Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold Complexes: Highly Active Catalysts for the Room Temperature Hydration of Alkynes

Article abstract of DOI:10.1002/adsc.201800605

The virtually quantitative, room temperature hydration of various terminal and internal alkynes in methanol/water requires between 0.01–0.05 molpercent of [AuCl(NHC)] activated with 1.5 equiv. of silver triflate and 45 equiv. of triflic acid (both relative to gold complex) with ton of up to 300.000. Iptycenyl-substituted NHC ligands play the key role and the most efficient NHC ligand is characterized by a hemispherical shape formed by two N-pentiptycenyl substituents. (Figure presented.).

Full text of DOI:10.1002/adsc.201800605

Title: Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold
Complexes: Highly Active Catalysts for the Room Temperature
Hydration of Alkynes
Authors: Herbert Plenio, Maximillian Heidrich, Marvin Bergmann, and
Dorian Müller-Borges
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold Complexes: Highly Active
Catalysts for the Room Temperature Hydration of Alkynes
Maximillian Heidrich, Marvin Bergmann, Dorian Müller-Borges and Herbert Plenio*
Organometallic Chemistry, Alarich-Weiss-Str. 12, TU Darmstadt, 64287 Darmstadt, Germany, e-mail:
plenio@tu-darmstadt.de, https://www.chemie.tu-darmstadt.de/plenio/akplenio/startseite/
Abstract: The virtually quantitative, room temperature hydration of various terminal and internal
alkynes in methanol/water requires between 0.01 – 0.05 mol% of [AuCl(NHC)] activated with 1.5
equiv. of silver triflate and triflic acid (both relative to gold complex) with ton of up to 300.000.
Iptycenyl-substituted NHC ligands play the key role and the most efficient NHC ligand is characterized
by a hemispherical shape formed by two N-pentiptycenyl substituents.
Keywords: gold catalyzed hydration, alkynes, NHC ligand, ketones, homogeneous catalysis
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/adscxxxx
Introduction
The hydration of alkynes providing the respective ketones is a well-known reaction, whose discovery
can be traced back to the 19th century.[1] Numerous metals (primarily mercury salts) are known to
catalyze the hydration of alkynes,[2] which used to require relatively harsh reaction conditions.[3] This
reaction was used for the industrial synthesis of acetaldehyde until the advent of the Wacker
process, led to the substitution of the ethyne based process.[3b]
Scheme 1. The gold-catalyzed hydration of alkynes.
The discovery of HAuCl4 for alkyne hydration by Utimoto et al. [4] and of the more effective “cationic”
gold-catalyzed alkyne hydration reactions[5] led to the seminal findings by Nolan et al.,[6] of highly
efficient NHC-gold complexes for the hydration of terminal and internal alkynes. Despite recent
[7]
progress, most of the known catalysts require relatively high catalyst loadings ≥ 0.1 mol% or large
amounts of additives such as silver salts[6] or organic salts[8] both with weakly coordinating anions or
high reaction temperatures or any combination of this.[9] Excellent room temperature activities were
1
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
recently reported for the hydration of 3-hexyne using as little as 0.01 mol% of gold complex[10]
–
however, at this catalyst loading up to 10 mol% of ionic additives are needed. Other room
temperature catalysts appear to be useful for terminal alkynes only and require high catalyst
loading.[7b, 11]
Figure 1. Gold complexes for alkyne hydration.
A very notable improvement of alkyne hydration was reported by Hammond/Xu et al. employing
gold complexes LAuCl with extremely bulky Buchwald type XPhos ligands using very small amounts of
Ag-triflate co-catalyst and acid promoter.[12] Among the various phosphines tested, the extremely
bulky Me3(OMe)tBuXPhos provides the most active gold catalyst, easily outperforming the Nolan
complex (Figure 1). Using the Hammond/Xu complex, the hydration of terminal alkynes can be done
at room temperature employing as little as 0.01 mol% [LAu] catalyst. However, the efficient
o
conversion of internal alkynes still requires prolonged heating to 80 C and slightly higher catalyst
loading. Since the steric bulk of either phosphine or NHC ligands[13] plays a very important role for the
activity of the respective [LAuX] in the hydration of alkynes, we became interested in applying
sterically highly demanding iptycenyl-NHC ligands for this reaction.[14] We recently reported the
synthesis of several bulky N-iptycenyl-diimines, NHC-ligands and the respective metal complexes
with unique steric properties.[14] Pentiptycene-substituted NHC-ligands were shown to be excellent
catalysts for the alternating ring-opening polymerization (aROMP) of norbornene and 1,5-
cyclooctadiene.[14a] Chiral bistriptycene-NHC-copper complexes were also employed in the
enantioselective borylation of ,-unsaturated esters. We want to present here new Iptycene-NHC-
gold complexes, some of which are extremely efficient in the room temperature hydration of
terminal and internal alkynes.
2
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
Results and Discussion
Synthesis of [AuCl(NHC)] complexes. In order to better understand the influence of ligand sterics on
the catalytic performance, several new azolium and azolinium salts containing bulky triptycenyl and
pentiptycenyl-substituents were synthesized (Schemes 2, 3).
Scheme 2. Synthesis of N-pentiptycenyl, N’-cyclohexyl-azolium salt. Reagents and conditions: a)
o
ClCH2COCl, NEt3, CH3CN, 12 h, rt; b) KI, Cy-NH2, DMF, 80 C, 24 h; c) BH3, THF, reflux, 24 h; d)
HC(OEt)3, NH4BF4, 120 oC, 48 h.
Scheme 3. Synthesis of a N-pentiptycenyl, N’-triptycenyl-azolinium salt. Reagents and conditions: a)
NaI, acetone, 24 h; b) aminotriptycene, DMAP, CH3CN, rt, 48 h; c) BH3, THF, reflux, 24 h.
The NHC precursors were converted into the respective [AuCl(NHC)] complexes, according to
established literature procedures.[15] Together with previously reported gold complexes (1, 2, 5), six
3
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
different complexes with systematically varied sterics were tested in the hydration of alkynes
(Scheme 4). [AuCl(NHC)] complex 1 contains two triptycenyl-substituents. The NHC ligand in complex
2[14b] is bulkier than in complex 1 since the respective two ortho-positions of the triptycene units
contain methyl groups instead of hydrogen atoms. Complex 2 is a mixture of syn- and anti-isomers,
with the latter isomer being a mixture of two enantiomers. Complex 3 possesses one N-pentiptycenyl
group and a cyclohexyl substituent. This latter complex appears interesting since the efficient
hydration of 3-hexyne employing [AuCl(NHC)] complexes with dissymmetric NHC ligands was
reported.[16] Complex 3 is a mixture of two isomers, differing with respect to the orientation of the
tBu –group. In Scheme 4 the isomer with the downward tBu group pointing away from the gold is
13
displayed, while the other isomer has the downward tBu group pointing towards the gold. The C
NMR spectrum of complex 3 shows separate resonances for the two isomers. Based on the NMR
spectra both isomers appear to exist in approx. equimolar amounts. In addition, each of the two
orientation isomers occurs as two enantiomers, leading to a mixture composed of four isomers for
complex 3. Complex 4 contains one pentiptycenyl and one triptycenyl substituent. The bulkiest gold
complexes are the previously reported complexes 5 and 6,[14d] which possess two N-pentiptycenyl
substituents, with different backbones attached to the heterocyclic unit.
Scheme 4. Synthesis of the [AuCl(NHC)] complexes 1- 6.
4
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
Catalytic hydration of alkynes. The gold complexes (Scheme 4) were first tested in the hydration of
phenyl acetylene employing the Hammond, Xu conditions (1.0 [Au]: 1.5 [Ag]: 45 H+),[12][17] i. e. 0.01
mol% of gold complex, 0.015 mol% of silver triflate and 0.45 mol% of triflic acid (Table 1). According
to Hammond, Xu et al. the acid appears to be important for suppressing the formation of inactive
gold-hydroxides.[18]
First the [AuCl(NHC)] complexes 1-6 were tested in the room temperature hydration of phenyl
acetylene under the reaction conditions specified in the legend of Table 1. Three complexes (6 > 5 >
2) perform significantly better than the other complexes (1, 3, 4). Obviously, the steric bulk of the
NHC ligands is essential for the catalytic activity and the two bulkiest ligands provide the most active
gold complexes (5, 6).
Table 1. Initial screening of [AuCl(NHC)] complexes 1- 6 in the room temperature hydration
of phenyl acetylene.
Conversion Conversion
Complex
4h/%
2
11
-
3
37
60
21 h/%
22
93
6
52
>99
>99
1
2
3
4
5
6
Reaction conditions: room temperature, 0.5 mmol
phenylacetylene, MeOH (solv.), H2O (1 eq), 0.01 mol%
OMe-Au catalyst, 0.015 mol% AgOTf, 0.45 mol% HOTf.
Interestingly, the omission of a single triptycene wing (complex 5, 6 compared to complex 4) leads to
a pronounced decrease in the catalytic activity. Based on the comparison of complexes 2 and 3, it
appears that evenly distributed steric bulk at the NHC ligand is very important for high catalytic
activity.[19] The backbone substituents also have an influence on the catalytic activity and additional
screening (Supporting Information, Figure S1) established the superior performance of complex 6
compared to complex 5. Complex 6 was tested in the conversion of a large number of different
terminal and internal alkynes (Table 2). This was also done to better understand the effect of steric
and electronic variations of the alkyne substrates on substrate conversion. In general terminal
alkynes are more reactive than internal alkynes. Alkyl alkynes (entry 2) are more reactive than the
related aryl alkynes (entry 3). The same reactivity trend is observed in the reactivity of internal
alkynes (di-n-propylacetylene (= 4-octyne) > phenyl-ethyl-acetylene > diphenylacetylene, Table 2,
entries 11, 12, 13).
Since the catalytic performance of the NHC-gold complexes critically depends on the use of sterically
very demanding NHC ligands,[20] one could have expected, the conversion of sterically very hindered
5
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
substrates, such as 2,4,6-triisopropyl-benzene (Table 2, entry 5), to be significantly retarded.
However, the influence of the ortho, ortho-substitution is not very pronounced, when comparing the
hydration of phenyl acetylene and 2,4,6-trimethylphenyl acetylene and 2,4,6-triisopropylphenyl
acetylene (Table 2, entries 3, 4, 5). The hydration of 2,4,6-triisopropylphenyl acetylene is only slightly
less efficient than the conversion of phenyl acetylene. With a view to the observed modest influence
of substrate sterics, this raises questions with respect to the established mechanism of the gold
catalyzed hydration,[10, 21] which typically involves the side-on coordination/activation of the alkyne to
the catalytically active gold center. Based on the X-ray crystal structure of the related complex
[AgCl(6)][14d] this side-on coordination of bulky alkynes should be much less facile for the very bulky
NHC ligands employed here – nonetheless, the bulkiest NHC ligands provide the most active
hydration catalyst and the very bulky 2,4,6-triisopropylphenyl acetylene is easily converted into the
respective ketone. However, the coordination of the alkyne to the cationic gold is normally not
considered to be a rate limiting step in alkyne alkoxylation. Instead the protodeauration or the attack
of the alcohol on the coordinated alkyne are considered to be rate limiting.[21-22] The rate of
protodeauration should increase with increasing steric bulk of the ligand, an accelerating effect of
steric bulk on the nucleophilic attack of the coordinated alkyne is less obvious.
More recent studies report on mass spectrometric studies in which binuclear gold species are
involved in the hydration of propargylic esters.[23] This mechanistic scenario appears less compatible
with the complexes with very bulky NHC ligands.[24]
In this context, it is also important to demonstrate (again) that the gold complex is essential for the
catalytic transformation of the alkyne - i. e. in the absence of gold complex 6, the hydration of the
alkyne does not take place (Table 2, entry 14).
Heteroatoms in the substrates can be tolerated and 3-furanyl-acetylene is converted very efficiently
into the respective ketone. On the other hand, the related sulfur-containing substrate (3-thiophenyl-
acetylene) and a nitrogen-containing substrate (3-pyridyl acetylene) do not react (even at much
higher catalyst concentrations). This is not unexpected since both the sulfur (in thiols) and the
nitrogen atom coordinate to Au, furthermore the basic nature of the pyridine interferes with the acid
cocatalyst. The electronic influence of a 4-R substituent in 4-R-phenyl acetylene was also tested
(Figure 2). It was found, that the hydration of alkynes occurs faster in the presence of electron-
donating 4-R groups (OMe) than with electron-withdrawing groups (4-R= CF3).
6
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
Table 2: Acid-promoted room-temperature hydration of Alkynes catalyzed by complex 6.
6
time Conversion
entry substrate
product
yield (%)a)
(mol%) (h)
(%)a)
1
2
0.01
0.01
1
>99
>99
55b)
67b)
2.5
3
4
0.02
0.02
3
>99
>99
96
96
20
5
0.02
20
92
91
6
7
0.02
0.02
1
>99
>99
96
99
20
8
9
0.02
0.02
20
20
>99
>99
99
83
10
11
0.02
0.05
20
20
>99
>99
72
55b)
99 (22:78
ratio)c)
12
0.05
20
>99
13
14
0.05
0d)
44
20
99
0
98
-
Reaction conditions: room temperature, 0.5 mmol phenylacetylene, MeOH (solv.), H2O (1 eq), 0.01 mol% (or as
designated in the table) complex 6, 0.015 mol% AgOTf, 0.45 mol% HOTf. a) The conversion was determined by GC
b)
using mesitylene as internal standard, yield corresponds to the amount of isolated product. low isolated yield
due to volatility of the reaction product; c) determined by 1H NMR spectroscopy; d) zero [Au], 0.015 mol% [Ag+], 0.45
mol% H+.
7
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
100
50
0
4-CF3-phenylacetylene
phenylacetylene
4-(OMe)-phenylacetylene
0
2
4
6
8
10 12 14 16 18 20
reaction time / h
Figure 2. Conversion-time curve for the room temperature hydration of three electronically variable
4-R substituted phenyl acetylenes (4-R= CF3, H, OMe) in the presence of complex 6 (conditions: 0.5
mmol alkyne, MeOH (solv.), H2O (1 eq), 0.02 mol% complex 6, 0.03 mol% AgOTf, 0.9 mol% HOTf).
To explore the limits of the present room temperature catalytic system reported here, the catalyst
loading was systematically lowered (Table 3) until the conversion turns out to be too slow. At 0.005
mol% complex 6 the conversion of 1-octyne is still virtually quantitative, at 0.002 mol% loading 67%
conversion is observed after 21 h.[25] The turnover numbers are in excess of 300.000. Upon further
lowering catalyst loading, the conversion of the alkyne is negligible.
Table 3: Room temperature hydration of 1-octyne with complex 6 at different catalyst loading.
Catalyst
6 /mol%
0.02
Conversion
Conversion 21
4h/%
>99
>99
97
21
<1
h/%
>99
>99
>99
67
0.01
0.005
0.002
0.001
0.0005
5
<1
<1
Reaction conditions: room temperature, 0.5 mmol 1-
octyne, MeOH (solv.), H2O (1 eq), x mol% Au catalyst, 1.5x
mol% AgOTf, 45x mol% HOTf.
Conclusions
Six different gold complexes [AuCl(NHC)] with iptycene-substituted NHC ligands were synthesized
and tested in the hydration of alkynes. The sterically most demanding NHC ligands provide the
catalytically most active complexes. Following activation of 1 equiv. [AuCl(NHC)] with 1.5 equivs. of
silver triflate and 30 equivs. of triflic acid, the [(bispentiptycenyl-NHC)AuCl] 6 is converted into an
8
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10.1002/adsc.201800605
Advanced Synthesis & Catalysis
extremely active catalyst for the room temperature hydration of alkynes. At 0.01- 0.05 mol% catalyst
loading terminal and internal alkynes provide the respective ketones in nearly quantitative yields.
Steric effects in the substrates do not have a very pronounced effect on the catalytic conversion, but
electron-rich alkynes react significantly faster than electron-deficient ones.
Experimental Section
Typical procedure for the gold catalyzed hydration of alkynes: To a small flask equipped with stirring
bar was first added the corresponding alkyne (1.0 mmol) followed by a MeOH/water mixture (600 µL,
29:1). In the case of insoluble alkynes, CH2Cl2 can be added to improve solubility. Stock solutions of
trifluoromethanesulfonic acid (0.01 M in methanol, 450 µL, 0.0045 mmol, 0.45 mol%), silver triflate
(0.01 M in methanol, 15 µL, 0.00015 mmol, 0.015 mol%) and of gold complex 6 (0.01 M in DCM, 10
µL, 0.0001 mmol, 0.01 mol%) were added. The reaction mixture was stirred at room temperature for
the designated time. After the reaction was completed, the solvents were evaporated. The residue
was dissolved in DCM (10 mL) and the resulting solution filtered over a short plug of silica, which was
washed with DCM (5 mL). The combined filtrates were evaporated under reduced pressure to afford
the respective ketone.
Acknowledgements
We are grateful for support of this work by the DFG via grant Pl 178/13-2.
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10
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Advanced Synthesis & Catalysis
Table-of-contents entry
Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold Complexes: Highly Active Catalysts for the
Room Temperature Hydration of Alkynes
Maximillian Heidrich, Marvin Bergmann, Dorian Müller-Borges and Herbert Plenio*
11
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