Pyridylidene ligand facilitates gold-catalyzed oxidative C-H arylation of heterocycles

Article abstract of DOI:10.3762/bjoc.11.295

Triaryl-2-pyridylidene effectively facilitates the gold-catalyzed oxidative C-H arylation of heteroarenes with arylsilanes as a unique electron-donating ligand on gold. The employment of the 2-pyridylidene ligand, which is one of the strongest electron-donating N-heterocyclic carbenes, resulted in the rate acceleration of the C-H arylation reaction of heterocycles over conventional ligands such as triphenylphosphine and a classical N-heterocyclic carbene. In situ observation and isolation of the 2-pyridylidene-gold(III) species, as well as a DFT study, indicated unusual stability of gold(III) species stabilized by strong electron donation from the 2-pyridylidene ligand. Thus, the gold(I)-to-gold(III) oxidation process is thought to be facilitated by the highly electron-donating 2-pyridylidene ligand.

Full text of DOI:10.3762/bjoc.11.295

Pyridylidene ligand facilitates gold-catalyzed oxidative
C–H arylation of heterocycles
Kazuhiro Hata1, Hideto Ito1, Yasutomo Segawa*1,2 and Kenichiro Itami*1,2,3
Full Research Paper
Open Access
Address:
Graduate School of Science, Nagoya University, Chikusa, Nagoya
64-8602, Japan, 2JST, ERATO, Itami Molecular Nanocarbon Project,
Beilstein J. Org. Chem. 2015, 11, 2737–2746.
1
doi:10.3762/bjoc.11.295
4
Nagoya University, Chikusa, Nagoya 464-8602, Japan and 3Institute
of Transformative Bio-Molecules (WPI-ITbM), Nagoya University,
Chikusa, Nagoya 464-8602, Japan
Received: 20 September 2015
Accepted: 09 December 2015
Published: 28 December 2015
Email:
This article is part of the Thematic Series "C–H
Functionalization/activation in organic synthesis".
Yasutomo Segawa* - ysegawa@nagoya-u.jp; Kenichiro Itami* -
itami@chem.nagoya-u.ac.jp
Guest Editor: R. Sarpong
*
Corresponding author
©
2015 Hata et al; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
carbene ligand; C–H arylation; gold catalysis
Abstract
Triaryl-2-pyridylidene effectively facilitates the gold-catalyzed oxidative C–H arylation of heteroarenes with arylsilanes as a unique
electron-donating ligand on gold. The employment of the 2-pyridylidene ligand, which is one of the strongest electron-donating
N-heterocyclic carbenes, resulted in the rate acceleration of the C–H arylation reaction of heterocycles over conventional ligands
such as triphenylphosphine and a classical N-heterocyclic carbene. In situ observation and isolation of the 2-pyridylidene-gold(III)
species, as well as a DFT study, indicated unusual stability of gold(III) species stabilized by strong electron donation from the
2
2
-pyridylidene ligand. Thus, the gold(I)-to-gold(III) oxidation process is thought to be facilitated by the highly electron-donating
-pyridylidene ligand.
Introduction
Over the past decade, gold salts and complexes have emerged gold-catalyzed oxidative C–H arylation of simple arenes with
as unique catalysts for the transformation of alkynes, alkenes arylsilanes have led the way to novel gold-catalyzed reactions
and allenes [1-30]. In most of the gold-catalyzed reactions, that could not be achieved with other transition metals [68,69].
phosphines, N-heterocyclic carbenes, pyridines and salen In these reactions, the oxidation of gold(I) to gold(III) is
ligands have been applied as ligands for controlling the stability thought to be a key step in the catalytic cycle consisting of
of catalysts, and chemo-, regio- and enantioselectivities of the transmetalation with arylsilane, C–H activation and reductive
reactions [31-36]. Recent advances in the gold-catalyzed reac- elimination [69]. While gold(I) complexes bearing various
tions are represented by oxidative coupling that is expected to ligands are used as gold(III) precursors, it remains unclear
proceed through a gold(I)/gold(III) catalytic cycle [37-81]. In whether ligands can still coordinate to the gold center or not
particular, the elegant works of Lloyd-Jones and Russell on under such oxidative reaction conditions. For example, tri-
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Beilstein J. Org. Chem. 2015, 11, 2737–2746.
phenylphosphine is easily oxidized to triphenylphosphine oxide phene are presented. In addition, direct observation and isola-
by a hypervalent iodine reagent that has been used as an oxidant tion of PyC-gold(III) complexes are described.
for gold-catalyzed C–H arylation [69]. Appropriate ligands that
are tolerant to the oxidative conditions would offer numerous
benefits such as high activity and stability of gold catalyst,
thereby achieving otherwise-difficult oxidative transformations
[
37-40].
Recently, we have introduced highly electron-donating triaryl-
-pyridylidene (PyC: pyridine-based carbene) [82-84] as a new
2
type of nonclassical N-heterocyclic carbene [85-102]. We
demonstrated that the PyC ligand is one of the strongest elec-
tron-donating carbene ligands to a gold(I) species (Figure 1)
Figure 1: Triaryl-2-pyridylidene (PyC) and PyC-gold(I) complex
AuCl(PyC)).
[
83]. The AuCl(PyC) complex is very stable, even in air and
(
moisture, and isolable by column chromatography on silica gel.
Thus we envisioned that a gold complex with strongly electron-
donating PyC would promote the gold(I)-to-gold(III) oxidation
process, facilitating oxidative coupling reactions. Herein we
Results and Discussion
Ligand effect of PyC in gold-catalyzed
report that the PyC ligand facilitates gold-catalyzed oxidative aromatic C–H arylation
C–H arylation of hereroarenes that has been known to be very In this study, we selected the gold-catalyzed oxidative C–H
sluggish with typical ligand systems [68-72]. In this paper, the arylation of arenes with arylsilanes [68,69], reported by Lloyd-
C–H arylation reactions of isoxazole, indole, and benzothio- Jones and Russell, to test the ligand effect of PyC (Table 1).
Table 1: Effect of ligand and oxidant in gold-catalyzed oxidative C–H arylation of isoxazole 1a.a
Entry
Au catalyst
Oxidant
Yield [%]b
3aa
4a
1
2
3
4
5
AuCl(PPh3)
AuCl(IPr)
IBA
IBA
10
0
7
0
AuCl(PyC)
AuCl(PyC)
AuCl(PyC)
AuCl(PyC)
AuCl(PyC)
AuCl(PyC)
AuCl(PyC)
IBA
30
4
12
5
PhI(OAc)2
PhI(OCOCF3)2
PhI(OH)(OTs)
IBA
3
4
6c
9
5
7d
13
0
5
8c
IBA
9
9e
IBA
3
36
aReaction conditions: 1a (0.20 mmol), 2a (0.20 mmol), Au catalyst (5 mol %), oxidant (0.20 mmol), (+)-10-camphorsulfonic acid (CSA, 0.20 mmol),
CHCl3/MeOH (10:1, 1.1 mL), 65 °C. bDetermined by GC analysis with n-nonane as an internal standard. cWithout CSA. dTsOH·H2O was used instead
of CSA. eCHCl3 (1.0 mL) was used as solvent.
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Likely due to the low stability of electron-rich heteroarene with arylsilane 2a (Table 1, entries 4–6) [69]. Using p-toluene-
substrates toward oxidative conditions [103-107], their original sulfonic acid (TsOH) instead of CSA was less effective
conditions usually do not work well for these substrates. For (Table 1, entry 7). It was clearly seen that both CSA and
example, when isoxazole (1a: 1 equiv) [108-111] was treated methanol had a significant effect on the reaction progress
with 1-bromo-4-(trimethylsilyl)benzene (2a: 1 equiv) in chloro- (Table 1, entries 8 and 9). Nevertheless, the highest yield
form/methanol solution at 65 °C in the presence of AuCl(PPh3) achieved by the use of AuCl(PyC) may be attributed to the
(
5 mol %), iodosobenzoic acid (IBA: 1 equiv) and (+)-10- highly electron-donating nature of the PyC ligand.
camphorsulfonic acid (CSA: 1 equiv), the corresponding C–H
Oxidative C–H arylation of heteroarenes with
arylation product 3aa was obtained in only 10% yield (Table 1,
entry 1). Although the application of IPr, a conventional NHC arylsilanes catalyzed by AuCl(PyC)
ligand, to the reaction did not afford 3aa at all (Table 1, entry Having discovered the positive effect of using PyC as a ligand,
2
), PyC promoted the reaction with higher yield of 4-arylisoxa- we further examined the C–H arylation of various heteroarenes
zole 3aa under these conditions (30%, Table 1, entry 3). In the with arylsilanes (Table 2). It should be noted that all of the
AuCl(PyC)-catalyzed reaction, 1a was fully consumed, and examined heteroarenes were not successfully applied in the
4
,4’-dibromobiphenyl (4a) derived from the homocoupling of previous gold-catalyzed C–H arylation. The reactions of 1a
arylsilane 2a was also detected. Furthermore, a significant with halogenated aryltrimethylsilanes 2a and 2b afforded
amount of methyl 2-iodobenzoate (5) was generated through the coupling products 3aa and 3ab in 14% and 15% isolated yields,
esterification of a co-product (2-iodobenzoic acid) with respectively (Table 2, entries 1 and 2) [112]. 5-Methylisoxa-
methanol. We also tested other iodine(III) reagents such as zole (1b) was arylated with bromo-, fluoro- and trifluoro-
PhI(OAc)2, PhI(OCOCF3)2 and PhI(OH)(OTs), but they all methyl-substituted aryltrimethylsilanes, 2a, 2b and 2c, respect-
resulted in lower yields than IBA mainly due to the formation ively, to give the corresponding 4-aryl-5-methylisoxazoles, 3ba,
of diaryliodonium PhI(4-BrC6H4)+ produced by the reaction 3bb and 3bc, respectively, in higher efficiency as compared
Table 2: AuCl(PyC)-catalyzed oxidative C–H arylation of heteroarenes with arylsilanes.a
Entry
1
1
2
3
Yieldb
14%
1
a
2a
2b
3aa
3ab
2
3
15%
55%
1
1
1
1
a
2a
2b
2c
b
b
b
3ba
3bb
3bc
4
5
54%
33%
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Table 2: AuCl(PyC)-catalyzed oxidative C–H arylation of heteroarenes with arylsilanes.a (continued)
6
13%
1
b
c
2
d
a
3bd
7
8
28%
2
1
3ca
17%
2
a
a
1
d
e
3da
9
44%
2
1
3ea
10
22%
2a
1f
3fa (C2):3fa’ (C3)
=
83:17
aReaction conditions: 1 (0.20 mmol), 2 (0.20 mmol), AuCl(PyC) (5 mol %), IBA (0.20 mmol), CSA (0.20 mmol), CHCl3/MeOH (10:1, 1.1 mL), 65 °C,
8–48 h. bIsolated yield.
1
with 1a (Table 2, entries 3–5). This may be due to the higher of methyl 2-iodobenzoate (5). The reaction plots with
tolerability of 1b than 1a toward undesired decomposition AuCl(PyC), AuCl(PPh3) and AuCl(IPr) are depicted in
[
113]. The introduction of the 3,5-dibromophenyl group onto Figure 2. Noteworthy observations are as follows: (i) the reac-
methylisoxazole 1b resulted in lower yield of heterobiaryl 3bd tion with AuCl(PyC) was fastest among those with three cata-
Table 2, entry 6). In the reaction of 5-phenylisoxazole (1c), the lysts (Figure 2a), (ii) the induction periods with regard to the
(
selective arylation at the C4 position occurred without any aryl- formation of 3ba were found in the reactions using AuCl(PyC)
ation at the phenyl group (Table 2, entry 7). 3,5-Dimethylisoxa- and AuCl(PPh3) (Figure 2a,b), and (iii) the oxidant consump-
zole (1d) showed low reactivity, likely due to the steric tion began at the reaction initiation for all catalysts (Figure 2c).
hindrance, but the reaction gave sterically congested hetero- In the reaction using AuCl(PyC), the coupling product 3ba was
biaryl 3da in 17% yield (Table 2, entry 8). In the case of the generated after a shorter induction period of about 3 h and
reaction of indole 1e, 3-arylindole 3ea was exclusively obtained reached 60% yield after 50 h (Figure 2a). On the other hand, the
in 44% yield (Table 2, entry 9). On the other hand, arylation of reaction using AuCl(PPh3) began after a longer induction period
benzo[b]thiophene (1f) mainly afforded 2-arylbenzothiophene (ca. 5 h), and the yield of 3ba did not exceed the yield with
3
fa along with a small amount of 3-arylbenzothiophene 3fa' AuCl(PyC) even after 100 h (see Supporting Information File 1
(
Table 2, entry 10).
for details). No coupling product was produced with AuCl(IPr)
although the consumption of about 10% of IBA was observed.
Reaction progress analysis
To further unveil the ligand effect of PyC, time-production Mechanistic considerations
profiles of coupling product 3ba were investigated for the reac- Based on the above results and the literature [68-75], we
tion of 1b and 2a with AuCl(PyC), AuCl(PPh3) and AuCl(IPr). propose the reaction mechanism of the gold-catalyzed C–H
The yield of 3ba was determined by GC analysis, whereas the arylation of heteroarenes with arylsilanes as shown in
consumption of IBA (oxidant) was estimated by the production Scheme 1. A gold(I) complex A is first oxidized to gold(III)
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Beilstein J. Org. Chem. 2015, 11, 2737–2746.
Figure 2: Yield–time profiles of 4-(4-bromophenyl)-5-methylisoxazole (3ba) and methyl 2-iodobenzoate (5) with AuCl(PyC), AuCl(PPh3) and
AuCl(IPr). (a) Yield of 3ba. (b) Magnified figure of (a). (c) Yield of 5. All yields were determined by GC analysis with n-nonane as an internal standard.
Scheme 1: Plausible reaction mechanism of gold-catalyzed oxidative C–H arylation of heteroarenes with arylsilanes.
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Beilstein J. Org. Chem. 2015, 11, 2737–2746.
species B by the iodine(III) reagent E derived from IBA by the
exchange of a hydroxy group with an existing acid such as
CSA, HCl and MeOH. We independently confirmed that the
esterification of 2-iodobenzoic acid takes place to give 5 under
the reaction conditions; 2-iodobenzoic acid was smoothly
converted to 5 in chloroform/methanol solution at 65 °C. Trans-
metalation of gold(III) complex B with arylsilane 2 affords
monoarylated gold(III) intermediate C. The electrophilic meta-
lation of heteroarene 1 with C with concurrent generation of an
acid (HX) produces diarylated gold(III) species D. Finally, the
reductive elimination from D releases the coupling product 3
along with the regeneration of gold(I) species A. The side reac-
tion leading to the homocoupling product of arylsilane 4
likely occurs via over-transmetalation of monoarylated
gold(III) species C with arylsilane 2 or disproportionation of C
[
67-81].
Oxidation process of gold
Figure 3: ORTEP drawing of AuCl3(PyC) with 50% probability.
Hydrogen atoms and solvent are omitted for clarity.
In all reaction progress experiments with the three gold cata-
lysts (Figure 2), the consumption of IBA (production of 5) was
observed to some extent even in the induction period. Taking
the possible reaction mechanism into consideration, the oxi- With the authentic AuCl3(PyC) in hand, we next carried out the
dation of gold(I) to gold(III) by the oxidant may occur during direct observation of PyC-gold(III) species under the catalytic
the induction period. While it is unclear what is oxidized in conditions. The treatment of AuCl(PyC) with 5-fluoroiodoso-
these reactions, we hypothesize that the highly electron- benzoic acid (5F-IBA) and CSA in CDCl3/CD3OD at 65 °C
donating PyC ligand facilitates the oxidation of gold(I) to resulted in the full consumption of 5F-IBA within 30 min
gold(III). As triphenylphosphine is known to be easily oxidized (monitored by 19F NMR). While the resulting mixture seemed
to triphenylphosphine oxide under the current oxidative condi- to contain several PyC-gold(III) complexes, the formation of
tions, the ligand-free gold(III) species is thought to be an active various gold(III) species bearing hydroxy, methoxy, sulfoxy
species in the arylation reaction with AuCl(PPh3) [69]. While and chloro groups made the analysis and isolation difficult.
the IPr-gold(I) complex is known to undergo oxidation to an However, the subsequent addition of excess LiCl enabled us to
IPr-gold(III) species [114], its inactiveness in the current reac- detect the gold(III) species as AuCl3(PyC) by 1H and 13C NMR
tion indicates that the electron-donating capability is not high analyses. The 1H NMR analysis revealed that about 90% of
enough to facilitate this process.
AuCl(PyC) was consumed and AuCl3(PyC) was produced in
0% NMR yield. Fortunately, the isolation from the messy
5
Direct observation and isolation of PyC-
gold(III) complex
crude mixtures was accomplished to give AuCl3(PyC) in 24%
isolated yield. We also conducted the same experiment
To verify our hypothesis that PyC accelerates the gold(I)-to- with the AuCl(IPr) complex. From the 19F and 1H NMR
gold(III) oxidation, we attempted the direct observation and the analyses, approximately half of AuCl(IPr) and oxidant
isolation of the PyC-gold(III) complex. First of all, the gold(III) 5F-IBA remained unreacted after heating for 30 min, and
complex AuCl3(PyC) was newly synthesized by treating AuCl3(IPr) was observed only in 34% 1H NMR yield [114]
AuCl(PyC) with PhICl2 (see Experimental section and (Scheme 2).
Supporting Information File 1 for details) [114]. The X-ray
crystallographic analysis was successfully accomplished with a These observations on gold(III) species support our hypothesis
colorless single crystal of AuCl3(PyC), which was recrystal- that the highly electron-donating PyC ligand strongly coordi-
lized from nitrobenzene and pentane (Figure 3) [115]. The nates to a gold center and promotes the gold(I)-to-gold(III) oxi-
X-ray crystal structure shows that the four gold bonds are in a dation by stabilizing a gold(III) species without dissociation. An
planar surface, and the pyridylidene face and the added two IPr-gold(III) complex is known to be stable, but the lower elec-
chlorine atoms are in vertical positions. The ligand arrange- tron-donation ability of IPr than that of PyC seems to result in
ment is quite similar to a series of reported NHC–AuCl3 the inefficient oxidation of AuCl(IPr). DFT calculations on the
complexes [114].
oxidation process of the AuCl(ligand) to AuCl3(ligand) also
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Beilstein J. Org. Chem. 2015, 11, 2737–2746.
Scheme 2: Direct observation and isolation of carbene-gold(III) complex. Mes = 2,4,6-Me3C6H2, Xyl = 2,6-Me2C6H3, Dip = 2,6-iPr2C6H3.
clarified the advantage of the PyC ligand over IPr by 1.0 mmol) in 1,2-dichloroethane (5.0 mL). The mixture was
.6 kcal mol–1 (see Supporting Information File 1 for details). stirred at room temperature for 2 h, and AuCl(SMe2) (11.5 mg,
3
While it still remains unclear how the PyC ligand affects the 0.10 mmol) was then added to the reaction mixture. The
transmetalation, C–H metalation and reductive elimination reaction mixture was further stirred overnight, and the addition
steps, we believe that the strongly electron-donating PyC not of CHCl3 (50 mL) to the mixture gave a white precipitate.
only facilitates gold(I)-to-gold(III) oxidation in catalysis but The suspension was filtered off and the filtrate was concen-
also prolongs the catalyst lifetime by preventing the ligand trated under reduced pressure. The crude product was purified
dissociation and formation of inactive gold nanoparticles.
by column chromatography on silica gel (eluents: MeOH/
CHCl3 1:20) and recrystallized from CHCl3/toluene at room
temperature to give a pure AuCl(PyC)/toluene complex
Conclusion
In summary, we have developed the oxidative C–H arylation of (286 mg, 40%) as a pale yellow crystal. The addition of CHCl3
heteroarenes with arylsilanes catalyzed by PyC-gold complex and the concentration in vacuum yielded a pure AuCl(PyC)
and revealed the advantageous features of using the PyC ligand. complex without toluene as white powder. The characterization
From the reaction progress, experiments and stoichiometric oxi- data for AuCl(PyC) corresponded to the reported values [83].
dation of gold(I) complexes, we conclude that the highly elec-
tron-donating PyC ligand promotes the gold(I)-to-gold(III) oxi- General procedure for AuCl(PyC)-catalyzed oxidative C–H
dation and stabilizes the gold(III) species, thereby facilitating arylation of heteroarenes with arylsilanes: AuCl(PyC)
the oxidative coupling reactions.
(6.4 mg, 0.010 μmol, 5.0 mol %), heteroarene 1 (0.20 mmol),
and aryltrimethylsilane 2 (0.20 mmol), 2-iodosobenzoic acid
(IBA, 53 mg, 0.20 mmol), 10-camphorsulfonic acid (CSA)
Experimental
Preparation of triarylpyridylidene-gold(I) chloride (47 mg, 0.20 mmol) and a stir bar were placed in a screw test
AuCl(PyC)]: A 10 mL Schlenk tube containing a stir bar was tube, and dry CHCl3/MeOH (1.0 mL/0.10 mL) was added under
[
dried under vacuum and filled with N2 after cooling to room N2 atmosphere. The tube was sealed with a cap equipped with a
temperature. Ag2O (232 mg, 1.0 mmol) and NBu4Cl·H2O Teflon®-coated silicon rubber septum, and the mixture was
(
1.39 g, 5.0 mmol) were added to the solution of 3,5-bis(2,6- stirred at 65 °C for 18–48 h. The reaction was quenched by ad-
dimethylphenyl)-1-mesitylpyridin-1-ium triflate (730 mg, dition of excess saturated aqueous NaHCO3, the aqueous layer
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Beilstein J. Org. Chem. 2015, 11, 2737–2746.
was extracted with CH2Cl2, and the combined organic layers Acknowledgements
were dried over Na2SO4, filtered, and concentrated under This work was supported by the ERATO program from JST
reduced pressure. The residue was purified by flash chromatog- (K.I.) and the Funding Program for KAKENHI from MEXT
raphy on silica gel to afford the coupling product 3 (Table 2).
(26810057 to H.I. and 23750038 to Y.S.). K.H. is a recipient of
the JSPS research fellowship for young scientists. ITbM is
Oxidation of AuCl(PyC): The oxidation of AuCl(PyC) was supported by the World Premier International Research Center
performed according to the literature [16]. PhICl2 (54.8 mg, (WPI) Initiative (Japan).
0
0
.20 mmol) was added into a solution of AuCl(PyC) (128 mg,
.20 mmol) in CH2Cl2 (2.0 mL) under N2 atmosphere. After References
stirring at room temperature for 19 h, the reaction mixture was
filtered through a pad of Celite®. The filtrate was poured into
hexane and the resulting precipitate was collected by filtration
to obtain pure AuCl3(PyC) as a white solid (140 mg, 99%). The
colorless single crystal used for X-ray diffraction analysis was
obtained by recrystallization from nitrobenzene and pentane.
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H NMR (CDCl3, 600 MHz) δ 8.17 (d, J = 2.1 Hz, 1H), 7.90 (d,
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2
1
1
.28 (s, 12H), 2.15 (s, 6H); 13C NMR (CDCl3, 150 MHz) δ
62.5 (CH), 149.6 (4°), 146.6 (CH), 144.7 (CH), 141.9 (4°),
41.5 (4°), 138.5 (4°), 136.4 (4°), 135.6 (4°), 135.6 (4°), 133.2
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(
(
(
4°), 132.2 (4°), 130.4 (CH), 129.9 (CH), 129.7 (CH), 128.3
4°), 128.3 (CH), 22.1 (CH3), 21.1 (CH3), 20.9 (CH3), 19.3
CH3); HRMS (ESI+) m/z: [M − Cl + MeOH]+ calcd for
9
1
1
1
1
.
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C31H35AuCl2NO, 704.1756; found, 704.1722.
2
2
In situ observation and isolation of AuCl3(PyC): AuCl(PyC)
(
12.8 mg, 0.020 mmol), 5-fluoroiodosobenzoic acid (5F-IBA,
.6 mg, 0.020 mmol) and CSA (4.6 mg, 0.020 mmol) were
5
5
3. Ohno, H. Isr. J. Chem. 2013, 53, 869–882.
placed in an NMR tube, and CDCl3/CD3OD (10:1, 0.60 mL)
was added under N2 atmosphere. The tube was sealed with a
cap equipped with a Teflon®-coated silicon rubber septum and
heated at 65 °C for 30 min. After cooling to room temperature,
LiCl (8.4 mg, 0.20 mmol) was added. 1,1,2,2-Tetra-
chloroethane was added as an internal standard and an NMR
yield of AuCl3(PyC) was estimated by 1H NMR spectroscopy.
The solvent was removed in vacuum, and the residue was
dissolved in EtOAc. The organic layer was washed with satu-
rated aqueous NaHCO3 and brine, dried over Na2SO4, filtered,
and concentrated in vacuum to afford the crude mixture. The
crude mixture was further washed with Et2O to give pure
AuCl3(PyC) as a white powder (3.4 mg, 24%, Scheme 2).
doi:10.1002/ijch.201300054
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1
1
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