Dendritic polymers containing a dimethylsilyl linked dihydroxybenzyl alcohol backbone: Divergent synthesis, aggregation, functionalization, and an evaluation of their applications in catalysis

Article abstract of DOI:10.1039/b301778h

A divergent synthetic approach to the construction of dimethylsilyl linked dihydroxybenzyl alcohol based dendrimers via acid-base hydrolytic chemistry of bis(dimethylamino)dimethylsilane with 3,5-dihydroxybenzyl alcohol, is reported. The peripheral OH gro

Full text of DOI:10.1039/b301778h

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Dendritic polymers containing a dimethylsilyl linked
dihydroxybenzyl alcohol backbone: divergent synthesis, aggregation,
functionalization, and an evaluation of their applications in catalysis
Olivier Bourrier and Ashok K. Kakkar*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal,
Quebec, H3A 2K6, Canada. E-mail: ashok.kakkar@mcgill.ca
Received 13th February 2003, Accepted 26th March 2003
First published as an Advance Article on the web 17th April 2003
A divergent synthetic approach to the construction of dimethylsilyl linked dihydroxybenzyl alcohol based
dendrimers via acid–base hydrolytic chemistry of bis(dimethylamino)dimethylsilane with 3,5-dihydroxybenzyl
alcohol, is reported. The peripheral OH groups lead to significant aggregation that was studied using FT-IR
and UV-Vis spectroscopies, light scattering and transmission electron microscopy. Different generations of
dendrimers could be easily functionalized at the periphery with phosphorus donor groups that were
subsequently bound to RhCl(1,5-C8H12). Such organometallic dendrimers were found to be active and
recoverable catalysts for hydrogenation of decene. The catalytic efficiency was found to be dependent on the
number of generations and reaction time. An uncontrolled reaction of Me2Si(NMe2)2 with 3,5-dihydroxybenzyl
alcohol in a multi-step addition process or one-pot reaction, led to the formation of hyperbranched polymers in
which the build-up of the polymeric backbone was found to be dependent on the preferential reactivity and
sterics at the benzylic center, as observed during dendrimer construction.
Such dendritic materials are of considerable value due to their
Introduction
ease of synthesis using a divergent methodology and versatility
in manipulating the peripheral architecture by functionaliza-
tion with a variety of useful moieties for targeted applications.
We demonstrate here a simple and highly tailorable synthetic
route to dendritic polymers containing a dimethylsilyl linked
3,5-dihydroxybenzyl alcohol (DHBA) backbone, examine their
physical properties, and the functionalization of their periph-
eries with a variety of end-groups including organometallic
centers for applications in supported metal catalysis. The
methodology is based on acid–base hydrolytic chemistry, and
capitalizes on the repetitive and controlled addition of an
aminosilane (Me2Si(NMe2)2) to backbones containing terminal
OH groups (DHBA), leading to the formation of Si–O links
with amine as the only by-product. It is an attractive route to
prepare a diverse range of dendritic polymers since the
peripheries of any generation can be easily functionalized
with, for example, trimethylsilyl groups, and phosphorus
donors for binding transition metal centers. An investigation
of the uncontrolled addition of dimethylaminosilane to DHBA
was also carried out in one pot, and step-by-step reactions, and
it led to the formation of hyperbranched polymers that
typically follow a dendritic growth.
The secondary interactions among the suitable periphery
situated end-groups of a dendrimer often affect its overall
shape.1i It has been demonstrated that the non-polar terminal
groups in poly(aryl ether) based dendrimers lead to a meso-
genic assembly while the polar end units lead to structures
resembling that of a micelle.5 The dendrimers reported here
contain OH groups at the periphery, and may undergo
extensive intermolecular hydrogen bonding at appropriate
dilutions. We have examined the behavior of these dendrimers
in THF using UV-Vis and FT-IR spectroscopies, transmission
electron microscopy and multi-angle light scattering studies,
and a discussion of their aggregation depending on the
concentration of the solutions, is reported.
Dendrimers constitute an important and intriguing class of
bulky monodisperse macromolecules that can offer a poten-
tially defect-free structure for a diverse range of applications.1
Due to their unique properties and enormous significance,
much effort has been devoted to the development of synthetic
routes that can lead to large quantities of these materials, and
incorporate required functionalities in the core or at the
periphery of a desired backbone.2 Numerous methodologies
have been proposed to synthesize these dendrimers, and
employ a divergent step-by-step approach or a convergent
one-step reaction. The divergent route has the advantage of
providing maximum control on the growth of the dendrimer,
and macromolecules with a higher number of generations can
be obtained. We have been investigating new divergent
synthetic routes to dendrimers, utilizing commercially available
or easily accessible reagents with high yield reactions,3 and
have reported the synthesis of dendrimers containing phos-
phorus and nitrogen based donor groups distributed through-
out their backbone.3a,c These globular macromolecules
containing donor sites are important precursors in the synthesis
of metallodendrimers due to the strong ability of phosphines to
ligate with a variety of transition metal centers. Studies of the
efficiency of dendrimer supported metal catalysts, in terms of
their activity, selectivity and recyclability in important organic
transformations, continue to emerge.3a,d
Poly(aryl ether) dendrimers developed by Hawker and
Frechet4 using 3,5-dihydroxybenzyl alcohol as the monomer
and a convergent synthetic methodology, constitute some of
the widely studied dendrimers due to their suggested applica-
tions in areas such as light amplification and harvesting.4c The
construction of similar dendrimers with a tailorable architec-
ture is an important goal in achieving desired functions
attributable to the specific properties of the dendritic frame-
works. We were intrigued by the design of dendritic polymers
in which the 3,5-dihydroxybenzyl alcohol based backbone units
are linked by flexible dimethylsilyl groups using a simple and
controlled coupling reaction in a divergent synthetic route.
Dendrimers functionalized with active transition metal
centers are gaining ongoing importance in the development
of efficient recyclable catalysts.3 a,c, d Different generations of
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DOI: 10.1039/b301778h

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Scheme 1
dimethylsilyl linked 3,5-dihyroxybenzyl alcohol dendrimers
were functionalized on the periphery with phosphine donor
ligands by reacting them first with Me2Si(NR2)2 followed
by HO–(CH2)3PPh2. The phosphine terminated dendrimers
were then metallated via a bridge splitting reaction with
[RhCl(cod)]2, and the catalytic activity of these dendrimers was
investigated for the hydrogenation of decene. The catalytic
activity was found to be dependent on factors such as the
number of generations in the dendrimer core, and the duration
of the hydrogenation reaction. After one batch of the reaction,
the organometallic dendrimer was precipitated and reintro-
duced into a fresh loading of decene in the hydrogenation
reactor. The activity was retained in the case of the third
generation dendrimer, however, it was found to increase upon
recycling of the functionalized dendrimers of generation 1
and 2.
DHBA was reacted with 3 equivalents of a mono-aminosilane,
Me3Si–NEt2, in tetrahydrofuran (THF) at room temperature
overnight. It led to a clean and almost quantitative conversion
to the corresponding trimethylsilyl terminated compound 1
(Scheme 1) as a grey liquid, with the elimination of NEt2H.
Since the DHBA backbone contains a benzylic OH and two
phenolic OH groups, we decided to first prepare model
compounds 2 and 3 containing DHBA at the core and benzylic
or phenolic terminal units. In the model studies, 1 equivalent of
DHBA was added slowly in a dropwise fashion to a solution of
3 equivalents of bis(dimethylamino)dimethylsilane in THF,
cooled to ice-bath temperatures. After stirring the mixture for
5 h, a solution of 3 equivalents of benzyl alcohol or phenol in
THF was added, and the stirred mixture was left to warm to
room temperature overnight. The removal of the solvent
yielded compounds 2 and 3 as colorless and highly viscous
liquids in good yields. The above reaction established that we
could tailor the reactivity of bis(dimethylamino)dimethylsilane
with DHBA using controlled temperature conditions and slow
dropwise additions, and avoid any side reactions leading to
hyperbranched polymer growth.
Results and discussion
Dendrimers
To prepare the first generation of dendrimers, a similar
reaction sequence was adopted. To an ice-bath cooled solution
of 3 equivalents of bis(dimethylamino)dimethylsilane in THF,
1 equivalent of DHBA was added dropwise over a period of
2–3 h. The stirred solution was then left to warm to room
temperature over a period of 5 h, and then transferred to an
addition funnel. This was then added slowly to a solution of
3 equivalents of DHBA in THF and stirred overnight.
Acid–base hydrolysis of aminosilanes with molecules contain-
ing acidic groups e.g., terminal OH groups in alcohols, leading
to almost quantitative conversion to the corresponding Si–OR
with elimination of amine, is well known.6 We have used this
chemistry to prepare a variety of supramolecular structures
ranging from simple monolayers to complex sol–gel net-
works.3b,7 The generality of this simple reaction has enabled us
to develop an efficient synthetic route to dendritic polymers. A
bifunctional linking unit [bis(dimethylamino)dimethylsilane]
and trifunctional backbone 3,5-dihydroxybenzyl alcohol
(DHBA) were chosen as reacting partners in a divergent
step-by-step build-up of dendrimers. To demonstrate the
reactivity of the aminosilanes with OH terminated molecules,
1
Removal of the solvent and characterization by H, 13C and
29Si NMR, MS and C & H analysis confirmed the synthesis of
first-generation dendrimer 4 as a solid in a good yield.
Continuation of this methodology of carrying out reactions
under controlled additions at room temperature at each step
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led to the preparation of second and third generation den-
drimers 5 and 6. The dendrimers remained in solution during
their syntheses, but their ability to go back into solution upon
isolation was reduced with an increasing in the number of
generations, most probably due to the kinetics of dissolution
getting slower with an increasing number of generations.
1H NMR spectroscopy and MALDI-TOF MS proved to be
extremely useful in the characterization of these dendrimers.
DHBA has an asymmetric location of OH groups, and
although the pKa of phenolic hydroxy group (10) is lower
than that of benzylic analog (15), the reactivities of benzylic
and phenolic OH groups were expected to be indistinguishable
under the reaction conditions employed. If there is no
preferential attack site on DHBA, one would expect that the
structures of the above dendrimers will have a random
distribution of benzylic and phenolic links to dimethylsilane.
However, it was noted from the 1H NMR spectra that in
generations 1–3, the residual OH groups on the periphery were
almost exclusively phenolic in nature and not benzylic. This
reactivity pattern could be explained using steric arguments.
The steric strain will be much less at the benzylic center in these
dendrimers making it the preferential site of attachment. It
becomes manifested more in the synthesis of hyperbranched
polymers, and will be discussed in detail in the later sections.
Aggregation
The interactions among dendritic molecules via suitable
periphery situated end groups impart interesting physical
properties, and may lead to aggregation in solution when their
concentration exceeds a certain threshold, often referred to as
the critical micellar concentration (c.m.c.). DHBA based
dendrimers (4–6) contain terminal OH groups (6, 12 and 24
respectively), and individual molecules may interact with each
other through hydrogen bonding, yielding aggregates. This
phenomenon was first examined using UV-Vis spectroscopy. A
steady decrease in turbidity of the solution was observed upon
successive dilutions, and upon reaching the c.m.c., the
absorption decreased dramatically. The scattering part of
each UV-Vis spectrum remained quite similar at c.m.c. and
below, suggesting that it was not affected very much by a
decrease in concentration of the dendrimers in solution. By
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Fig. 1 UV-Vis spectra of dendrimer 6 including the region of scattering
(300–350 nm) at concentrations of 1–10 mg mL21 in THF.
Fig. 3 FT-IR spectra of dendrimer 5 (3100–3800 cm21) in THF at
concentrations of 1–10 mg mL21
.
Fig. 2 Absorbance vs. concentration of dendrimer 6 (in mg mL21) in
THF at 340 nm.
plotting the intensity of scattered light at a given wavelength
against the concentration (Figs. 1 and 2), we were able to
determine the c.m.c. for the dendrimer generations 2 and 3. The
aggregation was seen to begin at a concentration of 3.7 mg mL21
for dendrimer generation 2 as well as generation 3.
As mentioned earlier, the aggregate formation in DHBA
based dendrimers may be attributed to intermolecular hydro-
gen bonds between dendrimer molecules. Infrared spectro-
scopy is a valuable tool in examining the existence of hydrogen
bonds, and it has been used to ascertain their presence in
dendrimers containing peripheral carbamate N–H groups.8
It was reported that there is a difference of 120 cm21 in the
N–H absorption between hydrogen bonded and non-hydrogen
bonded compounds. We obtained FT-IR spectra of den-
Fig. 4 Diameter (as determined from light scattering measurements) vs.
concentration in mg mL21 in THF for dendrimers 5 and 6.
drimer generations
2 and 3 at different concentrations
(1–10 mg mL21). Fig. 3 highlights the evolution of the IR
spectrum in the hydroxy group region at these concentrations.
A broad peak at 3300 cm21 for the terminal OH groups was
clearly observed at concentrations of 4–10 mg mL21. This
peak position suggests strong hydrogen bonding between
peripheral OH groups. At concentrations of 1–3 mg mL21
which are below the c.m.c., a peak due to OH stretching
appeared at 3570 cm21, which is indicative of the absence of
any hydrogen bonding between the peripheral OH groups.
Dynamic light scattering (DLS) has been extensively used to
assess the particle size of polymeric materials in solution, and in
particular micelles.9 DLS is also a useful technique to estimate the
radii of the particles in solution, and to study the effect of the
concentration of the solutions on the radii. If the formation of
aggregates is dependent on concentration, a noticeable change in
the size of the aggregates is expected. An evaluation of the size of
the DHBA based dendrimers was carried out using DLS
measurements at different concentrations of dendrimers of
generations 2 and 3: one below the c.m.c. (1 mg mL21), one
close to the c.m.c. (3 mg mL21), and one well above the c.m.c.
(12 mg mL21). Calculation of the diameter (Fig. 4) was done
using the CONTIN method.10 For dendrimer generations 2
and 3 at a concentration of 1 mg mL21, only one peak with a
particle size of 1.33 nm was observed. At 3 mg mL21, particles of
size 133 nm were detected, which indi- cated the formation of
aggregates. At 12 mg mL21 of dendrimer generation 2, a major
peak at 145 nm was observed. For dendrimer generation 3, at a
concentration of 12 mg mL21, similar aggregation was seen with
particles of 180 nm diameter. These experiments clearly
suggested that dendrimers of generations 2 and 3 show a
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of generations, we attempted to improve their solubility by
introducing Me3Si functionalities at the periphery. This could
easily be achieved by reacting, for example, dendrimer 4 with 6
equivalents of Me3Si–NEt2 at room temperature overnight,
leading to a clean and quantitative capping of the latter
dendrimer with Me3Si groups. The resulting compound (7) was
isolated as a liquid that was found to be soluble in common
organic solvents.
The terminal OH groups in dendrimers 4–6 were then used to
introduce donor phosphine groups at the periphery. The latter
could then bind transition metal centers for heterogenizing
homogeneous catalysts.5j To test this methodology, after
reacting 6 equivalents of Me2Si(NMe2)2 with 1 equivalent of
the first-generation dendrimer 4 in THF at room temperature,
6 equivalents of HO(CH2)3PPh2 in THF were added, and the
solution was warmed to room temperature. The solution
mixture was stirred at room temperature overnight, and the
solvent was then removed under vacuum to yield compound 8.
The latter showed a single peak in its 31P{1H} NMR spectrum
at 215.9 ppm. Phosphinated first-generation dendrimer 8 was
then reacted with [RhCl(cod)]2 in a bridge-splitting reaction to
link donor phosphine units to Rh(I) centers. This led to a
complete functionalization with metal centers (9) with no
residual free phosphine units on the periphery, as indicated by
the appearance of a doublet at 27.3 ppm in the 31P{1H} NMR
spectrum due to Rh–P coupling (JRh–P ~ 150 Hz). Using the
same reaction sequence as described above for generation 1,
dendrimers of generations 2 and 3 were functionalized with
HO(CH2)3PPh2 to give dendrimers functionalized at the
periphery with phosphines (10 and 11). Complete functiona-
lization with terminal phosphine units required stirring at 55 uC
to graft 12 equivalents of HO(CH2)3PPh2 to the second
generation dendrimer backbone (10), and a week of stirring at
55 uC to graft 24 equivalents on the third generation dendrimer
(11). Compounds 10 and 11 were then reacted with
[RhCl(cod)]2 to yield organometallic dendrimers 12 and 13.
Fig. 5 Transmission electron micrograph of dendrimer 5 in a solution
(3 mg mL21) of THF.
strong tendency to interact with each other through hydrogen
bonding that leads to the formation of large aggregates.
Transmission electron microscopy is another useful tool to
assess the physical characteristics of particles in solution.10 We
have also used this technique to evaluate the size of aggregates
formed by dendrimers reported here. A transmission electron
micrograph of dendrimer generation 2 is shown in Fig. 5. It shows
the presence of two particles of globular shapes and sizes 65 nm
(major) and y10 nm (minor component). A detailed evaluation
of different generations of these dendrimers is continuing.
Functionalization of dendrimers
Hydrogenation of decene
The terminal OH groups in the DHBA based dendrimers offer
opportunities to functionalize them with a variety of peripheral
moieties. Since the ability to redissolve dendrimers 4–6 upon
isolation as powders decreased with an increase in the number
Dendrimers offer unique opportunities to design supports for
recoverable heterogenized homogenous catalysts.3a,d Much
effort has been devoted to the synthesis of organometallic
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dendrimers in which active transition metal centers are present
in the core, corona or the entire backbone. A study of the
catalytic efficiency of DHBA based organometallic dendrimers
of generations 1–3 functionalized at the periphery with
phosphines ligated to RhCl(1,5-C8H12) was carried out for
the catalytic hydrogenation of decene. The hydrogen pressure
reactor was loaded with the organometallic dendrimer and
decene (1:200 molar ratio) dissolved in benzene, and catalysis
was carried out at room temperature at 20 bars of H2 for
periods of 0.5 to 5 h. The organometallic dendrimers were
found to be active catalysts for the hydrogenation of decene,
and the activity of the catalyst was found to increase with an
increase in the number of generations and the reaction time
(Table 1). For example, with the generation 1 organometallic
dendrimer, only 19% of decene was found to be converted to
decane after 0.5 h of the reaction which gradually increases to
97% after 5 h reaction time. The reactivity of the generation 2
organometallic dendrimer was found to be higher than that of
generation 1, and followed a similar pattern of increasing
activity with time (Table 1). Generation 3 dendrimer showed a
maximum conversion after 2 h of contact time, and was
retained upon further increase in time. Catalyst recovery and its
recycling ability were also investigated for generations 1–3
(Table 1). The recycled catalysts retained their efficiency, and
surprisingly, the recovered catalysts showed an increase in
activity upon recycling.
Hyperbranched polymers
The synthesis of globular macromolecules with defect free
structures has been the driving force for all the activity in the
field of dendrimers. It requires a careful execution of the steps
involved in the build-up of generations in a divergent manner,
and is a lengthy and cumbersome process. This could be a
handicap in any industrial scale synthesis and applications of
these otherwise very valuable materials. We made a compro-
mise and carried out the synthesis using our acid–base
hydrolytic chemistry under uncontrolled conditions, which
lead to macromolecules that have a hyperbranched structure
but with more defects than their corresponding dendrimers. To
achieve this goal, two sets of reactions were designed: i) a one-
step process in which equimolar quantities of reactive groups
are put together with the evolution of the macromolecule
examined after a set period of time; and ii) a multi-step reaction
of partners, as in the synthesis of dendrimers, but without the
dropwise addition or control of temperature.
In the one-pot reaction, solutions of 2 equivalents of DHBA
and 3 equivalents of Me2Si(NMe2)2 in THF were mixed
together, and stirred at room temperature overnight. Removal
of the solvent yielded a solid, the MALDI-TOF MS of which
showed a major peak at a mass of 589. This corresponds to the
species shown in Scheme 2, the formation of which could be
explained by considering a reaction pattern in which molecules
of DHBA first react with 1 equivalent of Me2Si(NMe2)2 each
preferably at the benzylic OH group position. One molecule of
the resulting compound then reacts with the other at the
phenolic OH group yielding a compound with MALDI-TOF
mass of 589 (after losing a NMe2 group).
Table 1 Hydrogenation of decene using functionalized dendrimers of
generation 1–3
Time of
Reaction/h
Conversion
(%)
9, [G-1]-[Rh(cod)Cl]6
12, [G-2]-[Rh(cod)Cl]12
13, [G-3]-[Rh(cod)Cl]24
0.5
1
2
5
0.5
1
2
5
0.5
1
2
5
2
2
2
2
2
2
19
47
48
97
23
65
71
98
26
65
93
93
48
85
71
93
93
98
9, [G-1]-[Rh(cod)Cl]6
Recycle 9, [G-1]-([Rh(cod)Cl]6
12, [G-2]-[Rh(cod)Cl]12
Recycle 12, [G-2]-[Rh(cod)Cl]12
13, [G-3]-[Rh(cod)Cl]24
Recycle 13, [G-3]-[Rh(cod)Cl]24
In the two-step uncontrolled methodology to prepare
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using Me3Si locks at the periphery after entrapment are
currently being pursued.
Experimental section
Materials and measurements
All manipulations were performed under a nitrogen atmo-
sphere using either standard Schlenk line techniques or
an Innovative Technology (Braun) Labmaster MB-150-M
dry box. All solvents were stored under nitrogen and used
after distillation over sodium. Chemicals were purchased
from the following and used as received: 3,5-dihydroxy-
benzyl alcohol (Aldrich), trimethylsilyldiethylamine (Aldrich),
bis(dimethylamino)dimethylsilane (Gelest), 3-hydroxypropyl-
diphenylphosphine (Organometallics, Inc.), chloro(1,5-cyclo-
octadiene)rhodium(I) dimer (Pressure Chemicals). NMR spectra
were measured on a 270 MHz JEOL spectrometer at ambient
temperature. NMR samples were prepared inside the dry box
using deuterated solvents (Cambridge Isotope laboratories,
Inc.). The chemical shifts in ppm are reported relative to
1
tetramethylsilane as an internal standard for H, 13C and 29Si
NMR spectra, and to H3PO4 for 31P{1H} NMR spectra. Mass
spectra were obtained on a Hewlett Packard 5973 mass
spectrometer, and MALDI-TOF spectra on a Kratos Kompact
Maldi 3.v.4.0.0 spectrometer using LiBr–dithranol or a-cyano-
4-hydroxycinamic acid as the matrices. Elemental analyses
were performed by Guelph Chemical Laboratories, Guelph,
Ontario, Canada. Infrared spectra were measured on a Bruker
IFS-48 Fourier transform infrared spectrometer using a
standard resolution of 4 cm21 for transmission. UV-Vis
spectra were recorded on a Hewlett Packard 8453 with a
resolution of 2 nm using quartz cuvettes. For dynamic light
scattering experiments, a system from Brookhaven Instru-
ments Corporation equipped with their BI-200SM goniometer,
BI-9000AT digital correlator, and a Compass 315-150 CW
laser light source from Coherent Inc. (operating at 532 nm,
150 mW), was used. The DLS experiments were carried out at
an angle of 90u. Transmission electron microscopy (TEM) was
carried out on a JEOL 2000FX microscope operating at an
acceleration voltage of 80 kV. Copper EM grids (400 mesh)
were precoated with carbon.
Scheme 2
hyperbranched polymers, a solution of 1 equivalent of DHBA
in THF was added directly to the solution of 3 equivalents of
Me2Si(NMe2)2 in THF. The mixture was stirred at room
temperature overnight. Then 3 equivalents of DHBA were
added, and the solution was left to stir at room temperature
overnight. Removal of the solvent yielded a sticky brown gel,
1
the H NMR spectrum of which indicated the presence of a
substantial amount of residual phenolic OH groups. The
MALDI-TOF MS of the product showed the expected mass of
728 for the first generation dendrimer, however, there were
additional peaks up to a maximum noticeable mass of 1910,
corresponding to the second generation dendrimer. A very
interesting feature to be noted in the MALDI-TOF MS was the
appearance of peaks that were separated by a unit mass of 197
which corresponds to the linking unit {(HO)2[C6H3](OCH2-
Si(CH3)2)}. Once again, this suggests a reaction pattern based
on the domination of the higher rate of reaction of
Me2Si(NMe2)2 with the benzylic OH groups, leading to the
formation of
a molecular species, {(HO)2[C6H3](OCH2-
Reaction of 3,5-dihydroxybenzyl alcohol with Me3Si-NEt2
Si(CH3)2)NMe2}. The latter then reacts with DHBA one OH
at a time to yield a dendritic polymer.
3,5-Dihydroxybenzyl alcohol (DHBA) (304 mg, 2.17 mmol)
was dissolved in 1 mL of dry THF, and added to a solution
of 3 equivalents of trimethylsilyldiethylamine (1.22 mL,
6.51 mmol) in 10 mL of dry THF. The mixture was stirred
at room temperature overnight. The solvent was removed
under vacuum, yielding a brown liquid (1, 714 mg, 96%).
1H NMR (270 MHz, C6D6): d (ppm) 0.07 (s, 9H, CH2O-
SiMe3), 0.18 (s, 18H, C6H3-OSiMe3), 4.51 (s, 2H, C6H3-
CH2OSiMe3), 6.49 (s, 1H, C6H3), 6.67 (s, 2H, C6H3). 13C NMR
(68 MHz, C6D6): d (ppm) 20.6, 0.0 (C6H3-OSiCH3 and C6H3-
CH2-OSiCH3), 64.2 (C6H3-CH2), 110.7, 111.4, 144.0, 156.7
(C6H3). 29Si NMR (54 MHz, C6D6): d (ppm) 18.1 and 18.7
(C7H5-OSiMe3 and C6H3-OSiMe3). EI-MS: m/z 356. Anal.
Calcd. for C16H32O3Si3: C, 53.93; H, 8.98. Found: C, 53.93; H,
8.71%.
Conclusions
It is demonstrated here that the simple acid–base hydrolytic
chemistry of 3,5-dihydroxybenzyl alcohol with Me2Si(NMe2)2,
is
a versatile divergent synthetic methodology for the
construction of dendrimers and hyperbranched polymers
containing aryl ether units linked by dimethylsilyl units.
Preferential attack at the benzylic OH group by the
aminosilane during the synthesis of hyperbranched polymers
leads to an unexpected control of the evolution of these
macromolecules. The terminal OH groups in the dendrimers
yield to hydrogen bonding, and lead to significant aggregation
at concentrations above 3 mg mL21. The peripheral OH groups
in the dendrimers of nth generations were functionalized with
i) Me3Si groups leading to significant improvements in their
solubilities; and ii) phosphine donor groups that were
subsequently used to bind Rh(I) centers. The resulting
organometallic dendrimers were found to be active and
recyclable catalysts for the hydrogenation of decene. The
catalytic efficiency was found to be dependent on dendrimer
generation and reaction time. Further evaluation of these
dendrimers as traps for important organic molecules during
aggregation and blocking the escape of the guest molecules by
Reaction of 3,5-dihydroxybenzyl alcohol first with
Me2Si(NMe2)2 and then benzyl alcohol
A solution of DHBA (104 mg, 0.74 mmol) in 10 mL of THF
was added dropwise to a solution of 3 equivalents of
bis(dimethylamino)dimethylsilane (0.40 mL, 2.22 mmol) in
5 mL of THF at ice-bath temperature. After stirring for 5 hours,
a solution of 3 equivalents of benzyl alcohol (0.24 mL,
2.22 mmol) in 5 mL of dry THF was added dropwise to the
above mixture, which was then stirred overnight. The solvent
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was removed under vacuum yielding a colorless liquid (2,
1
1.215 g, 89%). H NMR (270 MHz, C6D6): d (ppm) 0.13 (s,
C6H3), 9.13 (s, 12H, C6H3-OH). 13C NMR (68 MHz, DMSO-
d6): d (ppm) 22.5, 20.5, 1.4 (SiCH3), 63.5, 64.2, 67.5 (C6H3-
CH2), 143.1, 145.2, 158.7 (C6H3). 29Si NMR (54 MHz, DMSO-
d6): d (ppm) 219.9, 211.1, 210.9, 21.6 (C7H5-OSiMe2O-C7H5,
C7H5-OSiMe2O-C6H3, C6H3-OSiMe2O-C6H3). Mass spectrum
(MALDI-TOF) m/z 1912. Anal. Calcd for C88H116O30Si9: C,
53.26; H, 6.07. Found: C, 53.06; H, 6.68%.
12H, OSiMe2O), 0.20 (s, 6H, OSiMe2O), 4.67 (s, 6H, O-CH2-
C6H3), 4.74 (s, 2H, C6H3-CH2-O), 6.90–7.30 (m, 18H, C6H3).
13C NMR (68 MHz, C6D6): d (ppm) 24.0, 22.3 (SiCH3), 62.3,
66.5 (C6H3-CH2), 111.6, 140.9, 156.0 (C6H3), 125.9, 127.4,
128.2, 143.9 (C6H5). EI-MS: m/z 632. Anal. Calcd. for
C34H44O6Si3: C, 64.56; H, 6.96. Found: C, 65.50; H, 7.34%.
Third generation dendrimer: [G-3]-(OH)24 (6)
Reaction of 3,5-dihydroxybenzyl alcohol with Me2Si(NMe2)2
and then phenol
A similar procedure as described above for [G-2]-(OH)12 was
used to prepare the third generation dendrimer. A solution of
1 equivalent of DHBA (20 mg, 0.143 mmol) in 10 mL of THF
was added to a solution of 3 equivalents of bis(dimethylamino)-
dimethylsilane (0.0755 mL, 0.429 mmol) in 5 mL THF, and
stirred for 6 hours at ice bath temperature. The above mixture
was added to a solution of 3 equivalents of DHBA (60 mg,
0.429 mmol) in 10 mL of THF, and stirred overnight at room
temperature. It was then added to a solution of 6 equivalents of
bis(dimethylamino)dimethylsilane (0.155 mL, 0.858 mmol) in
5 mL of THF, and stirred overnight at room temperature. This
was then added to a solution of 6 equivalents of DHBA
(120 mg, 0.858 mmol) in 10 mL of THF, and after stirring
overnight, this solution was added to a solution of 12
equivalents of bis(dimethylamino)dimethylsilane (0.310 mL,
1.716 mmol) in 5 mL of THF, and stirred for 14 hours. Finally
this solution was added to a solution of 12 equivalents of
DHBA (240 mg, 1.716 mmol) in 15 mL of THF, and stirred
overnight. THF was removed under vacuum to afford a gel
A solution of DHBA (106 mg, 0.76 mmol) in 10 mL of dry
THF was added dropwise to a solution of 3 equivalents of
bis(dimethylamino)dimethylsilane (0.43 mL, 2.28 mmol) in
5 mL of dry THF. During the addition and the 6 hours of
stirring that followed, the solution was maintained at ice-bath
temperature. A solution of 3 equivalents of phenol (223 mg,
2.28 mmol) was prepared in 15 mL of dry THF. The latter was
stored on molecular sieves for about 5 hours, and then added
dropwise to the above mixture. It was stirred overnight at room
temperature, and then the solvent was removed under vacuum.
A clear grey liquid was obtained as the product (3, 1.16 g, 91%).
1H NMR (270 MHz, C6D6): d (ppm) 0.18 (s, 6H, OSiMe2O),
0.23 (s, 12H, OSiMe2O), 4.47 (s, 2H, C6H3-CH2O), 6.80–7.08
(m, 18H, C6H3, C6H5). 13C NMR (68 MHz, C6D6): d (ppm)
22.6 (SiCH3), 64.3 (C6H3-CH2), 102.1, 105.3, 143.5, 156.2
(C6H3), 117.0, 120.5, 128.7, 131.0 (C6H5). 29Si NMR (54 MHz,
C6D6): d (ppm) 25.4, 23.3, (C6H3-OSiO-C6H5, C6H3CH2-
OSiO-C6H5). CI-MS: m/z 590. Anal. Calcd. for C31H38O6Si3:
C, 63.05; H, 6.44. Found: C, 62.87; H, 6.60%.
1
(6, 0.550 g, 91%). H NMR (270 MHz, DMSO-d6): d (ppm)
0.12–0.24 (m, 126H, OSiMe2O), 4.66 (s, 44H, C6H3-CH2O),
6.14, 6.17, 6.26, 6.73 (m, 66H, C6H3), 9.36 (s, 24H, C6H3-OH).
13C NMR (68 MHz, DMSO-d6): d (ppm) 22.2 (m, SiCH3),
64.3, 67.6 (C6H3-CH2), 143.1, 155.4, 158.7 (C6H3). 29Si NMR
(54 MHz, DMSO-d6): d (ppm) 220.5 211.8, 21.7 (C7H5-
OSiMe2O-C7H5, C7H5-OSiMe2O-C6H3). Mass spectrum
(MALDI-TOF) m/z 4245.7 (including m/z Li1). Anal. Calcd
for C196H260O66Si21: C, 55.48; H, 6.13. Found: C, 55.02; H,
6.65%.
First generation dendrimer: [G-1]-(OH)6 (4)
A solution of DHBA (0.200 g, 1.43 mmol) in 5 mL of dry THF
was added dropwise to a solution of 3 equivalents of
bis(dimethylamino)dimethylsilane (0.774 mL, 4.29 mmol) in
5 mL of dry THF cooled to 0 uC. Stirring at ice-bath
temperature was maintained for an additional 6 hour period,
and the resulting solution was then warmed to room
temperature. The latter was then added to a solution of 3
equivalents of DHBA (0.600 g, 4.29 mmol) in 10 mL of dry
THF. The resulting solution was stirred overnight, and THF
was then removed under vacuum to afford a sticky gel (4, 0.936
g, 90%). 1H NMR (270 MHz, DMSO-d6): d (ppm) 0.10 (s, 6H,
OSiMe2O), 0.23 (s, 12H, OSiMe2O), 4.65 (s, 8H, C6H3-CH2O),
6.09–6.54 (m, 12H, C6H3), 9.07 (s, 6H, C6H3OH). 13C NMR
(68 MHz, DMSO-d6): d 22.4, 20.1, 1.4 (SiCH3), 63.6, 64.0
(C6H3-CH2), 101.4, 104.7, 143.3, 158.8 (C6H3). 29Si NMR
(54 MHz, DMSO-d6): d (ppm) 219.9, 211.1, 21.6 (C7H5-
OSiMe2O-C7H5, C7H5-OSiMe2O-C6H3, C6H3-OSiMe2O-
C6H3). Mass spectrum (MALDI-TOF) m/z 734.7 (including
m/z Li1). Anal. Calcd for C34H38O12Si3: C, 56.04; H, 6.04.
Found: C, 55.55; H, 6.57%.
[G-1]-(SiMe3)6 (7)
To a solution of the first generation dendrimer (4, 520 mg,
0.714 mmol) in 20 mL of THF, 6 equivalents of trimethyl-
silyldiethylamine (0.811 mL, 4.286 mmol) were added via
syringe, and the mixture was stirred overnight. THF was
removed under vacuum to afford a dark brown liquid (7,
1
0.801 g, 96%). H NMR (270 MHz, C6D6): d (ppm) 0.17 (s,
6H), 0.22 (s, 12H and s, 54H, OSiMe2O and OSiMe3), 4.68 (br
s, 8H, C6H3-CH2O), 6.61–6.81 (m, 12H, C6H3). 13C NMR
(C6D6) d 22.9, 20.1, 0.4 (SiCH3), 64.0 (C6H3-CH2),111.4,
111.5, 156.6 (C6H3). 29Si NMR (54 MHz, C6D6): d (ppm) 23.7,
23.3, 21.8, 21.8 (OSiMe2O), 18.86, 18.92 (OSiMe3). Mass
spectrum (MALDI-TOF) m/z 1168.8 (including m/z Li1).
FAB-MS: m/z 1160.
Second generation dendrimer: [G-2]-(OH)12 (5)
[G-1]-(PPh2)6 (8)
A solution of 1 equivalent of DHBA (50 mg, 0.357 mmol) in
10 mL of THF was added to a solution of 3 equivalents of
bis(dimethylamino)dimethylsilane (0.194 mL, 1.071 mmol) in
5 mL of THF, and stirred for 6 hours at ice-bath temperature.
The resulting solution was added to 3 equivalents of DHBA
(150 mg, 1.071 mmol) dissolved in 10 mL of THF. After stir-
ring overnight, the mixture was then added dropwise to a
solution of 6 equivalents of bis(dimethylamino)dimethylsilane
(0.388 mL, 2.142 mmol) in 5 mL of THF. The resulting solution
was then added to 6 equivalents of DHBA (300 mg, 2.142
mmol) dissolved in 15 mL of THF, and stirred overnight. The
removal of the solvent under vacuum afforded a sticky gel. (5,
To a solution of 6 equivalents of bis(dimethylamino)dimethyl-
silane (0.755 mL, 4.302 mmol) in THF, a solution of 4 (520 mg,
0.714 mmol) in 10 mL of THF was added dropwise over a
period of 3 hours. Once the addition was complete, the mixture
was stirred overnight at room temperature. A solution of 6
equivalents of 3-hydroxypropyldiphenylphosphine (1.051 g,
4.302 mmol) in 10 mL of THF was then added dropwise to the
above mixture. Stirring was continued overnight at room
temperature. The removal of THF afforded a white gel (8,
1.570 g, 87%). 1H NMR (270 MHz, C6D6): d (ppm) 0.09,
0.21 (br s, 54H, OSiMe2O), 1.75 (m, 12H, -O-CH2-CH2-), 2.07
(t, JH–H ~ 8.3 Hz, 12H, -CH2- PPh2), 3.63 (t, JH–H ~ 6.3 Hz,
12H, -O-CH2-CH2-), 4.69 (m, 8H, C6H3-CH2O), 6.86 (m, 12H,
1
0.587, 86%). H NMR (270 MHz, DMSO-d6) d 0.11, 0.21 (m,
54H, OSiMe2O), 4.60 (s, 20H, C6H3-CH2O), 6.18 (m, 30H,
J. Mater. Chem., 2003, 13, 1306–1315
1313

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C6H3), 7.08, 7.44 (m, 60H, P(C6H5)2). 31P{1H} NMR
(109 MHz, C6D6): d (ppm) 215.9 (s, PPh2). Mass spectrum
(MALDI-TOF) m/z 2529.5. Anal. Calcd for C136H164O18Si9P6:
C, 57.41; H, 6.72. Found: C, 56.94; H, 6.89%.
(109 MHz, C6D6): d (ppm) 27.1 (d, J
~ 149.0 Hz). Mass
Rh–P
spectrum (MALDI-TOF) m/z 8488.7 (including m/z Na1).
[G-3]-[Rh(cod)Cl]24 (13)
An orange solution of 12 equivalents of the chloro(1,5-
cyclooctadiene)rhodium(I) dimer (106 mg, 0.215 mmol) in
15 mL of THF was added dropwise to a solution of 11 (311 mg,
1.793 6 1025 mol). The mixture was left to stir overnight
before removing the solvent to afford an orange sticky solid
[G-1]-[Rh(cod)Cl]]6 (9)
To a solution of 8 (362 mg, 0.143 mmol) in 10 mL of THF, a
solution of 3 equivalents of chloro(1,5-cyclooctadiene)rho-
dium(I) dimer (211 mg, 0.429 mmol) in 15 mL of THF was
added dropwise. THF was then removed under vacuum to give
a yellow solid (9, 0.544 g, 95%). 1H NMR (270 MHz, C6D6): d
(ppm) 0.17 (br m, 60H, OSiMe2O), 1.70 (br, 12H, -O-CH2-
CH2-), 2.10 (br, 48H, -CH2-CH2-), 2.30 (br, 12H, -CH2-PPh2),
3.67 (br, 12H, -O-CH2-CH2-), 4.60 (br m, 8H, C6H3-CH2O),
5.77 (s, 24H, -CHLCH-), 6.86 (m, 12H, C6H3), 7.06, 7.69 (m,
60H, P(C6H5)2). 31P{1H} NMR (109 MHz, C6D6): d (ppm)
27.3 (d, JRh–P ~ 150.4 Hz). Mass spectrum (MALDI-TOF) m/z
4008.2.
1
(13, 0.230 g, 74%). H NMR (270 MHz, C6D6): d (ppm) 0.15
(br m, 270H, OSiMe2O), 1.67 (br, 48H, -O-CH2-CH2-), 2.11
(br, 192H, -CH2-CH2-), 2.34 (br, 48H, -CH2-PPh2), 3.56 (br,
48H, -O-CH2-CH2-), 4.69 (br m, 44H, C6H3-CH2O), 5.74 (s,
96H, -CHLCH-), 6.84 (m, 66H, C6H3), 7.00, 7.63 (m, 240H,
P(C6H5)2). 31P{1H} NMR (109 MHz, C6D6): d (ppm) 27.3 (d,
JRh–P ~ 150.4 Hz). Mass spectrum (MALDI-TOF) m/z
17368.7.
Synthesis of hyperbranched polymers
[G-2]-(PPh2)12 (10)
One-pot reaction. 2 Equivalents of DHBA (151.6 mg,
1.083 mmol) were dissolved in 10 mL of THF, and 3
equivalents of bis(dimethylamino)dimethylsilane (0.294 mL,
1.624 mmol) were directly added to the DHBA solution via
syringe. The mixture was stirred overnight at room tempera-
ture, and the solvent was then removed under vacuum to yield a
To a solution of 12 equivalents of bis(dimethylamino)di-
methylsilane (0.755 mL, 4.285 mmol) in 5 mL of THF, a
solution of 5 (682 mg, 0.357 mmol) in 15 mL of THF was added
dropwise. The mixture was stirred overnight. Then 1.05 g of
3-hydroxypropyldiphenylphosphine (12 equivalents, 4.285 mmol)
in 15 mL of THF were added. The reaction mixture was heated
for a period of 24 hours at a temperature of 55 uC. The solvent
was then removed under vacuum to afford a white gel (10,
1.593 g, 81%). 1H NMR (270 MHz, C6D6): d (ppm) 0.11 (br m,
126H, OSiMe2O), 1.71 (m, 24H, -O-CH2-CH2-), 2.08 (m, 24H,
-CH2-PPh2), 3.54 (t, JH–H ~ 6.29 Hz, 24H, -O-CH2-CH2-),
4.70 (m, 20H, C6H3-CH2O), 6.86 (m, 30H, C6H3), 7.15, 7.46
(m, 120H, P(C6H5)2). 31P{1H} NMR (109 MHz, C6D6): d
(ppm) 16.0 (s, PPh2). Mass spectrum (MALDI-TOF, LiBr,
dithranol) m/z 5518.7 (including m/z Li1).
1
grey gel (293 mg, 92%). H NMR (270 MHz, C6D6): d (ppm)
0.07, 0.20, 0.29 (s, 18H, OSiMe2O), 2.42 (s, -SiNMe2), 4.72 (br
m, 6H, C6H3-CH2O), 6.89 (br, 9H, C6H3-CH2O),. Mass
spectrum (MALDI-TOF, LiBr, dithranol) m/z 596.2 (including
m/z Li1). Anal. Calcd for C29H43O9Si3N: C, 53.94; H, 6.84.
Found: C, 53.38; H, 7.06%.
2-Step preparation of hyperbranched polymers. 3 Equivalents
of bis(dimethylamino)dimethylsilane (0.774 mL, 4.286 mmol)
were directly added via syringe to a solution of DHBA (200 mg,
1.429 mmol) in 10 mL of THF, and the mixture was stirred
[G-3]-(PPh2)24 (11)
overnight at room temperature. Then
a solution of 3
equivalents of DHBA (600 mg, 1.429 mmol) in 15 mL of
THF was added, and the mixture was stirred overnight at room
temperature. The solvent was then removed under vacuum to
afford a grey–brown gel (1.150 mg, 80%). 1H NMR (270 MHz,
DMSO-d6): d (ppm) 0.12, 0.23, 0.35 (m, OSiMe2O), 2.30 (s,
-SiNMe2), 4.65 (m, C6H3-CH2O), 6.16 (m, C6H3-CH2O), 9.17
(br, C6H3-OH). Mass spectrum (MALDI-TOF, LiBr, dithra-
nol) m/z 539, 735.1, 931.1, 1127.7, 1324, 1521, 1719.5, 1917.5
(including m/z Li1).
A solution of 170 mg of 6 (0.040 mmol) in 10 mL of THF was
added dropwise to a solution of 24 equivalents of bis(dimethyl-
amino)dimethylsilane (0.174 mL, 0.960 mmol) in 5 mL of
THF. The mixture was then stirred overnight at room
temperature. 24 equivalents of 3-hydroxypropyldiphenylphos-
phine (0.235 mg, 0.960 mmol) in 10 mL of THF were added
dropwise to the latter solution. Once the addition was
complete, the mixture was stirred at 55 uC under nitrogen
for 7 days to allow all the phosphine compounds to be grafted
on the dendritic backbone (11, 0.375 g, 87%). 1H NMR
(270 MHz, C6D6): d (ppm) 0.16 (br m, 270H, OSiMe2O), 1.75
(m, 48H, -O-CH2-CH2-), 2.10 (m, 48H, -CH2-PPh2), 3.69 (t,
JH–H ~ 6.2 Hz, 48H, -O-CH2-CH2-), 4.79 (m, 44H, C6H3-
CH2O), 6.86 (m, 66H, C6H3), 7.06, 7.45 (m, 240H, P(C6H5)2).
31P{1H} NMR (109 MHz, C6D6): d (ppm) 215.9 (s, PPh2).
Mass spectrum (MALDI-TOF) m/z 11465.6 (including m/z
Na1).
Hydrogenation of 1-decene
All of the hydrogenation reactions were performed using a
Parr 4560 Mini-reactor with constant experimental conditions:
pH ~ 20 bar, room temperature, ratio ndendrimer:ndecene ~ 1:200.
2
The product analysis was carried out using a GC-MS spectro-
meter (Hewlett Packard 6840 GC and HP 5973 mass spectro-
meter) using He as the carrier gas and an initial temperature of
50 uC with a ramp of 12 uC min21 stopping at 300 uC. Both the
catalyst and the decene were mixed in 20 mL of benzene inside
the pressure reactor. After the catalysis run, the solvent and the
decene–decane mixture were distilled by static vacuum distilla-
tion, leaving the dendritic catalyst behind. The benzene was then
[G-2]-[Rh(cod)Cl]12 (12)
211 mg of chloro(1,5-cyclooctadiene)rhodium(I) dimer (6
equivalents, 0.428 mmol) were dissolved in 15 mL of THF,
and added dropwise to a solution of 10 (393 mg, 0.0714 mmol)
in 10 mL of THF. The yellow solution was stirred overnight at
room temperature. The solvent was then removed under
vacuum to give a yellow solid (12, 0.588 g, 93%). 1H NMR
(270 MHz, C6D6): d (ppm) 0.21 (br m, 126H, OSiMe2O), 1.70
(br, 24H, -O-CH2-CH2-), 2.10 (br, 96H, -CH2-CH2-), 2.30
(br, 24H, -CH2- PPh2), 3.67 (br, 24H, -O-CH2-CH2-), 4.76
(m, 20H, C6H3-CH2O), 5.84 (s, 48H, -CHLCH-), 6.86 (m,
30H, C6H3), 7.15, 7.73 (m, 120H, P(C6H5)2). 31P{1H} NMR
1
removed by distillation to afford a colourless liquid. H NMR
(270 Hz, C6D6): d (ppm) 0.91 (6H, t, JH–H ~ 6.7 Hz, CH3), 1.28
(16H, br m, CH2). GC-MS: m/z 142, 7.45 min.
Acknowledgements
We would like to thank the NSERC of Canada and FCAR of
Quebec, Canada, for financial support.
1314
J. Mater. Chem., 2003, 13, 1306–1315

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