Dissymmetric gold(i) N-heterocyclic carbene complexes: A key unexpected structural parameter for highly efficient catalysts in the addition of alcohols to internal alkynes

Article abstract of DOI:10.1039/c0dt00932f

Gold(i) N-heterocyclic carbene complexes (Au-NHC) with symmetric (bis-benzyl, -propyl and -mesityl substituents) and dissymmetric (mesityl and alkyl - benzyl or propyl - substituents) NHC ligands were synthesized and tested as catalysts for the addition of methanol to 3-hexyne, as a representative internal alkyne. While symmetric ones - bis-alkyl (propyl or benzyl) or bis-mesityl systems - displayed low activity, dissymmetric Au-NHC systems with one alkyl (benzyl or propyl) and one mesityl groups on the NHC unit were unexpectedly highly active with rates and turnover numbers up to 294000 h ^-1 and 800000 mol.mol_Au^-1. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt00932f

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PAPER
Dissymmetric gold(I) N-heterocyclic carbene complexes: a key unexpected
structural parameter for highly efficient catalysts in the addition of alcohols to
internal alkynes†
Mohamed Bouhrara,a Erwan Jeanneau,b Laurent Veyre,a Christophe Cope´reta and Chloe´ Thieuleux*a
Received 30th July 2010, Accepted 7th January 2011
DOI: 10.1039/c0dt00932f
Gold(I) N-heterocyclic carbene complexes (Au-NHC) with symmetric (bis-benzyl, -propyl and -mesityl
substituents) and dissymmetric (mesityl and alkyl - benzyl or propyl - substituents) NHC ligands were
synthesized and tested as catalysts for the addition of methanol to 3-hexyne, as a representative internal
alkyne. While symmetric ones – bis-alkyl (propyl or benzyl) or bis-mesityl systems – displayed low
activity, dissymmetric Au-NHC systems with one alkyl (benzyl or propyl) and one mesityl groups on
the NHC unit were unexpectedly highly active with rates and turnover numbers up to 294 000 h-1 and
-1
800 000 mol.molAu
.
to 3000 TON) were also achieved with diaminocarbene gold(I)
catalysts.19
Introduction
Intermolecular addition of alcohols and water to alkynes is an
efficient route to functionalize hydrocarbon skeletons. The first
example of such a reaction was reported in the 1920’s using a
Hg(II)/H+ system,1 but since then, many other catalytic systems
based on Hg,2–6 Pt,7–10 Ag,9 Ir11 and Au12–16 have been reported to
catalyze the addition of nucleophiles to alkynes. Of these systems,
gold complexes were found to be excellent catalysts: Au(III)-
complexes were used to generate ketones and dimethylacetals
by hydration of alkynes in methanol12 and more recently Teles
et al.16 and Tanaka et al.14 reported that gold(I)-phosphane
complexes catalyzed the addition of alcohols to non-activated
internal alkynes. Such a system probably involves [L-Au+] cationic
gold(I) intermediates (where L is a phosphane, a phosphite, or
an arsine). More recently, isoelectronic Au(I)-NHC complexes17
have also turned out to be highly efficient Lewis acid catalysts.
For instance, Nolan et al.13 described the hydration of various
alkynes with Au(I)-NHC complexes in the presence of AgSbF6
as co-catalyst (eqn (1)), where they pointed out the need to
use a specific NHC ligand bearing two bulky bis-isopropyl-2,6-
phenyl substituents if one wants to obtain efficient catalysis with
high TON (i.e. 84 000). This reaction was also described using
water-soluble Au(I) complexes bearing sulfonated N-heterocyclic
imidazolylidene as ligands;18 good catalytic performances (up
(1)
Here, we describe the synthesis, the characterisation, and
the evaluation of the catalytic activity of gold(I) N-heterocyclic
carbene complexes in the addition of MeOH–H2O mixtures to
a representative internal alkyne, namely, 3-hexyne. The most
significant and surprising observation is that, while the symmetric
bis(propyl)-, bis(benzyl)- and bis(mesityl) systems are inactive
(Scheme 1), the dissymmetric ones bearing one mesityl and one
alkyl (propyl or benzyl) groups on the NHC ligands are highly
active, with TON up to 800 000 (Table 1).
Scheme 1 Synthesis of Au(I)-NHC complexes 1-Au, 2-Au, 3-Au, and
4-Au.
aUniversite´ de Lyon, Institut de Chimie de Lyon, C2P2 UMR 5265 (CNRS–
CPE Lyon – Universite´ Lyon 1) ESCPE Lyon 43, Bd du 11 Novembre
1918, F-69616, Villeurbanne, France. E-mail: thieuleux@cpe.fr; Fax: +33
472431795; Tel: (+33)472432795
Experimental section
bCentre de Diffractome´trie Henri Longchambon – Universite´ Lyon 1, 43, Bd
du 11 Novembre 1918, F-69616, Villeurbanne, France
General procedures
† Electronic supplementary information (ESI) available: Liquid state
NMR spectra of 1-Au, 2-Au, 3-Au and 4-Au and X-ray data for 2-Au.
CCDC reference number 763439. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00932f
All reactions were carried out using standard Schlenk techniques
under an argon atmosphere except for the synthesis of imidazolium
salts and the catalytic tests. Anhydrous solvents were distilled
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Table 1 Catalytic performances of Au(I)-NHC complexes in the reaction of methanol addition to 3-hexyne
Entry Catalytic system (mol% Au) Acid (0.2 mol% H+) Time (h) Conversion (%) Selectivity a/ba (%) TOF (s-1)/Conv.b (%) TOF (h-1)/Conv.b (%)
1
2
3
4
5
6
7
8
2-Au (0.1)
H2SO4
—
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
24
24
4
No reaction
No reaction
—
2-Au/AgBF4 (0.4)
2-Au/AgBF4 (0.4)
2-Au/AgOTf (0.4)
AgBF4 (0.1)
100
100
61/39
57/43
—
—
4
—
24
24
24
24
24
24
24
24
64
64
120
120
No reaction
No reaction
1.99/5.96
1.10/3.31
0.03/0.66
No reaction
1.70/5.09
0.68/2.05
12.41/3.72
7.32/2.20
81.75/2.45
63.67/1.91
Au(IMes)Cl/AgBF4
1-Au/AgBF4 (0.01)
2-Au/AgBF4 (0.01)
3-Au/AgBF4 (0.01)
4-Au/AgBF4 (0.01)
1-Au/AgBF4 (0.01)
2-Au/AgBF4 (0.01)
1-Au/AgBF4 (0.001)
2-Au/AgBF4 (0.001)
1-Au/AgBF4 (0.0001)
2-Au/AgBF4 (0.0001)
100
100
64
70/30
73/27
71/29
7,15 ¥ 103/5.96
3,97 ¥ 103/3.31
99/0.66
9
H2SO4
H2SO4
10
11
12
13
14
15
16
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
100
100
98
97
80
83/17
84/16
80/20
79/21
43/57
41/59
6.10 ¥ 103/5.09
2.46 ¥ 103/2.05
4.47 ¥ 104/3.72
2.64 ¥ 104/2.20
2.94 ¥ 105/2.45
2.29 ¥ 105/1.91
70
a Selectivities of the products a and b were calculated by integration of the corresponding GC signals after calibration using standard mixtures of a and
b. b The TOF was calculated based on 3-hexyne conversion after 5 min of reaction.
using appropriate drying agents or were purchased from Aldrich
and kept over molecular sieves. Mesitylimidazole was synthesized
according to the literature procedure.20 Sodium imidazolide, 1-
iodopropane, benzyl chloride, methanesulfonic acid, 3-hexyne,
Ag2O, [Au(SMe2)Cl] complexes were purchased from Sigma–
Aldrich and used as received. Liquid state 1H and 13C NMR
spectra were recorded on a Bruker 300 MHz. 1H and 13C chemical
shifts were referenced to solvent resonances and given in ppm.
NMR multiplicities are abbreviated as follows: s = singlet, d =
doublet, t = triplet and m = multiplet.
N-), 2.32 (s, 3H, CH3 para), 2.03 (s, 6H, CH3 ortho). 13C NMR
(300 MHz, CD2Cl2, ppm): 141.1 (Ccarbene), 138.9 (Carom-CH3 para),
134.4 (Carom-CH3 ortho), 134.3-129 (CHarom), 123.3 (CHimin-Naliph),
122.8 (CHimin-Narom), 52.8 (-CH2-N), 20.8 (CH3 para), 17.4 (CH3
ortho). MS (ES1+): m/z 277.2 [2]+, i.e. [2 - Cl]+, calculated 277.4.
Synthesis of N,N¢-bis(propyl)imidazolium iodide, 3. To a solu-
tion of sodium imidazolide (2.0 g, 22.2 mmol) in THF (40 mL)
was added dropwise 1-iodopropane (4.54 mL, 46.6 mmol). After
heating under reflux for 48 h, the reaction mixture was filtered
through Celite and evaporated to dryness under vacuum. The thus-
obtained residue was dissolved in CH2Cl2 (5 mL), and addition
of pentane (15 mL), resulting in the immediate formation of
an orange–brown oil. Removal of the supernatant and further
washing of the oil 3 times with pentane (10 mL) yielded 4.9 g of 3
(79%) after drying under vacuum. 1H NMR (300 MHz, CD2Cl2,
ppm): 9.93 (s, 1H, CHcarbene), 7.6 (s, 2H, CHimin-Naliph), 4.26 (t, 4H,
-CH2-N-), 1.88 (m, 4H, -CH2-), 0.89 (t, 6H, CH3-CH2). 13C NMR
(300 MHz, CD2Cl2, ppm): 136.1 (Ccarbene), 122.5 (CHimin-Naliph), 51.3
(-CH2-N), 23.6 (-CH2-), 10.4 (-CH2-CH3). MS (ES1+): m/z 153.1
[3]+, i.e. [3 - I]+, calculated 153.2.
Synthesis of 1-(propyl)-3-(mesityl)imidazolium iodide, 1. Repre-
sentative procedure. To a solution of mesitylimidazole (2.0 g,
10.7 mmol) in acetonitrile (60 mL) was added dropwise 1-
iodopropane (6.3 mL, 64.4 mmol). After heating under reflux
the reaction mixture for 12 h, the solvent and the excess of
1-iodopropane were removed under vacuum, and the resulting
orange oil was washed with pentane (20 mL) to yield essentially a
1
pure sample of 1 (3.1 g, 81%) after drying. H NMR (300 MHz,
CD2Cl2, ppm): 10.05 (t, 1H, CHcarbene), 7.86 (t, 1H, CHimin-Narom),
7.3 (t, 1H, CHimin-Naliph), 7.05 (s, 2H, CHarom), 4.58 (t, 2H, -CH2-
N-), 2.35 (s, 3H, CH3 para), 2.08 (s, 6H, CH3 ortho), 2.05 (m, 2H,
-CH2-), 1.00 (t, 3H, CH3-CH2). 13C NMR (300 MHz, CD2Cl2,
ppm): 141.4 (Ccarbene), 137.2 (Carom-CH3 para), 134.3 (Carom-CH3
ortho), 130.6 (CHarom), 129.7 (Carom-N), 123.5 (CHimin-Naliph), 123.2
(CHimin-Narom), 51.6 (-CH2-N), 23.8 (-CH2-), 20.9 (CH3 para), 17.6
(CH3 ortho), 10.3 (-CH2-CH3). MS (ES1+): m/z 229.2 [1]+ , i.e. [1 -
I]+, calculated 229.3.
Synthesis of N,N¢-bis(benzyl)imidazolium chloride, 4. Using
the procedure described for 3 and replacing iodopropyl by benzyl
chloride, 4 was obtained in 82% yield. 1H NMR (300 MHz,
CD2Cl2, ppm): 11.82 (s, 1H, CHcarbene), 7.44 (d, 2H, CHimin
-
Narom), 7.34 (m, 10H, CHbenzyl), 5.52 (s, 4H, -CH2-N-). 13C NMR
(300 MHz, CD2Cl2, ppm): 137.5 (Ccarbene), 133.5–126.9 (CHarom),
122 (CHimin-Narom), 46.4 (-CH2-N). MS (ES1+): m/z 249.1 [4]+, i.e.,
[4 - Cl]+, calculated 249.3.
Synthesis of 1-(benzyl)-3-(mesityl)imidazolium chloride, 2. Fol-
lowing the synthesis described above for 1 and replacing
iodopropyl by benzyl chloride the compound 2 was obtained in
Synthesis of silver(1-(propyl)-3-(mesityl)imidazol-2-ylidene) io-
dide, 1-Ag. Representative procedure. Silver(I) oxide (0.67 g,
2.91 mmol) and 1-(propyl)-3-(mesityl)imidazolium iodide 1 (2.0 g,
5.6 mmol) in CH2Cl2 (20 mL) were stirred for 4 h at room
1
95% yield: H NMR (300 MHz, CD2Cl2, ppm): 11.12 (t, 1H,
CHcarbene), 7.93 (t, 1H, CHimin-Narom), 7.65–7.36 (5H, CHbenzyl), 7.23
(t, 1H, CHimin-Naliph), 6.99 (s, 2H, CHarom), 5.86 (s, 2H, -CH2-
2996 | Dalton Trans., 2011, 40, 2995–2999
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temperature. After filtration through Celite, the solvent was
removed under reduced pressure, and the obtained residue was
dissolved in CH2Cl2 (5 mL). Addition of pentane (20 mL) resulted
in an immediate precipitation of a light orange solid. After
filtration and drying under vacuum, 2.3 g of 1-Ag was obtained
N), 122.1 (CHimin-Naliph), 120.6 (CHimin-Narom), 52.9 (-CH2-N), 24.3
(-CH2-), 20.9 (CH3 para), 17.5 (CH3 ortho), 10.7 (-CH2-CH3).
Synthesis of gold(1-(benzyl)-3-(mesityl)imidazol-2-ylidene) chlo-
ride, 2-Au. Using the procedure described above, the reaction of
Au(SMe2)Cl (0.7 g, 2.38 mmol) and 2-Ag (1.0 g, 2.38 mmol) in
CH2Cl2 (20 mL) yielded 1.0 g of 2-Au (83%). 1H NMR (300 MHz,
CD2Cl2, ppm): 7.4–6.94 (9H, CHarom), 5.5 (s, 2H, -CH2-N-), 2.37
(s, 3H, CH3 para), 2.06 (s, 6H, CH3 ortho), 1.81 and 1.54 (traces of
solvent). 13C NMR (300 MHz, CD2Cl2, ppm): 172.3 (Ccarbene), 139.9
(Carom-CH3 para), 135.5 (Carom-CH3 ortho), 135–127.6 (CHarom),
122.8 (CHimin-Naliph), 120.8 (CHimin-Narom), 54.9 (-CH2-N), 20.9
(CH3 para), 17.6 (CH3 ortho).
1
(90% yield). H NMR (300 MHz, CD2Cl2, ppm): 7.23 (d, 1H,
CHimin-Narom), 6.98 (2H, CHarom), 6.97 (1H, CHimin-Naliph), 4.04
(t, 2H, -CH2-N-), 2.36 (s, 3H, CH3 para), 1.91 (s, 6H, CH3
ortho), 1.79 (m, 2H, -CH2-), 0.85 (t, 3H, CH3-CH2). 13C NMR
(300 MHz, CD2Cl2, ppm): 139.4 (Carom-CH3 para), 135.6 (Carom
-
CH3 ortho), 134.9 (CHarom), 129.1 (Carom-N), 122.6 (CHimin-Naliph),
121.3 (CHimin-Narom), 53.4 (-CH2-N), 24.6 (-CH2-), 20.8 (CH3 para),
17.4 (CH3 ortho), 10.7 (-CH2-CH3).
Synthesis of gold(N,N¢-bis(propyl)imidazol-2-ylidene) iodide,
3-Au. Using the procedure described above, the reaction
of Au(SMe2)Cl (0.381 g, 1.29 mmol) and 3-Ag (0.5 g,
1.29 mmol) in CH2Cl2 (20 mL) yielded 0.54 g of 3-Au
(88%). 1H NMR (300 MHz, CD2Cl2, ppm): 6.97 (s, 2H,
CHimin-Naliph), 4.12 (t, 4H, -CH2-N-), 1.89 (m, 4H, -CH2-),
0.94 (t, 6H, CH3-CH2). 13C NMR (300 MHz, CD2Cl2,
ppm): 170.2 (Ccarbene), 120.4 (CHimin-Naliph), 53 (-CH2-N), 24.3
(-CH2-), 10.7 (-CH2-CH3).
Synthesis of silver(1-(benzyl)-3-(mesityl)imidazol-2-ylidene)
chloride, 2-Ag
Using the procedure described above, the reaction of Ag2O (0.77 g,
3.31 mmol) and 1-(benzyl)-3-(mesityl)imidazolium chloride 2
(2.0 g, 6.37 mmol) in CH2Cl2 (20 mL) yielded 2.6 g of 2-Ag
(97%).1H NMR (300 MHz, CD2Cl2, ppm): 7.41–7.00 (9H, CHarom),
5.39 (s, 2H, -CH2-N-), 2.35 (s, 3H, CH3 para), 1.99 (s, 6H, CH3
ortho). 13C NMR (300 MHz, CD2Cl2, ppm): 139.6 (Carom-CH3
Synthesis of gold(N,N-bis(benzyl)imidazol-2-ylidene) chloride,
4-Au. Using the procedure described above, the reaction of
Au(SMe2)Cl (0.376 g, 1.28 mmol) and 4-Ag (0.5 g, 1.28 mmol) in
dichloromethane (20 mL) yielded 0.58 g of 4-Au (95%). 1H NMR
(300 MHz, CD2Cl2, ppm): 7.4–7.29 (m, 10H, CHbenzyl), 6.94 (s, 2H,
CHimin-Narom), 5.4 (s, 4H, -CH2-N-). 13C NMR (300 MHz, CD2Cl2,
ppm): 171.4 (Ccarbene), 135.4–127.4 (CHarom), 121.1 (CHimin-Narom),
55.1 (-CH2-N).
para), 136 (Carom-CH3 ortho), 135.5–127.5 (CHarom), 123.3 (CHimin
Naliph), 121.3 (CHimin-Narom), 55.5 (-CH2-N), 20.8 (CH3 para), 17.4
(CH3 ortho).
-
Synthesis of silver(N,N¢-bis(propyl)imidazol-2-ylidene) iodide, 3-
Ag. Using the procedure described above, the reaction of Ag2O
(0.43 g, 1.86 mmol) and N,N¢-bis(propyl)imidazolium iodide 3
(1.0 g, 3.57 mmol) in CH2Cl2 (20 mL) yielded 1.2 g (87%) of 3-Ag.
1H NMR (300 MHz, CD2Cl2, ppm): 7.07 (s, 2H, CHimin-Naliph),
4.08 (t, 4H, -CH2-N-), 1.88 (m, 4H, -CH2-), 0.94 (t, 6H, CH3-
CH2). 13C NMR (300 MHz, CD2Cl2, ppm): 121.1 (CHimin-Naliph),
53.6 (-CH2-N), 24.8 (-CH2-), 10.8 (-CH2-CH3).
Methanol addition to 3-hexyne using Au-NHC complexes 1–4-
Au. Representative procedure. A mixture of methanol (0.36 mol),
methanesulfonic acid or H2SO4 (0.02 mmol) and 3-hexyne
(9 mmol) was heated to 60 ◦C before the gold catalyst (with ratio
from 0.4 mol% to 0.0001 mol% Au) and 1 equiv. of AgBF4 or
silver triflate were introduced. The concentrations of 3-hexyne,
3,3¢-dimethoxyhexane and 3-hexanone were determined by GC
using dodecane as an internal standard.
Synthesis of silver(N,N-bis(benzyl)imidazol-2-ylidene) chloride,
4-Ag. Using the procedure described above, the reaction of Ag2O
(0.423 g, 1.83 mmol) and N,N-bis(benzyl)imidazol-2-ylidene)
chloride 4 (1.0 g, 3.51 mmol) in CH2Cl2 (20 mL) yielded 1.3 g
(95%) of 4-Ag. 1H NMR (300 MHz, CD2Cl2, ppm): 7.34–7.24 (m,
10H, CHbenzyl), 7.05 (s, 2H, CHimin-Narom), 5.23 (s, 4H, -CH2-N-).
13C NMR (300 MHz, CD2Cl2, ppm): 136.1–127.8 (CHarom), 121.9
(CHimin-Narom), 55.6 (-CH2-N).
Results and discussion
The Au(I)–NHC complexes, 1-Au, 2-Au, 3-Au and 4-Au, were
synthesized in good yields in three steps according to a standard
procedure (Scheme 1) via: (i) formation of the imidazolium salts 1,
2, 3 and 4 by contacting 1.0 equiv. of iodopropane or benzyl chlo-
ride with mesitylimidazole for 1 and 2 and by reacting 2.0 equiv.
of iodopropane and benzyl chloride with sodium imidazolide for
3 and 4, followed by (ii) their conversion into the corresponding
silver carbene complexes 1-Ag, 2-Ag, 3-Ag and 4-Ag by reaction
of 1, 2, 3 or 4 with 0.5 equiv. of Ag2O before performing (iii) a
transmetallation step with Au(SMe2)Cl (1.0 equiv.) to yield 1-Au,
2-Au, 3-Au and 4-Au. All these compounds were characterized by
1H and 13C NMR (See Experimental section and ESI† for details
and NMR spectra) including X-ray diffraction on single crystals
Synthesis of gold(1-(propyl)-3-(mesityl)imidazol-2-ylidene) io-
dide, 1-Au. Representative procedure. A mixture of (dimethyl-
sulfide)gold(I) chloride, Au(SMe2)Cl, (0.63 g, 2.15 mmol) and
1-Ag (1.0 g, 2.15 mol) in CH2Cl2 (20 mL) was stirred for 4 h
at room temperature. After filtration through Celite, the solvent
was removed under vacuum, and the thus-obtained residue was
dissolved in CH2Cl2 (5 mL). Addition of pentane (20 mL) resulted
in an immediate precipitation of a light orangish solid. After
filtration, the solid dried in vacuum to yield 1.1 g (92%) of 1-
Au as an essentially pure material. 1H NMR (300 MHz, CD2Cl2,
ppm): 7.19 (d, 1H, CHimin-Narom), 7.03 (s, 2H, CHarom), 6.93 (d, 1H,
CHimin-Naliph), 4.25 (t, 2H, -CH2-N-), 2.36 (s, 3H, CH3 para), 2.03
(s, 6H, CH3 ortho), 1.96 (m, 2H, -CH2-), 0.99 (t, 3H, CH3-CH2).
1
for 2-Au (Scheme 2). To summarize the salient features, the H
NMR spectra of 1–4 exhibited the expected signals of the NHC
units including the characteristic peak of the imidazolium proton
at ca. 9–11 ppm. This signal disappeared upon reaction with Ag2O.
13C NMR (300 MHz, CD2Cl2, ppm): 171.7 (Ccarbene), 139.7 (Carom
CH3 para), 135 (Carom-CH3 ortho), 134.9 (CHarom), 129.2 (Carom
-
-
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Scheme 2 X-ray structure of 2-Au (CCDC 763439). Selected bond
˚
˚
distances and angles: Au–CNHC and Au–Cl distances are 1.95 A and 2.58 A,
respectively, the CNHC–Au–Cl angle is 179.0◦.
Fig. 1 3-Hexyne conversion versus time for 1-Au (empty symbols) and
2-Au (full symbols) (0.01mol%).
Further treatment with Au(SMe2)Cl led to the appearance of
the expected signal around 175 ppm in the 13C NMR spectrum
corresponding to the carbon of the NHC ligand directly bonded
to Au for 1-Au to 4-Au (See ESI†, Fig. S1–S25).
The catalytic performances of complexes 1-Au, 2-Au, 3-Au and
4-Au were then investigated in the reaction of methanol addition
to 3-hexyne. In a typical procedure, a methanolic solution of
methanesulfonic acid (or pure sulfuric acid) and 3-hexyne in a
0.02 : 1 ratio was heated to 60 ◦C, before the gold catalyst and
the silver salt (AgBF4 or AgOTf) were added. It is worthy of note
tha◦t the reaction had to be carried out at temperatures below
60 C, otherwise the reaction mixture turned purple, evidencing
the formation of gold nanoparticles.
In terms of catalytic activity, as previously reported by Teles
et al. both silver salt and acid co-catalysts were needed to generate
a catalytic system, based on the formation of the putative cationic
gold(I) catalytic species [AuL+] (Table 1, entries 1–3).16 The nature
of the silver salt was found to have no significant influence on
the conversion rate and the product selectivity (Table 1, entries 3
and 4). Similarly, the acid co-catalysts, namely methanesulfonic
acid or sulfuric acid (98%wt), had no major significant influence
on the reaction rates or on the final products selectivities (83%
vs. 70% of 3,3¢-dimethoxyhexane; Table 1, entries 7 and 11). The
ketone formation probably results from the competitive hydrolysis
of the intermediate product of methanol mono-addition or from
the direct hydration of hexyne (presence of water in H2SO4 as well
as the intake of water during the catalytic tests; see ESI† and Table
1, entries 2, 8 and 16).
observed with 2-Au but with slightly slower rates (Fig. 1). This
difference is further increased when using a 0.0001 mol% metal
loading, for which the conversion reached a plateau at 80 and 70%
after 3500 min of reaction for catalysts 1-Au and 2-Au, respectively
(Fig. S29, ESI†). In terms of initial rates measured after 15 min
(TOF) and overall turnovers (TON): 1-Au is slightly better than
2-Au: TOF = 82 and 64 s-1 (i.e. ca. 294 000 h-1 and 229 000 h-1)
-1
and TON = ca. 800 000 and 700 000 mol molAu for 1-Au and
2-Au, respectively. These catalytic performances are noteworthy,
in particular if one compares these with reported data (TON =
5000–10 000 and TOF = 700–1500 h-1).16
Conclusions
Overall, symmetric and dissymmetric gold(I) N-heterocyclic car-
bene complexes were prepared and tested as catalysts in the
methanol addition to 3-hexyne under mild conditions (60 ◦C).
It is worthy of note that we showed that symmetric NHC ligands,
whether bearing alkyl or aryl substituents, are almost inactive,
while, in contrast, the dissymmetric complexes having one alkyl
group and one mesityl group on the NHC unit are highly
active, reaching TOF of 290 000 h-1 and TON of 800 000 mol
-1
molAu for the best systems. While these catalytic performances
show that dissymmetric Au(I)-NHC complexes gave very high
and unexpected catalytic activity for the alcohol additions to
internal alkynes, the origin of their reactivity and the role of
the dissymmetric ligands are not clear. Further work is currently
underway to understand and to evaluate the scope of these systems.
To determine the catalytic performances of these Au–NHC
systems in the methanol addition to 3-hexyne, the reaction was
monitored with various catalyst systems and loadings. With a
catalyst loading of 0.01 mol%, a clear difference in the catalytic
performances was observed with respect to the nature of the
NHC substituents. Dissymmetric systems (1-Au and 2-Au) were
found to be highly active (full conversion of hexyne after 24 h)
with comparable initial rate (TOF) : 2 s-1 versus 1 s-1 for 1-Au
and 2-Au, respectively, while symmetric ones (3-Au and 4-Au)
exhibited only very poor catalytic activities. Note that having
two mesityl substituents is also detrimental to catalytic activity
as previously shown, which led earlier to the development of the
bulkier bis-2,6-di-isopropylphenyl NHC-Au(I) systems.13 In order
to determine and to compare the maximum catalytic performances
of the best systems based on dissymmetric complexes, experiments
were carried out using 0.001 and 0.0001 mol% of catalysts (Table
1, entries 13–16). Using 0.001 mol% of catalyst, a conversion of
ca. 73% was reached within 500 min with a quasi constant rate
allowing full conversion in 64 h for 1-Au. Similar results were
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