A Supramolecular Palladium Catalyst Displaying Substrate Selectivity by Remote Control

Article abstract of DOI:10.1002/chem.201804543

Inspired by enzymes such as cytochrome P-450, the study of the reactivity of metalloporphyrins continues to attract major interest in the field of homogeneous catalysis. However, little is known about benefitting from the substrate-recognition properties of porphyrins containing additional, catalytically relevant active sites. Herein, such an approach is introduced by using supramolecular ligands derived from metalloporphyrins customized with rigid, palladium-coordinating nitrile groups. According to different studies (NMR and UV/Vis spectroscopy, XRD, control experiments), the supramolecular ligands are able to accommodate pyridine derivatives as substrates inside the porphyrin pocket while the reactivity occurs at the peripheral side. By simply tuning a remote metal center, different binding events result in different catalyst reactivity, and this enzyme-like feature leads to high degrees of substrate selectivity in representative palladium-catalyzed Suzuki–Miyaura reactions.

Full text of DOI:10.1002/chem.201804543

A Journal of
Accepted Article
Title: A Supramolecular Palladium Catalyst Displaying Substrate
Selectivity by Remote Control
Authors: Rafael Gramage-Doria, Paolo Zardi, and Thierry Roisnel
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A Supramolecular Palladium Catalyst Displaying Substrate
Selectivity by Remote Control
Paolo Zardi, Thierry Roisnel and Rafael Gramage-Doria*[a]
Abstract: Inspired by enzymes, such as cytochrome P-450, the study
of the reactivity of metalloporphyrins continues to attract major interest
in homogeneous catalysis. However, little is known to take benefit
from the substrate recognition properties of porphyrins containing
additional, catalytically-relevant active sites. Herein, we introduce
such approach using supramolecular ligands derived from
metalloporphyrins customized with rigid, palladium-coordinating nitrile
groups. According to different spectroscopic studies (NMR, UV-vis, X-
ray diffraction, control experiments), the supramolecular ligands are
able to accommodate pyridine derivatives as substrates inside the
porphyrin pocket while the reactivity occurs in the peripheral side. By
simply tuning a remote metal center, different binding events result in
different catalyst reactivity: an enzyme-like feature leading to high
levels of substrate selectivity when applied to representative
palladium-catalyzed Suzuki-Miyaura reactions.
Surprisingly, little catalytic systems have been proved
successful in substrate selective catalysis, where the active
catalyst is able to discriminate between similar substrates by
modifying the substrate reactivity patterns.[11] Such a concept,
which is reminiscent of the high levels of substrate selection
processes occurring in enzymes, is extremely difficult to mimic in
non-natural transition metal catalysis[12] and it is mainly
associated to the high inner reactivity of transition metal
complexes, which make difficult to discriminate between
chemically similar substrates.[13] Herein we report a strategy that
is based on a set of supramolecular palladium catalysts that are
stereo-electronically identical with a minimal variation in the
nature of a cation located eight chemical bonds apart from the
catalytically active sites. The nature of the metal center has a
direct impact in the dynamic lability of the substrate-catalyst
interaction, thus making possible to control the reactivity and
substrate selectivity patterns of the catalyst in representative
Suzuki-Miyaura reactions.
We reasoned that metalloporphyrins appended with rigid,
coordinating nitrile groups could be suited for this purpose since
(1) different metal centers (A, Figure 1) can easily be embedded
in the porphyrin core affecting the strength of the apical binding of
pyridine-containing substrates,[14] (2) the stability of meso-
substituted metalloporphyrins in transition metal catalysis is well
known,[15] and (3) catalytically active metal centers, such as
palladium are known to coordinate to nitrile groups (Figure 1).[16]
Previously, Sanders reported porphyrin nano-rings acting as
Introduction
In the search for more efficient and sustainable chemical
transformations, homogeneous catalysis plays a prominent role.[1]
In this way, fine-tuning of transition metal catalysts by modification
of the stereo-electronic properties of the ligand(s) attached to the
metal center(s) is regarded as an essential process for further
developments.[2] As such, catalyst screening is largely dominated
by traditional trial and error methods[3] or through the selection of
potential leads by high-throughput instrumentation.[4] Recently,
important efforts have been also devoted to rationally design
powerful transition metal catalysts at will by computational
methods in order to anticipate and predict a catalytic outcome.[5]
As an appealing alternative, the emergence of transition metal
catalysis with supramolecular chemistry has led to a wide array of
unique systems to tackle important issues that traditional
approaches have difficulties to address.[6] For example, catalyst
encapsulation within covalent or supramolecular frameworks
resulted in an increase of the catalyst stability and reactivity as
well as a tool to control the chemo-, regio- and stereo-selectivity
of a reaction outcome.[7] Another approach is based on the use of
non-covalent interactions such as ion-pair, hydrogen bonding or
π-π interactions between the catalyst and the substrate to dictate
products selectivity.[8] These non-covalent interactions are also
being exploited to exponentially generate ligand libraries that are
challenging to assemble by covalent bonds.[9] Other strategies
deal with transition metal-based catalysts that are able to switch
between on/off states and those connected to sophisticated
artificial molecular machineries.[10]
substrate-preorganization
reactors
for
purely
organic
transformations (Diels Alder) and their impact in reaction rate and
product selectivity.[17] Later, Nishibayashi and Uemura reported
zinc-porphyrins decorated with phosphine rhodium and iridium
fragments as catalysts for hydrosilylation reactions, although the
substrate-recognition properties of the zinc-porphyrin core were
not exploited.[18]
Figure 1. Supramolecular palladium catalyst design. Ar = 2-CN-C6H4.
Results and Discussion
Synthesis of the supramolecular ligands. Starting from 2-
cyanobenzaldehyde and pyrrole, porphyrin L1 was synthesized
as a suitable framework to build up our supramolecular catalysts
(Scheme 1). Appropriate metal insertion reactions following well-
established protocols led to the synthesis of ligands L2-L4 that
[a]
Dr. P. Zardi, Dr. T. Roisnel, Dr. R. Gramage-Doria
Univ Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France.
E-mail: rafael.gramage-doria@univ-rennes1.fr
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under: XXX
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contain different metal centers (Zn, Ru and Cu) sitting inside the
porphyrin core (Scheme 1). They were fully characterized by NMR
and HRMS analysis (see details in the Experimental Section and
the Supporting Information). In addition, these studies revealed
that the ligands L1-L4 exist, as it could be anticipated,[14] as a
respectively, and their association constants (K1.1) are reported in
Table 1 (see details in Figures S11-S21 in the Supporting
Information). The different binding behaviour associated to the
bulkiness of the substrates P2-P4 and the nature of the metal
embedded in L2-L4 was evidenced. For instance, P2 did not bind
to any metalloporphyrin ligand due to important steric effects
imposed by the ortho-substitution pattern.[15c,21] On the other hand,
P3 and P4 did reversibly bind to both Zn-containing L2 and Ru-
mixture
of
four
possible
atropoisomers
(αααα, αβαβ, ααββ, αααβ) due to the steric restrictions imposed
by the ortho substitution pattern of the meso aryl groups that
prevent free rotation around these groups and the porphyrin core.
Although the atropoisomers are conformationnally stable at room
temperature, they easily interconvert each other by standing in
solution at high temperatures,[19] making them compatible to be
used in transition metal catalysis where usually high temperatures
are required. L2 was further characterized by a single-crystal X-
ray diffraction study that clearly showed that the nitrile groups are
located above and below the porphyrin plane leaving the zinc
center available for binding to pyridine derivatives (Scheme 1).[20]
For comparison purposes (vide infra), L5 -with no nitrile groups-
and L6 -with one nitrile group- were also synthesized and
characterized (Scheme1, see details in the Experimental Section
and the Supporting Information).
containing L3 in
a
1:1 stoichiometry with comparable
thermodynamic strength (K1.1 ca. 103-104 M-1, Table 1).
Importantly, 1H NMR analysis indicated that the interactions
between Ru-containing L3 with P3 and P4, respectively, are
kinetically less labile (thus more coordinative) as compared to the
interactions involving the Zn-containing L2 upon binding to P3 and
P4, respectively. Thereby, when a solution of L3 was titrated with
P3 and followed by 1H NMR spectroscopy, a slow ligand
exchange was witnessed at the NMR timescale leading to the
presence of free 3-bromopyridine (P3) and the pyridine-
coordinated assembly [L3•P3] without formal changes in the
chemical shifts for each set of proton signals. Conversely, the
same experiment being performed with Zn-containing L2 showed
only one set of signals even in the presence of an excess of P3,
thus proving the fast exchange behaviour for the assembly
[L2•P3] between bounded and unbounded bromopyridine.[22] Cu-
porphyrin ligand L4 displayed negligible binding to all
bromopyridines P2-P4.[23] Overall, the general trend for the
binding strength of the bromopyridine...metalloporphyrin
interaction could be summarized as Cu >> Zn > Ru.
Table 1. Binding constants K1.1 (M-1) of the assemblies between ligands L2-L6 and
different pyridine derivatives.[a]
X
Ar
N
X
K1.1
N
N
Ar
N
Ar
+
A
Ar
N
N
N
N
Ar
A
Ar
N
Ar
L2-L6
Ar = 2-CN-C6H4 or C6H5
N
P2-P5
X = Br or CH=CH2
Ar
[L P]
K1.1 [L•P]
L2
L3
L4
<1[b]
<1[b]
<1[b]
<1[b]
L5
L6
2-bromopyridine (P2)
3-bromopyridine (P3)
4-bromopyridine (P4)
4-vinylpyridine (P5)
<1[b]
<1[b]
<1[b]
<1[b]
2.1×103
5.5×103
1.2×104
8.4×103
7.1×104
1.1×105
1.7×104
2.4×103 [c]
n.d.[d]
1.5×103
3.1×103
1.1×104
Scheme 1. Synthesis of the supramolecular ligands L1-L6 used in this study.
Reaction conditions: (i) Propionic acid, reflux,
2 h; (ii) Zn(OAc)2•2H2O,
CHCl3:MeOH, reflux, 2 h; (iii) Ru3(CO)12, 1,3-dichlorobenzene, reflux, 1.5 h; (iv)
Cu(OAc)2•2H2O, CHCl3:MeOH, reflux, 5 h.
[a] Determined by UV/Vis titrations (CH2Cl2 as solvent) and 1H NMR (CDCl3 as
solvent) analysis, errors estimated to be <5%. [b] Negligible binding. [c] The binding
constant value may be underestimated likely due to partial decomposition of P4
during the titration; 1H NMR competition experiments showed that the association
constants for L2 and L3 with P4 are in the same order of magnitude (see details in
Figure S25 in the Supporting Information). [d] n.d. = Not determined.
Binding studies of the supramolecular ligands towards
different pyridine substrates. Next, we studied the
supramolecular capabilities of L2-L6 to bind via the embedded
metal centre (Zn, Ru, Cu) to different pyridine-containing
substrates that could be used in Pd-catalysed carbon-carbon
bond-forming reactions such as 2-bromopyridine (P2), 3-
For comparison purposes, the binding of L5 and L6 to the
different bromopyridines P2-P4 was also studied (Table 1). As for
the other porphyrin ligands, no binding was observed for P2 with
1
bromopyridine (P3) and 4-bromopyridine (P4). H NMR and UV-
vis titration studies were performed combining L2-L6 with P2-P4,
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L5 and L6, respectively. Interestingly, the binding of
bromopyridine P3 with L5 and L6, respectively, was 4-fold weaker
than with its structurally related ligand L2; and the binding of the
bromopyridine P4 with L5 and L6, respectively, was 13-fold
weaker than with its structurally related ligand L2. These findings
indicate that increasing the number of electron withdrawing nitrile
groups around the zinc centre increases the binding strength
towards the same type of substrate. This was exemplified in the
solid state as we managed to obtain single-crystals suitable for X-
ray diffraction studies for the assembly of L2 and L5 with 4-
vinylpyridine (P5), respectively (Figure 2). The Zn...N distance
between the pyridine derivative and the Zn centre in [L2•P5] was
much shorter (dZn N = 2.139 Å) than in [L5•P5] (dZn N = 2.183 Å),
indicating that the supramolecular ligand L2 binds to pyridine
derivatives stronger than unfunctionalized L5. This was inferred
in solution as well, since P5 binds 10-fold stronger to L2 than to
L5 (K1.1 = 1.1×105 vs K1.1 = 1.1×104, Table 1). It can be concluded
that the strong electron withdrawing ability of nitrile groups within
the porphyrin ring of L2 decreases the electron density of the Zn
centre and consequently, increases its binding strength to the
pyridine nitrogen as compared to L5. In addition, we noticed that
the increase of the binding strength of L2 to P3-P5 is likely a
consequence of the increase of the electron-donating properties
of the substituents in the pyridine ring from P3 to P5 (Table 1).
The same trend was observed for the binding of L5 and L6 to P3-
P5, respectively (Table 1).
[Pd(OAc)2]). This ensures that most of the palladium remains
coordinated to the nitrile groups of the ligands (see Figure S11 in
the Supporting Information for reaction optimization).[16]
The conversion of P3 for each one of the three reactions
was followed in time by GC and the corresponding kinetic profiles
are displayed in Figure 3a. The reactions in the presence of the
supramolecular ligands L2-L4 each gave a substantially different
kinetic profile as a consequence of the differences related to their
binding properties towards P3. The dynamic but strong enough
binding of P3 to the zinc center of L2 ensured the best catalytic
performance with almost full conversion after 40 minutes; while
the reaction with a ligand that is unable to interact with the
substrate P3, such as the copper-porphyrin L4, was slow with
90% conversion reached after 180 minutes. On the other hand,
the strong kinetic stability of the assembly [L3•P3] caused an
important catalyst deactivation when using L3 as ligand, with a
conversion of P3 of 35% after 3 hours. To determine the
cooperativity of Zn...N and Pd...N≡C interactions suggested in the
catalytic experiments for the best supramolecular ligand (L2), we
performed a set of control experiments with ligands lacking nitrile
groups (L5 at 5 mol%), with one nitrile group (L6 at 5 mol%) and
with mixtures of the individual components of L2 (L5 at 5 mol%
and benzonitrile at 20 mol%, that is a 1:4 ratio). The reaction was
also conducted without any ligand. All control experiments
provided significantly lower reaction rates as compared to the
supramolecular ligand L2 (Figure 3b). This highlights the positive
effect of (1) the substrate recognition of P3 inside the Zn-
porphyrin of L2 and (2) the increase of the palladium reactivity
due to the presence of four nitrile groups in the periphery, thereby
increasing the effective concentration of palladium at close
proximity of the substrate.[24]
...
...
dN...Zn = 2.139 Å
dN...Zn = 2.183 Å
N
N
Pd(OAc)2 (0.5 mol%)
Br
Ph
L (5 mol%)
Zn
+
Zn
PhB(OH)2
K2CO3, toluene, 80 o
C
N
N
P3
Figure 2. ORTEP drawing of the assemblies [L2•P5] (left) and [L5•P5] (right)
determined by single crystal X-ray diffraction studies with thermal ellipsoids at
50% probability. All hydrogen atoms are omitted for clarity.
Pd-catalyzed cross-coupling studies. Having established that
the structurally-equivalent supramolecular ligands L2-L4 were
able to bind in a different manner to bromopyridines P2-P4 by
exclusive tuning of the binding capabilities of the embedded metal
center (Zn, Cu, Ru), we investigated these ligands in
representative Pd-catalysed Suzuki-Miyaura reactions. Toluene
was used as solvent since its relative low polarity favors the
interaction between pyridine derivatives and metalloporphyrins to
occur.[15c] We also hypothesized that the low coordinating
properties of toluene will enhance the binding of nitrile groups to
catalytically active palladium ions. 3-Bromopyridine (P3) was thus
treated with phenylboronic acid in the presence of catalytic
amounts of [Pd(OAc)2], which is known to be a good pre-catalyst
for carbon-carbon bond-forming reactions, and ligands L2-L4,
respectively (Figure 3). As it is the case in many cross-coupling
reactions,[2,3] the catalytic system was formed in situ using a ten-
fold excess of ligand per palladium (5 mol% of L vs 0.5 mol% of
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Figure 3. (a) Conversion of P3 vs time using L2-L4, respectively. (b) Reaction
rates measured in the range of 20-40% conversion for control experiments.
Homocoupling was found to be <5%, thus conversion of P3 virtually
corresponds to the yield of product
L2. On the other hand, 3-bromopyridine (P3) reacted in a more
efficient manner (twofold increase in conversion) when using
supramolecular ligand L2 even in the presence of the competing
substrate B1 (Figure 5). Thereby, L2-Pd was able to promote
palladium catalysis preferentially with the substrate that is
remotely recognized by the supramolecular catalyst even in the
presence of any additional aryl bromides such as B1. It is
important to note that the supramolecular ligands L2-L4 could be
recovered at the end of the reactions via acid/base workup,
observing no Zn/Pd transmetallation, which indicates the
robustness of this approach (see details in the Supporting
Information).
In view of the unique features of our supramolecular design
and the importance of developing artificial enzyme-like catalysts
with substrate selectivity features,[6-13] we studied the behavior of
the supramolecular catalyst L2-Pd with the other bromopyridine
regioisomers: 2-bromopyridine (P2) and 4-bromopyridine (P4).
The conversion of each regioisomer P2-P4 was followed in time
by GC in the presence and absence of L2 (Figure 4). The
reactivity of P2 remained almost unaffected by the presence or
absence of L2 due to the inability of the ortho regioisomer P2 to
bind to the Zn center of L2.[25] P4, which is known to be the most
reactive regioisomer of the series, displays a kinetic profile
completely unaffected by the absence or presence of the
supramolecular ligand L2 with full conversion after 50 minutes in
both cases. Interestingly, when comparing the reactivity of P3 in
the presence or absence of the supramolecular ligand L2, it was
found that the supramolecular ligand L2 tuned the reactivity of P3
making this substrate as reactive as the most reactive regioisomer
of the series, namely P4, with full conversion at 50 minutes as well
(Figure 4).
Pd(OAc)2 (0.5 mol%)
Br
Ph
L2 (0 or 5 mol%)
+
PhB(OH)2
K2CO3, toluene, 80 o
C
N
N
Figure 5. Conversion of P3 and B1 vs time in the absence (dashed lines) and
presence (continuous lines) of L2 under competitive conditions. Homocoupling
was found to be <5%, thus conversion of substrates virtually corresponds to
yield of product.
P2-P4
Assessment of the substrate selectivity observed for
supramolecular ligand L2. First, the ability of the
supramolecular ligand L2 to bind to palladium ions was
determined by HRMS analysis by mixing L2 with the catalytically-
relevant [Pd(OAc)2] complex in a 1:4 ratio. The mass spectrum
showed a peak at m/z 884.046 2 ppm having exactly the isotopic
profile corresponding to [L2+Pd]+ (see Figures S31-S32 in the
Supporting Information). The same type of analysis performed
with L5, which lacks nitrile groups, revealed no binding of
palladium, which agrees well with Pd...N≡C binding in the case of
1
[L2+Pd]. The H NMR spectra obtained after titration of L2 with
Pd(OAc)2 in CDCl3 at room temperature revealed small downfield
shifts at
δ ca. 8.25-8.50 ppm (see Figure S28-S29 in the
Supporting Information) for the signals associated to the protons
Ho located in ortho position with respect to the nitrile groups
(Scheme 2) within L2. Similar observations have been reported
for related systems.[16l] Furthermore, the interaction of nitrile
groups with palladium ions has been experimentally evidenced by
1H NMR and HRMS; and supported by DFT calculations
elsewhere.[16d,e,n,o] Other analysis such as UV-vis, IR and 13C NMR
were unfortunately not conclusive due to the very small
spectroscopic changes associated to the Pd...N≡C interaction.[26]
On the other hand, the proton signals belonging to palladium-
coordinated acetate ligands were observed as a major singlet at
Figure 4. (a) Conversion of P2-P4 vs time in the absence of L2. (b) Conversion
of P2-P4 vs time in the presence of L2. Homocoupling was found to be <5%,
thus conversion of P2-P4 virtually corresponds to the yield of product.
The substrate selectivity displayed by the supramolecular
ligand L2 was further assessed by performing competition
experiments with 3-bromopyridine (P3) and bromobenzene (B1)
(Figure 5). In the presence of both substrates, bromobenzene
(B1) followed a similar kinetic trend regardless of the presence of
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δ
= 2.03 ppm during the titration of [Pd(OAc)2] with L2 (see Figure
S28 in the Supporting Information). By increasing the amounts of
palladium, there were very minor proton signals that begin to
appear at
δ ca. 1.90-2.05 ppm (see Figure S30 in the Supporting
Information), which could involve the presence of acetate ligands
bridging two palladium ions according to literature precedents.[27]
Nevertheless, these data shows that palladium ions are able to
simultaneously bind to the nitrile groups of L2 and the acetate
ligands as well. The fact that structurally related square planar
[Pd(RCN)2(RCO2)2] (R = alkyl, aryl) species are known in the
literature,[28] enabled us to propose a plausible structure of the
pre-catalyst A (Scheme 2) were two coordinating sites of the
palladium center are occupied with a nitrile group from the
supramolecular ligand L2 and an acetate ligand, respectively, and
the other two vacant sites might involve coordination of a nitrile
group (either from the same ligand -chelate- or another molecule
of ligand -dimer-) and acetate ligands as terminal or bridging
ligands.
Figure 6. Reaction of self-assembly [L2•P3] with [Pd(OAc)2] (top) and the
corresponding 1H NMR titration (bottom). The signals related to the porphyrin
ligand (left) and the coordinated pyridine signals (right) are shown. A spectrum
was registered for the initial [L2•P3] adduct (red line) and for each one of the
additions of [Pd(OAc)2] (from 0.2 equivalents to 0.8 equivalents). Ar = 2-CN-
C6H4. Blue arrows indicate the binding strength on each component (top).
Y
AcO
Pd
X
N
N
Ar
N
Ar
Ho
Ho
Pd(OAc)2
N
N
N
Ar
Ar
Zn
L2
Zn
N
N
N
N
Ar
Ar
A
According to preliminary molecular modelling (PM3-
minimized calculations, see details in the Supporting Information)
the oxidative addition of P3 to the palladium center and the further
reductive elimination steps are doable with the presence of
Zn…pyridine interactions (Figure 7). These calculations were done
with one palladium ion binding to a single nitrile group of the ligand,
which is comparable to the case in catalysis, where the ligand is
in excess with respect to palladium. This indicates that the high
substrate selectivity observed in the catalytic experiments by L2
for P3 compared to the other regioisomers P2 and P4 (Figure 4)
and the competing substrate B1 (Figure 5) is due to the ideal
substrate preorganization fit of the supramolecular ligand L2 via
a putative 12-membered palladacycle intermediate (Figure 7).
Such supramolecular ligands, with this unique action mode, might
be regarded as an alternative to the sensitive phosphine ligands
for cross-coupling reactions involving heterocyclic motifs.[29]
X = -OAc or 2-OAc
Y =
N
R from same or different L2
Scheme 2. Formation of a plausible pre-catalyst A upon treatment of L2 with
[Pd(OAc)2]. Ar = 2-CN-C6H4.
To verify that the palladium…nitrile binding was compatible
with the zinc…pyridine binding, 1H NMR spectroscopy studies
were performed by increasing amounts of palladium ions to a
solution of the ligand-substrate assembly [L2•P3] (Figure 6). For
instance, the addition of 0.2 equivalents of [Pd(OAc)2], which is
already beyond the ratio used in the catalytic experiments (Pd:L2
= 1:10), clearly indicates the persistence of the binding of
bromopyridine P3 to the zinc center of L2 as no downfield shifts
were observed for the signals belonging to the pyridinic protons
(Figure 6). Further increase of the amounts of [Pd(OAc)2] to the
solution of the ligand-substrate assembly [L2•P3] showed little
changes associated to the porphyrin signals and persistence of
the zinc-coordinated pyridinic signals albeit with a concomitant
decrease of their intensity (Figure 6). The stability of the
zinc…pyridine binding with the palladium…nitrile binding was
further exemplified upon titration of P3 to an equimolar solution of
L2 and [Pd(OAc)2], that is the opposite study as the one depicted
in Figure 6. The binding constant measured by UV-vis studies for
the ligand-substrate assembly [L2•P3] was found to be K1.1
=
8.2×103 M-1 (see Figure S23 in the Supporting Information), a
value which is almost the same -within the error of the
measurement- as the one observed without the presence of
palladium (K1.1 = 8.4×103 M-1, Table 1).
Figure 7. PM3-minimized molecular modelling for the plausible transition state
of the oxidative addition of P3 (left) and the plausible transition state prior to the
reductive elimination step (right). Color code: supramolecular ligand with one
nitrile group (blue), pyridine scaffold (red), palladium (gray) bromine atom (gold)
and phenyl ring (green).
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General procedure for zinc porphyrins: In air atmosphere, the
opportune free-base porphyrin (0.16 mmol) and Zn(AcO)2•2H2O (140 mg,
Conclusions
0.64 mmol) were dissolved in 25 mL of a CHCl3/MeOH 4:1 mixture. The
mixture was refluxed for two hours, when no free-base porphyrin was
detected by TLC analysis, and evaporated to dryness. The reaction crude
was filtered over a short pad of alumina using CH2Cl2 as eluent. The
fraction containing the porphyrin complex was evaporated to dryness.
L2: Quantitative yield. Analytical data matched those found in the
literature.[19a] 1H NMR (400 MHz, CDCl3) δ 8.80 – 8.74 (m, 8H), 8.47 – 8.22
(m, 4H), 8.15 – 8.04 (m, 4H), 8.04 – 7.86 (m, 8H). HRMS (ESI) m/z: [M]+
calcd. for C48H24N864Zn 776.14099, found 776.1401. UV-vis (CH2Cl2): λmax
(log ε) 421 (5.73), 549 (4.34).
In conclusion, we have introduced an approach to design
supramolecular ligands to target substrate selective reactions in
palladium catalysis. This was possible by merging the substrate
recognition properties of metalloporphyrins and the incorporation
of metal coordinating functionalities such as nitriles in a precise,
peripheral position to bind catalytically active palladium ions at
close proximity to the substrate. This strategy enabled to find a
suitable supramolecular palladium catalyst in which the substrate
recognition site is eight chemical bonds apart from the catalytically
active site. By screening a few cations (Cu, Zn, Ru) in the
substrate recognition site, the supramolecular ligand L2, which
contains zinc, was identified as the most promising one to tune
the reactivity of one regioisomer to make it as reactive as the most
reactive one even in the presence of competing substrates;
features that are reminiscent of enzymes.[12] Due to the ease of
modification of metalloporphyrin scaffolds and the multiple
possibilities in terms of peripheral design they offer, one could
anticipate that new reactions could be disclosed applying such
principles.
1
L6: Quantitative yield. H NMR (400 MHz, CDCl3) δ 8.98 (1H, d, J = 4.7
Hz,), 8.97 – 8.93 (6H, m), 8.73 (1H, d, J = 4.7 Hz), 8.35 (1H, d, J = 6.9 Hz),
8.23 (6H, m), 8.09 (1H, dd, J = 7.7, J = 1.2 Hz,), 7.98 (1H, td, J = 7.6, J =
1.5 Hz,), 7.90 (1H, td, J = 7.7, J = 1.2 Hz,), 7.77 (9H m,). 13C NMR δ 150.8
(C), 150.5 (C), 150.4 (C), 149.7 (C), 146.8 (s), 142.8 (C), 142.8 (C), 135.0
(s), 134.7 (CH), 134.7 (CH), 134.6 (CH), 134.5 (CHJ), 133.1 (CH), 132.5
(CH), 132.3 (CH), 131.7 (CH), 130.8 (CH), 128.6 (CH), 127.7 (CH), 126.8
(CH), 126.7 (CH), 126.7 (CH), 126.7 (CH), 126.7 (CH), 122.3 (C), 121.7
(C), 118.4 (C), 117.6 (C), 115.4 (C). HRMS (ESI) m/z: [M+Na]+ calcd. for
C45H27N5Na64Zn 724.14501, found 724.1454; [M+K]+ calcd. for
C45H27N5K64Zn 740.11895, found 740.1188. UV-vis (CH2Cl2): λmax (log ε)
420 (5.73), 549 (4.31).
Synthesis of L3: Porphyrin L1 (82 mg, 0.11 mmol) was dissolved in
1,3-dichlorobenzene (8 mL) and the solution was heated to reflux.
Ru3(CO)12 was added stepwise by 10 mg portion every 12 minutes. The
required amount of Ru3(CO)12 was 73 mg (0.12 mmol) and no free-base
was detected after 1.5 hours by TLC monitoring. The solvent was
evaporated and the reaction crude was purified by filtration over alumina
using CH2Cl2 and 9:1 CH2Cl2/CH3CN mixture as eluent. The product
fraction was evaporated to dryness obtaining L3 as a dark violet solid
(33mg, 34%). 1H NMR (400 MHz, acetone-d6) δ 8.60 – 8.51 (m, 8H), 8.49
– 8.00 (m, 12H).13C NMR was performed but no peaks significantly higher
than the noise were detected. HRMS (ESI) m/z: [M+Na]+ calcd. for
C49H24N8ONa102Ru 865.10087, found 865.1016; [M(H2O)+Na]+ calcd. for
C49H26N8O2Na102Ru 883.11144, found 883.1092. UV-vis (CH2Cl2): λmax
(log ε) 410 (5.18), 534 (4.10), 610 (3.49).
Experimental Section
General: Toluene and dichloromethane were purified by a MB SPS-800
purification system. Pyrrole and 1,3-dichlorobenzene were dried over
CaH2 and distilled prior to use. CDCl3 was filtered over alumina and stored
under argon over molecular sieves. The free-base prorphyrin for L6 was
synthesised by following a reported procedure.[30] All the other employed
chemicals were purchased from commercial sources and used as
received. Unless otherwise specified, reactions were carried out under
argon atmosphere employing standard Schlenk techniques and vacuum
line manipulations. 1H NMR and 13C NMR spectra were recorded on
Bruker GPX (400 MHz) spectrometer. 1H NMR spectra were referenced to
residual protiated solvent (δ = 7.26 ppm for CDCl3 and 2.05 ppm for
acetone-d6). 13C NMR spectra were referenced to CDCl3 (δ = 77.16 ppm).
The catalytic reactions were monitored using a Shimadzu 2014 gas
chromatograph equipped with EquityTM-1 Fused Silica capillary column
(30 m x 0.25 mm x 0.25 µm) and a FID detector; conversion and selectivity
were determined using dodecane as internal standard. Molecular
modelling calculations were performed using PM3-Spartan molecular
modelling program. UV-Vis absorption spectra were recorded using a
Specord 205 UV-vis-NIR spectrophotometer and quartz cuvettes of 1 cm
pathlength. Mass spectroscopy and microanalysis were performed in the
laboratories of the Centre Régional de Mesures Physiques de l’Ouest
(CRMPO, Universitè de Rennes 1, Rennes, France).
Synthesis of L4: In air atmosphere, porphyrin 1 (50 mg, 7.1×10-2 mmol)
and Cu(AcO)2•2H2O (43 mg, 0.20 mmol) were dissolved in 10 mL of a
CHCl3/MeOH 4:1 mixture. The mixture was refluxed for 5 hours, until the
free-base porphyrin was no longer detected by TLC analysis, and
evaporated to dryness. The reaction crude was filtered over a short pad of
alumina using CH2Cl2 as eluent. The fraction containing the porphyrin
complex was evaporated to dryness obtaining L4 as a red solid (30 mg,
55%). 1H NMR (400 MHz, CDCl3) δ 7.81 (br). 13C NMR was performed but
no peaks significantly higher than the noise were detected. HRMS (ESI)
m/z: [M+Na]+ calcd. for C48H24N8Na63Cu 798.13121, found 798.1309;
[M+K]+ calcd. for C48H24N8K63Cu 814.10515, found 814.1044. Elemental
Analysis calcd. for C48H24N8Cu • 4/3 H2O: C, 72.04; H, 3.36; N, 14.00;
found: C, 72.29; H, 3.56; N, 13.76. UV-vis (CH2Cl2): λmax (log ε) 415 (5.70),
539 (4.34).
Synthesis of the porphyrin compounds
Synthesis of L1: Under light cover, 2-cyanobenzaldehyde (1.89 g, 14
mmol) was dissolved in propionic acid (100 mL) and heated to reflux,
pyrrole (1.0 mL, 14 mmol) was added dropwise and the so-obtained dark
mixture was furtherly refluxed for 2 hours. The solvent was removed by
vacuum distillation and the crude was purified by chromatography (SiO2,
CH2Cl2 as eluent). The fraction containing the porphyrin was evaporated
to dryness and crystallized from a CH2Cl2/Heptane mixture to give L1 as a
dark violet solid (121 mg, 4.7%). Analytical data matched those found in
the literature.[31] 1H NMR (400 MHz, CDCl3) δ 8.75 – 8.67 (8H, m), 8.43 –
8.18 (4H, m), 8.17 – 8.07 (4H, m), 8.03 – 7.89 (8H, m).
General procedure for the binding constants measurements
A stock solution (A) of complex L (2.0 ×10-5 M for zinc poprhyrins L2, L5,
L6 and 6.0-1.0 ×10-6 M for L3) in dry dichloromethane was prepared and
employed as solvent for the preparation of a second solution (B) containing
the pyridine species P. Different aliquots of solution B were withdrawn and
diluted with solution A in order to reach a volume of 2 mL. UV-Vis spectra
of each sample were recorded using 3 mL quartz cuvettes with 10 mm
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path length. Binding constants were evaluated considering
a
1:1
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Acknowledgements
The research leading to these results has received funding from
the People Programme (Marie Curie Actions) of the European
Union’s Seventh Framework Programme (FP7/2007-2013) under
REA grant agreement n. PCOFUND-GA-2013-609102, through
the PRESTIGE programme coordinated by Campus France.
Financial support from CNRS, Université de Rennes 1, Rennes
Métropole, and Région Bretagne (Stratégie d’Attractivité Durable
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Conflict of interest
The authors declare no conflict of interest.
Keywords: supramolecular catalysis • porphyrins • palladium •
pyridines • cross-coupling
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Entry for the Table of Contents
FULL PAPER
Ground control: Supramolecular
catalysts made up from porphyrin
scaffolds with peripheral palladium
ions (see picture) modulate the
reactivity of structurally similar
substrates by tuning the properties of
the binding site (A), which stands
eight chemical bonds apart from the
active sites (Pd).
│Supramolecular Catalysis
Paolo Zardi, Thierry Roisnel, Rafael
Gramage-Doria*
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
Page No. – Page No.
A Supramolecular Palladium
Catalyst Displaying Substrate
Selectivity by Remote Control
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