Palladium complexes of phosphine functionalised carbosilane dendrimers as catalysts in a continuous flow membrane reactor

Article abstract of DOI:10.1039/a904455h

Phosphine functionalised carbosilane dendrimers have been synthesised and their palladium complexes used as catalysts in the allylic alkylation reaction performed in a continuous flow membrane reactor.

Full text of DOI:10.1039/a904455h

Palladium complexes of phosphine functionalised carbosilane dendrimers as
catalysts in a continuous flow membrane reactor†
Debby de Groot,a Eva B. Eggeling,b Janine C. de Wilde,a Huub Kooijman,c Richard J. van Haaren,a
Alexander W. van der Made,d Anthony L. Spek,c Dieter Vogt,b Joost N. H. Reek,a Paul C. J. Kamera and Piet
W. N. M. van Leeuwen*a
a
Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The
Netherlands. E-mail: pwnm@anorg.chem.uva.nl
Institute of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, Post Office Box 513, 5600 MD
b
Eindhoven, The Netherlands
c
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, University of Utrecht, Padualaan 8,
3584 CH Utrecht, The Netherlands
Shell International Chemicals BV Amsterdam, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
d
Received (in Cambridge, UK) 4th June 1999, Accepted 15th July 1999
Phosphine functionalised carbosilane dendrimers have been
synthesised and their palladium complexes used as catalysts
in the allylic alkylation reaction performed in a continuous
flow membrane reactor.
various generations (G0, G1, G2) with chlorodimethylsilane or
dichloromethylsilane followed by reaction with lithium me-
thyldiphenylphosphine–TMEDA7 (Scheme 1).§ All the phos-
phine functionalised dendrimers were obtained as wax-like
solids. Characterisation by 1H and 31P-{1H} NMR and MALDI-
TOF MS shows that the P-functionalised dendrimers were
obtained in at least 95% purity. The dendrimer with seventy-two
phosphine groups could not be prepared, probably because of
surface congestion.
Since Vögtle and co-workers synthesised branched structures in
1978,1 much research has been devoted to the synthesis and
investigation of dendrimeric molecules.2 One of the main
applications of dendrimers2h,i is in catalysis,3 allowing easy
recycling of the homogeneous catalyst by means of nano-
filtration.4 Here we report the synthesis of phosphine function-
alised carbosilane dendrimers and their use as catalysts in a
continuous process.
Dendrimers 7 and 8 have four and twelve endgroups,
respectively, each containing a chelating bidentate phosphine
ligand. The endgroups of dendrimers 4, 5 and 6 contain
monodentate phosphine ligands. Allylpalladium complexes of
We have chosen carbosilane dendrimers5 as a backbone for
our catalytic system because of their catalytic inertness. A
previous report describes the synthesis of these dendrimers.6
The second generation dendrimer is a white solid and crystals
suitable for X-ray analysis were grown from a diethyl ether–
methanol solution.‡ The structure (Fig. 1a) shows that the
molecule crystallised with all the bonds in a zigzag conforma-
tion. This dendrimer has a calculated molecular volume of 2414
Å3, which is anticipated to be large enough for separation from
a reaction mixture by nanofiltration.
these dendrimers were synthesised by reaction with [(h -
3
C3H7)PdCl]2. According to 1H and 31P-{1H} NMR the
phosphine dendrimers 7 and 8 co-ordinate in a bidentate way
resulting in well defined Pd complexes, while the monodentate
phosphine dendrimers give rise to a mixture of products.
All the Pd(allyl) dendrimer complexes were used as catalysts
in the allylic alkylation reaction of allyl trifluoroacetate and
sodium diethyl methylmalonate yielding diethyl allylmethylma-
lonate. The reaction was first carried out via a batch process. All
the dendrimeric catalysts showed a very high activity. Using a
substrate–Pd ratio of 2000 the yield after 30 min was over 80%,
and only small differences in reaction rates were observed for
the different catalysts.¶ The fact that the activity did not
decrease with increasing generation indicates that all active sites
act as independent catalysts. From molecular modelling (Fig.
1b) it was clear that indeed all the Pd(allyl) groups reside at the
outer surface of the dendrimer and should be easily accessible to
The phosphine functionalised carbosilane dendrimers were
synthesised by hydrosilylation of the double bonds of the
Fig. 1 Crystal structure of the second generation dendrimer (a) and a
modelled structure of the Pd(allyl) complex of 8 (b). Hydrogens and
counterions have been omitted for clarity. [See Electronic Supplementary
Information (ESI) for a colour version of this figure (Si in red, C in dark
blue, Pd in green, P in yellow)].
† Electronic supplementary information (ESI) available: full colour version
of Fig. 1. See http://www.rsc.org/suppdata/cc/1999/1623/
Scheme 1 Synthesis of phosphine functionalised carbosilane dendrimers.
Chem. Commun., 1999, 1623–1624
1623

= 108.1431(15), g = 106.6735(14)°, V = 6268.0(2) Å3, Z = 2, m(Mo-Ka)
= 0.2 mm21, 80242 reflections measured, 14833 independent, Rint
=
0.1352, (1.6° < q < 24.1°, T = 150 K). Only Si atoms were refined with
anisotropic displacement parameters. The outside of the molecule shows
considerable dynamic disorder, which gives rise to high displacement
parameters and unrealistic geometries. No satisfactory disorder models
could be obtained. Mild distance restraints were introduced for the most
unrealistic parameters. wR2 = 0.2949, R1 = 0.1369, S = 0.886, 20.38 <
Dr < 0.78 e Å23. CCDC 182/1331. See http://www.rsc.org/suppdata/cc/
1999/1623/ for crystallographic files in .cif format.
§ Selected data for 4: 1H NMR (CDCl3): d 7.5 (m, 16H, ArH), 7.3 (m, 24H,
ArH), 1.40 (s, 8H, SiCH2P), 0.26 (s, 16H, SiCH2CH2Si), 20.10 (s, 24H,
SiCH3). 31P-{1H} NMR (CDCl3): d 220.8. Dendrimers 5 and 6 have
similar NMR spectra. For 7: 1H NMR (CDCl3): d 7.5–7.1 (m, 80H, ArH),
1.26 (s br, 16H, SiCH2P), 0.19 (m, 16H, SiCH2CH2Si), 20.29 (s br, 12H,
SiCH3). 31P-{1H} NMR (CDCl3): d 222.5. Dendrimer 8 has similar NMR
spectra.
Fig. 2 Space time yield versus amount of solvent pumped through the
reactor of the continuous allylic alkylation reaction in a membrane
reactor.‡‡
¶ Room temperature, solvent: THF, [allyl trifluoroacetate] = 50 mM,
[diethyl methylmalonate] = 25 mM, [Pd] = 12.5 mM.
∑ Koch/SelRO MPF-60 NF membrane, Koch Membrane Systems, Düssel-
dorf, Germany, molecular weight cut-off (MWCO) = 400 Daltons.
** Room temperature, reactor volume: 20 ml, solvent: THF, [allyl
trifluoroacetate] = 50 mM, [diethyl methylmalonate] = 25 mM, [Pd] =
the nucleophile. Addition of a second portion of substrate after
nearly full consumption of the sodium diethyl methylmalonate
(over 90%) showed that the catalyst remained active.
These novel dendrimeric catalysts were studied in a continuous
process using a membrane reactor.∑ A solution of allyl
trifluoroacetate and sodium diethyl methylmalonate in THF
(including n-decane as an internal standard) was pumped
through the reactor. The allylpalladium complex of the largest
dendrimer with bidentate phosphines (8) was used as a
catalyst.**
12.5 mM, flow rate: 44 ml h21
.
†† Much higher turnover numbers can be reached in a batch reactor
compared to the membrane reactor, showing that in the membrane reactor
new problems are introduced that are not related to the dendrimeric
catalyst.
‡‡ The space time yield has been corrected for the background reaction.
In Fig. 2 the space time yield is plotted as a function of the
amount of solvent (expressed in reactor volumes) pumped
through the reactor. The reaction started immediately after
addition of the catalyst and reached its maximum space time
yield after one reactor volume. The space time yield slowly
dropped to zero after ca. 153 the reactor volume of substrate
solution had been pumped through the reactor. Regarding the
size of the catalyst, this decrease was unexpectedly rapid. The
retention in the membrane reactor of the second generation
dendrimer (molecular volume: 2414 Å3), which is much smaller
than the Pd catalyst (calculated molecular volume: ≈ 7600 Å3
(Fig. 1)), was determined to be 98.1%. Using this number the
decrease in catalyst activity is calculated to be only 25% after
flushing the reactor fifteen times. The observed decrease in
catalyst activity is therefore ascribed to decomposition†† of the
palladium compound and not to loss of the dendrimeric catalyst.
This is in agreement with the observation that samples taken
from the product flow were not catalytically active, indicating
that no active catalyst had gone through the membrane.
In conclusion, carbosilane dendrimers functionalised with
diphenylphosphine groups at the periphery have been syn-
thesised and characterised. Palladium complexes of these
dendrimers have been used as catalysts in the allylic alkylation
reaction. It has been shown that these dendrimeric catalysts can
be used in a continuous process using a membrane reactor.
Current work is aiming at the enhancement of the stability of
these catalysts, and the exploration of these systems in other
reactions.
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This work was supported in part (A. L. S.) by the Council for
Chemical Sciences of the Netherlands Organisation for Scien-
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Notes and references
¯
‡ Crystal data for 3: C104H172Si17, Mr = 1899.96, triclinic, space group P1,
a = 17.2970(4), b = 19.5782(4), c = 22.2298(5) Å, a = 106.1813(15), b
Communication 9/04455H
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Chem. Commun., 1999, 1623–1624