Tandem catalysis with a bifunctional site-isolated Lewis acid-Bronsted base metal-organic framework, NH2-MIL-101(Al)

Article abstract of DOI:10.1039/c2cc36678a

A metal-organic framework (MOF), NH₂-MIL-101(Al), which acts as a bifunctional, site-isolated Lewis acid-Bronsted base heterogeneous catalyst, catalyzes a tandem Meinwald rearrangement-Knoevenagel condensation reaction with remarkable substrate selectivity.

Full text of DOI:10.1039/c2cc36678a

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COMMUNICATION
Tandem catalysis with a bifunctional site-isolated Lewis acid–Brønsted
base metal–organic framework, NH2-MIL-101(Al)w
Renganathan Srirambalaji,z Soonsang Hong,z Ramalingam Natarajan, Minyoung Yoon,
Raghunandan Hota, Yonghwi Kim, Young Ho Ko and Kimoon Kim*
Received 13th September 2012, Accepted 19th October 2012
DOI: 10.1039/c2cc36678a
A metal–organic framework (MOF), NH2-MIL-101(Al), which
acts as a bifunctional, site-isolated Lewis acid–Brønsted base
heterogeneous catalyst, catalyzes a tandem Meinwald rearran-
gement–Knoevenagel condensation reaction with remarkable
substrate selectivity.
direct synthesis or by post-synthetic modification.6,8 Catalytic
reactions performed with MOFs, either by Lewis acidic
metal centers or functional ligands as active sites, include
transesterification, oxidation, cyanosilylation, aldol condensation
among others.6,8
Despite the rapidly growing interest in MOFs, the possibility
of combining more than one catalytic site in MOFs to perform
tandem reactions remains largely unexplored.9 The existing few
examples involve extensive post-synthetic modification (PSM)
of MOFs to incorporate additional functional groups, along
with the aid of external additional reagents if necessary. Herein,
we demonstrate a tandem reaction, heterogeneously catalyzed
by a site-isolated bifunctional MOF, NH2-MIL-101(Al),10a
without any post-synthetic incorporation of additional catalytic
moieties on it. Furthermore, as a first step, the reaction involves
remarkable substrate-selective epoxide opening.
As exemplified by many catalytic enzyme assemblies in
nature,1 site-isolation is a powerful strategy for performing
tandem reactions to synthesize highly complex molecules.2
Heterogeneous catalysis is a preferable and powerful way, by
which many fine chemicals are synthesized in industries.
Beyond their obvious advantages such as easy recovery of
catalysts, heterogeneous catalysts offer an extraordinary
opportunity for site-isolation of acidic and basic catalytic
functional groups to perform multi-component reactions
which would be inconceivable by their homogeneous analogues.3
This allows several reactions to be performed in tandem without
isolation of intermediates and self-quenching of catalysts, offering
an enormous economic advantage. In recent years, a few catalysts
that use site-isolated acidic and basic catalytic groups embedded
in mesoporous silica and polymer resins have been shown to
exhibit tandem synthetic transformations.4
The MOF used in this study, NH2-MIL-101(Al), belongs
to the MIL-10110b series, formed by 2-aminoterephthalate
ligands and trimeric Al3+ octahedral clusters mediated super-
tetrahedral (ST) building units resulting in a robust mesoporous
crystalline material (Fig. 1A).10a Amino moieties in the organic
Metal–organic frameworks (MOFs) are compounds
assembled from metal ions or clusters as ‘nodes’ and organic
ligands as ‘linkers’. Owing to their tunable pores, functional
walls, diverse structures and potential applications, MOFs are
a rapidly growing class of new materials.5 In particular, MOFs
are promising as heterogeneous catalysts6 as they offer several
advantages over other immobilized catalyst systems, including
atomistically well-defined structures7 with uniformly accessible
catalytic centers and their site isolation.6
MOFs can act as heterogeneous catalysts either by using
their coordinatively unsaturated metal centers, or by functional
groups incorporated in the linkers and/or metal centers through
Center for Smart Supramolecules, Department of Chemistry and
Division of Advanced Materials Science, Pohang University of Science
and Technology, San 31 Hyojadong, Pohang 790-784, Republic of
Korea. E-mail: kkim@postech.ac.kr; Fax: +82-54-279-8129;
Tel: +82-54-279-2113
w Electronic supplementary information (ESI) available: Experimental
details for preparation, characterization of materials. See DOI:
10.1039/c2cc36678a
Fig. 1 (A) A large cage in NH2-MIL-101(Al); (B) Lewis acidic sites
(Al3+ centers) and Brønsted basic sites (aminoterephthalate linkers);
(C) schematic representation of tandem organic reactions using a
site-isolated heterogeneous catalyst.
z R. Srirambalaji and S. Hong contributed equally to this work.
c
11650 Chem. Commun., 2012, 48, 11650–11652
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linker act as Brønsted bases and Al3+ metal centers act as Lewis
acid catalytic sites. Since the Brønsted basicity of amino groups
in NH2-MIL-101(Al) can catalyze a Knoevenagel condensation,
we decided to first explore the Lewis acidic nature of Al3+
centers for catalysis and subsequently exploit the site isolated
Lewis acidic and Brønsted basic catalytic groups in the framework
to perform a tandem transformation (Fig. 1B and C).
Table 1 Meinwald rearrangementa
Entry Epoxide
Product
Conversionb (yield) (%)
The catalyst was synthesized by following the recently
reported procedure and characterized by PXRD and N2 gas
adsorption experiments.10a It exhibited similar BET surface
area and total pore volume (see ESIw). We started exploring
the Lewis acidic nature of the Al3+ centers using epoxide
ring opening reactions. Epoxides are versatile substrates for
formation of industrially important complex organic compounds.11
In particular, the Meinwald rearrangement, an industrially
important atom economic way of opening of epoxides
catalyzed by Lewis acids, proceeds through a carbocation
intermediate, resulting in carbonyl compounds.12
1
o1c
o1c
2
3
o1c
We began our study with various epoxides as shown in
Table 1. In a typical experiment, the epoxide was taken in
an aprotic solvent such as acetonitrile along with 10 mol%
NH2-MIL-101(Al) catalyst and stirred at 60 1C for 48 h.
Aliphatic epoxides (Table 1, entries 1 and 2) remained unopened
even after 48 h. Styrene oxide 3 and trans-stilbene oxide 4 also
remained unopened under similar conditions. Surprisingly,
epoxides 5 and 6 underwent rearrangement to aldehydes 5a
and 6a, respectively, with more than 70% isolated yield.
Mechanistically (Scheme 1), epoxides 1 and 2 proceed via
aliphatic 31 carbocation; 3 and 4 via benzylic 21 carbocation;
and 5 and 6 via benzylic 31 carbocation intermediates.
4
5
o1c,d
80 (75)
75 (70)
6
Among these, the latter intermediates of 5 and 6 gain more
resonance stabilization than that of other substrates (1–4). To
the best of our knowledge, such selective opening of epoxide
using Lewis acid is unprecedented. Presumably, the amino
group present in the linker significantly reduced the Lewis
acidic nature of the Al3+ center; thus only epoxides that
generate a more stable benzylic 31 carbocation as an intermediate
resulted in corresponding aldehydes. A similar electronic effect of
an amino group attached to a linker reducing the Lewis acidity
of zirconium-based MOF (UiO-66) was recently reported.13
However, epoxides (2–6) underwent rearrangement to the corres-
ponding carbonyl compounds (2a–6a) with more strongly Lewis
acidic AlCl3 in our hands, or are known to undergo the same
rearrangement with other Lewis acids such as InCl3,14
and Cu(BF4)2.15 Interestingly, a mesoporous aluminosilicate
(Si/Al ratio = 14 : 1) opens all epoxides without substrate
selectivity by Meinwald rearrangement.16
Having established that the Lewis acidic Al3+ centers in
NH2-MIL-101(Al) can promote epoxide ring opening of
selective substrates, we decided to combine this feature with
the Brønsted basicity of the amine groups present in the linkers
to catalyze a nucleophilic addition to the resultant aldehydes
as a one pot tandem reaction. NH2-MIL-101(Al) is known to
catalyze a Knoevenagel condensation between aldehydes and
activated methylene nucleophiles. As anticipated, epoxide 5
(Table 2, entry 1) and malononitrile 7 along with 10 mol%
NH2-MIL-101(Al) in acetonitrile taken together in one-pot
(60 1C, 48 h) resulted in a tandem epoxide ring opening to
aldehydes, followed by condensation and subsequent
a
Conditions: reactions were carried out using 0.13 mmol of epoxide
b
with 10 mol% of the catalyst. Conversions were calculated using
1H NMR based on epoxides. Reactions were carried out for 48 h.
d
c
Other possible regioisomer (ketone) was also not detected.
Scheme 1 Mechanism for Meinwald rearrangement using Lewis acid
as a catalyst.
dehydration to 5b with 70% yield. Epoxide 6 (Table 2, entry 2)
also underwent a similar Meinwald rearrangement–Knoevenagel
condensation with 7 under identical conditions in good yield. In
1
both instances, H NMR analysis of the crude reaction mixture
after 24 h confirmed the presence of intermediate aldehyde
resonances, which disappeared completely after 48 h. Sub-
sequently, the catalyst was recovered from the reaction mixture
by centrifugation and the pure product was isolated by column
chromatography and characterized by 1H and 13C NMR analysis
and mass spectrometry.
Most importantly, when the catalyst was filtered from the
tandem reaction mixture of 5 and 7 after just 5 hours and the
mixture was kept in the supernatant liquid under similar
conditions (CH3CN, 60 1C, 48 h), compound 5 remained
c
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Table 2 Meinwald rearrangement–Knoevenagel condensationa
was tested by performing a tandem reaction between 5 and 7
using the recovered catalyst, which produced the final product
in 65% yield. The structural integrity of the catalyst remained
unaffected even after two cycles, as evidenced by PXRD
analysis (see ESIw).
Entry Substrate Nucleophile Product
Conversionb (yield) (%)
In summary, we have successfully demonstrated tandem
catalysis using a bifunctional MOF, NH2-MIL-101(Al), as a
site-isolated Lewis acid-Brønsted base catalyst. The Lewis
acidic sites exhibited remarkable substrate selectivity for
epoxide ring opening by Meinwald rearrangement via a
benzylic 31 carbocation intermediate to form aldehydes. The
subsequent Knoevenagel condensation of the aldehydes with
an active methylene nucleophile catalyzed by basic catalytic
sites completed a one-pot tandem reaction with high conversion.
Taking advantage of the aldehyde electrophile as an inter-
mediate, facile synthesis of more complex molecules using
different nucleophiles in tandem in a similar catalytic process
is anticipated. Work along this line is in progress.
1
5
6
7
7
80(70)
2
75(70)c
a
Conditions: reactions were carried out using 0.13 mmol of epoxide
b
and malononitrile 7 each with 10 mol% of the catalyst. Isolated
We gratefully acknowledge the Acceleration Research,
Brain Korea 21, and World Class University (Project No.
R31-2008-000-10059-0) of the National Research Foundation
of Korea and the Korean Ministry of Education, Science, and
Technology (MEST). We also thank J. Mark Kim for helpful
discussions.
c
yields based on epoxides. Reaction was carried out for 96 h.
Table 3 Control experimentsa
Entry Substrate Nucleophile Catalyst
Product Conversionc (%)
5a 85
1
2
3
4
5
5
5a
5
7
7
7
7
AlCl3
ATAb
ATA
AlCl3 +
ATA
5a/5b o1
5b
5a
65
85
Notes and references
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a
Conditions: All the reactions were carried out using 0.13 mmol of
epoxide or aldehyde and malononitrile 7 with 10 mol% of the catalyst.
3 B. Voit, Angew. Chem., Int. Ed., 2006, 45, 4238.
b
c
Dimethyl 2-aminoterephthalate. Conversions were calculated
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1
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6 J.-Y. Lee, O. M. Farha, J. Roberts, K. Scheidt, S. T. Nguyen and
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ring opening and Knoevenagel condensation reaction was
promoted by heterogeneous NH2-MIL-101(Al) as a catalyst.
As another control experiment, when the tandem reaction was
carried out with 5 and 7 along with AlCl3 (10 mol%) as a
catalyst (Table 3, entry 1) the epoxide ring opening occurred,
but the Knoevenagel condensation failed to take place. A
similar reaction with 10 mol% dimethyl 2-aminoterephthalate
(ATA), an ester analogue of the organic linker present in the
framework, as a catalyst under the same conditions left the
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c
11652 Chem. Commun., 2012, 48, 11650–11652
This journal is The Royal Society of Chemistry 2012