Silica-supported silver nanoparticles as an efficient catalyst for aromatic C-H alkylation and fluoroalkylation

Article abstract of DOI:10.1039/c8dt01090k

The efficient catalysis of oxidative alkylation and fluoroalkylation of aromatic C-H bonds is of paramount importance in the pharmaceutical and agrochemical industries, and requires the development of convenient Ag⁰-based nano-architectures with high catalytic activity and recyclability. We prepared Ag-doped silica nanoparticles (Ag^0/+@SiO₂) with a specific nano-architecture, where ultra-small sized silver cores are immersed in silica spheres, 40 nm in size. The nano-architecture provides an efficient electrochemical oxidation of Ag⁺@SiO₂ without any external oxidant. In turn, Ag⁺@SiO₂ 5 mol% results in 100% conversion of arenes into their alkylated and fluoroalkylated derivatives in a single step at room temperature under nanoheterogeneous electrochemical conditions. Negligible oxidative leaching of silver from Ag^0/+@SiO₂ is recorded during the catalytic coupling of arenes with acetic, difluoroacetic and trifluoroacetic acids, which enables the good recyclability of the catalytic function of the Ag^0/+@SiO₂ nanostructure. The catalyst can be easily separated from the reaction mixture and reused a minimum of five times upon electrochemical regeneration. The use of the developed Ag⁰@SiO₂ nano-architecture as a heterogeneous catalyst facilitates aromatic C-H bond substitution by alkyl and fluoroalkyl groups, which are privileged structural motifs in pharmaceuticals and agrochemicals.

Full text of DOI:10.1039/c8dt01090k

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S. V. Fedorenko, A. Mustafina, K. V. Kholin, I. R. Nizameev, S. O. Strekalova, V. Grinenko, T. Gryaznova, R.
Zairov, R. Mazzaro, V. Morandi, A. Vomiero and Y. H. Budnikova, Dalton Trans., 2018, DOI:
10.1039/C8DT01090K.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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DOI: 10.1039/C8DT01090K
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ARTICLE
Silica-Supported Silver Nanoparticles as an Efficient Catalyst for
Aromatic C-H Alkylation and Fluoroalkylation
Mikhail N. Khrizanforova, Svetlana V. Fedorenkoa, Asiya R. Mustafinaa, Kirill V. Kholina, Irek R.
Nizameeva, Sofia O. Strekalovaa, Valeriya V.Grinenkoa, Tatiana V. Gryaznovaa, Rustem R. Zairova,
Raffaello Mazzarob,c, Vittorio Morandiс, Alberto Vomierob and Yulia H. Budnikovaa*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The efficient catalysis of oxidative alkylation and fluoroalkylation of aromatic C-H bonds is of paramount importance in
pharmaceutical and agrochemical industry, and requires the development of convenient Ag0-based nano-architectures
with high catalytic activity and recyclability. We prepared Ag-doped silica nanoparticles (Ag0/+@SiO2) with specific nano-
architecture, where ultra-small sized silver cores are immersed into silica spheres, 40 nm in size. The nano-architecture
provides efficient electrochemical oxidation to Ag+@SiO2 without any external oxidant. In turn, Ag+@SiO2 5 mol. % results
in 100 % conversion of arenes into their alkylated and fluoroalkylated derivatives in a single step at room temperature in
nanoheterogeneous electrochemical conditions. Negligible oxidative leaching of silver from Ag0/+@SiO2 is recorded during
the catalytic coupling of arenes with acetic, difluoroacetic and trifluoroacetic acids, which enables the good recyclability of
the catalytic function of the Ag0/+@SiO2 nanostructure. The catalyst can be easily separated from the reaction mixture and
reused a minimum of five times upon electrochemical regeneration. The use of the developed Ag0@SiO2 nano-architecture
as heterogeneous catalyst facilitates the aromatic C-H bonds substitution by alkyl and fluoroalkyl groups, which are
privileged structural motifs in pharmaceuticals and agrochemicals.
alkylhalogenides, alkenes, and other available precursors,
upon chemical, photochemical15,30,31 and thermal initiation
(using transition metals as catalysts, such as Ag13, Pd19, Ni14,
Introduction
Alkylation reactions represent an important organic
transformation to form C−C bonds. From a structural-
diversification standpoint, substitution of simple C–H bonds
for alkyl and fluoroalkyl groups is an important operation for
organic synthesis, given the marked changes in chemical and
biological properties that can arise from the incorporation of
small aliphatic group1–12. Trifluoromethyl group is a privileged
structural motif in pharmaceutical and agrochemical sectors,
owing to its profound effect on properties such as lipophilicity,
Co17, etc18). Electrochemically induced catalysis has emerged
as an attractive alternative for the generation of alkyl radicals
or alkyl-metal intermediates from different alkyl precursors via
single- or two-electron transfer processes32–44
.
The addition of radicals to aromatic systems has an long-
lasting history, dating from an earlier era of organic chemistry.
The pioneering work of Minisci in the 1970s is worth noting to
highlight the C-H alkylation of electrondeficient (hetero)arenes
by alkyl radicals, generated through silver-catalyzed oxidative
decarboxylation of alkyl carboxylic acids, as a powerful
synthetic tool13 (Scheme 1). Since its inception, this
transformation has been extensively used in the context of
drug discovery, as many target molecules may be rapidly
accessed through variation of either the heterocycle or the
carboxylic acid component45. The traditional Minisci reaction
involves Ag(I)-catalyzed radical decarboxylation of alkyl
carboxylic acids at elevated temperatures with inorganic
persulfates (S2O82−) as oxidants13.
permeability and metabolic stability1–11
.
With the
advancement of transition metal chemistry during the last
decades, catalytic carbon−hydrogen bond (C−H) alkylation13-20
or fluoroalkylation6,8,21–30 has become an important tool for
the synthesis of complex molecules, and these reactions play a
critical role in chemical and pharmaceutical industry.
Generally, alkylation (fluoroalkylation) reactions proceed via
formation of alkyl-radicals emerging from carbon acids,
a.Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center
of RAS, 8 Arbuzov str., Kazan 420088, Russian Federation. E-mail: yulia@iopc.ru.
b.Department of Engineering Sciences and Mathematics, Luleå University of
Technology, 971 87 Luleå, Sweden
AgI NH
4 2S2O
,
(
)
8 (1:10)
700
+
R
R-COOH
10% H2SO4,
-CO2
C
N
N
:
5
1
c. CNR-IMM, Bologna Section, Via Piero Gobetti 101, 40139 Bologna, Italy
Scheme 1
† Electronic Supplementary Information (ESI) available: [Methods, electrochemical
synthesis details, Conditions and yields of arenes alkylation and fluoroalkylation].
See DOI: 10.1039/x0xx00000x
In the recent years, the synthetic conditions have been
widened by using many other compounds, including aromatic
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carboxylic acids46, aryl boronic acids and trifluoroborates47 as catalysis65, and high catalytic activity of ultra-small (2-5 nm)
View Article Online
60,65
radical precursors, oxidizing and promoting agents48. The Ag0 nanoparticles has been already docuDmOeI:n1t0e.1d039/C.8FDrTo0m10t9h0eK
direct silver-catalyzed trifluoromethylation of unactivated viewpoint of oxidative catalysis, Ag@SiO2 nanoarchitecture,
arenes and heteroarenes through
a
radical-mediated where silver in Ag+ or Ag2+ oxidized form is encapsulated into
mechanism using commercial photocatalysts by MacMillan30 is silica supports, seems to be a promising structure, since
worth regarding as appreciable achievement. The best similar approach was very successful for Ni3+@SiO2
product yields have been obtained for the electrondeficient nanocatalyst43. Unfortunately, photo-induced reduction of Ag+
(hetero)arenes. The recently described Minisci alkylations of to Ag0 causes the “aging” of Ag+@SiO2 colloids66. Thus,
electron-deficient pyrimidines with alkyl carboxylic acids is an Ag0@SiO2 nanoparticles are worth regarded as a starting point
elegant example49.
for more reproducible catalytic function, although they should
Electrochemical generation of radicals from carbonic acids be easily transformed into Ag+(Ag2+)@SiO2.
(Kolbe reaction) or alkylhalogenides is also known since a long Recyclability of nanoheterogeneous Ag0@SiO2 catalysts is one
time. In absence of catalyst and/or mediator, the reaction of the most important factors driving the research for its
requires high electrode potentials and thus it is rarely applied further application. Literature data indicate that the best
for alkylation, because it commonly yields to products of nano-architectures are Ag0@SiO2, where Ag0 cores located in
homo-coupling. Nevertheless, trifluoromethylation of the cavities of the mesoporous silica 67. The synthetic potential
fumaronitrile,50 and the Reformatsky reaction with aldehydes of AgNO3–SiO2 as a catalyst has been mostly over-looked, with
and ketones51 are fine examples of its successful application. just a handful of reports on its use as a reagent in organic
70,71
Carbonic acids are highlighted as more efficient reactive synthesis67-71. Thus, Marshall68,69 and Knight
showed that
agents versus organohalides in nickel- or palladium-mediated silica-supported AgNO3 (AgNO3–SiO2) catalyzes the
C-H aromatiс electrochemical perfluoroalkylation36,43. The dearomatizing spirocyclization of alkyne-tethered aromatics
recent success in electrochemical oxidative catalysis by silver far more effectively than the analogous unsupported reagent;
salts reveals facile synthetic routes of both efficient in many cases, reactions which fail using unsupported AgNO3
phosphorylation of arenes (heteroarenes) by dialkyl-H- proceed effectively with AgNO3–SiO2. More recent
phosphonates52 and С-H/P-H cyclization53 through the radical mechanistic studies reported by Taylor and Unsworth indicate
mechanism.
Following the requirements of green chemistry and the silica surface combined with a synergistic effect caused by
pharmaceutics, it is of vital importance to focus on atom- the silica support itself 67
that this is a consequence of silver nanoparticle formation on
.
economical catalytic reactions of C-C formation through direct The oxidative catalytic function of Ag0@SiO2 nanoparticles is
C-H substitution, avoiding the use of toxic metals (Ni, etc.) as based on Ag0→ Ag+ oxidation process. Ag+ ions should be
catalysts. To this end, we have focused on commercially efficiently adsorbed or specifically bound in silica supports to
available and chemically stable aliphatic carbon acids, inhibit their leaching. High level of leaching results in limited
including fluorosubsituted ones, as a source of alkyl and recyclability of Ag0@SiO2 in oxidative catalysis, when silica
fluoroalkyl species. In addition, environmentally-friendly silver spheres are surface decorated by Ag0-cores65. Thus,
a
catalysts are extremely attractive in conditions of convenient nano-architecture should be able to keep the
electrochemical oxidation, thanks to the possibility of balance between efficiency and recyclability of the oxidative
generation of active radicals without use of conventional catalytic function.
13
oxidizers as K2S2O8
.
For all these reasons, the present work introduces one-pot
Nanoheterogeneous
approach
for
C-H
alkylation and facile synthetic route to develop nano-architecture of
(fluoroalkylation), previously tested for Ni-catalyzed Ag0@SiO2 nanoparticles based on both surface decoration and
reactions41,43, is an exceptionally attractive goal to be achieved upload into silica matrix of ultra-small Ag0-cores. The
by means of Ag nanoparticles. In this respect, immobilizing Ag developed nano-architecture ensures the high recyclability of
particles on different supports is the common way to minimize the catalytic function of the Ag0-cores, via the shielding effect
aggregation, which, in turn, restricts recyclability and facile of silica shell on them. This is a prerequisite for its wide
manipulation of the catalyst54. In recent years, literature data application as recyclable and efficient catalyst of aromatic C-H
illustrate the use of different supports for silver deposition55– bonds oxidative alkylation and fluoroalkylation.
64, highlighting54 the impact of the nature of the supports on
the selectivity of Ag-catalyzed reactions. The highest catalytic
Results and discussion
Synthesis of Ag-doped silica nanoparticles and their morphology
efficiency for oxidative dehydrogenation of allyl alcohol to
acrolein was demonstrated for Ag@SiO261. For this reason, we
chose SiO2 supports in this study as a host to be decorated by
silver.
Nano-architecture of Ag@SiO2 nanoparticles is expected to
greatly influence the oxidative catalytic activity of silver.
Shape- and size-effects on catalytic activity of silver are well
documented in literature60,62. The size-effect is of particular
importance when Ag0 nanoparticles are applied in oxidative
The standard water-in-oil microemulsion procedure was
applied as a basis for the preparation of SiO2 particles, while
Ag doping and/or decoration was obtained by simple
admixture of AgNO3 to the synthetic mixture without
additional reductant. The synthetic procedure is schematically
represented in Fig.1. The color of the synthesized composite
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Table 1. Si and Ag concentration from ICP-OES data in the Ag@SiO samples
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DOI: 10.1039/C8DT01090K
before and after treatments (1) and (2).
Element content (mg L-1, ±10%)
Sample
Si (251.611 nm)
Ag (328.068 nm)
Ag@SiO2
Ag@SiO2 (1)
Ag@SiO2 (2)
81.1
81.1
89.9
4.77
4.69
0.122
0
2000
4000
6000
Magnetic field / G
Figure 2. ESR spectrum of Ag0@SiO2, g = 2.22, ∆Hpeak-peak
= 650 G. T = 295 K
Figure 1. Scheme of the synthesis of Ag@SiO2 NPs (top). TEM (A)
and STEM (B) micrographs of Ag@SiO2.
The paramagnetic properties of Ag@SiO2 are manifested by
the widened band in ESR spectrum (Fig.2), which points to the
presence of Ag0-clusters in Ag@SiO2, although the data above
are not conclusive to identify the oxidation state of silver in
Ag@SiO2.
colloids is very similar to that of the naked silica nanoparticles,
without any brown staining, which is peculiar for the
decoration of silica supports by specifically sized (>
15 nm) Ag0-
cores. Thus, the absence of the staining indicates that silver is
introduced into silica nanoparticles in ionic form or as ultra-
small Ag0-cores. The TEM and STEM analysis of the composite
nanoparticles indicates the ultra-small size of Ag0-cores, in
agreement with the lack of the staining (Fig.1).
Redox properties of Ag@SiO2
The redox properties of Ag@SiO2 are critical to evaluate their
suitability for application in oxidative catalysis. Only a few data
exist in the literature on oxidation potentials of Ag0/Ag+/Ag2+
in organic media, although silver salts and oxides are widely
applied as catalysts there. This is probably due to the poor
solubility of silver salts in organic media. The high standard
potential of silver(II), often referenced in the literature, refers
to acidic aqueous solutions (E0 = 1.98 V vs. normal hydrogen
electrode, NHE)73,74. Only a few works mention the potential
for Ag+/Ag0 , which is strongly dependent on the nature of the
solvent: 0.61 V in acetone and 1.08 V in CH2Cl2 (ref.
Ag/AgCl)75,76. However, due to the lack of data on anions in
silver salts and exact experimental conditions, these data are
not fully reliable. AgNO3 in a solution of HNO3 is oxidized
according to Ref. 77 at 1.5 V (ref. SSE) or 2.15 V (ref. Ag/AgCl)
on boron-doped diamond electrodes, while this oxidation
interferes with water oxidation, which occurs at almost the
same potentials. Moreover, the authors77 reveal the nitrate
ion decomposition, which is followed by brown NO2 gas
formation. The potentials of electrochemical oxidation of
Indeed, the TEM and STEM micrographs of the composite
nanoparticles (Fig.1A and B) reveal av. 40-45 nm sized silica
spheres decorated by ultra-small Ag-nanoparticles (1-3nm)
(sample labeled as Ag@SiO2). The microscopy images reveal
also some amount of Ag-cores synthesized out of the silica
supports. Thus, the applied synthetic conditions favored the
nucleation and growth of Ag cores both inside and outside the
silica spheres, since AgNO3 was added to the synthetic mixture
before the alkalization by ammonia (for more details see the
Exp. Section). The silver salt in the alkaline condition
transforms to [Ag(NH3)2]OH and/or Ag2O which, in turn, can
undergo a photochemical reduction. Thus, silver can be
introduced into silica matrix either in the form of Ag2O or in
the form of Ag0, which cannot be discriminated by TEM.
ICP-OES spectroscopy confirms the silver content in the
Ag@SiO2 system. Table 1 reports the concentration of silver
and silicon in Ag@SiO2 (0.2 g L-1) calculated from ICP-OES data
for the samples before and after different treatments, where
the treatment (1) is the electrochemical oxidation with C6H6
and CF3COOH, and the treatment (2) is the preparative
oxidation of Ag@SiO2 upon heating in CF3COOH solution. The
reasons for the treatments and the corresponding ICP-OES
data will be described later on in the text.
Ag(CH3COO) in acetonitrile were reported recently52,53
.
The redox properties of Ag0@SiO2 were studied by cyclic
voltammetry in solid phase with the use of carbon-paste
electrode (CPE)78. The comparison of the obtained results
(Fig.3 and Table 2) with the redox behavior of Ag2O as a
probable intermediate in conversion of AgNO3 to Ag0@SiO2
measured in the same conditions allows to identify the
oxidation state of silver (Ag0) in the composite nanoparticles,
which will be further labeled as Ag0@SiO2. In particular, CV of
Ag0@SiO2 (Fig.3b) reveals the lack of Ag(I) reduction to Ag(0),
Dried sample of Ag@SiO2 exhibits paramagnetic properties
peculiar for silver clusters impregnated in the matrix72.
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Table 2. Redox data for Ag0@SiO2 and Ag2O. Conditions: 25°С; Bu4NBF4
DOI: 10.1039/C8DT01090K
background salt; CH3CN, 100 mV/s, working electrode: CPE or GC.
while very small peak at -0.59 V (Fig.3b) most probably refers
to reduction of trace amounts of Ag+.
View Article Online
a
Еp /Еpс (V vs. Ag/AgCl) for silver samples
Ag0@SiO2 in CPE
Ag0@SiO2 in
suspension in CH3CN
1.17/0.79
Ag2O in CPE
1.35/-0.98
1.29/0.74
Nevertheless, the color changes are reversible, since both
color and spectra return to their initial state after the
reduction of the oxidized form of the nanoparticles during the
catalytic cycle. This confirms that Ag0@SiO2 are efficiently
oxidized and reduced to the initial structure, without
formation of insoluble precipitate (unsupported Ag+/Ag2+
salts) or larger nanoparticles (resulting in the typical rise of the
plasmon resonance absorption band).
Redox properties of Ag0-cores in Ag0@SiO2 are worth
comparing with the literature results on redox behavior of Ag0
in graphene oxide (GO, Ag0@GO) nanocomposite57. Ag0@GO
is oxidized very easily at about 0.1 V ref. Ag/AgCl, and its
reverse reduction occurs at -0.5V. Taking into account that the
oxidation of Ag0@SiO2 occurs at 1.29 V (Table 2), silica matrix
inhibits the oxidation of Ag0 in Ag0@SiO2, compared to
Ag0@GO. The specific redox properties of Ag0@SiO2
nanoparticles seem directly correlated to their morphology.
The specific morphology of Ag0@SiO2 can be revealed after
the dissolution of Ag0-cores by the oxidation upon the heating
in air in highly acidic conditions. The ICP-OES data confirm the
significant leaching of Ag0-cores from Ag0@SiO2 under
conditions of the treatment (2) (Table 1, last line). The TEM
and STEM analysis of the composite nanoparticles reveals the
nanosized pores at the silica spheres after the leaching (Fig. 4).
It is well known that leaching of any nanosized species from
silica matrix results in mesoporous structure. Thus, the
oxidative leaching of nanosized Ag-based templates can be
also regarded as a route for the development of mesoporous
silica spheres, as reported in Fig.4, C. Moreover, the
mesoporous structure revealed after the leaching (Fig.4)
highlights the presence of Ag0-cores in the internal pores of
silica supports in Ag0@SiO2.
Figure 3. CVs registered at CPE for a) Ag2O (5mg); b) Ag0@SiO2 (5mg) in similar
conditions; c) Ag0@SiO2 (5mg) isolated after catalytic reaction (C6H6+CH3COOH).
The electrochemical measurements with the use of CPE
provide more accurate evaluation of the redox behavior of
insoluble or poorly soluble compounds. However, it is worth
measuring the redox properties of Ag0@SiO2 nanoparticles
dispersed in acetonitrile due to their importance for
electrochemical catalysis performance. CV curves of Ag0@SiO2
in the acetonitrile colloids are presented in Fig.S 1 in SI.
The oxidation potential of Ag0@SiO2 is 120 mV larger in CPE
than in acetonitrile colloids (Table 2). This slight difference is
probably to be attributed to the difference in solvent/ionic
environment of Ag0@SiO2.
The comparison of the cyclic voltammograms of Ag2O and
Ag0@SiO2 (CPE) (Table 2, Fig. 3) reveals significant difference
between them, while the oxidation potential values are close
to each other (1.35 and 1.29 V). The indistinct small oxidation
peak with current intensity no more than 20% of the reduction
peak intensity points to an admixture of Ag0 in the Ag2O used
as received. This finding is in good agreement with literature
data on autocatalytic thermal decomposition of silver oxide to
silver and oxygen61. Thus, CV data of Ag0@SiO2 confirm Ag0 as
the predominant form of silver in the composite
nanoparticles.
It is worth noting that the paste-like mixture of dried
Ag0@SiO2 and phosphonium salt as binder was efficiently
applied for the preparative oxidation of Ag0@SiO2 (Fig.S2 in
SI). The oxidized nanoparticles tend to change the color from
white to greenish grey, as evident from UV-Vis spectra (Fig.S3
in SI).
Aside of the nature of the host, the specific morphology of the
Ag0@SiO2 system differentiates from the Ag0@GO structure,
where Ag0-cores are predominantly localized at the surface of
the graphene oxide matrix57.
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DOI: 10.1039/C8DT01090K
[CH3COOH]
[CF3COOH]
Figure 5. CVs of Ag0@SiO2 (5mg) suspended in CH3CN (1ml) on GC (a) in the
presence of СH3COOH (1:1, 1:2, 1:3, 1:4, 1:5, 1:6); (b) in the presence of
СF3COOH (1:1, 1:2, 1:3, 1:4, 1:5, 1:6). The arrows indicate the trend of the
increasing CH3COOH and CF3COOH concentration in (a) and (b), respectively.
Figure 4. TEM (A) and STEM (B) micrographs of Ag0@SiO2 after the dissolution of
Ag0 cores and schematic illustration of localization of Ag0-cores in mesoporous
silica spheres of Ag0@SiO2 (C).
Electrocatalytical properties of Ag0@SiO2
The oxidation of Ag0@SiO2 was studied at increasing acid
concentrations. The addition of CH3COOH to Ag0@SiO2
colloids results in some positive shifting of the oxidation
potential to 1.47 V (Fig.5 (a)), while the acid itself is oxidized at
2.2 V. The peak current tends to increase with the increase of
the acid concentration, reaching the maximum (icat/ip = 13,
where ip and icat are the peak current values without/with
acid), at 1:6 (Ag:CH3COOH) molar ratio. This result indicates
the quick regeneration of Ag0@SiO2 in the presence of alkyl
precursors. CF3COOH results in similar effect on the oxidation
peak current of Ag0@SiO2 (Fig.4 (b)). In particular, the
oxidation peak shifts to 1.47V (CF3COOH is oxidized at 2.31 V),
while the maximum for icat/ip (=11) is obtained under the same
1:6 (Ag:CF3COOH) molar ratio.
Neither potential values nor peak current tend to change
under the addition of benzene to Ag0@SiO2 in the acidified
solutions and no oxidation peaks of benzene are revealed.
Thus, the silver-mediated enhancement of the peak current
has an impact on the oxidative catalytic function of Ag+@SiO2
and fast regeneration of the catalyst at the anode.
The catalytic increases in the oxidation current of silver
derivatives in presence of any reagents in catalytic reactions
has never been reported. The effect of the acids on the
electrochemical behavior of CH3COOAg was also revealed by
cyclic voltammetry for comparison with Ag0@SiO2. It is worth
noting that even equimolar amounts of CH3COOH or CF3COOH
produce the immediate precipitation of Ag0, which appears as
the adsorption oxidation peak at 0.35 V (Fig.6). The latter
value is close to the oxidation potential reported for
Ag0@GO57. In a wide window of potentials above 2.2 V,
oxidation of the acids themselves can also be observed. There
are no catalytic waves in this case.
Figure 6. CVs of a) CH3COOAg (5mg) in the CH3CN (1ml) on GC (purple); in the presence
of 1 eq СH3COOH (blue) or in the presence of 1eq СF3COOH (red); b) The same for
Ag2O.
Similar precipitation of Ag0 followed by the narrow adsorption
peak close to zero potential without any catalytic peaks was
revealed when Ag2O is dispersed in acetonitrile (Fig.6b).
Thus, the precipitation results in the removal of Ag0 from the
reaction mixture, while the silica matrix facilitates the
regeneration of Ag0@SiO2. The specific electrochemical
behavior of Ag0@SiO2 under acidic conditions (Figs.5, 6)
indicates poor leaching of Ag+ from the silica supports under
the oxidation of Ag0 to Ag+. The TEM image of Ag@SiO2 after
the oxidative dissolution of Ag0 (Fig.4) reveals the formation of
mesoporous silica surface with very high surface activity. The
active silica surface provides efficient adsorption and/or
binding of Ag+, which results in the regeneration of the nano-
architecture of Ag0@SiO2 after the reduction of Ag+ to Ag0. The
quick regeneration is the reason for the acid-induced catalytic
enhancement of the silver oxidation peak current in presence
of alkyl(fluoroalkyl) precursors, which, in turn, is of great
impact on the catalytic function of Ag0@SiO2. It is worth
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Table 3. Yields of the alkylated arenes (1-6) in conditions
I (stoichiometric
View Article Online
noting that the catalysis by commonly applied Ag salts and
oxides13,46 requires great excess of an oxidizer in order to
return precipitated Ag0 to reaction mixture. In the present
case, instead, the quick regeneration of Ag0@SiO2 can be
explained by their specific morphology. Moreover, the silver-
mediated catalytic enhancement of the oxidation current is
the prerequisite for the catalytic function of Ag0@SiO2 in the
C-H substitution of arenes.
quantity of the nanocatalyst) and II (catalytic quantity of the nanocatalyst).
DOI: 10.1039/C8DT01090K
Blank SiO2 indicates blank test with empty, naked SiO2.
Product
Condi-
tions
I / II
Product yield [%] in repeated synthesis
1st
2nd
3rd
4th
5th
1
75/55
75/55
75/54
75/53
75/53
CH3
Blank
SiO2
I / II
12
-
2
52/45
10
55/43
-
54/39
54/39
53/44
54/45
54/45
52/35
53/35
53/43
52/43
53/45
50/33
52/30
53/43
50/40
53/40
Oxidative C-H alkylation and fluoroalkylation of arenes
CF2H
Blank
SiO2
I / II
We discovered that the oxidized and active form of Ag+@SiO2
taken in stoichiometric amounts can serve as oxidizing agent
producing the coupling reaction of arenes (benzene and
pyridine, coumarines, alkaloids) and RCOOH (Scheme 2, path
3
56/47
11
55/42
-
CF3
Blank
SiO2
I / II
I). However, the most important catalytic path to C-H
4-o
53/45
14
53/45
-
alkylation (fluoroalkylation) was realized with Ag0@SiO2 as
catalyst being used in conditions of electrochemical oxidation,
due to the rapid regeneration of Ag0/+1@SiO2 (Scheme 2, path
N
CH3
Blank
SiO2
I / II
5-o
57/49
9
55/48
-
I). These two application frameworks of Ag@SiO2 are
N
CF2H
designated in Scheme 2 as
I
and II, respectively.
Blank
SiO2
I / II
-
2
SiO
]
2
eqv.
[AgI
6-o
55/45
10
54/42
-
Ar-R
(I)
1:1, CH3CN
Ag0
N
CF3
Blank
SiO2
Ar-H
AgI
+
R
COOH
5%
V
-SiO
]
2
[AgI
Mixing the reagents with Ag+@SiO2 as oxidizer in the ratio
1:1:2 (benzene: HOC(O)CF3: Ag+@SiO2) results in the quick
production of the alkylated arenes with the yield up to 75%
(the yields were calculated from chromatomass spectrometry
for volatile compounds, and the 100% conversion of organic
acid was observed, for more details see SI). The application of
Ag+@SiO2 in situ electrochemically generated from Ag0@SiO2
within the framework II results in smaller, but still significant
yield equal to 45-55% (Table 3). The use of catalytic instead of
stoichiometric amounts of Ag (5 mol %) in the form of
Ag0@SiO2 along with easier and facile reaction indicates the
Ar-R
(II)
-
2F
+1,1
(ref. Ag/AgCl)
1:1, CH3CN
R
:
Ar
N
N
R
50-75
40-53
%
,
%
:
,
CF3
R
H
CF2
CH3
Scheme 2. Arene alkylations using Ag@SiO2 (stoichiometric I and catalytic II paths).
The product yields of the alkylated arenes produced within the
frameworks and II are collected in Table 3.
advantage of the framework II versus I (Scheme 2). It is also
I
worth noting that CO2 gas evolution was observed at room
temperature, and this confirms the active transformation of
acids with the elimination of alkyl radical and CO2.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 4. Total yields and selectivity of the alkylated heteroarenes (4-12) in the
conditions II using 5 % of the nanocatalyst.
was also studied, and the results are presented in Table 3. The
View Article Online
0
data indicate that the catalytic functionDoOfI:A1g0.1@03S9i/OC28DkeT0e1p0s90aKt
the same level for at least five cycles, which is in good
agreement with the negligible decrease of the silver content in
Ag@SiO2 after the catalytic cycle, according to the ICP-OES
data presented in Table 1. The data indicate rather poor
leaching of Ag from Ag@SiO2 after the reaction, which points
to the regeneration of the nanocatalyst within the catalytic
cycle. The TEM image after the catalytic cycle (Fig.S4 in SI)
confirms the regeneration of the nanocatalyst resulting from
the Ag0→Ag+→Ag0 process illustrated in Fig.4,C. Thus,
embedding of Ag0-cores into silica supports in Ag0@SiO2 is the
critical factor to inhibit the leaching of Ag+ from the silica
matrix after the oxidation. This facilitates the regeneration of
Ag0@SiO2 after the reduction, which, in turn, results in the
recyclability of the nanocatalyst.
Regioselectivity of the catalytic reaction was analyzed for the
series of heteroarenes (Table 4). The oxidative alkylation of
the alkaloids results in the single product of the C-H
substitution, while the alkylation of the pyridine and
coumarines favors the C-H substitution in ortho-position to N
of pyridine and C=O of coumarines. No products of
polyalkylation of benzene was observed.
The mechanism of aromatic C-H bonds alkylation requires a
separate study. Minisci’s assumption that his reaction is
triggered by Ag2+ resulting from the oxidation of AgNO3 was
not confirmed by further experimental studies (Scheme 1)13. It
is well-known that the oxidation of AgNO3 is accompanied by
the evolution of brown gas NO280. The absence of the NO2 gas
evolution in the Minisci reaction argues against the
assumption. Moreover, directed mechanistic studies of
external oxidant-free silver-catalyzed aromatic C-H
functionalization (phosphonation, phosphonation/cyclization)
in electrochemical conditions52,53 show that the reaction
proceeds at low anode potentials, while the ESR studies reveal
the lack of Ag2+ under these conditions. The silicate matrix,
where the silver nanoclusters are immobilized, leads to the
heterogeneity of the system, which complicates the study of
possible radical intermediates by the ESR method with or
without traps. However, the cyclic voltammetry method in the
conditions of Ag0@SiO2 oxidation (Fig. 5) points to intensive
regeneration of this immobilized Ag0 catalyst under the
increased amounts of acids. This fact can be explained by the
Scheme 3, where the key role of silicate matrix is to promote
the generation of R. radicals due to intramolecular electron
transfer in RCOOAg, followed by the oxidation of RCOO- and
regeneration of Ag0.
Product
aTotal product
yield [%]
79
bIsomer ratio
(substitution position)
4:1:1(o,m,p)
4
CH3
N
N
5
6
85
82
75
4:1:1(o,m,p)
4:1:1(o,m,p)
H
CF2
CF3
N
7
8:2:1:1(3,4,6,7)
5
8
4
6
7
3
CF3
O
O
8
77
75
83
9:2:1(3,4,6)
5:1:1(3,4,7)
-
5
4
6
3
CF3
O
O
8
9
5
8
4
3
CF3
7
O
O
10
11
12
O
H
N
N
CF3
N
N
O
87
81
-
-
O
N
N
N
N
CF3
O
O
O
N
N
4
N
CF3
N
O
aNMR yields and bselectivity obtained by 19F NMR analysis (GC-MS was
used for products 4 (o,m,p)) of the crude reaction mixtures.
The use of “naked” silica nanoparticles without the decoration
by Ag in conditions of oxidative electrolysis results in small
production of the alkylated arenes (benzene and pyridine)
(Table 3). This fact is worth correlating with the Kolbe reaction
data: in fact, the applied potential in these conditions was
about 2.3-2.6 V ref to Ag/AgCl, which is in good agreement
with the condition of the Kolbe process79
.
We did not find a dependence of the yields on the catalyst
loading in the range of 5-20% Ag0@SiO2. Since recyclability of
heterogeneous catalysts is the key factor for their practical
exploitation, the reuse of Ag0@SiO2 in the oxidative catalysis
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alkylation of aromatic C-H bonds. The developed route of
alkylation and fluoroalkylation is a very promising prerequisite
View Article Online
DOI: 10.1039/C8DT01090K
Si
Si
O
Si
O
Si
O
O
-e
for its further application in synthetic transformations of wide
circle of aromatic compounds.
O
O
O
O
O
O
Si
Ag0
Si
HSi
Si
AgI
Si
Si
O
O
O
O
Si
O
O
Si
HSi
O
Si
O
O
O
Conflicts of interest
O
O
Si
Si
There are no conflicts to declare.
-CO2
Si
Si
O
O
R
COOH
H
Acknowledgements
O
O
O
This work was supported by the Russian Science Foundation
grant no. 14-23-00016 (the idea, the teaching, the synthesis of
nanoparticles, electrochemical and catalytic experiments). The
microscopic studies of the nanocatalyst (SEM) were supported
by the Knut & Alice Wallenberg Foundation and the Kempe
Foundation (A.V. and R.M.) The LTU Labfund program and the
European Union’s Horizon 2020 research and innovation
programme under grant agreement No 654002 are
acknowledged for partial funding (A.V.).
Si
Si
Si
Ag
H
O
Si
O
O
R
OC(O)
Si
O
O
O
R
Si
H
-e
R
H
R
-H+
-2e
+
RCOOAr
+2H+
Notes and references
ArH
RCOOH
1
2
3
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