Novel catalysis of dendrimer-bound Pd(0) complexes: Sterically steered allylic amination and the first application for a thermomorphic system

Article abstract of DOI:10.1039/b107452k

Phosphinated dendrimer-bound Pd(0) complex catalysts show high stereoselectivity for allylic amination due to the surface congestion of dendrimers and can be easily recycled without loss of activity under thermomorphic conditions.

Full text of DOI:10.1039/b107452k

Novel catalysis of dendrimer-bound Pd(0) complexes: sterically steered
allylic amination and the first application for a thermomorphic system
Tomoo Mizugaki, Makoto Murata, Masahiko Ooe, Kohki Ebitani and Kiyotomi Kaneda*
Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka
University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Received (in Cambridge, UK) 16th August 2001, Accepted 22nd November 2001
First published as an Advance Article on the web 13th December 2001
Phosphinated dendrimer-bound Pd(0) complex catalysts
show high stereoselectivity for allylic amination due to the
surface congestion of dendrimers and can be easily recycled
without loss of activity under thermomorphic conditions.
13C{1H}, 31P{1H} NMR, IR and XPS.† The 31P{1H} NMR
spectrum of 2 did not show any signal for the residual free
phosphine at d –27 but only one singlet at d 8, and elemental
analysis of 2 gave a Pd+P+Cl ratio of 1+2+2, which strongly
supports the complete complexation of the chelate phosphine to
Pd in a cis form.4b After the reduction of 2, the above resonance
at d 8 was fully replaced by a new one around d 27. From XPS
analysis of 2, the band observed at 338 eV assignable to Pd
3d5/2 is comparable with that of a typical Pd(II) complex such as
PdCl2(PPh3)2, and the band at 336 eV for dendritic Pd complex
3 is due to a Pd(0) species.8
Dendrimers are novel macromolecules with monodispersed
molecular weights, precisely determined cascade structures,
and a specific number of the end groups.1 One of the promising
applications of metallo-dendrimers is in catalysis.2 Organome-
tallic dendrimers offer potential in building the gap between
homogeneous and heterogeneous catalysis because of their
structurally well-defined and specific number of active sites as
well as their advantage of facile recovery by nano-filtration or
solvent precipitation.3 The characteristic structures of den-
drimers have been expected to enhance selectivities, but, there
are few reports on positive catalytic effects due to the dendritic
structures.4 Here, we report the synthesis of dendrimer-bound
Pd(0) complexes and their unique catalysis for allylic substitu-
tion reactions.5 Furthermore, facile recovery of the dendritic Pd
complexes can be achieved by the use of a thermomorphic
system,6 which enables the temperature-induced phase separa-
tion of the homogeneous catalyst solution from a product phase.
Thermomorphic behavior has also been reported in some
catalytic systems, e.g. in a fluorous biphase by Horvath and
Rabai7 and for soluble PNIPAM-supported metal complexes by
Bergbreiter et al.6 To our knowledge, thermomorphic catalyst
systems including dendrimer-bound metal complexes have not,
as yet, been reported.
Catalytic performances of Pd(0) dendrimers 3 was examined
in allylic substitution reactions of allylic acetates with amines
[eqn. (1)].5‡ For example, the substitution of trans-cinnamyl
acetate with morpholine catalysed by 3b smoothly proceeded to
give the corresponding allylic amines in > 99% yield (l+b =
90+10) within 30 min. The dendritic catalysts 3 were soluble in
DMSO and it turned out to be the best solvent to study the
catalytic reactions while CH2Cl2 and THF were not effective
due to the low solubilities of the dendritic catalysts in these
solvents. In the case of Pd(PPh3)4, DMSO, CH2Cl2 and THF
were all good solvents. This substitution reaction using the
dendritic catalysts could be also extended to include soft carbon
nucleophiles and other allylic derivatives such as carbonates.
van Leeuwen and coworkers reported that increased re-
gioselectivity for the branched product was observed on going
to the higher generation of a core functionalised dendritic Pd
catalyst.9 In contrast, changing the generation of our surface
functionalised dendritic catalysts did not affect the regiose-
lectivity among linear and branched products.
Double phosphinomethylation of primary amino groups on
the 1st, 3rd, 4th and 5th generation of poly(propylene imine)
dendrimers were carried out giving 4, 16, 32 and 64 chelate
phosphines on the periphery (1a–d), respectively, as shown in
Scheme 1.3b Treatment of the dendrimers 1a–d with
PdCl2(PhCN)2 afforded dendrimer-bound PdCl2 complexes
2a–d. Subsequently, reduction of the Pd(II) dendrimers 2 with
hydrazine monohydrate in the presence of two equivalents of
8
PPh3 led to the formation of dendritic Pd(0) complexes 3a–d,
respectively, with retention of the parent dendritic structures.
These
Pd
complexes
were
characterized
by 1H,
The prominent catalysis of dendritic Pd complexes 3 was
further investigated in the amination of cis-3-acetoxy-5-carbo-
methoxycyclohex-1-ene with morpholine. Notably, the ster-
eoselectivity for a cis product increased with increasing the 3rd
to the 5th generation of dendrimers as shown in Fig. 1: a high
cis selectivity of 94% was obtained with 3d, while Pd(PPh3)4 as
a typical monomeric Pd(0) catalyst led to a slight excess of cis
product. The 1st and the 3rd generations of the dendritic
catalysts 3a and 3b showed moderate cis selectivity, re-
spectively. In order to clarify the origin of the stereoselectivity,
13C spin–lattice relaxation times (T1) of the dendritic ligands
1a–d were measured (Fig. 1).§ T1 of peripheral phenyl groups
on 1a–d decreased with increasing the generation of the
dendrimers, and this phenomenon shows that the dendrimer
surface becomes more congested for higher generations.10 In
the case of 3d, attaining high stereoselectivity of the substitution
reaction can be explained by steric steering of the nucleophilic
Scheme 1 Preparation of dendritic Pd(II) complexes 2 and Pd(0) complexes
3. Reagents and conditions: i, HCHO, HPPh2, toluene, 60 °C; ii,
PdCl2(PhCN)2, toluene, room temp.; iii, H2NNH2·H2O, PPh3, EtOH, room
temp.
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CHEM. COMMUN., 2002, 52–53
This journal is © The Royal Society of Chemistry 2002

attack to a surface (p-allyl)Pd intermediate, i.e. the active sites
would be strongly shielded from the endo attack of the
nucleophile. The moderate selectivity with 3a and 3b might be
due to the loosely packed dendrimer surfaces, which would
allow the partial dissociation of chelate phosphine ligands.11
Similar stereoselective performances in allylic aminations due
to steric hindrance of solid supports have also been observed by
Trost and others using polystyrene- and silica-bound Pd(0)
complex catalysts.12
Recycling of dendritic metal complex catalysts was at-
tempted by solvent precipitation3b or membrane filtration,3d
which often results in some losses of the catalytic activities
during the recovery and reuse processes. Fortunately, our
dendritic Pd(0) catalysts 3 could be easily recycled by using a
biphasic system of DMF and heptane and the system does not
need any special procedures such as membrane filtration.¶ The
two phases consisting of DMF and heptane became homoge-
neous when heated up to 75 °C, and then could be readily
separated by cooling the reaction mixtures to room temperature.
Extremely low solubility of the dendrimers 3 in the apolar
heptane solvent meant the catalysts were completely transferred
to DMF and the catalyst solution could be recycled after
decantation of the heptane phase containing products. For
example, in the allylic substitution of trans-cinnamyl acetate
with dibutylamine, the high catalytic activity was retained
during three reuse experiments: yields of the allylic amine in
heptane phase were 66% (1st), 99% (2nd), 99% (3rd) and 99%
(4th run), respectively.∑ This is the first application of dendritic
catalysts in a thermomorphic system.
In conclusion dendrimer-bound Pd(0) complexes have been
synthesised by reduction of dendritic Pd(II) complexes with
hydrazine. They show high stereoselectivity for allylic amina-
tion ascribed to the surface congestion of the dendrimers.
Employing a thermomorphic system makes it possible to
efficiently recycle the dendritic catalysts. We expect that the
above results could give a clue to more sophisticated catalyst
designs of dendrimers for highly selective organic syntheses.
This work is supported by the Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture of Japan (12750691 and 11450307). We are grateful to
the Department of Chemical Science and Engineering, Graduate
School of Engineering Science, Osaka University for scientific
support via the ‘Gas-Hydrate Analyzing System (GHAS)’ and
the Lend-Lease Laboratory System.
Notes and references
† Selected data: for 1b: 1H NMR (CDCl3): d 7.16–7.33 (m, 320H, Ph), 3.50
(br, 54H, NCH2P), 2.76 (br, 32H, CH2NCH2P), 2.17–2.34 (br, 92H,
CH2NCH2CH2, NCH2CH2CH2NCH2P), 1.42(br, 60H, NCH2CH2CH2N,
NCH2CH2CH2CH2N). 31P{1H} NMR (CDCl3): d 227. For 2b: 31P{1H}
NMR (DMSO-d6): d 8. XPS: 343.4 eV (Pd 3d3/2), 338.3 eV (Pd 3d5/2). IR
(CsI) 294 cm21 (cis Pd–Cl). For 3b: 31P{1H} NMR (DMSO-d6): d 27. XPS:
342.0 eV (Pd 3d3/2), 336.8 eV (Pd 3d5/2).
‡ Typical reaction conditions: trans-cinnamyl acetate (1.5 mmol), morpho-
line (1.8 mmol), catalyst (5 mmol of Pd atoms), DMSO (5 mL), 40 °C, Ar
atmosphere.
§ Because of the low solubility of the dendrimers 3, T1 for the dendrimers
1 in place of 3 was measured by the inversion–recovery method.
¶ Reaction conditions for the thermomorphic catalysis are as follows:
dendritic catalyst (10 mmol of Pd atoms), DMF (1.5 mL), dibutylamine (1.8
mmol), triethylamine (1.5 mmol), heptane (3 mL), and trans-cinnamyl
acetate (1.5 mmol) were added in a Schlenk type flask under Ar and stirred
at 75 °C for 1 h. For reuse experiments, heptane, allylic acetate, a
nucleophile and triethylamine were added subsequently after decantation of
the heptane phase. These procedures were repeated three times.
∑ The relatively low yield for the fresh run was due to the distribution of
allylic amine product in the DMF phase. No Pd leaching in the heptane
phase was observed during recycle experiments according to ICP (detection
limit is 0.1 ppm).
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Fig. 1 Generation dependence on selectivity in the allylic amination of cis-
3-acetoxy-5-carbomethoxycyclohex-1-ene with morpholine. Stereoselec-
tivity for cis isomer was compared at 30% conversion of the substrate.
Selectivity (%) = [(cis 2 trans)/(cis + trans)] 3 100. T1 values were
calculated for the C4 carbon (128 ppm) in the phenyl groups of the
corresponding dendrimers 1, respectively.
CHEM. COMMUN., 2002, 52–53
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