Spatially isolated palladium in porous organic polymers by direct knitting for versatile organic transformations

Article abstract of DOI:10.1016/j.jcat.2017.08.030

We report here a direct knitting Method for preparation of highly robust, effective while air- and moisture-tolerant, and readily recyclable three-dimensional (3D) porous polymer-Pd network (PPPd) from the widely used Pd(PPh₃)₄. Electro-beam induced Pd atom crystallization was observed for the first time in organic polymer and revealed the ultrafine dispersion of palladium atoms. Challenging types of Suzuki-Miyaura couplings, reductive coupling of aryl halides and oxidative coupling of arylboronic acid were successively catalyzed by PPPd in aqueous media. Also catalytically selective C–H functionalization reactions were achieved with orders of magnitude more efficient than conventional Pd homogeneous catalysts. The strategy developed here provides a practical method for easy-to-make yet highly efficient heterogeneous catalysis.

Full text of DOI:10.1016/j.jcat.2017.08.030

Journal of Catalysis 355 (2017) 101–109
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Spatially isolated palladium in porous organic polymers by direct
knitting for versatile organic transformations
Xinbo Wang a, Shixiong Min a,c, Swapan K. Das a, Wei Fan b, Kuo-Wei Huang a, Zhiping Lai a,
⇑
a Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
b Chemical Engineering Department, University of Massachusetts, Amherst, MA 01003, United States
c School of Chemistry and Chemical Engineering, Beifang University of Nationalities, Ningxia 750021, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here a direct knitting Method for preparation of highly robust, effective while air- and
moisture-tolerant, and readily recyclable three-dimensional (3D) porous polymer-Pd network (PPPd)
from the widely used Pd(PPh3)4. Electro-beam induced Pd atom crystallization was observed for the first
time in organic polymer and revealed the ultrafine dispersion of palladium atoms. Challenging types of
Suzuki-Miyaura couplings, reductive coupling of aryl halides and oxidative coupling of arylboronic acid
were successively catalyzed by PPPd in aqueous media. Also catalytically selective CAH functionalization
reactions were achieved with orders of magnitude more efficient than conventional Pd homogeneous cat-
alysts. The strategy developed here provides a practical method for easy-to-make yet highly efficient
heterogeneous catalysis.
Received 26 May 2017
Revised 23 August 2017
Accepted 30 August 2017
Keywords:
Heterogeneous catalysis
Porous organic polymers
Direct knitting
Suzuki-Miyaura cross coupling
CAH bond activation
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
porous organic polymers (POPs) are arguably one of the most ideal
supports with unique features including excellent chemical tun-
Development of heterogeneous catalysts that can integrate the
advantages of homogeneous catalysts has been the important tar-
get of chemists for decades yet still remains a major challenge
[1,2]. Besides high selectivity, activity and reusability, the scalabil-
ity and cost in preparation are also factors that must be considered
for practical usage. Although progresses in surface organometallic
chemistry [3,4], zeolitic supports of so-called ‘‘ship-in-bottle” cat-
alyst [5], high surface area material such as metal-organic frame-
works (MOFs) supported catalyst [6–8], single-atom catalyst
(SAC) based on atomically dispersed metal atoms on the surfaces
of metal crystal or graphene [9–11], have been well documented
and made an essential contribution to the progress of research in
heterogeneous catalysis [12–14], these methods are costly and
not readily scalable, thus do not meet industrial demands. Single
atom of palladium anchoring to a relatively cheaper carbon nitride
(C3N4) has been achieved recently by stepwise loading metal-salt
and a subsequent reduction [15], but introducing other coordina-
tion ligands, which may fully determine the properties of the cat-
alyst, is still challenging.
ability, insolubility but with adjustable wettability, recyclability,
and so on [16–18]. Many organometallic and transition metal coor-
dination catalysts contain electron-rich arene substituent groups,
which from the structural view point are able crosslinked to form
POP frameworks. While at the same time, their soluble nature
allows easy purification from aggregated clusters and other by-
products, which is unachievable when reduction occurs inside
the insoluble network. A series of heterogeneous catalysts sup-
ported on POPs have been reported and excellent selectivity and
higher activity than the corresponding homogeneous counterparts
were observed [19,20]. By crosslinking benzene and triph-
enylphosphine followed by dispersing PdCl2 into the framework,
a highly active and stable Pd (II) organometallic heterogeneous cat-
alyst was obtained by Tan et al. [21,22]. However, these catalysts
are prepared by the conventional step-by-step method as shown
in the top part of Fig. 1, where metal salts are loaded into the
POP structure. In Tan’s method, to obtain Pd in zero valence, a fur-
ther reduction step is required. All of these post treatments will
inevitable cause metal aggregation and result in decrease of reac-
tivity and selectivity.
Whereas supported noble metal heterogeneous catalysis has led
to a plethora of effective catalysts used in practical applications,
In this study, we utilized tetrakis(triphenyl phosphine) palla-
dium (Pd(PPh3)4) as precursor, and for the first time in one step
successfully obtained a POP structure with nearly single-atom dis-
persed Pd, through crosslinking the four triphenylphosphine
⇑
Corresponding author.
E-mail address: zhiping.lai@kaust.edu.sa (Z. Lai).
https://doi.org/10.1016/j.jcat.2017.08.030
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

102
X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
Fig. 1. Illustration of the difference between the conventional step-by-step approach and the direct knitting method.
ligands of Pd(PPh3)4, as illustrated in the bottom part of Fig. 1. Pd
(PPh3)4 is one of the most widely used homogeneous coordination
catalysts but suffering from air sensitivity and poor recyclability.
While the crosslinked catalyst, which was named as PPPd, showed
not only higher catalytic efficiency that surpassed its homogeneous
form, but also much better stability and recyclability that allowed
the catalytic reactions to be carried out in aqueous solutions under
open conditions.
the next cycle. Combing the recycled catalyst from X reaction times
was marked as Re-PPPd-X.
2.3. General procedure for Ullmann reductive homo-coupling reaction
Aryl halide (0.5 mmol), K3PO4 (1.5 mmol), PPPd (8 mg⁄1.4 wt%
Pd, 0.001 mmol), solvent (2 ml, H2O/EtOH v/v = 2:3) were mixed
in a 5 ml vessel and sealed. Then it was heated in an oil bath at
80 °C for 20 h. After cooled down to r.t., the mixture was filtered.
Take a small amount of the liquid for GC analysis. The solid part
was washed three times with ethanol. The combined liquid was
dried and purified by column to give pure coupling product.
2. Experimental section
2.1. Catalyst PPPd synthesis
2.4. General procedure for aryl boronic acid oxidative homo-coupling
reaction
Benzene (Aldrich, 0.8 g, 10 mmol), dimethoxymethane (Aldrich,
2.3 g, 30 mmol) and Pd(PPh3)4 (Aldrich, 0.6 g, 0.5 mmol) were
mixed together in 1,2-dicholoroethane (Aldrich, 20 ml). Protected
with nitrogen and cooled with ice, anhydrous FeCl3 (Aldrich,
4.8 g, 30 mmol) was added portion by portion. The reaction mix-
ture was then moved to oil bath and heated to 80 °C for 24 h. After
cooling down to r.t., the resulted precipitate was immersed and
washed with methanol until the filtrate was clear. Khaki solid pow-
der PPPd was obtained after drying in vacuum oven at 70 °C for
48 h.
Aryl boronic acid (0.5 mmol), K3PO4 (1.5 mmol), PPPd
(4 mg⁄1.4 wt% Pd, 0.0005 mmol), solvent (2 ml, H2O/EtOH v/
v = 2:3) were mixed in a 5 ml vessel and sealed. Then it was heated
in an oil bath at 80 °C for 6 h. After cooled down to r.t. for about
half an hour, the mixture was filtered. Take a small amount of
the liquid for GC analysis. The solid part was washed three times
with ethanol. The combined liquid was dried and purified by col-
umn to give pure coupling product.
With similar procedure, 50 g PPPd was produced in a scale-up
experiment.
2.5. General procedure for direct C-H bond selective alkoxylation and
halogenation reactions [23]
2.2. General procedure for Suzuki-Miyaura cross coupling reaction [22]
Benzoquinoline (0.5 mmol), PhI(OAc)2 (1.0 mmol), solvent
(4 ml) and PPPd (15 mg⁄1.4 wt% Pd, 0.002 mmol) were mixed
together in a 10 ml glass vessel under air. Then it was sealed and
heated in an oil bath. The reaction was monitored by GC–MS.
Inside an argon atmosphere glove box, halide (0.5 mmol), boro-
nic acid (0.75 mmol), K3PO4 (1.5 mmol), PPPd (4 mg, 1.4 wt% Pd,
0.0005 mmol), solvent (2 ml, H2O/EtOH v/v = 2:3) were mixed in
a 5 ml vessel and sealed. Then it was moved out of the glove box
and heated in an oil bath to 80 °C for 2–6 h. After cooled down to
r.t. for about half an hour, the mixture was filtered. Take a small
amount of the liquid for GC analysis. The solid part was washed
three times with ethanol. The combined liquid was dried and puri-
fied by column to give pure coupling product. For recycle experi-
ments, the solid part after washing was degassed and used into
2.6. Intermolecular regioselective C-H alkenylation of indole [24]
Indole (0.5 mmol), copper(ii) acetate (0.8 mmol), alkene
(1.5 mmol) and PPPd (8 mg⁄1.4 wt% Pd, 0.001 mmol) were mixed
together in a 10 ml glass vessel under air, then DMF/ DMSO
(9:1 v/v, 4 ml), was added and the vessel was sealed to heat by

X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
103
oil bath at 80 °C for 12 h. After cooled down to r.t. for about half an
hour, the mixture was filtered. Take a small amount of the liquid
for GC analysis.
(TEM) of the samples were recorded on a high resolution electron
microscope Titan ST TEM at 300 kV. Nitrogen physisorption iso-
therms were measured using a Micromeritics ASAP 2420 auto-
matic volumetric adsorption apparatus, at 77 K. The samples
were degassed at 393 K for 48 h under high vacuum prior to the
gas adsorption measurements.
2.7. Material characterization
13C and 31P CP/MAS measurements were performed using a Bru-
ker 400 M MAS system with Adamantane and NH4H2PO4 as inter-
nal reference for C and P respectively. Liquid NMR data were
recorded on Bruker 600 M (for 1H) and 150 M (for 13C) with CDCl3
or D6-DMSO as solvent. Fourier transform infrared spectroscopy
(FTIR) was performed on a Nicolet iS10 Smart FTIR spectrometer
with Smart OMNI Transmission ranging from 4000 cmÀ1 to
400 cmÀ1, Thermo Scientific, USA. CHN elemental analysis was car-
ried out in a Flash 2000 organic elemental CHN analyzer, Thermo
Scientific, USA. Thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) of samples were carried out in a TGA
instruments thermal analyzer Netzsch leading thermal analyzer
TG 209, under N2 flow of rate 20 ml/min. Scanning electron micro-
scope images (SEM) were obtained on Quant 200 SEM operated at
an accelerating voltage of 5 keV, spot size 3, SEM-EDS was per-
formed on the same SEM equipment with an accelerating voltage
of 10 keV, spot size 6. Transmission electron microscopic images
3. Results and Discussion
(Pd(PPh3)4) was directly knitted into a POP structure, termed as
PPPd, using the hyper-crosslinking method (experimental sec-
tion).[21] FTIR spectroscopy of PPPd (Fig. 2a) and 13C CP/MAS
NMR spectrum (Fig. 2b) displays a similar bone structure to KAPs
(Ph-PPh3)-Pd reported by Tao et al.[22] Specifically, absorption
peaks (Fig. 2a) around 3040, 2000–1600, 1250–1000, and 900–
690 cmÀ1 present the aromatic CAH bonds in stretching mode,
overtones region, in-plane blending, and ‘‘opp” bands respectively.
The peak at 2927 cmÀ1 can be attributed to the alkyl CAH stretch
vibration, indicating the existence of methylene linker formed
from dimethoxymethane. The existence of P-Ar was evidenced by
the P-Ar stretching 1130–1090 cmÀ1 medium in IR. The 13C CP/
MAS NMR spectrum revealed that PPPd exhibits two resonance
peaks at 136 and 127 ppm which can be attributed to the substi-
Fig. 2. Characterization of PPPd and recycled after 5 runs of Suzuki reaction (Re-PPPd). (a) FTIR; (b) 13C CP/MAS of PPPd; (c) 31P CP/MAS of Re-PPPd; (d) XPS of of PPPd; (e)
XPS of Re-PPPd; (f) Nitrogen adsorption isotherms at 77 K, insert pore width distribution based on NLDFT analysis of the adsorption isotherms.

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X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
tuted and non-substituted aromatic carbon respectively, and one
peak at 35 ppm belonging to the methylene linker carbon.[22]
However, the new bands at 1260 and 1184 cmÀ1 in the FTIR spec-
tra (Fig. 2a) as well as the chemical shift at 30 ppm in 31P CP/MAS
NMR (Fig. 2c) both suggest that (Pd(PPh3)4) were oxidized to form
P@O bonds in the POP structure. Elemental analysis (EA) measured
by inductively coupled plasma atomic emission spectroscopy (ICP-
AES) reveals a high Pd loading of 1.4 wt% in PPPd, and trace
amount of Fe left inside the material (200 ppm). The XPS spectra
(Fig. 2c) reveals mixed oxidation states (Pd0 and PdII) for Pd spe-
cies, which is due to the partial oxidation of (Pd(PPh3)4) that is con-
sistent to the FTIR and NMR results. Such a mixed-valence catalyst
is beneficial in carrying out versatile transformation reactions
compared to single valence catalysts [25–28].
Nitrogen adsorption measurement were conducted to analyze
the porous properties of PPPd. As shown in Fig. 2f, the isotherms
displayed a typical type I curve with a steep increase of nitrogen
gas uptake at a low relative pressure (P/P0 < 0.01), indicating the
abundant presence of permanent microporosity [6]. Moreover,
the slight hysteresis loop with gradual increasing gas uptake
reflects the presence of mesoporosity, while a second steep
increase at a high relative pressure (P/P0 = 0.9–1) could be attribu-
ted to the macropores. Consistently, the pore size distribution anal-
ysis of PPPd as shown in Fig. 2f (insert) gave two main pore size
distributions of 0.7 and 3.4 nm corresponding to the micro and
mesopores, respectively. Such a structure with mixed-sized pores
has been found to be beneficial for anchoring metal catalysts by
accelerating the mass transport of reactants and products
[22,29]. The BET surface area and pore volume of the as-
synthesized PPPd were determined to be 702 m2/g and
0.546 cm3/g, respectively. As demonstrated below, this novel por-
ous 3D architecture offers more accessible active sites and facili-
tate mass transport to achieve efficient catalysis [29,30].
Fig. 3a, b show scanning electron microscopy (SEM) images of
as-synthesized PPPd, in which a 10-mm 3D nanoflower-like struc-
ture was constructed from interconnected and slightly bended
nanosheets with thickness of ca. 80 nm. The macropores between
nanosheets are observable. The transmission electron microscopy
(TEM) image (Fig. 3c) shows that most of the nanosheets have
dimensions of several microns and an average thickness of about
Fig. 3. SEM images of PPPd at low (a) and high (b) magnifications. (c) TEM images of PPPd at low magnification. (d)–(g) are the SEM-EDS mapping images of PPPd, coated with
carbon. (h)–(m) are HRTEM images of PPPd at different irradiation time under 300 KVe: (h) 0 S, (i) 20 S, (j) 40 S, and (k) partially enlarged image of the lattice in (j) and the
insert image in (k) shows the lattice distance which matches Pd lattice. (l) a crystalline particle found after 1 h irradiation together with its FFT (m).

X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
105
100 nm, which are consistent with SEM results. This type of assem-
bly architecture allows for easy access to the available high surface
area thus benefiting catalysis, while the macropores between each
flake together with the micropores inside the nanosheet will ben-
efit mass transport [29,30]. SEM-EDS elemental mappings (Fig. 3d–
g) clearly indicate that the Pd(PPh3)4 moiety is uniformly dispersed
throughout the framework. An interesting electro-beam irradiation
induced metal atoms aggregation-crystallization was observed
under HRTEM imaging process. As shown in Fig. 3h–m, no metal
agglomerates could be observed at the beginning of imaging
(0 s), while an obscure crystalline lattice corresponding to Pd
(1 1 1) plane was developed after 20 s, and eventually 3–5 nm
crystalline Pd domains are observable due to the fusion of spatially
isolated Pd atoms. It is known that organic polymers could be
burned away under TEM beam, when the surface polymer was
burned out by the eletro-beem, the metal atoms were able to
assemble and crystallize. Such a time-dependent nanoparticle for-
mation process further revealed that the palladium atoms are spa-
tially isolated in the framework of PPPd. To the best of our
knowledge, this is the first direct observation of Pd crystalline
nanoparticle formation induced by electro-beam irradiation on a
polymer support.
ered by simple filtration, and no obvious loss in the catalytic
efficiency was observed after 10 repeated cycles (Fig. 4). To distin-
guish whether supported heterogeneous or dissolved homoge-
neous Pd species were the real responsible catalysts, ‘‘hot
filtration” tests were adopted. Here, the reaction of chlorobenzene
and phenylboronic acid (Table 2, entry 12) was stopped after 1 h,
the solid was filtered out from the hot solution and the mother fil-
trate was allowed to react for another 5 h under the similar condi-
tions. GC analysis of the mixture showed no significant progress in
the reaction occurred. Meanwhile, elemental analysis of the filtrate
after the first cycle shown Pd content less than 50 ppb, indicating
that no appreciable amount of Pd dissolved, which could account
for the preservation of catalytic activity. Characterization of the
recycled catalyst after 5 runs (Re-PPPd) produced identical spec-
trums as the fresh catalyst (Fig. 2). The BET surface area and the
pore volume slightly decreased to 657 m2/g and 0.538 cm3/g in
Re-PPPd, respectively. This was mainly due to the loss of microp-
orosity (Fig. 2f), which could possibly be occupied by some impu-
rities. It should be emphasized that unlike phosphine-coordinated
homogeneous organometallic catalysts that are commonly unsta-
ble in air, no observable change in the morphology or the catalytic
efficiency of the PPPd was found after exposure to air for several
months. The enhancement of catalyst’s stability is also observed
in other porous material supported initially low-stable catalyst
systems [31,32], most likely benefited from the spatial isolation
effect of the framework by inhibiting aggregation.
The Suzuki-Miyaura coupling reactions of Aryl iodide and aryl
boronic acid were first chosen as standards to test the reactivity
and recyclability of the PPPd. All of the substrates, involving triflu-
oromethyl, pyridine, furane, ferrocene functionalized or bis-
boronic acid, gave almost quantitative yield within 1 h in an aque-
ous medium (Table 1). Moreover, the catalyst can be easily recov-
The less active coupling partners for Suzuki-Miyaura reactions
were then also investigated. First, the multifunctionalized aryl bro-
Table 1
PPPd-catalyzed Suzuki-Miyaura coupling reactions of Aryl iodide and aryl boronic acid.
99% (10 cycles)
99% (98%)
99% (96%)
99% (95%)
98%
99%
GC yield, isolated yield in parentheses.
PhI
PhCl
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Cycles
Fig. 4. Reusability test of PPPd in Suzuki-Miyaura coupling reactions (left) and a typical reaction mixture as well as the liquid phase after filtration (right).

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X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
Table 2
PPPd-catalyzed Suzuki-Miyaura coupling reactions of challenging partners.
1, X = Br, 99%[a]
5, X = Br, 99%
2
[b], X = Br, 0%
3, X = Br, 98%
7, X = Br, 92%
4, X = Br, 100%
8, X = Br, 99%
6, X = Br, 100%
9, X = Br, 100% (98%)
13, X = Cl, 100%
10, X = Br, 100%
11, X = Br, 100% (98%)
15[c], X = Br, 99% (97%)
12, X = Cl, 99%
14, X = Cl, 97% [8 cycles]
16[c], X = Cl, 94% (96%)
17, X = Br, 98% (95%)
18[c][d], X = I, 60%
19[c][d], X = Br, 88%
[a]
[b]
[c]
[d]
GC yield with isolated yield in parentheses.
1 mol% Pd was used.
0.2 mol% Pd was used.
24 h.
mides were employed which again displayed encouraging reactiv-
ity and selectivity (Table 2, entry 1 and 3–11). It should be men-
tioned that substrates employed here with functions of fluoride,
furan, amine, indole, pyridine, carboxylic acid, and thiophene, are
all general and important intermediates for the synthesis of agro-
chemicals, pharmaceuticals and natural products. The bipolar
benzo[1,2,5]thiadiazole has wide applications in organic light-
emitting diodes (OLEDs) as an emissive group [33,34]. The excel-
lent activity and functional group tolerance of PPPd revealed its
high potential in real applications in these fields. No coupling pro-
duct was obtained when (2-bromophenyl) diphenylphosphane
was employed as substrate, possibly due to its sterically hindered
nature, suggesting that the reactions occur inside the pores. The
activation of the CACl bond (ꢀ79 kcal/mol) is much more difficult
than the CABr (ꢀ66 kcal/mol) and CAI (ꢀ52 kcal/mol) bonds due
to its slow oxidative addition to palladium, especially in heteroge-
neous catalysis system; thus, harsher reaction conditions are gen-
erally required [35]. For this reason, coupling of aryl chlorides and
aryl boronic acids is commonly selected as a standard reaction to
demonstrate the high reactivity of a catalyst [22,36]. At a Pd load-
ing of 0.1%, 99% yield of biphenyl was obtained after heating
chlorobenzene and benzeneboronic acid at 80 °C for 6 h (Table 2,
entry 12). Other aryl chlorides bearing electron-giving groups such
as –OMe or –CF3 gave high quantitative yields within 6 h. Notably,
the electron deficient 1-chloro-4-nitrobenzene also gave a high
and reproducible yield in 8 continuous cycles (Table 2, entry 14).
More impressively, coupling of benzyl halides with phenylboronic
acid, which is a rarely reported, challenging reaction but important
in heterogeneous catalysis [22], also performed very well (Table 2,
entry 15, 16). This further demonstrates the high activity of PPPd
for coupling benzyl halide and phenylboronic acid.
Encouraged by these impressive results, we further exposed the
substrate to vinyl and alkyl boronic acids. The alkylation of aryl
halides with vinyl- or alkylboron derivatives may provide a simple
and easy way to obtain the corresponding aryl-vinyl or aryl-alkyl
derivatives. Normally, these reactions are performed using 9-BBN
compounds as reactants, which are not air stable thus difficult to
operate, and the processes are not economical [37]. In the last dec-
ade, several palladium catalysts associated with electro-rich phos-
phine ligands have been developed to achieve the coupling of
alkylboronic esters or acids in homogeneous conditions. To the
best of our knowledge, very few cases have been reported for these
reactions in heterogeneous systems, which typically gave very low
yields [38,39]. In our system, the coupling of 2-bromo-6-
methylpyridine with (E)-(4-methoxystyryl) boronic acid occurred
smoothly, leading to a 95% isolated yield in the coupled product.
The coupling of aryl iodide and aryl bromide with n-butylboronic
acid was also successfully achieved in a longer reaction time. Inter-
estingly, the aryl bromide reacted faster than the aryl iodide, which
suggests that in the case of cross-coupling of aryl iodide with alkyl
boronic acid, oxidative addition may not be the rate-limiting step
of the catalytic cycle. Such phenomenon was also observed in a
homogeneous Suzuki cross-coupling system [40]. Importantly,
such reactions cannot be achieved with homogeneous catalyst of
Pd(PPh3)4.
The biaryl motif is of great importance in biological products, as
well as in asymmetric catalysis, thus stimulating numerous con-
struction methodologies [41]. In addition to the original and the
most widely used copper-mediated Ullmann couplings,
palladium-catalyzed reductive homo-couplings of aryl halides
have also been reported, and advantages were found [42]. One
big challenge in Pd mediated homo-coupling of aryl halides is

X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
107
the generation of Pd0 which always require inconvenient reaction
conditions [41]. By supporting palladium NPs on a porous MOF,
Jiang and co-workers reported a heterogeneous Pd°Catalyst, Pd/
MIL-101, which is efficient for Ullmann couplings and also found
that PdII is not active [36]. During our examination of the coupling
of aryl iodide and butylboronic acid, a significant amount of homo-
coupling biaryl product was observed. Considering the mixed-
valence nature of Pd0/PdII in PPPd, the biaryl compound must come
from the PdII-catalyzed Ullmann coupling reaction.
Further experiments quickly revealed that under the presence
of PPPd, aryl iodide, bromide, and chloride were all homo-
coupled to the corresponding biaryl products in excellent yields
(Table 3, entry 1–3). In comparison with the highly active PPPd cat-
alyst, Pd(PPh3)4 and PdCl2 were found not active under the same
conditions (Table 3, entry 3), indicating that the porous polymer
supported mix-valence palladium was essential for the Ullmann
coupling reaction. The oxidative homo-coupling of aryl boronic
acids was also conducted successfully, leading to biaryl products
under conditions similar to Suzuki coupling reactions but without
halides and under an air atmosphere (Table 3, entry 4–5). Further-
more, the exceptionally high activity and selectivity for the biaryl
synthesis in an aqueous solvent at a moderate temperature shows
great potential for practical applications.
Table 3
Palladium-catalyzed reductive homo-coupling of aryl halides and oxidative homo-
coupling of aryl boronic acid[a]
.
The direct conversion of CAH bonds into CAO, CAhalogen, CAC
bonds has attracted significant attention but remains a critical
challenge in organic chemistry. Recently, Cohen et al. reported a
thiocatechol-functionalized MOF supported recyclable Pd catalyst
that can selectively catalyze chelate-directed functionalization of
CAH bonds [23]. However, the synthesis of this MOF substrate
was costly and a high palladium loading (5–15 mol%) was required
to obtain good yields, which undoubtedly inhibits its practical
applications. To the best of our knowledge, although numerous
of porous polymer supported palladium catalysts have been
reported in the literature, applications in direct CAH bond func-
tionalization are quite rare. The outstanding performance of PPPd
in the reactions of CAC bond formation motivated us to further
investigate its catalytic activity in regioselective heterogeneous
CAH functionalization. To our delight, successful CAH oxidation
of benzoquinoline was achieved regioselectively to give benzo[h]
quinolin-10-yl acetate in a quantitative yield with iodobenzene
diacetate [PhI(OAc)2] as the oxidant and 0.4 mol% of the PPPd cat-
alyst in hot MeCN within 12 h under an air atmosphere. When the
solvent was changed to methanol (MeOH) or 2,2,2-trifluoroethanol
(CF3CH2OH), alkoxy functional groups of AOMe and AOCH2CF3
were successively and selectively added. When NXS was used as
Entry
Substrate
Product
Yield [%][b]
98 (96)
1
2
99 (97)
3
96 (95) < 10[c] < 5[d]
96 (93)
4[e]
5[e]
95 (94)
[a]
Reaction conditions: Aryl halide (0.5 mmol), K3PO4 (1.5 mmol), H2O/EtOH
(2:3 v/v, 2 ml), PPPd (0.2 mol% Pd), 80 °C, 20 h, under air.
[b]
GC yield, isolated yield in parentheses.
0.2 mol% Pd(PPh3)4 was used instead of PPPd.
0.2 mol% PdCl2 was used instead of PPPd.
Arylboronic acid (0.5 mmol), K3PO4 (1.5 mmol), H2O/EtOH (2:3 v/v, 2 ml), PPPd
[c]
[d]
[e]
(0.1 mol% Pd), 80 °C, 6 h, under air.
Table 4
Palladium-catalyzed direct CAH bond selective alkoxylation and halogenation reactions[a]
.
Entry
B
Solvent
MeCN
Product
Yield [%][b]
99
1[c]
no
2[d]
3[c]
4[e]
5[e]
MeOH
CF3CH2OH
NBS
MeOH
96–99
99
CF3CH2OH
MeCN
95
NCS
MeCN
86
[a]
[b]
[c]
[d]
[e]
Reaction conditions: benzoquinoline (0.5 mmol), PhI(OAc)2 (1.0 mmol), solvent (4 ml), PPPd (0.4 mol% Pd), under air.
Determined by GC–MS.
100 °C, 12 h.
70 °C, 6 h, 5 cycles.
No PhI(OAc)2 was used. NXS (1.25 mmol), 85 °C, 20 h. Multi-halogenation was observed when more NXS was applied.

108
X. Wang et al. / Journal of Catalysis 355 (2017) 101–109
Table 5
Palladium-catalyzed intermolecular regioselective CAH alkenylation of indole[a]
.
Entry
Catalyst
Pd (mol%)
Time (h)
Yield (%)
Ref.
1
2
Pd(OAc)2
PPPd
10
0.2
18
12
66
[24]
this work
88–92 (5 cycles)
[a]
Reaction conditions: Indole (0.5 mmol), Cu(OAc)2 (0.8 mmol), alkene (1.5 mmol), DMF/DMSO (9:1 v/v, 4 ml), air. Yield determined by GC.
the oxidant in replacement of PhI(OAc)2, good to excellent yields of
halogenation products were obtained (Table 4, entries 4 and 5).
Though only 0.4 mol% catalyst was used, PPPd-catalyzed CAH oxi-
dation of benzoquinoline showed higher activity than the reported
Pd(OAc)2 homogeneous catalysts [43] and much higher activity
than the Pd/MOF heterogeneous catalysts [23]. The higher reactiv-
ity of PPPd can be rationalized in the following manner. First, the
spatial-isolated palladium atoms in the porous network with high
surface area provide more active sites. Second, the fully organic
nature of the support with good solvent wettability can accelerate
the mass diffusion of organic reactant molecules thus promoting
the catalytic cycle. Third, phosphine oxide inside the framework
coordinates with palladium to keep the resting state of Pd in a high
active form [44]. Fourth, although the PdII/0 or PdII/IV cycle process
is still a debated topic in CAH activation [45], PPPd with both Pd0
and PdII can possibly serve both paths, thus enhancing the overall
conversion rate.
Intermolecular regioselective CAH alkenylation of N-free indole
was also achieved with good yield and selectivity (Table 5), under
similar reaction conditions as Pd(OAc)2-catalyzed homogeneous
systems, but with much lower Pd loading and shorter reaction
time.[24] As shown in table 5, 90% yields of C-3 alkenylation pro-
duct was obtained with only 0.2 mol% PPPd loading in 12 h, which
is two magnitudes more efficient than the reported Pd(OAc)2 sys-
tem. Moreover, the PPPd catalyst can be easily recovered and
reused without significant loss of activity after five runs.
Acknowledgement
Financial support was provided by KAUST competitive grant
URF/1/1723, Saudi Arabia.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2017.08.030.
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