Nanopalladium-Catalyzed Oxidative Heck Reaction of Fluorinated Olefins: H2O2 as a Green Oxidant

Article abstract of DOI:10.1007/s10562-020-03334-5

Pd-based nanocatalysts were prepared through immobilization of Pd(OAc)₂/phenanthroline on nanocarbon materials and subsequent pyrolysis. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electron microscopy (TEM). Pd-based nanocatalysts can efficiently catalyze the oxidative Heck reaction of tetrafluoropropylene by using H₂O₂ as a green oxidant and generate (Z)-β-fluoro-β-(trifluoromethyl)styrenes in excellent yield. The yield and Z/E selectivity of the titled reaction remained higher than 90% after four reaction cycles. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03334-5

Catalysis Letters
https://doi.org/10.1007/s10562-020-03334-5
Nanopalladium‑Catalyzed Oxidative Heck Reaction of Fluorinated
Olefns: H2O2 as a Green Oxidant
Yang Li1 · Ning Sun1 · Meng Hao1 · Cai‑Lin Zhang1 · Hong Li1 · Wen‑Qing Zhu1
Received: 26 January 2020 / Accepted: 26 July 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Pd-based nanocatalysts were prepared through immobilization of Pd(OAc)2/phenanthroline on nanocarbon materials and
subsequent pyrolysis. The catalysts were characterized by X-ray difraction (XRD), X-ray photoelectron spectroscopy (XPS)
and electron microscopy (TEM). Pd-based nanocatalysts can efciently catalyze the oxidative Heck reaction of tetrafuo-
ropropylene by using H2O2 as a green oxidant and generate (Z)-β-fuoro-β-(trifuoromethyl)styrenes in excellent yield. The
yield and Z/E selectivity of the titled reaction remained higher than 90% after four reaction cycles.
Graphic Abstract
F
CF3
Pd@NGr-C (5 mol%)
H2O2 (2 eq)
B(OH)2
R
R
F
CF3
NaHCO3 (2 eq)
1,4-dioxane, 80 oC,12 h
Synthesis of trifluoromethyl styrenes
Nano-catalyst
Green oxidant
High Z/E selectivity
Keywords Oxidative heck · Nanosized pd · Fluorinated olefn · Green oxidant · Trifuoromethylstyrenes
1 Introduction
such compounds. Among the various catalytic transforma-
tions applied for the synthesis of these compounds, palla-
dium-catalyzed cross-coupling reactions are of exceptional
importance. We have a long-standing interest in this area,
especially in palladium-catalyzed cross-coupling of aryl hal-
ides and related substrates [16–18].
The development of heterogeneous catalysts for organic
transformation is of interest for organic and material sci-
ences [1–6]. In general, such catalysts present advantages
over classical homogeneous systems due to their easy recov-
ery and the possibility of recycling [7–12]. This is of par-
ticular importance when the catalyst is based on scarce and
expensive noble metals. On the other hand, heterogeneous
catalysts often display lower reactivity and selectivity than
their homogeneous counterparts. Fluorine-containing aryl
compounds have important applications in medicinal chem-
istry [13–15]. Therefore, many research groups are commit-
ted to developing new chemical reactions for the synthesis of
In the past, palladium has been mainly supported on
“classic” inorganic oxides as well as carbon, silicon diox-
ide, and aluminum trioxide. Recently, the incorporation of
dopants into the matrix of the parent support has become a
highly active and interesting area in organic chemistry. More
specifcally, palladium-supported N-doped carbon materials
(Pd/PdO@NGr-C) have generated major interest, and some
interesting research has been performed by using this cata-
lyst [19, 20]. We thought that such doping could improve the
binding properties of the support and therefore minimize the
leaching of palladium atoms (small clusters).
* Yang Li
liyang@xpu.edu.cn
1
The Heck reaction is one of the most commonly employed
transformations for the formation of carbon–carbon bonds.
School of Environmental and Chemical Engineering, Xi’an
Polytechnic University, Xi’an 710048, China
Vol.:(0123456789)
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Y. Li et al.
Because of the mild reaction conditions, the availability
of reagents, and the broad functional group tolerance of
this transformation, it has found extensive use in synthetic
organic chemistry [21–23]. However, only a few oxidative
Heck reactions can tolerate a wider range of fuorinated sub-
strates because these cross-coupling reactions usually result
in low yields or completely inhibit the catalyst [24–28]. For
those reasons, a heterogeneous palladium catalyst in which
the metal is immobilized on a solid support to allow highly
efective catalysis of the oxidative Heck reaction is urgently
needed.
of two Pd species. The frst one has a binding energy of
approximately 341.1 eV and is assigned as Pd(0) (41%). The
second Pd species has a binding energy of approximately
343.8 eV and is assigned as Pd(II) in the form of palladium
oxide (59%). Three distinct peaks are observed in the N1s
spectra with electron-binding energies of 398.3 eV. 399.6 eV
and 400.7 eV (Fig. 1c). Finally, the peak with the lowest
binding energy can be attributed to pyridine-type nitrogen
(sp2 hybridized, 41.1%).
2.1 XPS Spectra
Based on our previous palladium-catalyzed fuorinated
olefn cross-coupling reaction [29, 30], herein, we report on
the use of Pd/PdO@NGr-C as a heterogeneous catalyst in an
oxidative Heck reaction using a wide range of arylboronic
acids and 2,3,3,3-tetrafuoroprop-1-ene. (Z)-β-Fluoro-β-
(trifuoromethyl)styrenes can be synthesized in high yield
by using H2O2 (30% water solution) as a green oxidant.
The electron-binding energy of 399.6 eV is characteristic of
a pyrrole-type nitrogen (sp3 hybridized, 24.5%). The peak at
400.7 eV is typical of quaternary N (sp2 hybridized, 24.3%).
In addition to those three nitrogen types, N oxide of pyri-
dinic N (small peak at 402.8 eV, 9.9%) was also observed,
with the oxygen atoms probably arising from the acetate
counterions. Additionally, the C1s spectra showed 3 peaks
(Fig. 1d): the sharp peak at 284.1 eV corresponds to the sp2
carbon with C=C, while the smaller peaks at 285.4 eV and
286.8 eV are assigned to C-N and C=N, respectively. The
peak observed at 289.8 eV is ascribed to the π-π* transition
typical for aromatic rings.
2 Results
The preparation of Pd/PdO@NGr-C involves three steps,
as shown in Scheme 1. First, palladium acetate (as the pal-
ladium precursor) and 1,10-phenanthroline were mixed in
ethanol with stirring at 60 °C for 1 h. Second, the result-
ing mixture was adsorbed on carbon and dried at 60 °C
overnight. Finally, the samples were subjected to pyrolysis
under vacuum at 800 °C for 2 h under a N2 atmosphere.
The amount of palladium determined by elemental analysis
was 4.6% wt%, while the nitrogen and carbon contents were
2.1 wt% and 86.2 wt%, respectively. The initial loading of
palladium was 5.1%. This shows that this synthesis process
does not have much metal loss.
2.2 XRD Pattern Analyses
As shown in Fig. 2, the X-ray difraction (XRD) studies con-
frmed the presence of palladium dispersed into the carbon
powder. By comparing the XRD patterns of the new material
with the XRD patterns of Pd/C, it is possible to identify the
characteristic broad peak of the amorphous carbon support
(approximately 2θ=24 °C), and the three peaks at 2θ=40°,
48° and 68° are assigned to palladium(0). On the other hand,
the peaks at 2θ=42° and 54° demonstrate the existence of
PdO. Rietveld refnement analysis further gives phase frac-
tions of 88.9% Pd and 11.1% PdO, and an average particle
size of 45 nm for PdO.
To investigate the structure of the catalyst in more detail,
several characterization methods were used. The nature of
the palladium and nitrogen species on the surface of the
catalyst was analyzed by X-ray photoelectron spectroscopy
(Fig. 1a). The Pd3d XPS data (Fig. 1b) reveal the presence
2+
Pd(OAc)2
N
N
ethanol
adsorbed on carbon
N
2 atmosphere
800 oC, 2 h.
2(OAc)-
Pd(Phen)2(OAc)2
Pd/PdO@NGr-C
Pd
60 oC, 1 h.
dried at 60 oC for 12 h.
N
N
2
N
N
Scheme 1 Preparation of nanosized Pd
1 3

Nanopalladium‑Catalyzed Oxidative Heck Reaction of Fluorinated Olefns: H2O2 as a Green…
Fig. 1 a XPS spectra for pal-
Survey
C 1s
ladium supported on N-doped
carbon (Pd/PdO@NGr-C), b
Pd3d XPS spectrum, c N 1 s
XPS spectrum, d C1s XPS
spectrum
Pd(II) of PdO
Pd(0)
Pd 3d
N 1s
O 1s
330
335
340
345
350
355
1200
1000
800
600
400
200
0
Binding energy (eV)
Binding energy (eV)
a
b
Π-Π* satellit
e
N Oxide
C=N
quatemary nitrogen
pyrrolic nitrogen
pyridinic nitrogen
C-N
C=C
276
280
284
288
292
392 394 396 398 400 402 404 406 408
Binding energy (eV)
Binding energy (eV)
c
d
Fig. 2 Rietveld refnement of the powder XRD pattern of palladium supported on N-doped carbon
some of the Pd particles (Fig. 3b, c). Since the HRTEM
images show the presence of Pd in an area without Pd
particles, these fnely distributed particles or clusters are
likely PdO particles. However, due to the low quality of the
HRTEM images, we are unable to distinguish Pd particles
from the PdO particles directly.
2.3 HRTEM Images
TEM analysis confrmed the presence of Pd nanoparticles
with a very broad range of sizes (Fig. 3a). On some particles,
several graphene layers formed as a result of carbonization
of the nitrogen ligand. These layers cover or partially cover
1 3

Y. Li et al.
Fig. 3 HRTEM images of palladium supported on N-doped carbon (Pd@NGr-C) showing a broad range of diferent particle sizes
We initially chose 4-methoxyphenylboronic acid 1a as a
substrate for the screening of this coupling transformation.
A solution of 4-methoxyphenylboronic acid 1a (1.0 mmol)
and 2,3,3,3-tetrafuoroprop-1-ene 2 (2.0 mmol) in MeCN
(2.0 mL) was stirred, and Pd/PdO@NGr-C (1 mol%) was
used as a catalyst, NaHCO3 was used as a base and H2O2
(30% water solution) was used as an oxidant. Only (Z)-β-
fuoro-β-(trifuoromethyl)styrene 3a was formed in a low
yield of 43%, albeit with a high Z/E ratio (Table 1, entry
1). Next, the infuence of catalyst loading was examined.
It could be seen that 5 mol% was the best loading for the
reaction (Table 1, entries 2–4). Third, the oxidant was
changed to O2 (1 bar), and the product formed in a yield
of 45% (Table 1, entry 5). Moreover, when the solvent was
varied across a series including water and DMSO, a better
yield was obtained when 1,4-dioxane served as the solvent
(Table 1, entries 6–10). Moreover, the infuence of the base
was examined, and it was observed that NaHCO3 was the
best base for the reaction (Table 1, entries 11–13). Finally,
when the temperature was raised to 80–100 °C, a good
yield of 3a (95%) was obtained (Table 1, entries 14–15). It
is gratifying that the common palladium carbon as a catalyst
also has catalytic activity for this reaction, but the yield is
relatively low (Table 1, entry 16).
2,3,3,3-tetrafuoroprop-1-ene 2 (2.0 equiv), NaHCO3 (2.0
equiv), H2O2 (2.0 eq), and 1,4-dioxane (2.0 mL) at 100 °C
for 12 h.
3 Discussion
3.1 Substrate Scope
To explore the scope of the coupling reaction, various arylb-
oronic acids were examined for coupling with 2,3,3,3-tetra-
fuoroprop-1-ene 2 by using Pd/PdO@NGr-C as a hetero-
geneous catalyst and H2O2 as a green oxidant. The results
are summarized in Table 2. Para-substituted phenylboronic
acids reacted smoothly with 2,3,3,3-tetrafuoroprop-1-ene
2 to aford the desired products in excellent yields (Table 2,
3a-3 g). Next, substrates having m- or o-substituents also
reacted with acceptable yields (Table 2, 3 h-3j). Disubsti-
tuted arylboronic acids were also tested, and good yields
were obtained (Table 2, 3 k-3 m). Furthermore, a carbazole-
derived boronic acid could also be converted into the cor-
responding product in moderate yield (Table 2, 3n).
It is well known that this kind of catalyst can be recy-
cled by centrifugation and then reused for an additional
process. After five runs without an obvious decrease in
either the yield or the conversion, the yield remained at
In summary, the best reaction conditions for this cou-
pling process were as follows: Pd/PdO@NGr-C (5 mol%),
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Nanopalladium‑Catalyzed Oxidative Heck Reaction of Fluorinated Olefns: H2O2 as a Green…
Table 1 Optimization reaction conditions
Entry
Catalyst (mol%)
Solvent
Oxidant
Base
Yielda (%)
Z/E
1
2
3
4
5
6
7
8
1
2
5
10
5
5
5
5
5
5
5
5
5
5
5
MeCN
MeCN
MeCN
MeCN
MeCN
H2O
1,4-dioxane
MePh
DMF
H2O2
H2O2
H2O2
H2O2
O2 (1 bar)
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
KHCO3
43
56
62
58
45
34
79
55
42
37
71
75
77
95b
87c
29
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
9
10
11
12
13
14
15
16
DMSO
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
K2CO3
NaHCO3
NaHCO3
NaHCO3
Pd/C
The optimal reaction condition is given in bold
Reaction conditions: arylboronic acid 1a (1.0 mmol), 2,3,3,3-tetrafuoroprop-1-ene 2 (2.0 mmol), catalyst (5 mol%), base (2.0 mmol), oxidant
(2.0 mmol), solvent (2.0 mL), 60 °C, 12 h.
aIsolated yield
b80 °C
c100 °C
approximately 92%, which indicated the good stability of
the Pd/PdO@NGr-C catalyst (Fig. 4).
solution), which shows that this catalytic cycle process
is heterogeneous catalysis, not a homogeneous catalyst.
We seriously conducted a recycling experiment on
the recovered catalyst. The content of palladium metal
in the solution after one cycle was tested. It can be seen
that there is only 1PPM. There is almost no loss. After
five trials, the yield decreased by about 10%, which may
be the operation loss of some catalysts in the catalyst
recovery experiment. After all, the amount of catalyst is
relatively small. In addition, we have collected the reac-
tion solution for ICP-MS measurement at the end of the
first catalytic cycle. The concentration of palladium ions
in the post-reaction liquid is very low (1 ppm in 2 mL
4 Materials and Methods
Unless otherwise mentioned, the solvents and reagents
were purchased from commercial sources (Macklin,
Heowns, Meryer Co. Ltd) and used as received. H, 13C
1
and 19F NMR spectra were recorded on 400 and 500 MHz
spectrometers. 1H NMR chemical shifts were determined
relative to internal (CH3)4Si (TMS) at δ 0.0 or to the signal
of the residual protonated solvent: CDCl3 δ 7.26. 13C NMR
1 3

Y. Li et al.
Table 2 Oxidative Heck reaction between 2,3,3,3-tetrafuoroprop-1-ene and arylboronic acids in the presence of Pd/PdO@NGr-C
1 3

Nanopalladium‑Catalyzed Oxidative Heck Reaction of Fluorinated Olefns: H2O2 as a Green…
Table 2 (continued)
Reaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), Pd/PdO@NGr-C (5 mol%), NaHCO3 (2.0 mmol), H2O2 (30%, 2.0 mmol), 1,4-dioxane
(2.0 mL), 80 °C, 12 h.
aIsolated yield
1 3

Y. Li et al.
layers were washed with brine and dried over Na2SO4, and
the organic solvent was evaporated by a rotatory evapora-
tor under atmospheric pressure. The crude products were
determined by 19F NMR using p-fuoroacetophenone as
an internal standard.
Conv
Yield
100%
98%
96%
94%
5 Conclusions
In summary, we developed a new strategy for the facile syn-
thesis of (Z)-β-fuoro-β-(trifuoromethyl)styrene derivatives
via a Pd/PdO@NGr-C-catalyzed oxidative Heck reaction
with 2,3,3,3-tetrafuoroprop-1-ene as a fuorinated olefn.
More importantly, H2O2 is used as a green oxidant during
this process. The wide scope and remarkable tolerance, par-
ticularly to a large number of important arylboronic acids,
make this strategy remarkably practical for the efcient syn-
thesis of functional styrenes.
92%
90%
1
2
3
4
5
Fig. 4 Recycling and reuse of Pd/PdO@NGr-C in Heck oxidation
Acknowledgements We are grateful for the fnancial support from the
National Natural Science Foundation of China (21503161), the Key
Research & Development Project in Shaanxi Province (2017ZDXM-
GY-040), and the Doctoral Scientifc Research Foundation of Xi’an
Polytechnic University (107020336).
chemical shifts were determined relative to internal TMS
at δ 0.0. For the isolated compounds, 19F NMR chemical
shifts were determined relative to CFCl3 at δ 0.0. 19F NMR
chemical shifts were determined relative to p-fuoroace-
tophenone at δ -107.5. Data for 1H, 13C and 19F NMR are
recorded as follows: chemical shift (δ, ppm), multiplicity
(s=singlet, d=doublet, t=triplet, m=multiplet, q=quar-
tet, br = broad). Mass spectra were obtained on a mass
spectrometer. High-resolution mass data were recorded on
a high-resolution mass spectrometer in ESI mode.
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