Tuning the Porphyrin Building Block in Self-Assembled Cages for Branched-Selective Hydroformylation of Propene

Article abstract of DOI:10.1002/chem.201702113

Unprecedented regioselectivity to the branched aldehyde product in the hydroformylation of propene was attained on embedding a rhodium complex in supramolecular assembly L2, formed by coordination-driven self-assembly of tris(meta-pyridyl)phosphine and zinc(II) porpholactone. The design of cage L2 is based on the ligand-template approach, in which the ligand acts as a template for cage formation. Previously, first-generation cage L1, in which zinc(II) porphyrin units were utilized instead of porpholactones, was reported. Binding studies demonstrate that the association constant for the formation of second-generation cage L2 is nearly an order of magnitude higher than that of L1. This strengthened binding allows cage L2 to remain intact in polar and industrially relevant solvents. As a consequence, the unprecedented regioselectivity for branched aldehyde products can be maintained in polar and coordinating solvents by using the second-generation assembly.

Full text of DOI:10.1002/chem.201702113

A Journal of
Accepted Article
Title: Tuning the porphyrin building block in self-assembled cages for
branched-selective hydroformylation of propene
Authors: Xiaowu wang, Sandra Nurttila, Wojciech Dzik, Rene Becker,
Jody Rodgers, and Joost Reek
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To be cited as: Chem. Eur. J. 10.1002/chem.201702113
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Tuning the porphyrin building block in self-assembled cages for
branched-selective hydroformylation of propene
Xiaowu Wang,‡[a,c] Sandra S. Nurttila,‡[a] Wojciech I. Dzik,[a] René Becker,[a] Jody Rodgers,[b] and Joost
N.H. Reek*[a]
which selectivity can be controlled by the second coordination
sphere, i.e. the supramolecular cage that is surrounding the active
Abstract: In this study unprecedented regio-selectivity to the
branched aldehyde product in hydroformylation of propene is attained
site. To date most examples of cage controlled catalysis involve
upon embedding a rhodium complex in a supramolecular assembly,
L2, formed by coordination-driven self-assembly of tris-(meta-pyridyl)-
organic transformations, such as acyl transfer reactions,[11] Diels-
Alder reactions,[12–15] imine forming reactions,[16] hydrolysis
phosphine and zinc(II) porpholactones. The design of the new cage is
reactions,[17,18] photoinduced rearrangements[19] and cyclization
based on the ligand-template approach, in which the ligand acts as a
reactions[20]. More recently, metal catalyzed reactions carried out
template for the cage formation. Previously we have reported the first
generation cage L1, in which zinc(II) porphyrin units were utilized
in molecular cages were also disclosed.[10,21–23] For example,
instead of porpholactones. Binding studies demonstrate that the
association constant for the formation of the second generation cage
L2 is nearly an order of magnitude higher than for L1. This
strengthened binding keeps the new cage L2 intact in polar and
encapsulation of a gold(I) phosphine complex in a supramolecular
host, resulted in an 8-fold increase in the catalytic activity of
hydroalkoxylation of allenes.[24] The same host was also capable
of encapsulating a cationic Ir(III) half-sandwich complex that was
active in the C-H activation of aldehydes and demonstrated both
substrate size and shape selectivity.[25] The effects of confinement
have also been studied in hydrogenation and hydroformylation
reactions. It was shown that by encapsulation of a Rh(I)
industrially interesting solvents. As
a
consequence, the
unprecedented regioselectivity for branched aldehyde products can
be retained in polar and coordinating solvents when using the second-
generation assembly.
norbornadiene complex by
a
self-folding cavitand,
a
hydrogenation catalyst capable of reducing norbornadiene was
obtained.[26] Interestingly, a large difference in product selectivity
between the encapsulated and free catalyst was observed, where
the free catalyst favoured dimer formation whereas the
encapsulated catalyst predominantly formed norbornene. In
another example it was reported that by utilizing monophosphane
rhodium complexes trapped inside cyclodextrins highly branched-
and enationselective hydroformylation of styrene could be
achieved.[27] Moreover, cyclodextrins have been employed as
reverse phase transfer catalysts allowing for both hydrogenation
of unsaturated alcohols[28] and hydroformylation of water insoluble
olefins[29,30] in aqueous media. Further examples of cage catalysis
include gold(I) catalyzed cyclization of acetylenic acid to enol
Introduction
Transition metal catalysis provides powerful tools for the selective
construction of chemical bonds and as such it is important for the
development of sustainable and economical routes to make
chemicals.[1–6] The traditional approach in transition metal
catalysis involves the optimization of catalyst properties by
modifying the ligands that are coordinated to the active metal
center. Typical properties of ligands that have proven influential
include the electronic[7] and steric effects[7] of the ligands, as well
as the bite angle[8]. More recently it has been recognized that
ligands that partake in the catalytic event can also be useful to
invoke new reactivity.[9] Enzymes, Nature’s catalysts, can be very
efficient and selective and as such they have been a source of
inspiration for scientists. Whereas this inspiration initially resulted
in systems in which substrate binding sites were connected to
catalytic centers,[10] more recently strategies have been explored
to place catalysts in confined spaces. This leads to systems in
lactone[26]
and
cobalt(II)
catalyzed
radical-type
cyclopropanation[27]. One of the challenges in the approach used
in the previous examples is that at least for part of the catalytic
cycle the metal complex and the substrate need to be co-
encapsulated, which is challenging particularly in the presence of
excess substrate and product.[22] In addition, it is essential that no
competing reaction pathways take place outside the cage. While
it has been demonstrated that this is possible in some cases,
leading to interesting examples of cage controlled activity and
selectivity, it is also clear that this is not a general strategy. We
previously introduced a more general strategy to encapsulate
catalysts in an efficient way that involves a ligand-template
approach to encapsulate catalytically active mental centers. The
key is that the catalyst is non-covalently linked to the surrounding
cage, and thereby the strategy is applicable to a variety of
catalytic systems.[28] The first example that we reported along
these lines was L1, which was formed by the assembly of three
Zn(II) meso-tetraphenylporphyrins (Zn(II)TPP) around the ligand-
template tris-(meta-pyridyl)-phosphine (P(m-py)3) by utilizing the
selective N-Zn coordinate bond. The phosphorus atom located in
the center of the cage defined by the three porphyrins is available
for metal coordination (see Figure 1).
[a]
Dr. X. Wang, S.S. Nurttila, Dr. W.I. Dzik, Dr. R. Becker, Prof. Dr. J. N.
H. Reek
Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904
1098 XH Amsterdam (The Netherlands)
E-mail: j.n.h.reek@uva.nl
[b]
[c]
‡
Dr. J. Rodgers
Eastman Chemical Company
200 South Wilcox Drive, Kingsport, TN (USA)
Present address: Qingdao Institute of Bioenergy and Bioprocess
Technology, Chinese Academy of Sciences, No. 189 Songling Road,
Laoshan District, Qingdao 266101 (P. R. China).
These authors contributed equally.
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Because the zinc-pyridine interaction is the strongest in
apolar solvents, only non-coordinating solvents like toluene and
dichloromethane were employed. With the increase of polarity or
coordinating ability of the solvent, the binding constant of the zinc-
pyridine interaction decreases, which could potentially lead to a
shift of the equilibrium to the non-encapsulated catalyst. However,
from an industrial point of view it would be preferable to move
away from toluene and chlorinated solvents, which have been
listed as problematic in the CHEM21 solvent selection guide.[37]
Many polar solvents, such as ketones, alcohols and various
esters, on the other hand, have been classified as industrially
recommended solvents.
Importantly, hydroformylation of propene is currently
performed in relatively polar solvents that potentially can
coordinate to the zinc porphyrin unit in competition with the pyridyl
group. We envisioned that by modifying the cage-forming
porphyrin in such a way that the strength of the zinc-pyridine
interaction is increased, the use of the cage could be extended to
more polar and industrially interesting solvents. Previously we
have shown that substituting the porphyrins at the phenyl group
is only possible at one of the meso-positions as para- and ortho-
substitution deformed the cage to a large extent.[29,38] Increasing
the binding constant in this manner was only successful to a
limited extent. The question raised is if the binding strength can
be increased by introducing modifications directly at the porphyrin
ring.
In this contribution, we demonstrate that the zinc-pyridine
binding constant of zinc(II) meso-tetraphenylporpholactones
(Zn(II)TPPL) is nearly an order of magnitude higher compared to
the parent Zn(II)TPP. The resulting self-assembled cage retains
the branched selectivity but can now also be applied in more polar,
industrially relevant solvents. Importantly, as the modification is at
the core of the porphyrin and no bulky substituents are introduced,
the cage formation is not disrupted.
Figure 1. First generation assembly L1 formed by the self-assembly of tris-
(meta-pyridyl)-phosphine and three equivalents of Zn(II) meso-
tetraphenylporphyrins.
By coordination of the center phosphine ligand to rhodium,
efficient hydroformylation catalysts[28,29] were obtained (see
Scheme 1). In the rhodium-catalyzed hydroformylation of 1-
octene the encapsulation resulted in a 10-fold increase in catalytic
activity. Remarkably, a preferential formation of the branched
aldehyde product was observed (l/b = 0.6; l/b = linear to branched
ratio; room temperature). This unusual selectivity was ascribed to
the encapsulation of the catalytically active species within the
cavity defined by the three porphyrins.
Results and Discussion
Scheme 1. Coordination of the center phosphine in assembly L1 to rhodium(I)
leads exclusively to encapsulated rhodium monophosphine complexes. When
applied in 1-octene hydroformylation, a selective formation of the branched
aldehyde product was observed (l/b = 0.6).
Formation of the cage
In order to increase the stability of the supramolecular cage in
polar and more competing solvents, we set out to modify the
porphyrin scaffold to strengthen the zinc-pyridine interaction. We
chose an oxidized form, Zn(II)TPPL (TPPL
tetraphenyl-8H-7-oxaporphyrin-8-one), of the
Zn(II)TPP (TPP = 5,10,15,20-tetraphenylporphyrin), that was
used in the first generation cage L1. As such, the zinc center is
more electron deficient, which we expected would result in a
higher binding constant with pyridine in a variety of solvents.
Zn(II)TPPL is thermally stable and was obtained by direct
oxidation of free base meso-tetraphenylporphyrin (TPP-2H) and
subsequent metalation in 30 % overall yield (see Scheme 2).[39]
The ligand-template approach was further extended to
asymmetric hydroformylation of internal alkenes.[30,31] Here, bulky
chiral pyridine-based phosphoramidite ligands were used in
combination with zinc(II) templates for the formation of
encapsulated rhodium(I) catalysts. The encapsulated catalysts
outperformed their non-supramolecular analogues in both activity
and enantioselectivity. Furthermore, the ligand-template
approach has been employed in many other ligand systems, such
as BIAN ligands, xanthene phosphine ligands and hybrid
bidentate ligands, demonstrating the strength and generality of
this specific approach.[23,32–34]
=
5,10,15,20-
porphyrin,
All initial hydroformylation reactions were conducted at
room temperature or slightly above, as the zinc-pyridine
interaction was anticipated to be weaker at elevated temperatures.
To extend the application window to more industrially relevant
conditions, assembly L1 was applied in 1-octene
hydroformylation at temperatures as high as 75 °C.[35]
Interestingly, the assembly retained its unprecedented branched
aldehyde selectivity, when higher partial pressures of CO were
applied. This demonstrates that even though the interactions are
weaker at elevated temperatures, the overall structure is
thermodynamically sufficiently stable for cage controlled
catalysis.[36]
Scheme 2. Direct oxidation and subsequent metalation of TPP-2H to yield
desired Zn(II)TPPL.
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between the cavity shape of assemblies L1 and L3 may be a
result of different crystal packing forces, and as such these are
not necessarily different in solution. Importantly, the average Zn-
N distance of the Zn(II)TPPL moiety to P(m-py)3 is shorter than
that of Zn(II)TPP moiety to P(m-py)3, which points to a stronger
binding of pyridyl groups to Zn(II)TPPL compared to Zn(II)TPP.
Upon mixing P(m-py)3 and three equivalents of Zn(II)TPPL
in toluene, a new assembly L2 was formed by selective pyridine-
zinc coordination (see Scheme 3). The selective coordination of
the pyridine groups of the phosphine to the zinc centers is
confirmed by various spectroscopic techniques (UV-vis and NMR)
and a solid-state structure (vide infra).
Binding studies
To confirm our hypothesis that the binding affinity of pyridine to
Zn(II)TPPL is higher than for Zn(II)TPP, we determined the 1:1
host-guest binding constants in various solvents by UV-vis
titration experiments (Table 1; for binding curves, see Supporting
Information, section 5.3). Upon coordination of pyridine to
Zn(II)TPPL the typical red shift of both the Soret and the Q bands
was observed in all used solvents; toluene, dichloromethane,
acetone and dioctyl terephthalate (DOTP).[40] All titration curves
exhibited isosbestic points, indicating that a simple transition from
one species (H = host) to another (HG = host-guest) takes place.
All the titration curves fitted well with the typical equilibrium
equation of a complex with a 1:1 stoichiometry, from which the
association constants were determined. As a typical example,
overlapped absorption spectra and the titration curve for the 1:1
binding between Zn(II)TPPL and pyridine in dichloromethane are
displayed in Figure 3a and 3b. The 1:1 stoichiometry for this
specific binding event was further confirmed by Job’s plot analysis
(see Supporting Information Figure S 23 and S 24).
Scheme 3. Three equivalents of Zn(II) meso-tetraphenylporpholactone and tris-
(meta-pyridyl)-phosphine assemble to form cage L2 in toluene.
Single crystals of sufficient quality for x-ray diffraction were grown
by slow vapor diffusion of pentane into a toluene solution of P(m-
py)3 and three equivalents of Zn(II)TPPL at room temperature
without taking precautions against air. The assembly crystallized
as the phosphine oxide adduct L3 (see Figure 2). The diffraction
data allows for unambiguous assignment of the cage’s
conformation, and confirms the formation of a novel assembly
based on Zn(II)TPPL. A comparison of the structures of the novel
assembly L3 and first generation assembly L1 is displayed in
Figure 2.
L1
L3
Figure 3. (a) Overlay of UV-vis spectra of the titration of Zn(II)TPPL (host) with
pyridine (guest), at a fixed host concentration of 16 µM in dichloromethane at
298 K. (b) Absorption variation at the right Q band versus equivalents of added
guest.
L1
L3
Figure 2. Comparison between the crystal structure of parent assembly L1
and the new assembly L3 in stick and space-filling model (left), and a
molecular structure of the novel assembly (right). Solvent molecules and
hydrogen atoms have been omitted for clarity. Color code: C, grey; N, blue; O,
red; P, orange; Zn, purple.
As expected, the binding constant of pyridine to Zn(II)TPPL
(K = 2.27 ∙ 104 M-1) is more than three times higher than for
Zn(II)TPP when dichloromethane is used as solvent. In toluene
this difference is even larger, although the absolute binding
constant for the pyridine-Zn(II)TPPL complex is slightly lower (K
= 1.40 ∙ 104 M-1). Surprisingly, in acetone, the binding constants
for pyridine to Zn(II)TPP and Zn(II)TPPL are almost identical, and
slightly below 1000. Interestingly, the binding constant in a solvent
that is industrially applied and is also rather polar, dioctyl
terephthalate, is more than three times higher for Zn(II)TPPL than
Zn(II)TPP. Most importantly, the binding constant of pyridine to
Zn(II)TPPL in this solvent is only three times smaller than pyridine
to Zn(II)TPP in toluene, the conditions of the previously reported
system. With these promising results in hand, we anticipated the
Previously, we reported the crystal structure of the
supramolecular assembly L1 in which all three porphyrin moieties
are engaged in mutual CH-π interactions.[38] However, in the
crystal structure of assembly L3, such CH-π interactions are
present only between two porpholactone moieties which are tilted
towards the axis going through the P=O bond. This results in the
environment around the phosphorus atom being more sterically
congested compared to the first generation L1 assembly. The Zn-
N distances of assembly L3 are 2.158(3) Å (Zn1-N1), 2.174(3) Å
(Zn2-N2) and 2.182(3) Å (Zn3-N3), respectively. The differences
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cage controlled catalysis would now also be possible in these
more polar solvents (vide infra).
Table 2. Association constants (K) for 1:3 binding of P(m-py)3 with
Table 1. Association constants (K) for 1:1 binding of pyridine with Zn(II)TPPL
Zn(II)TPPL or Zn(II)TPP in toluene-d8 at 298 K.
or Zn(II)TPP in different solvents at 298 K.
Zn(II)TPPL
Zn(II)TPP
Solvent
KZnTPPL / M-1
2.27 ∙ 104
1.40 ∙ 104
8.57 ∙ 102
1.02 ∙ 103
KZnTPP / M-1
6.92 ∙ 103
3.41 ∙ 103
7.05 ∙ 102
2.98 ∙ 102
KZnTPPL / KZnTPP
3.28
[a]
[a]
[a]
[a]
α1
α2
K / M-1
α1
α2
K / M-1
Dichloromethane
Toluene
1.2
1.2
1.51 ∙ 104
2.8
4.8
2.50 ∙ 103
4.11
[a] Cooperativity factors for binding of the second and third
Acetone
1.22
porphyrin/porpholactone unit; where K1 = 3 ∙ K; K2 = α1 ∙ K; K3 = ꢀꢁ ∙ α2 ∙ K.
Dioctyl terephthalate
3.42
Application of assemblies in hydroformylation of 1-octene
Next, the differences in the binding constants for the
formation of the cages, L1 and L2, were investigated. Previously
it was reported that positive cooperativity plays a role in the
formation of cage L1.[29] To investigate whether such an effect is
present in the self-assembly of the second generation cage L2,
3:1 host-guest titrations were performed separately for assembly
L1 and L2. The titration for L1 was repeated so as to allow for a
valid comparison where both sets of data are acquired and fitted
with the same procedure. As UV-vis turned out to not be a suitable
Scheme 4. Rhodium(I) catalyzed hydroformylation of 1-octene.
Once the new assembly L2 was thoroughly characterized, we
focused our efforts on the investigation of its catalytic
performance in hydroformylation of 1-octene (see Scheme 4). In
parallel the same catalytic reactions were performed with L1, so
that we could clearly study differences in activity and selectivity
between the first and second generation assembly. 1-octene is a
benchmark substrate and an excellent model compound for
tracking activity and selectivity of Rh(I) hydroformylation
catalysts.[3,43] Rhodium(I) complexes with the assemblies were
generated in situ and used as such in catalysis. In all reactions
five equivalents of assembly with respect to rhodium was used to
avoid formation of active and nonselective ligand-free rhodium
species. In all the reactions, a 1 h incubation time under syngas
was applied before substrate introduction, to allow for the
complete formation of the Rh-coordinated assembly (for details
on catalysis procedures, see Supporting Information, section 3).
Reactions were carried out both at room temperature and at
elevated, industrially relevant temperatures. The results for the
hydroformylation of 1-octene are reported in Tables 3 and 4.
technique for studying the 3:1 binding of the systems, 1H and 31
P
NMR spectroscopy was utilized instead (for further details
regarding the challenges with using UV-vis for studying 3:1
binding, see Supporting Information, section 5.1).
The use of NMR spectroscopy allowed us to monitor the
changes in the signals of tris(m-pyridyl)phosphine upon binding of
Zn(II)TPP and Zn(II)TPPL. Importantly, by tracking the phosphine
signals the supramolecular system can be saturated to the 3:1
complex, which we anticipated would allow us to further study the
cooperativity in both systems. 1H and 31P NMR signals of the
phosphine were monitored in parallel throughout the titration,
resulting in more reliable data for the further fitting procedures. In
addition, the application of NMR permitted us to increase the
absolute concentration of both the host and guest in solution,
leading to more informative titration curves.
During the formation of assembly L1, an up-field shift of all
1
four pyridine peaks in H NMR and the phosphorus peak in 31P
NMR was observed. In the formation of assembly L2 similar shifts
1
were detected in H NMR; however, in 31P NMR the phosphine
Table 3. Hydroformylation of 1-octene using Rh-catalysts based on
assemblies L1 and L2 at varied temperatures in toluene.
peak shifted down-field. These NMR shifts are as expected for
similar systems in literature, and are caused by the anisotropic
ring currents from the porphyrin/porpholactone moieties.[41,42] By
fitting the 1H and 31P NMR titration curves simultaneously, binding
constants for the formation of both assemblies could be derived
(for binding curves, see Supporting Information, section 6.2).
Interestingly, a positive cooperativity effect was found for both
assemblies L1 and L2, where the second and third binding event
is stronger than the first one. The calculated binding constants are
listed in Table 2 along with the cooperativity factors. For assembly
L2 a small cooperativity effect is present, but much less
pronounced than for L1. This is in correspondence with the
presence of fewer CH-π interactions in the crystal structure of L2
than L1.
Entry[a]
Assembly
T / °C
Conv / %
TON
l/b[b]
1
2
3
4
5
6
L1
L2
L1
L2
L1
L2
25
25
40
40
80
80
10
22
64
57
98
97
440
0.56
0.60
0.84
0.89
1.97
2.22
1080
3130
2760
4100
3720
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9[c]
None
75
Acetone
58
1.75
[a] Reagents and conditions: 2 µmol (0.02 mol%) Rh(acac)(CO)2, 10 µmol
(0.1 mol%) P(m-py)3, 30 µmol (0.3 mol%) porphyrin/porpholactone, 0.01 mL
DIPEA (N,N-Diisopropylethylamine), 5.5 mL dry toluene, 20 bar H2/CO (1:1),
incubation time 1 h, reaction time 16-18 h.
[a] Reagents and conditions: 0.5 µmol (0.02 mol%) Rh(acac)(CO)2, 2.5 µmol
(0.1 mol%) P(m-py)3, 7.5 µmol (0.3 mol%) porphyrin/porpholactone, 0.002
mL DIPEA (N,N-Diisopropylethylamine), 1.5 mL solvent, 80 bar H2/CO (1:1),
incubation time 1 h, reaction time 16-19 h.
[b] Ratio of linear and branched aldehyde.
[b] Ratio of linear and branched aldehyde.
In hydroformylation of 1-octene at room temperature, the
first and second generation assembly produce the product with
nearly the same selectivity (see Table 3). Interestingly, the
conversion of the substrate is twice as high for L2 compared to
L1, suggesting that the activity is different. However, at the higher
temperatures the conversion values are nearly identical. Both
assembled catalysts maintain a good selectivity for the branched
aldehyde at a reaction temperature of 40 °C, and at 80 °C both
catalyst systems lose their selectivity. This drop is expected and
is due to the zinc-pyridine interaction becoming weaker at higher
temperatures. We previously reported that by increasing the
partial pressure of CO in the gas mixture, higher selectivity for the
branched aldehyde can be maintained with assembly L1 at higher
temperature.[35] Subsequent reactions were therefore carried out
at elevated temperatures and pressures, and the effect of the CO
concentration was investigated in more detail in the
hydroformylation of propene (vide infra).
Next, the solvent scope of the reaction was evaluated in the
hydroformylation of 1-octene, using three different solvents at
high temperature (75 °C) and pressure (80 bar) (see Table 4).
Due to the higher binding constant of pyridine to Zn(II)TPPL in all
solvents explored, we expected the assembly L2 to perform better
when extending the reaction to more polar, and industrially
relevant solvents acetone and DOTP (see Figure 4). It turns out
that yet again assemblies L1 and L2 perform nearly equally well
in all three solvents, albeit the selectivity is slightly higher for the
first-generation assembly. In all cases, however, the cages out-
perform their non-encapsulated catalyst analogues, confirming
that even at high temperatures the cages are at least partly intact.
[c] No porphyrin/porpholactone was added, only P(m-py)3 was used.
Application of assemblies in hydroformylation of propene
Scheme 5. Rhodium(I) catalyzed hydroformylation of propene.
With the results of 1-octene hydroformylation in hand, we focused
our efforts on the hydroformylation of the more challenging
substrate propene (see Scheme 5). The challenge lies in the small
size of propene and the lack of directing functional groups, which
generally results in relatively low selectivity for the branched
aldehyde.
First, we evaluated the effect of the CO and H2 partial
pressure, respectively, on the hydroformylation of propene,
knowing that these parameters greatly affect both the activity and
selectivity of 1-octene hydroformylation. We explored this effect
for first generation assembly L1, for which the CO concentration
effect was earlier reported in the hydroformylation of 1-octene.[35]
Thus, we could directly conclude if the same effect is present for
propene. The results are displayed in Table 5 and 6. As expected,
an increase in the partial pressure of CO leads to a higher
selectivity for the branched aldehyde, whereas the activity and
productivity of the catalyst are decreased (Table 5, entries 1-4).
Table 5. Hydroformylation of propene using Rh-catalysts based on
assembly L1 at different partial pressures of CO at 70 °C.
[b]
Entry[a]
Assembly
TON
TOFmax
l/b[c]
pH2
bar
/
pCO
bar
/
ptot /
bar
Figure 4. The structure of industrially relevant solvent dioctyl terephthalate.
1
2
3
4
L1
L1
L1
L1
12.5
12.5
12.5
12.5
12.5
16.7
25
25
7600
6370
6180
5610
1500
950
880
690
1.40
1.30
1.20
1.12
Table 4. Hydroformylation of 1-octene using Rh-catalysts based on
assemblies L1 and L2 at different solvents under industrially relevant
conditions.
29.2
37.5
50
Entry[a]
Assembly
L1
T / °C
75
Solvent
Toluene
Toluene
Toluene
DOTP
Conv / %
>99
>99
>99
>99
>99
>99
10
l/b[b]
0.78
0.99
1.72
0.96
1.08
2.31
1.12
1.35
37.5
1
[a] Reagents and conditions: 2 µmol Rh(acac)(CO)2, 10 µmol P(m-py)3, 30
µmol porphyrin, 0.01 mL DIPEA (N,N-Diisopropylethylamine), 5.5 mL dry
toluene, 8 bar propene, 70 °C, reaction time 16 h.
[b] TOFmax = turnover frequency (mol mol-1 h-1), see SI for calculation of
2
L2
75
3[c]
4
None
L1
75
TOFmax
75
[c] Ratio of linear and branched aldehyde.
5
L2
75
DOTP
6[c]
7
None
L1
75
DOTP
Remarkably, a rather high selectivity (l/b = 1.12) was
preserved at
a temperature as high as 70 °C, clearly
demonstrating the correlation between a high CO concentration
and relative high preference for the branched aldehyde formation
in propene hydroformylation. The opposite effect is observed for
an increase in the partial pressure of H2, where the activity and
75
Acetone
Acetone
8
L2
75
28
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10.1002/chem.201702113
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productivity of the catalyst are increased at the cost of selectivity
(see Table 6). Similar effects are observed when performing
catalysis at a set pressure of 20 bar while varying the CO to H2
ratio (see Supporting Information, Table S 1).
Table 7. Hydroformylation of propene using Rh-catalysts based on
assemblies L1 and L2 at varied temperatures in toluene.
Table 6. Hydroformylation of propene using Rh-catalysts based on
assembly L1 at different partial pressures of H2 at 70 °C.
[b]
Entry[a]
Assembly
T / °C
25
TON
390
TOFmax
60
l/b[c]
0.94
0.84
1.12
1.11
[b]
Entry[a]
Assembly
TON
TOFmax
l/b[c]
pH2
bar
/
pCO
bar
/
ptot /
bar
1
2
3
4
L1
L2
L1
L2
1
2
3
4
L1
L1
L1
L1
12.5
16.7
25
12.5
12.5
12.5
12.5
25
7600
9050
4940
11500
1500
1840
4810
6600
1.40
1.44
1.58
2.10
25
480
75
29.2
37.5
50
70
5600
5570
690
500
70
37.5
[a] Reagents and conditions: 2 µmol Rh(acac)(CO)2, 10 µmol P(m-py)3, 30
µmol porphyrin/porpholactone, 0.01 mL DIPEA (N,N-
[a] Reagents and conditions: 2 µmol Rh(acac)(CO)2, 10 µmol P(m-py)3, 30
µmol porphyrin, 0.01 mL DIPEA (N,N-Diisopropylethylamine), 5.5 mL dry
toluene, 8 bar propene, 70 °C, reaction time 16 h.
[b] TOFmax = turnover frequency (mol mol-1 h-1), see SI for calculation of
Diisopropylethylamine), 5.5 mL dry toluene, 8 bar propene, 50 bar H2/CO
(1:3), reaction time 15-17 h.
[b] TOFmax = turnover frequency (mol mol-1 h-1), see SI for calculation of
TOFmax
TOFmax
[c] Ratio of linear and branched aldehyde.
[c] Ratio of linear and branched aldehyde.
Having concluded that a high CO partial pressure is
important for the branched selectivity not only in 1-octene but also
propene hydroformylation, we sought to explore the effect of
temperature on the reaction with both assemblies. Much to our
delight, the novel second generation assembly L2 demonstrated
a higher selectivity both at room temperature and at elevated
temperatures (see Table 7). This is to our knowledge the highest
selectivity for the branched aldehyde product in propene
hydroformylation reported to date. Interestingly, first generation
cage L1 outperforms L2 in benchmark 1-octene hydroformylation
in terms of branched selectivity, whereas the opposite effect is
observed in propene hydroformylation.
With these surprising results in hand, we attempted to find
an explanation for the selectivity differences between the
assemblies. As described earlier, the average Zn-N distance in
assembly L2 is shorter than in L1 in the solid state. Although in
solution, the shape of the new assembly is likely dynamic, we
assume that this Zn-N bond distance difference may still play an
important role in the catalytic performance. Preliminary volume
calculations based on the crystal structures of L1 and L2 were
carried out to shine light on the effect of going from Zn(II)TPP to
Zn(II)TPPL on catalysis results (for calculations, see Supporting
Information, section 10). Interestingly, the cavity volume of
assembly L2 is 44 % smaller than that of L1. Thus, by exchanging
a porphyrin for a porpholactone not only the binding strength, but
also the size of the cage is changed. This could be a plausible
explanation for the observed selectivity differences in catalysis. It
is likely that a smaller cage is more selective in the conversion of
the smaller propene, whereas the slightly larger capsule provides
a more branched-selective hydrofromylation catalyst for the larger
substrates such as 1-octene.
Finally, we carried out hydroformylation of propene with
both assemblies in toluene and in a more competing solvent,
DOTP, under industrially relevant conditions (see Table 8).
Interestingly, assembly L2 displays both higher activity and
selectivity in more polar and coordinating solvent, DOTP. This
effect can be directly attributed to the higher binding constant of
the zinc-pyridine interaction in DOTP for assembly L2. As
hypothesized, an increase in the binding constant between zinc
and pyridine allows for a more stable cage in polar solvents. By
this small change in the cage forming units, we have extended the
branched-selective hydroformylation of propene to more
industrially relevant conditions and solvents (DOTP). These
results are consistent with L2 being both smaller and more stable
than cage L1.
Table 8. Hydroformylation of propene using Rh-catalysts based on
assemblies L1 and L2 in different solvents at 70 °C.
[b]
Entry[a]
Assembly
Solvent
Toluene
Toluene
DOTP
TON
5600
5570
1273
4130
TOFmax
690
l/b[c]
1.12
1.11
1.45
1.28
1
2
3
4
L1
L2
L1
L2
500
107
DOTP
167
[a] Reagents and conditions: 2 µmol Rh(acac)(CO)2, 10 µmol P(m-py)3, 30
µmol porphyrin/porpholactone, 0.01 mL DIPEA (N,N-
Diisopropylethylamine), 5.5 mL dry toluene, 8 bar propene, 50 bar H2/CO
(1:3), 70 °C, reaction time 17 h.
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[b] TOFmax = turnover frequency (mol mol-1 h-1), see SI for calculation of
TOFmax
unless otherwise noted. 1-octene was filtered over basic alumina
before use.
[c] Ratio of linear and branched aldehyde.
X-ray single crystal determination
C144H90N15O7PZn3 + 2(C7H8)+ disordered solvent, Fw = 2553.73
(Derived values do not contain the contribution of the disordered
solvent), violet-redrough fragment, 0.32 x 0.20 x 0.11 mm, triclinic,
P-1 (no. 2), a = 18.0306(10), b = 20.7321(12), c = 21.3054(12) Å,
α = 95.628(3), β = 99.410(3), γ = 106.576(3), V = 7441.8(7) Å3, Z
= 2, Dx = 1.140 g cm-3 (Derived values do not contain the
contribution of the disordered solvent), µ = 0.548 mm-1 (Derived
values do not contain the contribution of the disordered solvent).
In total, 203052 reflections were measured on a Bruker D8 Quest
Eco diffractometer, equipped with a TRIUMPH monochromator
and a CMOS PHOTON 50 detector (λ = 0.71073 Å) up to a
resolution of (sinθ/θ)max = 0.83 Å-1 at a temperature of 150(2) K.
The intensity data were integrated with the Bruker APEX2
software.[44] Absorption correction and scaling was performed with
SADABS.[45] (0.64–0.75 correction range). In total, 26675
reflections were unique (Rint = 0.085), of which 19584 were
observed [I>2σ(I)]. The structure was solved with direct methods
using the program SHELXS-97[46] and refined with SHELXL-2013
against F2 of all reflections. One of the porpholactone moieties is
positionally disordered (rotation over a 90 degree axis) and the
lactone moiety was refined as occupying two sites with the
occupancy factors of 0.64 and 0.36. The structure contains voids
(1695 Å3 per unit cell) filled with disordered solvent molecules.
Their contribution to the structure factors was secured by back-
Fourier transformation using the SQUEEZE routine of the
PLATON package,[47] resulting in 226 electrons per unit cell. 1657
parameters were included in the least-squares refinement. Non-
hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were introduced in calculated
positions and refined with a riding model. R1/wR2 [I42s(I)]:
0.0606/0.1751. S = 1.020. Residual electron density between -
0.78 and 1.18 e Å-3. Geometry calculations and checking for
higher symmetry was performed with the PLATON program.[47]
Conclusions
This work describes the encapsulation of a monoligated rhodium
complex in a new supramolecular assembly based on P(m-py)3
and Zn(II)TPPL. The resulting supramolecular catalyst displays
the highest selectivity for the branched aldehyde in the
hydroformylation of propene (l/b = 0.84). In the current system,
porpholactone units are used to generate the second coordination
sphere around the active catalyst, whereas previously we have
used normal Zn(II)TPP. This small change in the electronics of
the porphyrin has a large effect on the binding constant and as
such also on the stability of the cage. In addition, X-ray analysis
of the assembly shows that the cage volume is also slightly
smaller. As a result of these differences, the new self-assembled
cage gives unprecedented branched selectivity in the
hydroformylation of propene, whereas the use of the cage based
on Zn(II)TPP gives higher branched selectivity for 1-octene.
Importantly, the increased zinc-pyridine interaction observed for
Zn(II)TPPL allows for the reaction to be performed in industrially
relevant solvent DOTP, while maintaining high selectivity in
propene hydroformylation. With these experiments we have
demonstrated that small changes made to the building blocks of
the assembly allows the fine tuning og the catalyst properties such
that these can be applied under industrially relevant conditions.
Experimental Section
General procedures
Synthesis of meso-tetraphenyl-2-oxa-3-oxoporphyrinato
zinc(II) (Zn(II)TPPL)
All reactions were carried out under an atmosphere of N2 using
standard Schlenk techniques unless otherwise stated. CH2Cl2
was distilled from CaH2 under N2, and pentane and toluene were
distilled from Na under N2. NMR spectra were recorded on Bruker
AMX 300 (300.1 MHz, 75.5 MHz and 121.5 MHz for 1H, 13C and
31P respectively), Bruker AMX 400 (400.1 MHz, 100.6 MHz and
162.0 MHz for 1H, 13C and 31P respectively) and Bruker AMX 500
(500.1 MHz, 125.8 MHz and 202.5 MHz for 1H, 13C and 31P
respectively). CDCl3 was used as a solvent unless otherwise
specified and the 1H NMR spectra were referenced to the solvent
residual signal. ESI-MS measurements were recorded on a JEOL
JMS SX/SX102A four sector mass spectrometer, UV-vis spectra
were recorded on a Shimadzu UV-2000 spectrophotometer, using
a 10 mm quartz cuvette. Gas chromatographic analyses of 1-
octene hydroformylation were performed on a Shimadzu GC-17A
apparatus. Gas chromatographic analyses of propene
hydroformylation were performed both on a Trace GC-ultra
apparatus (Thermo electron cooperation) and a Shimadzu GC-
17A apparatus. Kinetic data were recorded by Brooks 0254. X-ray
crystal diffraction data was collected on a Bruker D8 Quest Eco
single crystal diffractometer equipped with a CMOS Photon 50
detector, using Mo Ka radiation. All reagents were purchased
from commercial suppliers and used without further purification,
Modified literature procedure[39,48]
:
Step 1: To a stirred mixture of 5,10,15,20-tetraphenylporphyrin
(1.5 g, 2.4 mmol, 1 eq) and RuCl3 (248.9 mg, 1.2mmol, 0.5 eq) in
1,2-dichloroethane (DCE) (750 mL) and water (750 mL),
respectively, a DCE solution (20 mL) of 2,2’-bipyridine (187.4 mg,
1.2 mmol, 0.5 eq) was added. The solution was heated to 100 ºC.
A mixture of Oxone® (7.377 g, 12 mmol, 5 eq) and NaOH (480 mg,
12 mmol, 5 eq) was added in 5 portions over a period of 5 h. The
reaction was quenched with a saturated aqueous solution of
Na2S2O3, where after the organic layer was separated and the
aqueous layer was extracted twice with dichloromethane. The
combined organic layers were dried with Na2SO4, filtered, and
concentrated under vacuum. The residue was purified by column
chromatography (silica gel, eluent: CH2Cl2 / hexane = 2 : 1) to give
the product, 5,10,15,20-tetraphenylporpholactone, as a purple
solid (yield 45 %, 683.3 mg, 1.08 mmol). 1H NMR (500 MHz,
CDCl3, 293 K): δ = 8.80 (dd, J = 5.0, 1.7 Hz, 1 H), 8.76 (dd, J =
5.0, 1.8 Hz, 1 H), 8.70 (dd, J = 4.9, 1.7 Hz, 1 H), 8.60 (d, J = 4.9
Hz,1 H), 8.58 (dd, J = 4.6, 1.4 Hz, 1 H), 8.53 (d, J = 4.5 Hz, 1H),
8.13 (m, 4 H), 8.10 (m, 2 H), 7.98 (m, 2 H), 7.73 (m, 12 H), -1.66
(s, 1 H, NH), -2.03 (s, 1 H, NH).
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Step 2: 5,10,15,20-Tetraphenylporpholactone (290 mg, 0.458
mmol, 1 eq) and Zn(OAc)2 were suspended in 120 mL solvent
(CHCl3 / EtOH = 2 : 1). The reaction mixture was heated up to 70
ºC for 2 h. Afterwards, the reaction mixture was cooled down and
filtered through Celite. The filtrate was concentrated and purified
by column chromatography (silica gel, eluent: CH2Cl2, Rf = 0.44).
The bright green band was collected and all the solvent was
evaporated, which afforded a green purple solid in 80 % yield (255
mg, 0.366 mmol). 1H NMR (500 MHz, CDCl3, 298 K): δ= 8.72 (bs,
6 H),8.13 (bs, 6 H), 7.8 (bs, 14 H). Due to strong self-aggregation,
the peaks are broad and cannot be assigned. For better resolution
of peaks, 2 eq. of pyridine was added. 1H NMR (500 MHz, CDCl3,
298 K): 1H NMR (500 MHz, CDCl3, 293 K): δ = 8.74 (d, J = 4.7 Hz,
1 H, pyrrole-H), 8.66 (dd, J = 7.3 Hz, 4.6 Hz, 2 H, pyrrole-H), 8.60
(d, J = 4.5 Hz, 1 H, pyrrole-H), 8.54 (d, J = 4.5 Hz, 1 H, pyrrole-
H), 8.50 (d, J = 4.5 Hz, 1 H, pyrrole-H), 8.10 (d, J = 7.7 Hz, 4H),
8.05 (d, J = 6.7 Hz, 2H, Ph-H), 7.93 (d, J = 6.2 Hz, 2H, Ph-H),7.70
(m, 12H, Ph-H), 7.17 (t, J = 7.5 Hz, 2 H, p-Py), 6.58 (t, J = 7.5 Hz,
4 H, m-Py), 5.93 (bs, 4 H, o-Py).
tributylphosphite were added to quench the active rhodium
catalyst. 10-20 µL of the reaction mixture was diluted to 1 mL with
solvent. The vial was directly measured by GC without further
workup.
Acknowledgements
We thank the Eastman Chemical Company and the European
Research Council (ERC Adv. NAT-CAT Reek) for kind financial
support.
Keywords: propene hydroformylation • supramolecular catalysis
• cooperative binding • porpholactone • self-assembly
Typical procedure for catalysis
In a flame-dried schlenk under N2, metalloporphyrins (0.03 mmol),
a stock solution of P(m-py)3 (0.01 mmol) in toluene (c = 0.026 M),
neat diisopropylethylamine (DIPEA, 0.06 mmol),[49] a stock
solution of Rh(acac)(CO)2 (0.002 mmol) in toluene (c = 0.005 M),
substrate were added consecutively. Toluene was added to the
reaction mixture to reach the same total volume for all
experiments. The autoclave was evacuated and flushed with N2
three times. The above reaction mixture was transferred to the
autoclave with a syringe and stainless steel needle (~25 cm). The
autoclave was then flushed three times with 20 bar syngas. Then
the autoclave was pressurized to the required pressure,
immersed into a pre-heated oil bath and stirred with a fixed speed.
After a pre-set reaction time, the reaction mixture was cooled
down and the pressure was carefully released. 20 µL n-
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Tuning the porphyrin building block in self-assembled cages for branched-selective
hydroformylation of propene
Layout 1:
FULL PAPER
Cage controlled catalysis: the self-
assembly of trispyridylphosphine and
three zinc porpholactone units yields
a novel supramolecular cage that is
stable in both nonpolar and polar
solvents. When coordinated to
Dr. Xiaowu Wang,‡ [a] Sandra S.
Nurttila,‡ [a] Dr. Wojciech I. Dzik,[a] Dr.
René Becker,[a] Dr. Jody Rodgers,[b]
Prof. Dr. Joost N.H. Reek*[a]
Page No. – Page No.
rhodium(I), a catalyst is obtained,
which is capable of unprecedented
branched-selective hydroformylation
of propene in industrially interesting
solvents (see figure).
Title
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