A Porous Cu(II)-MOF with Proline Embellished Cavity: Cooperative Catalysis for the Baylis-Hillman Reaction

Article abstract of DOI:10.1021/acs.inorgchem.6b01211

l-Proline has been covalently attached in a rigid linear ligand, H₄L, having an isophthalate moiety at each terminal to form the chiral ligand, H₄LPRO. This linker has been used for the construction of a porous MOF, L_CuPRO

Full text of DOI:10.1021/acs.inorgchem.6b01211

Communication
pubs.acs.org/IC
A Porous Cu(II)-MOF with Proline Embellished Cavity: Cooperative
Catalysis for the Baylis-Hillman Reaction
Dinesh De, Tapan K. Pal, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
Scheme 1. Modification of the Ligand and Crystal Structure of
LCuPRO
ABSTRACT: L-Proline has been covalently attached in a
rigid linear ligand, H4L, having an isophthalate moiety at
each terminal to form the chiral ligand, H4LPRO. This
linker has been used for the construction of a porous
MOF, LCuPRO. The free L-proline moiety in the cavity of
the framework in the presence of imidazole as a cocatalyst
functions synergistically to catalyze the Baylis−Hillman
reaction between α,β-unsaturated carbonyl compounds
and aromatic aldehydes. High porosity of the framework is
proven by the nitrogen adsorption isotherm.
ransition metal complexes have been extensively used as
Thomogeneous catalysts over the years. These metal ions can
be used with linkers of different topologies to construct porous
metal−organic frameworks (PMOFs). When the metal ions in
these PMOFs possess open coordination sites, they can be used
along with the coordination space to efficiently act as
heterogeneous catalysts.1 Moreover, in PMOFs, a functional
group can sometimes be integrated in the ligand to furnish a
unique catalytic site.2 Incorporation of urea and pyrrolidine in
PMOFs are well documented.3 Importantly, with the incorpo-
ration of a chiral group in the ligand, the derived MOF will
become homochiral.4 Motivated by supramolecularly organized
homogeneous chiral catalysis reactions, the employment of chiral
functional group in the linkers to have homochiral organo-
catalytic PMOFs is quite fascinating and has been reported in the
literature.5
Most of the natural amino acids are cheap and may be
considered as an enantiopure ligand for the construction of the
chiral MOFs.6 But their flexible nature poses problems in
constructing MOFs with large cavities. An easier way would be
the attachment of a chiral amino acid covalently to a rigid linker.
Inspired by the excellent structure of our earlier framework LCu,7
we have modified the ligand H4L into an enantiopure ligand
H4LPRO by attaching a proline group (see Scheme 1) which has
been used in many asymmetric reactions.8 There are few L-
proline (hereafter only proline) based homochiral MOFs
reported in the literature. Thus, proline incorporation in MOF
was performed by post-synthetic coordination on open metal
sites in MIL-1019 and post-synthetic amide coupling10 or post-
synthetic click reactions.11 In a recent report, the proline moiety
has been covalently attached to a linker prior to MOF
construction for heterogeneous catalysis of asymmetric aldol
reactions.12
trated homochiral MOF, LCuPRO. This MOF has been
characterized by single crystal X-ray diffraction, powdered X-
ray diffraction, digestion NMR, and solid state circular dichroism
(CD) spectra (Supporting Information (SI)). The porous
LCuPRO together with cocatalytic amount of imidazole has
been used to explore the Baylis−Hillman (BH) reaction between
an aldehyde and an α,β-unsaturated carbonyl compound to form
α-methylene-β-hydroxy carbonyl derivatives. The BH reaction
has emerged as a synthetically highly versatile process13 with
atom economical carbon−carbon bond formation at the α
position with respect to the activated alkene. A number of studies
are reported in the literature on the homogeneous catalysis of the
BH reactions. Here, we show the BH reaction beingcatalyzed by a
Cu-MOF acting as a heterogeneous catalyst.
The ligand H4LPRO was synthesized in four steps via the
reaction of N-(tert-butoxycarbonyl)-L-proline and L1 by slightly
modifying a procedure described by Telfer et al.12a (Scheme S1,
SI). The ligand, H4LPRO, reacts with Cu(NO3)2·3H2O (1:3
molar ratio) solvothermally to form blue colored block shaped
crystals of LCuPRO (Scheme 1). Structural analysis by X-ray
crystallography divulges that the framework is isoreticular with
our previously reported PMOF, LCu.7 In the present case, due to
heavy disordering, the proline side chain in the PMOF LCuPRO
could not be located in the electron density map and thus could
not be established by single crystal X-ray structural analysis. Its
presence in the PMOF has been established by the combination
1
of H NMR and ESI-MS analysis of a digested sample of the
Herein, we report the synthesis of the enantiopure ligand
Received: May 17, 2016
H4LPRO (Scheme 1) for the construction of a noninterpene-
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b01211
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Inorganic Chemistry
Communication
1
PMOF (in D2SO4/DMSO-d6). The H NMR spectroscopy
(Figure S5, SI) unambiguously reveals that the H4LPRO ligand
does not decompose during the MOF synthesis and matches
perfectly with that of the pure ligand, H4LPRO in DMSO-d6
(Figure S2, SI). The integrity of the linker is further corroborated
by the ESI-MS analysis of the digested sample (Figure S6, SI),
which shows a prominent peak at m/z 519 corresponding to
[H4LPRO + H]+.
(BET surface area 1952 m2 g−1 and pore volume 0.98 cm3 g−1).
The reduced surface area and pore volume in LCuPRO are
considerably influenced by the flexible proline groups which
project into the channels of the structure and occupy the free
space.
The presence of free proline moiety in the framework led us to
explore the catalytic activity of the Baylis−Hillman reaction. Our
interest in the Baylis−Hillman reaction is due to the fact that it is
atom economical and forms specific carbon−carbon bonds along
with the generation of functional groups for further use.18 This
reaction can be done by using a Lewis base, typically a tertiary
amine like DABCO or a tertiary phosphine. But the reactions
suffer from low reaction rates, and therefore, Lewis bases in
combination with catalyst components capable of activating the
carbonyl functions are used to accelerate the reaction rate.19 The
amino acid, L-proline provides extensive opportunities as a
homogeneous organo-catalyst in various organic transforma-
tions.8 However, homogeneous catalysts are very often difficult to
recover, and/or they decompose during the catalytic reaction. In
order to surmount these limitations, researchers have developed
methods to fix the catalyst in a solid support. In recent times, the
supported-proline moiety is found to be a good heterogeneous
catalyst.20 In this regard, we sought to explore the possibility of
synergistic effects between two distinct cocatalytic entities,
LCuPRO and imidazole. The methyl vinyl ketone (MVK)-
based BH reaction (Table 1) is currently a subject of extensive
The parent PMOF LCu crystallizes in the trigonal space group
R3m. In the case of the PMOF L PRO, the diffraction data could
be found to be consistent with the trigonal space group R3. This
̅
Cu
̅
trigonal space group cannot be the true space group due to the
chirality of the proline moiety. But because of the lack of long-
range order of the chiral side arm and the rigid architecture of the
remaining part of the molecule, the structure equates to a
nonchiral space group with statistical disordering of the proline
group over four sites.12 Theasymmetric unit consists of half of the
LPRO4− ligand and Cu2 paddle-wheel SBU. Each Cu(II) ion in
the SBU exhibits square pyramidal coordination geometry with
equatorial coordination from four different bridging carboxylates
and an aqua ligand bound axially (Figure 1a).
Table 1. Results Obtained for the Baylis−Hillman Reactions
a
Catalyzed by LCuPRO/ Imidazole
Figure 1. (a) Coordination environment around Cu2+ ions in LCuPRO
(H atoms and side proline part of the ligand H4LPRO are not shown).
(b) PXRD pattern simulated from the CIF file (bottom) and PXRD
pattern of as synthesized LCuPRO (top).
b
entry
Ar
R1, R2
catalyst
% yield
1
4-NO2Ph
3-NO2Ph
2-NO2Ph
4-FPh
CH3, H
CH3, H
CH3, H
CH3, H
CH3, H
CH3, H
C2H5, H
OCH3, H
−(CH2)3−
CH3, H
CH3, H
CH3, H
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
LCuPRO/imidazole
H4LPRO/imidazole
imidazole
75
69
2
3
66
The Cu···Cu distance in the SBU is 2.658(1) Å, which
compares well with the similar reported structure.14 All Cu−O
bond lengths are comparable to the literature known Cu(II)-
containing complexes.15 These SBUs are propagated in all
directions without any interpenetration, forming a robust
structure with large cages (Scheme 1) of dimension 14.072 Å
(distance refers to atom-to-atom connection) occupied by DMF
and water as guests. The framework is also porous like LCu and
exhibits a void volume of ∼63.1% (including the proline side-
group; 62.2% for LCu) as calculated by PLATON.16 The
experimental powder X-ray diffraction (PXRD) pattern for
bulk samples of LCuPRO matched closely to the simulated
pattern (generated from the CIF file) confirming the bulk phase
purity (Figure 1b).
4
62
5
Ph
50
6
4-CH3Ph
4-NO2Ph
4-NO2Ph
4-NO2Ph
4-NO2Ph
4-NO2Ph
4-NO2Ph
41
7
64
8
57
9
61
10
11
12
32
trace
trace
LCuPRO
a
Reaction conditions: aldehyde (0.1 mmol), carbonyl (0.2 mmol) in 1
mL of CHCl3/THF (1:1), cat. LCuPRO (10 wt %), and imidazole (10
mol %), 25 °C for 24 h. Isolated yields after silica gel
b
chromatography.
The TGA curve of LCuPRO (Figure S9, SI) shows a weight loss
of 24.35% due to the removal of guest lattice DMF and
coordinated and lattice water molecules and is stable up to ∼300
°C. The compound is insoluble and stable in common organic
solvents as confirmed from PXRD measurements (Figure S10,
SI). To measure the permanent porosity of LCuPRO, the
activated LCuPRO was subjected to the nitrogen gas sorption
measurements at 77 K. The desolvated LCuPRO exhibits an initial
type-I isotherm up to the relative pressure of 0.7 and afterward
shifted to type-IV17 as in the case of LCu (Figure S11, SI). The
calculated Brunauer−Emmett−Teller (BET) surface area is
found to be 920 m2 g−1 with a pore volume of 0.82 cm3 g−1. Thus,
the surface area and pore volume are lower than those of LCu
investigation in asymmetric catalysis.18,21 In analogy to Shi et
al.,22 we seek the combination of a nucleophilic catalyst,
imidazole, with LCuPRO for the BH reaction. Among the variety
of BH reactions, that of MVK with 4-nitrobenzaldehyde is chosen
as a model reaction (entry 1). As shown in Table 1, imidazole (10
mol %) in the absence of LCuPRO (entry 11) catalyzes the
reaction between MVK and aldehyde (<4% yield, 48 h) very
sluggishly. Also, the use of LCuPRO alone (entry 12) provides no
product in the same time length. Yet, when the two are employed
together (entry 1), a facile reaction occurs (75% yield, 24 h).
Notably, the product is nearly racemic (Figure S12, SI),
indicating the influence of the L-proline chirality to be minimal.22
B
DOI: 10.1021/acs.inorgchem.6b01211
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Inorganic Chemistry
Communication
Based on these results, a range of aromatic aldehydes were
employed in the coupling of MVK under optimized reaction
conditions using LCuPRO with imidazole as the catalytic system.
The results are listed in Table 1. The corresponding adducts are
obtained in good yields. For 3-nitrobenzaldehyde, 2-nitro-
benzaldehyde, and 4-fluorobenzaldehyde, similar results are
obtained (entries 2−4). However, for benzaldehyde or 4-
methylbenzaldehyde, the products are obtained only in moderate
yields (entries 5 and 6). Further, we have extended different α,β-
unsaturated carbonyls such as ethyl vinyl ketone, methyl acrylate,
and cyclohexenone for the model BH reaction (entries 7−9). In
each case, the yield is fair to good. The formation of the desired
product was confirmed by the 1H NMR and 13C NMR (Figures
S18−S35, SI), and improvement of the reaction is monitored by
TLC. It should be pointed out that, in all cases, the products are
obtained with very low ee values (5−8%).
To demonstrate the heterogeneous nature of LCuPRO, the
filtration test was performed (see SI), which authenticated no
catalytically active species leached into the liquid phase, and the
conversion was only possible with the solid catalyst (LCuPRO)
along with cocatalyst. After being recovered using filtration and
washing, LCuPRO could be subsequently used in the successive
runs. The catalytic activity of LCuPRO experienced only a slight
degradation after three cycles (see Figure S16, SI); meanwhile, it
always retained its crystallinity, as determined by PXRD (Figure
S17, SI). N2 gas sorption measurement at 77 K of recovered
LCuPRO after three catalytic cycles does not show any significant
adsorption, presumably due to collapse of the framework during
activation (Figure S36, SI).
of Scientific and Industrial Research, New Delhi, India to D.D.
and T.K.P.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01211.
Hubberstey, P.; Champness, N. R.; Schroder, M. J. Am. Chem. Soc. 2009,
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Experimental details, characterization procedures, crystal-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from the
MNRE, New Delhi, India (to P.K.B.) and SRF from the Council
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DOI: 10.1021/acs.inorgchem.6b01211
Inorg. Chem. XXXX, XXX, XXX−XXX