The role of the dendritic support in the catalytic performance of peripheral pincer Pd-complexes

Article abstract of DOI:10.1039/c1nj20366e

To investigate the effects of the dendrimer backbone on catalysis, a series of monomeric and dendritic SCS-pincer Pd-complexes was synthesized and tested in two different Pd(ii)-catalyzed reactions. To this end, the three novel polar PAMAM dendrimer-immobilized SCS-pincer Pd-complexes 3, 4, and 5, and the two apolar carbosilane dendrimer-immobilized complexes 7 and 8 were compared to three monomeric analogues 1, 2 and 6. These complexes were investigated in the cross-coupling reaction between vinyl epoxide and styrylboronic acid and the auto-tandem reaction of cinnamyl chloride, hexamethylditin, and 4-nitrobenzaldehyde. The differences in catalytic rate and product selectivity for these complexes are described and discussed. For the cross coupling reaction, the PAMAM dendrimer-immobilized complexes were found to give a similar reaction rate, but a higher product selectivity than their monomeric counterparts. The carbosilane complexes showed a lower reaction rate and similar product selectivity. These observations are explained in view of dendrimer aggregation and peripheral group backfolding.

Full text of DOI:10.1039/c1nj20366e

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PAPER
The role of the dendritic support in the catalytic performance of
peripheral pincer Pd-complexeswz
Niels J. M. Pijnenburg,a Martin Lutz,b Maxime A. Siegler,b Anthony Spek,b
Gerard van Kotena and Robertus J. M. Klein Gebbink*a
Received (in Montpellier, France) 26th April 2011, Accepted 27th June 2011
DOI: 10.1039/c1nj20366e
To investigate the effects of the dendrimer backbone on catalysis, a series of monomeric and dendritic
SCS-pincer Pd-complexes was synthesized and tested in two different Pd(II)-catalyzed reactions. To this
end, the three novel polar PAMAM dendrimer-immobilized SCS-pincer Pd-complexes 3, 4, and 5, and
the two apolar carbosilane dendrimer-immobilized complexes 7 and 8 were compared to three
monomeric analogues 1, 2 and 6. These complexes were investigated in the cross-coupling reaction
between vinyl epoxide and styrylboronic acid and the auto-tandem reaction of cinnamyl chloride,
hexamethylditin, and 4-nitrobenzaldehyde. The differences in catalytic rate and product selectivity for
these complexes are described and discussed. For the cross coupling reaction, the PAMAM dendrimer-
immobilized complexes were found to give a similar reaction rate, but a higher product selectivity than
their monomeric counterparts. The carbosilane complexes showed a lower reaction rate and similar
product selectivity. These observations are explained in view of dendrimer aggregation and peripheral
group backfolding.
dendritic support. The role of the dendritic support on the
Introduction
overall catalytic performance of both types of dendritic
catalysts is not negligible.9 Except for electronic effects that
may result from the covalent connection of the dendrimers to
the organometallic catalyst, steric effects seem to play a more
pronounced role. In close proximity of the dendritic backbone,
the chemical micro-environment near the catalytic center may
significantly alter.10 Limited accessibility due to additional steric
bulk around a catalytic center might impede substrate binding
and therefore is an important reason for dendritic effects to be
observed on catalytic reaction rates or product selectivities.
Several examples that reveal positive dendritic effects in
terms of a higher product selectivity for dendritic catalysts in
comparison to non-dendritic catalysts have been reported.11–15
In these cases the increased steric crowding around the
catalytic reaction center seems to hamper the formation of
sterically unfavored (often branched) isomers in favor of less
sterically demanding (often linear) isomers, thus improving the
selectivity. Positive dendritic effects on reaction rate and
product yield have also been observed.16–20
Dendrimer-immobilized catalysts combine the benefits of
homogeneous catalysis, namely high activity, tunable solublility,
a
well-understood catalyst description and mild reaction
conditions, with the key benefit of heterogeneous catalysis:
the recyclability of the catalyst.1 Since the first report appeared
by van Koten and van Leeuwen,2 the field of dendrimer-
immobilized catalysts has expanded enormously. Several
comprehensive reviews have been published that provide an
overview on dendrimer-immobilized organometallic catalysts,
thereby focusing on the recyclability and the differences of
these dendritic catalysts in terms of reaction rate and/or
selectivity compared to their monomeric, non-dendritic
counterparts.3–8
Most dendritic catalysts consist either of a single catalytic
unit connected to the focal point of a dendritic wedge to
increase steric bulk around the reaction center, or of multiple
catalytic units that are connected to the periphery of a
a Organic Chemistry & Catalysis, Debye Institute for Nanomaterials
Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht,
The Netherlands. E-mail: r.j.m.kleingebbink@uu.nl;
The origins of these effects are diverse. For example, in a
recent paper Snelders et al. observed that higher generations of
oligocationic dendritic ligands can stabilize the mono-ligated
active species in the Suzuki–Miyaura cross-coupling reaction
of aryl chlorides with phenyl boronic acids.18 On the other
hand, cooperation between two or multiple catalytic centers
might be favored in the case of peripherally immobilized
catalysts. Jacobsen et al. reported a positive effect for the
Co–salen catalyzed hydrolytic kinetic resolution of epoxides,
Fax: +31-30-252-3615
b Crystal and Structural Chemistry, Bijvoet Center for Biomolecular
Research, Utrecht University, Padualaan 8, 3584 CH Utrecht,
The Netherlands
w Dedicated to Prof. Didier Astruc on the occasion of his 65th birthday.
z Electronic supplementary information (ESI) available. CCDC reference
numbers 824226, 824227, 824228. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1nj20366e
c
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a reaction known to be second order in Co.20 The Kharasch
addition, an atom transfer radical reaction, performed by
immobilized NCN-pincer nickel catalysts by Kleij et al. is
another example, although this has led to a negative dendritic
effect.19
consist of an ethylenediamine core that has been reacted
through a divergent synthesis protocol with methyl acrylate
and ethylenediamine to create higher generations of
dendrimers.23–25 Carbosilane dendrimers on the other hand
are apolar dendrimers that consist of C–Si and C–C bonds.
The central silicon atom and other peripheral silicon centers
act as the branching points in these dendrimers. Because of
their chemical inertness these dendrimers are often used in
catalysis.26,27 Carbosilane dendrimers are usually synthesized
in a divergent manner in an iterative two-step protocol using
alternate allylation and hydrosilylation steps.28
The choice of the type of dendritic support used for the
dendrimer immobilization of a catalyst is often not specifically
stated, and sometimes even seems quite randomly chosen.
Experience in the research group, dendrimer availability and
price, rather than arguments based on chemical properties are
often important parameters for the selection of the dendritic
support used for catalyst immobilization. As a result, only few
reports detail the specific effects of the use of a different
dendrimer support on one and the same reaction catalyzed
by identical catalytic sites.15,21 In addition, obtaining a
balanced overview of such effects by combining data from
different literature reports is difficult. For these reasons, we set
out to study the catalytic performance of dendrimer-immobilized
catalysts by maintaining a single catalyst type and instead
changing the dendritic support.
We have studied two SCS-pincer Pd-catalyzed reactions
with a series of different SCS-pincer Pd-catalysts. This series
consist of the ‘parent’ SCS-pincer Pd-complex 1, two monomeric
para-substituted SCS-pincer Pd-complexes (2 and 6) as models
for the dendrimer supported complexes, and a series of
PAMAM (3–5) and CS (7 and 8) dendrimer-supported pincer
Pd-complexes (Fig. 1).
This series of eight different SCS-pincer Pd-complexes was
tested in two different Pd-catalyzed reactions: (1) the cross
coupling of vinyl epoxide with phenylvinylboronic acid,29 and
(2) an auto-tandem reaction consisting of the stannylation of
cinnamyl chloride followed by allylation of 4-nitrobenzaldehyde
resulting in functionalized allylic alcohols (Fig. 2).30 In both
reactions, different reaction products can be formed; either a
linear/branched or a syn/anti product mixture. In this way, not
only the catalytic reaction rate, but also the product profile
In this study, two abundantly used but chemically different
types of dendrimers have been used as supports for catalyst
immobilization: polyamidoamine (PAMAM) dendrimers and
carbosilane (CS) dendrimers.22 PAMAM dendrimers are
possibly the most frequently used types of dendrimers in
materials science, biotechnology applications and catalysis
and are commercially available. These polar dendrimers
Fig. 1 Monomeric and dendritic SCS-pincer Pd-complexes 1–8 comprising either PAMAM or carbosilane dendritic scaffolds.
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Fig. 2 SCS-pincer Pd-catalyzed reactions: (a) cross coupling between vinyl epoxide and styrylboronic acid, and (b) auto-tandem reaction between
cinnamyl chloride, hexamethylditin, and 4-nitrobenzaldehyde.
can be used to parameterize the effect of the dendritic
supporting scaffold on the performance of the SCS-pincer
Pd-catalysts.
(NBS) in methyl acetate, prevents the use of the more com-
monly published, but very environmentally unfriendly NBS
bromination in carbon tetrachloride.32 The resulting 1-bromo-
3,5-bis(bromomethyl)benzene 9 was reacted under basic con-
ditions with thiophenol to yield SCS-pincer preligand 10.33
Subsequently, ligand 10 was lithiated upon addition of two
equivalents of tBuLi at ꢀ80 1C in diethylether. Quenching of
this mixture by addition of (gaseous) carbon dioxide at ꢀ80 1C,34
and protonation by addition of water lead to carboxylic acid
11 in 71% yield. This amphiphilic compound is hardly
soluble in organic solvents, therefore the next synthetic step
was performed in suspension. EDC coupling (EDC = 1-ethyl-
Results and discussion
Synthesis and analysis of SCS-pincer Pd-complexes
Synthesis. Monomeric pincer complex 2 and dendritic pincer
complexes 3–5 were synthesized via an amide coupling between
a primary amine (i.e. n-butylamine or commercially available
amino-terminated PAMAM dendrimers) and an activated ester
pincer derivative. This coupling reaction is based on an earlier
reported protocol in which an NCN-Pd-pincer active ester
compound was used to connect NCN-pincer Pd-complexes to
various amines via a robust amide linkage.31
3-(3-dimethylamino-propyl)carbodiimide;
a water soluble
carbodiimide) of 11 with N-hydroxysuccinimide in the
presence of triethylamine yielded active ester 12 in good yields.
Palladation of 12 was carried out using [Pd(MeCN)4](BF4)2 in
refluxing acetonitrile. After treatment of the resulting cationic
SCS-pincer Pd–NCMe complex with sodium chloride for 1 h,
the resulting para-succinimidyl ester functionalized SCS-
pincer Pd-complex 13 was isolated as an air-stable yellow powder.
The activated ester functionalized SCS-pincer Pd-complex
13 was synthesized starting from commercially available
1-bromo-3,5-dimethylbenzene (Fig. 3). Bromination of this
xylene via a light-induced reaction using N-bromosuccinimide
Fig. 3 Synthesis of succinimidyl ester functionalized SCS-pincer Pd-complex 13.
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In a last step, a solution of 13 was treated with 100 equivalents
of crosslinked (polyvinyl)pyridine (PVPy) for 3 h. This last
step was performed to chelate Pd(0) particles that might still be
present as byproducts after the introduction of the palladium
centers.35 These particles could potentially lead to undesired
competition in catalysis, therefore care was taken to avoid
these particles in the final batch of 13. After treatment with
PVPy beads, active ester SCS-pincer complex 13 was obtained
in 78% yield as a pale yellow powder.
was successfully characterized by ESI-MS. Signals contributed
to the [M ꢀ 2Cl]2+ fragment with m/z = 1201.0618 (calc. m/z =
1201.0865) and to the [M ꢀ 3Cl + Na + xMeCN]2+ (x = 2–4)
fragments were identified. For the higher generation dendritic
compounds 4 and 5, no correct mass spectra could be obtained.
Due to their macroscopic size these compounds could not be
detected either by ESI-MS or by MALDI-TOF MS.
Proton and carbon NMR analysis showed shifts of the
signals of the most peripheral CH2 groups of the PAMAM
dendrimer after coupling (1H NMR: from 2.8 ppm to 3.4 ppm;
13C NMR from 41 ppm to 39 ppm). Nearly identically IR
spectra for 3, 4, and 5, together with negative ninhydrin tests36
performed after the coupling reaction of succinimidyl
ester functionalized SCS-pincer Pd-complex 13 with the
Gx-PAMAM-NH2 dendrimers, hinting at the absence of
primary amine groups, conclude that for all generations
PAMAM dendrimers a complete coupling reaction has taken
place leading to fully metallated dendrimers 3–5.
Next, compound 13 was used to couple SCS-pincer
Pd-complexes to primary amines in dichloromethane or in a
mixture of dichloromethane and methanol under ambient
conditions (Fig. 4). The latter solvent was used to improve
the solubility of the dendritic products. As it turned out, 13 did
not undergo nucleophilic substitution by methanol under
these conditions. Using n-butylamine as nucleophile leads to
monomeric complex 2 in 82% yield. Succinimidyl ester 13 was
connected in the same way to the peripheral groups of
commercially available PAMAM dendrimers leading to
dendritic complexes 3–5. A slight excess of 1.25 equivalents
of pincer complex 13 per dendritic arm was used in this
protocol. The resulting dendritic complexes were purified from
the succinimide byproduct via passive dialysis and the dendritic
products were isolated in good yields (66–78%) as yellow
powders. The solubility of these dendrimers is not very high
in most common organic solvents. Polar aprotic solvents like
DMF and DMSO and a mixture of dichloromethane and
methanol are solvents in which these complexes showed good
solubility. These dendritic catalysts were neither soluble in
pure dichloromethane nor in pure methanol.
Catalytic results
Cross coupling of vinyl epoxide with phenylvinylboronic acid.
The ECE-pincer Pd-catalyzed cross coupling of vinyl epoxides
with boronic acids (reaction (1), Fig. 2) has been introduced by
Kjellgren et al.29 and has been studied in a later stage by
Bonnet using SCS-pincer Pd-complexes.37 This reaction allows
for a range of vinyl epoxides to be coupled to various boronic
acids using NCN-, SCS- and SeCSe-pincer Pd–Cl complexes.
These reactions proceed via either an SN2 or SN20 mechanism
and lead to a mixture of linear and branched products in a
ratio of 2.3 for NMeCNMe-pincer Pd-complexes29 and 3.8 for
SiPrCSiPr-pincer Pd-complexes.37
The synthesis of para-TMS SCS-pincer Pd-complex 6 and
carbosilane dendrimers 7 and 8 has been published earlier by
us,35 and will not be detailed here.
The dendritic catalysts 3, 4, and 5 do not dissolve in the
THF/water mixture used by Kjellgren29 and Bonnet.37 For
this reason, a mixture of CH2Cl2/MeOH (9 : 1 v/v) was used
for the catalytic tests; in this solvent mixture the reaction
substrates and catalysts are fully soluble, and reproducible
catalytic results were obtained. In the reaction setup, two
Analysis of PAMAM dendrimers 3, 4 and 5. To get insight in
the integrity and catalyst loading of dendritic complexes 3–5, a
variety of analytical techniques have been applied. The
smallest PAMAM dendrimer complex, i.e. G0 compound 3,
Fig. 4 Synthesis of monomeric complex 2 and dendritic complexes 3–5 by active ester chemistry.
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equivalents of caesium carbonate were used as base and
2 mol% of Pd were used as catalyst in 2 mL CH2Cl2/MeOH.
The linear product 6-phenylhexa-2,5-dien-1-ol and the
branched
product
4-phenyl-2-vinylbut-3-en-1-ol
were
observed as major reaction products. In addition, fractions
of styrene (hydrolysis product) and 1,4-diphenylbuta-1,3-diene
(homocoupling product) were found. After 3 h of reaction
time, the formation of 60–63% cross coupling products,
34–38% styrene, and 2–3% 1,4-diphenylbuta-1,3-diene were
typically found for all tested catalysts. The amount of hydrolysis
product that was found is higher than reported in the
literature,29 and is likely caused by the change of the reaction
medium from THF/H2O to CH2Cl2/MeOH. Without catalyst
(blank reaction) no product formation was observed.
The conversion of phenylvinylboronic acid in reaction (1) in
time catalyzed by the different SCS-pincer Pd-complexes is
depicted in Fig. 5a–c for the PAMAM-based catalysts 1–5, for
the series of carbosilane dendrimers 1 and 6–8, and for the
respective G0 and G1 PAMAM and CS dendrimers. Furthermore,
the ratio between linear and branched cross-coupling product is
shown in Fig. 5d for all catalysts.
In this particular reaction, the effect of both the presence
and the absence of a PAMAM-scaffold on the reaction rate
appears to be minimal (Fig. 5a). For all tested generations of
PAMAM dendrimers (catalysts 3–5) and for monomeric
catalyst 2 the observed overall reaction rates are identical,
and somewhat lower than for parent catalyst 1. Dendrimers
4 and 5 did show a somewhat higher conversion after
30 minutes, but this difference was cancelled out after about
1 h. The linear/branched selectivity increases significantly from
5.5–6.0 for the monometallic catalysts 1 and 2 to 7.0–8.5 for
the dendritic PAMAM catalysts 3, 4 and 5 (Fig. 5d). There
seems to be no direct relation with the size of the dendrimer or
the number of SCS-pincer Pd-units per dendrimer and the
observed l/b ratio, as this ratio increases from 2 to 3 (6.0 and
8.2 respectively), then decreases again for 4, and reaches a
maximum of 8.4 for 5. Amongst these catalysts, monomeric
catalyst 2 shows the lowest l/b ratio, which hints at a positive
influence of the dendritic catalyst structure on the l/b ratio.
Carbosilane dendrimers 7 and 8 showed a somewhat higher
initial reaction rate than the monomeric catalysts 1 and 6.
However, after 1 h the monomeric catalysts are significantly
faster than dendritic catalysts 7 and 8. These dendritic
catalysts showed a different overall reaction profile than all
other catalysts tested here. While the other catalysts displayed
a fairly S-type reaction profile, catalysts 7 and 8 showed a
more linear reaction profile in which the overall reaction rate
considerably dropped compared to monomeric catalysts 1 and 6
(Fig. 5b). The l/b selectivity for catalysts 1, 6, 7, and 8 showed
only small differences and all appeared close to the observed
ratio for parent catalysts 1 of 5.5. TMS-appended catalyst
6 showed in fact the highest l/b selectivity (5.9) amongst the
carbosilane series. The selectivity decreases for 7 to 4.9 and then
increases back for 8 close to the selectivity of 6.
Fig. 5 (a, b, c) Conversion of phenylvinylboronic acid in the
SCS-pincer Pd-complex catalyzed cross coupling with vinyl epoxide
at constant Pd-concentration. (d) Linear/branched product ratio of
this coupling using different SCS-pincer Pd-complexes.
When the catalytic performance of the polar PAMAM
dendrimers 3 and 4 was compared to the apolar carbosilane
dendrimers 7 and 8, the PAMAM dendrimers were found to
be superior by showing a higher reaction rate and a higher l/b
ratio (Fig. 5c + d).
Tandem catalysis. Next, the dendritic catalysts were tested in
auto-tandem reaction 2 (Fig. 2). These catalysts were tested
both in solution and in a compartmentalized setup using
membrane dialysis bags.35 Starting from a mixture of the three
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reaction substrates cinnamyl chloride, hexamethylditin and
4-nitrobenzaldehyde, a mixture of stereoisomers of the 1-(4-
nitrophenyl)-2-phenylbut-3-en-1-ol product was obtained. The
ratio of anti (RS and SR) and syn (RR and SS) products
depends on the particular catalyst that is used in the reaction.
In these experiments, three equivalents of cinnamyl chloride
and hexamethylditin were used with respect to the amount of
4-nitrobenzaldehyde. These substrate ratios were used because
earlier studies have shown that only under these conditions the
tandem reaction operates through a Pd(II) cycle and that the
formation and participation of Pd(0) is excluded.38 Again, a
mixture of CH2Cl2 and MeOH (9 : 1 v/v) has been used as solvent
system and no reaction was observed in the absence of catalysts.
Fig. 6 depicts the conversion of the cinnamyl chloride sub-
strate using the eight different SCS-pincer palladium complexes
1–8. The conversion of this substrate appeared to be almost
independent of the catalyst used (Fig. 6a–c). In all cases, the
conversion of cinnamyl chloride is complete within 2 h and all
kinetic curves seemingly follow a first order type reaction
profile. Amongst the dendritic catalysts, G0-PAMAM dendrimer
3 seems to be the most active one (Fig. 6c). Also for the second
step in this tandem reaction (i.e. the formation of the reaction
product 1-(4-nitrophenyl)-2-phenylbut-3-en-1-ol) all eight
tested catalysts, whether monomeric, dendritic, polar or apolar,
showed very similar reaction profiles. The anti/syn ratio of the
tandem products appeared to be between 1.4 and 1.5 for all
catalysts (Fig. 6d). Exceptions are monomeric catalyst 2 that
showed a slightly higher anti/syn ratio of 1.6 and the PAMAM
G2 catalyst 5 that showed a significantly higher anti/syn ratio of 1.9.
Discussion and conclusion
In this paper two series of dendritic catalysts were studied: one
series is based on the polar PAMAM scaffold (catalysts 3–5)
and the other series on the apolar CS scaffold (catalysts 7–8).
Together with monomeric catalysts 1, 2 and 6, which are the
parent catalyst 1 and two monometallic catalysts that are
electronically equivalent to the PAMAM-based (catalyst 2)
and the CS-based catalysts (catalyst 6), these dendritic
catalysts have been tested in two different Pd-catalyzed
reaction to investigate whether the dendritic support itself
plays a role in the catalytic parameters of these reactions.
For the Pd-catalyzed cross coupling of vinyl epoxide with
phenylvinylboronic acid (reaction 1) small but significant rate
and selectivity differences between monomeric catalysts, polar
PAMAM and apolar CS dendritic catalysts have been found.
Because the three monomeric catalysts 1, 2 and 6 showed very
similar characteristics for this reaction, these differences are
not caused by remote electronic effects on the catalytic center,
but rather by steric effects. The PAMAM-based dendritic
catalysts showed a very similar reaction rate compared to
the monomeric catalysts, while the carbosilane-based dendritic
catalysts were found to be considerably slower than their
monomeric counterparts. The product selectivity in this
cross-coupling reaction showed another trend. Here, the
monomeric catalysts and the carbosilane dendrimers showed
a linear/branched product ratio around 5–6, whereas the
PAMAM dendrimers showed a noticeably higher l/b ratio of
8–9: a small, but significant positive dendritic effect.
Fig. 6 (a, b, c) Conversion of cinnamyl chloride in the SCS-pincer
Pd-complex catalyzed auto-tandem reaction with hexamethylditin and
4-nitrobenzaldehyde at constant Pd-concentration. (d) Linear/
branched product ratio of this coupling using different SCS-pincer
Pd-complexes.
We believe that these small effects can be explained by
taking the solubility and the conformational behavior of the
dendritic supports into account. These considerations lead us
to propose that under the reaction conditions dendrimer
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aggregation takes place for the carbosilane dendrimers and
peripheral group backfolding occurs in PAMAM dendrimers,
which lead to the observed catalytic behavior of the dendritic
catalysts.
around the vinyl–Pd intermediate increases, the SN20 attack on
the terminal olefin of vinyl oxirane, i.e. the sterically least
hindered reaction pathway, would be favoured. Accordingly,
formation of the linear product isomer is even more favored
than formation of the branched product isomer under
backfolding conditions. This leads to an overall increase in
the linear: branched product ratio, even though reaction
kinetics are likely slower at a backfolding catalytic site.
For the auto-tandem reaction between cinnamyl chloride,
hexamethylditin and 4-nitrobenzaldehyde (reaction 2) the
differences in the reaction characteristics between the various
catalysts were smaller than for the cross coupling reaction. The
structural arguments for these differences, i.e. accumulation of
apolar carbosilane dendrimers and backfolding of PAMAM
dendrimers, still hold for the tandem reaction as it was carried
out in the same solvent system, but do not seem to have a large
impact on the reaction characteristics of this reaction. The
most striking observation for this tandem reaction is
the improved selectivity of the largest PAMAM-dendrimer
5 compared to all other tested catalysts. This positive dendritic
effect might again be caused by increased steric crowding
because of partial backfolding of the catalysts to the dendrimer
interior. A closer look at the mechanism of the tandem
reaction38 shows that the SCS-pincer Pd-Z1-allyl intermediate
attacks the electrophilic 4-nitrobenzaldehyde to form both
syn- and anti-products (Fig. 8). The distinction between the
formation of these products is the relative orientation of the
two reaction partners in the transition state. Due to the large
4-nitrobenzyl-group, the orientation is always favored to a
positioning that leads to an anti-product, since in all cases
these diastereoisomers are observed in excess. In a sterically
crowded environment, this effect is likely enhanced leading to
a higher selectivity for dendritic catalyst 5.
The apolar carbosilane catalysts are soluble in the dichloro-
methane/methanol (9 : 1, v/v) solvent mixture, which may be
considered as rather polar on the basis of the dielectric
constants of these solvents (e = 9.1 for CH2Cl2, and 33 for
CH3OH). It is known that the introduction of apolar polymers
into polar solvents leads to polymer entanglement. In this way,
macroscopic clusters of these polymeric materials are formed
in order to minimize the Gibbs free energy of the mixture.39,40
These materials show a tendency to self-assembly into clusters
that are solvated by the least polar solvent system, dichloro-
methane in our case. From the catalytic point of view, the
accumulation of dendritic catalysts by means of self-assembled
clusters is likely to lead to a decreased number of accessible
catalyst sites, and therefore a lower reaction rate for the cross-
coupling reaction catalyzed by the CS-based catalysts. For the
polar PAMAM dendrimers no aggregation in the reaction
medium is expected and therefore the observed reaction rates
are comparable to those of monomeric catalysts.
The differences in product selectivity in the cross coupling
reaction can be explained by the role of peripheral group
backfolding. With respect to PAMAM dendrimers, carbosilane
dendrimers are relatively small, rigid and sterically crowded.
For this reason no backfolding of peripheral groups takes
place for these type of dendrimers (see Elshakre et al.22 for a
structural study between carbosilane and PAMAM dendrimers).
The observed selectivity of these carbosilane based catalysts is
therefore similar to the tested monomeric catalysts: the catalysts
on the outside of the self-assembled clusters possess a similar
local reaction environment as the monomeric catalysts and
accordingly lead to a similar product selectivity. At the same
time, catalytic moieties located on the inside of a cluster are
not reached by the substrates, and therefore do not take part
in catalysis and hence do not affect product selectivity.
Because of the relatively long dendritic arms and the
possibility to form hydrogen bonds, backfolding of the peripheral
groups towards the interior of the PAMAM-dendrimers is a
well-known phenomenon.22,41–43 The peripheral catalysts that
are brought closely to the crowded center of the dendrimer, are
expected to experience a different and more crowded reaction
environment than monomeric catalysts or peripherally located
In conclusion, the role of the dendritic support in
dendrimer-immobilized homogeneous catalysis has been
investigated for a series of dendritic catalysts in which the
dendritic backbone was varied. For one of the catalytic
reactions that was studied the change in dendritic support
had a clear effect, whereas for the other reaction the effect was
minor. These observations indicate that the role of the
dendritic support in ‘directing’ a catalytic reaction seems very
much dependent on the specific reaction intrinsics. It would
therefore be of interest to explore reactions that have been
proven to show a positive dendritic effect with regards to
selectivity or reaction rate, with entirely different dendritic
scaffolds than in the original studies. Further studies, using
either very apolar or very polar substrates might also enhance
the observed effects that have been discussed here and
catalysts. In the catalytic cycle as published by Szabo 29
the
´
,
formation of the branched isomers takes place via a (direct) SN2
attack of a SCS-pincer Pd–vinyl intermediate on vinyl epoxide,
whereas for the linear isomers in this step a (conjugated) SN20
attack on vinyl epoxide occurs (Fig. 7). When steric crowding
Fig. 7 The product formation step in the catalytic cycle of the
cross-coupling as proposed by Szabo 29
Fig. 8 The product formation step in the catalytic cycle of the
cross-coupling as proposed by us.38
´
.
c
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2362 New J. Chem., 2011, 35, 2356–2365

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eventually might lead to a more rational choice of the
dendrimer support to be used in homogeneous catalysts.
with water (4 ꢁ 40 mL) to get rid of the formed urea product. The
product was purified via column chromatography (hexanes: THF
2 : 1 v/v) resulting in a white powder (1.09 g, 80%).
1H NMR (CDCl3, 300 MHz): d 7.86 (s, 2H, ArH), 7.47 (s,
1H, ArH), 7.30–7.17 (m, 10H, SPh), 4.05 (s, 4H, SCH2), 2.87
(s, 4H, C(QO)CH2). 13C NMR (CDCl3, 100 MHz) d 164.7,
156.9, 134.6, 131.1, 130.6, 126.1, 125.3, 124.8, 122.5, 120.9,
34.2, 21.2. ESI-HRMS for C25H21NO4S2 (m/z): [M + Na]+
486.0829 (calc. 486.0810).
Experimental section
General
All reactions were carried out using standard Schlenk techniques
under an inert dinitrogen atmosphere unless stated otherwise.
All solvents were carefully dried and distilled prior to use. All
standard reagents were purchased commercially and used with-
out further purification. Compounds 9 (synthesis described by
Amijs et al.,32 analytical data present in Paugam et al.),44
SCS-pincer Pd-complex 13. To a solution of SCS-pincer
ligand 12 (1.36 g, 2.93 mmol) in acetonitrile (40 mL) was
added [Pd(MeCN)4](BF4)2 (1.1 equiv., 1.43 g, 3.23 mmol). The
yellow solution was stirred for 16 h at reflux temperature,
whereupon the solvent was evaporated. Subsequently, the
resulting solids were suspended by adding acetone (40 mL).
NaCl (large excess) was added and the suspension was stirred
for 1 h. The reaction mixture was filtered and the volatiles were
removed in vacuo. The resulting solids were redissolved in
CH2Cl2 (50 mL) and extracted with water (50 mL). The
organic fractions were dried over MgSO4, filtered and treated
with polyvinylpyridine (PVPy; approximately 100 equiv.) to
scavenge eventual present Pd(0) impurities. The PVPy was
removed by filtration and the resulting clear solution was
concentrated. The product slowly precipitates from the
solution and was collected by decantation of the supernatant.
This solution was concentrated and the precipitated product
was collected again. This cycle was repeated five times. The
solid fractions were collected and yielded 1.38 g (78%) of a
pale yellow powder.
45
1033 and [Pd(MeCN)4](BF4)2 were prepared according to
literature procedures. The PAMAM dendrimers were
purchased by Dendritech as solutions in MeOH (G0: 39.36%
w/w, G1: 45.10% w/w, G2: 30.17% w/w) and used as received.
All other reagents were purchased from Acros Organics and
Sigma-Aldrich Chemical Co. Inc. and used as received. 1H
(300 MHz) and 13C (100 MHz) NMR spectra were recorded
on Varian spectrometers at 25 1C, chemical shifts are given in
ppm referenced to residual solvent resonances. UV-Vis spectra
were recorded on a Cary 50 Scan UV-visible spectrophotometer.
IR spectra (ATR) were measured with a Perkin Elmer Spectrum
One FT-IR instrument. High resolution mass spectroscopy
(HRMS) has been performed on a Waters LCT Premier XE
Micromass instrument using the electrospray ionization technique.
Syntheses
3,5-Bis(phenylsulfidomethyl)benzoic acid 11. To a cooled
solution (ꢀ80 1C) of 3,5-bis(phenylthiomethyl)bromobenzene
(10, 2.00 g, 4.98 mmol) in Et2O (60 mL) was added a 1.6 M
tBuLi solution in hexanes (2.0 equiv., 6.23 mmol, 9.97 mL).
The resulting yellow mixture was stirred for 5 min. Then, dry
CO2 (large excess) was bubbled through the solution. Immediately
a white precipitation was formed. The suspension was allowed to
reach room temperature and water (1 mL) was added resulting
in a clear solution. Then, the volatiles were removed and the
resulting slurry was taken up in dichloromethane (100 mL) and
extracted with an aqueous 4 M HCl solution (3 ꢁ 100 mL). The
organic fractions were collected and evaporated in vacuo.
The resulting syrup was redissolved in CH2Cl2 (3 mL) and
precipitated by slow addition of hexanes. The supernatant was
removed to yield a white powder (1.30 g, 71%).
1H NMR (DMSO-d6, 300 MHz): d 13.02 (bs, 1H, COOH),
7.79 (s, 2H, ArH), 7.61 (s, 1H, ArH), 7.32–7.21 (m, 6H, m,p-SPh),
7.19–7.16 (m, 4H, o-SPh), 4.28 (s, 4H, CH2). 13C NMR
(DMSO-d6, 100 MHz) d 167.6, 139.1, 136.2, 134.2, 131.6, 129.7,
129.3, 129.1, 126.8, 36.9. IR (ATR): nOH 2605 cmꢀ1, nCO
1687 cmꢀ1. ESI-HRMS for C21H18O2S2 (m/z): [M + Na+]
389.0667 (calc. 389.0646).
1H NMR (CD2Cl2, 300 MHz): d 7.79–7.72 (m, 6H, o-SPh +
ArH), 7.43–7.37 (m, 6H, m,p-SPh), 4.70 (bs, 4H, SCH2), 2.84
(s, 4H, C(QO)CH2). 13C NMR (CD2Cl2, 100 MHz) d 169.5,
162.1, 150.8, 132.1, 131.6, 130.4, 130.0, 129.9, 123.6, 121.8,
52.5, 25.9. ESI-HRMS for C25H20ClNO4PdS2 (m/z):
[2M
ꢀ
Cl]+
= 1172.9449 (calc.=1172.9440). UV/Vis
(CH2Cl2): lmax = 330.1 nm.
SCS-pincer Pd-complex 2. Active ester complex 13 (28.3 mg,
46.9 mmol) and butylamine (1.0 equiv., 4.6 mL, 46.9 mmol) were
dissolved in CH2Cl2 (2 mL). The reaction mixture was stirred
for 2 h. After performing a ninhydrin test on a TLC plate to
confirm that no primary amines were present in the solution,36
the reaction was stopped by diluting it to 10 mL CH2Cl2 and a
similar volume of water. This biphasic mixture was extracted
and the organic phase was washed two more times with water
and brine (2 ꢁ 10 mL). The organic fraction was dried over
MgSO4, filtered and concentrated in vacuo yielding a yellow
solid (21.6 mg, 82%).
1H NMR (CD2Cl2, 300 MHz): d 7.85 (m, 4H, o-SPh),
7.47–7.43 (m, 8H, ArH + m,p-SPh), 6.39 (t,3J = 4.5 Hz,
1H, NH), 4.67 (bs, 4H, SCH2), 3.38 (q, 3J = 6.9 Hz, 2H,
NCH2), 1.57 (m, 2H, NCH2CH2), 1.42 (m, 2H, CH3CH2), 0.97
(t, 3J = 7.2 Hz, CH3). 13C NMR (CD2Cl2, 100 MHz) d 174.2,
149.9, 132.2, 131.8, 130.2, 130.0, 129.9, 123.8, 120.8, 52.4, 40.1,
31.9, 20.4, 14.0. ESI-HRMS for C25H26ClNOPdS2 (m/z):
2,5-Dioxopyrrolidin-1-yl-3,5-bis(phenylthiomethyl)benzoate 12.
Benzoic acid 11 (1.00 g, 2.97 mmol) was dissolved in dichloro-
methane (40 mL). Subsequently triethylamine (1.2 equiv., 0.50 mL,
3.57 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC, 1.2 equiv., 0.68 g, 3.57 mmol) and N-hydroxysuccinimide
(1.2 equiv., 0.41 g, 3.57 mmol) were added to the solution and the
reaction mixture was stirred for 16 h. Water (40 mL) was added
to the reaction and the resulting biphasic solution was extracted
[M
ꢀ
Cl + = 567.0720 (calc.=567.0765).
MeCN]+
UV/Vis (CH2Cl2): lmax = 330.1 nm. IR (ATR): nmax 3344
m, 3056 w, 2957 m, 2929 m, 2857 m, 1652 s, 1586 m,
c
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1533 s, 1477 m, 1438 s, 1317 m, 1274 m, 1245 s, 1023 s, 902 m,
754 s, 740 s, 685 s.
General protocol for the cross coupling of vinyl epoxide with
phenylvinylboronic acid. A catalyst solution (2 mol% Pd,
0.016 mmol Pd centers) in a mixture of CH2Cl2 and MeOH
(9 : 1 v/v, 2 mL) was added to a solution of vinyl epoxide
(1.0 equiv., 0.80 mmol, 64 mL), phenylvinylboronic acid
(1.0 equiv., 0.80 mmol, 118.4 mg), Cs2CO3 (2.0 equiv.,
1.6 mmol, 521 mg), and hexamethylbenzene (internal
standard, 0.111 mmol, 14.4 mg) in a mixture of CH2Cl2 and
MeOH (9 : 1 v/v, 18 mL). The reaction mixture was stirred at
room temperature in a nitrogen environment. Aliquots of
50 mL for NMR/GC analysis were taken at regular time
intervals with an airtight syringe.
Synthesis of dendritic catalysts 3, 4 and 5
General procedure. To a mixture of CH2Cl2 and MeOH
(1 : 1 v/v; 10 mL) was added Gx-PAMAM-NH2 dendrimer
(solution in MeOH, purchased by Dendritech) and 1.25 equiv.
of 13 per dendritic arm. This solution was stirred at room
temperature. At regular intervals, a ninhydrin test on TLC was
performed to check the remainder of primary amines in the
solution.36 The reaction was stopped when primary amines
were no longer detected. The dendritic compound was purified
by passive dialysis. To this end, the reaction mixture was
concentrated to 5 mL and placed into a dialysis bag.
This bag was placed into a beaker containing a mixture of
CH2Cl2 : MeOH (200 mL; 1 : 1 v/v) and dialyzed for 2 h. This
procedure was repeated twice. The contents of the dialysis bag
were removed from the bag and evaporated to dryness to yield
the PAMAM-Gx-(SCS-Pd-Cl)n materials.
General protocol for the stannylation/electrophilic addition
tandem reaction. A catalyst solution (2 mol% Pd, 0.016 mmol
Pd centers) in a mixture of CH2Cl2 and MeOH (9 : 1 v/v, 2 mL)
was added to a solution of cinnamyl chloride (3.0 equiv.,
2.40 mmol, 0.34 mL), hexamethylditin (3.0 equiv., 2.40 mmol,
0.50 mL), 4-nitrobenzaldehyde (1.0 equiv., 0.80 mmol,
121 mg), and hexamethylbenzene (internal standard,
0.088 mmol, 14.4 mg) in a mixture of CH2Cl2 and MeOH
(9: 1 v/v, 10 mL). The reaction stirred at room temperature in a
nitrogen environment. Aliquots of 50 mL for NMR/GC analysis
were taken at regular time intervals with an airtight syringe.
PAMAM-G0-(SCS-Pd-Cl)4 3. Yield: 110 mg (66%).
1H NMR (CD2Cl2/CD3OD 1 : 1): d 7.77 (m, 16H, o-SPh),
7.45 (s, 8H, ArH), 7.38–7.32 (m, 24H, m,p-SPh), 4.60
(bs, 16H, SCH2), 3.40–3.30 (m, 16H, NHCH2CH2NH), 2.62
(m, 8H, NCH2CH2C(QO)), 2.40 (s, 4H, NCH2CH2N), 2.26
(m, 8H, NCH2CH2C(QO)). 13C NMR (CD2Cl2/CD3OD 1 : 1)
d 173.9, 168.1, 150.0, 132.2, 131.7, 131.1, 130.3, 129.8, 129.1,
121.1, 52.4, 50.9, 49.9, 40.2, 39.1, 33.4. UV/Vis (CH2Cl2):
lmax = 330.1 nm. IR (ATR): nmax = 3287 br, 3056 m,
2926 m, 2854 m, 1634 s, 1580 m, 1532 s, 1471 m, 1440 s,
1322 m, 1254 s, 1024 m, 908 m, 742 s, 685 s. ESI-HRMS for
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New J. Chem., 2011, 35, 2356–2365 2365