Water-medium C-H activation over a hydrophobic perfluoroalkane-decorated metal-organic framework platform

Article abstract of DOI:10.1016/j.jcat.2015.10.012

The use of water as reaction medium in the heterogeneous activation of C-H bonds has numerous advantages in terms of environmental benign, safety and cost efficiency impact. However, it is severely hampered because the reactants are difficult to dissolve in water and contact with the active sites of heterogeneous catalysts. Herein, we choose perfluoroalkane-functionalized mesoporous metal-organic framework (MOF) NU-1000 as a hydrophobic platform to encapsulate ultrafine palladium nanoparticles (Pd NPs) for C-H activation in water. The resultant Pd NPs stabilized by the perfluoroalkane exhibited high activity and regioselectivity in the direct C-H arylation of indoles in water. The introduction of perfluoroalkane chains into the mesoporous pores of NU-1000 provides hydrophobic surfaces to facilitate access of the reactants to the active sites to guarantee the high activity.

Full text of DOI:10.1016/j.jcat.2015.10.012

Journal of Catalysis 333 (2016) 1–7
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Water-medium C–H activation over a hydrophobic perfluoroalkane-
decorated metal-organic framework platform
⇑
Yuan-Biao Huang, Min Shen, Xusheng Wang, Ping Huang, Ruiping Chen, Zu-Jin Lin, Rong Cao
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155, Yangqiao Road West, Fuzhou 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of water as reaction medium in the heterogeneous activation of C–H bonds has numerous advan-
tages in terms of environmental benign, safety and cost efficiency impact. However, it is severely ham-
pered because the reactants are difficult to dissolve in water and contact with the active sites of
heterogeneous catalysts. Herein, we choose perfluoroalkane-functionalized mesoporous metal-organic
framework (MOF) NU-1000 as a hydrophobic platform to encapsulate ultrafine palladium nanoparticles
Received 3 September 2015
Revised 19 October 2015
Accepted 20 October 2015
(
Pd NPs) for C–H activation in water. The resultant Pd NPs stabilized by the perfluoroalkane exhibited
Keywords:
high activity and regioselectivity in the direct C–H arylation of indoles in water. The introduction of per-
fluoroalkane chains into the mesoporous pores of NU-1000 provides hydrophobic surfaces to facilitate
access of the reactants to the active sites to guarantee the high activity.
Metal-organic frameworks
Water-medium
Pd nanoparticles
C–H activation
Ó 2015 Elsevier Inc. All rights reserved.
Perfluoroalkane
1
. Introduction
The C2 arylated indoles are one of the most important building
leads to difficult to purify the products and has a negative impact
on the environment [19]. To overcome these drawbacks, Pd NPs
supported on hydrophobic mesoporous materials will provide a
promising way for highly efficient catalytic C–H activation in aque-
ous media. Nevertheless, large surface areas and mesoporous chan-
nels are required to guarantee a high dispersion of Pd NPs active
sites and facilitate reactants adsorption and diffusion. Meanwhile,
hydrophobic materials can enhance the adsorption of hydrophobic
organic reactants in aqueous solution, which leads to accelerated
reaction rate and enhanced reaction selectivity [20].
MOFs have been emerging as promising heterogeneous cata-
lysts due to their high surface areas, tunable pores and postmodi-
fication [25–45]. As known, the perfluoroalkane chains not only
can stabilize metal NPs [16,46,47], but also provide hydrophobic
environments where the organic substrates could be much more
blocks of bioactive molecules [1]. Direct construction of C–C bond
from inert C–H bond is a highly efficient and sustainable strategy
in the synthesis of arylated indoles due to no need of organometal-
lic activating groups [2–13]. Palladium-catalyzed direct C–H aryla-
tions of indoles have been investigated and made great progress
over the past few years [2–18]. Unfortunately, the reported direct
arylation methodologies were usually carried out in organic sol-
vents, which may be hazardous to health and the environment
[
16].
The use of heterogeneous catalysts in water-medium C–H acti-
vation offers particular environmental and sustainable advantages
because water is the most inexpensive and environmentally
benign solvent [19–22]. However, most of organometallic catalysts
are sensitive to the moisture and the expensive additives, such as
Ag salts or phosphine ligands are required. Thus, the catalysis of
the C–H arylations of indoles in water-medium with high effi-
ciency remains very challenging [23,24]. Moreover, the active sites
in heterogeneous catalysts are not as accessible as those in homo-
geneous catalysts. In addition, because of its low solubility in
water, most organic reactants are difficult to contact with the
active centers. Although introduction of co-solvents or surfactants
can accelerate reaction rate, the addition of auxiliary additives
accessible to the active sites in water [48,49]. NU-1000 (Zr
6 3
(l -
OH) (OH) (TBAPy) TBAPy = 1,3,6,8-tetrakis(p-benzoate)pyrene)
8
8
2
,
has mesopores (3.0 nm) with high thermal stability (up to
500 °C) and chemical stability in boiling water at different pH val-
ues (pH = 1À11, Fig. S7) [50], and the eight terminal OH groups on
Zr can be used for postmodification with perfluoroalkane chains
(Figs. 1 and S1). Therefore, the hydrophobic mesoporous MOFs
NU-1000 functionalized with perfluoroalkane chains was selected
to immobilize Pd catalyst for the generation of uniform NPs with
small size and the enhancement for catalytic activity in water.
Herein, ultrafine Pd NPs were encapsulated in the mesopores of
perfluoroalkane functionalized NU-1000 for the high activity in the
direct C–H arylation of indoles in water.
⇑
Corresponding author.
E-mail address: rcao@fjirsm.ac.cn (R. Cao).
http://dx.doi.org/10.1016/j.jcat.2015.10.012
021-9517/Ó 2015 Elsevier Inc. All rights reserved.
0

2
Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
Fig. 1. Schematic representation of introduction of perfluoroalkane chains into the pores of NU-1000 by SALI approach and immobilization of the Pd NPs into the pores of
perfluoroalkane functionalized NU-1000 using incipient-wetness impregnation method.
eluent). The identification was conducted by 1H and 13C NMR
measurement.
2
. Experimental
2
.1. Synthesis of perfluoroalkane functionalized NU-1000
For the measurement of the Pd leaching during the reaction, a
hot-filtration experiment was run to investigate whether the reac-
tion proceeded in a heterogeneous or homogeneous fashion. After
6 h, the catalyst was separated by hot filtration and the filtrate was
further treated under the same conditions for another 42 h. For the
recyclability test, the catalyst was recovered from each of the
reaction of N-methylindole with iodobenzene at 100 °C for 24 h,
washed with water and acetone several times, and then dried
under vacuum at 150 °C for the next use.
The perfluoroalkane functionalized NU-1000 (F9-NU-1000, F15-
NU-1000 and F19-NU-1000) was synthesized according to the lit-
erature method [51]. 60 mg NU-1000 (0.027 mmol) was loaded
in a 10 ml vial. Subsequently a 2.4 ml of 0.1 M solution of fluo-
roalkane carboxylic acid (0.24 mmol) in DMF was added to the
reaction vial, which was then sealed and heated at 60 °C for 24 h
with occasional swirling. The supernatant of the reaction mixture
was decanted and the MOF sample was soaked into fresh hot
DMF which was then filtered, washed sequentially with DMF, ace-
tone and diethyl ether (30, 20 and 15 ml each), and finally dried at
3
. Results and discussion
1
20 °C under vacuum for 12 h. Interestingly, the covalent ester
3.1. Preparation and characterization of catalysts
bonds in Fn-NU-1000 cannot be hydrolyzed in boiling water at
pH = 1–9 (Fig. S8). Notably, the perfluoroalkanes can be released
from the boiling water at pH = 11 for 24 h, while the parent mate-
rial NU-1000 retains the crystalline structure.
NU-1000 was synthesized and purified according to the litera-
ture [50]. A series of perfluoroalkanes with different chain length
decorated in the mesoporous channels of NU-1000, denoted as
Fn-NU-1000 (e.g., F9-NU-1000, F15-NU-1000 and F19-NU-1000),
was easily obtained using solvent-assisted ligand incorporation
2.2. Preparation of palladium catalysts embedded in NU-1000 and Fn-
(
SALI) approach (Figs. 1 and S1) [51]. Here n is the number of the
NU-1000
fluorine atoms in the perfluoroalkanes chains. After digestion with
1
9
D
2
SO
4
,
F NMR spectra of the functionalized NU-1000 proved the
Pd@NU-1000 and Pd@Fn-NU-1000 (n = 9, 15, and 19) were pre-
pared using incipient-wetness impregnation method. Typically, a
solution of Pd(acac) (14.5 mg, 0.30 ml CHCl ) was added dropwise
successful decoration with perfluoroalkanes (Figs. S2–S4). The
powder X-ray diffraction (PXRD) patterns of the obtained yellow
solids are coincident with that of the simulated NU-1000, confirm-
ing that the functionalized samples retain their crystallinity
2
3
into the activated NU-1000 (or Fn-NU-1000) powder (500 mg)
under vigorously stirring in the nitrogen flow. The mixture was
continuously stirred for 30 min. The solid was dried at 80 °C for
(
2
Fig. S5). The N adsorption measurements (Fig. 2a and Table S1)
2
À1
showed a large specific surface areas (SBET = 2742.6 m g ) of the
obtained NU-1000 [50,51]. Interestingly, although the samples
Fn-NU-1000 exhibited lower specific surface areas (Fig. 2a), they
still remain mesoporous after functionalized with fluoroalkane
chains (Table S1), which cannot prevent the loading of Pd catalysts.
More interestingly, compared to NU-1000, all the fluoroalkanes
functionalized samples showed lower water uptakes with increas-
ing fluoroalkane chain length (Fig. 2b), which indicated the meso-
pores become hydrophobic platforms [51].
1
2 h under vacuum and reduced in 5% H
2 2
/N flow at 200 °C for
3
h. The Pd loading on NU-1000 and Pd@Fn-NU-1000 (n = 9, 15,
and 19) was 1.14 wt%, 0.98 wt%, 1.20 wt% and 1.11 wt%, respec-
tively, according to the ICP.
2.3. Catalytic test
2
Typically, 1.0 ml H O was added into the mixture of indole
(
1.0 mmol), aryl halide (1.2 mmol), KOAc (3.0 mmol), and the pal-
ladium catalyst (1.0 mol% Pd). The reaction mixture was stirred at
00 °C for 24 h. After cooling to room temperature, most of the
Pd NPs embedded in NU-1000 and Fn-NU-1000 (here denoted
as Pd@NU-1000 and Pd@Fn-NU-1000, n = 9, 15, and 19) were pre-
pared using incipient-wetness impregnation method, followed by
1
products are present in the aqueous phase. The suspension was
centrifuged. To completely collect the products residual remaining
in the solid NU-1000, ethyl acetate was used to wash for three
times (5 ml  3). The aqueous phase was extracted three times
using ethyl acetate (5 ml  3). The two parts of organic phase were
combined and subsequently washed with water and brine and
2
treatment with H at 200 °C for 3 h. The PXRD patterns (Fig. S6)
show that there is no significant loss of crystallinity, and no sup-
plementary Bragg peaks appearance, which indicated that the
integrity of the frameworks of the MOFs was maintained after
the Pd loading. Moreover, the characteristic peak of Pd was indis-
tinguishable indicating the formation of small Pd NPs [30]. Com-
pared with the corresponding bare supports, the obvious
decreases in the surface areas of Pd@NU-1000, Pd@Fn-NU-1000
2 4
then dried over Na SO . The product was purified by silica gel
chromatography (mixture of light petroleum and ethyl acetate as

Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
3
Fig. 2. (a) Nitrogen sorption isotherms at 77.3 K for NU-1000 and Fn-NU-1000 samples. (b) Water isotherms of NU-1000 and Fn-NU-1000 samples at 298 K; Po is the
saturated vapor pressure at 298 K. (c) Nitrogen sorption isotherms at 77.3 K for Pd@NU-1000 and Pd@Fn-NU-1000 samples. (d) BJH pore size distributions for Pd@NU-1000
and Pd@Fn-NU-1000 samples.
indicate that the pores of the host frameworks were occupied by
dispersed Pd NPs (Figs. 2a, c, and S9, Table S1) [30]. Interestingly,
although a slight decrease of the pore sizes after the incorporations
of Pd NPs, all the MOFs composites are in the mesoporous ranges
except Pd@F19-NU-1000 functionalized with the long fluoroalkane
chains (Fig. 2d and Table S1). Therefore, the substrates still can
access into the catalytic active sites through the mesoporous
windows.
surface. The average particle size is very small (ca. 2.5 nm, Fig. 3d),
which could be ascribed to the confinement of the mesopores of
F15-NU-1000 [44] and the entrapment of the perfluoroalkane
chains [48,49]. Interestingly, TEM images (Figs. 3a and S12) and
HAADF-STEM images (Fig. 3b) show the average adjacent lattice
plane of Pd@F15-NU-1000 is ca. 2.74 nm (7 of lattice fringes are
19.18 nm), which implies that the structural integrity of F15-NU-
1000 was preserved after the Pd incorporation. It further proved
that NU-1000 functionalized with fluoroalkane chains was indeed
stable even under the irradiation of the TEM electron beam.
X-ray photoelectron spectroscopy (XPS) spectra (Fig. S11) show
5
/2
3/2
that the 3d
and 3d
peaks of Pd appear at 335.77 and
3
41.14 eV, respectively, which indicates that most of the palladium
is in the reduced form [32]. It should be noted that the weak inten-
sity of the Pd peaks is due to the low ratio of the Pd/Zr (1.20 wt%
Pd, Fig. S13). The other two peaks at 347.29 and 333.85 eV are
3.2. C–H arylation of indoles
It is well known that C–H arylation of free indole is more diffi-
cult than the N-substituted derivatives. Therefore, the free indole
and iodobenzene were employed as the substrates in the screening
of optimized reaction conditions in water. All the reactions were
performed in the air atmosphere and C2-arylation products were
obtained as the major regioisomers. Gratifyingly, the good GC yield
(81.3%) and high selectivity (C2/C3 = 22.4, the ratio of the C2 pro-
duct and C3 product) were obtained when using 1.0 mol% Pd cata-
lyst Pd@F15-NU-1000 at 100 °C after 24 h (Tables S2–S4 and
Fig. S16). In contrast, higher catalyst loadings (5 mol%) are
generally required in the homogeneous catalyst system even in
the organic solvents [19,20]. Although increasing the amount of
1
/2
3/2
ascribed to the Zr 3p and Zr 3p , respectively, while the Zr 3d
5
/2
3/2
peaks for Zr 3d
85.17 eV, respectively.
Transmission electron microscopy (TEM) (Figs. 3a and S12),
high-annular dark-field scanning TEM (HAADF-STEM) images
Figs. 3b and S13–S15) and HRTEM images (Fig. 3c) of Pd@Fn-
NU-1000 and Pd@NU-1000 show that the uniformly dispersed Pd
NPs are formed. Combined with the preparation method, N
and Zr 3d
are located at 182.78 eV and
1
(
2
adsorption results and TEM images, it can be deduced that most
of the Pd NPs are embedded in the pores of Fn-NU-1000 and
NU-1000, although a few of the particles are found on the outside

4
Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
Fig. 3. (a) TEM, (b) HAADF-STEM, (c) HRTEM images of Pd@F15-NU-1000 and (d) the size distribution of Pd NPs. Scale bars = 50, 20, 2 nm in (a), (b) and (c), respectively.
the catalyst to 2.0 mol% Pd led to a little higher yield, the selectivity
of C2/C3 decreased and a small amount of the by-product biphenyl
was produced (Table S4, entry 5).
As we expected, different perfluoroalkane chain length plays a
vital role in the catalysis in water. Although the mesoporous NU-
the hydrophilic pore environment inhibits the diffusion of the
organic substrates into the Pd active sites in water [49]. Interest-
ingly, perfluoroalkane chains functionalized catalysts efficiently
enhanced the C–H activation in water (Table 1, entries 2–4).
Pd@F9-NU-1000 gave approximately twice yields (Table 1, entry
2) than the nonfunctionalized catalyst (Table 1, entry 1). Satisfac-
torily, longer perfluoroalkane chains decorated Pd@F15-NU-1000
1
000 has higher surface area and pore volumes, unfortunately, it
exhibited lower activity (Table 1, entry 1). It can be ascribed to that
Table 1
a
Direct arylation of indole in water with different Pd catalysts.
Entry
Pd-based catalyst (wt% Pd)
Yield (%)b
Selectivity (C2/C3)c
1
2
3
4
5
6
7
Pd@NU-1000 (1.14)
12.5
24.3
87.2
84.2
11.1
33.7
27.1
9.3
11.1
21.1
20.3
6.3
Pd@F9-NU-1000 (0.98)
Pd@F15-NU-1000 (1.20)
Pd@F19-NU-1000 (1.11)
Pd(acac) @F15-NU-1000 (1.00)
2
Pd@MIL-101(Cr) (0.56)
19
15
Pd/MIX-MIL-53(Al) (1.53)
a
b
c
Conditions: 1.0 mmol indole, 1.2 mmol iodobenzene, Pd-based catalyst (1.0 mol% Pd), KOAc (3 mmol), air, 48 h.
GC yield.
Selectivity determined by GC–MS

Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
5
Table 2
a
Direct arylation of various indoles with PhI.
2a: 77.3% (22.3)
2
b: trace
2c: trace
2f: R1 = H, 72.1%
b
2d: 73.4% (19.7)
2e: 66.7% (11.2)
1
(
33.4); 2g: R = CH
3
,
74.2% (43.5)
1
b
b
1
2
2
h: R = H, 61.2% (21.1);
2j: 76.3% (32.2)
2k: R = H, trace;
1
1
i: R = CH
3
, 68.4% (17.7)
2l: R = CH
3
, trace
NC
N
H
N
2o: 6.5%b
2
m: trace
2
n: no product
a
Conditions: 1.0 mmol indole derivatives, 1.2 mmol iodobenzene, Pd@F15-NU-1000 (1.0 mol% Pd), KOAc (3 mmol), air, 24 h. Isolated yields of pure C2-isomers after flash
column chromatography are reported. Values in brackets refer to C2/C3 selectivity of the product determined by GC–MS.
b
GC yield.
gained high activity (87.1%) and selectivity (21.1, Table 1, entry 3).
Although Pd@F19-NU-1000 with longer chains showed a little
lower activity to the lower surface areas and pore volumes
for another 42 h. As expected, no further conversion was detected
after the removal of Pd@F15-NU-1000 (Fig. S16). After the workup,
there was no detectable Pd leaching into the filtrate by the induc-
tively coupled plasma atomic emission spectroscopy (ICP-AES)
analysis. Interestingly, after recycled for six runs of the reaction
of N-methylindole and iodobenzene, the Pd catalyst still exhibited
high activity (Fig. S17). The PXRD patterns revealed that the crys-
talline structure of the catalyst was still retained after six catalytic
cycles (Fig. S6). In addition, there was no Pd peak appearing in the
PXRD, which indicated that there was no aggregation of Pd NPs
during the reaction. Impressively, indeed, the TEM images of the
catalyst after six catalytic recycles showed that the size of Pd
NPs was similar to that before reaction (Mean size = 2.53 nm,
Fig. S18). The result may be contributed to that the Pd NPs are con-
fined into the pores of F15-NU-1000 and entrapped by the perflu-
(Fig. 2c and Table S1), it also gave 84.2% yield. These findings are
associated with that the water adsorption uptakes decrease from
a shorter to longer perfluoroalkane chains (Fig. 2b). It indicated
that the introduction of perfluoroalkane chains not only provides
hydrophobic inner surfaces to facilitate access of the organic reac-
tants to the active sites, but also inhibits the migration and aggre-
gation of Pd NPs in the reduction and catalysis processes.
Furthermore, the catalyst Pd@F15-NU-1000 still possesses larger
surface areas and accessible mesoporous pores, which ensures
the high dispersion of the active sites and facilitates the diffusion
of the substrates in the pores. Other supported palladium catalysts
also proved the results (Table 1, entries 6–7). Pd NPs supported on
the hydrophilic mesocages of MIL-101(Cr) [14,15] gave only 33.7%
yield (Table 1, entry 6), although it has large surface areas and pore
volumes. The microporous (0.86 nm) mixed-linker MOF MIX-MIL-
oroalkane chains. The N
2
sorption results exhibit that the Pd@F15-
NU-1000 catalyst still remained porous with high surface area
2
À1
(SBET = 975.9 m g ) and pore size (2.09 nm) after six catalytic
2
+
5
3(Al) (Al(OH)[BDC]
x
[H
2
N-BDC]1Àx) containing free amine group
cycles (Fig. S19). In contrast, the Pd counterparts Pd(acac)
ported on F15-NU-1000 (Pd(acac) @F15-NU-1000) showed low
activity and selectivity of the C2 arylation product (Table 1, entry
5). The TEM of Pd(acac) @F15-NU-1000 after catalysis (Fig. S20)
2
sup-
shows high thermal stability [52]. Well-dispersed palladium
nanoparticles (Pd NPs, 3.2 nm) supported on MIX-MIL-53(Al) (Pd/
MIX-MIL-53(Al)) were easily obtained using the ion-exchange
method, which showed high activity for Heck reactions [52]. How-
ever, a poor activity was also observed in the catalyst Pd/MIX-MIL-
2
2
indicated that larger NPs (11.1 nm) were in situ formed, which
were difficult to activate the substrates effectively. Thus, according
to these results, the reaction occurs in a heterogeneous fashion,
although it cannot completely exclude that the leached active Pd
species redeposited in the support after the coupling reaction [53].
Subsequently, the scope of the reaction was evaluated for a ser-
ies of indole and aryl iodide derivatives (Tables 2 and 3). The sub-
stituents on the N atom of indoles affect dramatically the activity
5
3(Al) (the ratio of the ligands is x = 0.5) (Table 1, entry 7) due to
the hydrophilic microporous channels [52].
A hot filtration experiment was performed to confirm the
heterogeneity of the catalyst. After the reaction was performed
6
h, Pd@F15-NU-1000 catalyst was separated by hot filtration
and the filtrate was monitored under identical reaction conditions

6
Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
Table 3
a
Direct arylation of various indoles with aryl halides.
X
Pd@F15-NU-1000
H
o
R3
N
KOAc, H
3
2
O (1 ml), 100 C
N
R1
R
R1
1
X = I, Br, Cl
2
Entry
1d
R3
Product
Yield (%)b selectivityc
Trace
H
2a
2d
H
Trace
2
d
3
4
H
H
77.3% (22.3)
73.4 (19.7)
2
a
N
2
d
5
6
4-Br
78.2% (26.3)
81.2 (22.3)
2p
4-Me
N
H
2q
7
4-Me
78.3 (19.4)
N
2
r
8
9
4-CF
4-CF
3
3
76.8 (17.4)
74.1 (16.2)
CF
3
N
H
2s
CF
3
N
2t
a
Conditions: 1.0 mmol indole derivatives, 1.2 mmol aryl iodides (except entries 1 and 2 using bromides and chlorides, respectively), Pd@F15-NU-1000 (1.0 mol% Pd), KOAc
(
3 mmol), air, 24 h.
b
Isolated yield.
c
Values in brackets refer to C2/C3 selectivity of the product determined by GC–MS.
d
4
8 h, X = Br (Cl), 1.2 mmol.
(
Table 2). Interestingly, the free N–H indole obtained a high isolated
cannot be activated to obtain the C2 arylation product (2n), while
2-methylindole can afford to a small amount of C3 arylation pro-
duct (2o), which indicates that a C3-to-C2 migration of Pd may take
place during the reaction [6]. Notably, the lower activity and
selectivity of the substrate functionalized with bulky substituent
on the N atoms also support a C3-to-C2 migration of Pd mechanism
[6].
The effects of a variety of substitutedaryl halides were alsoexam-
ined (Table 3). It was not surprised that the reaction of bromoben-
zene/chlorobenzene with free N–H indole or N-methylindole
afforded to traces of C2 arylation products due to the difficult activa-
tion of the C–Br and C–Cl bonds with high bond energy (Table 3,
entries 1 and 2). These findings have been found in other Pd
yield of 2a (77.3%) and selectivity (22.3). It should be noteworthy
that the same reaction catalyzed by Pd/MIL-101 at 120 °C afforded
only 18% yield of the desired C2 product in the organic solvent [14].
Although the indole derivatives on the N atoms with electron-
withdraw groups (Ac for 2b and Boc for 2c) cannot be effectively
activated, the electron-donating groups substituted indoles gave
high yields and selectivities of the C2 arylated products (2d and
2
e). The results indicated a support for the electrophilic palladation
mechanism [6]. In addition, the electron-donating substituents on
the phenyl segment of the free N–H indole and N-methylindole
gave better yields (72.1–74.2%) and selectivities (Table 2f–g) than
the corresponding substrates containing electron-withdraw groups
(Table 2h–i). It should be noted that C–Br bond of bromoindole can
2
(OAc) -catalyzed homogeneous catalysis in organic solvents [6,7].
be tolerated, affording a high isolated yield (76.3%) and chemose-
lectivity (32.2, 2j), which allows further functionalization through
cross coupling reactions. Unfortunately, replacing the benzene ring
of indole or N-methylindole with the pyridine group resulted in no
arylation (2k–2m), which may be ascribed to that Pd NPs are easily
coordinated with the pyridine groups in water [23]. 3-Methylindole
Interestingly, the C–Br bond was tolerated when 1-bromo-4-
iodobenzene reacted with free N-H indole to obtain 2p, which can
be further functionalized (Table 3, entry 5). Furthermore, the reac-
tions proceeded extraordinarily well with a variety of substituted
iodobenzenes, with indole or N-methylindole, giving C2-arylation
products (2q–2t) in good yields and selectivities (Table 3, entries 9).

Y.-B. Huang et al. / Journal of Catalysis 333 (2016) 1–7
7
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lyst for the direct C–H arylation of indoles in water using ultrafine
Pd NPs, which embedded in the hydrophobic mesoporous pores of
NU-1000 decorated with perfluoroalkanes. The methodology was
found to be general, and provided the desired C2-arylindoles with
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[
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