A facile 2: H -chromene dimerization through an ortho -quinone methide intermediate catalyzed by a sulfonyl derived MIL-101 MOF

Article abstract of DOI:10.1039/c8nj01354c

A MIL-101-SO₃H MOF was synthesized using commercially available materials. The as-synthesized MIL-101-SO₃H was characterized by SEM, XRD, FTIR, BET and TGA. An efficient and diastereoselective homo-dimerization of 2H-chromenes cataly

Full text of DOI:10.1039/c8nj01354c

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C8NJ01354C
New Journal of Chemistry
ARTICLE
A facile 2H-chromene dimerization through an ortho-quinone
methide intermediate catalyzed by a sulfonyl derived MIL-101 MOF
Xin Du,a Xiujuan Li,a Houliang Tang,b Wenyu Wang,c Daniele Ramella d and Yi Luana,*
Received 00th January 20xx,
Accepted 00th January 20xx
A MIL-101-SO3H MOF was synthesized using commercially available materials. The as-synthesized MIL-101-SO3H was
characterized by SEM, XRD, FTIR, BET and TGA. An efficient and diastereoselective homo-dimerization of 2H-chromenes
catalysis was achieved using the sulfonyl derived MIL-101 MOF as a catalyst. Benzopyranobenzopyran polycyclic structure
were generated in high yield and diastereoselectivity under mild catalytic reaction conditions. Furthermore, a regio- isomer
was observed when 2H-chromene bears different substitution group, furnished chromeno[2,3-b]chromene dimer in high
yield and good diastereoselectivity. A mechanism is proposed with a tandem rearrangement/hetero-Diels-Alder reaction
sequence is supported by evidence. In addition, the acid MIL-101 MOF catalyst can be recycled ten times without
compromising the yield and selectivity.
DOI: 10.1039/x0xx00000x
www.rsc.org/
highly tailorable nature, porous structure and large surface
area.10 Brønsted acid MOFs have been developed to serve as
the efficient catalysts for acid catalysis.11 A MOF structure
1 Introduction
Ortho-quinone methides (oQM)1 are composed of
a
bearing sulfonyl groups can serve as a low pKa catalytic
material.12 Among several choices of -SO3H derived MOF
materials, the synthesis of chemically stable MIL-101-SO3H MOF
has been reported in the literature.13 Moreover, MIL-101-SO3H
has been utilized as an efficient catalyst for several organic
reactions, thanks to its chemical and physical stabilities.14
With this communication, we wish to report a novel
synthetic utilization of MIL-101-SO3H MOF catalyst. This newly
synthesized MIL-101-SO3H catalyst, bearing aromatic sulfonic
acid groups, was obtained using commercially available
monosodium 2-sulfoterephthalate. An efficient homo-
dimerization of electron-rich 2H-chromene allows rapid access
to the benzopyranobenzopyran polycyclic and chromeno[2,3-
b]chromene core structure. The achievement of extremely high
diastereoselectivity using MIL-101-SO3H as a strong Brønsted
acid solid catalyst is unprecedented in literature. A variety of
electron-rich 2H-chromenes were tolerated under the optimal
reaction conditions for the homo and hetero- dimerization
reactions. Furthermore, the as-synthesized MIL-101-SO3H
catalyst can be readily filtered and separated from the reaction
solution; our study showed that recycling the catalyst up to ten
times doesn’t compromise yield or selectivity.
cyclohexadiene motif in conjugation with a carbonyl and a
methylene group, are highly polarized and are versatile
intermediates in organic synthesis,2 material chemistry3 and
biology.4 An ortho-quinone methide intermediate can undergo
1,4-conjugate addition reactions with nucleophiles and [4 + 2]
cycloaddition reactions with various dienophiles.5 Due to broad
range of applications of ortho-quinone methide intermediates,
numerous methods have been established for the their
synthesis.6 However, a methodology for quick in-situ generation
from 2H-chromene would allow practical, rapid access to a
variety of complex synthetic intermediates.7 The dimeric
flavonoid dependensin, which can be isolated from the
antimalarial
uvarza
dependens,
contains
a
benzopyranobenzopyran polycyclic ring structure.8 We
envisaged synthetic access to these natural product core
structures via benzopyran based starting materials.
A
rearrangement of 2H-chromene is expected to generate an
ortho-quinone methide intermediate, followed by hetero-Diels-
Alder reaction with dienophiles to prepare these unique
heterocyclic structures.9
In the past decade, metal-organic frameworks (MOFs) have
emerged as a new class of solid catalysts, by virtue of their
2 Experimental section
2.1 Preparations of MIL-101-SO3H support
a.School of Materials Science and Engineering, University of Science and
Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, P. R.
China, E-mail: yiluan@ustb.edu.cn.
b.Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue,
Dallas, 75275, TX, USA
MIL-101 and MIL-101-SO3H were prepared according to the
literature.15
c. The Broad Institute, 415 Main Street, Cambridge, MA02142, USA.
d.Temple University-Beury Hall, 1901, N. 13th Street Philadelphia PA 19122, United
States.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
2.2 Procedure of 2H-chromene homodimerization
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2H-chromenes were prepared according to the published
procedure.16 To a reaction vessel equipped with stir bar in air,
7-methoxy-2-(4-methoxyphenyl)-2H-chromene 1a (0.25 mmol)
and 1 mol% catalyst in 1.25 mL dichloromethane were added.
The mixture was stirred at room temperature for 12 hours. The
View Article Online
DOI: 10.1039/C8NJ01354C
1
crude product was then subjected to H NMR analysis for the
determination of diastereoselectivity. The product was purified
by flash chromatography over silica gel column. The reported
yields were calculated based on the isolated amount of product.
Fig. 2 SEM images of MIL-101-SO3H.
MIL-101-SO3H particles were synthesized via hydrothermal
method, in order to obtain particles very monodisperse in size.
SEM images of MIL-101-SO3H were acquired (Fig. 2). The
rectangular crystals of MIL-101-SO3H are evenly distributed
with a size of 200 nm.
2.3 Procedure of 2H-chromene heterodimerization
To a reaction vessel equipped with stir bar in air, 7-methoxy-2-
(4-methoxyphenyl)-2H-chromene 1a (0.25 mmol), 3,4-dihydro-
2H-pyran 8a (1.5 equiv., 89.2 mg, 0.375 mmol) and catalyst (0.1
mmol) in 2.5 mL dichloromethane were added. The mixture was
stirred at room temperature for 12 hours. The crude product
was then subjected to 1H NMR analysis for the determination of
diastereoselectivity. The reaction mixture was purified without
work-up by flash chromatography over silica gel column to
afford the heterodimer.
2.4 Reusability of the MIL-101-SO3H catalyst
For the recyclability test of the MIL-101-SO3H catalyst, the
catalytic reactions were performed under the same optimized
conditions in dichloromethane for 12 h using the recovered
MIL-101-SO3H catalyst. A leaching study of the MIL-101-SO3H
catalyst was conducted; the mother liquor was filtered and the
supernatant was re-evaluated using fresh starting material
under optimized catalytic conditions.
Fig. 3 Powder XRD patterns of (a) MIL-101 and (b) MIL-101-SO3H.
Additionally, powder X-ray diffraction (PXRD) studies were
conducted as part of the characterization process (Fig. 3) and
the data were compared with literature values.17 The laboratory
XRD data were not sufficient to refine the structural difference
of the MIL-101 and MIL-101-SO3H composites. MIL-101-SO3H
crystalline structure was highly similar to MIL-101 according to
the PXRD patterns.
3. Results and discussion
Fig. 1 The detailed structure of MIL-101-SO3H.
MIL-101-SO3H can be readily synthesized and purified according
to the reported methods as a crystalline powder.15 It is iso-
structural to MIL-101, and thus has the same pore structures
(Fig. 1).
Fig. 4 N2 adsorption-desorption isotherm of MIL-101-SO3H.
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The specific surface areas of the products were analyzed by 1). Firstly, a control experiment was performed VtioewsAtrtuicdleyOntlhinee
N2 adsorption-desorption measurements at 77 K. As shown in background rate in the dimerization reaDctOioI:n10o.1f0379-/mC8eNthJ0o1x3y5-42C-
Fig. 4, the trace of the MIL-101-SO3H showed a type-I isotherm. (4-methoxyphenyl)-2H-chromene 1a (Table 1, entry 1). No
N2 gas sorption isotherms reveal a Brunauer–Emmett–Teller conversion was observed for 2H-chromene 1a in the absence of
(BET) surface area of 1266 m2/g.
catalyst. HCl and FeCl3·6H2O showed modest yield at 1 mol%
catalyst loading and the reaction was slow due to the small
amount of catalyst (Table 1, entries 2-3). As expected, MIL-101
bearing no acidic functional group provided only traces of
product at 23 ºC in CH2Cl2 solvent (Table 1, entry 4). 1 mol% MIL-
101-SO3H showed almost quantitative conversion and over 99:1
diastereoselectivity for the dimerization of 2H-chromene 1a
.
The greatly enhanced acidity could allow the rearrangement of
2H-chromene, followed by the [4 + 2] cycloaddition to provide
the desired dimer 2a in excellent yield (Table 1, entry 5).
Toluene and acetonitrile gave comparable, but still lower yields.
Further solvent evaluation indicated that dichloromethane is
the most suitable reaction solvent for further dimerization
reaction studies (Table 1, entries 5-7). Oxygen-containing
solvents, such as ether and THF, gave decent yields of the
desired dimerization product. However, the yields were not
satisfying (Table 1, entry 8). It was noticed that the 7-methoxy
functional group on 7-methoxy-2-(4-methoxyphenyl)-2H-
chromene 1a is essential for the promotion of the dimerization
reaction. A 2H-chromene without the 7-methoxy group failed to
provide benzopyranobenzopyran product under the several
catalytic reaction condition we tested.
Fig. 5 TGA of MIL-101 and MIL-101-SO3H.
Additionally, the thermal and structural stabilities of MIL-
101 and MIL-101-SO3H were examined by thermal gravimetric
analysis (TGA). A weight loss at about 300 °C was observed
during the TGA of MIL-101-SO3H. The weight loss of the
modified samples MIL-101 showed similar thermal curves.18 The
TGA results are evidence of high thermal stability of the acid
derived MIL-101-SO3H sample, which ensures its stability in the
catalytic reaction temperature range.
Table 2. Homodimerization substrate scope.
Table 1. Optimization of dimerization using 2H-chromene
precursor.a
Entry
Catalyst
-
HCl(aq)
FeCl3·6H2O
MIL-101
MIL-101-SO3H
MIL-101-SO3H
MIL-101-SO3H
MIL-101-SO3H
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhCH3
CH3CN
THF
Yieldb
<5%
41%
53%
<10%
98%
67%
71%
59%
Drc
n.a.
1
2
3
4
5
6
7
8
>95:5
>95:5
n.a.
>99:1
>99:1
>99:1
>99:1
a Reaction condition: 0.20 mmol of 1a, 1 mol % catalyst, 0.2 M
in the solvent for 2 h at room temperature, Isolated yield,
Ratio was determined by 1H NMR analysis.
b
c
Reaction condition: 1 mol% MIL-101-SO3H, 0.2 M in the CH2Cl2
solvent for 2 h at room temperature. For product 2c and 2d, 12
h reaction time was required.
Several reaction factors such as nature of the catalyst,
temperature and solvent, were tested in order to reveal the
most suitable parameters for the dimerization reaction (Table
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The optimized reaction conditions were employed in the
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DOI: 10.1039/C8NJ01354C
reaction of substituted chromene compounds to widen the
substrate scope. Based on our preliminary studies of the
reaction, we expected the cyloaddition process to occur in
presence of electron-rich dienophiles utilizing 2H-chromenes as
oQM precursors.19 The reaction proceeded well with 1,3-
benzodioxol substituted chromenes 1b (Table 2, compound 2b).
Less electron rich 2H-chromenes 1c was also able to participate,
but at a slower rate (Table 2, compound 2c). Similar yield was
observed for 1,3-benzodioxol substituted chromenes 1d
,
despite the presence of a less electron-donating group installed
at C2 position (Table 2, compound 2d).
Table 3. Chromeno[2,3-b]chromene dimer formation.
Scheme 1. Synthesis of 5,7,8-trimethoxyflav-3-ene
6.
5,7,8-trimethoxyflav-3-ene is the essential starting
6
material for the rapid synthesis of natural product dependensin
(Scheme 1), and can be easily prepared in three steps from
commercially available starting materials. Potentially, the
dimerization of 2H-chromene could access the dimeric
flavonoid dependensin. Isolated from the antimalarial uvarza
dependens,
dependensin
also
contains
a
benzopyranobenzopyran polycyclic ring structure.8 5,7,8-
trimethoxyflav-3-ene
tetramethoxybenzenein
6
3
was synthesized from 1,2,3,5-
, which is commercial available at low
price. To a solution of 1,2,3,5-tetramethoxybenzene in dry THF,
AlCl3 was added. Acetyl chloride wasthen slowly added to the
mixture. The crude product was purified by silica gel column
chromatography
trimethoxyacetophenone
2-hydroxy-3,4,6-trimethoxyacetophenone
to
4
give
in 76% yield. To a stirred mixture of
and benzaldehyde
2-hydroxy-3,4,6-
4
in ethanol, aqueous potassium hydroxide in water was then
added. The precipitate was filtered, washed with water and
dried to give chalcone
borohydride was added to
trimethoxyflav-3-ene . After the removal of ethanol, the
5
in 90% yield. Lastly, sodium
a
stirred mixture of 5,7,8-
5
resulting solid was treated with 10% aqueous acetic acid and
extracted with toluene. The toluene solution was separated and
evaporated, followed by silica gel column chromatography to
Reaction condition: 1 mol% MIL-101-SO3H, 0.2 M in the CH2Cl2
solvent for 2 h at room temperature.
provide 5,7,8-trimethoxyflav-3-ene 6 in 71% yield.
To broaden the scope of this methodology, the optimized
reaction conditions was employed in the dimerization of other
substituted
2H-chromenes.
Interestingly,
unexpected
chromeno[2,3-b]chromene dimer was isolated as the only
product when less electron-rich chromenes were utilized. The
reaction proceeded smoothly with non-substituted chromene
1e, affording chromeno[2,3-b]chromene dimers 2e in good
yields (Table 3, 2e). Electron-deficient chromenes, such as 1f
and 1g, underwent dimerization in a similar fashion, providing
compounds 2f and 2g in good yields (Table 3, 2f and 2g). The
electron-rich chromene 1h bearing a –CH3O group at the C-6
position instead of C-7 position appeared to have the
appropriate reactivity profile (Table 3, 2h).
Scheme 2. The synthesis of dependensin natural product.
The dimerization reaction strategy was utilized in the
synthesis of dependensin (7) via the homodimerization of 5,7,8-
trimethoxyflav-3-ene 6. This new route for the synthesis of
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dependensin accounts for a reduction in the number of steps well, yielding the corresponding heterodimer 9a in 71% yield as
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and high stereocontrol (Scheme 2).20 Subjecting 5,7,8- the only diastereomer. At present, a limited substrate scope
DOI: 10.1039/C8NJ01354C
trimethoxy-3-flavene
MIL-101-SO3H afforded (±)-dependensin (7) as
6
to reaction in presence of 1 mol% of motivates us to optimize the dimerization reaction of various
single dienophiles.
a
diastereomer in 35% overall yield from 1,2,3,5-
tetramethoxybenzene.
Table 4. Substrate scope for the hetero-Diels-Alder reaction.
Scheme 3. Dimerization comparison for mechanism study.
oQM (E)-8 was synthesized to investigate the mechanism in
detail. (E)-8 and 1b homodimerize into the same product 2b in
similar yields and equal diastereoselectivities. This observation
suggests the equilibrium between chromene 1b and vinyl oQM
(E)-8.The homodimerization reaction proceeds well to give the
chromeno[2,3-b]chromene heterodimer 2b in high yield as a
single diastereomer, supporting our mechanistic hypothesis The
same product was isolated in comparable yield and equal
diastereoselectivity, which provides further evidence for the
inverse electron demand [4+2] pathway.
Scheme 4. Model substrate study for reaction mechanism study.
The hetero-Diels-Alder reaction promoted by MIL-101-SO3H
was carried out by using various dienophiles, which furnished
several [4+2] cycloadducts in high diastereoselectivities. We
further explored the scope of the cycloaddition reaction
pathway using the in situ generated oQM of chromene 1a with
a variety of dienophiles (Table 4). The hetero-Diels-Alder
reaction proceeded smoothly with dihydropyran 8a to produce
the corresponding acetal 9a (Table 4, entry 1). Under the
optimized reaction conditions, p-methoxystyrene 8b
successfully afforded chromane 9b in good yields (Table 4, entry
2). Dihydronaphthalene 8c yielded the [4 + 2] adduct 9c as a
Inspired by natural products such as the mulberrofurans21
and australisine22 we first evaluated hetero-Diels-Alder
reactions between 2H-chromenes 1a and 3,4-dihydro-2H-pyran
8a under MIL-101-SO3H catalysis. A hetero-Diels-Alder reaction
process involving oQM intermediate is proposed as the main
step of our reaction mechanism. We then designed further
experiments based on our mechanistic hypothesis. We first
postulated that a model substrate 2H-pyran 8a would react with
the oQM generated in situ from 2H-chromene 1a under the
acid-promoted conditions (Scheme 4). The reaction proceeded
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single diastereomer (Table 4, entry 3). Furthermore, electron
rich indene 8d was also a suitable dienophile (Table 4, entry 4).
Lastly, electron-rich flavene was also tested and the resulting
tetrahydrochromeno polycyclic compound was obtained in 90%
yield (Table 4, entry 5).
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DOI: 10.1039/C8NJ01354C
Scheme 5. Mix coupling between two 2H-chromenes.
Mix coupling between two different chromenes 10 and 11 is
possible (Scheme 5). The homodimerization reaction occurs
faster than the heterodimerization pathway. The hetero-Diels-
Fig. 7 MIL-101-SO3H catalyst filtration test.
A hot filtration test was run for the MIL-101-SO3H catalyzed
dimerization reaction of 1a; the solid catalyst was filtered after
4 h of reaction. The dimerization of 2H-chormene 1a did not
further proceed after the MIL-101-SO3H catalyst was removed
from the solution, which further suggests no acid leaches into
the solution (Fig. 7). Furthermore, inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis of the filtrate
showed the presence of 1.9 ppm of the Cr ion, which indicates
extremely low leaching of the metal ion during the dimerization
catalysis. SEM images and X-ray powder diffraction spectrum of
the MIL-101-SO3H catalyst were collected after ten reaction
cycles; these data were found to be indistinguishable from
those of the fresh catalyst (Fig. S1 and S2). This observation
suggests the high chemical stability of the MIL-101-SO3H
catalyst.
Alder reaction occurs faster with electron-rich substrate 11
.
4 Conclusions
In conclusion, a heterogeneous MIL-101-SO3H catalyst bearing
an aromatic sulfonic acid group was synthesized and utilized in
2H-chromene dimerization reaction. The structural morphology
of MIL-101-SO3H was fully characterized by SEM, XRD, TGA, FTIR
and BET. The newly developed MIL-101-SO3H showed high
activity and diastereoselectivity in the dimerization reaction at
only 1 mol% catalyst loadings, which provides higher TON and
TOF than several other heterogeneous Brønsted acid catalysts.
The high chemical stability of the MIL-101-SO3H catalyst was
due to its strong covalent bond, and did not suffer from leaching.
Further studies involving new synthetic applications of the MIL-
101-SO3H catalyst are in progress.
Fig.
6
MIL-101-SO3H catalyst recycling test for the
homodimerization of 1a
.
The acidic MIL-101-SO3H catalyst can be recycled up to ten
times without compromising the yield and selectivity of the
reaction. The recyclability of the MIL-101-SO3H catalyst was
evaluated using 7-methoxy-2-(4-methoxyphenyl)-2H-chromene
1a in 2 mL of CH2Cl2 at room temperature. As for MIL-101-SO3H,
the yield of desired benzopyranobenzopyran 2a remained 97%
after usage of the same recycled catalyst with 10 batches of
fresh reagent (Fig. 6).
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
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22 Q. J. Zhang, Y. B. Tang, R. Y. Chen and D. Q. Yu, Chem.
This work is supported by Beijing Natural Science Foundation
(2172037) and National Natural Science Foundation of China
(No. 51503016).
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Biodiversity, 2007, 4, 1533–1540.
DOI: 10.1039/C8NJ01354C
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New Journal of Chemistry
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DOI: 10.1039/C8NJ01354C
A facile 2H-chromene dimerization through an ortho-quinone methide
intermediate catalyzed by a sulfonyl derived MIL-101 MOF
Xin Du,a Xiujuan Li,a Houliang Tang,b Wenyu Wang,c Daniele Ramella d and Yi Luana,*
An efficient homo-dimerization of 2H-chromenes was achieved in high yield and
diastereoselectivity using MIL-101-SO3H MOF catalyst.