Tuning the Selectivity of AuPd Nanoalloys towards Selective Dehydrogenative Alkyne Silylation

Article abstract of DOI:10.1002/chem.201900493

The cross-dehydrogenative coupling of terminal alkynes and hydrosilanes catalyzed by AuPd nanoalloys is described. Metal nanoparticles are readily prepared in 15 minutes from commercially available and cheap starting materials by using a photochemical app

Full text of DOI:10.1002/chem.201900493

A Journal of
Accepted Article
Title: Tuning the Selectivity of AuPd Nanoalloys towards Selective
Dehydrogenative Alkyne Silylation
Authors: Maren Wissing and Armido Studer
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Tuning the Selectivity of AuPd Nanoalloys towards Selective
Dehydrogenative Alkyne Silylation
Maren Wissing[a] and Armido Studer*[a]
Abstract: The cross-dehydrogenative coupling of terminal alkynes
and hydrosilanes catalyzed by AuPd nanoalloys is described. Metal
nanoparticles are readily prepared in 15 minutes from commercially
alkynes and di- or trihydrosilanes using B(C6F5)3/DABCO as an
organic catalyst system.[9i]
In the field of heterogeneous catalysis of this reaction, only few
papers have been published. Along with KNH2/Al2O3 and MgO,
two reports on Au based systems have appeared: Mizuno
described a catalyst consisting of Au nanoparticles supported on
manganese oxide (Au/OMS-2) [10a] while Asao recently studied a
nanoporous gold catalyst.[10b] Both systems operate under similar
conditions at 80 °C in toluene and O2 was applied as the terminal
oxidant. Despite these recent advances, there is room for
improvement since exciting methods work at elevated
available and cheap starting materials using
a photochemical
approach. The ratio of Au and Pd in the alloys heavily influences their
reactivity. These cooperative nanoalloy catalysts tolerate a large
number of functional groups (e.g. free amines and acids), operate at
room temperature under air atmosphere at low loading (2 mol%), and
the cross-dehydrogenative coupling can easily be scaled up.
The cross-dehydrogenative coupling (CDC) between terminal
alkynes and hydrosilanes represents the most atom economic
and straightforward approach to access alkynylsilanes.[1] This
versatile structural entity not only serves as protecting group,[2]
but is also used in many organic transformations such as
cycloisomerizations,[3] cycloadditions[4] and carbon−carbon- and
carbon−heteroatom bond forming reactions.[5] Due to the mild
conditions generally employed in these processes, alkynylsilanes
represent highly versatile intermediates for late stage
functionalization and as substrates for cyclizations to form
complex molecular architectures.[6a-c] As examples, they have
been successfully applied to prepare electronically, mechanically
and structurally interesting materials, e.g. polymers with strong
triplet emission or high thermal stability.[6d-j]
temperatures
(100°C-120 °C),[8c,d,e,9e,h]
require
laborious
preparation of the catalyst,[9g,10a,b] lead to hydrosilylation
byproducts,[8a,b,c] need high catalyst loadings[9h] and most severely
show low tolerance towards functional groups.[8a-d,f,9a-c,9e-h]
Therefore, new easily accessible and bench-stable catalysts that
work under mild conditions are demanded.
As compared to their parent monometallic nanoparticles,
bimetallic systems often show altered catalytic performance in
terms of selectivity, activity and stability due to modification of
their electronic and/or structural properties as a result of
cooperative metal-metal interactions.[11] In recent years, AuPd
nanoalloys have been shown to catalyze different
transformations.[12] For example, Shishido and coworkers
described the hydrosilylation of alkynes or α,β-unsaturated
ketones to provide vinylsilanes or silyl enol ethers[13a] and also
showed that the hydrosilylation of allenes to internal
alkenylsilanes can be achieved with such nanoparticles.[13b] We
recently studied the cooperative activity of AuPd nanoalloys in
selective hydrogenation of internal alkynes to the corresponding
Z-alkenes.[14] The incorporation of Au into the Pd nanoparticle
framework led to an increase in both selectivity and activity.
Herein we show that AuPd nanoalloys efficiently catalyze the
cross coupling between alkynes and silanes to the corresponding
alkynylsilanes. Notably, Pd-catalyzed CDC of alkynes with silanes
is to the best of our knowledge currently unknown.
Alkynylsilanes are generally prepared by deprotonation of
terminal alkynes using Grignard reagents or strong lithium bases
and subsequent coupling with silyl-electrophiles.[7] However, this
established route suffers some drawbacks regarding process
economy, environmental impact as well as tolerance towards
commonly used functional groups. As mentioned above, the
direct dehydrogenative coupling of alkynes and hydrosilanes
would be most appealing considering atom economy and indeed
few reports along those lines have appeared. Pukhnarevich and
coworkers disclosed the first transition metal catalyzed coupling
of alkynes and hydrosilanes using H2PtCl6 in the presence of
iodine to give the corresponding alkynylsilanes.[8a] Since this work,
other homogeneous transition metal catalysts[8b-f], as well as
We commenced our studies by investigating the CDC of
phenylacetylene (1a) and dimethylphenylsilane (2) to give 3a as
a function of the AuPd nanoalloy composition (Scheme 1b). The
nanoparticles were readily prepared by mixing the precursor salts
HAuCl4 and Pd(OAc)2 at defined ratio in DMF and subsequent
irradiation at 254 nm for 15 minutes in the presence of the
strong bases or reducing agents such as alkoxides,[9a]
[9g]
Na/HMPT,[9b] MgO,[9c] NaOH,[9d] LiAlH4,[9e,f] KNH2/Al2O3
NaHBEt3
and
[9h]
have been shown to mediate this transformation.
Recently, Hou and Lou described the CDC between terminal
commercially available photoinitiator Irgacure D-2959 (I2959
,
10 equiv) and polyethylengycol (Mn=2000 g mol-1, PEG2000,
10 equiv). Upon irradiation with UV-light, I2959 fragments to give
ketyl radicals in a Norrish type 1 reaction. These radicals can act
as one-electron donors to reduce Au3+ and Pd2+ to the
[a]
M. Wissing, Prof. A. Studer
Organisch Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: studer@uni-muenster.de
]
corresponding metal atoms[15a while PEG2000 ensures
stabilization of the in situ generated nanoparticles (Scheme
1a).[15b] The thus obtained solution containing the nanoalloys was
Supporting information for this article is given via a link at the end of
the document
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10.1002/chem.201900493
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then directly used to catalyze the CDC of alkyne 1a with
hydrosilane 2.
It was found that increase of the amount of Au led to a better
selectivity towards formation of the targeted 3a with highest yields
(58 %) being achieved with Au0.3Pd0.7@PEG2000 and
Au0.4Pd0.6@PEG2000
as
catalysts
(entries
2-10).
Au0.4Pd0.6@PEG2000 was selected for further optimizations.
Several hydrogen scavengers were added to suppress styrene
formation (see Supporting Information, SI) and best results were
achieved with ethyl phenylpropiolate. The addition of amines like
HNMe2 (added as solution in THF) was also found to be beneficial
(see SI). Upon combining both additives the yield of 3a further
increased to 70 % (entry 12). Importantly, we found that AuPd
nanoalloys prepared with cheaper benzoin as photo-reducing
agent in place of I2959 show similar activity (entry 13). Therefore,
benzoin-derived nanoparticles were used in further studies.
Notably, all CDC reactions were clean and we did not observe
other byproducts than styrene and 5. Since starting 1a was fully
consumed, mass loss can be understood considering that some
alkyne 1a is irreversibly adsorbed on the particle surface, as
terminal alkynes are known to interact strongly with Au
surfaces.[16] The robustness of the process was documented by
running the CDC at 1 mmol scale in a round-bottom flask under
air. After 6 h, the alkyne 1a was fully converted and analytically
pure 3a was isolated in 78 % yield after work-up and purification
by column chromatography (entry 13).
Scheme 1. a) Synthesis of AuPd nanoalloys by simultaneous reduction of
precursor salts with ketyl radicals generated from commercially available
photoinitiator I2959 in the presence of PEG2000. b) CDC of phenylacetylene (1a)
and dimethylphenylsilane (2).
Table 1. CDC of 1a and 2. Variation of the AuPd nanoalloy composition
and additives.
The optimized catalyst system Au0.4Pd0.6@PEG2000 was
carefully analyzed by different spectroscopic techniques.
Transmission electron microscopy (TEM) pictures confirmed the
formation of small spherical nanoparticles with an average
diameter of 2.8±1.0 nm. 2d-energy dispersive X-ray (EDX)-
mapping shows no evident metal segregation within the particles,
which supports the formation of alloyed AuPd nanoparticles
(Figure 1 and SI). To further confirm that Au and Pd are randomly
alloyed, X-ray diffraction (XRD) analysis was conducted. The
XRD pattern only exhibits one set of peaks, which supports the
formation of mixed AuPd particles (see SI). In addition, X-ray
photoelectron spectroscopic (XPS) analysis showed that in
Au0.4Pd0.6@PEG2000 both metals are fully reduced. Binding
energies of Pd 3d5/2 = 335.4 eV, 3d3/2 = 340.5 eV and Au
4f7/2 = 83.7 eV, 4f5/2 = 87.4 eV fit well with previously reported
values for Au0 and Pd0.[17a,b]
Entry
Catalyst
Conversion
[%]
3a
4
5
[%][a]
[%][a]
[%][a]
1
Pd
>99
>99
>99
>99
>99
>99
>99
>97
>99
67
33
34
32
33
22
17
28
27
35
49
24
28
9
13
5
3
3
3
4
3
5
4
3
-
2
3
Au0.1Pd0.9
Au0.2Pd0.8
Au0.3Pd0.7
Au0.4Pd0.6
Au0.5Pd0.5
Au0.6Pd0.4
Au0.7Pd0.3
Au0.8Pd0.2
Au0.9Pd0.1
Au
40
44
4
58
5
58
6
50
7
42
8
27
9[b]
10[b]
11[b]
12[c]
13[c,d,e]
9
6
38
-
Au0.4Pd0.6
Au0.4Pd0.6
>99
>99
70
1
2
77 (78)[e]
10
[a] Averaged values over 4 experiments. All reactions were carried out
in DMF (1.7 mL) at rt under air with 1a (50 µmol) and 2 (75 µmol).
Conversion and selectivity were determined by GC analysis using
mesitylene as internal standard. [b] Reaction was carried out only once.
[c] Ethyl phenylpropiolate (1 equiv) and HNMe2 in THF (0.5 equiv) were
added. [d] Benzoin was used as photoinitiator for the formation of
Au0.4Pd0.6@PEG2000. [e] Value in parentheses corresponds to isolated
yield when the reaction was conducted in 1 mmol scale. Reaction at
0.15 mmol scale provided 3a with 86 % yield (not shown).
The first experiment was conducted with Pd-free Au nanoparticles
which showed no activity for the formation of silylalkyne 3a and
starting 1a (62 %), dimethylphenylsilanol and styrene (28 %) were
detected by GC-analysis (Table 1, entry 11). In contrast, using
“bare” Pd nanoparticles the starting material was fully converted
after 18 h to a mixture of targeted alkynylsilane 3a (33 %), 5 (13 %,
- and E/Z--dimethylphenylsilylstyrene) and styrene 4 (34 %)
(entry 1). We next switched to alloy catalysts and systematically
increased the concentration of Au in the Pd-based nanoparticles.
10 nm
10 nm
Figure 1. High-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) image and EDX-mapping for
Au0.4Pd0.6@PEG2000. green: EDX signal for Pd, red: EDX signal for Au.
With the optimized reaction conditions in hand, the scope and
limitations of the CDC were investigated by first testing various
terminal alkynes, keeping 2 as the silyl component. The thienyl
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alkyne 1b, bearing a thiophene moiety known to interact with Au
nanoparticles,[18] reacted efficiently to 3b (87 %). Aromatic
alkynes bearing electron donating (1d,g) or electron withdrawing
groups (1c,e,f) were smoothly converted to the corresponding
alkynylsilanes (43 % - 71 %). Notably, substrates bearing a free
alcohol (3g, 3n) or amino group (3d) are eligible for this CDC
reaction. The lower yield obtained for 3g is due to loss of material
during purification. Sterically more demanding alkynes (3h, 3i)
also proved to be competent coupling partners and the silylation
was efficient also for long chain aliphatic alkynes such as 3l
(63 %). Cyclic aliphatic alkynes 1j and 1k reacted well to give 3j
and 3k in 77 % and 73 % yield, respectively. Aliphatic alkynes
bearing chloro, methoxycarbonyl, siloxyl or silyl functionalities
were smoothly converted to the corresponding alkynylsilanes 3m,
3p, 3r and 3s. Free acid and nitrile moieties are tolerated and 1q
and 1o engaged in the CDC to give 3q and 3o in 51 % and 73 %
yield. Substrates bearing internal double or triple bonds reacted
to the corresponding alkene 3t and alkyne 3u. Hydrogenation of
the -bonds in these substrates was not observed, as checked by
GC-MS.
We also evaluated the scope with respect to the hydrosilane using
1a as the alkyne component. Trialkylsilanes 6a, 6b and 6c
reacted with 1a to the corresponding CDC products 7a-c in
40 - 65 % yield. The alkene moiety in 6b remained untouched
under the applied conditions. As compared to the silylation with 2,
slightly lower yields were achieved with the trialkylsilanes and
hence an aryl moiety at the silane seems to be beneficial. Indeed,
with silanes 6d-f bearing phenyl groups higher yields were noted
(69 % - 79 %). Steric effects play a role, as for the sterically
demanding silanes 6c and 6f the reaction temperature had to be
increased to 45 °C.
Scheme 3. CDC of various hydrosilanes 6a-f and 1a. [a] Reaction was
conducted for 6 h. [b] Reaction was conducted at 45 °C. [c] Reaction was
conducted for 15 h.
Finally, some mechanistic studies were conducted. CDC with the
deuterium labeled DSiMe2Ph or PhCCD proceeded without any
measurable change in kinetics and selectivities in both cases. The
experiment with DSiMe2Ph showed that deuterium was
incorporated in the hydrogen scavenger ethyl phenylpropiolate.
This indicates that the hydrosilane gets activated by the particles
and likely hydrogen is adsorbed on the particle surface. Notably,
no evolution of hydrogen gas was observed. The thus available
hydrogen is then likely transferred mainly to the internal alkyne.
H-transfer to the substrate terminal alkyne cannot be fully
suppressed and the corresponding alkene is observed as side
product in most cases. For d1-phenylacetylene no transfer of
deuterium to ethyl phenylpropiolate was found. Hence, alkyne
CH activation does likely not occur. To test whether the side
products 5 arise from hydrogenation of the product silylalkyne 3a
or hydrosilylation of the alkyne 1a, the alkynylsilane 3a was added
in a reaction of alkyne 1d and silane 2. Formation of the
hydrogenated alkenylsilane
5 was not observed even for
monometallic Pd nanoparticles. This suggests that byproducts 5
are formed through direct hydrosilylation. This is in agreement
with the fact that also -dimethylphenylsilylstyrene was identified
as a side product. Considering all these results, the mechanism
probably involves activation of the SiH bond at the particle
surface and that the surface bound silyl group then
regioselectively inserts into the alkyne in the second step. The
surface
bound
metal-alkenyl-species
is
then
likely
deprotonated[19] by the added base to give the product 3a. When
the reaction is carried out under Ar atmosphere, the
transformation is incomplete and
a
larger amount of
hydrosilylation side product and styrene are formed. Since H2
formation was not observed, O2 is the terminal oxidant
Scheme 2. Substrate screening for the reaction between various terminal
alkynes (1a-u) and 2. [a] Reaction was conducted for 6 h.
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In summary, AuPd-based nanoparticle catalysts for selective
dehydrogenative cross coupling between alkynes and
hydrosilanes were developed. While monometallic Au
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nanoparticles showed low efficiency in this CDC. Notably, this is
the first report of Pd nanoparticles showing activity in the CDC of
silanes with alkynes. Efficiency was significantly improved by
using readily prepared AuPd nanoparticle alloys as catalysts.
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Acknowledgements
Sebastian Lamping (Organic Chemistry Institute, University of
Mꢀnster) is acknowledged for performing XPS measurements,
Maximilian Niehues (Organic Chemistry Institute, University of
Mꢀnster) for recording TEM- and HAADF-STEM-images, Dr. Uta
Rodehorst (Münster Electrochemical Energy Technology Institute,
University of Mꢀnster) for performing XRD measurements, and
Grete Hoffmann and Dr. Dirk Leifert (Organic Chemistry Institute,
University of Mꢀnster) for fruitful discussions. We also thank the
Deutsche Forschungsgemeinschaft DFG (SFB858) for supporting
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Keywords: nanoalloys • cross-dehydrogenative coupling •
photochemistry • AuPd nanoparticles • catalysis
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Tuning the Selectivity of AuPd
Nanoalloys towards Selective
Dehydrogenative Alkyne Silylation
Two is better than one! Cross-dehydrogenative coupling (CDC) of terminal
alkynes and hydrosilanes catalyzed by AuPd nanoalloys is reported. Au
nanoparticles do not catalyze that transformation, but the alloys with cooperatively
interacting Au and Pd metals show good activity. Various functional groups are
tolerated and these CDC work at room temperature under air atmosphere.
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