Asymmetric hydroformylation of olefins catalyzed by rhodium nanoparticles chirally stabilized with (R)-BINAP ligand

Article abstract of DOI:10.1016/j.molcata.2007.12.008

Rhodium nanoparticles have been conveniently synthesized by one-pot chemical reduction of aqueous rhodium chloride (RhCl₃·3H₂O) dispersed in toluene solution in the presence of amphiphilic tetraoctylammonium bromide (TOAB) and chiral

Full text of DOI:10.1016/j.molcata.2007.12.008

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
Asymmetric hydroformylation of olefins catalyzed by rhodium
nanoparticles chirally stabilized with (R)-BINAP ligand
a
b
a
a
a
a,∗
Difei Han , Xiaohong Li , Huidong Zhang , Zhimin Liu , Gengshen Hu , Can Li
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
b
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
Received 28 September 2007; received in revised form 28 November 2007; accepted 1 December 2007
Available online 15 December 2007
Abstract
Rhodium nanoparticles have been conveniently synthesized by one-pot chemical reduction of aqueous rhodium chloride (RhCl
3
·3H
2
O) dispersed
in toluene solution in the presence of amphiphilic tetraoctylammonium bromide (TOAB) and chiral (R)-BINAP at ambient conditions. The resulting
highly dispersed rhodium nanoparticles show small and narrowly distributed core sizes (1.5–2.0 nm). The chirally stabilized rhodium nanoparticles
3
1
were also immobilized on silica by impregnation to give the corresponding supported catalysts. P MAS NMR results and IR spectra of adsorbed
CO confirm that chiral (R)-BINAP ligands stabilize the nanoparticles by coordinative interaction between phosphine and rhodium, and produce
chirally catalytic active sites on the rhodium nanocatalysts. Chirally stabilized catalysts exhibit high regioselectivity and chiral induction ability
for the asymmetric hydroformylation of olefins. The supported rhodium nanocatalysts show the increased activities compared to the unsupported
−1
catalysts (e.g. from 12 to 22 h for the hydroformylation of styrene). One hundred percentage regioselectivity of branched aldehyde and up to
9% ee were obtained for the hydroformylation of vinyl acetate.
2007 Elsevier B.V. All rights reserved.
5
©
Keywords: Rhodium nanoparticles; Enantioselective hydroformylation; Chiral stabilization; Coordination; Phosphorus ligands
1
. Introduction
able influence on their catalytic performance. However, chiral
ligand-stabilized nanoparticles have been seldom synthesized
or applied to enantioselective catalysis. Fujihara and coworkers
The synthesis and characterization of metal nanoparticles
have been intensively studied because of their potential appli-
cations in materials sciences, biological sciences, and chemical
platforms[1,2]. Mostofthemetalsubgroupelementscanbeused
to prepare metal nanoparticles [3]. The stabilization of metal
nanoparticles with surfactants or ligands can greatly influence
their particle sizes, electron, and other properties as catalysts,
photoelectric materials, molecular machines, and bio/chemical
sensors [4]. Particularly, the stabilized metal nanoparticles
have been extensively utilized as catalysts for hydrogenations,
hydrosilylations, hydrogenolysis, oxidation, McMurry, Suzuki
and Heck-Type couplings and other organic synthesis reac-
tions [5–7]. Various ligands, such as surfactants, phosphorus or
nitrogen-containing ligands, and thiols/thioethers, can be used
as stabilizer to prepare the metal nanoparticles and show remark-
[8,9] reported the synthesis of chirally stabilized gold and pal-
ladium nanoparticles and their application in the heterogeneous
asymmetric hydrosilylation and carbon–carbon coupling reac-
tions. Jansat et al. [10,11] synthesized ruthenium nanoparticles
stabilized by diphosphite and N-donor ligands and applied these
chirally stabilized catalysts to enantioselective allylic alkylation,
hydrogenation and transfer hydrogenation reactions. Hyeon and
coworkers [12] investigated the coordination chemistry of chi-
ral phosphine ligands on palladium nanoparticles in detail.
Researches indicate that coordinated chiral ligand may lead to
chiral induction on the metal surface and stabilize the nanopar-
ticles due to their strong chelating effect [13–16]. However, the
rhodium nanoparticle catalysts stabilized with chiral phospho-
rus ligands appear to be an attractive and unexplored area for
the development of effective heterogeneous chiral catalysts.
Chiral hydroformylation is one of the most challengable
organometallic catalytic reaction and also a very useful method
for the asymmetric synthesis of optically active aldehy-
∗
Corresponding author. Tel.: +86 411 84379070; fax: +86 411 84694447.
E-mail address: canli@dicp.ac.cn (C. Li).
URL: http://www.canli.dicp.ac.cn (C. Li).
1
381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.12.008

1
6
D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
des [17,18]. Chiral rhodium organometallic complexes were
extensively developed for homogeneous asymmetric hydro-
formylation reactions. Recently, we developed chirally modified
rhodium catalysts for the heterogeneous asymmetric hydro-
formylation of olefins [19]. Chiral phosphorus ligands were in
situ introduced to modify the Rh/SiO2 catalysts. Our Surface
Enhanced Raman Spectroscopy (SERS) investigation on lig-
and modification indicates that the ligands are adsorbed onto
the metal surface by coordination interaction [20]. The coor-
dination of chiral ligands to surface rhodium sites may form
chiral environment and further produce chiral induction on the
metal surface. These promote us to develop chirally stabilized
rhodium nanoparticles and research their application in asym-
metric hydroformylation.
In this paper, we present the synthesis of novel rhodium
nanoparticles stabilized with chiral phosphorus ligands and the
corresponding supported catalysts. The coordination of phos-
phines to the nanoparticles was characterized by 31P solid state
NMR and IR spectra using CO as a probe. The chirally stabilized
catalysts were then applied in the asymmetric hydroformyla-
tion of styrene and vinyl acetate under mild conditions, giving
highregioselectivityandchiralinductionabilityforthebranched
aldehydes.
0.2188 mmol, dissolved in 25 ml toluene) was added and the
solution was stirred vigorously for another 0.5 h at room temper-
ature. Aqueous sodium borohydride (NaBH4 0.05 g, 1.3 mmol,
dissolved in 1.2 ml H2O) was freshly prepared and added into
reaction system immediately. The organic phase turned black
and was further stirred for 3 h under argon atmosphere. The mix-
ture was washed with water (2× 50 ml) and the organic layer was
separated to obtain rhodium nanoparticle solution. The resulting
solid was washed with H2O, brine and methanol/water mixture
and gave rhodium nanoparticles (Rh-BINAP).
2.2.2. Preparation of chirally stabilized rhodium
nanoparticles supported on silica, Rh-BINAP/SiO2
The rhodium nanoparticles precursor containing organic
layer was added to vacuum activated SiO2 (the average pore
2
size is 9.7 nm, SBET = 375 m /g) and impregnated overnight. The
solvent was removed in vacuum to yield a black-gray solid. The
resulting solid was washed according aforementioned method
to give the supported rhodium nanoparticles (Rh-BINAP/SiO2).
The rhodium nanoparticles stabilized with achiral PPh3 (Rh-
PPh3 and Rh-PPh3/SiO2) were synthesized according to the
similar methods for comparison.
2
.2.3. Silica immobilized homogeneous complex,
+
−
2
. Experimental
[Rh(COD)BINAP] OTf /SiO2
+
−
[
Rh(COD)BINAP] OTf was synthesized according to
literature method. In a Schlenk tube under Ar, 0.1 mmol
Rh(COD)Cl]2 was dissolved in 10 ml CH2Cl2 and 0.23 mmol
2
.1. General
[
Unless otherwise mentioned, all the manipulations were car-
AgCF3SO3 was added. The resulting mixture was stirred at
room temperature for 1 h then the precipitate was filtered. To
the filtrate, 0.23 mmol (R)-BINAP in 5 ml CH2Cl2 was added
dropwise. The resulting mixture was stirred at RT for 3 h and
then 30 ml Et2O was slowly added. The precipitate was fil-
trated and dried under reduced pressure. The obtained powder
was washed with Et2O and vacuum-dried overnight to obtain
the Rh-diphosphine complexes. One grams of activated SiO2
was suspended in 5 ml CH2Cl2. The Rh diphosphine complex
(0.1 mmol) was dissolved in 1 ml CH2Cl2 and added to the
SiO2/CH2Cl2 suspension. The reaction mixture was stirred at
RT overnight and the recovered solid was washed with MeOH.
The supported homogeneous catalyst was vacuum-dried at RT
overnight and characterized by 31P MAS NMR and IR spec-
troscopy.
ried out with the use of standard Schlenk techniques. All the
solvents used for reactions were analytical grade and treated
by standard methods. Styrene and vinyl acetate were dis-
tilled before use. Chiral ligands were used as obtained without
ꢀ
ꢀ
further purification. (R)-(+)-2,2 -Bis(diphenylphosphino)-1,1 -
binaphthyl ((R)-BIANP, >98%), and other ligands and reagents
were purchased from Acros or Alfa Aesor. Rhodium trichloride
hydrate (Rh > 40 wt.%) and Bis(1,5-cyclooctadiene)rhodium (I)
chloride ([Rh(COD)Cl]2) were used as received. Activated SiO2
◦
was calcined at 540 C for 3 h before use.
2
.2. Catalysts preparation
2
.2.1. Preparation of rhodium nanoparticles stabilized with
chiral ligands, Rh-BINAP
Rhodium nanoparticles catalysts stabilized with chiral phos-
phorus ligand were prepared using a modification method
derived from the literature procedures [8,11]. Optically active
2.3. Characterization of chirally stabilized rhodium
nanoparticles
ꢀ
ꢀ
(R)-2,2 -bis-(diphenylphosphino)-1,1 -binaphthyl ((R)-BINAP)
Transmission Electron Microscope (TEM) images of the
catalyst samples were taken on a JEM-2000EX electron micro-
scope. Solid-state 31P MAS NMR spectra were accumulated on
a Bruker DRX-400 spectrometer. IR spectra of absorbed CO
on the chirally modified catalysts were collected on a Fourier
transform infrared spectrometer (Thermo Nicolet NEXUS 470)
was chosen as the chiral ligand for the stabilization of rhodium
nanoparticles owing to its excellent coordination and chi-
ral induction abilities. Rhodium trichloride hydrate (RhCl3,
0
0
.0270 g, 0.1 mmol) and tetraoctylammonium bromide (TOAB,
.0465 g, 0.085 mmol) were dissolved in 2 ml deionized water
−
1
and 2 ml toluene, respectively. The RhCl3 solution was drop-
wise added to the surfactant solution and the mixture was
stirred vigorously for 1 h under purged argon. When the rhodium
color was transferred into organic phase, (R)-BINAP (0.1360 g,
with a resolution of 4 cm
and 64 scans in the region
−
1
of 4000–1000 cm . The chirally stabilized catalyst or pre-
modified catalyst was transferred into an in situ IR cell, and the
sample was purged with argon to remove the solvent at 120 C.
◦

D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
17
Infrared spectra in the transmission mode were collected at reac-
tion temperature (60 C) after the exposure of the pre-modified
catalyst to carbon monoxide.
by calcination and hydrogen reduction at elevatory temperature.
The chiral phosphorus ligands were in situ introduced to reac-
tion or pre-adsorbed to modify the supported rhodium catalysts.
These catalysts can give chiral inductivity for the hydroformy-
lation of olefins. The silicas support is crucial to obtain high
dispersed rhodium nanoparticles. It is attractive for us to develop
the rhodium nanoparticles without the support and research the
corresponding catalytic performance for the asymmetric hydro-
formylation.
Herein, we firstly prepared rhodium nanoparticles catalysts
(Scheme 1). They can be conveniently synthesized by one-pot
chemical reduction method in the absence of silica support
[21]. Aqueous rhodium chloride was increasingly dissolved
and highly dispersed in toluene solution in the presence of
amphiphilic TOAB and the chiral stabilizer (R)-BINAP. Then
the catalytic precursor was reduced by NaBH4 at ambient condi-
tions. The solution turned from red to black immediately, which
indicated the formation of Rh(0) species. The rhodium nanopar-
ticles were stabilized with the TOAB and chiral ligands and
remained stable without aggregation.
Then the as-synthesized chirally stabilized rhodium nanopar-
ticles were also immobilized on silica support activated in
vacuum by adsorption. This supported catalyst was used for
comparison with the free nanocatalysts and the supported cat-
alysts prepared by traditional impregnation method. For the
chirally stabilized methods, the chiral ligand was introduced
in the synthesis procedure of rhodium nanoparticles. It is much
different from that of chirally modified catalyst, in which the
chiral ligand was introduced to modify the supported rhodium
catalyst pre-reduced by hydrogen.
◦
2
.4. Hydroformylation test
2
.4.1. Asymmetric hydroformylation on chirally stabilized
rhodium nanoparticles catalysts
The as-synthesized catalysts should be pretreated under a
flow of H2 at 453 K for 3 h before the heterogeneous asymmetric
hydroformylation. The chirally stabilized catalyst (0.002 mmol
Rh) together with the olefin substrate (1.2 mmol styrene or vinyl
acetate in 3 ml anhydrous toluene) were transferred into an auto-
clave under argon atmosphere. The hydroformylation reaction
◦
werecarriedoutunder50 atmsyngasat60 Cfor4 h. Conversion
and regioselectivity (branched to linear ratio) were determined
by gas chromatography (6890N, Agilent) equipped with a chi-
ral capillary column (Chiral-Dex ␤-225) and the enantiomeric
excess of the branched aldehyde was determined by Jones oxi-
dation of product to the corresponding carboxylic acid followed
by chiral GC analysis. The products configurations were deter-
mined by comparison of the retention time with the authentic
samples.
2
.4.2. Homogeneous asymmetric hydroformylation
In a typical homogeneous asymmetric hydroformylation
experiment, an autoclave was filled with 0.004 mmol phospho-
rus ligand (0.001 mM stock solution in toluene). 0.001 mmol
[
Rh(COD)Cl]2 (0.001 mM solution in toluene) were added
underargon(L/Rhratioof2.0)andthesolutionwasstirredfor2 h
to in situ form the chiral catalyst. The substrate (2.0 mmol) was
added and the subsequent operations were carried out according
to the heterogeneous reaction procedure except for a reaction
time of 10 h. Before GC analysis, the reaction product was
concentrated and purified by flash column chromatography.
3.2. Characterization of chirally stabilized rhodium
nanoparticles
3.2.1. TEM characterization
The TEM images of chirally stabilized Rh nanoparti-
cles (Fig. 1a) and the corresponding silica-supported catalyst
(Fig. 1b) show the highly dispersed rhodium nanoparticles
with narrowly distributed core sizes. The average sizes of chi-
rally stabilized rhodium nanoparticles were determined as about
1.5–2.0 nm. The particle size is similar to that of rhodium cat-
alyst supported on silica (Fig. 1c, Rh/SiO2, 69% dispersion
and 1.7 nm diameter) prepared by incipient wet impregna-
tion method, which was used for chirally modification. The
presence of chiral phosphorus ligands benefits the formation
and stabilization of rhodium nanoparticles, and provide more
uniformly dispersed particle size. These results indicate that
chiral stabilization is an efficient method to prepare highly dis-
persed rhodium nanoparticles in the absence of large surface
support.
2
.4.3. Recycle
The recycle experiments of catalyst and hot filtrate experi-
ment were also performed. When the first cycle reaction was
over, the reaction system was centrifuged and the solution was
carefully removed under argon atmosphere. Fresh vinyl acetate
substrate and solvent were added to the autoclave together with
the recovered solid catalyst and then the next cycle was carried
out under the same hydroformylation reaction conditions. On
the other hand, filtrate recycle experiment was also performed.
The reaction solution of the first cycle reaction with an incom-
plete conversion was filtered carefully and directly used in the
next catalytic cycle.
3
. Results and discussion
3
.1. Synthesis of chirally stabilized rhodium nanoparticles
3.2.2. 31P MAS NMR characterization
31
P MAS NMRandCO-IRspectroscopiesare effective meth-
In our previous work, we reported the synthesis and catalytic
ods to investigate the metal–ligand mutual interaction of solid
catalysts. The interaction between the rhodium nanoparticles
and the chiral (R)-BINAP ligands was investigated by P MAS
properties of silica-supported rhodium nanoparticles modified
with chiral phosphorus ligands [19]. The supported nanocata-
lysts were prepared by impregnation and consequently treated
3
1
NMR spectroscopy (Fig. 2). The broad signal at about 34.0 ppm

1
8
D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
Scheme 1. The schematic synthesis of chirally stabilized rhodium nanoparticles.
for the chirally stabilized nanoparticles (Rh-BINAP/SiO2,
Fig. 2a) is assigned to the (R)-BINAP coordinated to rhodium
atoms, which is similar to that of chirally modified catalysts
linearly adsorbed CO on the surface Rh(0) sites, appears as the
maincomponentforthechirallystabilizedcatalyst. Theseresults
indicate that adsorbed phosphines occupy surface Rh sites and
therefore inhibit the adsorption of CO on the rhodium surface,
especially for the chirally stabilized catalysts. The IR frequency
of the adsorbed CO red-shifts a lot to lower frequencies in the
presence of (R)-BINAP, which is due to the electron donor effect
of phosphorus atom.
It is well-known that the phosphorus atom of ligand can
donate its lone pair electrons to metal and accept the d orbital
feedbackfromatransitionmetalatom. OurSERSresearchonthe
phosphorus ligand modification also confirms that the ligands
adsorb on the metal surface through coordination interaction
(
BINAP-Rh/SiO2, Fig. 2b) and the supported organometallic
+
−
complex ([Rh(COD)BINAP] OTf /SiO2, Fig. 2c). This result
indicates that the stabilization of rhodium nanoparticles with
chiral phosphorus ligand can form quite strong P–Rh coordina-
tion interaction analogous to that of the rhodium organometallic
complex. Simultaneously, the surface acid sites of support can
compete with rhodium nanoparticles to adsorb the basic phos-
phine ligands. The signal of free (R)-BINAP (Fig. 2e) shifts to
lower field when the chiral ligands are physically adsorbed on
the silica support (Fig. 2d).
[20]. The ␴-donor ability of phosphorus atom is stronger than
3
.2.3. ATR-IR characterization
Interaction between the chiral diphosphine stabilizers with
the ␲-acceptor ability, and the coordination may change the sur-
face charge property of metal nanoparticles. This result agrees
to the results of CO-IR results (Fig. 3). For the chirally stabi-
lized rhodium nanoparticles in this work, the coordination of
phosphine ligands can increase the electron density of metal
centre, further increase the ␲-back-donation from metal to other
coordinative molecule. Therefore, the electron-rich surface may
provide better ability for the coordination and activation of olefin
and CO substrates.
the rhodium nanoparticles was also characterized by IR spec-
troscopy using CO as the probe molecule (Fig. 3). CO-IR spectra
of supported organometallic complexes (Fig. 3a) and Rh/SiO2
−
1
(Fig. 3b) show νCO stretching vibration at about 2100 cm
corresponding to the rhodium gem-di-carbonyl species as the
main band. These bands greatly decrease and red-shift to
lower wavenumbers for BINAP-modified Rh/SiO2 (Fig. 3c)
and BINAP-stabilized rhodium catalyst (Fig. 3d). The com-
The characterization results of NMR and IR spectra indicate
that the chiral stabilization via the coordination of phosphines
−
1
paratively weak band (1986 cm ) attributed to the stretching

D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
19
+
−
Fig. 3. IR spectra of adsorbed CO on (a) [Rh(COD)BINAP] OTf /SiO2, (b)
reduced Rh/SiO2, (c) BINAP pre-modified Rh/SiO2 and (d) Rh-BINAP/SiO2 at
◦
6
0 C.
Scheme 2. Asymmetric hydroformylation of olefins on chirally stabilized
rhodium nanoparticle catalysts.
3.3. Hydroformylation test
The effects of different preparation methods were tested for
Fig. 1. TEM images of (a) Rh-BINAP (b) Rh-BINAP/SiO2 (c) BINAP-Rh/SiO2.
the asymmetric hydroformylation of styrene and vinyl acetate
using the chirally stabilized catalyst, its supported analogue
and the chirally modified catalyst (Scheme 2). Homogeneous
to rhodium may modify the surface electronic state and pro-
duce chirally catalytic active sites on the rhodium nanoparticles.
Then, the possibility of the asymmetric induction on the chirally
stabilized catalysts were studied.
catalyst and PPh -stabilized catalyst were also investigated for
3
comparison.
3.3.1. Hydroformylation of styrene
The catalytic performance of chirally stabilized rhodium cat-
alysts was investigated for the hydroformylation of styrene as
the model reaction (Table 1). The introduction of chiral lig-
and to rhodium catalysts causes higher regioselectivity and
obvious chiral induction ability than the unmodified supported
catalysts. Chirally stabilized rhodium nanocatalyst (Rh-BINAP)
−
1
gave 12 h
S-enantiomer). When supported on silica, the chirally stabi-
lized catalyst (Rh-BINAP/SiO2) exhibits an increased activity
TOF, 92% branched selectivity and 25% ee
(
−
1
(
22 h ), which is higher than the chirally modified Rh/SiO2
catalyst (13 h ) and the corresponding homogenous catalyst
11 h ). The heterogeneous chiral catalysts exhibit much better
−
1
−
1
(
Fig. 2. 31P MAS NMR spectra of (a) Rh-BINAP/SiO2, (b) BINAP-Rh/SiO2,
branched regioselectivity (89:11–92:8) than the homogeneous
catalyst (85:15). The supported catalyst stabilized with achiral
+
−
31
(
c) [Rh(COD)BINAP] OTf /SiO2, (d) BINAP-SiO2 and (e) P NMR BINAP.

2
0
D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
Table 1
rality and chelation ability. And the competitive adsorption of
carbon monoxide possibly reduces the coordination of PPh3
to the rhodium catalyst under reaction conditions [22]. The
Asymmetric hydroformylation of styrene on chirally stabilized rhodium
nanoparticlesa
b
−1
Eec,d (%)
Entry
Catalyst
Conv. (%)
TOF (h
)
b/n
PPh -stabilized nanocatalyst exhibit higher activity than that
3
of the unmodified catalyst. This result is even more attractive
1
2
3
4
5
6
Rh/SiO2
Rh-BINAP
Rh-BINAP/SiO2
BINAP-Rh/SiO2
Homo.
99
5
9
6
5
212
12
22
13
11
67:33
92:8
89:11
92:8
85:15
85:15
0
25(S)
26(S)
30(S)
31(S)
0
considering that some of the active sites was occupied by phos-
phines for the ligand-stabilized catalysts, while all the surface
sites can be accessed for the unmodified rhodium catalysts.
The electron-donating coordination of PPh3 may increase the
surface electron density and thus activate the surface rhodium
sites, which can benefit the coordination and activation of
substrates. These may be the reasons for the high activity
Rh-PPh3/SiO2
99
239
a
Reactionswerecarriedoutwith2 ␮molrhodiumcatalystand1.2 mmololefin
◦
in toluene (3 ml) at 60 C under 50 bar syngas (CO:H2 = 1:1) for 4 h.
b
Calculated by the rhodium dispersion.
Determined by means of GC with a chiral ␤-cyclodextrin column.
The products configurations are determined by comparison of the elution
c
and regioselectivity of racemic reaction on the PPh -stabilized
3
d
nanocatalyst.
order with the authentic samples.
The chirally stabilized rhodium catalysts exhibited lower
activity than the ligand-free and PPh3-stabilized rhodium cat-
alysts. This indicates that the coordination of chiral phosphorus
ligands to the rhodium nanoparticles may form steady chi-
ral environment on the surface, and simultaneously inhibit the
access and complexation of substrates to the surface metal
sites. It is recognized that the steady coordination of chelat-
ing biphosphine ligands to the surface sites should account for
the decreased activity, although it may produces chiral induc-
tion environment on the rhodium surface and also activate the
catalytic sites by electron donation. Accordingly, the chirally
stabilized nanoparticles can induce chirality for the asymmetric
hydroformylation of olefins.
PPh3 ligand (Rh-PPh3/SiO2) was also tested for comparison.
No chiral inductivity and lower regioselectivity against higher
activity (239 h ) were obtained. The presence of chiral ligand
can remarkably influence the hydroformylation performance of
rhodium nanoparticles.
−
1
3
.3.2. Hydroformylation of vinyl acetate
The hydroformylation of vinyl acetate was also performed
on chirally stabilized rhodium catalysts (Table 2). The chi-
rally stabilized catalyst (Rh-BINAP) exhibits superior branched
regioselectivity (99:1) and up to 59% enantioselectivity, which
are much higher than those of the hydroformylation of
styrene. Its supported catalyst (Rh-BINAP/SiO2) gave complete
3.3.3. Recycle
−
1
branched aldehyde and an increased activity (5 h ). While, a
slight reduction of enantioselectivity (56% ee) was observed
on the supported catalyst. The chirally stabilized catalysts
exhibit higher regioselectivity and enantioselectivity than the
corresponding homogeneous catalyst. Achiral PPh3-stabilized
rhodium nanoparticles (Rh-PPh3/SiO2) shows high racemic
Catalyst recycling experiments were performed to test the
stability of the heterogeneous catalysts. The durability of the
catalytic system was investigated by employing it in sev-
eral consequent hydroformylation reactions for vinyl acetate
(Table 3). The reused catalyst exhibited similar catalytic per-
formance to the first catalytic cycle. After a few cycles, reaction
performance can be maintained by newly adding (R)-BINAP
ligand.
To further confirm that the catalyst is stable and the
catalytic reaction indeed occurs on the chirally stabilized
catalyst, the reaction of the hot filtration was performed.
The catalyst was separated from the reaction mixture after
the previous reaction. Then colorless filtrate was maintained
under the same hydroformylation conditions. Although the
−
1
hydroformylation activity (126 h ), which is even higher than
−
1
the ligand-free catalyst (90 h ) with almost complete regiose-
lectivity of branched product.
It is difficult for monodentate PPh3 to form stable and rigid
chiral environment on metal surface due to the lack of chi-
Table 2
Asymmetric hydroformylation of vinyl acetate on chirally stabilized rhodium
nanoparticlesa
Table 3
b
−1
Eec,d (%)
Entry
Catalyst
Conv. (%)
TOF (h
)
b/n
a,b
Recycleofchirallystabilizedcatalystforthehydroformylationofvinylacetate
1
2
3
4
5
6
Rh/SiO2
Rh-BINAP
Rh-BINAP/SiO2
BINAP-Rh/SiO2
Homo.
42
1
2
7
1
90
3
5
15
3
135
98:2
99:1
100:0
100:0
98:2
99:1
0
c
−1
d
Cycle
Conv. (%)
TOF (h
)
b/n
Ee (%)
59(S)
56(S)
59(S)
54(S)
0
1st
2.0
2.8
2.5
2.5
4
6
5
5
99/1
99/1
99/1
99/1
55
51
55
50
2nd
e
3rd
Rh-PPh3/SiO2
56
4th
a
a
Reactionswerecarriedoutwith2 ␮molrhodiumcatalystand1.2 mmololefin
Rh:L:Sub. = 1:1:600.
Reactions were performed in toluene (3 ml) at 60 C under 50 bar syngas
◦
b
◦
in toluene (3 ml) at 60 C under 50 bar syngas (CO:H2 = 1:1) for 4 h.
b
Calculated by the rhodium dispersion.
Determined by means of GC with a chiral ␤-cyclodextrin column.
The products configurations are determined by comparison of the elution
(CO:H2 = 1:1) for 4 h.
c
c
Calculated by the rhodium dispersion.
Determined by means of GC with a chiral ␤-cyclodextrin column.
(R)-BINAP was added.
d
d
e
order with the authentic samples.

D. Han et al. / Journal of Molecular Catalysis A: Chemical 283 (2008) 15–22
21
mechanic stirring probably damages the catalysts to some
extent, no further changes in conversion and enantioselectiv-
ity were observed after the filtrate reaction for 4 h. These
results indicate that the catalytic activity is definitely attributed
to the reaction on the chirally stabilized rhodium catalyst
and no obvious change was observed for the catalysts on
use.
nearby coordinated CO, which influence the branched to lin-
ear ratio and enantio-differentiation, respectively. It is presumed
that the hydroformylation reaction proceeds the catalytic recycle
through surface multi-site synergetic mechanism on the chirally
stabilized rhodium nanocatalyst (Scheme 3).
4. Conclusion
This work reports the preparation of rhodium nanoparticles
stabilized with chiral diphosphorus ligand and their silica-
supported analogues. The presence of chiral phosphorus ligands
can result in the formation of chirally stabilized rhodium
nanoparticles with uniformly high dispersity (1.5–2.0 nm).
3
.4. Mechanism Discussion
Thephosphorusligandcanbeadsorbedontothemetalsurface
via the coordination interaction between the phosphorus atoms
and the metal atoms [20,23]. In this work, the 31P MAS NMR
and CO-IR results indicate that the (R)-BINAP ligands steadily
adsorb onto the metal surface through coordination and produce
chirally stabilized active sites on the rhodium nanocatalyst. As
a result, the chirally stabilized rhodium catalysts exhibit chiral
induction ability for the hydroformylation of olefins. The biden-
tate phosphines of chiral ligand may chelate to nearby rhodium
sites to give stable coordination structure. Correspondingly,
the chirality of coordinated ligand transfers to the nanocata-
lyst and form chiral environment on the rhodium surface. The
coordinated phosphines and abundant spillover hydrogen can
increase the electron density of rhodium surface and benefit the
adsorption and activation of olefins and CO. The rigid chiral
environment formed by chelated (R)-BINAP ligand together
with the nanocatalyst surface exhibits good regioselectivity
and enantioselectivity for hydroformylation. The activation and
reaction of substrates may not necessarily occur on the same
rhodium atom coordinated by the ligand, but on the neighboring
active sites of the catalyst. The coordinated C C bond can eas-
ily interact with abundant hydrogen atoms and activated CO on
the catalyst surface. The surrounding chiral environment may
stereoselectively direct the addition of hydrogen to alkene and
the migratory insertion of the as-formed alkyl group into the
31
P MAS NMR and CO-IR results of (R)-BINAP-stabilized
nanocatalysts indicate that the diphosphine ligands strongly
adsorb onto rhodium surface sites through the coordination
interaction, leading to the formation of chiral environ-
ment on the rhodium nanoparticles. Therefore, the chirally
stabilized rhodium nanoparticles can catalyze the asymmet-
ric hydroformylation of olefins under mild conditions and
exhibit high regioselectivity and chiral inductivities (92:8
b/n, 26% ee for styrene and 99:1 b/n, 59% ee for vinyl
acetate, respectively). When supported on silica, the chi-
rally stabilized rhodium nanocatalyst showed the increased
−
1
−1
activities (12 h
vs. 22 h ) for the hydroformylation of
styrene. Achiral PPh3-stabilized rhodium nanoparticles exhib-
ited high racemic activity and increased regioselectivity. A
surface multi-site synergetic mechanism was presumed for
the asymmetric hydroformylation on the chirally stabilized
rhodium nanocatalyst. Chiral stabilization method through the
coordination of chiral ligand could be used as a potential con-
venient strategy for the preparation of heterogeneous chiral
catalysts.
Acknowledgements
Financial support for this work was provided by the National
Natural Science Foundation of China (NSFC, Grants 20621063
and 20423004).
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