Visible-Light-Enabled Preparation of Palladium Nanoparticles and Application as Catalysts for Suzuki-Miyaura Coupling

Article abstract of DOI:10.1021/acs.orglett.7b03892

Silyl ketones were used for the preparation of palladium nanoparticles (PdNPs) starting with Pd(OAc)₂ in dimethylformamide under irradiation with a visible light-emitting diode (LED). Variation of the silyl ketone structure allowed adjustment of the PdNP diameter (1.9 or 5.2 nm). The in situ-formed PdNPs were further stabilized with polyvinylpyrrolidone and then applied as recyclable catalysts in the Suzuki-Miyaura coupling of arylboronic acids with aryl iodides to obtain substituted biphenyls in excellent yields.

Full text of DOI:10.1021/acs.orglett.7b03892

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Enabled Preparation of Palladium Nanoparticles and
Application as Catalysts for Suzuki−Miyaura Coupling
Florian Masing,† Harald Nusse,‡ Jurgen Klingauf,‡ and Armido Studer*,†
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†Institute of Organic Chemistry, University of Munster, Corrensstrasse 40, 48149 Munster, Germany
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‡Institute of Medical Physics and Biophysics, University of Munster, Robert-Koch-Strasse 31, 48149 Munster, Germany
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̈
S
* Supporting Information
ABSTRACT: Silyl ketones were used for the preparation of
palladium nanoparticles (PdNPs) starting with Pd(OAc)2 in
dimethylformamide under irradiation with a visible light-emitting
diode (LED). Variation of the silyl ketone structure allowed
adjustment of the PdNP diameter (1.9 or 5.2 nm). The in situ-
formed PdNPs were further stabilized with polyvinylpyrrolidone and
then applied as recyclable catalysts in the Suzuki−Miyaura coupling of
arylboronic acids with aryl iodides to obtain substituted biphenyls in
excellent yields.
Scheme 1. (a) Photoinitiators for Light-Mediated
Preparation of Metal Nanoparticles; (b) Photoactive Silyl
Ketones Investigated; (c) Novel Concept for PdNP
Preparation
ilyl ketones have been successfully applied as visible-light
1
Sphotoinitiators in polymer chemistry. Upon irradiation,
they undergo homolytic Norrish Type I C−Si or C−C bond
cleavage, and the radical fragments generated then initiate the
polymerization process.1 Along these lines, visible-light photo-
initiators have been successfully applied to photocuring of
dental fillings, adhesives, and coatings.1,2
In recent years, several processes for the light-mediated
preparation of metal nanoparticles have been developed.3
Scaiano and co-workers first disclosed the potential of using
commercially available photoinitiators such as Irgacure-2959
(1) for the preparation of metal nanoparticles (e.g., Ag, Au, Cu)
under UV irradiation (Scheme 1a).4 Irradiation of I-2959 leads
to Norrish Type I C−C bond cleavage to generate ketyl
radicals, which are able to reduce metal salts to the
corresponding metal nanoparticles.4 I-2959 was also immobi-
lized into polymers, enabling the in situ preparation of polymer-
coated Au and Pd nanoparticles by UV irradiation.5
Furthermore, the commercially available bis(acyl)phosphine
oxide (BAPO) photoinitiator 2 has been applied successfully to
UV-light-mediated Ag, Au, and Pd nanoparticle synthesis.6
However, expensive special equipment (e.g., UV reactors,
quartz glass cuvettes) and harmful UV light are essential in
these processes, limiting the universal applicability of these
interesting methods. In contrast, the photoinitiator camphor-
quinone (CQ, 3) allows the preparation of Ag− or Au−
polymer nanocomposites under irradiation with blue light.7
Motivated by that study, we aimed to develop a process for the
preparation of Pd nanoparticles (PdNPs) using visible-light
irradiation, since visible-light sources are cheap, more energy
efficient than UV lamps, and easily accessible. The development
of a simple and reliable process for PdNP preparation with
visible light-emitting diodes (LEDs) should therefore be a
significant improvement over existing methods. It is known that
silyl ketones show Norrish Type I activity in the visible
spectrum, but surprisingly, they have not been tested for light-
mediated metal nanoparticle preparation to date (Scheme 1b).1
Herein we present first results on visible-light-enabled PdNP
synthesis using silyl ketones as reagents (Scheme 1c). We also
show that the PdNPs thus obtained can be stabilized with
commercially available polyvinylpyrrolidone (PVP) and then
applied as catalysts for the Suzuki−Miyaura coupling reaction.8
These Pd@PVP catalysts operate at low loading (0.1 mol %)
and can be readily recycled.
Received: December 14, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03892
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
PdNP formation was studied with easily accessible phenyl-
(trimethylsilyl)methanone (4) and bis(dimethyl(phenyl)silyl)-
methanone (5) as photoactive reagents (see the Supporting
Information (SI)). Acyl silane 4 exhibits UV/vis absorption in
the λ = 350−450 nm region (λmax = 418 nm), whereas the UV/
vis absorption of bis(silyl) ketone 5 is red-shifted to the λ =
425−575 nm region (λmax = 544 nm) (see the SI for UV/vis
spectra). For nanoparticle preparation, acyl silane 4 or bis(silyl)
ketone 5 was diluted in dimethylformamide (DMF), and
Pd(OAc)2 was added under an argon atmosphere. The molar
ratio of the silyl ketones with respect to Pd(OAc)2 was set as
40:1. The reaction mixture was then irradiated with LEDs at λ
= 420, 462, or 520 nm under vigorous stirring. When acyl silane
4 was used, irradiation at λ = 420 nm for 30 min provided a
brown solution, indicating the formation of PdNPs. Indeed,
transmission electron microscopy (TEM) revealed the
formation of small nanoparticles with a mean diameter of 5.2
0.9 nm (Figure 1a). The oxidation state of the PdNPs was
irradiation of Pd(OAc)2 in DMF for 3 h, documenting the
importance of both the silyl ketone and light for successful NP
preparation.11 Irradiation (λ = 420 nm) of a mixture of 4,
Pd(OAc)2, (2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and
DMF for 30 min provided benzoyl-TEMPO derived from
trapping of benzoyl radicals by TEMPO, showing that
homolytic Norrish Type I C−Si bond cleavage had occurred.
This indicates that the metal salt reduction process likely
follows a radical pathway.1 However, Pd salt reduction by a
siloxycarbene intermediate generated by a light-induced [1,2]-
Brook rearrangement currently cannot be ruled out.1,12
To test the catalytic activity of these PdNPs, they were used
as catalysts in Suzuki−Miyaura cross-coupling of phenylboronic
acid (6a) with iodobenzene (7a) to form biphenyl (8aa).8 The
activity of Pd@5PVP (1.9 0.5 nm) was investigated first, and
8aa was obtained in 83% isolated yield after 3 h at 50 °C with
0.5 mol % catalyst loading (Table 1, entry 1). Surprisingly, the
Table 1. Suzuki−Miyaura Coupling with Pd@PVP Catalysts
a
and Recycling Experiments
b
entry
Pd@PVP
cycle
time (h)
yield of 8aa (%)
1
2
3
4
5
Pd@5PVP
Pd@4PVP
Pd@4PVP
Pd@4PVP
Pd@4PVP
Pd@4PVP
Pd@4PVP
Pd@PVP
1
1
2
3
4
1
1
1
3
83
93
93
83
98
94
92
90
1
1
1.5
1.5
1
Figure 1. TEM images of the prepared PdNPs: (a) Pd@4 and (b)
Pd@5.
c
6
d
7
f
4
8
0.5
investigated by X-ray photoelectron spectroscopy (XPS), which
showed the characteristic binding energies for Pd0 (Pd 3d5/2 at
335.1 eV and Pd 3d3/2 at 340.4 eV), proving the complete
photochemical reduction of Pd ions to Pd0 (see the SI).9 When
bis(silyl) ketone 5 was used, a dark-violet reaction mixture
resulted after 1 h of irradiation at λ = 462 or 520 nm. TEM
analysis revealed in both cases that small nanoparticles were
formed. Under irradiation with blue light, PdNPs with a mean
diameter of 1.9 0.6 nm were obtained, and irradiation with
green light resulted in PdNPs with a similar diameter of 1.9
0.5 nm (Figure 1b).
Notably, the beginning of PdNP formation under LED
irradiation was observed rapidly after 5−10 min using 4 and 5.
The in situ-formed “naked” PdNPs prepared with ketones 4
(Pd@4) and 5 (Pd@5) showed high stability in DMF.
However, after evaporation of the solvent, aggregation to
larger particles was noted. Therefore, the PdNPs were further
stabilized by addition of commercially available PVP (molar
mass = 10 000 g/mol).10 The PdNP−polymer hybrids Pd@
4PVP and Pd@5PVP could be readily isolated by solvent
evaporation without aggregation. Dynamic light scattering
(DLS) measurements indicated that these PdNPs were coated
with a thin PVP layer (∼2.0 nm for Pd@4PVP and ∼1.8 nm for
Pd@5PVP).
a
Reaction with 0.2 mmol of iodobenzene and 0.3 mmol of
b
phenylboronic acid. Isolated yields after column chromatography.
c
Reaction with 1 mmol of iodobenzene and 1.5 mmol of phenyl-
d
f
boronic acid. Reaction with 0.1 mol % Pd@4PVP. The Pd@PVP
catalyst was prepared according to a literature prcedure.8d,15
larger Pd@4PVP (5.2
0.9 nm) showed higher catalytic
activity, and 8aa was obtained in 93% yield after 1 h under
otherwise identical conditions (Table 1, entry 2).13 Possible
reasons for the lower reactivity of Pd@5PVP might be stronger
adsorption of reactive intermediates on the NP surface or
deactivation of the catalyst by fragments derived from the
bis(silyl) ketone 5.13 Notably, Pd@4PVP was readily recycled.
After completion of the reaction, the EtOH/H2O solvent was
evaporated, and 8aa was extracted with 3:1 pentane/CH2Cl2.
Pd@4PVP was not soluble in pentane/CH2Cl2 and could be
dissolved in EtOH after extraction of 8aa. By means of this
simple protocol, product 8aa was obtained in 83−98% yield
over four cycles (Table 1, entries 2−5).
Leaching experiments were also conducted. After 0.5 h, the
Pd@4PVP catalyst was precipitated, and the Pd content of the
supernatant was analyzed with inductively coupled plasma mass
spectrometry (ICP-MS) (see the SI). The Pd content was
found to be 0.1 ppm. When this “leached Pd” was applied as
the catalyst, no biphenyl was formed after 1 h at 50 °C, and
only a 30% yield of biphenyl was obtained after 3 days. Both
experiments indicate that Pd@4PVP rather than leached Pd is
the catalyst in this reaction.8,14
To document the necessity of light for nanoparticle
preparation, the reactions with 4 and 5 were repeated in the
dark. As expected, formation of PdNPs was not observed after
30 min of stirring in these cases. With 5, however, NP
formation was observed after more than 30 min of stirring in
the dark. Furthermore, no nanoparticles were generated upon
B
DOI: 10.1021/acs.orglett.7b03892
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Furthermore, the reaction was carried out on a 1 mmol scale,
and 8aa was isolated in 94% yield (Table 1, entry 6). The
coupling also proceeded well at a lower catalyst loading (0.1
mol %), affording 8aa in 92% yield; however, reaction time had
to be extended to 4 h in that run (Table 1, entry 7).
In addition, the activity of the photochemically generated
PdNPs was compared with the activity of Pd@PVP (DLS
diameter = 5.4 1.7 nm) prepared according to a traditional
literature procedure.8d,15 The “traditional” particles turned out
to be slightly more active, affording 8aa in 90% isolated yield
after 0.5 h (Table 1, entry 8). However, the method used to
prepare these established NPs is more time-consuming and
elaborate compared with the method introduced herein.
Unfortunately, bromobenzene and chlorobenzene did not
react with 6a to form 8aa using these PdNP catalysts (the
experiments were conducted for 16 h at 80 °C with 1 mol %
Pd@4PVP or Pd@5PVP).
bromophenylboronic acid (6b) with 7a gave 4-bromobiphenyl
(8ba) in poor yield (29%). However, the chlorinated and
fluorinated boronic acids 6c and 6d provided the corresponding
biphenyls 8ca (99%) and 8da (94%) in excellent yields,
showing that the bromo substituent in 6b is detrimental to the
reaction outcome. Formyl- and acyl-substituted boronic acids
6e and 6f were transformed in excellent yields to the targeted
biphenyls 8ea (98%) and 8fa (94%). 2-Tolylboronic acid (6g)
afforded biphenyl derivative 8ga in nearly quantitative yield.
Sterically hindered 2,6-dimethylphenylboronic acid (6h) could
also be coupled with 7a to give biphenyl 8ha, although a lower
yield was noted (66%). The reaction was slow (18 h), and the
catalyst loading had to be increased to 1 mol %. Finally, 3,4-
(methylenedioxy)phenylboronic acid (6i) was transformed to
biphenyl 8ia (96%).
In summary, a novel method for the preparation of PdNPs
with silyl ketone photoinitiators was developed. Simple mixing
of silyl ketones with Pd(OAc)2 in DMF and subsequent
irradiation with visible light provided small PdNPs. Depending
on the photoinitiator used, the diameter of the PdNPs could be
adjusted to 1.9 or 5.2 nm. The PdNPs were further stabilized
with commercially available polyvinylpyrrolidone, and the
palladium−polymer hybrid materials obtained were carefully
characterized by TEM, XPS, DLS, NMR, and IR. The Pd@PVP
hybrid material was also applied as a catalyst for Suzuki−
Miyaura coupling of various boronic acids with a series of aryl
iodides.
Next, the substrate scope was investigated using Pd@4PVP
as the catalyst at 0.5 mol % loading (Scheme 2). Aryl iodides
a
Scheme 2. Substrate Scope
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03892.
Detailed experimental procedures and spectroscopic data
for all products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*studer@uni-muenster.de
ORCID
Armido Studer: 0000-0002-1706-513X
Notes
The authors declare no competing financial interest.
a
Reaction with 0.2 mmol of aryl iodide and 0.3 mmol of arylboronic
ACKNOWLEDGMENTS
acid. Isolated yields after column chromatography are presented.
■
b
c
Reaction at 50 °C. Reaction with 1 mol % catalyst.
Sebastian Lamping (Institute of Organic Chemistry, University
of Munster) is acknowledged for performing XPS measure-
̈
ments. Oliver Bolle Bauer (Institute of Inorganic and Analytical
7b−h were systematically varied, and coupling with 6a
provided the substituted biphenyls 8ab−ah. The cross-coupling
of 6a with 4-iodotoluene (7b) afforded 4-methylbiphenyl (8ab)
in 96% yield. The constitutional isomers of iodoanisole (7c−e)
were readily converted to the corresponding 4-, 3-, and 2-
methoxybiphenyls (8ac−ae) in excellent yields (90−93%).
Also, electron-deficient aryl iodides bearing ethyl ester (7f),
nitro (7g), and nitrile (7h) functional groups could be coupled
to give the substituted biphenyls 8af−ah in good to excellent
yields (80−98%).
Chemistry, University of Munster) is acknowledged for
performing ICP-MS experiments. We thank the Deutsche
Forschungsgemeinschaft (SFB 858) for funding this work.
̈
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DOI: 10.1021/acs.orglett.7b03892
Org. Lett. XXXX, XXX, XXX−XXX