Grafting of water-soluble phosphines to dendrimers and their use in catalysis: Positive dendritic effects in aqueous media

Article abstract of DOI:10.1039/b906393p

Generations 1 to 3 of dendrimers ended by water-soluble phosphines are synthesized and their ruthenium complexes are used as catalysts in aqueous media; a slightly positive dendritic effect on the regioselectivity is observed for hydration of alkynes and a large positive dendritic effect is observed in the biphasic (water/heptane) catalysed isomerisation of allylic alcohols to ketones where the products are easily isolated and the catalyst can be reused several times, even the first generation.

Full text of DOI:10.1039/b906393p

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Grafting of water-soluble phosphines to dendrimers and their use in catalysis:
positive dendritic effects in aqueous media†
Paul Servin,a,b Re´gis Laurent,a,b Luca Gonsalvi,c Mar Tristany,a,b Maurizio Peruzzini,*c
Jean-Pierre Majoral*a,b and Anne-Marie Caminade*a,b
Received 31st March 2009, Accepted 14th April 2009
First published as an Advance Article on the web 20th April 2009
DOI: 10.1039/b906393p
Generations 1 to 3 of dendrimers ended by water-soluble
phosphines are synthesized and their ruthenium complexes
are used as catalysts in aqueous media; a slightly positive den-
dritic effect on the regioselectivity is observed for hydration
of alkynes and a large positive dendritic effect is observed in
the biphasic (water/heptane) catalysed isomerisation of allylic
alcohols to ketones where the products are easily isolated
and the catalyst can be reused several times, even the first
generation.
one nitrogen, introducing a positive charge able to enhance the
solubility in water without killing the catalytic properties. Thus,
the grafting of PTA to the terminal groups of dendrimers might
lead to water-soluble organometallic dendritic catalysts. The type
of internal structure of dendrimers we decided to use is built from a
cyclotriphosphazene core, with phenoxy hydrazino thiophosphate
branches (1-Gn,6 from generation 1 to 3 (n = 1–3)). Such type of
internal structure was previously shown compatible with catalytic
experiments in several organic solvents, and even able to favour
them.7 Furthermore, it was shown that the presence of positive (or
negative) charges on each terminal groups induces the solubility in
water, or at least in aqueous media for this type of skeleton.8 The
grafting of PTA necessitates the presence of potentially alkylating
groups on the surface of dendrimers; benzylchlorides are known
to be good alkylating groups of amines, thus they were our first
targets.
Catalysis is a unique tool to carry out chemical transformations,
and it is well established that subtle changes in experimental
parameters could have a dramatic effect on the outcome of
catalytic reactions. A relatively large number of dendritic catalysts
have been proposed already, illustrating the growing importance
of this field of research.1 However, whole sections of catalysis have
been scarcely explored up to now using dendritic catalysts; this is
in particular the case for catalyses performed in aqueous media.2
This fact might appear surprising since water is abundant, cheap,
non-toxic and environment friendly, often able to induce dramatic
improvements of the catalytic efficiency;3 it is presumably due to
the difficulty to have dendritic organometallic catalysts soluble
in water (or even compatible with water). Water-solubility of
dendrimers is generally obtained by introducing charges (positive
or negative) on the terminal groups. In the case of dendritic
catalysts, the catalytic entities often constitute the terminal groups,
and the presence of charges might totally modify the catalytic
properties. In a first attempt to solve this dilemma, charges
and phosphino complexes were randomly grafted to the surface
of PAMAM dendrimers,4 but this method do not afford well-
defined catalysts. To the best of our knowledge, we report here
the first examples of dendrimers bearing both an organometallic
complex and a permanent positive charge on each terminal group,
enough remote from the catalytic centers to preserve their catalytic
properties.
Starting from the aldehyde terminal groups of dendrimers
1-Gn, the first reaction we carried out to obtain benzyl chloride
terminal groups is the reduction, to afford the benzylic alcohols
2-Gn. In first attempts, NaBH4 was used, but the reaction was
slow and the work-up tedious, thus BH3,SMe2 was preferred. The
completion of the reaction is shown in particular by 1H NMR, with
the disappearance of the signal corresponding to the aldehyde at
9.9 ppm on behalf of CH2 groups (4.4 ppm). Compounds 2-Gn
are isolated in 72% (2-G3) to 80% yield (2-G1). Then the benzyl
alcohol terminal groups are transformed into benzylic chlorides
3-Gn by reaction with SOCl2, under anhydrous conditions. The
1
completion of the reaction is shown in particular by H NMR,
which displays a shift of the signal corresponding to the CH2
groups from 4.4 ppm for 2-Gn to 4.6 ppm for 3-Gn, but also
by 13C NMR (shift from 63 ppm for 2-Gn to 45 ppm for 3-Gn,
also for the CH2 groups). Dendrimers 3-Gn were isolated in 80–
85% yield. Reaction with PTA affords cleanly the phosphino
dendrimers 4-Gn; neither dialkylation on two nitrogen atoms of
PTA, nor alkylation of the phosphine was observed, as expected.
The grafting of PTA induces an important deshielding of its signal
in the 31P NMR spectra, from -99 ppm for PTA alone to -80 ppm
when grafted to the dendrimer in 4-Gn. The completion of the
reaction is in particular shown in 1H NMR spectra by the shielding
of the signal corresponding to the CH2 benzyl group from 4.6 ppm
for 3-Gn to 4.1 ppm for 4-Gn. These PTA-dendrimers were isolated
in very good yields (from 97% for 4-G1 to 88% for 4-G3).
PTA (1,3,5-triaza-7-phosphaadamantane)5 is known both to
afford water-soluble complexes and to be easily alkylated at
aCNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de
Narbonne, F-31077, Toulouse, France. E-mail: majoral@lcc-toulouse.fr,
caminade@lcc-toulouse.fr; Fax: +33 5 6155 3003
bUniversite´ de Toulouse, UPS, INPT, LCC, F-31077, Toulouse, France
cICCOM CNR, Istituto di Chimica dei Composti Organometallici, Via
Madonna del Piano, 10 Polo Scientifico di Sesto Fiorentino, 50019, Sesto,
Fiorentino, Italy. E-mail: mperuzzini@iccom.cnr.it; Fax: +39 05 5522 5203
At this step, several types of metals could be complexed by the
PTA terminal groups, but we decided to choose the ruthenium
derivative [Ru(p-cymene)Cl2]2, due to the versatility of the type
of reactions this metal is able to catalyse. The complexation at
room temperature affords cleanly the ruthenium complexes 5-Gn
† Electronic supplementary information (ESI) available: Experimental
details for the synthesis and characterization of dendrimers. Procedures
for catalytic experiments. See DOI: 10.1039/b906393p
4432 | Dalton Trans., 2009, 4432–4434
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Scheme 1 The 3 or 4 steps needed for the synthesis of dendrimers terminated by PTA (4-Gn) or PTA-Ru(p-cymene)Cl2 groups (5-Gn), starting from
aldehyde terminal groups (1-Gn). All steps are shown only with the first generation dendrimers, but were carried out also with generations two and three,
and with the model compound.
(Scheme 1). 31P NMR shows unambiguously the complexation of
PTA which induces a very large deshielding of the corresponding
signal from -80 ppm for 4-Gn to -14 ppm for 5-Gn.9 All complexes
were isolated in moderate yields after work-up (52% to 68%
yield). The solubility in pure water of compounds 5-Gn is high
for 5, moderate for 5-G1 but very poor for 5-G2 and 5-G3, at
room temperature. Nevertheless, catalytic experiments are often
performed at high temperature, and only low quantities of catalysts
are needed, thus the poor solubility should not hamper the
catalytic experiments.
Compounds 5-Gn have a hydrophilic surface and a hydrophobic
interior, thus these dendritic complexes should be particularly
useful for catalytic reactions performed in mixtures of water–
organic solvent. Metal-catalysed hydration of alkynes provides
an important route to carbonyl compounds,10 with complete
atom economy, thus it appeared particularly interesting to study
in a first attempt. In general, addition of water to terminal
alkynes follows Markovnikov’s rule, leading mainly to ketones.
We have used 5 mol% of Ru-catalysts to perform the hydration
of phenylacetylene in water–isopropanol (1 : 3 in volume, one
phase) mixtures. The percentage of catalysts refers to the number
of ruthenium groups; it means that for comparing the efficiency
of the monomer and of the first generation, 12 equivalents of
5 are compared with 1 equivalent of 5-G1. The percentages of
conversion, measured by relative integration of the 1H NMR
signals after 48 h at 90 ◦C, are average: the first generation is
better than the monomer, but a decreasing efficiency is observed
with generations 2 and 3. On the other hand, the selectivity for
the ketone increases from the monomer to the third generation
from 91 to 98%, showing a slightly positive dendritic effect
(Fig. 1). In view of these average results, we did not try to
modify the reaction conditions, and we did not attempt to
recover and reuse these dendritic catalysts for this type of
reactions.
Fig. 1 Hydration of phenylacetylene using monomer 5 and dendrimers
5-Gn (n = 1–3) as catalysts; % of conversion and selectivity (a slightly
positive dendritic effect is observed on the selectivity).
The second type of catalysed reaction in aqueous media we
studied is the isomerisation of allylic alcohols. Without catalyst,
such reaction necessitates two steps (reduction of alkene and
oxidation of alcohol), whereas it is performed one-pot with an
organometallic catalyst.11 The isomerisation of 1-octen-3-ol into
octan-3-one is carried out using 1 mol% of Ru-catalysts and 2%
of Cs2CO3 as co-catalyst,12 in mixtures water–heptane (1 : 1) at
75 ◦C and under vigorous stirring (no reaction occurs in the
absence of water). Before stirring, analysis of both phases and
of the inter-phase indicates that octanol is in the organic phase,
the co-catalyst in water, and the dendrimer both in the aqueous
phase and at the interface. Upon vigorous stirring both phases
mix, but they are rapidly recovered by decantation. Such system
should allow catalyst recycling, as we will see later. The reactions
are monitored by gas chromatography (GC) on the organic phase,
and are stopped after 8 h. A clearly positive dendritic effect is
observed in this case for the conversion, on going from 38% with
the monomer 5 to 98% with the third generation 5-G3 (Fig. 2, left
part).
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Fig. 2 Isomerisation of 1-octan-3-ol using the monomer 5 and den-
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grey bar) is the first reuse.
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In conclusion, the grafting of a water-solubilising phosphine
(PTA) affords special dendrimers having a hydrophilic surface and
a hydrophobic interior. Their complexation with Ru derivatives
affords catalysts usable for the hydration of acetylenes, but also for
the isomerisation of allylic alcohols. In this later case, the catalytic
efficiency increases as the generation of the dendritic catalysts
increases, inducing a clearly positive dendritic effect. Such positive
effect was very rarely observed previously for catalytic entities
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media, to the best of our knowledge.
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P1 is the closest to the N3P3 core): 4: -81.8; 5: -27.6; 4-G1: -80.6 (PTA),
11.3 (N3P3), 64.8 (P1); 5-G1: -14.6 (PTA), 11.4 (N3P3), 64.8 (P1); 4-G2:
-80.7 (PTA), 11.7 (N3P3), 65.3 (P1 and P2); 5-G2: -14.6 (PTA), 11.2
(N3P3), 64.8 (P2), 65.0 (P1); 4-G3: -79.5 (PTA), 12.1 (N3P3), 66.4 (P1, P2
and P3); 5-G3: -12.8 (PTA), 14.0 (P0), 68.7 (P3), 68.9 (P1 and P2). See
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Acknowledgements
Thanks are due to the European Community (contract MRTN-
CT-2003-503864, AQUACHEM) for financial support (in partic-
ular for providing a grant to P. Servin and M. Tristany), and to
the CNRS.
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4434 | Dalton Trans., 2009, 4432–4434
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The Royal Society of Chemistry 2009
©