Enantioselective catalysis with a chiral, phosphane-containing PMO material

Article abstract of DOI:10.1039/c2cc31247f

A novel bistriethoxysilyl-BINAP monomer was prepared and co-condensed with a biphenylene-bridged siloxane precursor in the presence of surfactant templates to give periodic mesoporous organosilicas (PMOs) functionalized with BINAP. Complexation of ruthenium followed by asymmetric catalytic hydrogenation and asymmetric transfer hydrogenation were carried out, and demonstrated that high levels of activity and selectivity are achievable with the chiral material.

Full text of DOI:10.1039/c2cc31247f

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COMMUNICATION
Enantioselective catalysis with a chiral, phosphane-containing PMO materialw
Tomohiro Seki, Kevin McEleney and Cathleen M. Crudden*
Received 20th February 2012, Accepted 22nd March 2012
DOI: 10.1039/c2cc31247f
A novel bistriethoxysilyl-BINAP monomer was prepared and
co-condensed with a biphenylene-bridged siloxane precursor in
the presence of surfactant templates to give periodic mesoporous
organosilicas (PMOs) functionalized with BINAP. Complexation
of ruthenium followed by asymmetric catalytic hydrogenation and
asymmetric transfer hydrogenation were carried out, and demon-
strated that high levels of activity and selectivity are achievable
with the chiral material.
Fig. 1 Organic-bridged siloxane precursors with biphenyl and
binaphthyl structure.
Organic–inorganic hybrid materials have attracted attention
from a variety of disciplines since they combine the functionality
important that the placement of the ligand within the wall can be
network.1 In particular, polysilsesquioxanes can be prepared
controlled and that the porosity of the material is not disrupted to
of the organic spacer with the mechanical strength of an inorganic
permit appropriate access to the eventual catalyst. As compared
using templated approaches that give fine control over micro,
to typical grafting methods, the ability to incorporate ligands
meso and macroscopic ordering, providing a wide array of
structured organic–inorganic hybrid materials (Fig. 1).2–4
directly in the walls of porous materials is expected to provide
considerable advantages in this regard, and also with respect
Although amorphous materials can be prepared without the
decreased cleavage of the ligand from the support.
benefit of a sacrificial template and still have high surface areas,
BINAP is one of the most practical and widely used chiral
bidentate phosphane ligands.8 Its use in the enantioselective hydro-
burying active sites inside the wall and decreasing permeation of
pore structures can be expected to be significantly disordered,
substrates.5 On the other hand, surfactant-templated synthetic
genation of alkenes and carbonyl compounds with Rh or Ru
catalysts was the subject of the 2001 Nobel Prize in Chemistry.9
methods permit greater control over morphology and porosity,
Considering its importance in organic synthesis, then, it is not
with the ultimate example being the 2002 report by Inagaki and
surprising that there have been various supported versions
co-workers who showed that a biphenylene-bridged (2) periodic
reported.10 For example, van Koten and co-workers reported
mesoporous organosilica (PMO) materials could be prepared
that displayed molecular scale order in the walls.2 Sayari and
the immobilization of BINAP on amorphous silica using di-ureyl
functionalized BINAPO monomer (4)2, and Yang and co-workers
co-workers succeeded in preparing PMOs employing the same
biphenylene-bridged precursor under acidic conditions in 2007.6
co-condensed 4 with tetramethylorthosilicate (TMOS, Si(OMe)4)
under acidic conditions employing the tri-block copolymer P123
Previous work from our group showed that a mixed PMO could
as a surfactant template to obtain BINAP-supported in all silica
be synthesized in which a functional chiral organic monomer can
porous materials.11 However, mesoporous materials prepared
be introduced directly into the backbone by the co-condensation
of 2 and 3.7 It was the first example of a chiral PMO which
from monomers that have the siloxane groups directly attached
to the backbone of BINAP have not been prepared, and the
employed an axially chiral molecule, and in which a chiral dopant
synthesis of the dual hybrid PMO materials employing such a
was employed to induce chirality in the material via p–p stacking
interactions between aromatic systems in the solid state.
functionalized chiral ligand has also not yet been realized.
Herein, we describe the synthesis of the title material in
One of the main goals for the development of these materials in
which an immobilization of BINAP was successfully achieved
our lab and others is for their use as heterogeneous catalysts if
appropriate ligands can be introduced.8 In this case, it is especially
via the co-condensation of 1 along with 2 under acidic
conditions employing Brij76 surfactant permitting us to adjust
the pore diameter to about 26 A. In addition, we present the
Department of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada K7L 3N6.
application of the resulting functional material to heterogeneous
asymmetric hydrogenation both under high-pressure hydro-
genation conditions and atmospheric hydrogen transfer.
Functionalized BINAPO monomer 1 was prepared from
commercially available BINAP 5 in three steps (Scheme 1).
Protecting the phosphanes of BINAP as a bisphosphane oxide
E-mail: cruddenc@chem.queensu.ca; Fax: +1-613533-6669;
Tel: +1-613533-6000
w Electronic supplementary information (ESI) available: Material
preparations and characterizations including N2 isotherms, CP MAS
NMR spectra and powder XRD spectra are available. See DOI:
10.1039/c2cc31247f
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6369–6371 6369

Table 1 Physical characteristics of BINAPO-PMOs
Molar
Surface
Pore
Pore volume/
Entry Surfactant ratio of 1/2 area/m2 gꢀ1 a size/Ab cm3 gꢀ1
1
2
3
4
Brij76
rac-15/85
(S)-15/85
(R)-15/85
(R)-5/95
0/100
(S)-7.5/92.5 701
(R)-7.5/92.5 786
(R)-11/89
785
626
703
708
668
25.1
26.4
24.2
26.9
27.0
18.1
16.5
77
0.549
0.461
0.626
0.574
0.540
0.345
0.347
0.61
5
6c
7c
8d
P123
331
a
b
Calculated from BET plot. BJH adsorption average pore size.
c
d
TEOS (Si(OEt)4) was employed instead of 2. Compound 4 and
TMOS (Si(OMe)4) were co-condensed, reported by Wang et al.11
Scheme 1 Synthesis of the 5,50-bistriethoxysilyl-BINAPO (1).
(BINAPO, 6) was accomplished by oxidation with hydrogen
peroxide. Following this, bisiodination at the 5- and 50-positions
gives bisiodoBINAPO 7. Direct installation of the triethoxysilyl
moieties on the binaphthyl structure was performed via a
Rh-catalyzed Masuda reaction. This last step proved to be a
significant challenge since the Masuda reaction resulted in bis-
silylation along with monosilylation/monoreduction. The desired
bissilylated ligand precursor 1 could only be obtained cleanly after
chromatographic separation of the monosilylated byproduct.
Thus the low yield of this product resulted from a combination
of the poor selectivity in the Masuda coupling and losses during
the chromatography of the sensitive bis siloxane. Regardless, since
all other steps are high yielding (>90%), and the lowest yielding
step was the final step, this route provided sufficient material to
prepare and test PMO materials containing 1 as an additive.
It must be noted here that prior to the synthesis of 1, we
attempted to prepare 4,40-functionalized BINAP by several routes.
Although both bromination and iodination at the 4,40-position
could be achieved, successive rhodium catalyzed silylation did not
give any of the desired product, only reduction.
elevated temperatures,12 conditions that can result in dramatic
losses of surface area and porosity in mesoporous materials,
possibly by agglomeration of the silane on the surface of the
materials. To address this, we have found that ultra high purity
trichlorosilane is needed, along with the use of triethylphosphite
as an oxygen scavenger.13 The reaction was furthermore carried
out at 80 1C on material that had previously been deactivated
by silylation of the surface. 31P CP MAS NMR was used to
probe the success of the reduction as the signal at 30 ppm for
BINAPO shifted to ꢀ15 ppm for the trivalent phosphane of
BINAP (Fig. S-4A, ESIw). The reaction occurred without loss
of mesoporosity or order (Table S-1, entry 2, ESIw). It is notable
that clean and complete conversion of the phosphane oxide to
the phosphane implies a remarkable complete accessibility of all
BINAP functionalities contained within the material.
With the reduction successfully completed, complexation of
ruthenium on the BINAP unit was carried out following literature
conditions14 after drying the PMO under high vacuum at 80 1C in
an oil bath overnight prior to use. Successive filtration and washing
with copious amounts of methanol followed by dichloromethane
gave a pale red powder. This powder was stored in a vial filled with
argon, but still could be handled in open air when it was dry.
31P CP MAS NMR analysis of this material (Ru/5%(R)-
BINAP-PMO) showed successful complexation (Fig. S-4, ESIw).
Further analysis of the material by pXRD indicated that the
materials exhibit some degree of crystallinity within the walls due
to local alignment of the organic groups (Fig. S-3, ESIw). This
increased order will likely add to the stability of the material. It is
notable that preparing a locally aligned PMO in acidic conden-
sation condition with Brij76 template is quite rare, thus the use of
a rigid binapthyl precursor 1, which matches perfectly in size with
the bulk biphenylene monomer 2, is likely responsible for the
highly ordered walls observed in these materials. The amount of
ruthenium doped on the solid catalyst was determined to be
0.084 mmol gꢀ1 by ICP-MS elemental analysis. The TEM images
of Ru/5%(R)-BINAP-PMO exhibited cleanly ordered long nano
channels and hexagonal structure (Fig. 2).
Thus with a viable route to compounds 1 and 2, these were
mixed in a 15/85 and 5/95 molar ratio along with surfactant Brij-76
as the structure-directing agent under acidic conditions (Molar
ratio: Si/water/HCl/NaCl/EtOH/Brij-76 = 1.00/600/8.40/19.0/
18.4/0.533). After extraction of the surfactant, nitrogen adsorption
analysis of this material showed that it was highly ordered with a
pore diameter of ca. 26 A (Table 1, entry 1–4. Fig. S-1, 2 (ESIw)).
Similar materials were prepared employing TEOS (Si(OEt)4) as the
bulk constituent (1/TEOS = 7.5/92.5 molar ratio) (entries 6 and 7).
These primarily silica materials had less ordered and smaller
pores making them micro- rather than mesoporous, but they
were characterized by high surface areas and good pore volumes.
These results clearly showed that installing the functionality in
the wall structure gives functionalized PMOs with larger surface
areas, compared with the PMO which was functionalized via
monomers prepared with longer, more flexible tethers (entry 8).
To generate an effective heterogeneous catalyst from this
material, three post-condensation steps were employed: TMS
capping of free silanols, phosphane oxide reduction, and com-
plexation of ruthenium. 29Si CPMAS NMR of the silylated
materials showed large T3-sites, appearing concomitantly with
disappearance of T1-sites, implying successful capping of the
silanols (Table S-1, entry 1. Fig. S-4, B, ESIw).
Having prepared a well ordered, mesoporous organosilica
material containing BINAP-modified Ru catalyst in the walls,
we then set out to test its catalytic activity in the ruthenium-
catalyzed asymmetric hydrogenation of b-ketoesters. Since the
condensation points for the bis-trialkoxysilyl BINAP monomer
are the 5,50-positions which are not on the atropisomeric axis, and
since there are no flexible spacer units between the BINAP and
The reduction of phosphane oxides to phosphanes in
solution is typically carried out with excess trichlorosilane at
c
6370 Chem. Commun., 2012, 48, 6369–6371
This journal is The Royal Society of Chemistry 2012

Scheme 2 Asymmetric transfer hydrogenation of a simple ketone
(4-methoxyacetophenone).
studies noting the base-sensitivity of mesoporous materials,
the mesostructure of this material was highly maintained after
being subjected to a solution of basic isopropyl alcohol at high
temperature for an extended time (Table S-1, entry 5, ESIw).15
In conclusion, BINAP and biphenylene-bridged hybrid
periodic mesoporous organosilicas (PMOs) were synthesized by
co-condensation of a novel 5,50-bistriethoxysilyl BINAP monomer
with 4,40-bistriethoxysilylbiphenyl, templated by Brij76 surfactant
under acidic conditions. The resulting PMOs had well ordered
mesopores and crystal-like local alignment of the organic groups
within the walls. Since the chiral BINAP unit was polymerized via
rigid linkers at the 5,50 positions, which is off the axis of chirality,
the potential existed for detrimental effects on the bite angle of the
ligand. However, no negative effects on asymmetric catalysis were
observed for two different hydrogenation reactions. Further studies
to apply this material to other catalytic reactions are in progress.
Fig. 2 TEM images of Ru doped 5%(R)-BINAP-PMO.
Table 2 Asymmetric catalytic hydrogenation under high-pressure
hydrogen gas
Entry
R1, R2
Yield [%]a
e.e. [%]b
1
2
3
4
5
6
7c
Me, Me
99
99
99
99
99
99
99
99
99
99
99
99
99
99
Me. Et
Et, Me
Me, iPr
Me, 4-MeOPh
Me, Me
b
Ru/5%(R)-BINAP-PMO was employed.a Isolated yield. Determined
by supercritical fluid chromatography (SFC) using CHIRALPAK OD-H
column with 1B20% MeOH additive at 50 1C, 2 mL minꢀ1, 200 bar.
Notes and references
Homogeneous Ru/BINAP was employed under the same conditions.11
c
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(entry 2). Another notable advantage from this heterogeneity is
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6369–6371 6371