A microporous binol-derived phosphoric acid

Article abstract of DOI:10.1002/anie.201109072

No slow down: A microporous recyclable heterogeneous catalyst made from a 1,1'-binaphthalene-2,2'-diol (binol)-derived phosphoric acid chloride is as active as the corresponding homogeneous catalyst when using the same mass of both in different reactions. Reaction rates, yields, and enantioselectivities are comparable. Copyright

Full text of DOI:10.1002/anie.201109072

Angewandte
Chemie
DOI: 10.1002/anie.201109072
Organocatalysis
A Microporous Binol-Derived Phosphoric Acid**
Dipti S. Kundu, Johannes Schmidt, Christian Bleschke, Arne Thomas,* and Siegfried Blechert*
Dedicated to Professor Ekkehard Winterfeldt on the occasion of his 80th birthday
During the last decade, organocatalysis has emerged as an
important tool for a rapidly expanding range of synthetic
transformations.[1] Like metal catalysis, organocatalysis also
provides tremendous opportunities in the field of asymmetric
catalysis.[2] In particular, binol-derived phosphoric acid catal-
ysis[3] has been applied successfully for many asymmetric
reactions, such as transfer hydrogenation,[4] hydrocyana-
tions,[5] Aldol-type[6] and Friedel–Crafts-type reactions,[7] or
the Baeyer–Villiger reaction[8] in recent years. Their success in
homogeneous reactions is undisputed but the drawback is still
the recovery of the expensive catalysts. Therefore it is
desirable to design heterogeneous catalysts,[9] especially
because often up to 20 mol% of the catalyst has to be used.
Over the years, several methods have been developed to
immobilize catalysts.[10] In the field of organocatalysis, several
catalysts have already been immobilized successfully, such as
proline[11] and diarylprolinol silyl ether.[12] Very recently,
polymer-supported catalysts based on binol (1,1’-binaphtha-
lene-2,2’-diol)-derived phosphoric acids have been prepared
by the Rueping group.[13] However, these types of polymer-
supported catalysts often show slower reaction rates com-
pared to their homogeneous counterpart because of lower
accessibility of the catalytic center.[14] Recently we reported
on a concept using binol-derived tectons to generate a micro-
porous polymer network.[15]
Sonogashira,[18] Suzuki,[19] Yamamoto,[20] or oxidative cou-
pling can be applied.[21] We chose 1,1’-binaphthalene-2,2’-diyl
hydrogenphosphate (BNPPA) with 9-anthracenyl as bulky
groups in 3,3’-positions as a starting tecton. This compound is
known to catalyze Strecker reactions,[9] Friedel–Crafts alky-
lation, and aza-ene-type reactions[22] with good selectivity. To
couple BNPPA units, we decided to introduce a 3-thiophenyl
group at the 10-position of the anthracene units. Thiophenes
of this type can be coupled under mild oxidative coupling
condition using FeCl3. As the thiophene unit is connected at
3-position, it has two free sites (2,5-positions) for oxidative
coupling,[23] enabling the formation of a network (Scheme 1).
Such a polymer network, which only contains the
molecular catalyst, ensures a high density and accessibility
of catalytic centers and thus fast reaction rates. A variety of
functional organic tectons have been applied recently to
generate microporous polymers.[16] Also few examples have
been applied as organocatalysts or in asymmetric catalysis.[17]
To build up porous polymers from catalytically active tectons,
polymerizable groups have to be introduced. Depending on
the functionality, different types of polymerization, such as
[*] D. S. Kundu, Dr. C. Bleschke, Prof. Dr. S. Blechert
Technische Universitꢀt Berlin, Institut fꢁr Chemie, Sekr. C3
Strasse des 17. Juni 135, 10623 Berlin (Germany)
E-mail: blechert@chem.tu-berlin.de
Scheme 1. Strategy to assemble a chiral microporous polymer.
Starting from dimethylated (R)-binol, the diboronic acid 2
was synthesized as precursor for the Suzuki coupling reac-
tion.[24] Double coupling with 3-(10-bromoanthracen-9-yl)-
thiophene led to structure 4 (Scheme 2) in good yields.
Subsequent cleavage of the methyl ether with BBr3 and
treatment with phophorous oxychloride gave the BNPPA
chloride 5. The corresponding phosphoric acid 1, obtained in
overall 74% yield by hydrolysis of 5, is not suitable for
oxidative coupling owing to solubility issues. However, the
FeCl3-mediated reaction is successful, with the highly soluble
5 affording polymeric acid chloride 6. Hydrolysis of 6 with
Dr. J. Schmidt, Prof. Dr. A. Thomas
Technische Universitꢀt Berlin, Institut fꢁr Chemie, Sekr. C2
Hardenbergstrasse 40, 10623 Berlin (Germany)
E-mail: arne.thomas@tu-berlin.de
[**] This work was supported by the Cluster of Excellence “Unifying
Concepts in Catalysis” and the “Berlin International Graduate
School of Natural Sciences and Engineering”. Binol=1,1’-binaph-
thalene-2,2’-diol.
Supporting information for this article, including details of the
monomer synthesis and characterization, HPLC, and NMR, is
available on the WWW under http://dx.doi.org/10.1002/anie.
201109072.
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We expected the new monomer BNPPA 1 should have
similar steric effects at the phosphoric acid center like the
commercially
available
(R)-3,3’-bis(9-anthracenyl)-1,1’-
binaphthyl-2,2’-diyl hydrogenphosphate 8. Therefore, we
compared 1 with 8 and network 7. The porous polymer
network was first tested as a catalyst in the asymmetric
transfer hydrogenation of prochiral benzoxazines. The
3,3’phenanthrenyl-substituted BNPPA catalyst has been
shown to afford the best result, with 98% enantioselectivity
for asymmetric transfer hydrogenation of 3-phenyl-2H-1,4-
benzoxazine.[25] Using the commercially available 9-anthra-
cenyl substituted BNPPA (8) as a catalyst, we obtained 95%
ee (Table 1). Enantioselectivity of this reaction could be even
increased by the new BNPPA (1) to 99% ee. With network 7,
we achieved an ee of 98%, which is comparable to the result
of catalyst 1 and even better than 8. This result shows that
nearly no loss in selectivity occurs when switching over to
heterogeneous catalysis.
Table 1: Asymmetric hydrogenation of 3-phenyl-2H-1,4-benzoxazine.
Scheme 2. Synthesis of the polymer network. Conditions: i) [Pd-
(PPh3)4], K2CO3, THF/H2O, 808C; ii) a) BBr3, CH2Cl2, b) poly(4-vinyl
pyridine), POCl3, CH2Cl2; iii) THF, 1m HCl, RT; iv) FeCl3, toluene,
acetonitrile; v) aqueous HCl.
Entry
Catalyst
Conversion[a]
[%]
ee [%][b]
1
2
3
8
1
99
99
99
95 (S)
99 (S)
98 (S)
aqueous HCl and multiple washing with ethanol, THF, and
CHCl3 gave the desired iron-free material 7 as a powder,
which was insoluble in solvents such as ethanol, THF, CHCl3,
and CH2Cl2 (80% yield).
Nitrogen sorption of the polymer network 7 showed an
isotherm typical for microporous materials. The surface area,
calculated by the BET method, is 386 m2 gÀ1, which should
enable good accessibility of the catalytically active centers
(Figure 1).
network 7
[a] Yield of isolated product 91%; conversion checked by 1H NMR
spectroscopy. [b] ee determined by HPLC using an Chiracel OD-H
column.
In a recent example,[13] Rueping and co-workers immobi-
lized binol-derived phosphoric acid structure into a polymer-
supported catalyst. Using 5 mol% of their polymer-supported
catalyst, a 20–24 h reaction time was required to achieve full
conversion and 94% ee for asymmetric hydrogenation of 3-
phenyl-2H-1,4-benzoxazine. With our catalyst, the same
reaction was complete within 2 h with 5 mol% of the soluble
catalyst 1 or with the same weight percentage of the insoluble
network 7. The reaction rate with heterogeneous catalyst 7
was monitored by kinetic experiments and found to be as fast
as with homogeneous catalyst 1 owing to the microporous
nature and high surface area of our heterogeneous catalyst.
The heterogeneous catalyst 7 can be easily separated by
centrifugation and reused for further repeating runs. For each
run, 99% conversion and 98% ee was observed. Even after 10
runs, the catalyst works without any loss in activity or
selectivity (Figure 2). Furthermore, we performed hot extrac-
tion experiments to check if the catalysis is truly heteroge-
neous in nature or not. After 50% conversion of substrate 9,
we filtered out catalyst 7, and even after 24 h we observed no
further conversion of substrate 9 to product, which shows that
the catalysis is exclusively heterogeneous.
Figure 1. Nitrogen adsorption isotherm of polymer network 7.
*
*
Adsorption, desorption.
2
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Chemie
ene-type reactions, and many other reactions are catalyzed in
this fashion. Terada and co-workers have reported the
addition of enecarbamate and enamide to N-benzoylaldi-
mine.[22] This is an efficient method to prepare chiral b-amino
ketones. To demonstrate the broader scope of network 7, we
also carried out an aza-ene-type reaction (Scheme 3). Addi-
tion of enecarbamate 12 to freshly prepared N-benzoylaldi-
mine and subsequent hydrolysis afforded the b-amino ketone
13 in 82% yield and 81% ee (R).
&
Figure 2. Repeated runs using recycled network 7 as catalyst. % ee,
&
% conversion.
To establish the versatility of our BNPPA network, it was
also used in asymmetric transfer hydrogenation reactions with
other substrates. The tetrahydroquinoline structure is a very
important structural motif in many natural products and
bioactive molecules, and binol-derived phosphoric acids have
been used in the asymmetric transfer hydrogenation reaction
of 2-aryl quinolines.[26] Therefore, polymer network 7 was
used as heterogeneous catalyst in the reduction of 2-aryl
quinolones. The results (Table 2) show that the performance
of the polymer network is again comparable to the homoge-
neous reaction and gives high selectivity for different aryl
substituents.
Table 2: Asymmetric hydrogenation of 2-aryl quinolines.
Scheme 3. Asymmetric Friedel–Crafts alkylation of pyrrole and an aza-
ene-type reaction catalyzed by BNPPA-based network 7.
In conclusion, we have synthesized a new chiral BNPPA-
derived heterogeneous catalyst with high permanent surface
area, which is highly active and selective in asymmetric
organocatalysis, such as transfer hydrogenation, aza-ene-type
reactions and the asymmetric Friedel–Crafts alkylation of
pyrrole. The catalyst is stable, easily separable, and can be
reused several times. This porous material is a new type of
a heterogeneous phosphoric acid with a porous structure with
all the benefits of heterogeneous catalysis without any loss in
activity and selectivity. The reaction rates are comparable
with those of the soluble catalyst.
Entry
Catalyst
R
Conversion
[%]
ee
[%]
1
2
3
4
1
phenyl
phenyl
1-naphthyl
3-methoxyphenyl
99
99
99
99
98 (S)
97 (S)
94 (S)
87 (S)
polymer 7
polymer 7
polymer 7
Apart from hydrogenation reactions, BNPPA-derived
À
Received: December 22, 2011
Revised: February 16, 2012
Published online: && &&, &&&&
catalysts are extensively used for C C bond-formation
reations. Sheng and co-workers reported an efficient
Brønsted acid catalyzed asymmetric Friedel–Crafts alkylation
of pyrroles.[27] Pyrroles are often present as the core in many
natural products and pharmaceuticals.[28] Network 7 was used
for asymmetric Friedel–Crafts alkylation of unprotected
pyrrole with nitroalkene. The reaction in a mixture of DCM
and toluene at room temperature afforded a yield of 91%
with a selectivity of 96% ee (R).
Keywords: asymmetric catalysis · heterogeneous catalysis ·
.
organocatalysis · phosphorus · porous polymers
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Organocatalysis
No slow down: A microporous recyclable
heterogeneous catalyst made from a 1,1’-
binaphthalene-2,2’-diol (binol)-derived
phosphoric acid chloride is as active as
the corresponding homogeneous catalyst
when using the same mass of both in
different reactions. Reaction rates, yields,
and enantioselectivities are comparable.
D. S. Kundu, J. Schmidt, C. Bleschke,
A. Thomas,* S. Blechert*
&&&& — &&&&
A Microporous Binol-Derived Phosphoric
Acid
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!