s, Caryl-H); 132.3
(
(
s, Caryl-Me); 123.3 (d, 3JCAg = 5.4 Hz) (CHim); 22.9 (s, CH ); 16.6
3
s, CH ); 15.8 (s, CH ). ESI-MS: m/z observed 528.038, calcd. for
3
3
2−
C42H44N O S NaAg [M-2Na]
528.040 (see Fig. S2). IR (ATR)
3. Results and discussion
4
12 4
1
ꢀ
(SO ) = 1183, 1052 cm .
3
3.1. Synthesis and properties of the water-soluble NHC ligand
2
.4. Synthesis of [Ag(sSIMes) ], 2b
precursors and their Ag(I)- and Au(I)-complexes
2
+
−
This compound was prepared from Ag O and sSIMesH Cl
The bulky, basic N-heterocyclic ligands, IMes (1,3-
2
(
450 mg, 0.82 mmol) using the same procedure as for the synthesis
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)
and
SIMes
of [Ag(sIMes) ]. White solid, yield 347 mg, 77%.
(1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) are often
used in organometallic catalysis by transition metal–NHC
complexes [12]. In the course of our studies on the use of
NHC-complexes in aqueous catalysis we conceived that water-
soluble analogs of these ligands could be successfully used in
water or in biphasic systems. For that reason, disulfonated 1,3-
2
1
H NMR: (360 MHz, H O saturated with NaCl, D O capillary), ı
2
2
[
6
ppm]: 7.03 (s, 2H, ArH); 4.02 (s, 4H, CH ); 2.87 (s, 6H, CH ); 2.51 (s,
2 3
13
1
H, CH ); 1.90 (s, 6H, CH ). C{ H} NMR (90 MHz, H O saturated
3
3
2
1
with NaCl, D O capillary), ı [ppm]: 206.7 (dd,
J
= 192.4 Hz,
2
C109Ag
1
JC107Ag = 166.9 Hz) (NCim-N); 139.3 (s, Caryl-N); 138.5 (s, C-SO Na);
3
+
−
1
36.7 (s, Caryl-Me); 136.4 (s, Caryl-Me); 135.3 (s, Caryl-H); 132.7 (s,
bis(2,4,6-trimethylphenyl)imidazolium chloride (sIMesH Cl , 1a)
Caryl-Me); 50.8(CH2im); 22.9 (s, CH ); 16.9 (s, CH ); 15.9 (s, CH ).
and disulfonated 1,3-bis(2,4,6-trimethylphenyl)imidazolinium
3
3
3
+
−
ESI-MS: m/z observed 530.055, calcd. for C42H48N O S NaAg [M-
chloride (sSIMesH Cl , 1b) (Fig. 1) were prepared by sulfonation of
4
12 4
2
−
−1
+
−
+
−
2
Na] 530.054 (see Fig. S2). IR (ATR) ꢀ(SO ) = 1179, 1068 cm
.
the corresponding imidazolium salts, IMesH Cl and SIMesH Cl .
Sulfonation with fuming sulfuric acid proceeded cleanly at or
below room temperature. Neutralization with 50% NaOH, evap-
oration to dryness, and extraction by dry MeOH afforded 1a and
1b in yields of 72% and 81%, respectively, in pure form. These
compounds were previously obtained [42] by sulfonation with
3
2.5. Synthesis of [AuCl(sIMes)], 3a
In a Schlenk flask [Ag(sIMes) ] (300 mg, 0.27 mmol) was
2
dissolved in 36 mL deoxygenated dry methanol. A solution of

4
C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
+
−
Fig. 1. 1,3-bis(2,4,6-trimethyl-3-sulfonatophenyl)imidazolium chloride (sIMesH Cl , 1a) and 1,3-bis(2,4,6-trimethyl-3-sulfonatophenyl)imidazolinium chloride
+
−
(
sSIMesH Cl , 1b).
Fig. 2. Synthesis of [Ag(sIMes)2] (2a) and [AuCl(sIMes)] (3a).
chlorosulfonic acid at 100 C for 3 h. Although the H and 13C NMR
◦
1
tively. These complexes are well soluble in water. Their 13C NMR
spectra compare well to those of analogous complexes described
in the literature. For example, Nolan and co-workers reported the
characteristic 13C resonances for the carbene and imidazole carbon
atoms in [AuCl(IMes)] and [AuCl(SIMes)], respectively (all singlets,
ı/ppm): 173.4, 122.17 and 195.0, 50.67 [65]. The same values for
3a and 3b are 172.8, 124.3 and 194.9, 51.1, respectively. ESI-TOF
data (recorded in D O) of 1a and 1b prepared by us agree with
those of the same compounds obtained by Roy and Plenio et al.
42] the signals seemed to us unusually broad or split. Recording
the spectra in H O saturated with NaCl (and using a capillary
filled with D O providing a lock signal) led to sharpening of the
signals and even to the collapse of the two singlets observed in
D O (with no NaCl) for the unique proton on the C2 atom of the
2
[
2
2
MS showed peaks for [M-2Na]2 ions with precise match of the
−
2
imidazolium ring (9.04 and 9.03 ppm to 8.91 ppm for 1a; 9.23 and
observed and calculated isotope distribution patterns (Fig. S3).
9
.24 ppm to 8.88 ppm for 1b). We suppose that in solution there is
an equilibrium between the closed (zwitterionic) forms of these
imidazolium arylsulfonates and the fully dissociated salts where
there is no specific interaction between the imidazolium cation and
the pendant sulfonate anion. A high NaCl concentration is expected
to favor the latter (in our case NaCl was in about 40 times excess
relative to the imidazolium salts). Nevertheless, this phenomenon
was not investigated further. Electrospray mass spectra (ESI-TOF
MS) showed strong peaks corresponding to [M–Cl]+ cations and
the calculated isotope distribution pattern perfectly matched the
observed ones both for 1a and 1b (Fig. S1).
3
.2. Hydration of water-insoluble alkynes with [AuCl(sIMes)]
(
3a) and [AuCl(sSIMes)] (3b) as catalysts
In order to compare the catalytic properties of 3a and 3b to those
of the sulfoalkyl-substituted Au–NHC complexes investigated by
us earlier [61] we attempted hydration of several aromatic and
aliphatic alkynes (Fig. 3), mostly terminal ones but hydration of
diphenylacetylene, as a representative internal alkyne was also
examined.
It was found, that both 3a and 3b were highly active cat-
alysts in these reactions. The reactions proceeded according to
Markovnikov’s rule and afforded methyl-ketones as sole products.
Fig. 4 shows the time-course of hydration of 4-ethynylanisole,
while Table 1 contains characteristic data of the reactions with 3a
as catalyst. It should be mentioned here, that although the cat-
alysts work in purely aqueous solutions with the water-soluble
alkyne, propargyl alcohol, for solubility reasons in most cases
methanol/water 1/1 mixtures were used as solvent for the largely
water-insoluble aromatic alkynes.
In methanolic suspensions 1a and 1b react readily with Ag O
2
leading to the formation of the corresponding silver(I)–NHC com-
plexes [Ag(sIMes) ] (2a) and [Ag(sSIMes) ] (2b) (Fig. 2) which can
2
2
be isolated by filtration followed by evaporation of the filtrate to
dryness. Chloride removal is facilitated by the excess of silver, and
in these compounds one of the sulfonate groups acts as a counter-
+
ion to Ag . These zwitterionic complexes are stable in the solid form
for long periods in the dark and can be used for carbene transfer
reactions. NMR parameters are very similar to those of close analogs
described in the literature. For example, Arduengo et al. reported
It can be seen from Table 1, that all three alkynes reacted cleanly
with 4-ethynylanisole being the most reactive (see also Fig. S4).
1
3
[
63] for [Ag(IMes) ]CF SO the following C NMR signals (ı/ppm):
2 3 3
1
1
1
83.6 (dd, J
= 208.57 Hz, J
= 187.95) (NCim-N); 124.1 (d
C109Ag
C107Ag
3
JCAg = 5.39 Hz) (CH ), while those for [Ag(sIMes) ] were observed
im
2
at 182.8 ppm (dd JC109Ag = 205.9 Hz, 1JC107Ag = 178.4 Hz) (NCim-N);
1
3
1
23.3 (d, JCAg = 5.4 Hz) (CHim). These compounds were also char-
acterized by ESI-TOF mass spectrometry and peaks of [M-2Na]
2−
ions where detected with complete match of the observed and
calculated isotope distribution pattern (Fig. S2).
[
AuCl(sIMes)] (3a) and [AuCl(sSIMes)] (3b) were obtained by
carbene transfer [64] from 2a and 2b, respectively to [AuCl(tht)] in
dry methanol/acetone (Fig. 2). After filtration and evaporation to
dryness, 3a and 3b were obtained in 84% and 83% yields, respec-
Fig. 3. Hydration of alkynes catalyzed by 3a and 3b.

C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
5
Table 1
a
Hydration of aromatic alkynes with 3a as catalyst with and without acid co-catalyst .
TOF (h 1
−
)
Entry
Alkyne
3a (mol%)
H2SO4 (mol%)
Time (min)
Conv. (%)
1
2
3
4
5
6
7
8
9
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
0.1
0.1
0.1
0.2
0.4
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
–
–
–
–
–
10
10
-
10
10
–
–
10
10
30
90
120
120
120
30
90
90
30
90
13
31
36
52
95
83
99
28
80
95
38
98
78
100
260
206
180
130
119
1660
660
187
1600
633
760
653
1560
666
b
Phenylacetyleneb
4-Ethynyltoluene
4-Ethynyltolueneb
4-Ethynyltolueneb
4-Ethynylanisole
4-Ethynylanisole
4-Ethynylanisoleb
4-Ethynylanisoleb
1
1
1
1
1
0
1
2
3
4
30
90
30
90
a
Conditions: 5 mmol alkyne, 0.005 mmol 3a, 2.5 mL MeOH, 2.5 mL H2O.
b
2
.5 mL 0.2 M H2SO4, reflux.
in Fig. 4, where the substrate/catalyst concentration ratio was 1000
curve a) and 770 (curve b), however, the absolute concentrations
(
of the substrate and catalyst were much different in the two exper-
iments (1.0 mM and 0.13 mM catalyst, respectively). The latter
concentration corresponds to as little as 97 ppm 3a in the reaction
mixture so this catalyst belongs to the most active Au(I)–NHC com-
plexes known for alkyne hydration [59,60]. Remarkably, although
the reaction slowed down in dilute solution, still the conversion
was close to quantitative at longer reaction times. Initial turnover
frequencies of the catalyst (TOF = (mol reacted substrate) (mol
−
1
−1
, calculated from the conversion in the first 30 min)
catalyst)
are very high ranging from 260 h (entry 1) to 760 h (entry 11)
h
−
1
−1
−
1
under acid-free conditions, and are in the range of 1560–1660 h
in the presence of sulfuric acid (entries 6, 9, 13; see also Fig. S4).
AuCl(sSIMes)], 3b, also showed high catalytic activities in
[
hydration of aromatic alkynes (Table 2). Again, an acid co-catalyst
was not necessary (see e.g. entry 8), however, 10 mol% sulfu-
ric acid increased the rate so much that 99% conversions were
achieved in 30 min with all three substrates, corresponding to an
average turnover frequency of 1980 h (entries 3, 6, 9; see also
Tables S1 and S2).
Fig. 4. Hydration of 4-ethynylanisole catalyzed by 3a. Conditions: (ꢀ) 5.0 mmol 4-
ethynylanisole, 0.005 mmol 3a, 2.5 mL MeOH, 2.5 mL H2O, reflux; (᭹) 0.5 mmol 4-
ethynylanisole, 6.5 × 10− mmol 3a, 2.5 mL MeOH, 2.5 mL H2O, reflux.
4
−
1
An important feature of the reaction is in that neither an acid
co-catalyst nor a silver(I) activating agent for chloride removal
is needed. For example, 98% conversion was achieved in case of
The aliphatic terminal alkyne, 1-octyne also reacted smoothly
and with catalyst loadings of 0.5–1 mol% was converted to 2-
octanone with 89–100% conversions in 30–90 min reaction times
with no acid co-catalyst added. 3a and 3b showed about the same
catalytic activity (Table S3). The highest turnover frequencies (aver-
4
-ethynylanisole without addition of an acid (entry 12) at a sub-
strate/catalyst ratio of 1000. In this respect 3a (and also 3b, see
later) belong to the very few Au(I)-based hydration catalysts which
are active in acid-free solutions [58,59]. Nevertheless, sulfuric acid
has a beneficial effect on the conversions with all three substrates
−
1
−1
(3a) and 354 h
age for the first 30 min, no acid) were 400 h
3b).
Hydration of diphenylacetylene was also attempted with the
(
(
entries 6, 7, 9, 10, 13, 14), while it is completely ineffective in the
new water-soluble Au–NHC catalysts. The reactions proceeded
slowly both with 3a and 3b even at high catalyst loadings with
acid co-catalyst (maximum conversion to 2-phenylacetophenone
was 38% with 5 mol% 3b in 6h; see also Table S4). Nevertheless,
absence of the Au(I)–NHC catalyst. Increased catalyst concentra-
tions led to faster reactions, and close to quantitative conversion
could be achieved with 0.4 mol% catalyst even with the less reac-
tive phenylacetylene (entry 5). The effect of dilution is also shown
Table 2
a
Hydration of aromatic alkynes with 3b as catalyst, with and without acid co-catalyst .
TOF (h 1
−
)
Entry
Alkyne
H2SO4 (mol%)
Time (min)
Conv. (%)
1
2
3
4
5
6
7
8
9
Phenylacetylene
Phenylacetylene
Phenylacetyleneb
4-Ethynyltoluene
4-Ethynyltoluene
4-Ethynyltolueneb
4-Ethynylanisole
4-Ethynylanisole
4-Ethynylanisoleb
–
–
10
–
–
10
–
30
90
30
30
90
30
30
90
30
12
45
99
30
64
99
53
91
99
240
300
1980
600
349
1980
1060
606
–
10
1980
a
Conditions: 5 mmol alkyne, 0.005 mmol 3b (0.1 mol%), 2.5 mL MeOH, 2.5 mL H2O.
b
2
.5 mL 0.2 M H2SO4, reflux.

6
C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
Fig. 5. Temperature dependence of the hydration of phenylacetylene catalyzed by
3a. Conditions: 5.0 mmol phenylacetylene, 0.005 mmol 3a, 2.5 mL MeOH, 2.5 mL
0
.2 M H2SO4, 3 h.
Fig. 6. Hydration of propargyl alcohol in aqueous solution, catalyzed by 3b. Con-
ditions: 5 mmol propargyl alcohol, 2.5 mL 0.2 M H2SO4, 60 C. Concentration of
catalyst: 0.1 mol% (᭹), 0.2 mol% (ꢁ), 1.0 mol% (ꢀ).
Table 3
Hydration of phenylacetylene in water-methanol mixtures of various composition .
◦
a
H2O (v/v %)
0b
Conversion (%)
TOF (h−1)
∼
98
91
99
97
90
49
46
50
49
45
reactions showed the same time course and led to the same high
conversions (Fig. S5).
2
5
7
5
0
5
1
00
3.3. Hydration of propargyl alcohol
a
Conditions: 0.5 mmol phenylacetylene, 2 mol% 3a, 1 h, V(MeOH + H2O) = 3.0 mL,
reflux.
Propargyl alcohol is soluble in water and its catalytic hydra-
tion was studied without organic co-solvents. The reaction gave
hydroxyacetone (Fig. 3) as the sole product. The reaction was fol-
b
Wet (undried) methanol.
1
13
lowed by H NMR and the product was also identified by C NMR
spectroscopy. Both 3a and 3b catalyzed the reaction although in
this case 3a was markedly less active than 3b. (Fig. S6). The time-
course of the reaction with different amounts of 3b as catalyst is
shown in Fig. 6.
this low but distinct catalytic activity makes 3a and 3b superior in
comparison with the analogous Au(I) complexes with NHC-ligands
derived from sulfoalkylimidazolium precursors [61] which did not
catalyze the hydration of diphenylacetylene at all.
◦
Below 40 C the hydration of phenylacetylene catalyzed by 3a
As can be seen in Fig. 6, the reactions started with a fast ini-
tial phase which turned into saturation at medium conversions
30–40%) at small catalyst concentrations. Conversely, with 1 mol%
was very slow but the conversion increased exponentially as a func-
tion of the temperature (Fig. 5). From the corresponding Arrhenius
plot a virtual activation energy of 45.6 kJ/mol can be calculated.
However, since the reaction mechanism is not known in detail, but
certainly consists of several steps, this value can be taken only as
the average temperature coefficient of the reaction rate.
We studied the effect of the water concentration in methanolic
solutions on the hydration of phenylacetylene to see whether 3a
and 3b are could be used in purely aqueous solutions. The results
are collected in Table 3.
(
of 3b, 5 mmol propargyl alcohol was hydrated to hydroxyacetone
◦
with 95% conversion in 90 min at 60 C, demonstrating the synthetic
utility of the new water-soluble Au(I)–NHC catalysts in neat water
as solvent.
3.4. General discussion
According to the data, with low substrate amount (0.5 mmol)
and with 2 mol% of 3a already in wet (undried) methanol the
reaction gave high conversion and the same was observed on
increasing the water concentration in the solvent up to 100%. This
shows that the catalyst is active in neat water. However, with
high amount (5 mmol) of substrate and with only 0.1 mol% cata-
lyst, increasing the water concentration in the solvent led to gradual
decrease ofconversionand in purewater only0.06% conversionwas
observed instead of 22% detected under the same conditions but
with methanol/water 1/1 mixtures. We attribute this low conver-
sion to the very low solubility (456 mg/L [66]) of phenylacetylene
in water which leads to the formation of a two-phase reaction mix-
ture in which the catalyst and the substrate are confined to different
phases. Since the reaction in such systems can take place only in
the interphase region the relative concentration of the catalyst in
the aqueous phase may become the governing factor of the reaction
rate.
The new Au(I)–NHC complexes 3a and 3b showed excellent cat-
alytic properties in hydration of terminal alkynes, both aromatic
and aliphatic, in methanol–water mixtures as solvent. The catalytic
activities compare favourably to those reported for other water-
soluble gold(I) catalysts in the literature. For example, Laguna
−
1
−1
and co-workers observed a TOF of 1000 h
and 1060 h in
the hydration of phenylacetylene using the preformed alkynyl-
gold(I) complexes [Au(C CR )(mtppts)] and [Au(C CR )(mtppts)]
1
2
(R = tert-butyl, R = 3-thiophenyl), respectively [47]. Under closely
1
2
comparable conditions 3a and 3b catalyzed the same reaction with
−
1
−1
TOFs 1660 h and 1980 h . These activities are also much higher
than those obtained with sulfoalkyl-substituted imidazolylidene
complexes of gold(I) studied by us earlier which generally showed
−
1
turnover frequencies in the range of 20–140 h and only in case
of 4-ethynylanisole was an initial TOF 400 h
−
1
determined [61].
Conversely, the catalytic activities of 3a and 3b fall behind those
+
of preformed cationic [Au(PR )] catalysts [58] or behind those of
3
+
+
It is of interest that in solid state the catalysts are stable on air
for long times. Hydration of phenylacetylene was repeated with the
same batch of catalyst (3a) stored for eight month on air; the two
Au(I)-precatalysts transformed to [Au(NHC)] [59,60] or [Au(PR )]
3
[49] species under reaction conditions (although in the first two
cases much different reaction conditions were applied). However,

C.E. Czégéni et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 1–8
7
it should be noted that while the in situ [AuCl(IPr)]/AgSbF catalyst
reaction rates and conversions can be achieved also in the absence
of acid co-catalysts with no need for activation by silver(I) addi-
tives. These findings show that the environmentally benign water is
a suitable solvent for transition metal–NHC complexes and encour-
age further research on the use of such complexes in aqueous
organometallic catalysis.
6
proved extremely efficient for hydration of a variety of aliphatic and
aromatic, terminal and internal alkynes including diphenylacety-
lene, reaction of this latter substrate was not catalyzed at all by
the strictly analogous [AuCl(IMes)]/AgSbF catalyst (2 mol%) in 1,4-
6
◦
dioxane/water 2/1 solvent, at 120 C in 20 h [59]. In this comparison
the 38% conversion of diphenylacetylene to 2-phenylacetophenone
with 5 mol% 3b in 6 h (methanol/water 2/1, 50 mol% H SO co-
catalyst, reflux at 74 C) signals a moderate catalytic activity. It is
2
4
Acknowledgements
◦
also remarkable that the neutral 3a and 3b showed outstanding
The research was supported by the EU and co-financed by
the European Social Fund through the Social Renewal Operational
Programme under the projects TÁMOP-4.2.1/B-09/1/KONV-2010-
◦
activity at 74 C allowing catalyst loadings down to the 100 ppm
+
+
level. Formation of cationic [Au(sIMes)] and [Au(sSIMes)] is very
likely in the polar aqueous media, and catalytic hydrations can be
run in the absence of a silver(I) salt for chloride abstraction.
An important question on the mechanism of catalytic hydra-
tions in methanol–water mixtures is whether methanol takes part
in the reaction [58]. One possible pathway involves the formation
of enol ethers and ketals hydrolysis of which leads to ketones.
The other possibility is the direct addition of water on the coor-
dinated alkyne. The recent findings of Leyva and Corma with the
0
007 and TÁMOP-4,2,2-08/1-2008-0012 (CHEMIKUT). The finan-
cial support of TEVA Hungary Ltd. is also highly appreciated. The
authors are grateful for the support by Hungarian National Research
and Technology Office - National Research Fund (NKTH-OTKA K
6
8482).
Appendix A. Supplementary data
ꢀ
ꢀ
cationic [Au(L)]NTf2 catalyst (L = 2-(dicyclohexylphosphino)-2 ,6 -
dimethoxybiphenyl), NTf = bis(trifluoromethansulfonyl)imidate
support the enol ether/ketal route [58]. We have found that fast
hydration of propargyl alcohol could be carried out with both
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.03.009.
2
3
a and 3b as catalysts in homogeneous aqueous solution in the
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