Privileged phosphine-based metal-organic frameworks for broad-scope asymmetric catalysis

Article abstract of DOI:10.1021/ja500090y

A robust and porous Zr metal-organic framework (MOF) based on a BINAP-derived dicarboxylate linker, BINAP-MOF, was synthesized and post-synthetically metalated with Ru and Rh complexes to afford highly enantioselective catalysts for important organic transformations. The Rh-functionalized MOF is not only highly enantioselective (up to >99% ee) but also 3 times as active as the homogeneous control. XAFS studies revealed that the Ru-functionalized MOF contains Ru-BINAP precatalysts with the same coordination environment as the homogeneous Ru complex. The post-synthetically metalated BINAP-MOFs provide a versatile family of single-site solid catalysts for catalyzing a broad scope of asymmetric organic transformations, including addition of aryl and alkyl groups to α,β-unsaturated ketones and hydrogenation of substituted alkene and carbonyl compounds.

Full text of DOI:10.1021/ja500090y

Communication
pubs.acs.org/JACS
Privileged Phosphine-Based Metal−Organic Frameworks for Broad-
Scope Asymmetric Catalysis
Joseph M. Falkowski,‡,† Takahiro Sawano,‡,† Teng Zhang,‡ Galen Tsun,§ Yuan Chen,⊥ Jenny V. Lockard,⊥
and Wenbin Lin*,‡
‡Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States
§Department of Chemistry, CB#3290, University of North Carolina, Chapel Hill, North Carolina 27599, United States
⊥Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
ABSTRACT: A robust and porous Zr metal−organic
framework (MOF) based on a BINAP-derived dicarboxyl-
ate linker, BINAP-MOF, was synthesized and post-
synthetically metalated with Ru and Rh complexes to
afford highly enantioselective catalysts for important
organic transformations. The Rh-functionalized MOF is
not only highly enantioselective (up to >99% ee) but also
3 times as active as the homogeneous control. XAFS
studies revealed that the Ru-functionalized MOF contains
Ru-BINAP precatalysts with the same coordination
environment as the homogeneous Ru complex. The
post-synthetically metalated BINAP-MOFs provide a
versatile family of single-site solid catalysts for catalyzing
a broad scope of asymmetric organic transformations,
including addition of aryl and alkyl groups to α,β-
unsaturated ketones and hydrogenation of substituted
alkene and carbonyl compounds.
Figure 1. Post-synthetic metalation of BINAP-MOF (1) and
representative catalytic activities of metalated MOFs.
construction of MOF-based asymmetric catalysts, attributed to
the difficulty in synthesizing bridging ligands based on chiral
bisphosphines and the sensitivity of phosphines to the conditions
used in typical MOF crystal growth. Lin et al. reported an
amorphous Zr-phosphonate system containing Ru-BINAP
precatalysts for hydrogenation of both β-keto esters and aromatic
ketones.9 However, only a small fraction of the catalysts on the
surfaces of these Zr-phosphonate particles is active in asymmetric
hydrogenation reactions.
We report here the first chiral MOF based on a BINAP-derived
dicarboxylate linker (L) and its post-synthetic metalation to
afford highly enantioselective catalysts for hydrogenation and
aryl/alkyl addition reactions (Figure 1). This BINAP-MOF
contains the Zr6O4(OH)4(O2CR)12 cluster secondary building
unit (SBU) and adopts the same framework topology as UiO-66
reported by Lillerud et al.10 The UiO structure provides an ideal
platform to design MOF-based heterogeneous catalysts due to
their stability under a range of reaction conditions. We
demonstrate that BINAP-MOF is a versatile precursor to
multiple catalytic systems through the judicious choice of post-
synthetic metalation conditions. The metalated MOF materials
are efficient catalysts for asymmetric hydrogenation of
substituted alkene and carbonyl compounds and addition of
arylboronic acids and AlMe3 to α,β-unsaturated ketones.
The BINAP-derived dicarboxylic acid, H2L, was prepared from
4,4′-I2-BINAP11 in a multistep sequence as shown in Scheme 1.
s an emerging class of porous molecular materials,1 metal−
Aorganic frameworks (MOFs) provide a highly tunable
platform to engineer heterogeneous catalysts for important
reactions, e.g., asymmetric organic transformations, that cannot
be achieved with traditional porous inorganic materials.2 Among
the asymmetric MOF catalysts reported to date, the most
efficient examples all contain a privileged chiral ligand as the
means for enantio-differentiation.3 The first MOF catalyst with
significant enantiomeric excesses (ee’s) contained the C2-
symmetric 1,1′-bi-2-naphthol (BINOL).4 The post-synthetically
generated Ti-BINOLate moiety in the chiral MOF was
responsible for high ee’s observed for diethylzinc additions to
aromatic aldehydes.4b Subsequently, a Mn-salen-based MOF was
used for asymmetric epoxidation of alkenes.5 Since these reports,
multiple stereoselective MOF catalysts have been developed
based on BINOL- and salen-based ligands.6
Of the pantheon of privileged chiral ligands, 2,2′-bis(diphenyl-
phosphino)-1,1′-binaphthyl (BINAP) has received the most
attention in the homogeneous catalysis community.7 Since the
design of this C2-symmetric ligand by Noyori et al. in 1980,
BINAP has been used as a source of chirality in many late
transition metal-catalyzed reactions and is the gold standard
when developing new chiral bisphosphine ligands.8 Despite the
importance of the BINAP ligand, it has not yet been used in the
Received: January 9, 2014
Published: March 31, 2014
© 2014 American Chemical Society
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Communication
a
Scheme 1. Synthesis of H2L Starting from 4,4′-I2-BINAP
a
Reagents: (i) H2O2, acetone, 85% yield; (ii) Pd(PPh3)4, CuI, PPh3,
methyl 4-ethynylbenzoate, THF/TEA, 67% yield; (iii) HSiCl3, TEA,
m-xylene, 75% yield; (iv) NaOH, THF, EtOH, 95% yield.
4,4′-I2-BINAP was oxidized with hydrogen peroxide and coupled
with methyl 4-ethynylbenzoate via a Pd-catalyzed Sonogashira
reaction to yield 4,4′-bis(methyl-4-carboxyphenylethynyl)-
BINAP oxide. Reduction of the BINAP oxide following by
saponification led to H2L in 41% overall yield.12 Due to the air-
sensitive nature of the phosphine, solvothermal crystal growths
of BINAP-MOF were carried out in an air-free environment. A
mixture of equimolar H2L and ZrCl4 in dimethylformamide
(DMF) and a small amount of trifluoroacetic acid was degassed
in a glass tube, flame-sealed under vacuum, and heated at 120 °C
for 3 days, to yield the BINAP-MOF (1) as colorless octahedral
crystals in 44% yield.
1 crystallizes in the F23 chiral space group. The asymmetric
unit of 1 contains half of the L ligand and one-twelfth of the
Zr6O4(OH)4 SBU. The MOF contains both octahedral and
tetrahedral cages with edges measuring 23 Å (Figure 2a), but the
naphthyl and diphenylphosphino moieties could not be located
on the electron density maps due to the free rotation of the C−C
single bonds around the ethynyl groups of the L ligand. The
solvent-accessible void space was calculated to be 76.3% using
PLATON. Thermogravimetric analysis (TGA) of 1 indicated a
solvent content of 60% (Figure S1, Supporting Information
[SI]), whereas a combination of TGA and NMR solvent analyses
gave the complete formula of Zr6(OH)4O4L6·126DMF·156H2O
for 1 (Figure S2). Dye uptake measurements showed that 13.5 wt
% of brilliant blue R-250 could be loaded into the channels
(Figure S3), indicating the presence of large open channels in 1
that can accommodate large dye molecules and metalating
agents. N2 adsorption measurements did not show porosity for 1,
presumably due to framework distortion upon removal of solvent
molecules, which has been observed frequently for mesoporous
MOFs with large open channels.6b,13
Post-synthetic metalation of 1 was performed by treating with
1 equiv of [Rh(nbd)2](BF4) to afford 1·Rh or with 4.9 equiv of
Ru(cod)(2-Me-allyl)2 followed by HBr to afford 1·Ru (relative to
the L equivalents in 1, SI).4b,14 Powder X-ray diffraction (PXRD)
indicated that the crystallinity of 1 was maintained in both 1·Rh
and 1·Ru after the metalation reactions. Inductively coupled
plasma mass spectrometry (ICP-MS) analyses of the Zr:Rh and
Zr:Ru ratios of the digested metalated MOFs gave Rh and Ru
loadings of 33% and 50% for 1·Rh and 1·Ru, respectively.
Although 1·Rh and 1·Ru appear to remain single crystalline (SI),
the rotational disorder of the L ligands and the partial metalation
make it impossible to study the Ru and Rh coordination
environments by single-crystal X-ray crystallography. Instead, we
resorted to X-ray absorption fine structure spectroscopy (XAFS)
Figure 2. (a) Post-synthetic metalation of BINAP-MOF (1) to form 1·
Ru and 1·Rh. (b) PXRD patterns of pristine 1 (simulated from the CIF
file, black; experimental, red) and freshly prepared 1·Ru (blue). (c)
PXRD patterns of pristine 1 (black), freshly prepared 1·Rh (red), and 1·
Rh recovered from AlMe3 addition reactions (blue). The broad peaks at
2θ ≈ 20° are from the glass capillary tubes. (d) XANES spectra for the
Ru K-edge of 1·Ru (black) and Ru(Me2L)(MeOH)2Br2 (red). Inset:
Experimental EXAFS spectra in R for 1·Ru (black) and Ru(Me2L)-
(MeOH)2Br2 (red), showing their similarity. (e) Experimental EXAFS
spectra in R (solid traces) for 1·Ru and Ru(Me2L)(MeOH)2Br2 and fits
(dashed lines). The data for Ru(Me2L)(MeOH)2Br2 are shifted
vertically by 3 units for clarity.
to determine the Ru coordination environment in 1·Ru.15 Ru K-
edge spectra were collected for powder samples of Ru(Me2L)-
(MeOH)2Br2 and 1·Ru. Comparison of these data, depicted in
Figure 2d,e, reveals nearly identical coordination environments
of the Ru ions in the two systems. Further analysis through XAFS
data fitting shows that the first coordination shell peak in each
case arises from the combination of Ru−P and Ru−O single
scattering paths, while the second peak is dominated by the
longer Ru−Br scattering path. The scattering path distances and
degeneracies derived from these fits are consistent with distorted
octahedral coordination of the Ru centers with two P atoms of
the L ligand, two methanol solvent molecules, and two Br atoms
(Table S2).
A broad scope of catalytic activities was obtained with 1
through the judicious choice of metalating agents. 1·Rh showed
excellent activity in conjugate additions of arylboronic acids to 2-
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yields for 6a and 6b, respectively. At the same catalyst loadings,
the conversions and yields observed for 1·Rh are higher than
those for the Rh-Me2L homogeneous control. The ee’s seen for
1·Rh are comparable to those for the Rh-Me2L homogeneous
control. 1·Rh is thus an excellent single-site solid asymmetric
catalyst for multiple organic addition reactions.
Table 1. Asymmetric Additions of Arylboronic Acids to 2-
Cyclohexenone by 1·Rh and Homogeneous Catalysts
a
We have carried out a number of experiments to demonstrate
the “heterogeneous” nature of 1·Rh. First, the PXRD of 1·Rh
recovered from the reaction between AlMe3 and 2-cyclo-
hexenone (2a) remained the same as those of freshly prepared
1 and 1·Rh (Figure 2c). Second, at a 0.4 mol% catalyst loading,
the 1·Rh catalyst could be recovered and reused for AlMe3
addition to 2a with only slight decreases in conversions and
enantioselectivities. The conversions/ee’s for three consecutive
runs (with recovered 1·Rh) are 96/99%, 95/98%, and 87/96%,
respectively. Third, the amount of Rh and Zr leaching into the
supernatant during the AlMe3 addition reaction to 2a is <0.4%
and 1.0%, as determined by ICP-MS. Further, the amount of Rh
and Zr leaching into the supernatant during phenylboronic acid
addition to 2a is <0.9% and 0.2%, as determined by ICP-MS.
Finally, the 1·Rh recovered from the reaction between 2a and m-
methylcarboxyphenylboronic acid (85% isolated yield, 99% ee)
could be reused to catalyze addition of p-acetylphenylboronic
acid (65% isolated yield, 99% ee), whereas removal of 1·Rh 1 h
after the reaction between 2a and m-methylcarboxyphenyl-
boronic acid completely stopped the reaction (Scheme S1). All of
the above evidence strongly supports the notion that 1·Rh is a
recoverable and reusable, highly active, enantioselective single-
site solid catalyst.
cat.
loading
(mol%)
isolated
b
entry
Ar
catalyst
1·Rh
1·Rh
yield (%) ee (%)
1
Ph
1
1
1
3
3
1
1
1
3
3
3
80
85
99
0
>99
>99
99
2
m-MeCO2Ph
p-MeC(O)-Ph
Ph
3
1·Rh
Rh-H2L
c
4
N/A
N/A
>99
>99
>99
>99
99
c
d
5
Ph
Rh-Me2L
7
6
Ph
Rh-BINAP
Rh-BINAP
Rh-BINAP
Rh-BINAP
Rh-BINAP
Rh-BINAP
29
34
46
85
87
87
7
m-MeCO2Ph
p-MeC(O)-Ph
Ph
8
9
10
11
m-MeCO2Ph
p-MeC(O)-Ph
>99
a
Reaction conditions: 2a (1 equiv), 3 (3 equiv), NEt3 (1 equiv),
catalyst (1 or 3 mol% Rh),1,4-dioxane (0.04 M), H2O (0.04 M) at 40
°C for 20 h. Determined by chiral HPLC. The Rh-H2L and Rh-
Me2L complexes are insoluble in the reaction solvents. Yield
b
c
d
determined by NMR integration.
Table 2. Asymmetric AlMe3 Additions to α,β-Unsaturated
Ketones Catalyzed by 1·Rh and Homogenous Control
Catalyst
1·Ru is highly active in hydrogenation of β-keto esters (Table
3)8b,18 and substituted alkenes (Table 4).19 Under H2 at 40 bar,
Table 3. Asymmetric Hydrogenation of β-Keto Esters by 1·Ru
and Ru(Me2L)(DMF)2Cl2
a
a
entry
R1
R2
catalyst
yield
ee (%)
1
2
3
4
5
6
Me
Et
Me
Et
1·Ru
1·Ru
1·Ru
quant.
quant.
quant.
quant.
quant.
quant.
97
94
a
b
Determined by GC. Isolated yields are much lower due to the
Me
Me
Et
tBu
Me
Et
96
relatively low boiling points of the allylic alcohols 6.
Ru(Me2L)(DMF)2Cl2
Ru(Me2L)(DMF)2Cl2
Ru(Me2L)(DMF)2Cl2
>99
>99
>99
cyclohexenone (Table 1).16 At 1 mol% catalyst loadings, 1·Rh
afforded conjugate addition products 4 in nearly quantitative
conversions with 80−99% isolated yields (entries 1−3, Table 1).
Under the same conditions, neither the Rh-Me2L nor Rh-H2L
control gave appreciable amounts of products due to their
insolubility in the reaction solvents, whereas the Rh-BINAP
homogeneous control gave modest isolated yields of 29−46%
(entries 6−8, Table 1). In fact, 3 mol% of the Rh-BINAP catalyst
was needed to afford the addition products in yields comparable
(entries 9−11, Table 1) to those produced by 1 mol% of 1·Rh.
The ee’s of the conjugate addition products (99% or higher) are
similar between 1·Rh and the homogeneous control, but the
activity of 1·Rh is ∼3 times as high as that of the homogeneous
control. We believe that site isolation of the active catalysts in 1·
Rh is responsible for its higher catalytic activity, by preventing
any intermolecular catalyst deactivation pathways.
Me
tBu
a
Determined by GC.
Table 4. Asymmetric Hydrogenation of Substituted Alkenes
by 1·Ru and Ru(Me2L)(DMF)2Cl2
a
a
entry
1
R1
NHAc
R2
catalyst
yield
ee (%)
H
1·Ru
1·Ru
1·Ru
quant.
quant.
quant.
quant.
quant.
quant.
85
70
91
88
81
96
b
2
NHAc
Ph
H
3
4
5
6
CH2CO2Me
NHAc
H
Ru(Me2L)(DMF)2Cl2
Ru(Me2L)(DMF)2Cl2
Ru(Me2L)(DMF)2Cl2
1·Rh also showed excellent activity in additions of AlMe3 to
α,β-unsaturated ketones to afford chiral allylic alcohols 6 (Table
2).17 At 0.4 mol% catalyst loadings, 1·Rh afforded allylic alcohols
in nearly quantitative conversions and 71% and 68% isolated
NHAc
Ph
H
CH2CO2Me
a
b
Determined by GC. H2 (40 bar).
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1·Ru converted methyl acetoacetate to the corresponding
alcohol in a quantitative yield with ee’s as high as 97%. By
varying the post-synthetic metalation conditions, ee’s as high as
98% could be obtained for hydrogenation of tert-butyl aceto-
acetate, albeit in lower yields (51%, SI). 1·Ru is active in
hydrogenating a broad range of β-keto esters, but the ee’s of the
hydrogenation products are 2−5% lower than those obtained
using Ru(Me2L)(DMF)2Cl2.
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ASSOCIATED CONTENT
* Supporting Information
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available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
wenbinlin@uchicago.edu
■
(15) Attempts to measure Rh K-edge spectra of the analogous Rh
complex and 1·Rh were unsuccessful due to absorption signal
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NSLS.
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Author Contributions
†J.M.F. and T.S. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
W.L. thanks NSF (CHE-1111490) for financial support. J.M.F.
was supported by a U.S. DOE Office of Science Graduate
Fellowship (DE-AC05-06OR23100). J.V.L. acknowledges the
ACS Petroleum Research Fund for financial support (grant no.
52148-DNI5). Use of the National Synchrotron Light Source,
Brookhaven National Laboratory, was supported by the U.S.
DOE Office of Science (contract no. DE-AC02-98CH10886).
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