Chiral porous organic frameworks for asymmetric heterogeneous catalysis and gas chromatographic separation

Article abstract of DOI:10.1039/c4cc07648f

Three chiral robust diene-based porous organic frameworks (POFs) are prepared. POF-1 is shown to be an efficient heterogeneous catalyst after metallation for asymmetric conjugation addition with up to 93% ee, and it can also function as a new chiral stationary phase for gas chromatographic separation of racemates. This journal is

Full text of DOI:10.1039/c4cc07648f

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Chiral porous organic frameworks for asymmetric
heterogeneous catalysis and gas chromatographic
separation†
Cite this: Chem. Commun., 2014,
50, 14949
Received 28th September 2014,
Accepted 13th October 2014
Jinqiao Dong,a Yan Liu*a and Yong Cui*ab
DOI: 10.1039/c4cc07648f
www.rsc.org/chemcomm
Three chiral robust diene-based porous organic frameworks (POFs) porous POFs based on enantiopure diene ligands and show that
are prepared. POF-1 is shown to be an efficient heterogeneous the solids could be utilized as recyclable heterogeneous catalysts,
catalyst after metallation for asymmetric conjugation addition with after being post-modified with Rh(I) ions, for asymmetric
up to 93% ee, and it can also function as a new chiral stationary 1,4-addition reactions with up to 93% ee and as a new chiral
phase for gas chromatographic separation of racemates.
stationary phase (CSP) for gas chromatographic separation of
racemic molecules.
Porous organic frameworks (POFs), including covalent organic
As shown in Scheme 1, POFs 1–3 were synthesized by the
frameworks (COFs), are constructed from well-designed organic Suzuki–Miyaura cross coupling polycondensation of bis-triflate
precursors through coupling or condensation reactions,1,2 and (1R,4R)-bicyclo[2.2.1]hepta-2,5-diene and three connectors in
have emerged as a new class of porous materials for diverse the presence of a Pd catalyst under alkaline conditions. Such
applications.3–5 In particular, just like their metal–organic frame- polycondensation reactions lead to three inherent porous poly-
work (MOF) counterparts,6 POFs may serve as a solid platform for mers with built-in chiral diene platforms. We examined the
incorporating molecular catalytic modules into heterogeneous cross-coupling polycondensation reaction using different con-
catalyst systems by taking advantage of their permanent porosity ditions, including solvent, base and Pd catalyst, with the aim of
and the ability to tune their compositions at the molecular achieving maximal surface areas for the resulting materials.
level.1d,e,5 POFs, despite the amorphous nature for most of them, As optimal conditions, polycondensation in 1,4-dioxane/H2O
are unique because of their high thermal and chemical stability (4 : 1 v/v) in the presence of K2CO3 as base and Pd(dppf)Cl2 as
in comparison with MOFs. Although lots of POFs have been catalyst for 72 h allows for the preparation of the polymers with
reported, only a small fraction of them are chiral species and good surface areas. After repeated rinsing with water, THF,
versions featuring accessible functional sites that induced enantio- EtOH, CH2Cl2 and acetone, POFs 1–3 were rigorously washed by
selectivity remain scarce.1,7 In fact, only several asymmetric POF
catalysts have been generated either via a post-synthesis modifica-
tion strategy or by direct incorporation of catalytically competent
bridging units into the frameworks.7
As a recently emerging ligand class, chiral olefins have been
widely utilized as steering ligands for transition-metal-catalyzed
asymmetric reactions.8 In particular, C2-symmetric chiral diene
ligands bearing internal olefins and rigidly bicyclic frameworks
confer higher activities and enantioselectivities than phosphorus
ligands in rhodium- and iridium-catalyzed asymmetric addition
reactions.9 We report here the synthesis of three robust chiral
a School of Chemistry and Chemical Engineering and State Key Laboratory of Metal
Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail: yongcui@sjtu.edu.cn, liuy@sjtu.edu.cn; Fax: +86 21-5474-1297
b Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin 300072, China
† Electronic supplementary information (ESI) available: Experimental details and
Scheme 1 Synthesis of chiral POFs 1–3.
spectroscopic data. See DOI: 10.1039/c4cc07648f
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Fig. 1 N2 adsorption isotherms (filled symbols) and desorption isotherms
(open symbols) of 1–3 at 77 K.
Soxhlet extraction for 24 h with THF, EtOH, CH2Cl2 and acetone
as solvents, respectively, to remove any entrapped molecules
and impurities and then dried under vacuum overnight.
The permanent porosity of 1–3 was demonstrated by N2
adsorption measurements at 77 K (Fig. 1). All of them exhibit a
type I sorption behavior with Brunauer–Emmett–Teller (BET)
surface areas of 471, 312 and 252 m2 gÀ1, respectively, with total
pore volumes of 0.393, 0.304 and 0.295 cm3 gÀ1. The pore size
distribution calculated using nonlocal density functional theory
reveals three pore size distributions at around 7.1, 6.9 and 6.8 Å.
The 13C CP/MS NMR spectrum of 1 displayed four peaks at 125.31,
121.98, 46.17, 30.36 ppm assignable to chiral diene linkages and
signals at 64.63, 145.50, 130.56, 128.92 and 128.08 ppm owing to
the tetraphenylmethane units (Fig. 2). 13C CP/MS NMR spectra of
2 and 3 also suggested the successful incorporation of diene units
in their polymers (Fig. S6, ESI†).
These POFs are insoluble in water, hydrochloric acid (6 M),
sodium hydroxide (8 M) and common organic solvents that were
tested, consistent with their cross-linked networks. TGA show
that the decomposition of these framework starts at B550 1C.
The PXRD patterns indicated that these POFs are amorphous in
nature. Field emission scanning electron microscopy (FE-SEM)
revealed that they consist of spherical particles and display
rough surfaces and appear to be aggregates of much smaller
particles (Fig. 3). The average particle sizes of 1–3 were about 40,
400 and 550 nm, respectively. Interestingly, the microporous
structure of 1 can even be directly visualized by high resolution
Fig. 3 (a) SEM and (b) TEM images of 1, SEM images of (c) 1-Rh and (d)
1-Rh after five cycles and SEM images of (e) 2 and (f) 3.
tunneling electron microscopy (HR-TEM), as shown in Fig. 3b.
One clear feature of the porous texture is that the micropores are
present homogenously and are similar in size.
We have utilized 1–3 for heterogeneous asymmetric catalysis
by taking advantage of the accessible chiral diene groups.
[Rh(C2H4)2Cl]2 can react with the dienes in the polymers or
its analogues to afford Lewis acidic [(diene)Rh(C2H4)]+ species,9
which are active catalysts for the addition of carbon-based
nucleophiles to a,b-unsaturated ketones and esters. Post-
synthetic metalation of the POFs was performed by treating
with 0.5 equiv. of [Rh(C2H4)2Cl]2 (relative to the diene equi-
valents in polymers) in 1,4-dioxane at room temperature to
afford 1-, 2- and 3-Rh. Inductively coupled plasma optical emis-
sion spectrometer (ICP-OES) analyses of the digested metalated
polymer gave the Rh loading of 20.5, 19.0 and 16.5 wt% for
1-, 2- and 3-Rh, respectively. 1-Rh gave a decreased BET surface
area (186.0 m2 À1) compared with 1, whereas 2-Rh and 3-Rh
g
only gave surface sorption. After optimization of reaction
conditions, 1-Rh was found to be an active catalyst for the
1,4-addition of arylboronic acid to 2-cyclohexenone in 1,4-dioxane/
H2O (10 : 1 v/v) in the presence of KOH at 50 1C. Specifically, 4 mol%
loading of 1-Rh catalyzes the addition of phenylboronic acid to
2-cyclohexenone to give the 1,4-addition product (6a) in nearly
quantitative conversion with 93% isolated yield and 91% ee
in 8 h. Under similar conditions, the yield and ee observed for
1-Rh are comparable with those obtained using the homo-
geneous analogue (94% yield and 96% ee), although the hetero-
geneous reaction required longer time because of slow-mass
diffusion (8 h vs. 1 h).9 Notably, relatively few heterogeneous
catalysts are active in asymmetric conjugate 1,4-addition reac-
tions.6e,10 Very recently, Lin et al. reported the use of a Rh-BINAP
MOF for the catalytic addition of phenylboronic acid to
2-cyclohexenone, affording the product in 99% ee but with
moderate activity (80% yield in 20 h).10
Fig. 2 The solid-state 13C CP/MS NMR spectrum of 1 (signals with * are
sidebands).
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With the optimal reaction conditions in hand, the substrate substrate 1-pyrenylboronic acid was subjected to the reaction,
scope of the reaction was examined. A range of arylboronic almost no desired product 6l was detected after 12 h, whereas
acids could react with 2-cyclohexenone to produce the corre- the homogeneous diene catalyst could still give the product in
sponding products (6a–6j) in 88–93% yields with 71–93% ee. 96% yield. This result may suggest that this bulky substrate
Arylboronic acids with an electron-donating methoxy group, cannot access the catalytic sites in the framework due to its large
regardless of its relative position (ortho, meta or para), gave the diameter. It is thus likely that the catalytic reactions are hetero-
products 6b–d in B90% yield with 81–87% ee. Whereas aryl- geneous and may occur within the POF.
boronic acids with electron-withdrawing groups such as –F, –Cl
All of the products obtained with (R,R)-1-Rh have the absolute
and –CO2Me and –CF3 gave the products 6f–j in B90% yield R configuration, consistent with that observed in the homo-
with 71–93% ee. The highest enantioselectivity of 93% ee was genous control system. This suggests that the POF catalyst relies
attained with the phenylboronic acid bearing a 4-trifluoromethyl on the intrinsic chiral environments of the active diene sites to
group. Besides, 1-Rh was also active in the addition of phenyl- exert stereocontrol. The walls of the pores in the solid catalyst
boronic acid to linear enones, producing 6m and 6n in 84% yield may provide preferential secondary interactions between the sub-
with 80% ee and 55% yield with 90% ee, respectively. Hetero- strate and the framework, leading to shape- and size-selectivities
cyclic enones were also converted into the addition products, that are not achievable in homogeneous systems.
producing 6o and 6p in 73% yield with 74% and 78% yield with
The supernatant from the reaction of phenylboronic acid to
40% ee, respectively. We also examined the catalytic activities of 2-cyclohexenone after filtration through a regular filter did not
2- and 3-Rh, both of which exhibited lower activity and enantio- afford any additional addition product, suggesting the hetero-
selectivity than 1-Rh under otherwise identical conditions. For geneous nature of the reaction system. Upon completion of the
example, 4 mol% loading of 2-Rh and 3-Rh catalyzed the conju- reaction, polymer 1 could be readily recovered by centrifugation
gated addition of 3-methoxyphenylboronic acid to 2-cyclohexenone and, after washing with THF and heating under vacuum, reused
affording the product in 78% yield with 76% ee and 76% yield with for the next cycle by the addition of 0.004 mol [Rh(C2H4)2Cl]2
68% ee, respectively (Table 1).
without significant loss of activity and enantioselectivity.11 The
To investigate whether the catalysis by 1-Rh occurred pre- yield/ee’s for the five consecutive runs are 91/86%, 90/85%,
dominantly within the pores of the solid or just on the surface, 90/84%, 88/84% and 87/84%, respectively.
competitive size selectivity studies were carried out. The addition
We also examine the utility of the diene-based POF-1 as a
of 1-naphthylboronic acid to 2-cyclohexenone gave the product new chiral stationary phase (CSP) for gas chromatographic (GC)
6k in 57% yield in 8 h. When a sterically more demanding separation of racemic compounds. The 1-coating open tubular
column (15 m  250 mm i.d.) for GC was prepared by a dynamic
coating method.12 The average of the McReynolds constants
Table
1
Asymmetric conjugation addition of arylboronic acids to
2-cyclohexenone catalyzed by 1-Rha,b,c
is calculated to be 135, indicative of the moderate polarity of
the CSP.12 SEM images showed that the fabricated column had
an approximately 1 mm thick coating on the inner wall (Fig. S8,
ESI†). The performance of the CSP was evaluated by GC
separation of racemates of secondary alcohols and amines.
After many attempts, racemic mixtures of 1-phenylethanol
and 1-phenylethylamine were resolved at 150 1C, with separa-
tion factors (a) of 1.13 and 1.17 and retention factors (k1) of 0.23
and 0.21, respectively (Fig. 4). There are the same elution orders
for the two analytes, in which S enantiomers are eluted after R.
All enantioseparations have short retention times, and it might
be suitable to perform a quick analysis of the enantiomers.
Notably, the 1-coating GC column remained active for separa-
tion of 1-phenylethanol after 30 h working time. In addition,
1-phenylpropanol and 1-phenylpropylamine could also be partly
a
Reactions were carried out with enone (0.2 mmol), arylboronic acid
(0.4 mmol), 4 mol% 1-Rh, and 50 mol% KOH in 1,4-dioxane (1.0 mL,
10 : 1 v/v) for 8 h at 50 1C. Isolated yield. ee values were determined
by HPLC.
b
c
Fig. 4 GC separation of 1-phenylethanol (left) and 1-phenylethylamine
(right) on the 1-coating column at 150 1C.
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This work was supported by NSFC (21025103, 21371119,
21431004 and 21401128), the ‘‘973’’ Program (2014CB932102
and 2012CB8217) and SSTC-12XD1406300 and 14YF1401300.
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14952 | Chem. Commun., 2014, 50, 14949--14952
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