Rhodium(I) complexes with N-heterocyclic carbenes bearing a 2,3,4,5-tetraphenylphenyl and its higher dendritic frameworks

Article abstract of DOI:10.1039/b612385f

Novel rhodium(i) complexes with N-heterocyclic carbenes bearing a 2,3,4,5-tetraphenylphenyl and its higher dendritic frameworks were synthesized and the complexes were used as catalysts in rhodium-catalyzed hydrosilylation of α,β-unsaturated ketones to show high 1,4-adduct selectivity. The Royal Society of Chemistry.

Full text of DOI:10.1039/b612385f

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COMMUNICATION
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Rhodium(I) complexes with N-heterocyclic carbenes bearing a 2,3,4,5-
tetraphenylphenyl and its higher dendritic frameworks{
Hiromichi Sato, Tetsuaki Fujihara, Yasushi Obora, Makoto Tokunaga, Junya Kiyosu and Yasushi Tsuji*
Received (in Cambridge, UK) 30th August 2006, Accepted 29th September 2006
First published as an Advance Article on the web 25th October 2006
DOI: 10.1039/b612385f
In this communication, we have selected an N-heterocyclic
carbene (NHC) as a ligand. A use of NHC would be beneficial
because stronger bonds to metals as compared with conventional
ligands such as phosphines diminish their dissociation from metal
centers.7 Here, we designed and synthesized novel rhodium(I)
complexes with NHC ligands bearing a TPPh and its higher
dendritic moieties. These complexes are active catalysts and
provide 1,4-adducts predominantly in the hydrosilylation of
a,b-unsaturated ketones with Ph2SiH2, which has opposite
regioselectivity to the one obtained with conventional Rh-
phosphine and Rh-NHC catalyst systems.
Novel rhodium(I) complexes with N-heterocyclic carbenes
bearing a 2,3,4,5-tetraphenylphenyl and its higher dendritic
frameworks were synthesized and the complexes were used as
catalysts in rhodium-catalyzed hydrosilylation of a,b-unsatu-
rated ketones to show high 1,4-adduct selectivity.
Dendritic frameworks have received considerable attention to
develop new materials1 including catalysts.2 Among them, a
2,3,4,5-tetraphenylphenyl (TPPh) moiety developed by Mu¨llen and
co-workers have been utilized as building blocks for polyphenylene
nanomaterials.3 We are interested in their spatially spread and
rigid structures. Palladium acetates with pyridine ligands having a
TPPh moiety (1a) and its higher dendritic framework (1b)
successfully suppressed Pd black formation in the air-oxidation
of alcohols (Scheme 1).4 The diamine (2) with TPPh groups was
effectively employed as a ligand in a palladium-catalyzed kinetic
resolution of various biaryl compounds.5 Recently, we found that
the phosphines (3) having a TPPh moiety are effective ligands to
utilize unactivated aryl chlorides in palladium-catalyzed Suzuki–
Miyaura and Mizoroki–Heck reactions.6
Imidazolium salts (4a and 4b) having the TPPh and its higher
dendritc frameworks were synthesized in three steps via the
amination with benzophenone imine5,8,9 followed by the diimine
formation10 (Scheme 2). The dendritic moieties of 4b and 5b have
Scheme 1 Ligands bearing a 2,3,4,5-tetraphenylphenyl (TPPh) moiety.
Department of Energy and Hydrocarbon Chemistry, Graduate School of
Engineering, Kyoto University, and CREST, Japan Science and
Technology Agency (JST), Kyoto 615-8510, Japan and Catalysis
Research Center and Division of Chemistry, Graduate School of Science,
Hokkaido University, Sapporo, 001-0021, Japan.
E-mail: ytsuji@scl.kyoto-u.ac.jp; Fax: +81-75-383-2514
{ Electronic supplementary information (ESI) available: Experimental
procedures and data for characterization of 4 and 5. See DOI: 10.1039/
b612385f
Scheme 2 Synthesis of NHC rhodium(I) complexes.
Chem. Commun., 2007, 269–271 | 269
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Table 1 Hydrosilylation of a,b-unsaturated ketones catalyzed by 5a
the methyl substituents to ensure good solubility of these
materials.4 The MALDI-TOF-MS spectra of 4a and 4b showed
peaks at m/z = 981 ([M-Cl]+) and 2782 ([M-Cl]+), respectively.
Rhodium(I) complexes with the corresponding NHC ligands (5a
and 5b) were synthesized in good yields (69 and 70%, respectively)
by the carbene-transfer method11 using NHC–Ag species derived
from 4a and 4b (Scheme 2). The new complexes (5a and 5b) were
fully characterized, and their mass spectra showed peaks at m/z =
1227 (FD-MS: [M]+) and 2992 (MALDI-TOF-MS: [M-Cl]+),
respectively. In 13C{1H} NMR spectra of 5a and 5b, the
coordinated carbene carbons appeared with distinct 13C–103Rh
couplings at 184.4 ppm (1JRh–C = 48.7 Hz) and 184.3 ppm
(1JRh–C = 47.7 Hz), respectively, indicating their tight coordination
to the rhodium atoms in solution.
Entry Substrate
Catalyst Yieldb/% Selectivityb (1,4-/1,2-)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
5a
5b
5c
5a
5b
5c
5a
5b
5c
5a
5b
5c
5a
5b
5c
77
91
92
67
62
88
69
76
53
96
46
43
38
51
72
91
88
8
82
81
9
93
100
60
94
50
12
16
4
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
9
12
92
18
19
91
7
0
40
6
50
88
84
96
100
The molecular structure of 5a has been successfully determined
by X-ray crystal structural analysis (Fig. 1a).{ It is evident that the
TPPh moiety of the NHC ligand spatially spreads out. The
structure of 5a shows that the longest distance between the edge of
the two TPPh groups is 2.9 nm. The Rh–C(carbene) bond length
0
˚
of 5a (2.010(5) A) is similar to those of other Rh(I)–NHC
a
a,b-Unsaturated ketone (1 mmol), Ph2SiH2 (1.2 mmol),
5
complexes.12,13b As for 5b, all the trials to obtain single crystals
suitable for crystallographic analysis were unsuccessful. Therefore,
the optimized structure of 5b (Fig. 1b) were obtained by ONIOM
calculation14 (B3LYP/LANL2DZ:UFF) using an initial structure
based on the X-ray structure of 5a. As a result, 5b is a bigger nano-
sized complex with the longest distance between the edge of the
two dendritic frameworks being 4.9 nm.
(0.01 mmol), CH2Cl2 (1 cm3), rt, 24 h. By GC analysis after the
desilylation with K2CO3–MeOH.
b
catalyst afforded the 1,4-adduct (cyclohexanone) in 91% selectivity
with a smaller amount of the 1,2-adduct (2-cyclohexen-1-ol) in
77% total yield after the desilylation (entry 1). The complex 5b as a
catalyst also afforded the 1,4-adduct in 88% selectivity (entry 2).
Noteworthy is that these 1,4-selectivities obtained with Ph2SiH2 in
entries 1 and 2 were totally opposite to one obtained with
RhH(PPh3)4 as a catalyst,16b in which only the 1,2-adduct was
afforded exclusively. Furthermore, in contrast to 5a and 5b, 5c as a
catalyst afforded the 1,2-adduct as a major product in 92%
selectivity (entry 3). Thus, the TPPh moieties of 5a and 5b play a
critical role to reverse the typical regioselectivity associated with
the dihydrosilane. Similar effect of the TPPh to reverse the
regioselectivity was confirmed in the hydrosilylation of 3-octen-2-
one: 5a and 5b afforded the 1,4-adduct (2-octanone) in 82 and 81%
selectivities, respectively (entries 4 and 5), while 5c provided the
1,2-adduct (3-octen-2-ol) in 91% selectivity (entry 6). In the
hydrosilylation of benzylideneacetone, 5a and 5b provided the 1,4-
adduct (4-phenyl-2-butanone) in 93 and 100% selectivities,
respectively (entries 7 and 8), whereas selectivity of the 1,2-adduct
(4-phenyl-3-buten-2-ol) increased considerably by using 5c as a
catalyst (entry 9). This selectivity with 5c was comparable to that
obtained with RhCl(PPh3)3 as the catalyst.16a With 4-methyl-3-
penten-2-one having the two substituents at the b-position, 5a
realized the good 1,4-selectivity (entry 10), while 5b lowered both
the regioselectivity and yield of the products (entry 11). The steric
bulk associated with the higher dendritic framework of 5b might
affect the selectivity and catalytic activity. In the hydrosilylation of
3-methyl-2-cyclohexen-1-one, even 5a and 5b as well as 5c all
provided the high 1,2-selectivities (84–100%), possibly due to
significant steric congestion at the b-position (entries 13–15).
Recently, we have synthesized analogous Rh(I) NHC complexes
(6a–d in Scheme 3)17 with very flexible dendritic frameworks
consisting of the Fre´chet-type benzyl ether moieties.18 We are
interested how rigid4–6 and flexible17–19 dendritic frameworks
As shown in Fig. 1, the complexes 5a and 5b have the very
unique rigid and spatially spread structures. Thus, catalytic
behavior of 5a and 5b was intriguing and compared with the
representative Rh(I) NHC complex, RhCl(COD)[(Mes)2(C3H2N2)]
(5c) (Scheme 2).13 The hydrosilylation15 of a,b-unsaturated ketones
with Ph2SiH2 was carried out in the presence of a catalytic amount
(1 mol%) of 5 in CH2Cl2 at room temperature (Table 1).§ It is well-
established that regioselectivity in the Rh-catalyzed hydrosilylation
of a,b-unsaturated ketones was preferentially determined by the
nature of silanes employed:16 use of di- or tri-hydrosilanes such as
Ph2SiH2 or PhSiH3 lead to 1,2-adducts giving allylic alcohols after
the desilylation, whereas mono-hydrosilanes such as Et3SiH or
Me2PhSiH afford 1,4-adducts giving ketones. The hydrosilylation
of 2-cyclohexen-1-one with Ph2SiH2 in the presence of 5a as a
Fig. 1 Molecular structures of 5. (a) Crystal structure of 5a. (b) The
optimized structure of 5b calculated by ONIOM method.
270 | Chem. Commun., 2007, 269–271
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9.7 6 1022 dm3), and bibenzyl (0.25 mmol, 45.6 mg) as an internal
standard was placed in a 10-cm3 Schlenk tube under an Ar atmosphere and
the resulting solution was stirred for 5 min. Then, Ph2SiH2 (1.2 mmol,
0.22 dm3) was added via a syringe and the reaction mixture was stirred at
room temperature for 24 h. After removal of volatiles under vacuum at
230 uC, a residue was dissolved in Et2O (1 cm3). The solution was further
stirred for 1 h with K2CO3 (1 mg) and MeOH (1 cm3). Total yield (77%)
and a ratio (91/9) of cyclohexanone and 2-cyclohexen-1-ol were determined
by a GC (Shimadzu CPB10 column, 25 m length, 0.25 mm id).
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Scheme 3 Hydrosilylation of 2-cyclohexen-1-one catalyzed by flexible
dendrimer complexes 6.
operate in the same hydrosilylation of a,b-unsaturated ketones. In
the reaction of 2-cyclohexen-1-one, 6a–d with a series of dendritic
moieties of different generations all afforded the 1,2-adduct mainly
in similar selectivities (61–69%, Scheme 3). In contrast to 5a and 5b
having the rigid moieties, 6a–d could not change the regio-
selectivity inherent to Ph2SiH2. These flexible dendrimer moieties
tend to fold back around an active site.17 The rigid and spatially
spread structures of 5a and 5b might be crucial for causing the 1,4-
regioselectivity.
In conclusion, novel rhodium(I) NHC complexes having a
TPPh and its higher dendritic frameworks were synthesized and
fully characterized. These complexes are efficient catalysts in the
hydrosilylation of a,b-unsaturated ketones with Ph2SiH2which has
the opposite regioselectivity to the one obtained with conventional
Rh-phosphine or Rh–NHC catalysts. Further studies on transi-
tion-metal catalysts bearing rigid and flexible dendritic frameworks
are under investigation.
Notes and references
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18 C. J. Hawker and J. M. Fre´chet, J. Am. Chem. Soc., 1990, 112,
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{ Single crystals of 5a?C5H12 suitable for X-ray diffraction study were
obtained by diffusion of n-pentane into 5a in CH2Cl2. Crystal data for
5a?C5H12: C88H76ClN2Rh, M = 1299.9, T = 113 K, triclinic, space group
¯
˚
P1 (No. 2), a = 14.85(2), b = 20.51(2), c = 25.04(2) A, a = 79.38(7), b =
˚
3
21
,
71.05(9), c = 81.10(10)u, U = 7054(1) A , Z = 4, m(Mo Ka) = 3.30 cm
Unique reflections 31888, Observed reflections 16076 (I . 3s(I)), R1, wR2 =
0.056, 0.164. GOF = 1.01. CCDC 617650. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b612385f
§ General procedure for the hydrosilylation (entry 1 in Table 1): A mixture
of 5a (0.01 mmol, 12 mg), CH2Cl2 (1 cm3), 2-cyclohexen-1-one (1 mmol,
19 B. S. Balaji, Y. Obora, D. Ohara, S. Koide and Y. Tsuji,
Organometallics, 2001, 20, 5342.
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Chem. Commun., 2007, 269–271 | 271