A homochiral metal-organic framework as an effective asymmetric catalyst for cyanohydrin synthesis

Article abstract of DOI:10.1021/ja411887c

A homochiral metal-organic framework (MOF) of an enantiopure 2,2′-dihydroxy-1,1′-biphenyl ligand was constructed. After exchanging one proton of the dihydroxyl group for Li(I) ions, the framework is shown to be a highly efficient and recyclable heterogene

Full text of DOI:10.1021/ja411887c

Communication
pubs.acs.org/JACS
A Homochiral Metal−Organic Framework as an Effective Asymmetric
Catalyst for Cyanohydrin Synthesis
Ke Mo, Yuhua Yang, and Yong Cui*
School of Chemistry and Chemical Technology and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, China
S
* Supporting Information
cyanosilylation of aldehydes to generate chiral cyanohydrins.14
ABSTRACT: A homochiral metal−organic framework
(MOF) of an enantiopure 2,2′-dihydroxy-1,1′-biphenyl
ligand was constructed. After exchanging one proton of the
dihydroxyl group for Li(I) ions, the framework is shown to
be a highly efficient and recyclable heterogeneous catalyst
for asymmetric cyanation of aldehydes with up to >99% ee.
Compared with the homogeneous counterpart, the MOF
catalyst exhibits significantly enhanced catalytic activity
and enantioselectivity, especially at a low catalyst/substrate
ratio, due to that the rigid framework could stabilize the
catalytically active monolithium salt of biphenol against its
free transformation to catalytically inactive and/or less
active assemblies in reactions. The synthetic utility of the
cyanation was demonstrated in the synthesis of (S)-
bufuralol (a nonselective β-adrenoceptor blocking agent)
with 98% ee.
The real active species was shown to be a monomeric lithium
binolate aqua complex, but there exists a dynamic mixture of
binolate/Li species [(binolate)x-Li, where x refers to the molar
ratio of binol-to-Li] in solution, which is detrimental to the
catalyst activity and selectivity.14 Similar catalytic behavior was
also observed in the biphenolate/Li system (Scheme S1). We
report here the synthesis of an enantiopure tetracarboxylate
ligand of biphenol (H4L) to build a chiral MOF and show that
incorporation of biphenol into a porous network, after partially
exchanging its hydroxyl protons for Li ions, effectively enhanced
reaction rates and enantioselectivity of cyanation of aldehydes,
owing to the framework prohibited the free transformation of the
catalytically active assemble to inactive and/or less active species
(Scheme 1).
Scheme 1. Synthesis of MOF 1 and Its Post-Synthetic
Modification
etal−organic frameworks (MOFs) are crystalline hybrid
Msolids composed of organic struts and inorganic nodes
and have emerged as a novel class of porous materials.1,2 Thanks
to their catalysis friendly features such as large surface areas and
porosity, tunable and functionalizable pore walls, MOFs have
more advantages to be a kind of heterogeneous catalyst,
especially for asymmetric catalysis than traditional organic and
inorganic porous materials.3−5 Although lots of chiral MOFs
have been described,6 versions featuring accessible catalytic
active sites that induced high enantioselectivity remain scarce.7
An appealing strategy for making chiral MOF catalysts involves
using homogeneous asymmetric catalysts (or precatalysts) such
as Ti-binolate and metallosalen complexes as organic linkers.
Even though, the heterogenized catalysts are typically less
effective, with limited substrate scope, than their homogeneous
analogs.8,9 Remarkably, in several instances it is also possible to
generate MOFs with enhanced catalytic performance, e.g., by
providing confined spaces for reactants and eliminating multi-
molecular catalyst deactivation pathways5e,f or by bringing active
sites into a high local density arrangement and close proximity to
facilitate synergistic interactions with substrates.8e It is therefore
highly desirable to control the assembly of a chiral molecular
catalyst into a MOF to improve its catalytic performance, so that
predictive structure−property relationships can be established.4
Chiral diols such as 1,1′-biphenol and 1,1′-binaphthol (binol)
are privileged catalyst structures utilized in a wide range of
reactions that continue to expand.10 Li(I) complexes of binol,
e.g., could catalyze a variety of organic reactions, such as
The ligand H4L was prepared in an overall 76% yield by the
suzuki cross-coupling of dimethyl-5-(pinacolboryl)isophthalate
and (R)- or (S)-5,5′,6,6′-tetramethyl-3,3′-dibromo-biphenyl-
2,2′-diol12 and followed by hydrolysis with NaOH. Heating a
mixture of H4L and Zn(NO3)2·6H2O (a 1:2 molar ratio) in
DMSO, THF, and water at 100 °C afforded single crystals of
[Zn4O(L)3/2]·16H2O·4THF (1) in good yield. The product was
stable in air and insoluble in water and common organic solvents
and was formulated based on elemental analysis, IR spectrosco-
py, and thermogravimetric analysis (TGA). The phase purity of
the bulk sample was established by comparison of its observed
and simulated powder X-ray diffraction (PXRD) patterns.
(S)-1 exhibits a 3D chiral nanoporous framework and
crystallizes in the cubic chiral space group I213, with the
asymmetric unit containing one-third of the formula. The basic
Received: November 21, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja411887c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
building unit in 1 is a [Zn4(μ4-O)(L)3/2] cluster with one
crystallographic C3 axis running through one Zn atom and the
central oxygen atom. Four zinc atoms are each tetrahedrally
coordinated to one μ4-O atom and three carboxylate oxygen
atoms from three L lignds. The L ligand exhibits an exo-
octadentate coordination fashion, binding to four Zn4O clusters
via four bidentate carboxylate groups. The two phenyl rings are
twisted along the pivotal 1,1′-bond with a dihedral angle of
74.0(1)°.
PXRD pattern as the pristine sample. This result indicates that
the structural integrity and open channels of 1 are maintained in
solution.
Exchange of hydroxyl protons of 1 was achieved by replacing
(via soaking) the initially present guest molecules with more
volatile THF molecules and then stirring the evacuated MOF in a
solution of n-BuLi in toluene containing a small amount of
water.15 Note that lithiated MOF catalysts of this type have been
used before, but only with lithiation, no catalysis was reported.15
Mono- and dilithium salts of 1-Li(H2O)2 (1-Li) and 1-
Li2(H2O)4 (1-Li2) were prepared from 1:2 and 1:4 molar ratios
of 1 (per formular unit) and n-BuLi, respectively. The formation
of the lithium salts was suggested by inductively coupled plasma
optical emission spectrometer (ICP-OES). PXRD showed the
post-modified MOF remained crystallinity. 1-Li gave a decreased
BET surface area (124 m2/g) compared with 1.
As show in Figure 1a, three pairs of 12 Zn4O clusters that are
related by C3 symmetry and bridged by carboxylate groups merge
After optimization of reaction conditions, 1-Li was found to be
an active catalyst for cyanation of aldehydes with Me3SiCN in the
presence of a catalytic amount of water in toluene at −78 °C. The
co-existence of water as coactivator is significant to the
enantioselectivity of this cyanation, which is similar to the
homogeneous reactions.14a Specifically, 0.5 mol % loading of
(R)-1-Li with 0.15 mol % water catalyzes cyanation of
benzylaldehyde to give 97% conversion with 98% enantiomeric
excess (ee) of the desired product in 45 min. If the dilithium salt
1-Li2 was used as catalyst, 90% conversion with 79% ee of the
product was obtained, indicating that 1-Li was more active and
enantioselective than 1-Li2, consistent with the observed higher
activity of Me4L-Li in the homogeneous system. The chiral
nature of the product is determined by the handedness of the
catalyst, as evidenced by that cyanation of benzylaldehyde by (S)-
1-Li gave the R enantiomer over the S enantiomer (96% ee, Table
1, entry 2).
Figure 1. (a) A triply stranded cage in 1 encapsulated by 10 Zn4O
clusters (the cavity is highlighted by a colored sphere). (b) 3D structure
of 1 viewed along the a-axis (the Zn atoms are shown in polyhedron).
Aromatic aldehydes with an electron-withdrawing group gave
the desired product almost quantitatively with enantioselectiv-
ities up to >99% ee (Table 1, entries 3−5). Introduction of an
at two more Zn4O cores to generate a D3-symmetric cage. The
cage can also be viewed as a triply stranded structure with two
Zn4O cores located at opposite positions surrounded by three
metallomacrocycles, each of which is built of ten Zn4O clusters
bridged by isophthalate linkers. The cage has an irregular open
cavity with a maximum inner width of ∼1.8 nm (considering van
der Waals radii). The hexahedral aperture on each face has a
diagonal distance of ∼1.6 × 1.6 nm. Sharing the hexahedral
windows with neighboring cages leads to multidirectional zigzag
channels in the framework. The cage cavities are periodically
decorated with the dihydroxy groups of biphenyl backbones that
are accessible to guest molecules.
Calculations using the PLATON program indicate that 71.9%
of the total volume of 1 is occupied by solvent molecules.13
Circular dichroism (CD) spectra of 1 made from R and S
enantiomers of the H4L ligand are mirror images of each other,
indicative of their enantiomeric nature. TGA revealed that the
guest molecules could be readily removed in the temperature
range from 80 to 150 °C and the decomposition of the
framework starts at ∼400 °C. The crystallinity of the framework
remains intact upon removal of guest molecules. After soaking 1
in water for 2 days, PXRD indicated the sample retained its
crystallinity. The CO2 sorption measurement of the apohost 1 at
273 K gave a BET surface area of 172 m2/g. 1 could adsorb ∼2.0
methyl orange (MO, ∼1.47 × 0.53 × 0.53 nm in size) per
formula unit in solution, causing the initially colorless crystals
becoming dark orange, and the inclusion solid exhibited the same
a
Table 1. Cyanation of Aldehydes Catalyzed by (R)-1-Li
b
c
entry
Ar
conv (%)
ee (%)
1
2
C6H5
C6H5
97
97
90
99
99
99
99
97
95
98
99
99
67
98 (S)
96 (R)
d
e
3
C6H5
79 (S)
>99 (S)
92 (S)
>99 (S)
81 (S)
>99 (S)
99 (S)
98 (S)
94 (S)
>99 (S)
67 (S)
nd
4
p-CIC6H4
p-BrC6H4
p-NO2C6H4
m-MeOC6H4
3-pyridyl
5
6
7
8
9
2-fural
10
11
12
13
14
15
C6H5CHCH
1-naphthyl
2-naphthyl
9-anthral
f
coronenyl
coronenyl
<5
g
60
nd
a
For reaction details see Experimental section in SI, catalyst and water
loading based on aldehyde (30 mol % water loading based on the
b
c
d
catalyst). Calculated by 1H NMR. Determined by HPLC. Catalyzed
e
f
by 0.5 mol % (S)-1-Li. Catalyzed by 0.5 mol % (R)-1-Li2. Reaction
g
time: 3 h. Catalyzed by 1 mol % (R)-Me4L-Li.
B
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Journal of the American Chemical Society
Communication
electron-donating −OMe group into the phenyl ring of the
substrate led to decreased stereoselectivity (81% ee, entry 7). 3-
Pyridylaldehyde and 2-furylaldehyde bearing a heterocycle gave
excellent results up to >99% ee (entries 8 and 9). High
enantioselectivity was also attained with the vinyl-type aromatic
aldehyde cinnamaldehyde, affording 98% conversion and 98% ee.
1- and 2-naphthaldehydes were readily converted to the products
with 94% and >99% ee, respectively. For the bulky substrate, 9-
anthralaldehyde, moderate conversion (67%) and ee (67%) were
obtained.
To determine whether catalysis by 1-Li occurs predominantly
within the cavities or instead on the external surface, competitive
size selectivity studies were performed. A sterically more
demanding substrate coronenyl aldehyde was synthesized and
subjected to conditions of cyanation reaction. Less than 5%
conversion was observed after 3 h, much lower than the 60%
conversion obtained by using Me4L-Li in 45 min. This result may
suggest that this bulky substrate cannot access the catalytic sites
in the framework due to its large diameter. It is thus likely that the
catalytic reactions are heterogeneous and may occur within the
MOF. This point is further indicated by the fact that ground and
unground particles of 1-Li (∼180 nm in size) exhibited almost
the same reactivity (97% vs 96% conversions in 45 min) in
cyanation of benzaldehyde.
We examined the heterogeneity and recyclability of the MOF
catalyst. The supernatant from cyanation of benzlaldehyde after
filtration through a regular filter did not afford any additional
product. To evaluate the stability of the solid catalyst, we
investigated recycled and reused 1-Li in cyanantion of
benzaldehyde. Upon completion of the reaction, the catalyst
could be recovered in nearly quantitative yield and used
repeatedly without significant loss of activity for the following
four runs (>96% conversions and 98, 98, 97, and 97% ee for 1−4
runs, respectively). The conversions versus reaction time of the
fresh and recycled catalysts are given in Figure S9. The two lines
are almost coincide with each other, further confirming the
recyclability of the catalyst. After five cycles, the MOF catalyst
remained highly crystalline and still could adsorb ∼1.9 MO per
formula unit in solution, indicating maintenance of its porous
structure. ICP-OES analysis of the product solution indicated
little loss of the metal ions (∼0.001%) from the structure per
cycle, either as molecular species or as particles too small to be
removed by filtration through Celite.
To study the confinement effect of the MOF on the molecular
catalyst, we compared the activities of Me4L-Li and 1-Li at
different C/S ratios (the molar ratio of biphenol-Li to aldehyde).
With a C/S ratio of 1:100, Me4L-Li catalyzed cyanations of
benzaldehyde, p-Br-benzaldehyde and 1-naphthylaldehyde in 45
min affording 83% conversion with 92% ee of the product, 91%
conversion with 90% ee, and 90% conversion with 90% ee,
respectively (Table 2). It appears that the MOF catalyst gave
improved reactivity and enantioselectivity related to its
homogeneous counterpart under similar conditions, although it
required mass diffusion in the porous material. The difference in
the reactivity was clearly manifested in the reaction kinetics. The
conversion versus reaction time of the hetero- and homogeneous
catalysts suggested that the former is more active than the latter,
especially in the beginning 20 min (Figure S9). For example, the
conversion of benzaldehyde (76%) with 1-Li is about twice of
that (39%) with Me4L-Li in 10 min. Notably, the difference in
catalytic performances became larger at a low C/S ratio. For
instance, when the C/S ratio was decreased from 1:100 to
1:1000, 1-Li still could catalyze cyanation of benzaldehyde giving
Table 2. Cyanation of Aldehydes Catalyzed by 1-Li and Me4L-
Li at Different Catalyst/Substrate Ratios
a
heterogeneous
homogeneous
b
c
d
c
d
C/S
Ar
conv (%)
ee (%)
conv (%)
ee (%)
C6H5
97
99
99
84
88
86
98
92
94
96
92
92
83
91
90
60
65
63
92
90
90
86
82
87
1:100
p-BrC6H4
1-naphthyl
C6H5
1:1000
p-BrC6H4
1-naphthyl
a
For reaction details see Experimental section in SI. Thirty mol %
b
water loading based on the catalyst. Molar ratio of biphenol-Li to
aldehyde. Calculated by H NMR. Determined by HPLC.
c
d
1
84% conversion with 96% ee of the product in 45 min, whereas
Me4L-Li (0.1 mol % loading) only gave 60% conversion with
86% ee. Cyanations of p-Br-benzaldehyde and 1-naphthylalde-
hyde followed the same trend with the heterogeneous catalyst
giving higher levels of activity and enantioselectivity than the
molecular catalyst at a low C/S ratio (Table 2).
The catalyticaly active monomer of biphenolate/Li is subtle
changes in reaction conditions that have a big influence on the
catalytic performance. This is due to the coexistence of different
kinds of species during the catalytic process as a result of a
dynamic mixture of biphenolate/Li species equilibrating with
each other in solution.14 This makes the usage of catalyst
inefficient, thus reducing the catalyst activity, especially at low
catalyst loading. In contrast, immobilization of the catalyst on
MOF 1 may help to keep the monomer in its active form by fixing
one L ligand and one Li ion in a proper position, prohibiting the
free transformation of different assemblies of biphenolate/Li.
This would favor the usage efficiency of the catalysts, which may
contribute to its high activity. On the other hand, geometrical
constrains imposed by the chiral amphiphilic cages of 1 may exert
steric influence on the binding substrate of biphenol-Li active
centers and so impart enantioselectivity. In solution, many metal
catalysts would convert themselves among active, less active,
and/or unactive forms because of their labile coordination bonds,
but it remains difficult to control the composition of equilibrium
mixtures to optimize catalyst activities.16 This work highlights the
potential of modifying catalytic performances of metal catalysts
with structural dynamism in solution by using MOFs as support
structures. An in-depth investigation of the mechanistic aspect of
this catalytic system is still underway.
Chiral cyanohydrins are well-known natural products and
versatile synthetic intermediates for pharmaceuticals and agro-
chemicals.17 Although great efforts have been made to
heterogenize molecular catalysts for asymmetric cyanation, the
heterogeneous catalysts usually resulted in a significant lowering
of their enantioselectivity, which, with a few exceptions, did not
exceed 90%.3b,18 To demonstrate the synthetic utility of our
cyanation approach, we examined the preparation of an optically
enriched (S)-bufuralol (a potent β-adrenergic blocker with
peripheral vasodilating activity) on a gram scale.19 As shown in
Scheme 2, 7-ethylbenzofuran-2-carbaldehyde (1 g) could be
transformed into a chiral (S)-benzofuryl eyanohydrin in 95%
yield with 99% ee. The addition of tert-butyl alcohol to the O-
protected derivative of (S)-benzofuryl cyanohydrin afforded an
C
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Journal of the American Chemical Society
Communication
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Scheme 2. Synthesis of (S)-bufuralol from the cyanation
product
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In summary, we have presented a homochiral biphenol-based
MOF decorated with chiral dihydroxyl auxiliaries. Such
dihydroxyl groups accessible via the open MOF channels could
be partially exchanged by Li ions to generate a highly efficient and
recyclable heterogeneous catalyst for asymmetric cyanation of
aldehydes with up to >99% ee. The direct assembly of biphenol
into the MOF may stabilize the catalytically active monolithium
salt of biphenol against deactivation to catalytically inactive or
less active assemblies of biphenolates/Li in reactions, enhancing
the catalyst activity and enantioselectivity, especially at a low C/S
ratio. This cyanation approach is suitable for process chemistry to
ensure the practical gram-scale cyanohydrin synthesis.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org
■
S
AUTHOR INFORMATION
Corresponding Author
yongcui@sjtu.edu.cn
■
(9) (a) Ma, L.; Falkowski, J. M.; Abney, C.; Lin, W. Nat. Chem. 2010, 2,
838. (b) Tanaka, K.; Oda, S.; Shiro, M. Chem. Commun. 2008, 820.
(c) Tanaka, K.; Otani, K.-i. New J. Chem. 2010, 34, 2389. (d) Wu, C. D.;
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Lin, W. Angew. Chem., Int. Ed. 2007, 46, 1075. (f) Ma, L.; Wu, C.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSFC-21025103 and 21371119,
“973” Program (2014CB932102 and 2012CB8217) and SSTC-
12XD1406300.
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(12) Yuan, G.; Zhu, C.; Liu, Y.; Xuan, W.; Cui, Y. J. Am. Chem. Soc.
2009, 131, 10452.
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