Acid, silver, and solvent-free gold-catalyzed hydrophenoxylation of internal alkynes

Article abstract of DOI:10.3762/bjoc.9.235

A range of arylgold compounds have been synthesized and investigated as single-component catalysts for the hydrophenoxylation of unactivated internal alkynes. Both carbene and phosphine-ligated compounds were screened as part of this work, and the most efficient catalysts contained either JohnPhos or IPr/SIPr. Phenols bearing either electron-withdrawing or electron-donating groups were efficiently added using these catalysts. No silver salts, acids, or solvents were needed for the catalysis, and either microwave or conventional heating afforded moderate to excellent yields of the vinyl ethers.

Full text of DOI:10.3762/bjoc.9.235

Acid, silver, and solvent-free gold-catalyzed
hydrophenoxylation of internal alkynes
Marcia E. Richard, Daniel V. Fraccica, Kevin J. Garcia, Erica J. Miller,
Rosa M. Ciccarelli, Erin C. Holahan, Victoria L. Resh, Aakash Shah,
Peter M. Findeis and Robert A. Stockland Jr.*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2002–2008.
Department of Chemistry, Bucknell University, Lewisburg PA, 17837,
USA
doi:10.3762/bjoc.9.235
Received: 01 July 2013
Email:
Accepted: 11 September 2013
Published: 02 October 2013
Robert A. Stockland Jr.* - rstockla@bucknell.edu
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis II".
Keywords:
acid-free; catalysis; gold; gold catalysis; hydrophenoxylation;
silver-free; single component; solvent-free
Guest Editor: F. D. Toste
© 2013 Richard et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A range of arylgold compounds have been synthesized and investigated as single-component catalysts for the hydrophenoxylation
of unactivated internal alkynes. Both carbene and phosphine-ligated compounds were screened as part of this work, and the most
efficient catalysts contained either JohnPhos or IPr/SIPr. Phenols bearing either electron-withdrawing or electron-donating groups
were efficiently added using these catalysts. No silver salts, acids, or solvents were needed for the catalysis, and either microwave
or conventional heating afforded moderate to excellent yields of the vinyl ethers.
Introduction
The use of gold compounds to promote hydroelementation reac- obtained upon heating the reaction mixtures (dichloromethane
tions has grown tremendously over the past few years [1-5]. or THF solutions) to 100 °C for 24–146 hours. Given the
Using this approach, a host of different organic substrates have current interest in the synthetic community regarding the devel-
been successfully functionalized. Despite the intense interest in opment of sustainable transformations [6-10] that are practical
this chemistry the addition of phenols to internal alkenes has and operationally straightforward, we decided to attempt to
rarely been investigated [1,5]. Recently, Sahoo reported the generate the active gold species in situ from a single-compo-
hydrophenoxylation of alkynes using a multicomponent cata- nent precursor. Furthermore, one of the current goals of devel-
lyst comprised of AuCl3, JohnPhos as the supporting ligand, oping sustainable and “greener” organic reactions is focused on
and various silver/potassium salts as promoters [1]. He found the elimination of solvents and additives from the synthetic
that moderate to excellent yields of the vinyl ethers could be methodology [10]. Thus, the design of gold catalyzed reactions
2002

Beilstein J. Org. Chem. 2013, 9, 2002–2008.
that proceed under solvent-free conditions and without the addi- (t-BuXPhos)AuCl afforded incomplete conversion and the for-
tion of silver salts or acidic promoters would be of interest to mation of an intractable mixture of gold compounds when
those charged with designing sustainable organic reactions.
(IMes)AuCl was used. After some experimentation with the
reaction conditions and different mineral bases, the key issue
From the standpoint of the gold catalyst, there are several ways appeared to be the use of THF as the solvent. Changing from
to generate an active species. One of the first reports on this THF to iPrOH afforded high yields of (IMes)Au(C6H4t-Bu)
type of catalysis entailed the use of strong acids to remove the after heating at 50 °C for 20 minutes. Once a successful
methyl group from a methylgold compound and generate a protocol was determined for the model system, the chemistry
(LAu+) species that promoted the addition reaction [11]. Addi- was extended to other (NHC)AuCl compounds (Figure 1). It
tionally, these cationic gold species can be generated through was anticipated that incorporating electron-donating groups into
the reaction of chlorogold compounds with silver salts [1-4]. the aryl fragments on the gold would facilitate the cleavage of
Recently, there has been growing interest in developing the organic group from the gold center through protodeauration;
approaches that generate a catalytically active species under thus, –C6H4t-Bu and –C6H4OMe were selected as substituents.
acid-free and silver-free conditions [12,13]. To this end, we In general, the use of 4-tert-butylphenylboronic acid gave
have synthesized and investigated a series of arylgold com- higher yields of the arylgold compounds than 4-methoxy-
pounds as single-component catalysts for hydroelementation phenylboronic acid. Once isolated by column chromatography
reactions under acid, silver, and solvent-free conditions.
(basic alumina), the arylgold compounds were relatively stable
white solids. The IMes and SIMes examples (4–6) were the
We recently reported the synthesis of arylgold compounds using least stable of the group, and although no decomposition was
a focused microwave reactor (Scheme 1) [14]. Although noted when stored in the solid state under nitrogen (6 months)
microwave-assisted reactions are legion in organic chemistry, they begin to decompose within 24 hours in solution (THF). In
they are rare in organometallic synthesis [15-21]. Our initial contrast, the IPr and SIPr examples were quite stable in both
report on this chemistry was focused on gold compounds solution and the solid state.
bearing Buchwald-type phosphine ligands such as JohnPhos
and t-BuXPhos. These bulky ligands were selected since they Once the arylgold compounds were isolated, the efficacy of
were found to inhibit dynamic phosphine exchange between both phosphine and carbene-ligated species in hydrophenoxyla-
gold complexes [22]. The chemistry was operationally straight- tion reactions was investigated (Table 1). For these screening
forward, and simply heating the reagents in the microwave for reactions, 4-nitrophenol, diphenylacetylene, and 5-decyne
20–30 minutes afforded high yields of the arylgold compounds. served as representative substrates. These alkynes were selected
In this report, we extend the chemistry to the synthesis of aryl- since unactivated internal alkynes remain some of the most
gold compounds containing carbene ligands.
challenging substrates to functionalize through hydroelementa-
tion reactions. Initially, we selected compound 1 as the gold
catalyst and used a focused microwave reactor to heat the reac-
Results and Discussion
The synthesis of the carbene-ligated arylgold compounds was tions. After some experimentation, we discovered that heating
accomplished using (NHC)AuCl (NHC = IMes, SIMes, IPr, the phenol and alkyne with gold catalyst in the absence of
SIPr) precursors. For the initial screening runs, (IMes)AuCl and solvent to 130 °C for 20 minutes generated excellent yields of
4-tert-butylphenylboronic acid were chosen as representative the vinyl ethers. Control reactions revealed that the arylgold
substrates. The same protocol that was successful for the syn- compound was essential to the success of the reaction as its
thesis of arylgold compounds from (JohnPhos)AuCl or removal lead to quantitative recovery of starting materials. The
Scheme 1: Microwave assisted synthesis of arylgold compounds.
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Beilstein J. Org. Chem. 2013, 9, 2002–2008.
Figure 1: Synthesis of arylgold compoundsa,b. aChlorogold precursor (0.32–0.37 mmol), 2 equiv arylboronic acid, 2 equiv Cs2CO3, 50 °C, 20 min,
iPrOH (1.5 mL), microwave irradiation. bIsolated yields. cTHF was used as the solvent.
Table 1: Hydrophenoxylation of unactivated internal alkynes.a
Table 1: Hydrophenoxylation of unactivated internal alkynes.a
(continued)
14
15
16
17
18
6
64
86
89
90
95
45
92
95
91
93
7
8
9
1 Conventional heating
aDiphenylacetylene or 5-decyne (0.28 mmol), 4-nitrophenol
(0.56 mmol), catalyst (14.0 μmol, 5%), 130 °C, 20 min, no solvent,
microwave irradiation. bBased upon 1H NMR spectroscopy using
anisole as an internal standard.
Conv.b
R = Ph Bu
Entry
Gold catalyst
1
None
0
0
2
(JohnPhos)AuCl
0
0
generation of the active gold catalyst in these reactions is
proposed to occur through a protodeauration reaction between
the arylgold compounds and the phenols. The observation of
tert-butylbenzene in the reaction mixture supports this proposal.
Furthermore, to probe whether or not simple chlorogold-coordi-
nation compounds could promote this reaction, JohnPhosAuCl
and other gold compounds were investigated under the same
reaction conditions (Table 1, entries 2–7). In all cases, the
chlorogold compounds were unable to catalyze the addition
reaction and analysis of the crude reaction mixtures revealed
only starting materials. Conventional heating was also investi-
3
(t-BuXPhos)AuCl
0
0
4
(IMes)AuCl
0
0
5
(SIMes)AuCl
0
0
6
(IPr)AuCl
0
0
7
(SIPr)AuCl
0
0
8
1
98
62
79
77
47
63
95
48
60
70
50
48
9
2
10
11
12
13
2 + t-BuXPhos
3
4
5
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Beilstein J. Org. Chem. 2013, 9, 2002–2008.
gated during the screening phase of the project. Plunging a conversions observed with these catalysts. Furthermore,
reactor vial containing 1, 4-nitrophenol, and either internal changing to conventional heating did not prevent the decompo-
alkyne into a preheated oil bath (130 °C, 20 min) followed by sition of 4, 5, and 6 during the addition reaction. In contrast, the
rapid cooling also afforded high yields of the vinyl ethers complexes bearing IPr and SIPr carbene ligands 7–9 afforded
(Table 1, entry 18).
high yields of the vinyl ethers. Examination of the reaction
vessels from these reactions did not reveal the formation of
The effectiveness of different gold catalysts on the addition metallic gold. There was little difference between the saturated
reaction was also studied (Table 1). While 1 was effective at (SIPr) and unsaturated (IPr) carbene ligands, and there was only
promoting the addition reaction, an arylgold compound bearing slight difference between catalysts bearing –C6H4t-Bu and
a bulkier Buchwald ligand (2) afforded lower yields of the vinyl –C6H4OMe aryl groups (8,9).
ether. Adding an additional equivalent of t-BuXPhos increased
the yield slightly; however, 1 was still a superior catalyst. Building on the results from the screening reactions, the scope
Increasing the electron-donating ability of the substituent on the of the hydrophenoxylation reaction was investigated using a
aromatic group attached to the gold center in the t-BuXPhos range of phenols with diphenylacetylene and 5-decyne serving
based catalyst 3 slightly increased the yield of the vinyl ether. as representative internal alkynes (Figure 2). Microwave and
conventional heating were used in these reactions. Based upon
To investigate the effectiveness of carbene-ligated gold com- the initial studies, the phosphine ligated catalyst 1, as well as
pounds on the addition reaction, catalysts 4–9 were screened NHC ligated catalysts 7–9 afforded high yields of the vinyl
(Table 1). When IMes or SIMes based catalysts 4, 5, or 6 were ethers. After a bit of experimentation, it was observed that 1
used, lower yields of the vinyl ethers were observed. Exam- afforded the highest conversions in microwave-assisted reac-
ining the reaction vessels when 4, 5, and 6 were used revealed tions, while 7 generated the highest yields in conventionally
the formation of a significant amount of metallic gold. These heated reactions. The addition reaction was tolerant of both
metallic deposits were not observed when using phosphine electron-withdrawing and electron-donating groups, and the
ligated gold compounds 1–3. This observed decomposition/ ortho-substituted substrate 2-nitrophenol was also successfully
reduction to elemental gold could be a rationale for the lower used in the addition reactions. The resulting vinyl ethers were
Figure 2: Hydrophenoxylation of alkynesa,b. aAlkyne (0.28 mmol), phenol (0.56 mmol), 130 °C, 20 min, no solvent. bIsolated yields. cMicrowave
heating with 1 as the catalyst (14.0 μmol, 5%). dConventional heating with 7 as the catalyst (14.0 μmol, 5%).
2005

Beilstein J. Org. Chem. 2013, 9, 2002–2008.
purified by column chromatography and isolated as powders or chemical shifts were determined by reference to residual non-
viscous oils.
deuterated solvent resonances. The alkene geometry was deter-
mined using 1H–1H NOESY experiments. All coupling
The regioselectivity of the addition reaction was investigated constants are listed in Hertz. Microwave-assisted reactions were
using 1-phenyl-1-hexyne as a representative unsymmetrical carried out in sealed vessels using a CEM Discover equipped
alkyne, 4-nitrophenol as the model phenol, and 1 and 7 as cata- with an external IR (surface) temperature sensor. The
lysts. As shown in Scheme 2, both catalysts generated the addi- PowerMax setting on the Discover was off. Conventionally
tion product in excellent yields. Catalyst 1 gave a slightly heated reactions were carried out in an oil bath. HRMS data
higher yield of the addition product, but 7 was more selective. were obtained on a Thermo Scientific Exactive Plus LC–MS
The regioselectivity of the reaction using our arylgold precata- system (ESI).
lysts is the opposite of what Sahoo found using his catalyst
system [1] and is similar to the regioselectivity found by Nolan General method for the arylation reactions: general proce-
using his hydroxide bridged precatalysts [5].
dure A: For a typical reaction, (NHC)AuCl, 2 equiv of the aryl-
boronic acid, 2 equiv of Cs2CO3, and a magnetic stirring bar
were added to a 10 mL reactor vial. After exchanging the air for
Conclusion
In summary, we have synthesized several carbene-ligated aryl- nitrogen, isopropanol was added by syringe, the mixtures were
gold compounds and investigated the efficacy of both phos- triturated on a stirring plate for 3 minutes, and irradiated. The
phine and carbene-based gold catalysts towards the hydrophe- following settings were used for each experiment: Temperature
noxylation of unactivated internal alkynes. Using these single = 50 °C, time = 20 min, initial power level = 25 W. The initial
component catalysts, a range of phenols have been successfully power setting listed for each reaction was maintained until the
added to unactivated internal alkynes. No silver salts, acids, or desired temperature was reached. The power was then reduced
solvents were needed for the catalysis, and either microwave or for the remainder of the reaction to maintain the temperature.
conventional heating and afforded moderate to excellent yields The reaction time listed is the total irradiation time (no ramping
of the vinyl ethers.
periods). Note: High initial high levels of microwave power (to
rapidly heat the sample) lead to significant decomposition (gold
metal). After cooling to room temperature, the volatiles were
Experimental
General considerations: Unless specified, all solvents were removed, and the title compounds were purified by column
dried using a Grubbs-type solvent purification system. (John- chromatography (basic alumina). The arylgold compounds were
Phos)AuCl and (t-BuXPhos)AuCl, were prepared by displace- dissolved in CH2Cl2 and dried over molecular sieves. After
ment of dimethyl sulfide from Me2SAuCl by JohnPhos or filtration, they were dried under vacuum to afford white
t-BuXPhos [23,24]. The (NHC)AuCl (NHC = SIMes, IMes, powders.
SIPr, IPr) [19,25] precursors as well as arylgold compounds 1–3
[14] were prepared following literature procedures. The aryl- Preparation of (IMes)Au(4-C6H4t-Bu) (4): General proce-
boronic acids, alkynes, phenols, and Cs2CO3 (powder) were dure A was followed with (IMes)AuCl (0.20 g, 0.37 mmol),
obtained from Aldrich and used as received. NMR spectra were 4-tert-butylphenylboronic acid (0.135 g, 0.76 mmol), cesium
collected on a Varian DirectDrive 600 MHz NMR spectrom- carbonate (0.24 g, 0.74 mmol), and isopropanol (1.5 mL). Chro-
eter (1H: 599.77 MHz and 13C: 150.81 MHz) 1H and 13C{1H} matography: basic alumina (37.5 g), gradient hexane/THF
Scheme 2: Regioselectivity of the addition reaction using arylgold precatalysts. Alkyne (0.28 mmol), phenol (0.56 mmol), 130 °C, 20 min, no solvent.
Microwave heating with 1 or 7 as the catalyst (14.0 μmol, 5%). The yield of the addition product and ratio of regioisomers was determined by 1H NMR
spectroscopy using anisole as an internal standard.
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Beilstein J. Org. Chem. 2013, 9, 2002–2008.
(90:10–50:50), Rf 0.60 (hexane/THF 50:50), yield = 0.18 g of a (s, Ar-CH), 32.2 (s, -CH2-), 31.3 (s, -CH2-), 28.9 (s, -CH2-),
white powder (76.1%). HRMS: [M + Na]+ calcd for 24.9 (s, -CH2-), 22.3 (s, -CH2-), 22.1 (s, -CH2-), 13.8 (s, -Me).
C31H37AuN2Na, 657.2520; found, 657.2509. Spectral data:
1H NMR (CDCl3, 25 °C) δ 7.07 (s, 4H, Ar-H), 7.04 (s, 2H,
Supporting Information
=CH), 6.98 (br s, 4H, Ar-H), 2.34 (s, 6H, -Me), 2.16 (s, 12H,
-Me), 1.19 (s, 9H, -Me); 13C{1H} NMR (CDCl3, 25 °C) δ 196.0
Supporting Information File 1
(s, carbene C), 165.4 (s, quat), 146.6 (s, quat), 139.9 (s, Ar-CH),
139.0 (s, quat), 135.4 (s, quat), 134.9 (s, quat), 129.2 (s,
Ar-CH), 123.6 (s, Ar-CH), 121.7 (s, =CH), 34.0 (s, quat), 31.4
(s, -CMe3), 21.1 (s, -Me), 18.0 (s, -Me).
Detailed synthetic procedures for the synthesis of the
arylgold compounds and vinyl ethers as well as NMR
spectra for all new compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-235-S1.pdf]
General method for the catalyst screening reactions:
microwave and conventional heating: general procedure B:
A reactor vial (10 mL) was charged with the LAuAr species
(1–9, 0.014 mmol), alkyne (0.28 mmol), phenol (0.56 mmol),
and a magnetic stirring bar. After exchanging the air for
nitrogen, the samples were irradiated in a focused microwave
reactor or heated in an oil bath. For the reactions carried out in
the microwave reactor, the initial power setting listed for each
reaction was maintained until the desired temperature was
reached. No ramping periods were used in these reactions; thus,
the reaction time listed is the total irradiation time (not the time
at the desired temperature). After cooling, CDCl3 was added to
the reaction mixtures until homogeneous solutions were
obtained (≈2 mL). Anisole (internal standard, 0.28 mmol) was
added to the solutions and the extent of each reaction was deter-
mined by 1H NMR spectroscopy.
Acknowledgements
The authors thank the National Science Foundation for the
funds to purchase the NMR spectrometer (CHE-0521108) and
Bucknell University for an Undergraduate Research Fellowship
(E.J.M.). Acknowledgment is made to the donors of the Amer-
ican Chemical Society Petroleum Research Fund for support of
this research (51819-UR1 to R. A. S.). The authors thank
Professor David Rovnyak and Brian Breczinski for helpful
discussions on NMR spectroscopy.
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