Highly selective hydroformylation of vinylarenes to branched aldehydes by [Rh(cod)Cl]2 entrapped in ionic liquid modified silica sol-gel

Article abstract of DOI:10.1002/ejoc.200700458

A co-entrapped mixture of [Rh(cod)Cl]₂ and Na[Ph ₂P-3-(C₆H₄SO₃)] within a silica sol-gel matrix modified with ca. 5% of 1-butyl-3-[3-(trimethoxysilyl)propyl] imidazolium chloride catalyzes, in n-heptane, the hydroformylation of a variety of vinylarenes. At 50°C and under 6.9 bar each of H₂ and CO the reaction is high-yielding and highly selective. Non-hindered substrates give >95 % of branched aldehydes and only <5% of the linear isomers. The ceramic catalyst is leach-proof and recyclable. It does not lose its high catalytic activity and selectivity for at least four runs. The selectivity depends on the pressure of the gases, the temperature and the solvent. The electronic nature has no influence on the selectivity, but the latter is diminished by steric effects. Upon omission of the sol-gel component, the catalyst deteriorates and practically loses its activity after the first half-life of the reaction. In the absence of the ionic liquid, the catalyst undergoes substantial leaching. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700458

FULL PAPER
DOI: 10.1002/ejoc.200700458
Highly Selective Hydroformylation of Vinylarenes to Branched Aldehydes by
[Rh(cod)Cl]2 Entrapped in Ionic Liquid Modified Silica Sol-Gel
Khalil Hamza[a] and Jochanan Blum*[a]
Keywords: Hydroformylation / Catalyst recycling / Ionic liquids / Rhodium / Silica sol-gel / Branched aldehydes
A co-entrapped mixture of [Rh(cod)Cl]2 and Na[Ph2P-3-
(C6H4SO3)] within a silica sol-gel matrix modified with ca.
5% of 1-butyl-3-[3-(trimethoxysilyl)propyl]imidazolium chlo-
ride catalyzes, in n-heptane, the hydroformylation of a vari-
ety of vinylarenes. At 50 °C and under 6.9 bar each of H2 and
CO the reaction is high-yielding and highly selective. Non-
hindered substrates give Ͼ95% of branched aldehydes and
only Ͻ5% of the linear isomers. The ceramic catalyst is
leach-proof and recyclable. It does not lose its high catalytic
activity and selectivity for at least four runs. The selectivity
depends on the pressure of the gases, the temperature and
the solvent. The electronic nature has no influence on the
selectivity, but the latter is diminished by steric effects. Upon
omission of the sol-gel component, the catalyst deteriorates
and practically loses its activity after the first half-life of the
reaction. In the absence of the ionic liquid, the catalyst un-
dergoes substantial leaching.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
recycling of the expensive rhodium catalyst,[6] and the appli-
cation of an ionic liquid that frequently enhances catalytic
processes,[7] and that in our case, promotes also the highly
selective hydroformylation to yield branched products. In
this research we concentrated on the transformation of vi-
nylarenes by a mixture of [Rh(cod)Cl]2 and Na[Ph2P-3-
(C6H4SO3)] entrapped within a sol-gel matrix confined with
ca. 5 mol-% of silica-bound imidazolium-based ionic liquid.
The ceramic catalyst, so formed, promotes the transforma-
tion of a series of styrene derivatives to branched aldehydes
in Ͼ95% selectivity (Scheme 1).
Introduction
Hydroformylation of terminal olefins is a very useful re-
action in the chemical industry.[1] Since this process yields
usual two kinds of products, linear aldehydes (or alcohols),
and the corresponding branched isomers, much work has
been performed in order to make this process a more selec-
tive one. However, most of the studies were directed to the
selective preparation of the linear regioisomers, which are
starting materials for a variety of polymers, detergents, cos-
metics and other widespread products. The investigation of
selective hydroformylation to yield branched products de-
served much less interest despite the fact that branched al-
dehydes are starting compounds for the asymmetric synthe-
sis of many pharmaceuticals.[2] In fact, the number of stud-
ies that lead to substantial selectivity in favor of branched
products is very limited. Good results were obtained mainly
with terminal olefins bearing electron-withdrawing substit-
uents[2,3] or with substrates capable of chelating to the cata-
lysts[4] such as vinyl acetate, butenylphenylphosphane and
some N-allylamides. Selective hydroformylations towards
branched products in which neutral or electron-rich alkenes
have been used are even rarer.[5] In this study, aimed at the
improvement of the selectivity towards branched products,
we have combined two domains of modern catalysis: the
use of sol-gel technology that enables facile recovery and
Scheme 1.
Results and Discussion
In a series of experiments we have hydroformylated sty-
rene in the presence of a variety of rhodium complexes and
tertiary phosphanes co-entrapped in a sol-gel matrix (from
tetramethoxysilane) modified with a silyloxylated imid-
[a] Department of Organic Chemistry, The Hebrew University,
Jerusalem 91904, Israel
Fax: +972-2-6513832
E-mail: jblum@chem.ch.huji.ac.il
4706
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4706–4710

Hydroformylation of Vinylarenes by [Rh(cod)Cl]2
azolium ionic liquid (cf., e.g., ref.[8]). The best results, in carbon monoxide pressures at a temperature range between
terms of yield and selectivity were obtained when the molar 25 and 100 °C. These experiments are summarized in
ratio ion liquid/tetramethoxysilane was between 1:20 and Tables 3, 4, and 5.
1:30. A particularly efficient catalyst was obtained by mix-
ing 0.0608 mmol of [Rh(cod)Cl]2, 0.1207 mmol of Na[Ph2P-
3-(C6H4SO3)], 32 mmol of prehydrolyzed Si(OMe)4 and
1.13 mmol of 1-butyl-3-[3-(trimethoxysilyl)propyl]imid-
azolium chloride (prepared as shown in Scheme 2) in THF
for 6 h followed by aging at room temperature for 12 h and
drying at 13–130 Pa at 80 °C. No loss of rhodium or phos-
phorus was observed during the immobilization process.
Scheme 2.
The average surface area of several samples of the heterog-
Table 1. Hydroformylation of styrene by [Rh(cod)Cl]2 and
Na[Ph2P-3-(C6H4SO3)] to give C6H5CH(CH3)CHO (b = branched)
and C6H5(CH2)2CHO (l = linear).[a]
enized catalyst was shown by N2/BET measurements to be
292 m2/g, the pore diameter being 27 Å. When one-half of
this catalyst was mixed with 1 mmol of freshly distilled sty-
rene in 15 mL of n-heptane and heated at 50 °C under
6.9 bar H2 and 6.9 bar CO for 12 h, GC analysis of the
resulting mixture indicated the formation of 97.13% of 2-
phenylpropanal and 2.85% of the 3-phenyl isomer. An ex-
periment with a fivefold amount of the reactants gave a
mixture of the two aldehydes that were isolated on a prepar-
ative 5% Apiezon L/Chromosorb W column followed by
immediate transformation into the corresponding 2,4-dini-
trophenylhydrazones. In this experiment, 88% of branched
and 2% of linear aldehydes were isolated. Upon completion
of the hydroformylation, the ceramic material was washed
with n-heptane followed by sonication with dichlorometh-
ane and drying under reduced pressure. This operation did
not remove any rhodium or phosphorus from the matrix
and allowed the reuse of the catalyst system for further hy-
droformylation of styrene without loss of the catalytic ac-
tivity or of its selectivity for four consecutive runs. In the
fifth run, however, some deterioration of the catalyst started
to take place. These results are summarized in Table 1. Sim-
ilar hydroformylation reactions (including catalyst recycling
experiments) were conducted with several substituted sty-
renes, as well as with 2-vinylnaphthalene. The results are
listed in Table 2. Many examples reported in the literature
indicate the pronounced electronic effect on the selectivity
in hydroformylation (see, for example, ref.[2]). The results
listed in Table 2 show, however, that in our system the elec-
tronic effect is negligible. Substrates with electron-donating
and those with electron-attracting groups showed similar
conversions with a similar selectivity of branched product,
indifferently whether these groups are located at the meta
or para position to the vinyl function. On the other hand,
the steric effect associated with an ortho substitution, as in
2-chlorostyrene, causes a reduction in both the rate and the
selectivity (cf. Entries 5, 6 and 7 in Table 2). The relatively
low yield in the hydroformylation of 3-bromostyrene (Entry
4) and of 2-vinylnaphthalene (Entry 9) may result from the
bulkiness of the substrates. The small pore size of the ma-
trix (vide supra) may prevent the substrate from fast pen-
etrating through the sol-gel backbone to reach the metal
catalyst.[9] The selectivity, however, seems not to be affected
by this bulkiness. The reaction conditions employed in
Table 2 were selected after running model experiments with
Run Total yield of Yield of branched Yield of linear Ratio b/l
no. aldehydes (%)
aldehydes (%)
aldehyde (%)
1
2
3
4
5
100
99.5
99.5
99.5
80
97.1
96.7
96.8
96.8
77.0
2.9
2.8
2.7
3.0
3.0
34:1
35:1
36:1
32:1
26:1
[a] Reaction conditions: 1 mmol of substrate, 15 mL of n-heptane,
1.2 g of ceramic catalyst prepared from 16.45 mmol of prehy-
drolyzed Si(OMe)4 and 0.56 mmol of 1-butyl-3-[3-(trimethoxysilyl)-
propyl]imidiazolium chloride that contained 0.0304 mmol of
[Rh(cod)Cl]2 and 0.0603 mmol of Na[Ph2P-3-(C6H4SO3)]; 6.9 bar
of H2, 6.9 bar of CO; 50 °C; 12 h.
Table 2. Hydroformylation of some vinylarenes under comparable
conditions (b = branched; l = linear).[a]
[a] Reaction conditions as in Table 1. [b] In the first run. The yields
are determined by GC and are the average of at least two experi-
styrene in several solvents, under different hydrogen and ments that did not differ by more than 3%.
Eur. J. Org. Chem. 2007, 4706–4710
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K. Hamza, J. Blum
FULL PAPER
Table 3 indicates that the hydroformylation of styrene Table 5. Effect of the temperature on the hydroformylation of styrene
and 4-chlorostyrene (b = branched; l = linear).[a]
proceeds well with high selectivity in producing branched
products both in aliphatic and in aromatic hydrocarbons.
However, in THF substantial hydrogenation of both the
starting vinylarene, as well as the aldehydes was found to
take place. In 1,2-dichloroethane the reaction became com-
pletely non-selective.
Entry
Substrate
Reaction temp. (°C) Total yield of Ratio
aldehydes (%) b/l[b]
1
2
3
4
5
6
7
C6H5CH=CH2
C6H5CH=CH2
C6H5CH=CH2
20
50
16
100
99
40:1
34:1
14:1
10:1
31:1
12:1
6:1
75
C6H5CH=CH2
100
50
99
96
4-ClC6H4CH=CH2
4-ClC6H4CH=CH2
4-ClC6H4CH=CH2
Table 3. Effect of several solvents on the hydroformylation of sty-
rene by a mixture of [Rh(cod)Cl]2 and Na[Ph2P-3-(C6H4SO3)] en-
caged in ionic liquid modified silica sol-gel (b = branched; l = lin-
ear).[a]
75
99
100
99.5
[a] Reaction conditions as in Table 2, except that the hydrofor-
mylation was run at different temperatures. [b] In the first run.
Entry
Solvent
Total yield of Yield of b Yield of l
Ratio
aldehydes (%)
(%)
(%)
b/l
and forms an inactive deposit (mirror) of metallic rhodium.
In a typical experiment, the hydroformylation stopped after
68% of the styrene had reacted. By comparison of the pro-
files of the reactions by the non-entrapped and by the en-
trapped catalyst (Figure 1) one can realize that both reac-
tions start at a similar rate of styrene consumption that fol-
lows a first-order rate law but only the immobilized catalyst
leads the reaction to completion. The selectivity for
branched products also decreases to some extent when the
heterogenized catalyst is replaced by the homogeneous one
[the branched (b)/linear (l) ratio becomes 19:1 as compared
with 34:1 in the reaction with the entrapped rhodium com-
plex]. Of course the catalyst cannot be reused under homo-
geneous conditions. Under such conditions, the addition of
the ionic liquid component causes a solubility problem and
leads to a further decrease in the yield (40% with a selectiv-
ity of 21:1). In the absence of the tertiary phosphane hardly
any hydroformylation takes place. Substitution of the salt of
the sulfonated phosphane by PPh3 causes substantial metal
leaching. We assume that sodium 3-(diphenylphosphanyl)-
benzenesulfonate may undergo ion exchange with the ionic
liquid forming an immobilized rhodium complex with a
carbene-like ligand. The latter may be responsible for the
1
2
3
4
5
6
n-heptane
n-hexane
n-pentane
toluene
100
99
98.8
98.5
55[b]
99.3
97.1
95.9
95.5
94.7
53.7
55.3
2.9
3.1
3.3
3.8
1.3
44.0
34:1
31:1
29:1
25:1
42:1
1.3:1
THF
1,2-dichloroethane
[a] Reaction conditions as in Table 2, except that the solvent was
different. [b] The conversion was 95%. Side products (40%) consisted
mainly of ethylbenzene, 2- and 3-phenylpropanol, and polymeric ma-
terial.
Changes in the pressure were found to hardly effect the
reaction rate. In all experiments listed in Table 4 the total
yield was Ͼ96%. The selectivity, however, increased slightly
parallel to the increase in the pressure of the gases. Thus,
under a total pressure of 6.9 bar, styrene gave only 89.5%
of the branched 2-phenylpropanal, whereas under 55.2 bar
the yield rose to 97.8%.
Table 4. Effect of the pressure on the hydroformylation of styrene by
a mixture of [Rh(cod)Cl]2 and Na[Ph2P-3-(C6H4SO3)] encaged in
ionic liquid modified silica sol-gel (b = branched; l = linear).[a]
Entry
Initial H2
pressure (bar)
Initial CO
pressure (bar)
Ratio b/l[b]
1
2
3
4
3.45
6.9
13.8
27.6
3.45
6.9
13.8
27.6
8.5:1
29:1
31:1
45:1
[a] Reaction conditions: 1 mmol of styrene, 15 mL of n-pentane,
1.25 g of ceramic catalyst containing 0.0304 mmol of [Rh(cod)Cl]2,
0.0603 mmol of Na[Ph2P (3-C6H4SO3)] and the sol-gel-ionic liquid
combination described in the Experimental Section; 50 °C; 12 h. [b]
In the first run.
The influence of the temperature is more pronounced
(see Table 5). While the total yield increased along with the
elevation of the temperature, a reverse correlation was
found between the temperature and the selectivity for
branched products. A temperature of 50 °C was therefore
selected as optimal for our experiments.
It is noteworthy, that all four components of the catalyst
system, the rhodium complex, the sulfonated phosphane,
the modified ionic liquid and the silica sol-gel matrix,
proved essential for the selective hydroformylation to yield
branched products. In principal, the hydroformylation of
styrene takes place also under homogeneous conditions (in
the absence of the sol-gel material). However, the non-im-
Figure 1. Concentration/time profiles for the formation of 2-phen-
ylpropanal from styrene by the sol-gel entrapped [Rh(cod)Cl]2/
Na[Ph2P(3-C6H4SO3)]/ionic liquid catalyst (---᭡---) and by the
mobilized catalyst deteriorates at 50 °C within less than 2 h non-entrapped (sol-gel-free) catalyst (---᭿---).
4708 Eur. J. Org. Chem. 2007, 4706–4710
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Hydroformylation of Vinylarenes by [Rh(cod)Cl]2
high conversion and b/l selectivity.[5d] In this respect we re-
call the observation that various transition metal com-
pounds are able to coordinate with imidazolium nitrogen
atoms forming useful catalyst (see, for example, ref.[10]).
(10 mL) for 15 min and dried again at 130 Pa to constant weight
(3.5 h). Ca. 2.5 g of an orange ceramic material was obtained. The
combined washings were subjected to atomic absorption analysis
and were found to be rhodium-free (sensitivity limit 1 ppm).
Catalytic Hydroformylation: A 100 mL miniautoclave equipped
with a mechanical stirrer and a sampling device was charged at
50 °C with the heterogenized catalyst (containing 3.04ϫ10–2 mmol
of the dirhodium complex and 0.0603 mmol of the tertiary phos-
phane), the substrate (1 mmol) and an appropriate solvent (15 mL).
The autoclave was sealed and purged with hydrogen and then pres-
surized with H2 (usually to 6.9 bar) and with the same pressure of
CO. The stirred reaction mixture was then heated (usually at 50 °C)
for 12 h. After cooling to 0 °C, the excess gases were released and
the remaining mixture filtered and the solid washed with heptane
(2ϫ30 mL). The filtrate was concentrated and analyzed by GC,
MS and NMR and compared with authentic samples. The filtered
ceramic catalyst was refluxed for 20 min in heptane (20 mL) and
sonicated for 15 min with CH2Cl2 (20 mL). The combined wash-
ings were concentrated and subjected either to ICP or atomic ab-
sorption analysis for metal leaching analysis. The filtered solid was
dried at 130 Pa at 80 °C for 5 h prior to its use as catalyst for a
further run of hydroformylation.
Conclusions
In this study we have shown that the co-entrapment of a
rhodium complex, a sulfonated tertiary phosphane and an
ionic liquid within silica sol-gel, forms a stable and recycla-
ble hydroformylation catalyst. The ceramic catalyst pro-
motes the transformation of vinylarenes under mild condi-
tions to predominantly branched aldehydes with high selec-
tivity. The study is in line with the recent attempts to de-
velop sol-gel encapsulated ionic liquids that may show a
unique catalytic activity in a highly selective fashion.[11]
Experimental Section
General: The various styrene derivatives, 1-vinylnaphthalene and 1-
butylimidazole were purchased from Sigma–Aldrich. The vinyl-
arenes were distilled prior to the application. (3-Chloropropyl)tri-
methoxysilane and tetramethoxysilane were obtained from Gelest
Silanes & Silicones and used without further purifications. Di-µ-
chlorobis[(1,2,5,6-η)-1,5-cyclooctadiene]dirhodium[12] and sodium
3-(diphenylphosphanyl)benzenesulfonate[13] were prepared accord-
ing to literature procedures. The following analytical instruments
were used: Bruker Vector 22 FTIR spectrometer, Bruker AMX-300
NMR instrument, Hewlett Packard model Agilent 4890D gas chro-
matograph, Hewlett Packard model 4989A mass spectrometer
equipped with an HP gas chomatograph model 5890 series II, an
Optima 3000 instrument was used for inductively coupled plasma
(ICP) measurements, a Perkin–Elmer spectrophotometer model
403 equipped with a Juniper rhodium cathode lamp was used for
leaching analysis, and a Micrometrics ASAP 2020 instrument was
used for N2/BET surface area and pore diameter measurements of
the sol-gel matrices.
Acknowledgments
We gratefully acknowledge the support of this study by the Israel
Science Foundation through grant no. 269/06.
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1
for 12 h and proved to be analytically pure. H NMR (300 MHz,
CDCl3): δ = 0.50 (m, 2 H), 0.89 (t, J = 7.2 Hz, 3 H), 1.31 (m, 2
H), 1.82 (m, 2 H), 1.95 (m, 2 H), 3.48 (s, 9 H), 4.27 (t, J = 7.2 Hz,
2 H), 4.32 (t, J = 7.4 Hz, 2 H), 7.45 (br. s, 1 H), 7.64 (br. s, 1 H),
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Eur. J. Org. Chem. 2007, 4706–4710
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4709

K. Hamza, J. Blum
FULL PAPER
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Received: May 22, 2007
Published Online: July 25, 2007
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4710
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