Dendrimer N-heterocyclic carbene complexes with rhodium(I) at the core

Article abstract of DOI:10.1039/b506927k

Novel dendrimer N-heterocyclic carbene complexes with rhodium(I) located at the core were synthesized, and a positive dendrimer effect was found in the hydrosilylation of ketones catalyzed by them. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506927k

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Dendrimer N-heterocyclic carbene complexes with rhodium(I) at
the core{
Tetsuaki Fujihara, Yasushi Obora, Makoto Tokunaga, Hiromichi Sato and Yasushi Tsuji*
Received (in Cambridge, UK) 17th May 2005, Accepted 27th July 2005
First published as an Advance Article on the web 19th August 2005
DOI: 10.1039/b506927k
8
and 3) (eqn. (1)) and the new compounds (1b–d) fully
Novel dendrimer N-heterocyclic carbene complexes with
rhodium(I) located at the core were synthesized, and a positive
dendrimer effect was found in the hydrosilylation of ketones
catalyzed by them.
characterized. The rhodium(I) complexes with dendrimer NHC
1
1
ligands: RhCl(COD)[(G
)] (2b), RhCl(COD)[(G
RhCl(COD)[(G (C )] (2d) (COD 5 1,5-cyclooctadiene)
were synthesized in good yields (74, 72, 73 and 71% respectively)
)
(C
H
N
)] (2a), RhCl(COD)[(G
) -
1 2
0
2
3
2
2
(
C
3
H
2
N
2
2
)
2
(C )] (2c) and
3 2 2
H N
3
)
2
3 2 2
H N
Dendrimers are intriguing molecules owing to the distinct physical
and chemical properties caused by their well-defined hyper-
1
branched frameworks. In particular, the development of organo-
12
by the carbene-transfer method using the NHC–Ag species
derived from 1a–d (eqn. (2)). The newly isolated complexes (2b-d)
were fully characterized, their FD-MS spectra showing molecular
metallic dendrimers is imperative, since the dendritic moieties will
bring about unique catalytic environments to realize novel catalytic
performance. Thus, a number of organometallic dendrimers with
13
1
ion peaks at m/z 5 918, 1762 and 3466 respectively. In the C{ H}
NMR spectra of 2b-d, the coordinated carbene carbons appeared
as sharp doublets at 183.3 ppm (JRh–C 5 50.8 Hz), 183.4 ppm
2
catalytic sites at either their core or periphery have been reported.
Among them however, examples of the positive dendrimer effect,
in which the dendrimers play active roles in enhancing catalytic
(
50.7 Hz) and 183.3 ppm (47.6 Hz) respectively. 2a–d were also
1
highly stable both to air and moisture in the solid state. The H
NMR spectra of 2a–d measured in CDCl did not change over two
weeks, indicating they are also considerably stable in solution.
3
ability, have been limited. We have recently reported dendrimer
3
4
5
ligands with a pyridine or phosphine functionality at their core,
and found that palladium acetates with the dendrimer pyridine
ligands successfully suppress Pd black formation in the air
4
oxidation of alcohols.
N-Heterocyclic carbene (NHC) ligands have attracted consider-
able attention in both homogeneous catalysis and organometallic
6
chemistry. Stronger bonds to the metals associated with NHC
ligands, compared to conventional ligands such as phosphines,
diminish their dissociation from metal centers. As for dendrimer
7
NHC complexes, there is the only one precedent, a complex
with four ruthenium metal atoms located at the periphery of a
low generation dendrimer. The complex worked as a recyclable
metathesis catalyst, but the dendrimer effect for different
generations was not explored. In the present study, we synthesized
a series of dendrimer (zeroth to the third generations: G –G )
ð1Þ
0
3
NHC complexes with rhodium(I) at the core, and employed
the complexes as catalysts for the hydrosilylation of ketones. In the
catalytic reaction, the evident positive dendrimer effect using
the dendrimer NHC complexes was observed for the first time; the
yields of the products increased with increasing dendrimer
generation.
A series of imidazolium salts bearing Fr e´ chet-type polybenzyl
8
9
ether dendrimers (G
0 3 0 2 3 3 2 1 2
–G ): (G ) (C H N )Br (1a), (G ) -
(
C H N )Br (1b), (G ) (C H N )Br (1c) and (G ) (C H N )Br
3
3
2
2 2
0
3
3
2
3 2
3
3
2
1
(1d) were synthesized by the reaction of imidazole with 2 equiv.
of the corresponding dendrimer bromides G –Br (n 5 0, 1, 2
n
(
2)
Catalysis Research Center and Division of Chemistry, Graduate School
of Science, Hokkaido University, CREST, Japan Science and
Technology Agency (JST), Sapporo, 001-0021, Japan.
E-mail: tsuji@cat.hokudai.ac.jp; Fax: +81-11-706-9156
{
Electronic supplementary information (ESI) available: Synthetic
methods and analytical data for 1b–1d and 2b–2d. See http://dx.doi.org/
0.1039/b506927k
1
4
526 | Chem. Commun., 2005, 4526–4528
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Fig. 1 ORTEP drawing of 2b with thermal ellipsoids drawn at a 50%
probability level.
The molecular structure of 2b has been successfully determined
by X-ray structural analysis (Fig. 1).{ The rhodium complex is a
Fig. 3 Hydrosilylation of ketones with H SiPh
2
2
catalyzed by 2. (a)
Yields of 1-phenylethanol from actophenone after 20 h: 15% with 2e as
the catalyst, 11% with 2f, 62% with 2a, 72% with 2b, 84% with 2c and
92% with 2d. (b) Yields of cyclohexanol from cyclohexanone after 3h: 21%
with 2e as the catalyst, 14% with 2f, 66% with 2a, 84% with 2b, 90% with
2c and 99% with 2d.
2.2 nm-sized molecule and has a distorted square-planar coordina-
tion geometry. The Rh–C bond length of 2b (2.042(4) s) is similar
to those of other Rh–NHC complexes.
13
The electrochemically active rhodium cores of 2a–d could be
sensitive to an electrode, depending on the dendrimer moiety
(
G –G ). Therefore, cyclic voltammograms (CV) of these com-
3
Catalytic performance as well as the dendrimer effect of
11,15
2a–d were examined by the hydrosilylation of ketones.
0
plexes were measured in CH Cl with n-Bu NClO as a
2
2
4
4
14
13a
RhCl(COD)[Me (C H N )]
supporting electrolyte. The G0 complex (2a) exhibited the
2
3
2
2
(2e) and RhCl(COD)[(Mes)2-
16
irreversible oxidation process that can be assigned to a metal-
(
C H N )] (2f) (see eqn. (2)) were also employed as catalysts
3
2
2
centered oxidation peak (Fig. 2(a)). The G and G complexes
1
2
for comparison. The reaction was carried out in the presence of a
catalytic amount (0.5 mol%) of 2 and diphenylsilane (1.2 equiv.) in
THF.§ In the hydrosilylation of acetophenone, 2e and 2f showed
only low catalytic activities; yields of 1-phenylethanol after 20 h
were 15% and 11% respectively (Fig. 3(a)). The catalytic activity
(
2b and 2c respectively) also showed similar irreversible oxidation
peaks (Fig. 2(b) and 2(c)), although the peak height of 2c decreased
noticeably compared with 2b. Finally, the G complex (2d) did
3
not exhibit a distinguishable oxidation peak (Fig. 2(d)). These
different electrochemical responses suggest that electron transfer
from the Rh core to the electrode was hampered by the large
dendritic structures.
0 3
was enhanced by introducing G –G substituents onto the NHC
ligand; the product was obtained in 62, 72, 84 and 92% yields
with 2a–d as the catalyst respectively. It is noteworthy that the
yields gradually increased with increasing dendrimer generation.
This positive dendrimer effect was also found in benzene and
dichloromethane solvents. The same behavior was observed in the
reaction of cyclohexanone (Fig. 3(b)). The rhodium–NHC
complexes 2e and 2f afforded cyclohexanol after 3 h in only
21 and 14% yields respectively, whereas 2a–d provided the product
in 66, 84, 90 and 99% yields respectively. Again, the yield of the
product increased with increasing dendrimer generation. Aromatic
rings of G –G might interact with the rhodium metal and cause
0
3
the positive dendrimer effect, as suggested by the lowest energy
18
1
7
conformation of 1d calculated by CONFLEX5 /MMFF94s
Fig. 4). Interestingly, the dendrimer effect was influenced by the
(
concentration of the catalytic reaction. When the hydrosilylation
of acetophenone was carried out under more dilute conditions
2
3
23
(
0.29 mol dm , cf. 0.41 mol dm in Fig. 3), the yield of
-phenylethanol decreased considerably to 44% after 46 h (cf. 62%
after 20 h in Fig. 3) with the lower generation (G ) catalyst (2a),
1
0
1 3
while with the higher generation (G –G ) catalysts (2b–2d), the
product yields (70, 83 and 89% yields after 46 h respectively) were
almost comparable to those in Fig. 3 (72, 84 and 92% respectively).
In conclusion, a series of novel dendrimer NHC complexes
with rhodium(I) located at the core (2a–d) were synthesized and
Fig. 2 Cyclic voltammograms of (a) 2a, (b) 2b, (c) 2c and (d) 2d in
23
23
23
2 2 4 4
CH Cl (1 6 10 mol dm ) containing n-Bu NClO (0.1 mol dm ) at
2
1
varying scan rate (50, 100, 200, 300, 400 and 500 mV s ).
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 4526–4528 | 4527

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2
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5
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CONFLEX5/MMFF94s. A white circle indicates the C ring.
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3 3 2
H N
7
characterized. The positive dendrimer effect was evident in the
hydrosilylation reactions of ketones they catalyzed. The yields of
the corresponding alcohols increased with increasing dendrimer
generation. Further studies on the mechanism of the dendrimer
effect and applications to other catalytic reactions are currently
under way.
8 C. J. Hawker and J. M. J. Fr e´ chet, J. Am. Chem. Soc., 1990, 112,
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9
1
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1
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Notes and references
{
C
Crystal data for RhCl(COD)[(G
1 2 3 2 2 2 2 2 2
) (C H N )]?CH Cl (2b?CH Cl ):
¯
H54Cl Rh, M 5 1004.30, T 5 113 K, triclinic, space group P1
54
3 2 4
N O
(
no. 2), a 5 10.936(8), b 5 12.458(9), c 5 17.56(1) s, a 5 98.94(2),
3
b 5 101.480(10), c 5 91.966(11) u, U 5 2310(3) s , Z 5 2, m(Mo-Ka) 5
21
5.92 cm , observed reflections 12898 (I . 3s(I)), R1 5 0.062, wR2 5 0.159.
GOF 5 1.013. CCDC 269270. See http://dx.doi.org/10.1039/b506927k for
crystallographic data in CIF or other electronic format.
§
was placed in a 10 cm Schlenk tube under an Ar atmosphere. Anhydrous
General procedure for hydrosilylation: A rhodium catalyst 2 (0.005 mmol)
3
3
3
THF (2.0 cm , or 3.0 cm for the diluted conditions), acetophenone
1 mmol) and tridecane as an internal standard (0.25 mmol) were added by
syringe under an Ar flow. After the reaction mixture had been stirred for
min, diphenylsilane (1.2 mmol) was added via a syringe. The reaction
1
5 (a) W. A. Herrmann, L. J. Goossen, C. K o¨ cher and G. R. J. Artus,
(
Angew. Chem., Int. Ed. Engl., 1996, 35, 2805; (b) W-.L. Duan, M. Shi
and G.-B. Rong, Chem. Commun., 2003, 2916; (c) O. Niyomura,
M. Tokunaga, Y. Obora, T. Iwasawa and Y. Tsuji, Angew. Chem., Int.
Ed., 2003, 42, 1287.
5
mixture was stirred at room temperature (22 ¡ 1 uC) for 20 h (3 h in the
case of cyclohexanone). The desilylation was performed with HCl/MeOH
1
6 (a) P. A. Evans, E. W. Baum, A. N. Fazal and M. Pink, Chem.
Commun., 2005, 63; (b) A. M. Seayad, K. Selvakumar, M. Ahmed and
M. Bellar, Tetrahedron Lett., 2003, 44, 1679.
2
3
3
2 mol dm , 1 cm ) and the mixture stirred for 1 h. After neutralization
(
with NaHCO , the reaction mixture was passed through a short silica gel
3
column. The yield of the product was determined by GC using the internal
standard method.
1
7 CONFLEX5: (a) H. Goto and E. Osawa, J. Am. Chem. Soc., 1989, 111,
950; (b) H. Goto and E. Osawa, J. Chem. Soc., Perkin Trans. 2, 1993,
87.
8
1
1
(a) G. R. Newkome, C. N. Moorefield and F. V o¨ gtle, Dendrimers and
Dendrons: Concepts, Syntheses, Applications, Wiley-VCH, Weinheim,
18 MMFF94s: (a) T. A. Halgren, J. Comput. Chem., 1999, 20, 720; (b)
T. A. Halgren, J. Comput. Chem., 1996, 17, 490.
4
528 | Chem. Commun., 2005, 4526–4528
This journal is ß The Royal Society of Chemistry 2005