Metal-Organic Frameworks Invert Molecular Reactivity: Lewis Acidic Phosphonium Zwitterions Catalyze the Aldol-Tishchenko Reaction

Article abstract of DOI:10.1021/jacs.7b10928

The influence of metal-organic frameworks (MOFs) as additives is herein described for the reaction of n-alkyl aldehydes in the presence of methylvinylketone and triphenylphosphine. In the absence of a MOF, the expected Morita-Baylis-Hillman product, a β-hydroxy enone, is observed. In the presence of MOFs with UMCM-1 and MOF-5 topologies, the reaction is selective to Aldol-Tishchenko products, the 1 and 3 n-alkylesters of 2-alkyl-1,3-diols, which is unprecedented in organocatalysis. The (3-oxo-2-butenyl)triphenylphosphonium zwitterion, a commonly known nucleophile, is identified as the catalytic active species. This zwitterion favors nucleophilic character in solution, whereas once confined within the framework, it becomes an electrophile yielding Aldol-Tishchenko selectivity. Computational investigations reveal a structural change in the phosphonium moiety induced by the steric confinement of the framework that makes it accessible and an electrophile.

Full text of DOI:10.1021/jacs.7b10928

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Communication
Metal-Organic Frameworks Invert Molecular Reactivity: Lewis Acidic
Phosphonium Zwitterions Catalyse The Aldol-Tishchenko Reaction
Gerald Bauer, Daniele Ongari, Xiaoying Xu, Davide Tiana, Berend Smit, and Marco Ranocchiari
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10928 • Publication Date (Web): 02 Dec 2017
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Metal-Organic Frameworks Invert Molecular Reactivity: Lewis
Acidic Phosphonium Zwitterions Catalyse The Aldol-Tishchenko
Reaction
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[
a]
[b]
[a]
[b,c]
[b]
Gerald Bauer , Daniele Ongari , Xiaoying Xu , Davide Tiana , Berend Smit , Marco
[
a]
Ranocchiari *
[
a] Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, CHꢀ5232 Villigen PSI, Switzerland
[b] Laboratory of Molecular Simulation, EPFL Valais/Wallis, CHꢀ1951 Sion, Switzerland
c] School of Chemistry, University College Cork, College Rd, Cork, Ireland
EꢀMail: marco.ranocchiari@psi.ch
[
*
Supporting Information Placeholder
ABSTRACT: The influence of metalꢀorganic frameworks
topology that catalyse the Knøvenagel condensation of
nitrobenzaldehydes. The common underlying feature is the
7
(MOFs) as additives is herein described for the reaction of nꢀalkyl
aldehydes in the presence of methylvinylketone and
triphenylphosphine. In the absence of a MOF, the expected
MoritaꢀBaylisꢀHillman product – a βꢀhydroxy enone – is
observed. In the presence of MOFs with UMCMꢀ1 and MOFꢀ5
topologies, the reaction is selective to AldolꢀTishchenko products
anchoring of reaction intermediates to the framework which
consequently alters the reaction pathway. We show that we can
use the MOF’s porous environment to completely alter the
reactivity of a catalytic intermediate from nucleophile to
electrophile, yielding an as yet unprecedented catalytic pathway.
ꢀ
the 1 and 3 nꢀalkylesters of 2ꢀalkylꢀ1,3ꢀdiols ꢀ that is
Hereby, phosphines play a central role in catalytic processes to
achieve high reactivity and selectivity. Although their main
application lies in the field of transition metal catalysed
unprecedented in organocatalysis. The (3ꢀoxoꢀ2ꢀ
butenyl)triphenylphosphonium zwitterion, a commonly known
nucleophile, is identified as the catalytic active species. This
zwitterion favours nucleophilic character in solution, whereas
once confined within the framework, it becomes an electrophile
8
9
reactions, e.g. the hydroformylation of olefines and the
asymmetric 1 synthesis of fine chemicals and bioactive
0
compounds, phosphorous compounds gain increasing interest in
11
yielding
AldolꢀTishchenko
selectivity.
Computational
organocatalytic reactions.
Phosphines as organocatalysts
investigations reveal a structural change in the phosphonium
moiety induced by the steric confinement of the framework that
makes it accessible and an electrophile.
facilitate the reaction with unsaturated carbon atoms to form
11d
phosphonium zwitterions.
Such species are reactive towards
nucleophilic attack and catalyse a variety of CꢀC bond forming
reactions like the Michael addition and the MoritaꢀBaylisꢀHillman
11a, 11d
(MBH) reaction.
The phosphonium ion activates the
adjacent carbon atoms. Free phosphonium cations are also active
12
Metalꢀorganic frameworks (MOFs) are becoming increasingly
relevant for catalytic applications. Their structural versatility,
Lewis acid catalysts. The low lying σ* orbitals of the PꢀC bonds
1
13
make the phosphorous electrophilic. Even though phosphonium
1
4
tuneable pore size and modularity gives a nearly infinite number
cations have shown to catalyse different coupling reactions, they
are rarely – if ever – the reactive moiety when placed in a
zwitterion. Hence, electrophilic reactions, like the Aldolꢀ
2
of structures. MOFs feature active sites as intrinsic parts of the
inorganic nodes or organic linkers. Reactive intermediates may
1
a
15
also be trapped inside the pores. These features can be thought
of providing hostꢀguest properties similar to enzymes giving them
potential beyond simple heterogenisation of homogeneous
catalysts. Reactivity and selectivity of reactions can be tuned by
Tishchenko (AT) reaction, are usually not accessible via
phosphonium zwitterions.
In this contribution, we describe a triphenylphosphonium
zwitterionic species that features electrophilicity only when MOFs
are present. These findings show that MOFs can completely alter
the reactivity of an organocatalyst from a nucleophile in solution
to an electrophile in the framework, enabling the AT reaction: an
as yet unprecedented reaction in organocatalysis. The role of the
framework was subsequently studied by experimental and
computational methods. This work shows the capability of MOFs
to completely switch the reactivity of the phosphonium
zwitterions, thus enabling otherwise inaccessible reaction
pathways.
3
exploiting the environment around the active site. For instance, a
chiral binaphthyl copper MOF with phosphoric acid functionality
can reverse the stereoselectivity in the FriedelꢀCrafts reaction
4
between indoles and imines. The cobalt salenꢀcatalysed
intramolecular epoxide ring opening in the presence of a MOF
results in the formation of the 6ꢀmembered ring, whereas the5
homogeneous analogue yielded the 5ꢀmembered ring product.
Molecular confinement within a phosphine MOF with IRMOFꢀ9
topology has also proven to sterically induce intermediate
6
selectivity to determine which reactions occur. One can also tune
the environment around active sites to affect regioisomer
reactivity as demonstrated by amino MixMOFs with IRMOFꢀ9
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role of the MOF in forming the AT product. The formation of an
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imine between the dimethyl aminoterephthalate and the aldehyde
was not observed. In addition, the aminoꢀfree UMCMꢀ1 (Table 1,
Entry 6) also showed to induce the selectivity change towards AT
reaction, which excludes the formation and the involvement of
imines as catalytically relevant entities (Table S8). When MVK
and/or PPh were omitted from the reaction, neither reaction
3
19
occurred. Previous studies showed that PPh reacts with MVK
3
1
6, 19, 23
to form the zwitterionic species 4 (Figure 2).
The presence
of 4 was independently detected by UPLC/MS in the crude
19
reaction mixture. Molecule 4 is an active intermediate in the
MBH reaction and normally acts as a nucleophile. However, in
the presence of amino MOFs with MOFꢀ5 and UMCMꢀ1
topologies, this zwitterion becomes an electrophile as required in
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Figure 1. Structures and molecular formulas of the MOFs used in
this work: (a) MixMOFꢀ5ꢀNH and (b) MixUMCMꢀ1ꢀNH . bdc =
2
2
15
1,4ꢀbenzenedicarboxylate;
benzenedicarboxylate, btb
abdc
=
=
2ꢀaminoꢀ1,4ꢀ
the AT reaction.
4,4′,4′′,ꢀbenzeneꢀ1,3,5ꢀtriylꢀ
trisbenzoic acid (hydrogen and nitrogen atoms were omitted for
clarity).
Table 1. Reactivity and selectivity of various nꢀaliphatic
aldehydes in the presence of PPh3 and MVK using different
MixMOFꢀNH systems as coꢀcatalysts.
2
Phosphines are commonly used as organocatalysts in the MBH
reaction: electron deficient olefines, such as methyl vinyl ketone
(
MVK), react in the presence of the nucleophilic PPh and
3
1
6
aldehydes to form βꢀhydroxy enones. The influence of amino
containing MixMOF systems with MOFꢀ5 topology as coꢀ
17
catalysts in the PPh ꢀcatalysed MBH reaction was investigated
3
at first. Experiments in solution of nꢀpentanal and MVK with
catalytic amounts of PPh showed 15% conversion of the starting
3
aldehyde with a selectivity of >99% towards the corresponding
MBH product, the 4ꢀhydroxyꢀ3ꢀmethyleneꢀ2ꢀoctanone (3-C4)
(Table 1, Entry 1). When the same reaction was performed with nꢀ
b
Entry
MOF
Aldehyde
Conv.
Selectivity [%]
butanal as substrate in the presence of MixMOFꢀ5ꢀNH (13 mol%
2
a
18
[%]
NH ; Figure 1a) three products were observed. In addition to the
2
1-C
n
n
3-C
n
Other
expected MBH product, the AT products, the 1ꢀ and 3ꢀ butanoic
2-C
15
acid esters of 2ꢀethylꢀ1,3ꢀhexandiol (1-C4 and 2-C4), were
observed as major products with 76% selectivity (Table 1, Entry
2). The formation of 1-C4 and 2-C4 was confirmed by
1
2
3
4
5
6
7
8
9
--
n-pentanal
(R=C H )
3 7
15
47
70
87
14
84
58
56
34
0
>99
14
14
13
30
21
13
14
38
0
1
9
independent synthesis and analysis of the AT products.
20
MixMOF-5-NH
2
n-butanal
(R=C
76
82
84
63
75
84
84
62
10
4
The addition of the amino containing MILꢀ101(Al) and
(13 mol% NH
2
)
2 5
H )
2
1
19
DMOFꢀ1ꢀNH2, yielded only the MBH product 3-C . When
4
2
1ꢀ22
MixUMCMꢀ1ꢀNH (28 mol% NH ; Figure 1b)
was used as
2
2
MixUMCM-1-NH
(28 mol% NH2)
2
2
n-butanal
additive, the conversion increased to 70% and the AT selectivity
to 82% (Table 1, Entry 3). The role of the amino substituent was
further investigated. MixUMCMꢀ1ꢀNH2 with 28 to 100 mol%
(R=C H )
2
5
MixUMCM-1-NH
(52 mol% NH
n-butanal
(R=C
3
)
2 5
H )
NH were employed (Table 1, Entries 3 ꢀ 5). Increasing the amino
2
2
content in the MOF to 52 mol% NH (Table 1, Entry 4) enhanced
2
UMCM-1-NH
100 mol% NH
2
n-butanal
(R=C
7
the AT selectivity to 84% with an 87% conversion. The fully
functionalised UMCMꢀ1ꢀNH2 (Table 1, Entry 5) significantly
reduced conversion to 14% with an AT selectivity of 63%.
Reaction with nonꢀfunctionalised UMCMꢀ1 showed a conversion
of 84% with a respective AT selectivity of 75% (Table 1, Entry
6). These results indicate that the amino residue influence AT
reactivity and selectivity. The UMCMꢀ1 with around 50 mol%
(
2
)
2 5
H )
UMCM-1
n-butanal
R=C
4
(
2 5
H )
MixUMCM-1-NH
(28 mol% NH
2
2
2
n-pentanal
(R=C
3
2
)
3 7
H )
NH , equivalent of one functional group per pore, is optimal for
2
conversion and selectivity.
MixUMCM-1-NH
28 mol% NH
n-hexanal
(R=C
2
(
2
)
4 9
H )
The substrate scope was then extended using a series of
different nꢀaliphatic aldehydes with increasing chain lengths. The
use of nꢀpentanal and nꢀhexanal reduced the overall conversion to
MixUMCM-1-NH
28 mol% NH
n-heptanal
(R=C
0
(
2
)
5 11
H )
5
8% and 56%, respectively, with an AT selectivity of 84% in both
cases (Table 1, Entries 7 and 8). nꢀHeptanal yielded a conversion
of 34% with a corresponding AT selectivity of 62% (Table 1,
Entry 9). Increasing the chain length of the aldehyde limits the
conversion, giving further evidence that AT catalysis takes place
inside the framework. Blank reactions using dimethyl
aminoterephthalate – as a substitute for the amino containing
[
a] Conversions were determined via GC or UPLC: the starting material was calibrated prior
to the analyses, [b] The amount of product were determined via GC: the values are based on
the C-ratios of the respective products.
2+
19
MOF – and Zn precursors were also performed (Table S8). All
reactions yielded only the MBH product underlining the pivotal
2
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organocatalysis literature. Hence, we focused on Hꢀbond
stabilisation and studied stability of 4 in the presence of
MixUMCMꢀ1ꢀNH (50 mol% NH ; system A) and in solution
1
2
3
4
5
6
7
8
9
2
2
with dimethyl aminoterephthalate (system B; Figure 3). The
optimised geometry of systems A and B was computed using
DFT. Inside the MOF, the zwitterion can adopt two different
configurations: (A1) pointing towards the pore or (A2) pointing
towards the channel (Figure 3). Starting from these
configurations, classical molecular dynamics (MD) was used to
verify the strength of the Hꢀbond and the possibility of a transition
between the states A1 and A2. All the simulations consider liquid
nꢀbutanal as explicit solvent. In both MixUMCMꢀ1ꢀNH systems
2
1
1
1
1
1
1
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(
A1, A2) and the unhindered system (B) the H bond was found to
be stable, i.e., the zwitterion stayed bound to the amine group for
the entire simulation (50 ns). A transition between the pore and
the channel conformation was not observed, suggesting that a
possible transformation from A1 to A2 only goes via the cleavage
of the hydrogen bond.
Figure 2. Formation of (3ꢀoxoꢀ2ꢀbutenyl) triphenylphosphonium
(4) as catalytic active species: (A) MVK (0.25 mmol) + PPh3
(0.16 mmol) in d ꢀTHF (0.5 mL), (B) Formation and interaction 4
8
with MixUMCMꢀ1ꢀNH (28 mol% NH ).
2
2
To further prove that (a) the catalytic intermediate is trapped
within the MOF and that (b) leads to AT raectivity, MixUMCMꢀ
1ꢀNH (28 mol% NH ) was pretreated with PPh and MVK in
2
2
3
tetrahydrofuran (THF) overnight. It was then intensively washed
to remove the excess MVK and PPh . A solution with nꢀbutanal in
3
THF was introduced thereto. Under these conditions exclusively
19
the AT product was formed (Table S8). The absence of the
MBH product excludes the leaching of the catalytic intermediate
from the framework. Furthermore it shows the central role of the
MOF in the selectivity change. The interaction between active
species and the MOF was then investigated. A stoichiometric
31
mixture of PPh and MVK in d ꢀTHF was measured by P NMR
3
8
spectroscopy showing the formation of a new species at 23.7 ppm,
which corresponds to the zwitterionic phosphonium species, (3ꢀ
19,
oxoꢀ2ꢀbutenyl)triphenylphosphonium (Figure 2, spectrum A).
23b
When dimethyl aminoterephthalate was added to the solution,
1
9
the signal shifted downfield to 24.2 ppm. This is indicative of an
interaction between the zwitterion 4 and the amino moiety.
Subsequently, the presence of 4 in the MOF structure was
confirmed by pretreating MixUMCMꢀ1ꢀNH (28 mol% NH ) with
2
2
PPh and MVK in THF after removal of the excess PPh and
3
3
31
MVK. Solid state P NMR spectroscopy (Figure 2, Spectrum
19
B) shows the formation of three species at 21.9, 22.9 and 23.7
ppm with chemical shifts comparable to solution spectra of the
zwitterion 4 demonstrating that the zwitterion is trapped relatively
Figure 3. Two different configurations in which the zwitterion (4)
can be found inside MixUMCMꢀNH , anchored by an H bond to
the amino group: 4 is trapped inside the pore (A1); 4 points
toward the channel (A2). The dimethyl aminoterephthalate system
(B, “solution”). H atoms were omitted for clarity reasons.
1
strong within the MOF pores. H NMR spectroscopy after
2
digestion of 4 within MixUMCMꢀ1ꢀNH (42 mol% NH ) revealed
2
2
a formula [Zn O(btb)4/3(bdc)0.58(abdc)0.42(4) ] with 0.64 ratio
4
0.27 n
19
between 4 and amino groups (Figure S9ꢀS11). The structure of
the MOF is maintained upon reaction with intermediate 4, and a
surface area decrease of 50% is observed (Figures S3 and S4).
19
To derive different reaction pathways the C···C and the O···P
To understand the role of the framework in the activation of the
AT reaction pathway, we compared the catalytically active
phosphonium zwitterion 4 in solution as well as in the UMCMꢀ1
environment using a mix of density functional theory (DFT) and
force field calculations. Intermediate 4 can either bind to defect
19
(
Figure S12) distances were investigated in systems A1, A2 and
B. These distances represent the first step of MBH and AT
reaction pathway, respectively. In this analysis we consider that
the transition state of the reactions is reached when the dCcꢀCβ and
24
2+
dOꢀP distances are smaller than the respective C···C and O···P van
sites in the crystal lattice, enabling a direct coordination to Zn
der Waals distances, i.e., 3.47 Å and 3.36 Å. The probability of
the transition state can therefore be compared between the
different systems. The probability of the C···C distance between
the zwitterion and the carbonyl to be shorter than the respective
van der Waals distance is in a similar range for the system A2 and
B (Table S9). Hence, the steric confinement of the MOF does not
play a major role in the suppression of the MBH pathway. On the
ions, or via hydrogen bonding to the amine group dispersed within
the framework. The coordination of the Znꢀsites cannot be
excluded a priori but it was shown exprimentally that the amine
group plays a central role in the anchoring of 4. Further evidence
of the stabilising role of hydrogen bonds in phosphonium
zwitterions is also evidenced in the enantioselective phosphine
3
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other hand, the dOꢀP distance shows a remarkable enhancement
inside the MOF: in system A2 the probability related to the
formation of the AT precursor is 26ꢀtimes higher than in systems
B. This difference in the binding of the O···P arises from the
interaction between the phenyl groups of the phosphonium and
the steric confinement of the MOF. The three phenyl groups cause
a significant steric hindrance, which results in the shielding the
phosphorous from the nꢀbutanal. However, the MOF limits the
freedom of movement of 4, distorting the tetrahedral
configuration of the phosphonium moiety. This distortion enables
the oxygen attack by the nꢀbutanal due to the attractive
electrostatic interaction between the oppositely charged O
61ꢀ74; (c) Wang, C.; Liu, D.; Lin, W. J. Am. Chem. Soc. 2013, 135,
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4170.
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Zheng, M.; Liu, Y.; Wang, C.; Liu, S. B.; Lin, W. B. Chem.
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Zhang, T.; Song, F.; Lin, W. Chem. Commun. 2012, 48, 8766.
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.
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.
0
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Coupling Reactions, Second Completely Revised and Enlarged Edition;
Volume 2. WileyꢀVCH Verlag GmbH & Co. KGaA: Weinheim, Germany,
(ꢀ0.278) and P (0.523) atoms. On the other side, the configuration
with the zwitterion shielded inside the pores of the MOF (A1), are
shown not to be reactive, giving similar results to the B system
(Table S9). MD simulations confirm the active role of the MOF in
alternating the zwitterion's reactivity in A2 as result of the steric
interaction arising between between the ligands of the frameworks
and the phenyl rings.
2
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0.
Beller, M.; Blaser, H.ꢀU. Organometallics as Catalysts in the
We have shown that MOFs can effectively bind reaction
intermediates and influence the reactivity of catalytic systems. In
our case the MoritaꢀBaylisꢀHillman (MBH) reaction of nꢀaliphatic
Fine Chemical Industry. SpringerꢀVerlag Berlin Heidelberg, 2012; p 156
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1619; (b) Xiao, Y.; Sun, Z.; Guo, H.; Kwon, O. Beilstein J. Org. Chem.
aldehydes with methyl vinyl ketone and PPh can be switched to
3
2014, 10, 2089ꢀ2121; (c) Fraile, A.; Parra, A.; Tortosa, M.; Alemán, J.
Tetrahedron 2014, 70, 9145ꢀ9173; (d) Wang, T.; Han, X.; Zhong, F.; Yao,
W.; Lu, Y. Acc. Chem. Res. 2016, 49, 1369ꢀ1378.
exclusively yield the AldolꢀTishchenko (AT) reaction in the
presence of amino containing MixMOFs. This change in
reactivity was shown on a series of different nꢀaliphatic aldehydes
1
2.
774.
3.
Bayne, J. M.; Stephan, D. W. Chem. Soc. Rev. 2016, 45, 765ꢀ
in
various
framework
systems.
The
(3ꢀoxoꢀ2ꢀ
butenyl)triphenylphosphonium zwitterion (4), a commonly known
nucleophile, was identified as catalytic active species.
1
(a) LaFortune, J. H. W.; Johnstone, T. C.; Perez, M.;
Winkelhaus, D.; Podgorny, V.; Stephan, D. W. Dalton Trans. 2016, 45,
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Chem. 2010, 291, 349ꢀ393.
MixUMCMꢀ1ꢀNH confines the zwitterionic organocatalyst and
2
influences the geometry around the tetrahedral phosphonium
moiety. Simulations suggested the MOF to affect the fine
structure around the phosphonium through new steric interactions
between the host (MOF) and the guest (zwitterion), which opens
the phosphonium moiety to nucleophilic attack. This work shows
a novel way of doing catalysis where MOFs can be used as
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1
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1
6.
Menozzi, C.; Dalko, P. I., Organocatalytic Enantioselective
ACKNOWLEDGMENTS
Morita–Baylis–Hillman Reactions. In Enantioselective Organocatalysis,
WileyꢀVCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007; pp
151ꢀ187.
This work was supported by a grant from the Swiss National
Supercomputing Centre (CSCS) under Project no. s611. The
research of D.O. was supported by the European Research
Council (ERC) under the European Union's Horizon 2020
research and innovation programme (grant agreement No.
1
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(a) Luan, Y.; Zheng, N. N.; Qi, Y.; Tang, J.; Wang, G. Catal.
Sci. Tech. 2014, 4, 925ꢀ929; (b) Miao, Z. C.; Qi, C.; Wensley, A. M.;
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1
8.
Mol% NH
2
refers to % aminoterephthalate to total
6
66983, MaGic). D.T. and G.B. acknowledge the National Centre
terephthalate content.
of Competence in Research (NCCR) “Materials' Revolution:
Computational Design and Discovery of Novel Materials
19.
20.
See the supporting information.
SerraꢀCrespo, P.; RamosꢀFernandez, E. V.; Gascon, J.;
(MARVEL)” of the Swiss National Science Foundation (SNSF)
Kapteijn, F. Chem. Mater. 2011, 23, 2565ꢀ2572.
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Wang, Z.; Tanabe, K. K.; Cohen, S. M. Inorg. Chem. 2009, 48,
which funded their work at EPFL and PSI.
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96ꢀ306.
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