Chiral porous hybrid solids for practical heterogeneous asymmetric hydrogenation of aromatic ketones

Article abstract of DOI:10.1021/ja0348344

Chiral porous zirconium phosphonates containing Ru-BINAP-DPEN moieties were synthesized via a molecular building-block approach, and characterized by a variety of techniques including TGA, adsorption isotherms, XRD, SEM, IR, and microanalysis. These hybrid solids were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with remarkably high ee values of up to 99.2%. These solid catalysts can also be easily recycled and reused for eight times without the loss of activity and enantioselectivity. Ready tunability of such a molecular building-block approach will allow the optimization of these hybrid materials and promise to lead to other practically useful heterogeneous asymmetric catalysts. Copyright

Full text of DOI:10.1021/ja0348344

Published on Web 08/28/2003
Chiral Porous Hybrid Solids for Practical Heterogeneous Asymmetric
Hydrogenation of Aromatic Ketones
Aiguo Hu, Helen L. Ngo, and Wenbin Lin*
Department of Chemistry, CB#3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received February 24, 2003; E-mail: wlin@unc.edu
Scheme 1
Asymmetric reduction of prochiral olefins, ketones, and imines
is one of the most powerful methods for the production of optically
active compounds.1 Among these methodologies, Ru and Rh
complexes of chiral chelating bisphosphines, particularly 2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl (BINAP) and its derivatives,
were widely used for the hydrogenation of a wide range of
substrates with high chemo- and enantioselectivity.2 However, their
practical applications in industrial processes were often hindered
due to high costs of both noble metals and chiral ligands as well
as difficulties in removing trace amounts of toxic metals from the
organic products.
Heterogenization of these highly enantioselective catalysts has
proven effective in overcoming these problems. The heterogenized
catalysts can potentially combine the advantages of both homoge-
neous and heterogeneous systems and therefore provide easily
recyclable and reusable solid catalysts that have uniform and
precisely engineered active sites similar to those of their homoge-
neous counterparts. Many heterogenization approaches have been
by the microanalysis results. The IR spectra exhibit strong and broad
explored, including attaching the chiral catalysts to organic
polymers, dendrimers, membrane supports, and porous inorganic
oxides and immobilization via biphasic systems.3 The heterogenized
catalysts afforded by these methods are, however, typically less
effective than their homogeneous counterparts.
peaks at 950-1150 cm-1 for the P-O stretches and intense and
broad O-H stretching vibrations at ∼3350 cm-1 for the H2O
solvates. N2 adsorption measurements (Figure 1) indicate that both
Zr-Ru-L1 and Zr-Ru-L2 are highly porous with rather wide pore
size distributions. Zr-Ru-L1 exhibits a total BET surface area of
328 m2/g with a microporous surface area of 60 m2/g and a pore
volume of 0.65 cm3/g (by BJH method). Zr-Ru-L2 exhibits a total
BET surface area of 400 m2/g with a microporous surface area of
81 m2/g and a pore volume of 0.98 cm3/g (by BJH method). SEM
images show that both solids are composed of submicrometer
particles, while PXRD indicates that both solids are amorphous.
With the built-in Ru-BINAP-DPEN moieties, porous solids
of Zr-Ru-L1 and Zr-Ru-L2 exhibited exceptionally high activity
and enantioselectivity in hydrogenation of aromatic ketones.
Acetophenone was hydrogenated to 1-phenylethanol with complete
In an effort to rationally design functional hybrid solids based
on organic linkers and metal nodes,4 we have explored the synthesis
of chiral porous zirconium phosphonates for applications in
asymmetric catalysis. Such hybrid materials will combine the robust
metal phosphonate frameworks5 with highly enantioselective metal
complexes of the pendant chiral bisphosphines3 and will find
practical applications in heterogeneous asymmetric catalysis. Herein
we report the synthesis of novel chiral porous solid catalysts for
practically useful enantioselective hydrogenation of unfunctionalized
aromatic ketones. Remarkably, BINAP-derived porous Zr phos-
phonates provide enantioselectivity superior to their parent homo-
geneous counterpart Ru-BINAP-DPEN (DPEN is 1,2-diphenyl-
ethylenediamine) system developed by Noyori et al.6
Treatment of (R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl-
4,4′-bis(phosphonic acid), L1-H4, with [Ru(benzene)Cl2]2 followed
by (R,R)-DPEN afforded the phosphonic acid-substituted Ru-
BINAP-DPEN intermediate, which was directly reacted with Zr-
(OtBu)4 under reflux conditions to give chiral porous Zr phospho-
nate of the approximate formula Zr[Ru(L1)(DPEN)Cl2]‚4H2O (Zr-
Ru-L1). The solid precatalyst Zr-Ru-L2 with a 6,6′-disubstituted
BINAP was similarly prepared and also has an approximate formula
of Zr[Ru(L2)(DPEN)Cl2]‚4H2O. These chiral porous Zr phospho-
nates have been characterized with a variety of techniques including
TGA, adsorption isotherms, XRD, SEM, IR, and microanalysis
(Scheme 1).
TGA results indicated that Zr-Ru-L1 and Zr-Ru-L2 lost 5.1
and 5.4% of their weights by 200 °C, respectively (expected 5.42%
for loss of four water molecules). These formulations are supported
Figure 1. N2 adsorption isotherms for Zr-Ru-L1 and Zr-Ru-L2 at 77
K. The inset shows BET plot for Zr-Ru-L1 in the mesoporous region.
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J. AM. CHEM. SOC. 2003, 125, 11490-11491
10.1021/ja0348344 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Heterogeneous Asymmetric Hydrogenation of Aromatic
Table 2. Recycling and Reuse of Zr-Ru-L1 Solid Catalyst for
Ketonesa
Hydrogenation of 1-Acetonaphthonea
run
1
2
3
4
5
6
7
8
ee (%)
99.0 99.0 99.1 99.0 99.0 99.2 99.1 99.0
85
conversion (%) 100 100 100 100 100 100 95
solid
loading KOBu
(%)
a The reactions were carried out with 0.1 mol % solid loading and 1%
KOtBu under 700 psi H2 pressure for 20 h.
t
Zr−Ru−L1
ee (%)
Zr−Ru−L2
ee (%)
substrate
(%)
Ar ) Ph, R ) Me
0.1
1
1
1
1
1
1
1
1
96.3 (97.1)b 79.0 (81.3)b
current plasma spectroscopic studies indicated that less than 0.2%
of Ru metal has leached into the organic product for each round of
hydrogenation. We have also reused the Zr-Ru-L2 system for
hydrogenation of 1-acetonaphthone three times with complete
conversions and enantioselectivity of 96.3, 95.7, and 94.7%,
respectively.
In summary, we have designed and synthesized novel chiral
porous Zr phosphonates containing Ru-BINAP-DPEN moieties.
The Zr-Ru-L1 solid catalyzes heterogeneous asymmetric hydro-
genation of aromatic ketones with practically useful, remarkably
high activity and enantioselectivity (up to 99.2% ee). These solid
catalysts can be readily recycled and reused without the loss of
activity and enantioselectivity. The tunability of such a molecular
building-block approach promises to lead to other practically useful
heterogeneous asymmetric catalysts.
Ar ) 2-naphthyl, R ) Me 0.1
97.1
99.2
96.0
94.9
97.0
93.1
90.6
99.2
98.9
82.1
91.5
79.9
59.3
79.5
83.9
-
Ar ) 4′-tBu-Ph, R ) Me
Ar ) 4′-MeO-Ph, R ) Me 0.1
Ar ) 4′-Cl-Ph, R ) Me
Ar ) 4′-Me-Ph, R ) Me
Ar ) Ph, R ) Et
Ar ) Ph, R ) cyclo-Pr
Ar ) 1-naphthyl, R ) Me 0.1
0.1
0.1
0.1
0.1
0.1
1
0.4
95.8
0.02
0.005 0.02 98.8 (70)c
0.005 0.02 98.6d
a All of the reactions were carried out in 20 h, and the ee values were
determined by GC on a Supelco â-Dex 120 column. The absolute
configurations of the products are identical to those obtained by the Ru-
(R)-BINAP-(R,R)-DPEN catalyst. All the conversions were >99% as
judged by the integrations of GC peaks. b Homogeneous reactions. c 70%
conversion. d 40 h reaction time.
Acknowledgment. We thank NSF (CHE-0208930) for financial
support. W.L. is an A. P. Sloan Fellow, a Beckman Young
Investigator, a Cottrell Scholar of Research Corp, and a Camille
Dreyfus Teacher-Scholar.
conversion and 96.3% ee in 2-propanol with 0.1 mol % loading of
Zr-Ru-L1 solid. This level of enantioselectivity is significantly
higher than that observed for the parent Ru-BINAP-DPEN
homogeneous catalyst which typically gives ∼80% ee for the
hydrogenation of acetophenone under similar conditions.6a-b In
comparison, the Zr-Ru-L2 solid gives 79.0% ee for the hydro-
genation of acetophenone under the same conditions. As Table 1
shows, the Zr-Ru-L1 solid has also been used to catalyze a series
of other aromatic ketones with uniformly and remarkably high ee’s
of 90.6-99.2% and complete conversions. Although the Zr-Ru-
L2 solid is also highly active for the hydrogenation of aromatic
ketones, the enantioselectivity of Zr-Ru-L2 is modest and similar
to that of parent Ru-BINAP-DPEN homogeneous catalyst.7
Aromatic ketones can also be hydrogenated with much lower
catalyst loading. For example, with only 0.02 mol % solid loading
of Zr-Ru-L1, 1-acetonaphthone can be hydrogenated with com-
plete conversion and 98.9% ee in 20 h. When the solid loading
was decreased to 0.005 mol %, it took longer reaction time (40 h)
for the hydrogenation of 1-acetonaphthone to complete (98.6% ee).
The TOF is calculated to be ∼500 h-1 at complete conversion and
∼700 h-1 at 70% conversion.8
Supporting Information Available: Experimental procedures and
10 figures (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
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We have also successfully reused both the Zr-Ru-L1 and Zr-
Ru-L2 systems for asymmetric hydrogenation of 1-acetonaphthone
without the deterioration of enantioselectivity. As shown in Table
2, the Zr-Ru-L1 system was used for eight cycles of hydrogena-
tion without any loss of enantioselectivity. The activity did not
decrease for the first six runs, but began to drop at the seventh
run. This loss of activity may not reflect the intrinsic instability of
the Zr-Ru-L1 solid catalyst. The catalyst recycling and reuse
experiments were conducted without rigorous exclusion of air, and
the oxygen sensitivity of the ruthenium hydride complexes may
have contributed to the loss of activity after multiple runs. Direct
(7) We believe that very different ee values observed for the Zr-Ru-L1 and
Zr-Ru-L2 systems are a result of the substituent effects on BINAP. The
ee values similar to those listed in Table 1 have been observed for
homogeneous Ru-L1-DPEN and Ru-L2-DPEN systems.
(8) Under identical conditions, we have obtained a TOF of 1725 h-1 (69%
conversion) and 1250 h-1 (complete conversion) for the homogeneous
Ru-(R)-BINAP-(R,R)-DPEN catalyst.
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