Defect porous organic frameworks (dPOFs) as a platform for chiral organocatalysis

Article abstract of DOI:10.1016/j.jcat.2017.09.014

Porous organic frameworks (POFs) are emerging as an important class of porous materials. The absence of functional groups in POFs, however, renders them relatively nonspecific as porous materials for applications such as in heterogeneous chiral catalysis.

Full text of DOI:10.1016/j.jcat.2017.09.014

Journal of Catalysis 355 (2017) 131–138
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Defect porous organic frameworks (dPOFs) as a platform for chiral
organocatalysis
Zu-Jin Lin a,b, Jian Lü a, Lan Li a, Hong-Fang Li a, Rong Cao a,
⇑
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
b Department of Applied Chemistry, College of Life Science, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous organic frameworks (POFs) are emerging as an important class of porous materials. The absence of
functional groups in POFs, however, renders them relatively nonspecific as porous materials for applica-
tions such as in heterogeneous chiral catalysis. Although pre- and post-synthetic modifications have been
developed for the functionalization of POFs, the introduction of functional groups into POFs remains a
great challenge. Herein we have advanced a facile and versatile strategy to uniformly incorporate tar-
geted functional groups into defect porous organic frameworks (dPOFs) by one-pot copolymerization
of low-connected functional and primitive multi-connected building blocks. Based on this strategy, four
proline-functionalized dPOFs were readily synthesized and developed as new platforms for heteroge-
neous chiral organocatalysis. The as-prepared dPOFs show higher catalytic activity and superior enantios-
Received 20 April 2017
Revised 13 September 2017
Accepted 18 September 2017
Keywords:
Defect porous organic frameworks (dPOFs)
Heterogeneous catalysis
Aldol reaction
electivity than that of their homogeneous counterpart L-proline in the catalytic direct aldol reaction
Asymmetric catalysis
between 4-nitrobenzaldehyde and acetone, and could be reused at least five times without significant
loss of catalytic activity and enantioselectivity.
Porous organic polymers (POPs)
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
appropriate multi-connected organic building blocks for the con-
struction of the aforementioned topological nets. In this context,
Porous organic frameworks (POFs) have emerged as a new gen-
eration of porous materials. Unlike porous inorganic materials and
porous inorganic-organic materials, POFs are usually synthesized
by the self- or cross-condensation of multi-connected organic
building blocks, and are thus composed of organic moieties linked
through strong covalent bonds [1,2]. The rich variety of organic
building blocks combined with the diverse polymerization reac-
tions has led to various types of novel POFs, including crystalline
covalent organic frameworks (COFs) [3–5], hypercrosslinked poly-
mers [6], covalent triazine-based frameworks (CTFs) [7], conju-
gated microporous polymers (CMPs) [8], polymers of intrinsic
microporosity (PIMs) [9], porous aromatic frameworks (PAFs)
[10], and porous polymers networks (PPNs) [11]. From a topologi-
cal point view, the underlying topologies of the POFs that are
currently available only belong to a few types of topological nets,
including a 1D quasiregular chain, 2D hcb, sql, gra, bnn and 3D
ctn, bor, dia, aco, and lta [12]. Given that the underlying topology
plays a guiding role in the cross-linking of multi-connected organic
building blocks, POF design becomes a judicious selection of
symmetric multi-connected (3-, 4- and 6-c) organic building blocks
with special spatial conformation have been extensively applied in
the construction of POFs [2,13]. At the same time, POFs have shown
impressive applications in gas storage and separation and hetero-
geneous catalysis. The absence of functional groups such as cat-
alytic sites, however, renders POFs relatively nonspecific as
porous materials for applications such as heterogeneous catalysis
and others. A facile and viable approach to incorporate targeted
functional groups into POFs will greatly expand the applications
of POFs.
Recently, two strategies, pre- and post-synthetic modification
(PSM) methods were developed to functionalize POFs. The former
approach relies on the pre-design and synthesis of multi-
connected molecular building blocks with desired functional
groups [14,15], and the latter depends on the synthesis of starting
building blocks bearing the reactive species such as alkynes [16],
hydroxyls [17,18], azides [19] or amines [20,21] for further post-
modification. Therefore, both of them might suffer from limitations
such as requirement of drastic synthesis conditions, tedious purifi-
cation processes, and multistep synthesis during the syntheses of
pre-designed multi-connected organic building blocks. More
importantly, the pore sizes of the modified POFs are unavoidably
significantly reduced in comparison to those of the pristine POFs
⇑
Corresponding author.
E-mail address: rcao@fjirsm.ac.cn (R. Cao).
https://doi.org/10.1016/j.jcat.2017.09.014
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

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Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
due to the ligating functional groups pointing toward the pores.
This often results in significant reductions in both the accessibility
of functional groups and the efficiency of mass-transport process,
with concomitant tremendous detriment to POF performances in
such as heterogeneous catalysis. Thus, to functionalize a POF by
the introduction of targeted functional groups into pores, one
should first consider the effects of the embedded functional
groups. They have a dual role: they may endow the new functions
to a POF, but they can also block the pores. A good balance between
these dual roles in a POF would truly extend their applications in
host-guest chemistry or heterogeneous catalysis.
reagents and chemicals were purchased from Aladdin Industrial
Corporation and used directly unless stated otherwise.
Thin-layer chromatography (TLC) plates were visualized by
exposure to ultraviolet light. Flash column chromatography was
carried out with silica gel (300–400 mesh). Nuclear magnetic reso-
nance (NMR) spectra were recorded at ambient temperature on a
BRUKER AVANCE III spectrometer, where the chemical shifts (d in
ppm) were referenced to a residual proton of the solvent as stan-
dard. Fourier transform infrared (IR) spectra were recorded on Per-
kinElmer Spectrum One as KBr pellets in the range 4000–400 cmꢀ1
.
Elemental analyses (C, H, N, and S) were carried out on an Elemen-
tar Vario EL III analyzer. Thermogravimetric analyses (TGA) were
performed under N2 atmosphere on an SDT Q600 thermogravimet-
ric analyzer, with a heating rate of 10 °C minꢀ1. Powder X-ray
diffraction (PXRD) data were recorded on a Rigaku MiniFlex2
In nature, ‘‘ideal structure”, with an infinite periodic repetition
of identical groups of atoms in space doesn’t exist, even in crys-
talline materials. For example, MOFs, one of a typical crystalline
materials, many of them are known to contain defects [22]. Intrigu-
ingly, defects in MOFs can be engineered to tune the physical-
chemical properties of MOFs, opening up new avenues for their
practical application [23,24]. For instance, through co-assembling
of metal ions, ligands, and functional ligand-fragments (denoted
as ‘‘metal-ligand-fragment coassembly strategy” in the original
article), Zhou et al. have successfully introduced both the func-
tional groups and mesoporous into a microporous MOF while pre-
served the MOF parent structure [25]. Inspired by the engineering
defects in MOFs, herein, we report a facile and versatile strategy to
construct defect porous organic frameworks (dPOFs, the structure
is ‘‘defect” as compared to that of parent POFs from a topological
view) with uniform decoration by accessible targeted functional
groups through the one-fell-swoop copolymerization of an appro-
priate ratio of low-connected functional building blocks and prim-
itive multi-connected ones in the de novo synthesis. The dual roles
of functional groups can be well balanced via tuning the feeding
ratios of the initial building blocks. The new procedure for the syn-
thesis of the functional dPOFs may greatly extend the applications
of POFs since the low-connected, especially for 1- and 2- connected
functional building blocks, are often readily available or easily pre-
pared. Based on this strategy, four proline-functionalized dPOFs
(dPOF-1–4) were rationally designed and utilized as heteroge-
neous organocatalysts for the catalytic direct asymmetric aldol
reactions. Remarkably, the as-synthesized dPOFs show both higher
catalytic activity and better enantioselectivity than that of their
diffractometer working with Cu K
a radiation, and the recording
speed was 1° minꢀ1 over the 2h range of 5–50° at room tempera-
ture. Scanning electron microscopy (SEM) images were carried
out on a SU-8010. High resolution transmission electron micro-
scope (HRTEM) images were taken on a FEI TECNAI G2 F20 micro-
scope at an accelerating voltage of 220 kV. Nitrogen sorption
isotherms were measured at 77 K using a Micrometrics ASAP
2020 surface area and pore size analyzer. The Brunauer-Emmett-
Teller (BET) method was utilized to calculate the specific surface
areas. Pore size distribution data were calculated from the N2 sorp-
tion isotherms based on the DFT model in the Micrometrics ASAP
2020 software package (assuming slit pore geometry). Prior to
the measurements, the samples were degassed at 80 °C for 10 h.
The solid-state NMR spectra were measured on a Bruker AVANCE
400 spectrometer using densely packed powders of the samples
in 4 mm ZrO2 rotors spinning at 12 kHz rate. High performance liq-
uid chromatography was performed on a HITACHI L-2000 with
Daicel chiral AD-H and AS-H columns with i-PrOH/n-hexane as
the eluent.
2.2. Synthesis of dPOF-1–4
A mixture of Br2-L1-Boc (x mmol%, x = 25, 50 and 75) and
tetrakis(4-bromophenyl)methane (Br4tpm, (100-x) mmol%, total
of 1 mmol) was added to a solution of 2, 20-bipyridyl (936 mg,
6 mmol, 1.5 eq.), bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2,
1650 mg, 6 mmol, 1.5 eq.), and 1,5-cyclooctadiene (COD, 0.75 mL,
6 mmol, 1.5 eq.) in anhydrous DMF/THF (60 mL/30 mL). The mix-
ture was stirred at room temperature under a nitrogen atmosphere
for three days. Then, the mixture was cooled in an ice bath, drop-
wise added concentrated HCl solution (15 mL), and stirred for
overnight. The precipitate was collected, washed with DMF
(3 ꢁ 10 mL), water (3 ꢁ 10 mL), and methanol (3 ꢁ 10 mL), respec-
tively, and soxhlet extracted in methanol for 48 h, dried in vacuum
to give off-white solids. Deprotection of Boc groups was performed
by using 4 M HCl in methanol (10 mL) for 4 h at room temperature.
After rotary evaporation, the excess HCl were removed by triturat-
ing the residue with methanol (saturated with ammonia). The solid
was filtrated and soxhlet extracted in methanol for 24 h, and dried
in vacuum at 80 °C for overnight to obtain dPOF-1–3 (the feeding
molar ratios of building blocks between Br2-L1-Boc and Br4tpm are
1:3, 1:1 and 3:1 for dPOF-1, dPOF-2 and dPOF-3, respectively) as
off-white solids. The synthetic procedure of dPOF-4 was the same
as that for dPOF-1 except that Br2-L1-Boc (0.25 mmol) was
replaced by Br-L2-Boc (0.25 mmol).
homogeneous counterpart L-proline in the direct asymmetric aldol
reaction between p-nitrobenzaldehyde and acetone, and these cat-
alysts could be reused for at least five times in a role without sig-
nificant loss of catalytic activity.
2. Materials and methods
2.1. General
N,N-dimethylformamide (DMF), tetrahydrofuran (THF), 1,5-
cyclooctadiene (COD), and acetone were degassed before use. All
reactions involving moisture sensitive reactants were performed
under a nitrogen atmosphere using oven dried glassware. Anhy-
drous methanol, anhydrous dichloromethane (DCM), Boc-L-
proline, 4-dimethylaminopyridine (DAMP), N,N0-dicyclohexylcarbo
diimide (DCC), and trifluoroacetic acid (TFA) were purchased from
J&K Scientific Ltd. 2,20-bipyridyl, and bis(1,5-cyclooctadiene)nickel
(0) [Ni(COD)2] were purchased from Alfa Aesar. Tetrakis(4-
bromophenyl)methane (Br4tpm) and PAF-1 was synthesized
according to literature [10,26]. The synthetic procedures of (S)-
tert-butyl-2-(2,5-dibromophenylsulfonylcarbamoyl)pyrrolidine-1-
carboxylate (Br2-L1-Boc), (S)-N-(2,5-dibromophenylsulfonyl)pyrro
lidine-2-carboxamide (Br2-L1) and (S)-tert-butyl 2-(2-bromophe
nylsulfonylcarbamoyl)pyrrolidine-1-carboxylate (Br-L2-Boc) were
shown in supporting information in detail. All other solvents,
2.3. Catalytic test
Typical procedure of the asymmetric aldol reaction is described
as following. To a mixture of the anhydrous solvent (4 mL) and the
ketone donor (1 mL) was added the aldehyde (0.05 mmol) followed

Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
133
by the catalyst (10 mol%), and the resulting mixture was stirred at
room temperature for 24 h. The mixture was centrifuged, washed
with ethyl acetate for three times, and the combined organic
supernatant was dried over anhydrous MgSO4, concentrated in
rotary evaporation, and dried on vacuum overnight to get the
crude product. The pure aldol products were obtained by flashed
chromatography (200–300 mesh silica gel, mixture of ethyl acet-
ate/petroleum ether = 1:4). The conversion and the diastereoselec-
tivity were determined by 1H NMR analysis of the crude aldol
product. The enantiomeric excess value was determined by high
performance liquid chromatography.
With the default diamondoid structure, PAF-1 has an ultrahigh sur-
face area (SBET = 5600 m2 gꢀ1) and high physicochemical stability.
To date, surface function of PAF-1 can be achieved by pre-
modification of the tetrahedral building block (Fig. 1a). For exam-
ple, Farha, Hupp, Nguyen and their co-authors accessed five func-
tionalized PAFs (i.e. PAF-1–CH3, ACH2OH, ACH2NH2, ACH2-
phthalimide, and ACH2N@CMe2) through separate coupling of
the corresponding functionalized Br4tpm building blocks [27].
The syntheses of the functionalized Br4tpm building blocks, how-
ever, are tedious and exhaustive; more importantly, it is challeng-
ing to introduce functional groups into the 4-connected Br4tpm
building blocks. Through PSM modification, sulfonic acid, hydroxyl,
alkyl and amino groups can be grafted in the pores of PAF-1
(Fig. 1b) [28,29]. Nevertheless, the pore sizes significantly
decreased. For example, the pore sizes decreased from 13.6 Å for
PPN-6 to 7.5 Å for sulfated framework (PPN-6-SO3H) and 6.1 Å
for lithium sulfated framework (PPN-6-SO3Li) [26]. The PSM pro-
cess was largely limited since large reagents cannot penetrate into
the pores. In addition, the extent that a PAF-1 particle can be post-
synthetically modified largely depends on the mass transportation
of the reagents in the whole morphological structures, which may
result to the uneven distribution of the functional groups since the
outer ‘‘shell” would be more highly functionalized than the inner
‘‘core”.
2.4. Recycle use of dPOFs
The catalysts (dPOF-1 or dPOF-4) were recovered via cen-
trifuge, washed with ethyl acetate for three times and simply dried
on vacuum at 60 °C overnight before reuse.
3. Results and discussion
3.1. Design of dPOFs
To elucidate our inspiration, herein we take one of the classical
POF, PAF-1 (also named PPN-6 by Zhou group) [10,26], as a model
POF for explanation in details. PAF-1 was synthesized by coupling
of 4-connected tetrahedral rigid building block Br4tpm (Fig. 1).
To introduce the accessible functional groups such as active cat-
alytic sites, dPOFs could be synthesized by the copolymerization of
Fig. 1. Strategies for introducing functional groups into PAF-1; (a) the pre-modification process. (b) Post-synthetic modifications (PSM). dPOFs with targeted functional
groups synthesized by one-pot copolymerization of the primitive tetrahedral building blocks and the low-connected such as (c) 1-connected or (d) 2-connected functional
building blocks.

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Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
an appropriate amount of 1- or 2- connected functional building
blocks with 4-connected tetrahedral building block Br4tpm in the
de novo synthesis.
2.1, 1.2 and 7.8 tpm moieties in dPOF-1, dPOF-2, dPOF-3, and
dPOF-4, respectively.
The default diamondoid structure can be partially formed and
located around the functional building blocks in the resulting
dPOFs, which provides widely open and interconnected pores that
efficiently prevent the formation of ‘‘dead space” and facilitate the
accessible to the functional groups (Fig. 1c and d).
3.3. Structural characterization
No residual bromine was observed in the as-prepared dPOFs
based upon elemental analyses, indicating the efficiency of the
Yamamoto cross-coupling and the completion of the reactions.
The successful incorporation of L1 or L2 units was further con-
firmed by the N/S elemental ratios calculated from the EA results,
where the experimental values are 2.50, 2.12, 2.01 and 2.07 for
dPOF-1, dPOF-2, dPOF-3, and dPOF-4 (Table S1), respectively.
These values are slightly larger than the idealized value calculated
from the L1 or L2 moieties (i.e., 2), presumably because the residue
of a trace amount of 2,20-bipyridine is occupying in the pores of
dPOFs. To monitor the reaction procedures, the as-synthesized
dPOFs as well as initial building blocks were studied by Fourier
transform infrared (FT-IR) spectroscopy. As shown in Figs. S2 and
S3, the disappearance of the two diagnostic C-Br stretching vibra-
tion absorption bands (532 cmꢀ1 and 509 cmꢀ1 for Br4tpm; 513
and 495 cmꢀ1 for Br2-L1-Boc; 554 cmꢀ1 and 541 cmꢀ1 for Br-L2-
Boc) in the as-prepared dPOF-1–4 preliminarily demonstrated that
most of the bromide functional group in the starting materials had
been consumed by phenyl-phenyl coupling. To further reveal the
local structures of the obtained polymers, solid-state 13C cross-
polarization magic-angle spinning nuclear magnetic resonance
(CP/MAS NMR) were carried out (Fig. 2). The NMR spectra of the
dPOFs were dominated by five pronounced signals at approxi-
mately 145, 139, 131, 126, and 64 ppm, which are in accordance
with that of PAF-1 where the corresponding signals are 146, 140,
131, 125, and 64 ppm [10]; therefore, these five signals can be
3.2. Synthetic procedure
As shown in Scheme 1a, the syntheses of proline-functionalized
dPOF-1–3 were carried out by the nickel(0)-catalyzed Yamamoto-
type Ullmann cross-coupling reaction of the proline-functionalized
2-connected building block Br2-L1-Boc and the 4-connected tetra-
hedral building blocks Br4tpm. Treating with hydrogen chloride in
methanol (4 M) and subsequently treating with saturated ammo-
nium in methanol finally obtained targeted proline-
functionalized polymers dPOF-1–3 (the feeding molar ratios of
building blocks between Br2-L1-Boc and Br4tpm were 1:3, 1:1
and 3:1 for dPOF-1, dPOF-2 and dPOF-3, respectively). The syn-
thetic procedure of dPOP-4 was the same as that for dPOP-1
except that the 2-connectd building block Br2-L1-Boc was replaced
by the 1-connected building block Br-L2-Boc (Scheme 1b). dPOF-
1–4 are insoluble in water and common organic solvents and dis-
play an excellent chemical stability since they are isolated from
the harsh reaction conditions. The components of dPOF-1–4 were
determined by EA, and the real contents of the L1 or L2 moieties
were calculated from the amount of sulfur. The results suggest that
every L1 or L2 moiety was inter-connected to approximately 4.4,
Scheme 1. The syntheses of proline-functionalized (a) dPOF-1–3 and dPOF-4.

Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
135
Fig. 2. 13C solid-state cross-polarization magic angle spinning NMR spectra of
dPOF-1–4; signals from tpm moieties (*) and proline-functionalized L1 or L2
moieties (ꢂ).
unambiguously assigned to the carbon atoms from the tpm moi-
eties. The signal at approximately 173 ppm was assigned to the
carbon atom in the carbonyl group, and the signals at approxi-
mately 47, 32 and 23 ppm were assigned to the pyrrole rings in
the proline-species; these signals are strengthened by the content
of L1 moieties increasing from dPOF-1 to dPOF-3. Although the sig-
nals of some carbon atoms from the L1 moieties, such as the carbon
atoms from the phenyl, could not be exactly assigned, the NMR
spectra clearly indicated the tpm moieties and the functional L1
or L2 moieties are uniformly distributed in the skeleton of these
dPOFs. To explore the thermal stability of dPOFs, thermogravimet-
ric analysis (TGA) was performed under nitrogen atmosphere. Only
a slight weight loss before 200 °C were observed, revealing that the
high thermal stability of these dPOFs (Fig. S4). In order to probe the
long-range structure of dPOFs, powder X-ray diffraction (PXRD)
was performed (Fig. S5). The PXRD patterns showed that the inten-
sity of the broad peaks at ca. 10, 16, and 22 deg are obviously
weaken from dPOF-1 to dPOF-3, which indicates that the drop of
long-range order of the structures with the increase of the L1 moi-
eties. Scanning electron microscopy (SEM) images of dPOF-1–4
revealed that only spherical aggregates of small particles with sizes
of 300–500 nm are observed (Fig. S6), which to some extent indi-
cates the phase purity of these materials. Energy-dispersive X-ray
spectroscopy (EDS) showed that no bromine residues remained
in dPOF-1–4 (Fig. S8), and the EDS elemental mapping displayed
a homogeneous distribution of nitrogen and sulfur elements in
the dPOFs (Fig. S9), both of which further indicate the good
cross-coupling between the two initial building blocks.
Fig. 3. N2 sorption isotherms of dPOF-1–4 and (b) the pore size distribution of
dPOF-1–4 as well as PAF-1.
for dPOP-4. The pore size distribution calculated from nonlinear
density functional theory (NLDFT) shows that the as-synthesized
PAF-1 displays a narrow distribution at approximate 11.8, 12.7,
13.6, and 14.8 Å, in accordance with that of previously reported
[26]. The pore size distribution of dPOF-1–4, however, is distinct
from that of PAF-1 (Fig. 3b). On the one hand, dPOF-1–4 exhibit
fractional pores as same as PAF-1 (i.e., 11.8, 12.7, 13.6, and
14.8 Å), indicating the presence of the default diamondoid struc-
ture in dPOFs similar to that of PAF-1; on the other hand, some
small pores at approximately 8.0, 6.8, 5.9, and 5.0 Å for dPOF-1–
3 and 8.2, 7.3, 6.8, 5.9, and 5.0 Å for dPOP-4 are also observed.
Compared with PAF-1, the presence of small pores may result from
the protrusion of the flexible proline-based species in the L1 or L2
moieties into the partial pores of the structure. Importantly, an
appropriate feeding molar ratio of functional building blocks to
primitive building blocks is vitally important to prepare targeted
functional dPOFs since surface area, total pore volume, and amount
of characteristic diamondoid structure progressively decrease with
the increase in low-connected functional moieties.
3.4. Gas sorption measurements
To investigate the porosity of the dPOFs, the as-prepared sam-
ples of dPOF-1–4 as well as PAF-1 were activated under dynamic
vacuum at 80 °C overnight and the N2 sorption isotherms were col-
lected at 77 K. As shown in Fig. 3a, the resulting isotherms show
type I adsorption isotherms featured by a sharp uptake at the
low-pressure region. The Brunauer-Emmett-Teller (BET) surface
areas obtained from experimental data are 3087, 1484, 886, 326
and 1741 m2 gꢀ1 for PAF-1, dPOF-1, dPOF-2, dPOF-3, and dPOF-
4, respectively. The total pore volume obtained using the t-plot
method is 2.66 cm3 gꢀ1 for PAF-1, 0.85 cm3 gꢀ1 for dPOF-1,
0.43 cm3 gꢀ1 for dPOF-2, 0.20 cm3 gꢀ1 for dPOF-3 and 1.36 cm3 gꢀ1
3.5. Heterogeneous asymmetric organocatalysis
Recently, chiral organocatalysts have rapidly developed into a
vital tool for the asymmetric synthesis of chiral compounds [31].
However, high loading of organocatalysts, together with laborious
separation processes in homogeneous systems has generally
obstructed their practical applications. One of the most promising

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Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
strategies, the so-called ‘‘immobilization” of organocatalysts, may
address these problems by facilitating product separation and cat-
alyst reuse [32]. Various supports such as linear polymers, poly-
styrene spheres (PS), dendrimers, and silica have been applied in
the immobilization of chiral organocatalysts; nevertheless, most
of these catalysts often suffer from low catalytic activities as a
result of inefficient access to the catalytic sites [33]. To overcome
this issue, porous metal-organic framework (MOF)-based
organocatalysts have been developed [34–40]; but their low chem-
ical stabilities limited their applications as chiral organocatalysts.
With high surface areas and excellent physicochemical stability,
POFs have been proposed as a very promising supports in the
immobilization of organocatalysts [41,42]; however, due to the
lack of facile methods to integrate the organocatalytic sites into
the skeletons of a POF, only several POF-based organocatalysts
have been reported so far [43–50]. We envisage that dPOFs may
serve as a platform for heterogeneous chiral organocatalysis since
the active catalytic sites can be readily incorporated into dPOFs.
dPOF-1 was reduced from 10 to 5 mol%, the conversation largely
decreased from 99% to 56% and the isolated yield greatly
decreased from 83% to 43%, while the ee value only slightly
reduce from 83.1% to 79.4% (Table 1, Entry 8). Further experi-
ments indicated that the enantioselectivity is almost independent
to the amount of dPOF-1 and reaction time (Table 1, Entries 5, 8–
10). Control experiment demonstrated that the parent framework
itself (PAF-1) is completely inactive, indicating that the proline-
based L1 species are responsible for the observed catalytic activ-
ity (Table 1, Entry 11). Noticeably, the removal of dPOF-1 catalyst
by centrifugation after 5 h almost completely shut down the
reaction, affording only 3.6% additional conversion and almost
the same isolated yield and ee value after stirring for another
19 h, indicating that no leaching of catalytically active species
and the intrinsic heterogeneous nature of catalyst dPOF-1
(Table 1, Entry 12). The heterogeneous catalyst dPOF-1 has sig-
nificantly higher catalytic activity but slightly lower ee value
than its homogeneous acylsulfonamide catalyst ((S)-N-(2,5-dibro
mophenylsulfonyl)pyrrolidine-2-carboxamide, Br2-L1) (Table 1,
Entry 13); the higher activity may be attributed to the uniform
distribution of the proline-based L1 species towards the pores
of dPOF-1, while the slightly lower enantioselectivity presumably
originates from the restricted movement of the substrates on the
small pores of dPOF-1. Remarkably, the catalysts dPOF-1, dPOF-2
and dPOF-4 showed both higher activity and better enantioselec-
L
-Proline and its derivatives are well-known asymmetric
organocatalysts, accelerating a variety of enantioselective organic
reactions such as aldol reactions [30]. To evaluate the catalytic
activity of dPOF-1–3, we selected the direct asymmetric aldol
reaction between p-nitrobenzaldehyde and acetone as a mode
reaction. Solvent screening showed that dPOF-1 possesses the
best catalytic performance in neat acetone (Table 1, Entries 1–
5); with a 10 mol% dPOF-1 loading, dPOF-1 achieved over 99%
conversion in 24 h with a maximum isolated yield of 83% and a
ee value of 83.1%. Under such conditions, the catalytic perfor-
mances of dPOF-2–3 were also assessed (Table 1, Entries 5–6).
Among them, dPOF-2 resulted in a negligible difference in both
conversion and isolated yield but modest enantioselectivity in
comparison with dPOF-1, whereas dPOF-3 exhibited the lowest
catalytic activity and the modest enantioselectivity (Table 1,
Entry 7). The catalytic activity and enantioselectivity decreased
with an increasing density of the proline-based L1 species; pre-
sumably, because of the reduction in surface area and the
increase in narrow pore ratios from dPOF-1 to dPOF-3, decreas-
ing the accessibility of the catalytic sites and lowering the effi-
ciency of the mass transport. In addition, the effect of the
catalyst loading was examined; when the amount of catalyst
tivity than that of another homogeneous
L-proline counterpart,
even when 20 mol% -proline was used under the optimized reac-
L
tion conditions previously reported by List et al. (Table 1, Entries
14–16) [30].
Using the optimized reaction conditions, the substrate scope of
the direct asymmetric aldol reaction of ketones with nitroben-
zaldehyde was investigated, and the results are summarized in
Table 2. As depicted in Table 2, the as-prepared dPOF-1 could well
catalyze the direct asymmetric aldol reactions between nitroben-
zaldehyde and acetone, cyclopentanone and cyclohexanone with
excellent isolated yields (71–83%) and enantioselectivity (ee 66–
85%). When butanone was employed as the ketone, only modest
isolated yields (30–34%) of the corresponding 1-hydroxy-1-(nitro
phenyl)pentan-3-one were obtained because another condensa-
tion product, 4-hydroxy-3-methyl-4-(nitrophenyl)butan-2-one,
Table 1
Optimization of the reaction of 4-nitrobenzaldehyde with acetone.a
Entry
Solvent
Catalyst
Catalyst loading (mol%)
Time (h)
Conversionb (%)
Yieldc (%)
eed (%)
1
2
3
4
5
6
7
8
DMSO
dPOF-1
dPOF-1
dPOF-1
dPOF-1
dPOF-1
dPOF-2
dPOF-3
dPOF-1
dPOF-1
dPOF-1
PAF-1
10
10
10
10
10
10
10
5
10
10
10
10
10
10
24
24
24
24
24
24
24
24
5
15
24
5
24
24
55
39
52
55
>99
98
89
43
23
42
47
83
85
64
43
44
62
0
86.6
59.1
74.7
79.2
83.1
77.8
74.2
79.4
79.8
80.3
–
Methanol
Acetonitrile
DCM
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
56
9
59.8
83.7
0
63.4
>99
>99
10
11
12e
13
14
dPOF-1
Br2-L1
44
35
40
79.0
92.7
47.9
L
-proline
-proline
15
DMF
20
24
>99
65
68f
78
74.5
76f
83.6
L
16
Acetone
dPOF-4
10
24
>99
a
Typically, the reactions were performed with 4-nitrobenzaldehyde (76 mg, 0.5 mmol), acetone (1 mL) and catalyst (1–10 mol%) in corresponding solvents (4 mL, 80 vol%)
at room temperature.
b
Determined by 1H NMR of the crude product.
Isolated yields after column chromatography.
Determined by HPLC on chiral columns.
The supernatant was stirred in inert atmosphere for another 19 h.
c
d
e
f
Reported by the Ref. [30].

Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
137
Table 2
Results of reaction of various ketones with nitrobenzaldehyde.a
OH
O
O
O
R1
10 mmol%
R1
NO2
NO2
+
R2
dPOF-1
R2
Entry
Substrate
R1
R2
Isolated yieldb (%)
syn/antic
eed (%)
1
2
3
4
5
6
7
8
9
4-NO2
3-NO2
2-NO2
4-NO2
2-NO2
4-NO2
3-NO2
2-NO2
4-NO2
Me
Me
Me
Et
H
H
H
H
H
83
75
73
30
34
74
73
74
71
–
–
–
–
83.1
84.3
83.1
72.6
85.5
66.4e
84.7e
84.8e
74.9e
Et
–
A(CH2)3A
A(CH2)3A
A(CH2)3A
A(CH2)4A
49/51
48/52
37/63
27/73
a
b
c
Typically, the reactions were performed with catalyst (dPOF-1, 10 mol%) and corresponding aldehyde (0.5 mmol) in neat ketone (5 mL) for 24 h at room temperature.
Combined yields of isolated diastereomers.
Determined by NMR spectra of the isolated products.
Determined by chiral-phase HPLC analysis.
The anti product.
d
e
was competitively generated. The porous materials based on
heterogeneous organocatalysts for the aldol reaction reported to
date were summarized in Table S4. Among them, dPOF-1 is one
of the heterogeneous chiral organocatalysts with the highest cat-
alytic activity and best enantioselectivity.
The recyclability of the dPOFs catalysts was assessed by using
dPOF-1 and dPOF-4 as model organocatalysts and employing the
direct aldol reaction of 4-nitrobenzaldehyde with acetone as a
model reaction. dPOF-1 and dPOF-4 can be readily isolated from
the reaction suspension by simple centrifugation. The recycled
solids could be reused at least five runs with the retention of their
catalytic activities and only a slight decrease in enantioselectivi-
ties, which could be attributed to the covalent anchoring of catalyt-
ically active sites as well as framework robustness (Fig. 4, and
Tables S2 and S3). Only a slightly reduction in surface areas and
no obvious change of the pore sizes were observed for recycled
dPOF-1 and dPOF-4, even after five catalytic runs (Figs. S10–
S13). Moreover, SEM images showed that the recycled dPOF-1
and dPOF-4 could well retain their spherical shapes (Fig. S14).
These results show that dPOF-1 and dPOF-4 could well retain their
structural integrity after five catalytic runs.
4. Conclusion
In summary, we developed a facile and versatile strategy to con-
struct dPOFs facilitating to immobilization of targeted functional
groups by copolymerization of an appropriate amount of low-
connected such as 1- or 2-connected functional and primitive
multi-connected building blocks in the de novo synthesis. Based on
this strategy, dPOF-1–4 ligated with proline-functionalized groups
were readily built. The as-synthesized dPOFs showed high chemical
stability, high porosity and excellent guest-molecule accessibility
due to the present of diamondoid structure neighboring the func-
tional groups in their skeletons. Serving as a heterogeneous chiral
organocatalyst, dPOF-1 shows outstanding performances in the cat-
alytic direct asymmetric aldol reactions. Remarkably, dPOF-1,
dPOF-2, and dPOF-4 display higher catalytic activity and superior
enantioselectivity than those of their homogeneous L-proline coun-
terpart in the direct asymmetric aldol reactions between 4-
nitrobenzaldehyde and acetone, and these heterogeneous dPOFs-
based chiral organocatalysts could be reused for at least five times
without significant loss of catalytic activity and enantioselectivity.
Our strategy may thus facilitate the design of targeted functional
dPOFs combining of permanent porosity and functional groups
accessibility and would greatly extend the applications of POFs.
Fig. 4. The recycled experiment of (a) dPOF-1 and (b) dPOF-4.

138
Z.-J. Lin et al. / Journal of Catalysis 355 (2017) 131–138
[18] N. Huang, X. Chen, R. Krishna, D. Jiang, Angew. Chem., Int. Ed. 54 (2015) 2986.
Acknowledgements
[19] A. Nagai, Z. Guo, X. Feng, S. Jin, X. Chen, X. Ding, D. Jiang, Nat. Commun. 2
(2011) 536.
This work was financially supported by the 973 Program
(2014CB845605), NSFC (21521061, 21331006, 21520102001,
21303205 and 21571177), the ‘‘Strategic Priority Research Pro-
gram” of the Chinese Academy of Sciences (XDA09030102), Fujian
Agriculture and Forestry University (118360020), Outstanding
Youth Research Training Program of Fujian Agriculture and For-
estry University (XJQ201616), and the State Key Laboratory of
Structural Chemistry, Fujian Institute of Research on the Structure
of Matter, Chinese Academy of Sciences (20170028).
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