Metalloporphyrin-Based Hypercrosslinked Polymers Catalyze Hetero-Diels–Alder Reactions of Unactivated Aldehydes with Simple Dienes: A Fascinating Strategy for the Construction of Heterogeneous Catalysts

Article abstract of DOI:10.1002/chem.201601151

We describe a novel and intriguing strategy for the construction of efficient heterogeneous catalysts by hypercrosslinking catalyst molecules in a one-pot Friedel–Crafts alkylation reaction. The new hypercrosslinked polymers (HCPs) as porous solid catalysts exhibit the combined advantages of homogeneous and heterogeneous catalysis, owing to their high surface area, good stability, and tailoring of catalytic centers on the frameworks. Indeed, a new class of metalloporphyrin-based HCPs were successfully synthesized using modified iron(III) porphyrin complexes as building blocks, and the resulting networks were found to be excellent recyclable heterogeneous catalysts for the hetero-Diels–Alder reaction of unactivated aldehydes with 1,3-dienes. Moreover, this new strategy showed wide adaptability, and many kinds of homogeneous-like solid-based catalysts with high catalytic performance and excellent recyclability were also constructed.

Full text of DOI:10.1002/chem.201601151

DOI: 10.1002/chem.201601151
Communication
&
Heterogeneous Catalysis
Metalloporphyrin-Based Hypercrosslinked Polymers Catalyze
Hetero-Diels–Alder Reactions of Unactivated Aldehydes with
Simple Dienes: A Fascinating Strategy for the Construction of
Heterogeneous Catalysts
Zhiyu Dou,*[b] Li Xu,[a, b] Yongfeng Zhi,[a] Yuwei Zhang,[a] Hong Xia,[c] Ying Mu,[a] and
Xiaoming Liu*[a]
Heterogeneous catalysts are desirable for practical application
Abstract: We describe a novel and intriguing strategy for
due to their convenience of recovery and recycling. In general,
the construction of efficient heterogeneous catalysts by
however, the heterogeneous process causes a significant de-
hypercrosslinking catalyst molecules in a one-pot Friedel–
crease in catalytic activity and selectivity, attributed to the
Crafts alkylation reaction. The new hypercrosslinked poly-
complexity of multi-step syntheses and difficulty of tailoring
mers (HCPs) as porous solid catalysts exhibit the com-
the active centers. Thus, the development of a new judiciously
bined advantages of homogeneous and heterogeneous
chosen strategy for the construction of porous solid catalytic
catalysis, owing to their high surface area, good stability,
systems, that combine the advantages of homogeneous and
and tailoring of catalytic centers on the frameworks.
heterogeneous catalysis, is a crucial and emerging task for ad-
Indeed, a new class of metalloporphyrin-based HCPs were
vancing the research progress of porous polymer materials
successfully synthesized using modified iron(III) porphyrin
and catalysis.
complexes as building blocks, and the resulting networks
Our strategy is to employ homogeneous catalysts with ex-
were found to be excellent recyclable heterogeneous cat-
cellent catalytic properties as monomers for the construction
alysts for the hetero-Diels–Alder reaction of unactivated
of novel heterogeneous catalysts by
a hypercrosslinking
aldehydes with 1,3-dienes. Moreover, this new strategy
showed wide adaptability, and many kinds of homogene-
ous-like solid-based catalysts with high catalytic per-
formance and excellent recyclability were also construct-
ed.
method, which exhibits a wide applicability to homogeneous
catalytic molecules. Thus, a series of homogeneous-like solid-
based catalysts, which demonstrate well the combination of
advantages of both homogeneous and heterogeneous cata-
lysts, can be obtained. Here, we first utilized iron(III) porphyrin
complexes as building blocks and constructed a series of met-
alloporphyrin-based HCPs with inherently hierarchical porosi-
ties and single-site catalytic centers through a one-pot Friedel–
Crafts alkylation reaction promoted by anhydrous FeCl3
(Scheme 1, FeP-HCPs). In particular, this synthetic strategy has
a wide applicability to many metalloporphyrin molecules with
different substitutions, including at the adjustable position and
electronic properties, and the obtained FeP-HCPs have flexible
active centers similar to homogeneous analogues. Interestingly,
we demonstrated that FeP-HCPs can efficiently catalyze the
hetero-Diels–Alder reaction between simple dienes and unacti-
vated aldehydes, with high activity, wide substrate scope, and
excellent catalyst recyclability, leading to the formation of
pyran structural units as products in high yields.
Over the last few years, intensive efforts have been devoted to
immobilizing homogeneous catalysts by introducing them on
or into nanoporous materials[1] such as crystalline metal organ-
ic frameworks (MOFs),[2] covalent organic frameworks (COFs),[3]
conjugated microporous polymers (CMPs),[4] polymers of intrin-
sic microporosity (PIMs),[5] and hypercrosslinked polymers
(HCPs),[6] which leads to the formation of porous solid catalysts.
[a] L. Xu, Y. Zhi, Dr. Y. Zhang, Prof. Y. Mu, Prof. X. Liu
State Key Laboratory for Supramolecular Structure and Materials
College of Chemistry, Jilin University
Changchun, 130012 (P. R. China)
E-mail: xm_liu@jlu.edu.cn
Metalloporphyrin complexes possess unique chemical, mate-
rial, biological, and catalytic functionalities, and they represent
an excellent class of molecule-based solids.[7] More importantly,
metalloporphyrin-based porous materials can effectively limit
the deactivation of the catalyst framework in the catalytic reac-
tion. We designed and successfully synthesized the FeP-HCPs
with a facile Friedel–Crafts alkylation method. Typically, a substi-
tuted iron(III) porphyrin monomer was crosslinked using form-
aldehyde dimethylacetal as a crosslinking agent promoted by
anhydrous FeCl3 under solvated thermal conditions (1,2-di-
[b] Dr. Z. Dou, L. Xu
Department of Chemistry and Chemical Engineering
Changchun University of Science and Technology
Changchun, 130022 (P. R. China)
E-mail: douzhiyu79@sina.cn
[c] Prof. H. Xia
State Key Laboratory on Integrated Optoelectronics
College of Electronic Science and Technology
Jilin University, Changchun 130012 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601151.
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Scheme 1. Synthesis of metalloporphyrin-based porous hypercrosslinked
polymers.
chloroethane (DCE), 808C, 24 h). The resulting solids were
washed with various solvents to afford a series of FeP-HCPs in
excellent yields. No changes were observed in these materials
upon exposure to air, humidity, and organic solvents. FeP-HCPs
were unambiguously characterized by Fourier transform infra-
red spectroscopy, electronic adsorption spectroscopy, thermog-
ravimetric analysis, and elemental analysis (see Figures S1–S4
in the Supporting Information). Powder X-ray diffraction meas-
urements (PXRD) reveal that FeP-HCPs are amorphous, with no
definite signals evident (see Figure S5). Field-emission scanning
electron microscopy (FE-SEM) was performed to investigate
the morphology of the polymer networks (see Figure S6). The
SEM image of FePp-H-HCP samples displayed rather rough surfa-
ces and the particles were on the order of micrometer in size
(Figure 1a). The size distribution of the FePp-H-HCP samples at
room temperature in a methanol suspension was 615.1Æ
140 nm (Figure 1b). High-resolution transmission electron mi-
croscopy (HR-TEM) images proved the existence of hierarchical
porosities (Figure 1c). The elemental mapping by energy-dis-
persive X-ray specroscopy (EDS) analysis displayed a homoge-
neous distribution of iron (see Figure S7). X-ray photoelectron
spectroscopy (XPS) was conducted for the FeP-HCPs to deter-
mine the their chemical composition (see Figure S8).
Figure 1. a) FE-SEM image of a typical FePp-H-HCP sample on mica; b) DLS
profile of the FePp-H-HCP; c) TEM image of the FePp-H-HCP.
*
*
Figure 2. a) Nitrogen adsorption ( ) and desorption ( ) isotherm profiles of
FePp-H-HCP at 77 K; b) pore size distribution of FePp-H-HCP on the N2 adsorp-
tion isotherms.
fluences of solvent and reaction temperature on the yield in
the cycloaddition reaction were first investigated. Heating
a mixture of benzaldehyde with 2,3-dimethyl-1,3-butadiene
and AgBF4 in benzene at 808C for 12 h in the presence of
FePp-H-HCP led to the formation of pyran 3aa in 95% yield (see
the Supporting Information, Table S1, entry 4). In sharp con-
trast, low and moderate yields were obtained in THF, dioxane,
and toluene, respectively. Control experiments were also per-
formed in the absence of FePp-H-HCP or AgBF4, in both cases
providing no detected target product. Besides catalytic activity,
reusability is also crucial for an outstanding heterogeneous cat-
alyst performance. As shown in Figure S12, the catalyst was
easily isolated by centrifugation and reused at least five times
without any significant loss of the catalytic activity and original
shape, all of which is attributed to the built-in character of the
covalently linked catalytic sites on the metalloporphyrin-based
porous skeleton. In addition, we filtered the FePp-H-HCP from
the hot solution at 60% conversion, the isolated solution did
not exhibit any further reactivity, which clearly is an indication
of the heterogeneous catalytic nature of this new porous solid
catalyst. In relation to this, we were curious if the catalyst
would be applicable a larger scale synthesis. As shown in
The permanent porosity of the polymers was revealed by ni-
trogen sorption measurements at 77 K (see Figure S9). The
curve resulting from FePp-H-HCP exhibits a steep adsorption
and hysteresis loop at high relative pressure, suggesting the
co-existence of both micro- and mesopores in the network
(Figure 2a). Based upon the calculations of the Saito–Flory
method, the pore sizes are distributed around 0.51, 1.06, 2.04,
and 2.41 nm, respectively. The Brunauer–Emmett–Teller (BET)
surface area of FePp-H-HCP is estimated at 735 m2 gÀ1 with
a pore volume of 0.678 cm3 gÀ1 (Figure 2b).
Owing to the high concentration of iron porphyrin-based
catalytic sites in new HCP networks, their heterogeneous cata-
lytic characters are highly desired. It is well known that the
hetero-Diels–Alder reaction is one of the most powerful syn-
thetic methods to construct the pyran structural units that has
been widely applied in the synthesis of natural products.[8] Re-
cently, an important breakthrough in this field (i.e., that a cat-
ionic iron(III) porphyrin can efficiently catalyze the hetero-
Diels–Alder reaction of unactivated aldehydes and simple
dienes)[9] motivated us to explore the catalytic activity of new
FeP-HCPs as recyclable solid catalysts for this reaction. The in-
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Scheme S1 in the Supporting Information, we examined this
possibility using 1a (1.06 g, 10 mmol) as a substrate in 40 mL
of solvent at 808C. A good yield (87.2%) was also afforded in
the scaled-up reaction. Notably in this case, the turnover
number (TON) reaches 4359 with a turnover frequency (TOF)
of 91 hÀ1 after 48 h without loss of catalytic activity in the fol-
lowing runs (see Figure S13).
large BET surface areas of 419, 902, and 977 m2 gÀ1, respective-
ly. For the cycloaddition of benzaldehyde with 2,3-dimethyl-
1,3-butadiene, these FeP-HCPs exhibited excellent catalytic per-
formances, and corresponding yields were 91% for FePp-Br-HCP,
96% for FePp-OMe-HCP, and 97% for FePo-OMe-HCP. To further in-
vestigate the scope of the process, we tested the reaction be-
tween benzaldehyde and simple isoprene (Table 2). The FePp-H
-
The excellent catalytic activity of the cationic iron(III) porphy-
rin-based HCP encouraged us to further explore the generality
of the catalytic system; several benzaldehydes with different
steric and electronic characters were tested (Table 1). Benzalde-
hydes with electron-withdrawing substituents including bro-
mide, trifluoromethyl, and nitrile afforded pyrans in good
yields. In particular, a nitro substituent group on benzaldehyde
was tolerated under the present reaction conditions, and the
corresponding product 3ea was obtained in excellent yield
upon a considerable elongation of reaction time (five days).
Benzaldehydes 1 f, 1g, and 1h, all of which have electron-do-
nating substituents such as methoxy or methyl groups, can ef-
ficiently react with 2a to produce the desired cycloaddition
compounds. In addition, naphthaldehyde 1i also worked well
and the reaction afforded the corresponding substituted pyran
in 81% yield. Furthermore, the cationic iron(III) porphyrin-
based porous network was also found to be effective for the
cycloaddition of aliphatic aldehydes with 2a. For example, cy-
clohexanecarbaldehyde (1j) reacted with 2a to give pyran 3ja
in 94% yield.
HCP catalyst achieved only 30% conversion under the opti-
mized reaction conditions, which is lower than that of the cy-
cloaddition reaction with 2a. The reaction in the presence of
FePp-OMe-HCP gave the corresponding pyran in moderate yield
(55%). It outperforms that of FePp-H-HCP, which may be attrib-
uted to the higher BET surface area of FePp-OMe-HCP. FePp-Br-HCP,
however, exhibits higher conversion (65%) than that of both
FePp-H-HCP and FePp-OMe-HCP for the same cycloaddition reac-
tion, although the p-H and p-OMe variants have larger surface
areas. This could be attributed to the fact that the Br in the
porphyrin network of FePp-Br-HCP is an electron-withdrawing
group, which could decrease the electron density on the iron
center and increase the catalytic activity. It is noteworthy to
mention here the fascinating strategy can construct new kinds
of porous heterogeneous catalysts, which combine the advan-
tages of both homogeneous and heterogeneous catalysts.
Furthermore, this hypercrosslinking homogeneous catalytic
molecule strategy can be useful for the construction of various
heterogeneous catalysts with excellent catalytic performances,
including heterogeneous organocatalysts, asymmetric hetero-
geneous organocatalysts, metal-free heterogeneous photocata-
lysts, etc. (see Schemes S2 and S3). For instance, a series of
chiral porous HCPs based on 3,3‘substituted-1,1’-binaphthalen-
2,2’-diyl-phosphoric acid have also been synthesized using this
strategy. Similar to the homogeneous small molecule ana-
logue, these HCPs possess an adjustable chiral Brønsted acid
center, and exhibit good asymmetric benzoxazine reduction in
the presence of a Hantzsch ester (see Table S2). This new con-
struction strategy for porous heterogeneous catalysts is superi-
or to the previously reported immobilizing methods, which
usually need specific polymerizable groups in monomers and
an expensive metal catalyst for the synthesis process.
Indeed, the catalytic activity of the FePp-H-HCP network in
the hetero-Diels–Alder reaction was similar to that of the
homogeneous small complex.[9] More importantly, this new
synthetic strategy was also used to construct a series of other
FeIII porphyrin-based polymer networks by tailoring the active
center. The FePp-Br-HCP, FePp-OMe-HCP, and FePo-OMe-HCP all have
Table 1. FePp-H-HCP-catalyzed cycloaddition of various aldehydes with
2,3-dimethyl-1,3-butadiene.[a]
In summary, we have dedicated the judiciously chosen strat-
egy of hypercrosslinking homogeneous catalytic molecules to
the construction of porous catalytic HCPs, which have been
demonstrated as highly efficient heterogeneous catalysts once
synthesized using the one-pot Friedel–Crafts alkylation reac-
tion. Owing to high surface area, good stability, and specially
tailored catalytic centers, new HCP architectures as porous
solid catalysts exhibit the combined advantages of both homo-
geneous and heterogeneous catalysis, which can be difficult to
achieve with previously reported synthetic methods. Specifical-
ly, a new class of metalloporphyrin-based HCPs with specially
designed built-in catalytic centers were constructed using
modified iron(III) porphyrin complexes as building blocks, and
the resulting FeP-HCPs were found to be excellent recyclable
heterogeneous catalysts for the hetero-Diels–Alder reaction of
unactivated aldehydes with 1,3-dienes. Considering the huge
amount of homogeneous catalytic molecules and the broad
applicability of the Friedel–Crafts alkylation reaction, our strat-
[a] FePp-H-HCP (5.0 mmol of aldehyde), AgBF4 (7.5 mmol of aldehyde), al-
dehyde 1 (1.0 mmol), and diene 2a (2.0 mmol); yields determined by GC
using undecane as the internal standard.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51473064, 61435005, and 21274050).
X.L. is also grateful for support by the Frontiers of Science and
Interdisciplinary Innovation Project of Jilin University (No.
450060481015).
Keywords: hetero-Diels–Alder reaction
·
heterogeneous
catalysis · hypercrosslinked polymers · metalloporphyrin-based
networks · polymers
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Received: March 10, 2016
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COMMUNICATION
&
Heterogeneous Catalysis
Z. Dou,* L. Xu, Y. Zhi, Y. Zhang, H. Xia,
Y. Mu, X. Liu*
&& – &&
Holey HCPs: Porous solid catalysts were
constructed from hypercrosslinking
polymers (HCPs), and exhibited the
combined advantages of homogeneous
and heterogeneous catalysis. New met-
alloporphyrin-based polymers were
found to be excellent recyclable hetero-
geneous catalysts for the hetero-Diels–
Alder reaction of unactivated aldehydes
with 1,3-dienes. This strategy demon-
strated a wide applicability, and many
kinds of homogeneous-like solid cata-
lysts were constructed.
Metalloporphyrin-Based
Hypercrosslinked Polymers Catalyze
Hetero-Diels–Alder Reactions of
Unactivated Aldehydes with Simple
Dienes: A Fascinating Strategy for the
Construction of Heterogeneous
Catalysts
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