Homochiral Porous Framework as a Platform for Durability Enhancement of Molecular Catalysts

Article abstract of DOI:10.1021/acs.chemmater.7b01878

Self-quenching and vulnerability of active sites are major issues posed for practical applications of highly efficient chiral organometallic catalysts. Here, we demonstrate an effective strategy to address these challenges by constructing them into homoch

Full text of DOI:10.1021/acs.chemmater.7b01878

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Article
Homochiral Porous Framework as a Platform for
Durability Enhancement of Molecular Catalysts
Qi Sun, Zhifeng Dai, Xiangju Meng, and Feng-Shou Xiao
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01878 • Publication Date (Web): 20 Jun 2017
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Chemistry of Materials
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Homochiral Porous Framework as a Platform for Durability Enhance-
ment of Molecular Catalysts
1
1
Qi Sun, Zhifeng Dai, Xiangju Meng, and Feng-Shou Xiao*
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Key Laboratory of Applied Chemistry of Zhejiang Province and Department of Chemistry, Zhejiang University
Hangzhou 310028, P.R. China.
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ABSTRACT: Self-quenching and vulnerability of active sites are major issues posed for practical applications of highly
efficient chiral organometallic catalysts. Here, we demonstrate an effective strategy to address these challenges by
constructing them into homochiral porous frameworks, which renders them with extraordinary resistance against
deactivation yet fully retains the intrinsic catalytic activities and selectivilies under heterogeneous systems.
Representatively, after partial metalation of the porous chiral phosphoramidite ligand-based frameworks (Phos-HPFs)
with Rh species, the afforded catalysts exhibit dramatically enhanced durability while maintaining the activity and
selectivity of the homogeneous counterparts in assymetric hydrogenation of olefins. The rigid framework of Phos-HPFs
can isolate the active sites, thus preventing the self-quenching to form coordinatively saturated complexes, while the
active sites surrounded by dense free chiral ligands in Phos-HPFs can inhibit them from decomposing into metallic
particles. Our work thereby highlights the advantages of HPFs for the deployment of catalysts, which offers an
opportunity for enhancing the utilization efficiency rather than merely having the benefits of easy separation and
recycling of the chiral catalysts.
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INTRODUCTION
resistance against decomposition has not been carefully
explored thus far. We envision that the construction of the
chiral ligands into rigid porous solid materials, then par-
tially post-synthetic metalated with catalytic species is
supposed to address the aforementioned concerns simul-
taneously, considering that the rigid framework can effec-
tively isolate the metal species while closely and densely
surrounded chiral ligands are beneficial for improving the
resistance to decomposition of the complexes in HPFs, as
proposed in Figure 1.
Chirality is an essential attribute of nature.1-6 The need
for enantiomerically pure insecticides and pharmaceuti-
cals drives the search for new catalysts that can perform
asymmetric reactions with high efficiency and superb en-
7
-18
antiomeric selectivity.
Homogeneous asymmetric ca-
talysis has the advantage of higher selectivity and is less
affected by the limitations of the slow transport of reac-
tants and products relative to the heterogeneous catalysis.
However, besides the high cost of chiral catalysts, their
utilization in chemical industry has been mainly impeded
by the low utilization efficiency as a result of low stabil-
1
9,20
ity.
Numerous organometallic catalysts have suffered
from deactivation caused by two pathways. Firstly, part of
the metal species is locked up in a catalytically inactive co-
ordinatively saturated form, due to the propensity of het-
eroleptic complexes to homoleptic complexes by irreversi-
ble ligand redistribution. Secondly, these metal species
are usually vulnerable under the catalytic conditions,
21
Figure 1. Schematic illustration of enhanced durability
of active sites in the HPFs, while self-quenching and de-
composing of active sites in the homogeneous catalysts.
which are thus prone to decomposing into metal col-
2
2-25
loids.
Therefore, avoidance of these unwanted trans-
formations is essential toward the realization of highly ef-
ficient asymmetric catalysis and is of particular economic
and academic importance.
Phosphoramidites constitute one of the most useful
moieties in the current scenario of asymmetric catalysis,
because of their availability and exceptional levels of ste-
reo-control as well as possibilities for the fine-tuning of
To tackle the first challenge on molecular catalysts,
there is a method of confining them into heterogeneous
systems to spatially isolate the catalytic species, thereby
3
2-36
2
6-29
the steric and electronic properties.
Nonetheless, they
prohibiting their self-quenching.
The solution to the
also suffer from the aforementioned deactivation path-
second issue is a “self-healing” strategy using free ligands
for reconstitution of the damaged catalyst, which is favor-
able to prevent the decomposition of metal sites into
3
3
ways. Herein, to test our hypotheses, we have synthe-
sized a series of homochiral phosphoramidite based po-
rous frameworks (Phos-HPFs, Figure 2) via polymeriza-
tion of vinyl functionalized ligands (Figures. S1 and S2).
The porous organic polymer platform was chosen for this
2
2,30,31
metal colloids.
Nevertheless, the catalytic system
which features non-self-quenching and enhanced
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study due to their amenability to design, porosity and Figure S7) conducted in N
Page 2 of 8
atmosphere shows the decom-
2
1
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chemical stability.
37-57
Initial studies reveal that the em-
position of MonoPhos-HPF starts from 200 C, indicating
its good thermal stability.
polyment of Phos-HPFs as modification ligands has a pro-
found effect on the catalyst durability and provides addi-
tional recyclability. While the homogeneous analogues
exhibit poor durability for asymmetric hydrogenation of
olefins, the Rh metalated Phos-HPFs demonstrate excel-
lent durability due to the inhibition of deactivation path-
ways. Our work thus underscores the advantages of utility
homochiral porous frameworks as a promising platform
for the deployment of heterogeneous chiral catalysts, and
shows great potential for allowing molecular chiral cata-
lysts to develop from novel discoveries into practical ap-
plications.
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2
2
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2
Monomer Homochiral porous framework
BET (m /g)
499
A
B
C
MonoPhos-HPF
i-PrPhos-HPF
PrPhos-HPF
545
481
Figure 2. Structures of vinyl-functionalized phospho-
ramidite monomers and textural parameters of the corre-
sponding homochiral framework.
13
31
Figure 3. (A) C MAS NMR, (B) P MAS NMR, (C) SEM
image, (D) TEM image, (E) N sorption isotherms col-
2
28
lected at 77 K of MonoPhos-HPF, and (F) CD spectra of
the MonoPhos-HPF synthesized from opposite enantio-
mers of BINOL.
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3
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RESULTS AND DISCUSSION
Materials Preparation, Physiochemical Charac-
terization, and Local Structure Analysis. The
BINOL-derived phosphoramidite monomers with differ-
ent substitutes can be easily obtained by treating vinyl-
functionalized BINOL with corresponding amine in one
or two steps as shown in Figure S2. Treatment of the vi-
nyl-functionalized BINOL with hexamethylphosphorous
triamide gave rise to the methyl substituted phospho-
ramidite ligand directly. Phosphorylation of phosphorus
The local chemical structure of the MonoPhos-HPF was
1
3
31
characterized by C and P MAS NMR spectroscopy (Fig-
13
ures. 3A and 3B). The C MAS NMR spectrum of Mono-
Phos-HPF shows four connected peaks ranged from 126.3
to 153.7 ppm and a peak at 35.3 ppm, which are assigned
to the aromatic carbon and N connected methyl groups,
respectively. The successful transformation from the vi-
nyl-functionalized phosphoramidite monomers into
highly polymerized polymer is verified by the disappear-
ance of peaks in the range of 110 to 120 ppm, which are
related to vinyl groups and the concomitant emergence of
a strong peak at 42.1 ppm, which is attributed to the pol-
trichloride (PCl ) with vinyl-functionalized BINOL fol-
3
lowing by reacting with secondary amines led to corre-
sponding substituted phosphoramidites. Dipropylamine
and diisopropylamine were chosen in this work to demon-
strate the proof-of-concept; but in principle, various types
of secondary amines can be introduced. All of the new lig-
ands were characterized by NMR spectroscopy (Figure
S3).
5
8
13
ymerized vinyl groups. The detailed assignments for C
3
1
MAS NMR signals are presented in Figure S8. P MAS
NMR spectrum gives a single peak at 151.1 ppm (Figure
3
B) in good agreement with that of the monomer (Figure
Under solvothermal conditions in THF at 100 °C, the
polymerization of the monomers in the presence of azobi-
sisobutyronitrile (AIBN), afforded the homochiral porous
frameworks of phosphoramidite ligands as white solids
S3). These results demonstrate that the phosphoramidite
moieties are stable during the polymerization process.
Morphology of the MonoPhos-HPF was examined by
SEM and TEM techniques (Figures 3C and 3D). Particles
on the order of hundreds of nanometers in size display ra-
ther rough surfaces and appear to be aggregates of much
smaller particles with dimensions of around ten nanome-
ters. Nitrogen sorption measurements collected at 77 K
(
Figures S4 and S5). The resultant frameworks are stable
in air and insoluble in water and common organic sol-
vents. X-ray diffraction studies indicate that these poly-
mers are amorphous in nature (Figure S6). As a repre-
sentative sample among the synthesized HPFs based on
phosphoramidite, the preparation of the HPF constructed
by N,N-dimethylphosphoramidite (MonoPhos-HPF) is il-
lustrated thoroughly. Thermogravimetric analysis (TGA,
(
Figure 3E) indicate that the MonoPhos-HPF is highly po-
2
rous with a BET surface area and pore volume of 499 m /g
and 0.66 cm /g, respectively. Pore size distribution shown
3
in Figure S9 suggests that the majority of the porosity is
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from 10 to 5, affording the methyl 2-acetamidoacrylate
in the micropore region. A small percentage of the poros-
1
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1
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ity is in the mesopore and macropore region, likely com-
ing from the inter-particle spacing in the MonoPhos-HPF.
conversion from 48.7% to higher than 99.5%. Similar cat-
alytic activities were reached, when the ratio of the ligand
to Rh at the range of 5 to 2, but further lowering this ratio
from 2 to 1, a slightly decreased conversion was observed
The chirality of the MonoPhos-HPF was assessed by ex-
amination of their circular dichroism spectra (CD spectra)
in solid state that had been converted to KBr pellets. Fig-
ure 3F reveals that CD spectra of the MonoPhos-HPF
made from opposite enantiomers of BINOL are mirror
images of each other, in support of their enantiomeric na-
ture. Furthermore, related absorptions are also observed
in the solid state UV-Vis spectrum of MonoPhos-HPF,
confirming that these signals are not artificial (Figure
S10). These results disclose the stability of the chiral con-
(
from >99.5% to 92.1%). During these reactions, we found
that the catalysts with ligand/Rh at 2 and 1 turned to black
from light yellow, signaling the formation of Rh colloids.
ICP-OES results also reveal that over 10 wt% Rh species
had leached into the supernatant for both of the catalysts
tested. These results indicate the high concentration of
the ligands around the metal complexes is necessary to
prevent them decomposing and leaching.
59,60
figuration towards the polymerization conditions.
To further demonstrate the importance of the existence
of dense free ligands in the vicinity of active sites on boost-
ing the catalyst stability and to exclude the possibility of
deactivation as a result of crowded active sites in the
HPFs-based catalysts with ligand/Rh at 2 and 1, we syn-
thesized a porous polymer with low MonoPhos moieties
concentration by copolymerization of divinylbenzene
(DVB) with vinyl-functionalized MonoPhos in a mass ra-
Table 1. Asymmetric hydrogenation of methyl 2-acetam-
a
idoacrylate over various catalysts.
Entry
1
2
Catalyst
Time (h)
Conv. (%)
>99.5
98.5
>99.5
>99.5
32.3
>99.5
>99.5
>99.5
41.3
>99.5
>99.5
>99.5
47.5
ee (%)
97.7
97.7
98.4
97.7
98.0
94.3
94.5
94.2
94.1
94.2
94.5
84.8
84.6
2
tio of 4/1 to afford PDVB-1/4MonoPhos (BET: 521 m /g).
Rh/MonPhos-HPF
Rh/MonPhos-HPF
Rh/MonPhos
3
3
3
b
After coordinating with Rh(COD)
2
BF , the resultant cata-
4
lyst (Rh/PDVB-1/4MonoPhos, the ratio of MonoPhos
moieties to Rh is about 1) showed an obvious catalyst de-
composition in the asymmetric hydrogenation of methyl
2-acetamidoacrylate, as demonstrated by the fact that the
catalyst turned from yellow to black. Taking into account
that Rh/MonoPhos-HPF (the ratio of MonoPhos moieties
to Rh is about 5) and Rh/PDVB-1/4MonoPhos have simi-
lar surface areas, as well as type and density of active sites,
the significant difference in the stability should be as-
cribed to the existence of dense free ligands arranged
around the catalytic species. These ligands proximate to
active sites are expected to continuously repair broken
metal-ligand bonds during catalysis and thereby enhance
3
c
4
5
6
7
8
9
Rh/MonPhos-HPF 24
c
Rh/MonPhos
Rh/PrPhos-HPF
Rh/PrPhos
Rh/PrPhos-HPF
Rh/PrPhos
24
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12
12
72
72
c
c
10 Rh/i-PrPhos-HPF
11
Rh/i-PrPhos
1
3
2 c Rh/i-PrPhos-HPF
c
1
Rh/i-PrPhos
a
Reaction conditions: methyl 2-acetamidoacrylate (143
Cl (5 mL), H (1 bar), RT, Rh(COD) BF
4.1 mg, 1 mol%). Recycles for 5 times. methyl 2-acet-
amidoacrylate (1.43 g, 10 mmol), Rh(COD) BF (2.0 mg,
.05 mol%), CH Cl (10 mL), H (10 bar), and RT.
mg, 1 mmol), CH
2
2
2
2
4
b
c
(
62
the durability of the catalyst.
2
4
Based on the above results, the catalyst with the ratio of
the ligand to Rh at 5 was chosen for further detailed stud-
ies (hereafter denoted Rh/MonoPhos-HPF). At first, we
investigated the recyclability of the catalyst, which is a
crucial performance metric for cost-effective industrial
processes. Analysis of the reaction solution after each cy-
cle by ICP-OES showed that the Rh species leaching was
below the detection limit of 0.1 ppm. In addition, the su-
pernatant from hydrogenation of methyl 2-acetam-
idoacrylate after centrifugation did not afford any addi-
tional product, highlighting the heterogeneous nature of
the catalytic process. Impressively, the catalyst could be
recycled for at least 5 times without a drop in product
yield and enantioselectivity (Figure 4A). No Rh colloids
were observed in the recycled catalyst, confirming its ex-
cellent stability (Figure S12).
0
2
2
2
Catalytic Performance Investigation. To evaluate
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4
4
the performance of Phos-HPFs as solid ligands in the en-
antioselective catalysis, the Rh-catalyzed asymmetric hy-
drogenation of prochiral olefins as models were chosen,
as it is a significant type of reaction for the production of
6
1
fine chemicals. The catalysts were prepared in situ by
combining a catalyst precursor Rh(COD)
ymeric ligands, followed by stirring under N
2
BF
4
and the pol-
for 2 h. For
2
an initial screening, we turned our attention to the hydro-
genation of dehydroamino acid esters with a focus on the
benchmark substrate methyl 2-acetamidoacrylate. Initial
hydrogenation experiments were performed at 1 bar of H
pressure in CH Cl , using 1 mol% of catalyst based on Rh
2
2
2
with a ratio of ligand to Rh at 5.0. We observed that em-
ploying MonoPhos-HPF as the modification ligand gave a
desired product yield higher than 99.5% with an enanti-
oselectivity of 97.7% after 3 h (Table 1, entry 1).
5
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5
5
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With respect to the homogeneous controls, when the ra-
tio of the MonoPhos to Rh was 2 or 1, both of the catalytic
systems afforded full methyl 2-acetamidopropanoate con-
version as well as 98.4% ee; nevertheless obvious catalyst
decomposition was observed as black precipitates formed
during the reaction; when we increased this ratio to 3 or
higher, the reactions were completely ceased. The first
scenario can be attributed to the excellent acceptor prop-
erties of phoshoramidites that renders them with strong
Encouraged by the above result, we next investigated
the effect of the free ligands on the catalytic performance
by adjusting the ratio of ligand to Rh in MonoPhos-HPF.
As shown in Figure S11, activity increased with the decre-
ment of the ratio of MonoPhos in MonoPhos-HPF to Rh
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Figure 4. Catalytic performance comparison of Rh/MonoPhos-HPF and Rh/MonoPhos. (A) Plots of conversion and ee
values of asymmetric hydrogenation of methyl 2-acetamidoacrylate in the recycle of Rh/MonoPhos-HPF under the con-
ditions of Table 1 ertry 1. (B and C) Plots of GC conversion vs time using Rh/MonoPhos-HPF and Rh/MonoPhos catalysts
with S/C ratio at 500 and 2000, respectively. (B) Reaction conditions: methyl 2-acetamidoacrylate (2.5 mmol), CH
2
Cl
2
1
(5 mL), H (1 bar), RT, Rh(COD) BF (2.0 mg), and MonoPhos-HPF (14 mg) or MonoPhos (3.5 mg). (C) Reaction condi-
2
2
4
1
16
tions: methyl 2-acetamidoacrylate (10 mmol), CH
HPF (14 mg) or MonoPhos (3.5 mg).
2
Cl
2
(10 mL), H
2
(10 bar), RT, Rh(COD)
2
BF (2.0 mg), and MonoPhos-
4
1
18
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20
2
2
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3
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4
4
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The conversion steadily increased along with the time in-
crease in the presence of Rh/MonoPhos-HPF. In striking
contrast, Rh/MonoPhos was deactivated within 8 h and
prolonging the reaction time did not lead to any fruitful
product formation (Figure 4C and Table 1, entries 4 and
binding ability to Rh species and are thus more prone to
self-quenching via multimolecular deactivation pathways
forming coordinatively saturated complexes. Meanwhile,
the yielded low-ligated or naked Rh species are inclined to
be decomposed and agglomerated into metallic Rh nano-
particles. While, in the second case, the presence of an ex-
cess of phoshoramidites, catalytically inactive tris- and
5
).
The enhanced activities of the heterogeneous catalysts
relative to their homogeneous analogues were also ob-
served over Rh/PrPhos-HPF and Rh/iPrPhos-HPF. How-
ever, they gave lower activities and ee values under iden-
tical reaction conditions in comparison with that of
Rh/MonoPhos-HPF catalyst. Changing from dimethyl- to
di-n-propylamine led to a small decrease in ee value (Ta-
ble 1, entries 6-9), whereas the use of the di-i-propyla-
mine-derived ligand led to a clear drop in selectivity (Ta-
ble 1, entries 10-13). Interestingly, when aromatic en-
amide was employed as a reagent, an opposite ee trend
was observed. Specifically, employing i-PrPhos-HPF gave
rise to the acetylated amine ee value at up to 98.0%, while
using MonoPhos-HPF and PrPhos-HPF afforded ee val-
ues of 86.3% and 94.4%, respectively. These results are
well comparable with their homogeneous counterparts
(Table 2). Given the modular nature of phoshoramidites
and the de novo synthesis of homochiral frameworks, we
can thus readily accommodate the ligands in HPFs for
specific substrates, similar to molecular ones while greatly
extending their durability as well as the additional benefit
of recyclability.
3
3
tetraphoshoramidite Rh species might be predominant.
Consequently, it did not come as a surprise that the reac-
tion ceased when the ratio of ligand to Rh was increased
to higher than 3 and the decomposition of the catalysts
occurs when the ratio of ligand to Rh ratio was lower than
2
in the homogeneous hydrogenation. In contrast, when
the MonoPhos-HPF was employed as the modification
ligand, the considerable strain of polymer chains as a re-
sult of the high cross-linking levels can not only inhibit the
formation of the coordinatively saturated complexes, even
at high ratios of ligand to Rh, but also eliminate self-
quenching pathways by site isolation. More importantly,
the presence of free ligands in the Rh/MonoPhos-HPF,
which closely and densely surround the active sites in the
homochiral porous framework, in turn enhances the re-
sistance of active sites against decomposition, thereby
preserving high activity for the catalyst over a long dura-
tion of reaction time.
To further illustrate the benefit of the use of homochiral
porous frameworks as modification ligands, we compared
the activities of Rh/MonoPhos-HPF and Rh/MonoPhos
at high S/C ratios (the molar ratio of reagent to Rh). The
difference in catalyst durability between Rh/MonoPhos-
HPF and Rh/MonoPhos was clearly manifested in the re-
action kinetics. Time-dependent GC conversion curves in-
dicate that, in spite of the required mass diffusion of reac-
tants in polymer channels, Rh/MonoPhos-HPF showed
comparable activity to that of Rh/MonoPhos (Mono-
Phos/Rh = 2) at low conversion, while afforded superior
activity at high conversion. Specifically, at 0.2 mol% cata-
lyst loading, the hydrogenation of methyl 2-acetam-
idoacrylate was completed in 10 and 18 h for Rh/Mono-
Phos-HPF and Rh/MonoPhos, respectively (Figure 4B).
Notably, the difference became larger at higher S/C ratios.
For instance, with S/C ratio at 2000, Rh/MonoPhos-HPF
is at least 5 times as active as the homogeneous control.
Table 2 Asymmetric hydrogenation of N-(1-phenylvi-
a
nyl)acetamide over various catalysts.
49
50
51
52
53
5
5
56
Entry
Catalyst
Rh/MonPhos-HPF
Rh/MonPhos
Time (h)
Conv. (%)
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
ee (%)
86.3
86.7
94.4
94.4
98.0
98.2
1
4
4
4
4
5
5
2
3
4
5
6
Rh/PrPhos-HPF
Rh/PrPhos
Rh/i-PrPhos-HPF
Rh/i-PrPhos
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a
Reaction conditions: The catalysts used in all reactions
with hydrogen gas (balloon), and the solution was stirred
at room temperature for a predetermined period of time.
After the reaction, the catalyst was taken out from the sys-
tem by centrifugation and the liquid was passed through
a short column before analysis by gas chromatography
(Kexiao gas chromatography equipped with a flame ioni-
zation detector and a Supelco γ-DEX 225 capillary col-
umn). For recycling of the catalyst, the catalyst was sepa-
1
2
3
4
5
6
7
8
9
1
1
12
13
14
15
16
17
18
19
20
2
were in-situ prepared, the ratio of ligand to the Rh for ho-
mogeneous and heterogeneous is same to that given in Ta-
ble 1, N-(1-phenylvinyl)acetamide (161 mg, 1 mmol),
Rh(COD)
and RT.
2
BF
4
(4.1 mg, 1 mol%), THF (5 mL), H (10 bar),
2
CONCLUSION
rated by centrifugation, washed with CH
der N and then used directly for the next catalytic reac-
tion without drying.
Asymmetric hydrogenation over heterogeneous cata-
lysts under high H pressure. The procedures are similar
2
2
Cl (5×5mL) un-
In summary, we have demonstrated an effective strat-
egy to enhance the durability of molecular chiral catalysts
via the construction of homochiral porous frameworks, as
exemplified by the phoshoramidites. The spatial isolation
of catalytically active species in the HPFs effectively pre-
vents the self-quenching and the dense ligand proximate
to the metal complexes generates a decomposition re-
sistant system, which therefore leads to superior perfor-
mance over homogeneous counterparts. The ready tuna-
bility of such a modular approach based on chiral ligands
and homochiral framework synthesis promises to enable
a number of chiral solid catalysts with unique reactivity
and useful enantioselective functions, which are not trivi-
ally achieved with traditional heterogeneous and homoge-
neous systems. The strategy presented herein therefore
holds a great promise to advance the practical utility of
chiral catalysis.
2
2
to those of the reactions operated under atmospheric con-
ditions except that the in situ prepared catalyst was trans-
ferred into a 100 mL stainless steel autoclave equipped
with a Teflon liner, which contained corresponding sub-
strate and a magnetic stirring bar.
Asymmetric hydrogenation over homogeneous cata-
lysts. The procedures are similar to those of using homo-
chiral porous frameworks as modification ligands except
that soluble ligands were used.
2
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ASSOCIATED CONTENT
Supporting Information. Material synthesis; Characteri-
zation details; XRD, TEM, UV-Vis, and NMR; and support-
ing figures. This material is available free of charge via the
Internet at http://pubs.acs.org.”
EXPERIMENTAL SECTION
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Material synthesis
AUTHOR INFORMATION
Corresponding Author
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Synthesis of MonoPhos-HPF. 1.0 g of A (Figure 2) was
dissolved in 10 mL of THF, followed by addition of 25 mg
of azobisisobutyronitrile (AIBN). The mixture was trans-
ferred into an autoclave at 100 C for 24 h. After being
*
fsxiao@zju.edu.cn
Notes
washed with CH
2
Cl and dried under vacuum, the poly-
2
The authors declare no competing financial interest.
mer was obtained and denoted as MonoPhos-HPF.
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Synthesis of PrPhos-HPF. 1.0 g of B (Figure 2) was dis-
solved in 10 mL of THF, followed by addition of 25 mg of
azobisisobutyronitrile (AIBN). The mixture was trans-
ferred into an autoclave at 100 C for 24 h. After being
ACKNOWLEDGMENT
We acknowledge National Natural Science Foundation of
China (21333009 and 21422306).
washed with CH
2
Cl and dried under vacuum, the poly-
2
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