Amphiphilic Zwitterionic phosphine based Au(I)-complex as efficient and recyclable catalyst for hydration of alkynes free of additional additives

Article abstract of DOI:10.1016/j.mcat.2018.01.035

The unique amphiphilic Zwitterionic P,O-hybrid ligand (L1) containing phosphino-fragment and ?SO₃^? group was synthesized and firstly applied in Au-catalyzed hydration of alkynes. Without the aid of any auxiliary additive such as acid or silver salt, L1-based Au-catalyst exhibited excellent activity towards hydration of alkynes to yield ketones with 100% selectivity according to Markovnikov's rule. On the other hand, L1-based Au-catalyst could be recycled for 4 runs in room temperature ionic liquid of [Bmim]PF₆ without obvious activity loss, and also exhibited wide generality to the hydration of different alkynes.

Full text of DOI:10.1016/j.mcat.2018.01.035

Molecular Catalysis 448 (2018) 171–176
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Amphiphilic Zwitterionic phosphine based Au(I)-complex as efficient and
recyclable catalyst for hydration of alkynes free of additional additives
a
a
a
a
b
a,⁎
Xia Chen , Xu Ye , Wen-Yu Liang , Qing Zhou , Giang Vo-Thanh , Ye Liu
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North
Zhongshan Road, Shanghai 200062, PR China
b
UMR 8182, Bat. 420, Université Paris-Sud 11, Orsay Cedex, France
A R T I C L E I N F O
A B S T R A C T
−
Keywords:
The unique amphiphilic Zwitterionic P,O-hybrid ligand (L1) containing phosphino-fragment and −SO
3
group
Zwitterionic phosphines
Hybrid ligand
Hydration of alkynes
Au(I)-complexes
was synthesized and firstly applied in Au-catalyzed hydration of alkynes. Without the aid of any auxiliary ad-
ditive such as acid or silver salt, L1-based Au-catalyst exhibited excellent activity towards hydration of alkynes
to yield ketones with 100% selectivity according to Markovnikov’s rule. On the other hand, L1-based Au-catalyst
could be recycled for 4 runs in room temperature ionic liquid of [Bmim]PF
6
without obvious activity loss, and
Recyclability of Au-catalysts
also exhibited wide generality to the hydration of different alkynes.
−
1
. Introduction
Au(I)-complexes [(NHC)Au+X, X = BArF
(tetrakis(3,5-bis(tri-
−
−
−
−
−
fluoromethyl)phenyl)borate), BF
4 6 2 4
, SbF , OTf , NTf , ClO ,
−
−
The hydration of terminal alkynes which is the nucleophilic addi-
OTs , TFA ] for the efficient hydration free of silver salts and acids.
However, in the preparation of such kinds of Au(I)-complex catalysts,
the silver salts or strong Bronsted acids were still required without
exception [20–22]. Encouragingly, Xu [26] made an advance to de-
tion of water to alkynes following Markovnikov’s rule to generate the
ketones represents an environmentally benign route to form carbonyl
compounds because of water as reagent [1,2]. Acid-catalyzed hydration
of electron rich alkynes such as alkynyl ethers and alkynyl thioethers
has been well known to carry out satisfactorily [3–6]. However, the
hydration of simple alkynes is quite sluggish and need toxic mercury(II)
salts as co-catalysts along with the strong acids to improve the reaction
rate [1,2,7–11]. More recent attentions have been focused on Au-cat-
alyzed hydration of simple alkynes, due to the unique alkynophilicity of
Au-complex catalyst with π-acidity [2,12,13] and the environmentally
begin consciousness by replacing the uses of highly toxic mercury(II)
salts [14–17]. Typically, an Au(I)-complex with the linear configuration
is diagonally coordinated by one phosphine or carbene (L) and one
I
velop a unique acid- and silver-free catalytic system of (CyNC)Au Cl/KB
(C
ylate- and sulfonate-related Au(I)-complexes enabled the efficient hy-
dration with the presence of Lewis acidic BF ·Et O completely without
6 5 4
F ) for hydration. And Schmidbaur [27] discovered that carbox-
3
2
the track of any silver salt. But to the best of our knowledge, a complete
silver- and (Lewis/Bronsted) acid-free Au(I)-catalyzed hydration under
mild conditions is still lacking in the literature. On the other hand, due
to the strict requirements to the Au(I)-catalysts in terms of hydro-
philicity as well good stability, the recycling uses of the corresponding
Au(I)-catalysts has been rarely reported. The homogenous acid- and
silver-free (NHC)AuOTf catalyst reported by Belanzoni [21] was the
only example so far that was recyclable under the solvent-free condi-
tion. And Carriedo [28] and Li [29] respectively reported the porous
organic polymer-supported NHC-Au heterogeneous catalysts.
−
−
−
+
−
halide (X = Cl or Br ) in the form of L-Au -X . In order to cleave
−
+
Au(I)-X bond to expose Au -center to activate the alkynes, the silver
salts and/or strong acids have to be added additionally [17–19], which
greatly sacrificed the merits of this green hydration. Hence, many ef-
forts have been made to develop the Au(I)-catalysts which are co-
ordinated by the weak anionic ligands such as NTf
place of X to eliminate the use of auxiliary additives such as silver
It was believed that the uses of the poorly coordinated counter
anions instead of X can render the corresponding Au(I)-complexes
more activity towards hydration free of additives, due to the facilely
−
−
−
−
2
, OTf , TFA in
−
+
salts and/or strong Bronsted acids [20–26]. For example, Corma [20]
reported several (R
exposed Au -center to activate alkynes for the subsequent addition of
I
t
H
2
O molecules. In addition, Au+-center has to be coordinated by a
3
P)Au NTf
2
complexes (R
3
P = SPhos, PPh
3
, P Bu
3
)
as the active catalysts for the hydration of a wide range of alkynes to the
corresponding ketones. Gatto [21] developed a series of carbene-ligated
proper ligand like phosphine or carbene in order to prevent Au-catalyst
against deactivation. So we are inspired to develop a hybrid P,O-ligand
⁎
Corresponding author.
E-mail address: yliu@chem.ecnu.edu.cn (Y. Liu).
https://doi.org/10.1016/j.mcat.2018.01.035
Received 14 December 2017; Received in revised form 25 January 2018; Accepted 29 January 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

X. Chen et al.
Molecular Catalysis 448 (2018) 171–176
containing one hard (O-) donor and one soft (P-) donor with flexible
ligations to Au -center, in which the O-donor with weak coordinating
(C40
35 1 2
H O P , %): C 81.12, H 6.13 (Calcd., C 80.93, H 5.94).
+
ability can reversibly release during the catalytic cycle providing the
unsaturation site at the metal center as widely reported before
2.2.3. Synthesis of L3 ligand
L3 were prepared according to the literature methods [41].
[
27,30–35].
In this paper, we succeeded in developing an amphiphilic
2.2.4. Synthesis of Au-L1 complex
Zwitterionic P,O-hybrid ligand (L1) containing phosphino-fragment
In N2 atmosphere, L1 (0.35 g, 0.5 mmol) dissolved in dry di-
chloromethane (20 mL) was added to a solution of AuCl(THT) (0.18 g,
0.55 mmol) in dry dichloromethane (3 mL). The mixture was stirred
vigorously at room temperature for 4 h. Then diethyl ether was added
to the mixture solution to precipitate the white solids, which were
collected after washing by diethyl ether, and then dried under vacuum
as the product of Au-L1 (0.41 g, yield 90%). 1H NMR (δ, 500 MHz,
CD Cl ): 7.93 (d, J = 7.5 Hz, 1H), 7.73–7.67 (m, 6H), 7.57–7.52 (m,
−
and −SO
3
group. Without the addition of any auxiliary additive such
as acid or silver salt, L1-based Au-catalyst exhibited excellent activity
towards hydration of alkynes to yield ketones. In addition, as an ionic
phosphine, L1 based Au-catalyst in combination of the room tempera-
ture ionic liquid (RTIL) as the reaction medium could be easily re-
covered and recycled for 5 runs. This method has been accepted as an
efficient alternative to immobilize the transition metal catalysts for
recovery and recycling in green chemistry [36–40].
2
2
5H), 7.36–7.33 (m, 2H), 7.30–7.23 (m, 5H), 7.09 (t, J = 7.5 Hz, 1H),
.91 (t, J = 7.5 Hz, 4H), 6.77 (br, s, 1H), 6.63–6.60 (m, 1H), 4.03–3.97
6
3
1
2. Experimental
(m, 2H), 2.52–2.50 (m, 2H), 2.17–2.15 (m, 2H), 1.75 (s, 6H). P NMR
−1
(
δ, 500 MHz, CDCl
(w), 2975 (w), 1436 (s), 1402 (s), 1216 (vs), 1109 (s), 1099 (s), 1039
s), 694 (m). CHN-elemental analysis found for Au-L1
(C42H38Au Cl O P S , %): C 54.18, H 4.22 (Calcd., C 54.06, H 4.10).
3
): 28.35, 23.51. FT-IR (ν, KBr pellet, cm ): 3070
2.1. Reagents and analysis
(
The chemical reagents were purchased from Shanghai Aladdin
1
1 4 2 1
Chemical Reagent Co. Ltd. and Alfa Aesar China, which were used as
1
received. The solvents were distilled and dried before use. The H and
2.3. X-ray crystallography
3
1
3 4
P NMR spectra (85% H PO sealed in a capillary tube as an internal
standard) were recorded on a Bruker ARX 500 spectrometer at ambient
temperature. Gas chromatography (GC) was performed on a SHIMA-
Intensity data were collected at 296(2) K for Au-L1 on a Bruker
SMARTAPEX II diffractometer using graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å). Data reduction included absorption cor-
rections by the multi-scan method (Table 1). The structures were solved
by direct methods and refined by full matrix least-squares using
SHELXS-97 (Sheldrick, 1990), with all non-hydrogen atoms refined
anisotropically. Hydrogen atoms were added at their geometrically
ideal positions and refined isotropically.
DZU-2014
equipped
with
a
DM-Wax
capillary
column
(
30 m × 0.25 mm × 0.25 μm). GC-mass spectrometry (GC–MS) was
recorded on an Agilent 6890 instrument equipped with an Agilent 5973
mass selective detector. FT-IR spectra were recorded on a Nicolet
NEXUS 670 spectrometer. The amount of Au and P in the sample was
quantified by using an inductively coupled plasma optical emission
spectrometer (ICP-OES) on an Optima 8300 instrument (the detection
limit of 0.1 μg/g).
2.4. General procedures for hydration of alkyne
2
2
4
.2. Synthesis
In a typical experiment, phenylacetylene (1 mmol, or the other al-
kyne), 3 mL MeOH, deionized water (1 mL), AuCl(THT) (0.005 mmol)
and L1 (0.025 mmol) (or the other ligand) were sequentially added in a
safe glass pressure reactor. The obtained mixture was heated in oil bath
at 120 °C for 3 h. Upon completion, the reaction mixture was cooled to
room temperature. The solution was analyzed by GC to determine the
conversions (n-dodecane as internal standard) and the selectivities
.2.1. Synthesis of L1 ligand
2
In N atmosphere, the solution of Xantphos (1.73 g, 3.0 mmol) in
0 mL of dry toluene (refluxed with sodium and distilled freshly before
use) was treated with 1,3-propanesultone (0.40 g, 3.3 mmol). The ob-
tained mixture solution was stirred vigorously for 12 h at 70 °C. Then
the white precipitates were formed gradually. The precipitates were
collected after filtration and then washed with diethyl ether to give a
Table 1
1
white solid as the product of L1 (1.89 g, yield 90%). H NMR (δ,
The crystal data and structure refinement for Au-L1.
2 2
500 MHz, CD Cl ): 7.93 (d, J = 7.5 Hz, 1H), 7.73-7.67 (m, 6H),
Au-L1
7.57–7.52 (m, 5H), 7.36–7.33 (m, 2H), 7.30–7.23 (m, 5H), 7.09 (t,
J = 7.5 Hz, 1H), 6.91(t, J = 7.5 Hz, 4H), 6.77 (br, s, 1H), 6.63–6.60 (m,
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
1 1 4 2 1
C42H38Au Cl O P S
933.14
1
6
H), 4.03–3.97 (m, 2H), 2.52–2.50 (m, 2H), 2.17-2.15 (m, 2H), 1.75 (s,
3
1
Monoclinic
P2(1)/n
14.2090(6)
16.7516(7)
22.8631(9)
90
101.6170(10)
90
5330.5(4)
4
2 2
H). P NMR (δ, 500 MHz, CD Cl ): 26.38, −21.21. FT-IR (ν, KBr
−1
pellet, cm ): 3060 (w), 2977 (w), 1436 (s), 1402 (vs), 1225 (vs), 1124
(
(
s), 1111 (s), 1039 (s), 696 (m). CHN-elemental analysis found for L1
1, %): C 72.12, H 2.59 (Calcd., C 71.99, H 5.47).
b (Å)
C
42
H
38
O
4
P
2
S
c (Å)
o
α ( )
o
β ( )
2
.2.2. Synthesis of L2 ligand
In N atmosphere, the solution of Xantphos (1.74 g, 3.0 mmol) in
00 mL dry diethyl ether was treated with MeOTf (methyl tri-
o
λ ( )
2
3
V (Å )
1
Z
d
calc (g cm−3)
1.163
2.939
fluoromethanesulphonate, 0.54 g, 3.3 mmol). The obtained mixture
solution was stirred vigorously for 3 h at −50 °C. Then the white pre-
cipitates were formed gradually. The precipitates were collected after
μ (Mo-Kα) (mm−1)
T (K)
296(2)
0.71073
61129
0.0440
0.0262
0.0672
1856
λ
filtration to give a white solid as the product of L2 (2.0 g, yield 90%).
Total reflections
Unique reflections (Rint
1
H NMR (δ, 500 MHz, CDCl
3
): 7.92 (d, J = 8.0 Hz, 1H), 7.70-7.68 (m,
H), 7.60–7.58 (m, 8H), 7.50 (d, J = 9.5 Hz, 1H), 7.34–7.25 (m, 7H),
)
R
wR
1
[І > 2σ(I)]
2
(all data)
2
7
6
5
.07 (t, J = 8.0 Hz, 1H), 6.93 (t, J = 7.5 Hz, 4H), 6.77–6.72 (m, 1H),
F(000)
Goodness-of-fit on F
3
1
.60–6.58 (m, 1H), 2.89 (d, J = 14.0 Hz, 3H), 1.75 (s, 6H). P NMR (δ,
00 MHz, CDCl ): 22.22, −21.02. CHN-elemental analysis found for L2
2
1.070
3
172

X. Chen et al.
Molecular Catalysis 448 (2018) 171–176
Table 2
a
Au-catalyzed hydration of phenylacetylene under different conditions .
Entry
Catalyst precursor
Ligand
Additive
P/Aub
Temp. (°C)
Conv. (%)c
Sel. (%)d
TON
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
Au-L1
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
AuCl(THT)
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
–
Xantphos
L2
L3
L2
L1
–
–
–
–
–
–
–
1/1
3/1
5/1
6/1
5/1
5/1
5/1
5/1
5/1
5/1
1/1
5/1
5/1
5/1
5/1
5/1
100
100
100
100
120
80
80
120
80
29
60
75
48
97
30
95
< 5
21
23
30
–
100
100
100
100
100
100
100
–
100
100
100
–
58
120
150
96
194
60
190
–
42
46
60
–
e
AgOTf
AgOTf
AgOTf
–
–
–
–
n-C
–
0e
1
2
3
4
80
100
120
120
120
120
120
28
–
35
50
100
–
100
100
56
–
70
100
5
6
8
H17SO
3
Na
f
a
b
c
d
e
f
2
AuCl(THT) 0.005 mmol (0.5 mol%), phenylacetylene 1 mmol, H O 1 mL, AgOTf (if required) 0.005 mmol, MeOH 3 mL, time 3 h.
P/Au representing the molar ratio of phosphino-fragment to Au.
Determined by GC with n-dodecane as internal standard.
Determined by GC with normalization method which was calibrated by the commercial sample of acetophenone.
4 h.
2
TBAF 0.025 mmol.
Fig. 1. The single crystal structure of Au-L1 (H-atoms and the solvent molecules were
o
omitted for clarity). Selected bond distances (Å) and bond angles ( ): Au-L1, Au1-P1
2
.2343(8); Au1-Cl1 2.2855(9); P1-Au1-Cl1 174.61(3).
(
0.005 mmol), and L1 (0.025 mmol) sequentially to form a homogenous
reaction solution. Upon completion at the appointed conditions, the
reaction solution was treated with diethyl ether (3 mL × 3) to com-
pletely extract the reactants and products out of the IL phase. The
combined organic phase was analyzed by GC and ICP-OES analyses.
Scheme 1. The structures of Au(I)-complexes ligated by Xantphos and L1.
(
normalization method), and the products were further identified by
GC–MS spectrometry.
6
The lower phase containing [Bmim]PF , MeOH and water was directly
used without further treatment for the next run. Due to the partial loss
of MeOH and water into the organic phase during the extraction, in the
2.5. Protocols for the catalyst recycling experiments in [Bmim]PF
6
3rd and 5th runs, 1 mL of MeOH and 0.5 mL of the deionized water
were added additionally.
In the course of the recycling experiments, the room temperature IL
of [Bmim]PF was selected the co-solvent besides of MeOH. In the first
run (with the fresh catalyst), [Bmim]PF (3 mL) was mixed with MeOH
3 mL), deionized water (1 mL), phenylacetylene (1 mmol), AuCl(THT)
6
6
(
173

X. Chen et al.
Molecular Catalysis 448 (2018) 171–176
Scheme 2. The proposed catalytic mechanism over Au-L1 for hydration of phenylacetylene.
Table 3
Table 4
a
The recycling uses of L1-based Au-catalyst for hydration of phenylacetylene.
Hydration of the different terminal alkynes catalyzed by L1-AuCl(THT) system .
Run Ligand RTIL
Conv. (%)b Sel. (%)b TONc Accumulative TON
Entry
1
Alkyne
Product of ketone
Conv. (%)b
99
Sel. (%)b
100
1a
2
3
L1
–
–
–
–
[Bmim]PF
99
94
96
96
87
100
100
100
100
100
198
184
192
192
174
940
6
–
–
–
–
d
2
3
99
99
100
100
4
5
d
a
2
AuCl(THT) 0.005 mmol, P/Au = 5/1 (molar ratio), phenylacetylene 1 mmol, H O
1
mL, MeOH 3 mL, [Bmim]PF
6
3 mL, time 3 h, temperature 120 °C.
4
5
99
99
100
100
b
Determined by GC with n-dodecane as internal standard.
TON, turn over mumber in 3 h in each run.
c
d
1
mL MeOH and 0.5 mL deionized water were added additionally.
6
7
8
99
99
0
100
100
–
3
. Results and discussion
The catalytic performance of L1-based Au-catalyst was investigated
free of any auxiliary additives with hydration of phenylacetylene as a
model reaction (Table 2). Since phenylacetylene and Au-L1 were highly
polar and insoluble in water, the solvent of MeOH was added to admit
the accessibility of the substrates to the catalytic site. In addition,
MeOH with more nucleophilicity was believed to be able to promote
hydration of alkynes due to its faster attack to the metalated-alkyne
9
99
74
100
100
10
a
2
AuCl(THT) 0.005 mmol, L1 0.025 (P/Au = 5/1 molar ratio), alkyne 1 mmol, H O
intermediate than that of H
dramatically influenced the reaction rate. The best molar ratio of 5 gave
5% yield of acetophenone selectively according to Markovnikov’s rule
2
O [20]. Obviously the molar ratio of P/Au
1 mL, MeOH 3 mL, time 3 h, temperature 120 °C.
b
Determined by GC and GC–MS.
7
at 100 °C in 3 h (Entry 3). The increased temperature up to 120 °C re-
sulted in the highest yield of 97% of acetophenone (TON = 194) (Entry
[tetrahydrothiophene-gold(I) chloride] and L1 at molar ratio of 1/1
(Entries 11 vs 1). In contrast, the uses of analogues of L1 such as
Xantphos, L2, and L3 in place of L1 all corresponded to the poor yields
of acetophenone (Entries 12–15). Especially over the diphosphines of
Xantphos and L3, no reaction occurred under the applied conditions
5
) while at 80 °C only 30% yield was obtained (Entry 6). However, the
prolonged reaction to 24 h at 80 °C also gave 95% yield of the product
Entry 7). Unexpectantly, the addition of AgOTf badly deteriorated the
(
activity of the catalyst alone with the serious precipitation of Au-na-
noparticles, implying the rapid deactivation of the Au-catalyst (Entries
8 3
(Entries 12 and 14). The physical mixture of L2 and n-C H17SO Na also
led to the uncompetitive result in comparison to L1 (Entries 15 vs 5),
8
–10). It was found that the as-synthesized Au-L1 exhibited the same
which implied the intramolecular concerted effect of phosphino-frag-
−
activity as the one in situ formed by mixing AuCl(THT)
ment and −SO
3
in L1 on the catalytic performance of Au-complex.
174

X. Chen et al.
Molecular Catalysis 448 (2018) 171–176
group was synthesized and found efficient to
−
3
As for the diphosphines of Xantphos, it had been verified that the Au
I)-center of the corresponding complexes as shown in Scheme 1 was
fragment and −SO
(
promote Au-catalyzed hydration of alkynes without the addition of any
either chelated by two phosphino-fragments[Au-Xantphos-a: P1-Au1-
P2, 116.73(3) ] [42] or coordinated by one-phosphino-fragment along
auxiliary additives for the first time. In the developed L1-based AuCl
o
+
(THT) catalytic system, the active P-Au species could be protected
−
3
with the developed Au(I)-Au(I) aurophilic interaction [Au-Xantphos-b:
Au–Au bond, 2.9947(4) Å] [43]. In this way, the fused ring-config-
urations rendered Au-Xantphos-a or Au-Xantphos-b the good stabi-
timely by available SO
group again against deactivation (Scheme 2).
On the other hand, the amphiphilic nature of L1 rendered both H O
2
molecules and alkynes better accessibility to P-Au+ catalytic sites,
−
lity. Hence the dissociation of Cl to release the Au(I)-center to activate
which also gave rise to the reaction rate. As an ionic ligand, L1-based
Au-catalyst not only could be recycled in RTIL medium of [Bmim]PF6,
but also exhibited wide generality to the hydration of different terminal
alkynes including aryl ones and linear aliphatic ones.
alkyne substrate was quite difficult without the aid of the auxiliary
+
+
scavenger (H or Ag ) over the diphosphines such as Xantphos and L3,
leading to the poor activities towards hydration of phenylacetylene as
observed in Table 2 (Entries 12 and 14). Comparatively, L1 was a ty-
−
3
pical mono-phosphine tailed with SO
group. The complexation of L1
Acknowledgement
and AuCl(THT) led to the formation of a complex of Au-L1. Au-L1 is a
Zwitterionic mononuclear Au(I)-complex containing a linearly co-
ordinated Au(I)-center (Fig. 1) .
This work was financially supported by the National Natural Science
Foundation of China (Nos. 21673077 and 21473058).
As for Au-L1 complex, the strong electron-withdrawing effect
coming from the positive-charged phosphonium might be an inherent
Appendix A. Supplementary data
+
−
+
driving force to rupture Au -Cl bond. Then the active P-Au species
could be formed without the requirement of the addition of auxiliary
acids or silver salts. This conjecture was tentatively supported by the
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.01.035.
−
following fact. When the equal equivalent F (coming from TBAF) was
added into the reaction system to quench the Lewis acidity of phos-
phonium site due to its strong fluorophilicity [44,45], the activity of L1-
based Au-catalyst deteriorated obviously along with only 50% conver-
sion of phenylacetylene (Table 2, Entry 16 vs 5). In the catalytic cycle,
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−
3
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2
group rendered H O molecule better accessibility
4
123–4563.
to the Au-catalytic sites, which was believed to be another merit of L1
to promote the reaction rate.
[
[
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[
[
[
of L1-based AuCl(THT) system in [Bmim]PF
6
(1-butyl-3-methylimida-
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[
[
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[
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6
) in the
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[
(
(
95–99%) without discrimination on the steric and electronic effects of
[
the para-positioned substituents (Entries 1–7). As for para-ethy-
nylbenzonitrile, the presence of cyano group in excess as a kind of
strong σ-donors dramatically competed with L1 to coordinate to Au-
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and 74% (Entry 10). The increased carbon-chain obviously decelerated
the reaction rate.
[
[
6
2
[
[
[
[
[
[
4
. Conclusion
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[
The Zwitterionic P,O-hybrid ligand (L1) containing phosphino-
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176