Porphyrin macrocyclic catalysts for the processive oxidation of polymer substrates

Article abstract of DOI:10.1021/ja908524x

(Figure Presented) A novel cavity-containing porphyrin catalyst based on a previously reported clip architecture, substituted on the outer face with urea terminated tails, has been synthesized, and its properties toward the epoxidation of polybutadiene have been studied. It is shown that the presence of the urea tails provides efficient shielding of the manganese porphyrin against destruction and selectively directs the oxidation process to the inside of the catalyst cage, allowing for processive oxidation of a polymer substrate without the need of an additional axial ligand.

Full text of DOI:10.1021/ja908524x

Published on Web 01/14/2010
Porphyrin Macrocyclic Catalysts for the Processive Oxidation of Polymer
Substrates
Cyrille Monnereau, Pilar Hidalgo Ramos, Alexander B. C. Deutman, Johannes A. A. W. Elemans,
Roeland J. M. Nolte,* and Alan E. Rowan*
Institute for Molecules and Materials, Radboud UniVersity Nijmegen, Heijendaalseweg 135,
6525 AJ Nijmegen, The Netherlands
Received October 7, 2009; E-mail: r.nolte@science.ru.nl; a.rowan@science.ru.nl
Natural proteins and enzymes have been widely used as
blueprints for the construction of synthetic catalysts.1 As mimics
for the monooxygenase cytochrome P450, myriads of modifications
have been introduced into the skeleton of catalytic porphyrins with
the purpose to improve the degree of control over the catalytic
reactions and to reach efficiencies comparable to that of natural
enzymes.2 For example, nitrogen containing axial ligands have been
used to enhance the rate of oxidation catalysts by activating the
manganese-oxygen bond of Mn(III) porphyrins;3 electron donating
or withdrawing groups of different nature have been added to the
meso positions of these porphyrins to tune the electronic properties;
i.e. the reactivity of the catalytic center;4 and shape-selective or
chiral binding pockets have been introduced to improve the chemo-
and stereoselectivities of the reaction.5 Furthermore, bulky substit-
uents have been attached to the porphyrin to provide spatial
shielding, thus preventing deactivation of the catalyst by dimer-
ization reactions.6
We have previously reported on the synthesis and study of a
diphenylglycoluril-appended porphyrin catalyst Mn2 (Figure 1a).7
Because of its open ring structure, this compound is capable of
binding to a polymer chain, e.g. polybutadiene, and thread on it.8
Kinetic studies have revealed that the catalyst can convert the double
bonds of polybutadiene into epoxide functions while gliding along
the polymer chain in a pseudorotaxane fashion9 (Figure 1c). Because
of this unique feature, this catalyst can be called a processive
enzyme mimic.10 In spite of the very promising results in terms of
guest complexation, motion along the polymer chain, and oxidation
of polymer substrates, the system has some drawbacks hampering
its practical use as a processive catalyst. For instance, a large excess
of a bulky coordinating ligand (a pyridine derivative) is required
to efficiently shield the outer face of the catalyst, thereby facilitating
the catalytic reaction to take place on the inside through a rotaxane
mechanism. However, even at high concentrations of axial ligand,
the outer face of the catalyst remained partially accessible, and as
a result deactivation of the catalyst, probably by a dimerization
reaction, still occurred. This led to premature stalling of the
epoxidation process.
With this problem in mind, we decided to construct a new
analogue of Mn2 with ethylureapropoxy tails covalently attached
to the outer ortho-positions of the porphyrin meso-phenyl groups
(compound Mn1, Figure 1a). These tails are meant to protect the
exposed face of the cavity and to prevent the destruction of the
catalyst. Preliminary studies carried out with the zinc analogue of
Mn1 showed that the tails adopt a folded conformation over the
porphyrin roof, with the urea moieties displaying a coordinative
interaction with the zinc center (Figure 1b). We expected therefore
that similar coordination behavior would be present in the case of
the manganese derivative, thereby forcing the catalytic reaction to
take place on the inside of the cage.
Figure 1. (a) Molecular structures of Mn1, Mn2, Py, and SiPy. (b)
Computer-modeled structure of Mn1 with one of the urea-functionalized
tails coordinating to the manganese center. (c) Schematic representation of
the processive catalytic epoxidation of polybutadiene by Mn2.
The synthesis of Mn1 is outlined in Scheme 1. In the UV-vis
spectrum of Mn1 (see Supporting Information (SI)) the location
of the absorption bands is clearly indicative of the occurrence of
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J. AM. CHEM. SOC. 2010, 132, 1529–1531 1529

C O M M U N I C A T I O N S
Scheme 1. Synthetic Route To Obtain Porphyrin Catalyst Mn1
intramolecular coordination of the urea functionalities located at
the end of the porphyrin substituents to the porphyrin manganese
center. When compared to its nonsubstituted counterpart Mn2 in
similar concentration conditions, Mn1 shows a typical broadening
of the Soret band, with a simultaneous blue shift and slight increase
of the absorption maxima of the Q-bands. These features are
typically observed in the UV-vis spectra of manganese por-
phyrins when an axial ligand binds to the metal center and are,
in the case of Mn1, seemingly independent of the porphyrin
concentration, which proves that the process of axial coordination
is intramolecular. Additional evidence for this coordination
comes from mass spectroscopic analysis. Mn2 is characterized
by the presence of a methanol adduct of the manganese porphyrin
cation in the ESI-MS spectrum (100% isolation), which we
attribute to axial coordination of methanol to the manganese
center stabilized by inclusion inside the cavity of Mn2. Such
an adduct is completely absent in the mass spectrum of Mn1,
suggesting that binding of methanol to the manganese center is
impossible, or at least unfavorable, presumably because of
competition with the intramolecular binding of the urea functions
in Mn1, while the chloride counterion is presumably coordinating
inside the cavity.
proceeded to 100% completion following kinetics that could be
fitted almost perfectly to a first-order substrate conversion reaction.
In contrast, a substantial deviation from first-order was observed
in all reactions performed with Mn2, and fitting of the kinetic curves
required the use of eq 1, which includes a term (kbleach) accounting
for a bimolecular (second order) process of catalyst destruction,
i.e. the detrimental formation of a µ-oxo dimer of the catalyst (see
SI for more details about eq 1).
Compound Mn1 was tested as a catalyst in the epoxidation of
polybutadiene (98% cis, average MW ) 200.000 amu) in
chloroform. The catalytic reactions were performed using
iodosylbenzene as an oxidant, under conditions similar to those
described in our previous studies with Mn2.7 Pyridine (Py) and
4-{[(tert-butyldiphenylsilyl)oxy]methyl}pyridine (SiPy) (Figure 1a)
were used as axial ligands. At given time intervals, the reaction
mixtures were filtered to remove unreacted iodosylbenzene, and
the filtrate was directly analyzed by 1H NMR. The results are
presented in Figure 2, and the values measured for the initial rates
and cis/trans ratios of the reaction products are collected in Table 1.
Experiments performed with manganese(III) 5,10,15,20-tetrakis-
(o-methoxyphenyl) porphyrinato chloride (MnTMPP) as a reference
catalyst are also included in Table 1 for comparison.
Figure 2. Kinetics of polybutadiene epoxidation by Mn1 (full lines) and
Mn2 (dotted lines): first-order fit Mn1; first-order fit Mn1+Py; fit Mn2 to
eq 1; fit Mn2+py to eq 1; fit Mn2 + SiPy to eq 1.
Table 1. Initial Turnover Frequencies (T.O.F.) and Cis/Trans
Product Ratios Found in the Various Catalyzed Epoxidation
Experiments
axial ligand
sa
SiPy
Py
porphyrin
catalyst
initial
T.O.F.b
cis/
trans
initial
T.O.F.b
cis/
trans
initial
T.O.F.b
cis/
trans
Mn1
Mn2
MnTMPP
120
95
73
30/70
30/70
60/40
120
180
270
30/70
30/70
85/15
150
5200
330
30/70
97/3
85/15
Figure 2 shows the differences between the catalytic reactions
performed with Mn1 and the reference compound Mn2. In the case
of Mn1, independent of whether pyridine ligands were used in the
reaction mixture or not, conversion of polybutadiene always
b
a In the absence of axial ligand. In h-1; extrapolated from the slope
of the tangent to the catalytic curve at the origin.
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C O M M U N I C A T I O N S
enhancement for Mn1 is probably unrelated to any binding of Py
to the porphyrin cage, but presumably the result of slight changes
in the polarity of the medium due to the addition of the non-
negligible amounts of Py (up to 2 vol %). From the data above it
can be concluded that in the case of Mn1 the epoxidation reaction
is efficiently and exclusively directed toward the inside of the cage
catalyst. In other words, when Mn1 is used as a catalyst, the
oxidation of polybutadiene will only occur after the formation of
an initial rotaxane intermediate, which is favorable for a processive
sequence of reaction steps.
In conclusion, we have shown that the attachment of ethyl-
ureapropoxy tails to the outside of the cavity containing porphyrin
complex is a suitable procedure not only to increase the stability
of the catalyst, viz. by preventing degradation via dimerization but
also to force the catalytic reaction to take place within the cavity
of the catalyst in a pseudorotaxane fashion. We expect that these
features can be advantageously used in the future development of
efficient processive catalysts.
[P] ) [S]0{1 - (1 + [Cat]0 · kbleach · t)(-kcat/kbleach)}
(1)
Moreover, an additional series of experiments carried out with
a large excess (8000 equiv with respect to catalyst) of substrate at
low concentrations of the porphyrin catalyst (see SI) and in the
absence of axial ligand showed that the conversion was almost
complete after 96 h for Mn1, whereas the oxidation process stalled
after a turnover number (T.O.N.) of 1800 for Mn2. These results
strongly suggest that the urea tails of the catalyst have a beneficial
shielding effect and increase the stability of Mn1. The shielding
effect was confirmed by the fact that additional batches of substrate
could be successfully converted.11
As can be seen in Table 1, the use of an axial ligand in the
epoxidation reactions leads to remarkable differences between Mn1
and the other catalysts. For the reference catalyst MnTMPP, the
activation of the metal center by a pyridine ligand results in an
enhancement of its catalytic activity by a factor of 4 to 5 depending
on the type of pyridine ligand (Table 1, entry 3).
In the case of Mn2 the use of the bulky SiPy ligand, which
coordinates to the outside of the catalyst, gives a small yet
significant double enhancement of the initial turnover frequency
(T.O.F.) (Table 1, entry 2). In contrast, addition of a SiPy ligand
to Mn1 does not affect the catalytic reaction at all. The turnover
frequencies in the presence or absence of this ligand were identical
within experimental error (Table 1, entry 1). This indicates that no
complexation takes place between SiPy and the manganese center
of Mn1, presumably as the result of intramolecular coordination
of the urea tails to this center.
The use of the smaller Py ligand as a promoter of the catalytic
reaction led to an even more pronounced difference in behavior
between Mn1 and Mn2. In the latter case, Py binds to the inside
of the porphyrin cavity, forcing the oxidation reaction to take place
on the outside, which is more accessible to the polymeric substrate.
This different arrangement of the catalytic complex leads to striking
differences in the activity and stereoselectivity of the reaction, with
the initial TOF being enhanced by a factor of more than 50
compared to the TOF in the absence of an axial Py ligand. Also
the stereoselectivity of the reaction changes dramatically, i.e. in
the direction of the cis-epoxide product (cis/trans ratio changes from
30:70 to 97:3) (Table 1, entry 2). Such changes are not observed
in the case of Mn1: under analogous reaction conditions, only a
very limited rate enhancement of the reaction rate was measured
(TOF changes by a factor of 1.5), while no change in the cis/trans
selectivity was found (Table 1, entry 1). This is clear evidence that
the binding of Py in the cavity of Mn1 is prevented. We attribute
the latter to a competition between this ligand and the urea moieties
of the tails for binding to the manganese center. The presence of
four urea groups covalently anchored to the porphyrin cage complex
results in a high effective molarity of these groups, efficiently
counterbalancing the normally 2 to 3 orders of magnitude higher
binding of Py inside the cavity of the catalyst.7 The marginal
Acknowledgment. Financial support was provided by the Royal
Netherlands Academy of Science (to R.J.M.N.), by NRSCC (to
C.M., R.J.M.N., and A.E.R.) and by NWO-CW (to J.A.A.W.E. and
A.E.R.).
Supporting Information Available: Experimental procedures,
derivation of eq 1, UV-vis spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
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