Phosphine-stabilized, oxide-supported rhodium catalysts for highly efficient silylative coupling reactions

Article abstract of DOI:10.1007/s11164-015-1983-8

Oxide-supported rhodium catalysts with excellent activity in silylative coupling reactions have been developed. Reductive pretreatment of the catalysts in the presence of 0.5 equiv triphenylphosphine under a hydrogen atmosphere enhanced and stabilized the

Full text of DOI:10.1007/s11164-015-1983-8

Res Chem Intermed
DOI 10.1007/s11164-015-1983-8
Phosphine-stabilized, oxide-supported rhodium
catalysts for highly efficient silylative coupling reactions
Shinji Tsukada1 Kenji Wada2 Hiroki Miura1,3,4
•
•
•
Saburo Hosokawa4 Ryu Abe1 Masashi Inoue1
•
•
Received: 8 July 2014 / Accepted: 24 September 2014
ꢀ Springer Science+Business Media Dordrecht 2015
Abstract Oxide-supported rhodium catalysts with excellent activity in silylative
coupling reactions have been developed. Reductive pretreatment of the catalysts in
the presence of 0.5 equiv triphenylphosphine under a hydrogen atmosphere en-
hanced and stabilized the catalytic activity. Of the catalysts examined, ceria-sup-
ported rhodium had the highest activity in the homo-coupling of vinylsilanes to
bis(silyl)ethenes at 170 ꢁC. A zirconia-supported catalyst selectively gave E-1-aryl-
2-silylethenes by cross-coupling of vinylsilanes with styrenes at 130 ꢁC, and a high
turnover frequency of [8200 h-1 was achieved at 170 ꢁC. Spectroscopic studies
revealed that well-dispersed surface rhodium(I) species predominantly formed on
ceria or zirconia were transformed into rhodium hydride species, which are believed
to be responsible for the high activity. These catalysts were recyclable without loss
of activity, and leaching of rhodium species from the catalysts was not observed.
Keywords Supported catalyst ꢀ Rhodium ꢀ Ceria ꢀ Zirconia ꢀ
Phosphine-modification ꢀ Vinylsilane ꢀ Styrenes ꢀ Silylative coupling
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-1983-8)
contains supplementary material, which is available to authorized users.
&
Kenji Wada
wadaken@med.kagawa-u.ac.jp
1
2
3
4
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Department of Chemistry for Medicine, Graduate School of Medicine, Kagawa University,
1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793, Japan
Department of Urban Environmental Sciences, Tokyo Metropolitan University,
1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
123

S. Tsukada et al.
Introduction
The development of environmentally-benign catalytic organic transformation with
high atom efficiency and high selectivity is extremely important in modern
chemistry [1, 2]. To achieve these objectives, transition metal complexes have been
widely used as homogeneous catalysts [3]. Silylative homo-coupling of alkenyl-
silanes and cross-coupling with other alkenes are the most efficient routes to silyl-
substituted alkenes, which are important feedstock for functional materials and
chemicals [4]. Several rhodium [5], ruthenium [6–9], iridium [10], cobalt [11], and
iron-complex catalysts [12] have been reported to be active in these reactions, but
these metal-complex catalysts have several inherent disadvantages, for example
high catalyst-production costs, difficulty recovering and recycling the catalysts after
the reaction, and contamination of the products by heavy metal species. From
practical and environmental perspectives, the development of simple solid catalysts
with activity comparable with or superior to that of homogeneous complex catalysts
is highly desired [13–16]. We recently developed Ru/CeO2-based catalysts with
excellent activity in a variety of C–C and C–N bond-forming reactions which
substantially broadened the scope of reactions for which solid oxide-supported
catalysts could be used [15, 16].
Herein, we report the development of the first recyclable oxide-supported
rhodium catalysts that are quite effective for silylative coupling reactions. A high
turnover frequency ([8200 h-1) was achieved in the reaction of vinylsilane with
styrene. Effects of reductive pretreatment in the presence of small amounts of
phosphines and the state of the active rhodium species on the surface of the catalysts
are discussed.
Experimental
Preparation of supported rhodium catalysts
Supported catalysts were prepared by the impregnation method from a variety of
supports and Rh(acac)3 (Aldrich; acac = acetylacetonato). In a typical procedure,
1.0 g metal oxide was added to a solution of 0.10 mmol (10 mg as Rh) Rh(acac)3 in
10 cm3 THF under air at room temperature. After impregnation and drying
overnight in air at 80 ꢁC, the resulting powder was calcined in air at 400 ꢁC for 30
min to afford the Rh/support catalyst. The loading level was fixed at 1.0 wt % as Rh
metal unless otherwise noted.
Modification of the catalysts by use of triphenylphosphine
For modification of the Rh/support catalysts by use of PPh3, 129 mg (0.013 mmol
as Rh) of Rh/support and PPh3 (1.6 mg, 0.0060 mmol) were heated in a 20 cm3
glass Schlenk tube at 100 ꢁC for 20 min under a hydrogen atmosphere (1 atm)
without solvent. The resulting catalysts were denoted xPPh3–Rh/support (x = molar
123

Phosphine-stabilized, oxide-supported rhodium catalysts…
ratio of PPh3 to Rh). These phosphine-modified Rh/supports were immediately used
for the reaction without exposure to open air.
General procedure for the silylative coupling reaction
of dimethylphenylvinylsilane
All of the reactions were performed by the use of hot stirrers equipped with cooling
blocks for reflux of the solvent. In a typical reaction procedure dimethylphenylvinyl-
silane (0.50 mmol) in mesitylene (1.0 cm3) was placed in a Schlenk tube containing
the phosphine-modified Rh(1.0 wt %)/support catalyst (0.013 mmol as Rh) under an
Ar atmosphere contained in a balloon. The reaction mixture was stirred at 170 ꢁC for
18 h, and then cooled rapidly in an ice bath. After the reaction, the solid catalyst was
removed by passing the mixture through a 0.45 lm PTFE filter, and the filtrate was
concentrated under reduced pressure. The products were isolated by column
chromatography (silica gel, 10:1 v/v hexane–EtOAc), identified by GC–MS, 1H and
13C NMR spectroscopy, and quantified by GLC analysis with naphthalene as internal
standard.
Further experimental details are available in supplementary material.
Results and discussion
Effects of supported rhodium catalysts for silylative homo-coupling
of vinylsilanes
Supported rhodium catalysts effective for silylative homo-coupling of
dimethylphenylvinylsilane (1a) have been developed (Eq. 1). Ceria-supported
catalysts (Rh/CeO2, 1.0 wt % as Rh) were prepared from Rh(acac)3 (acac = acety-
lacetonato) by a conventional impregnation method followed by calcination in air at
400 ꢁC for 30 min. The preliminary experiments have revealed that unmodified Rh/
CeO2 had activity in the homo-coupling of 1a at 170 ꢁC for 18 h, but independent
runs gave the products in very different yields (11–58 %). After further investi-
gation, we found that reductive pretreatment of Rh/CeO2 in the presence of PPh3 at
100 ꢁC for 20 min greatly improved the activity and the reproducibility of the
catalysis. The thus-pretreated catalysts were denoted xPPh3–Rh/support, where
x shows the molar ratio of PPh3 to rhodium species. We have reported that
PPh3–Ru/CeO2 catalysts modified in a similar manner had excellent activity in the
arylation [17] and alkylation [18] of aromatic C–H bonds. As shown in Table 1,
entry 2, reaction with 0.5PPh3–Rh/CeO2 at 170 ꢁC for 18 h gave a mixture of E-1,2-
bis(silyl)ethene (E-2a) and 1,1-bis(silyl)ethene (2a0) in 72 % yield (E-2a:2a0 =
70:30), with no sign of formation of Z-2a. The optimum amount of PPh3 for the Rh/
CeO2 catalyst was found to be 0.5 equiv. Treatment with larger or smaller amounts
of PPh3 reduced the catalytic activity. The rhodium catalysts were supported on a
variety of oxides; ceria-supported catalysts had the highest activity, and zirconia and
alumina-supported catalysts had moderate activity (Table 1, entries 4 and 5).
Magnesia, silica, and titania-supported catalysts were not effective (Table 1, entries
123

S. Tsukada et al.
6–8). Ceria-supported palladium and ruthenium catalysts, and phosphine-modified
rhodium oxide and Rh(acac)3, were not effective (Table 1, entries 9–12).
One of the most advantageous features of solid catalysts is their high
recyclability [14–16]. After coupling of 1a at 170 ꢁC for 18 h, the solid ceria-
supported rhodium catalyst was separated from the reaction mixture by centrifu-
gation and washed with diethyl ether. The resulting powder was calcined in air at
400 ꢁC for 30 min to recover Rh/CeO2, on which PPh3-modification in H2 was
performed. The thus-recovered 0.5PPh3–Rh/CeO2 had almost the same activity as
the fresh catalyst, indicative of the recyclability of this catalyst (Scheme 1).
Inductively coupled plasma atomic emission spectroscopy (ICP–AES) analysis
indicated that rhodium species were not leached into the filtrate after catalytic
reaction at 170 ꢁC for 18 h. It is important to investigate whether or not the reaction
actually occurs on the solid surface [19, 20]. As shown in Fig. 1, removal of the
solid catalyst by hot filtration after reaction for 2 h completely stopped the further
progress of the reaction. These results strongly suggest that these solid catalysts act
heterogeneously.
Supported rhodium catalysts for cross-coupling of vinylsilanes and styrenes
These supported rhodium catalysts also had excellent activity for cross-coupling of
vinylsilanes and styrenes to E-1-aryl-2-silylethenes and ethene. Table 2 and Eq. 2
Table 1 Effects of catalysts on silylative homo-coupling of 1a
a
Entry
Catalyst
Time (h)
Conv. (%)
Yield of E-2a ? 2a0 (%)a
E-2a:2a0
1
CeO2
18
18
3
0
94
47
36
35
11
12
13
6
0
–
2
0.5PPh3–Rh/CeO2
0.5PPh3–Rh/CeO2
0.5PPh3–Rh/ZrO2
0.5PPh3–Rh/Al2O3
0.5PPh3–Rh/MgO
0.5PPh3–Rh/SiO2
0.5PPh3–Rh/TiO2
0.5PPh3–Pd/CeO2
0.5PPh3–Ru/CeO2
0.5PPh3–Rh2O3
Rh(acac)3
72
70:30
68:32
65:35
64:36
60:40
62:38
62:38
–
3
41
4
3
28
5
3
22
6
3
2
7
3
4
8
3
9
9
18
18
18
18
0
10
11
12
a
41
3
Trace
Trace
Trace
–
–
2
–
Determined by GLC
123

Phosphine-stabilized, oxide-supported rhodium catalysts…
Rh/CeO2
3. reduced at 100 o
C
1. washed by Et2O
2. calcined in air
under H2 for 20 min
with PPh3
at 400 oC for 30 min
(0.5 equiv to Rh)
0.5PPh3-Rh/CeO2
(0.013 mmol as Rh)
mesitylene (1.0 cm3),
under Ar, 170 oC, 18 h
- CH2=CH2
SiMe2Ph
SiMe2Ph
+
PhMe2Si
PhMe2Si
PhMe2Si
2a'
1a
E-2a
(0.50 mmol)
Conv. of
1a
Yield of
E-2a + 2a'
1st use
2nd use
88%
84%
71%
68%
Scheme 1 Use of the Rh/CeO2 catalyst after recycling
Fig. 1 Time course of the
homo-coupling of 1a without
a and with b hot filtration 2 h
after initiation. Reaction
conditions 1a 0.50 mmol,
mesitylene 1.0 cm3,
0.5PPh3–Rh/CeO2 catalyst
0.013 mmol as Rh, at 170 ꢁC
under Ar, internal standard
(naphthalene, ca 50 mg)
100
80
60
40
20
0
(a)
(b)
Hot filtration of the catalyst
0
5
10
15
20
Time (h)
show effects of different 0.5PPh3–Ru/support catalysts on the reaction of 1a with
styrene 3a. Similar to catalysis by homogeneous rhodium complexes [21, 22], the
cross-coupling proceeds much more smoothly than homo-coupling of 1a (Table 1):
reaction with 0.5PPh3–Rh/ZrO2 at 140 ꢁC was complete within 3 h to give the
1
desired product E-4aa in 87 % yield (Table 2, entry 1). The H NMR spectrum of
the crude product showed that only the E-isomer was produced. The 0.5PPh3–Rh/
Al2O3 catalyst had moderate activity (Table 2, entry 2), and the yield of E-4aa by
use of the ceria-supported catalyst was low (Table 2, entry 3). Silica, titania, and
magnesia-supported catalysts did not have significant activity (Table 2, entries 4–6).
These catalysts were applicable in reactions of 1a with styrene derivatives, as
shown in Table 3. Styrenes with electron-donating and withdrawing substituents at
para or meta positions reacted smoothly to give the corresponding products in high
123

S. Tsukada et al.
Table 2 Effects of supported
rhodium catalysts on the
silylative cross-coupling of 1a
with 3a
Entry
Support
Conv. of 1a (%)a
Yield of E-4aa (%)a
1
2
3
4
5
6
ZrO2
Al2O3
CeO2
SiO2
97
68
20
1
87
57
12
1
TiO2
MgO
6
1
a
Determined by GLC on the
basis of 1a
17
1
Table 3 Scope of styrene derivativesa
95 % (1 h)
[99 % (1 h)
[99 % (1.5 h)
Trace (18 h)
96 % (40 min)
42 % (18 h)
Trace (48 h)
a
Reaction conditions: 1a 0.50 mmol, styrenes 1.5 mmol, mesitylene 1.0 cm3, catalyst 0.5PPh3–Rh/ZrO2,
0.013 mmol as Rh, at 170 ꢁC under Ar. GLC yields on the basis of 1a are shown
yields. In contrast, this catalytic system did not enable reaction of ortho-substituted
styrenes and aliphatic terminal alkenes, for example 1-dodecene.
From a practical perspective, it is very important the reaction can be performed
on a large scale without solvent. As shown in Eq. 3, reaction of 60 mmol 1a with
excess 3a proceeded very efficiently in the presence of a small amount of
0.5PPh3–Rh/ZrO2 and was complete within 1 h, affording E-4aa with a high TOF,
[8200 h-1, on the basis of the total amount of Rh in the catalyst.
123

Phosphine-stabilized, oxide-supported rhodium catalysts…
Characterization of supported rhodium catalysts
To understand effects of the supports and of phosphine modification on catalytic
activity, the rhodium catalysts were characterized by a variety of methods. Results
from a study of nitrogen gas-adsorption by the unmodified catalysts and their X-ray
powder diffraction (XRD) patterns are summarized in Table S1 and Fig. S1 of the
supporting information (ESM). There was no significant correlation between the
surface areas of the supports and the catalytic activity of the supported rhodium
catalysts listed in Table 1, entries 3–8.
The reducibility of surface rhodium species supported on different metal oxides
was evaluated by a study of H2-temperature-programmed reduction (TPR). The
results, shown in Fig. S2 of the ESM, indicate that most of rhodium species on these
catalysts, the exception being Rh/MgO, are reduced below 150 ꢁC; the rhodium
species on MgO are hardly reducible and require temperatures higher than 250 ꢁC.
The effects of supports and phosphine modification on the state of surface rhodium
species were investigated by X-ray photoelectron spectroscopy (XPS). The surface
compositions of the catalysts before and after the phosphine modification are
summarized in Table S2 in the ESM. The surface composition of the rhodium species
on unmodified catalysts markedly depended on the support and decreased in the order:
Rh=TiO2 [ Rh=CeO2 ꢀ Rh=ZrO2 [ Rh=Al2O3 ꢀ Rh=MgO [ Rh=SiO2. As
a
whole, the catalytic activity depended on the surface rhodium compositions except
for Rh/TiO2: the lower activity of the titania-supported catalyst could be because of
the predominant formation of rhodium(0), not rhodium(I), during phosphine
modification (see below). After phosphine modification, the presence of a small
amount of phosphorus species was confirmed for all the catalysts. These catalysts had
P 2p peaks at 132–133 eV. The P 2p binding energy of PPh3 has been reported to be
131 eV [23], suggesting electron donation from P to the metallic species. We also
examined the solution phase by means of 31P NMR after the reaction and removal of
the solid catalyst, but phosphorus species were not detected. These results suggest that
PPh3 is adsorbed on the surface of these catalysts.
Figure 2 shows Rh 3d XP spectra of a variety of supported catalysts before and after
phosphine modification. The Rh 3d5/2 binding energies reported for Rh2O3 (308.8 eV)
and for metallic Rh (307.2 eV) [23] are marked in the figure for comparison. For all the
catalysts before phosphine modification Rh 3d5/2 peaks at 308.8–309.5 eV were
observed, indicative of the presence of high-valent rhodium species on the surface.
PPh3 modification shifted these peaks toward lower binding energy, however. The Rh
3d5/2 peaks of the 0.5PPh3–Rh/TiO2 and 0.5PPh3–Rh/SiO2 catalysts were at 307.5 eV,
indicating that most of the rhodium species on these catalysts are zero-valent.
Rhodium species on magnesia were found to be marginally reduced, however, which
is consistent with the conclusion from the H2-TPR study reported above. Very active
catalysts in these reactions, i.e. 0.5PPh3–Rh/CeO2 and 0.5PPh3–Rh/ZrO2, had the Rh
3d5/2 peaks between those of Rh2O3 and metallic Rh. Also, a shoulder appeared in this
region in the spectrum of 0.5PPh3–Rh/Al2O3. Several rhodium(I) complexes have
been reported to be effective for silylative coupling reactions [5, 21, 22]. Therefore, we
deduce that, similarly to homogeneous rhodium complex catalysts, surface rhodi-
um(I) species could be responsible for the excellent catalytic activity, and phosphine
123

S. Tsukada et al.
(a)
(b)
Rh2O3 Rh Metal
Rh2O3 Rh Metal
Rh/TiO2
Rh/SiO2
Rh/TiO2
Rh/SiO2
Rh/MgO
Rh/CeO2
Rh/MgO
Rh/CeO2
Rh/ZrO2
Rh/Al2O3
Rh/ZrO2
Rh/Al2O3
Binding Energy(eV)
Binding Energy(eV)
Fig. 2 Rh 3d XP spectra of Rh/support a before and b after phosphine modification
Fig. 3 TEM image of 0.5PPh3–
Rh/ZrO2
modification would promote the formation and stabilization of such rhodi-
um(I) species. The amount of substrate-accessible rhodium(I) species exposed on
the surface would also have important effect on catalytic activity.
123

Phosphine-stabilized, oxide-supported rhodium catalysts…
The transmission electron microscope (TEM) image of 0.5PPh3–Rh/ZrO2 shown
in Fig. 3 reveals the formation of very fine rhodium particles ca 1 nm in diameter.
Formation of small rhodium particles could be one reason for the excellent catalytic
activity.
Possible reaction mechanism
For transition metal-catalyzed silylative coupling reactions, a mechanism involving
the formation of silylrhodium species (Scheme 2) has been generally accepted [4,
21]. For non-hydride rhodium or other transition metal complexes it has been
proposed that hydride species are generated by oxidative addition of vinyl C–H
bonds of substrates [5] or by orthometallation on PPh3-coordinated metallic species
[6]. After insertion of vinylsilane in an Rh–H bond, Si transfer to rhodium with
elimination of ethene would occur to generate silylrhodium species. Insertion of
vinylsilane or styrene in an Rh–Si bond and subsequent b-hydrogen elimination
would give the coupled products and Rh–H species.
To investigate the likelihood of these mechanisms, effects of successive addition
of substrates on catalytic activity at a relatively low temperature, 140 ꢁC, were
examined. Scheme 3 shows the experimental procedures, and Fig. 4 shows time
courses of reactions using 0.5PPh3–Rh/ZrO2 as catalyst. When substrates 1a and 3a
were both added at the same time (Exp. A), an induction period of ca 3 h was
observed, which indicates that the catalytically active species are not produced
solely by phosphine modification. Addition of 3a after heating the catalyst at 140 ꢁC
for 2 h in the presence of 1a (Exp. B) significantly shortened the induction period,
Scheme 2 Possible reaction
mechanism
RhI
R'-H
(R' = PhCH=CH-,
PhMe2SiCH=CH-,
3,5-Me2C6H3CH2-, etc)
SiMe2Ph
SiMe2Ph
H
SiMe2Ph
Ph
RhIII R'
SiMe2Ph
Ph
RhIII
H
R'
RhIII
R'
SiMe2Ph
Ph
SiMe2Ph
R'
RhIII
RhIII R'
SiMe2Ph
RhIII
R'
Ph
CH2=CH2
123

S. Tsukada et al.
Exp. A
+ 1a (1.0 mmol), 3a (3.0 mmol),
mesitylene (2.0 cm3)
under Ar, 140 oC
+ 1a (1.0 mmol),
mesitylene (2.0 cm3)
under Ar, 140 oC, 2 h
+ 3a (3.0 mmol),
under Ar, 140 oC
Exp. B
0.5PPh3-Rh/ZrO2
+ 3a (3.0 mmol),
mesitylene (2.0 cm3)
(0.013 mmol as Rh)
+ 1a (1.0 mmol),
under Ar, 140 oC
Exp. C
under Ar, 140 oC, 2 h
+ 1a (1.0 mmol),
+ 3a (3.0 mmol)
under Ar, 140 oC
mesitylene (2.0 cm3)
under Ar, 140 oC, 2 h
Exp. D
Scheme 3 Experiments on successive addition of the substrates
100
80
60
40
20
0
Exp. D
Exp. A
Exp. B
Exp. C
0
5
10
15
20
25
30
Time (h)
Fig. 4 Time courses of the reactions after successive addition of the substrates in the presence of an
internal standard (naphthalene, ca 50 mg)
which strongly suggests that 1a contributes to generation of the catalytically active
species. Pre-treatment of the catalyst with styrene, however, greatly reduced the
catalytic activity (Exp. C), indicating that initial coordination of styrene to the
rhodium species substantially hampers the catalysis. Interestingly, heating of
the catalyst in mesitylene substantially reduced the induction period and increased
the yield of the coupled products (Exp. D), and similar results were obtained by the
heating of the catalyst in p-xylene at 140 ꢁC (Fig. S3 in the ESM). These results
indicate that these solvents and vinylsilane are important in the formation of
catalytically active species.
The catalyst preheated in p-xylene was analyzed by diffuse reflectance infrared
Fourier transform (DRIFT) study. As shown in Fig. S4 in the ESM, a band at
approximately 2000 cm-1, attributed to tRh–H [24], was observed when the
0.5PPh3–Rh/ZrO2 catalyst was heated in p-xylene at 140 ꢁC for 30 min; this band
was not observed when the catalyst treated in p-xylene-d10. Activation of benzylic
123

Phosphine-stabilized, oxide-supported rhodium catalysts…
C–H bonds has been reported to occur on the transition metal surface under mild
conditions [25]. Furthermore, catalytic homocoupling of 1a in p-xylene-d10 gave
deuterated products E-2a-d1 and 2a0-d1 and deuterated substrates 1a-d1 and 1a-d2.
After reaction at 160 ꢁC for 1 h, the 1a-to-1a-d1-to-1a-d2, E-2a-to-E-2a-d1, and 2a0-
to-2a0-d1 ratios were 73:21:3, 81:19, and 83:16, respectively. These results are
consistent with the idea that p-xylene acts as a hydride source as a result of C–H
bond cleavage, as shown in Scheme 2.
Conclusion
Simple modification of oxide-supported rhodium catalysts under H2 in the presence
of a small amount of triphenylphosphine enhanced and stabilized catalytic activity
in the homo-coupling of vinylsilanes and the cross-coupling of vinylsilanes with
styrenes. For homo-coupling of vinylsilanes, the ceria-supported catalyst was most
active. The zirconia-supported catalyst was quite effective for cross-coupling of
vinylsilanes with styrenes to give E-1-aryl-2-silylethenes, and an extremely high
turnover frequency exceeding 8200 h-1 was achieved. Hot filtration tests indicated
that these catalysts act heterogeneously, and ICP–AES analysis revealed that
rhodium species were not leached into the solution. Spectroscopic analysis of the
catalysts suggested that surface rhodium(I) species are crucial to the excellent
catalytic activity and that the reactions are initiated by surface rhodium hydride
species. Solvents such as mesitylene or p-xylene are believed to act as a hydride
sources. Because of these characteristic features, these rhodium catalysts are quite
attractive synthetically, industrially, and environmentally.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (no. 24360333)
from the Ministry of Education, Sports, Culture, Science and Technology, Japan and the JST ALCA
program.
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