Synthesis of new NHC-rhodium and iridium complexes derived from 2,2′-diaminobiphenyl and their catalytic activities toward hydrosilylation of ketones

Article abstract of DOI:10.1016/j.tet.2007.03.150

Four new N-heterocyclic carbene (NHC)-rhodium and iridium complexes derived from 2,2′-diaminobiphenyl have been successfully synthesized and their structures have been unambiguously characterized by X-ray diffraction. Their catalytic activities for hydrosilylation of ketones with diphenylsilane were investigated. It was found that NHC-rhodium complex (6) is the best one among the four catalysts for the reduction of ketones.

Full text of DOI:10.1016/j.tet.2007.03.150

Tetrahedron 63 (2007) 4874–4880
Synthesis of new NHC–rhodium and iridium complexes derived
from 2,20-diaminobiphenyl and their catalytic activities
toward hydrosilylation of ketones
Tao Chen,a Xu-Guang Liua and Min Shia,b,
*
aSchool of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, China
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China
Received 9 February 2006; revised 20 March 2007; accepted 23 March 2007
Available online 27 March 2007
Abstract—Four new N-heterocyclic carbene (NHC)–rhodium and iridium complexes derived from 2,20-diaminobiphenyl have been success-
fully synthesized and their structures have been unambiguously characterized by X-ray diffraction. Their catalytic activities for hydrosilyla-
tion of ketones with diphenylsilane were investigated. It was found that NHC–rhodium complex (6) is the best one among the four catalysts for
the reduction of ketones.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
chirality.11,12 RajanBabu reported the first chelated bis-
NHC complexes of Pd(II) and Ni(II) derived from 2,20-
bis-bromomethyl-[1,10]binaphthalenyl, in which the two
chelating N-heterocyclic carbenes oriented in a trans-
geometry.13 In 2005, Crabtree synthesized a new chiral
bis-NHC ligand from (S)-1,1-binaphthalenyl-2,20-diamine
(BINAM) in two steps.14 Recently, novel Pd(II) complexes
bearing a seven-membered N-heterocyclic carbene ligand
from 2,20-diaminobiphenyl were disclosed by Stahl et al.15
Previously, we also reported the preparation of bis-NHC–
Rh(III) and bis-NHC–Pd(II) complexes derived from opti-
cally active BINAM, and demonstrated their high catalytic
activities and chiral induction abilities in asymmetric reduc-
tion of ketones with diphenylsilane and oxidative kinetic
resolution of secondary alcohols using molecular oxygen
as a terminal oxidant.16 On the basis of these backgrounds,
we tried to explore more such bis-NHC ligands from other
diamine sources. In this paper, we wish to report the synthe-
sis of four new N-heterocyclic carbene (NHC) rhodium and
iridium complexes based on a new bis-NHC ligand from
2,20-diaminobiphenyl and their application to the hydro-
silylation of ketones with diphenylsilane. These interesting
bis-NHC–metal complexes have been unambiguously char-
acterized by X-ray diffraction.
N-Heterocyclic carbene ligands (NHCs), first reported by
1
2
¨
€
Ofele and Wanzlick and Schonherr, and later isolated and
characterized in the stable free state by Arduengo3 and co-
workers, have been developed rapidly in the latest decade
due to their stability to air and moisture and their strong
s-donor but poor p-acceptor abilities.4 Transition-metal
complexes with the NHCs have been proven to be effective
catalysts for the Suzuki and Heck reaction,5,6 aryl amination,7
hydrosilylation,8 olefinmetathesis,9 andmethaneoxidation.10
In addition to excellent catalytic activity in some reactions,
NHC catalysts can also tolerate high reaction temperature
since they decompose at slower rates than other ligands
such as tertiary phosphines. The use of polydentate NHC li-
gands has allowed the preparation of new metal complexes
whose stabilities are further improved by the chelating
effect. Most polycarbenes reported so far are bidentate and
pincer-type biscarbene ligands that were first used to coordi-
nate to Pd and later were applied to other transition metals
such as Rh and Ir. These chelated bis-NHCs not only yield
more stable metal complexes, but also give interesting
features. These features can provide fine-tuning of topo-
logical properties such as steric hindrance, bite angles, and
2. Results and discussion
Keywords: N-Heterocyclic carbene complexes; Rhodium; Iridium; 2,20-Di-
aminobiphenyl; Hydrosilylation; Ketones.
The synthesis of bis-NHC ligand 5 is shown in Scheme 1.
Using 2,20-diaminobiphenyl as the starting material to react
* Corresponding author. Fax: +86 21 64166128; e-mail: mshi@mail.sioc.
ac.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.150

T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
4875
NH2
NH2
NH NO2
NH NO2
Br
Pd-C/H2
EtOAc, 60 °C,
8 h, 94%
Pd2(dba)3, DPE-phos
+
Cs2CO3, toluene,
80 °C, 48 h, 99%
NO2
1
2
I-
+
N
N
N
N
cat. TsOH,
HC(OEt)3
100 °C, 16 h,
95%
N
N
NH NH2
NH NH2
CH3I
CH3CN,
+
N
80 °C, 5 h
N
I-
4
3
Scheme 1. Synthesis of bis-NHC ligand 5.
5
with 2-bromo-nitrobenzene in toluene at 80 ꢀC, product 2
was given in 99% yield after 48 h. Reduction of 2 by means
of Pd/C–H2 in ethyl acetate at 60 ꢀC produced compound
3 in 94% yield. Subsequent cyclization with triethyl ortho-
formate catalyzed by toluenesulfonic acid (TsOH) at
100 ꢀC afforded benzimidazole 4 in 95% yield after 16 h.
Quaternization of the benzimidazole ring of 4 with methyl
iodide gave the corresponding dibenzimidazolium salt 5 as
a solid.
Different from the preparation of an axially chiral bis-NHC–
Rh complex reported previously by us,8b it was found that
upon treatment of the dibenzimidazolium salt 5 with
[Rh(COD)Cl]2 in a similar procedure, the corresponding
cis-chelated bis-NHC–Rh(III) complex 6 was exclusively
obtained in high yield. Complex 6 could be isolated by silica
gel column chromatography as an orange solid in 83% yield
(Scheme 2). In addition, cis-chelated bis-NHC–Ir(III) com-
plex 7 could be obtained in 64% yield with the same proce-
dure. This procedure was unsuccessful on the preparation of
the bis-NHC–Ir(III) complex using bis-NHC ligand derived
from BINAM.17 Complexes 6 and 7 were stable at ambient
atmosphere and their structures are determined by spectro-
scopic data, microanalysis, and X-ray diffraction (Figs. 1
and 2).18,19 The special feature of the bis-NHC ligand 5 com-
pared to bis-NHC ligand derived from BINAM is the less
steric hindrance of the biaryl groups, which allowed the
C–C bond between the two phenyl rings of the bis-NHC
ligand 5 rotating more easily. In turn, this flexible rotation
facilitated the formation of the cis-chelated bis-NHC–metal
complexes.
Figure 1. ORTEP drawing of bis-NHC–Rh(III) complex 6.
Interestingly, upon treatment of the dibenzimidazolium salt
5 with [Rh(COD)Cl]2 or [Ir(COD)Cl]2 in tetrahydrofuran
(THF) under reflux in the presence of KOtBu, NHC–Rh(I)
complex 8 and NHC–Ir(I) complex 9 were produced in
14% and 12% yields, respectively, along with some uniden-
tified by-products (Scheme 3). Presumably, the (N-hetero-
cyclic carbene) intermediate was oxidized to urea by trace
of oxygen in the reaction system.20 These complexes were
stable under ambient atmosphere and their structures were
determined unambiguously by X-ray diffraction. The
ORTEP drawing of 8 and 9 are shown in Figures 3 and 4.21,22
I-
+
N
N
I
N
N
N
N
O
NaOAc, [M(COD)Cl]2, KI
CH3CN, reflux, 24 h
M
+
N
O
Next, these four new NHC–Rh and Ir complexes 6, 7, 8, and 9
were investigated as potential catalysts in the hydrosilylation
of ketones. Initial studies on the reaction conditions were car-
ried out with acetophenone 10a as a substrate in a variety of
solvents. The results of these experiments are summarized in
Table 1. Catalysts 6 and 8 showed good catalytic activities to
give the corresponding reduction product 11a in 90% and
83% yields using 2.0 mol % ofcatalyst, respectively (Table 1,
I
N
I-
5
6, M = Rh, Yield: 83%
7, M = Ir, Yield: 64%
Scheme 2. Synthesis of bis-NHC–Rh(III) and bis-NHC–Ir(III) complexes 6
and 7.

4876
T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
I-
+
N
N
N
N
KOtBu, [M(COD)Cl]2
THF, reflux, 24 h
O
M
I
+
N
N
N
N
I-
5
8, M = Rh, Yield: 14%
9, M = Ir, Yield: 12%
Scheme 3. Synthesis of NHC–Rh(I) and NHC–Ir(I) complexes 8 and 9.
entries 1 and 3). Catalyst 9 showed a moderate activity
(Table 1, entry 4). While the NHC–Ir(III) complex 7 showed
low catalytic activity, only giving 11a in 10% yield after 48 h
(Table 1, entry 2). Toluene (PhMe) was found to be the best
solvent compared to CH2Cl2, CH3CN, Et2O, and THF,
affording 11a in 92% yield using 1.0 mol % of catalyst 6 at
room temperature (Table 1, entries 7 and 8).
With the optimized conditions in hand, we subsequently
investigated the substrate scope of the hydrosilylation of
Figure 2. ORTEP drawing of bis-NHC–Ir(III) complex 7.
Figure 3. ORTEP drawing of NHC–Rh(I) complex 8.
Figure 4. ORTEP drawing of NHC–Ir(I) complex 9.

T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
4877
by us.8b For a,b-unsaturated ketone 10j and 1-tetralone
10k, 67% and 73% yields can be obtained, respectively
(Table 2, entries 9 and 10). Dialkyl ketones 10l, 10m, and
10n also can be reduced in high yields under identical con-
ditions (Table 2, entries 11–13).
Table 1. NHC–Rh or Ir complexes catalyzed hydrosilylation of ketones
O
OH
1) complexes 6-9, 2 equiv Ph2SiH2, r.t., 48 h
2) hydrolysis
10a
11a
Entrya
Catalyst
Solvent
Yieldb (%) (11a)
In conclusion, we have synthesized four new N-heterocyclic
carbene complexes of rhodium and iridium from a new bis-
NHC ligand based on 2,20-diaminobiphenyl. Among them,
the bis-NHC–Rh(III) complex 6 was found to be a fairly
effective catalyst for the hydrosilylation of ketones with di-
phenylsilane to give the corresponding alcohols in moderate
to high yields. Efforts to modify the biaryl backbone for
asymmetric catalysis are currently in progress.
1
2
3
4
5
6
7
8
9
10
6 (2 mol %)
7 (2 mol %)
8 (2 mol %)
9 (2 mol %)
6 (2 mol %)
6 (2 mol %)
6 (2 mol %)
6 (1 mol %)
6 (1 mol %)
6 (1 mol %)
THF
THF
THF
THF
CH2Cl2
CH3CN
PhMe
PhMe
Et2O
90
10
83
58
44
27
95
92
71
69
THF
3. Experimental section
3.1. General remarks
a
The reaction was carried out with acetophenone (0.5 mmol), 1 or 2 mol %
of catalyst, and 2 equiv of Ph2SiH2 in 2 mL of solvent at room temperature
for 48 h and then hydrolyzed by MeOH and 1 N HCl.
Isolated yields.
b
CH2Cl2 and CH3CN were freshly distilled from calcium
hydride under argon (Ar) atmosphere; THF, PhMe, and Et2O
were distilled from sodium (Na) under argon (Ar) atmo-
sphere. Melting points were determined on a digital melting
point apparatus and temperatures were uncorrected. 1H
NMR spectra were recorded on a Varian Mercury vx
300 MHz spectrometer for solution in CDCl3 with tetra-
methylsilane (TMS) as an internal standard; coupling con-
stants J are given in hertz. 13C NMR spectra were recorded
on a Varian Mercury vx 75 MHz spectrophotometers with
complete proton decoupling spectrophotometers (CDCl3:
77.0 ppm). Infrared spectra were recorded on a Perkin–
Elmer PE-983 spectrometer with absorption in cmꢁ1. Flash
column chromatography was performed using 300–400
mesh silica gel. For thin-layer chromatography (TLC), silica
gel plates (Huanghai GF254) were used. Elementary analysis
was taken on Elementar vario EL III. Mass spectra were
recorded by EI, and HRMS was measured on a HP-5989
ketones 10 using bis-NHC–Rh(III) complex 6. Various
ketones 10 with electron-rich, electron-neutral, and elec-
tron-poor substituents on the benzene ring can be smoothly
reduced to the corresponding alcohols 11 in good to high
yields under mild conditions (Table 2, entries 1–8), whose
results were consistent with the previously reported ones
Table 2. Bis-NHC–Rh(III) complex 6 catalyzed hydrosilylation of ketones
1) 2.0 equiv Ph SiH , complex (1 mol%),
PhMe, r.t., 48 h
6
OH
2
2
O
R1
R2
R1
10
R2
2) hydrolysis
11
Entrya
10 (R1/R2)
Products
Yieldb (%)
1
2
3
4
5
6
7
8
10b (C6H5/Et)
11b
11c
11d
11e
11f
11g
11h
11i
83
91
93
85
87
88
92
79
10c (4-ClC6H4/Me)
10d (4-BrC6H4/Me)
10e (3-FC6H4/Me)
10f (4-MeC6H4/Me)
10g (4-MeOC6H4/Me)
10h (2-naphthyl/Me)
10i (C6H5/CH2Cl)
23
24
instrument. [Rh(COD)Cl]2 and [Ir(COD)Cl]2 were pre-
pared employing the published procedures.
3.1.1. Synthesis of compound 2. Under argon atmosphere,
a mixture of 2,20-diaminobiphenyl (0.92 g, 5 mmol), 2-
bromo-nitrobenzene (3.03 g, 15 mmol), Pd2(dba)3 (0.12 g,
0.125 mmol), bis(2-diphenylphosphinophenyl) ether (DPE-
phos) (0.20 g, 0.375 mmol), and Cs2CO3 (5.20 g, 16 mmol)
was stirred in toluene (40 mL) at 80 ꢀC for 48 h. After the
reaction mixture was cooled to room temperature, the reac-
tion was quenched by the addition of 100 mL of H2O. The
mixture was extracted with EtOAc (3ꢂ50 mL), dried over
anhydrous Na2SO4, and filtered. The solvent was removed
under reduced pressure and the residue was purified by silica
gel flash column chromatography, eluting first with petro-
leum ether/ethyl acetate (20:1) to remove excess raw mate-
rial, and then eluting with petroleum ether/ethyl acetate (4:1)
to give 2 as a red solid; yield: 2.1 g (99%). Mp 191–192 ꢀC;
IR (CH2Cl2): n 3347, 3085, 2925, 1615, 1577, 1505, 1448,
O
9
10j
11j
67
73
O
10
10k
11k
O
11
12
10l
11l
90
O
10m
11m
11n
89
93
1
1345, 1255, 1148, 1037, 766 cmꢁ1; H NMR (300 MHz,
O
13
10n
CDCl3, TMS): d 6.51–6.54 (m, 2H, ArH), 7.08–7.13 (m,
4H, ArH), 7.29–7.33 (m, 2H, ArH), 7.39–7.43 (m, 6H,
ArH), 7.87 (d, J¼6.3 Hz, 2H, ArH), 9.18 (s, 2H, NH); 13C
NMR (75 MHz, CDCl3): d 114.8, 117.4, 123.7, 125.8,
126.2, 129.0, 132.0, 132.9, 135.4, 136.6, 141.9; MS (EI)
a
All of the reactions were carried out under the optimized reaction condi-
tions: 1 mol % bis-NHC–Rh(III), 2 equiv Ph2SiH2, in PhMe at room tem-
perature for 48 h and then hydrolyzed by MeOH and 1 N HCl.
Isolated yields.
b

4878
T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
m/z (%): 426 (M+, 100), 344 (M+ꢁ82, 12), 255 (M+ꢁ171,
13), 241 (M+ꢁ185, 9); Anal. Calcd for C24H18N4O4 re-
quires: C, 67.70; H, 4.25; N, 13.14%. Found: C, 67.41; H,
4.01; N, 13.31%.
CDCl3, TMS): d 2.03 (s, 3H, Me), 4.35 (s, 6H, Me), 6.56
(d, J¼8.1 Hz, 2H, ArH), 6.70 (d, J¼7.2 Hz, 2H, ArH),
7.07 (t, J¼7.2 Hz, 2H, ArH), 7.15 (t, J¼7.2 Hz, ArH),
7.22–7.39 (m, 6H, ArH), 8.17 (d, J¼8.1 Hz, 2H, ArH);
MS (EI) m/z (%): 703 (M+ꢁ127, 9), 644 (M+ꢁ186, 25),
576 (M+ꢁ254, 11), 517 (M+ꢁ313, 100), 268 (M+ꢁ562, 17);
Anal. Calcd for C30H25I2N4O2Rh requires: C, 43.40; H,
3.04; N, 6.75%. Found: C, 43.72; H, 3.04; N, 6.27%.
3.1.2. Synthesis of compound 3. A mixture of 2 (852 mg,
2 mmol), 10% Pd–C (100 mg) in 100 mL of EtOAc was
stirred under H2 atmosphere (15 atm) at 60 ꢀC for 8 h. After
cooling to room temperature, Pd–C was removed by filtra-
tion. The solvent was evaporated under reduced pressure.
The residue was purified by silica gel flash column chroma-
tography (eluent: petroleum ether/ethyl acetate 4:1–1:1) to
give 3 as a white solid; yield: 688 mg (94%). Mp 235–
237 ꢀC; IR (CH2Cl2): n 3329, 3281, 2926, 2319, 1727,
3.1.6. Synthesis of bis-NHC–Ir(III) complex 7. A mixture
of
5
(134 mg, 0.20 mmol), [Ir(COD)Cl]2 (67 mg,
0.10 mmol), NaOAc (134 mg, 1.60 mmol), and KI (66 mg,
0.40 mmol) was stirred in CH3CN (12 mL) under reflux
for 24 h. After cooling, volatiles were removed under re-
duced pressure and the residue was purified by silica gel
flash column chromatography (eluent: petroleum ether/ethyl
acetate 1:1) to give a yellow solid, bis-NHC–Ir(III) complex
7. Yield: 117 mg (64%). A single crystal suitable for X-ray
crystal analysis was obtained by recrystallization from a
saturated solution of petroleum ether/ethyl acetate (1:1).
Mp>250 ꢀC (dec); IR (CH2Cl2): n 3058, 2950, 2374,
1616, 1507, 1262, 999, 748 cmꢁ1 1H NMR (300 MHz,
;
DMSO-d6): d 4.71 (br, 4H, NH2), 5.99 (br, 2H, NH), 6.50–
6.57 (m, 4H, ArH), 6.69–6.72 (m, 2H, ArH), 6.81–6.92
(m, 6H, ArH), 7.12–7.21 (m, 4H, ArH); MS (EI) m/z (%):
366 (M+, 100), 259 (M+ꢁ107, 46), 166 (M+ꢁ200, 15);
HRMS (EI) calcd for C24H22N4 (M+) requires 366.1844,
found: 366.1850.
1711, 1474, 1326, 1094, 954, 744 cmꢁ1
;
1H NMR
3.1.3. Synthesis of compound 4. Compound 3 (366 mg,
1.0 mmol), toluenesulfonic acid (20 mg), and triethyl ortho-
formate [HC(OC2H5)3] (10 mL) were heated at 100 ꢀC for
16 h. After the reaction mixture was cooled to room temper-
ature, 40 mL of petroleum ether was added to precipitate
white solid, filtered, and the precipitate was washed with
light petroleum ether to give 4 as a white solid; yield:
365 mg (95%). Mp 166–167 ꢀC; IR (CH2Cl2): n 3430,
3054, 2925, 2844, 1712, 1614, 1492, 1227, 1009, 787,
(300 MHz, CDCl3, TMS): d 2.05 (s, 3H, Me), 4.24 (s, 6H,
Me), 6.52 (d, J¼8.1 Hz, 2H, ArH), 6.67 (dd, J¼7.2,
1.5 Hz, 2H, ArH), 7.03–7.11 (m, 4H, ArH), 7.18 (t, J¼
7.2 Hz, 2H, ArH), 7.24–7.27 (m, 2H, ArH), 7.35 (dt, J¼
7.2, 1.5 Hz, 2H, ArH); MS (EI) m/z (%): 920 (M+, 11), 793
(M+ꢁ127, 38), 733 (M+ꢁ186, 9), 666 (M+ꢁ254, 30), 607
(M+ꢁ313, 100); Anal. Calcd for C30H25I2IrN4O2 requires:
C, 39.18; H, 2.74; N, 6.09%. Found: C, 39.37; H, 2.51; N
5.81%.
1
737 cmꢁ1; H NMR (300 MHz, CDCl3, TMS): d 6.51 (d,
2H, J¼8.1 Hz, ArH), 6.77 (t, 2H, J¼7.5 Hz, ArH), 7.01 (t,
2H, J¼7.5 Hz, ArH), 7.21 (s, 2H, NCHN), 7.26–7.28 (m,
2H, ArH), 7.47–7.53 (m, 4H, ArH), 7.55–7.60 (m, 2H, ArH),
7.71 (d, J¼7.5 Hz, 2H, ArH); 13C NMR (75 MHz, CDCl3):
d 109.2, 119.4, 122.3, 123.3, 125.9, 128.6, 129.6, 132.5,
132.8, 133.5, 133.6, 141.7, 142.4; MS (EI) m/z (%): 386 (M+,
100), 268 (M+ꢁ118, 51), 193 (M+ꢁ193, 5); Anal. Calcd for
C26H18N4 requires: C, 80.81; H, 4.69; N, 14.50%. Found: C,
80.48; H, 4.53; N, 14.40%.
3.1.7. Synthesis of NHC–Rh(I) complex 8. A mixture of 5
(134 mg, 0.20 mmol), [Rh(COD)Cl]2 (48 mg, 0.10 mmol),
KOtBu (50 mg, 0.45 mmol), and KI (66 mg, 0.40 mmol)
was stirred in THF (12 mL) under reflux for 24 h. After cool-
ing, volatiles were removed under reduced pressure and the
residue was purified by silica gel flash column chromato-
graphy (eluent: petroleum ether/ethyl acetate 5:1–3:1) to
give an orange solid, Rh(I) complex 8. Yield: 22 mg
(14%). A single crystal suitable for X-ray crystal analysis
was obtained by recrystallization from a saturated solution
of petroleum ether/ethyl acetate (4:1). Mp>250 ꢀC (dec);
IR (CH2Cl2): n 3061, 2956, 2925, 2317, 1722, 1494, 1384,
3.1.4. Synthesis of dibenzimidazolium salt 5. Compound 4
(386 mg, 1.0 mmol) and CH3I (0.48 mL, 8.0 mmol) in
CH3CN (10 mL) were stirred at 80 ꢀC for 5 h. After cooling
to room temperature, volatiles were removed under reduced
pressure and the obtained solid compound 5 was used for the
next reaction without further purification. MS (ESI) m/z:
543.1 (M+ꢁI), 208.1 (M+ꢁ2I)/2.
1
1338, 1081, 963, 746 cmꢁ1; H NMR (300 MHz, CDCl3,
TMS): d 0.83–0.98 (m, 2H, CH2), 1.07–1.15 (m, 1H, CH2),
1.38–1.53 (m, 3H, CH2), 1.69–1.71 (m, 1H, CH2), 1.78–1.91
(m, 1H, CH2), 2.16–2.23 (m, 1H, CH), 2.52–2.65 (m, 1H,
CH), 3.33 (s, 3H, Me), 4.20 (s, 3H, Me), 4.78–4.79 (m,
1H, CH), 4.88–4.92 (m, 1H, CH), 6.37–6.53 (m, 1H, ArH),
6.87–6.93 (m, 2H, ArH), 7.04–7.09 (m, 4H, ArH), 7.15–
7.23 (m, 5H, ArH), 7.46–7.51 (m, 2H, ArH), 8.06–8.21
(m, 1H, ArH), 8.74–8.76 (m, 1H, ArH); MS (ESI) m/z:
641.2 (M+ꢁI); HRMS (ESI) calcd for C36H34IrN4O
(M+ꢁI) requires 641.1788, found: 641.1753.
3.1.5. Synthesis of bis-NHC–Rh(III) complex 6. A mixture
of
5
(134 mg, 0.20 mmol), [Rh(COD)Cl]2 (48 mg,
0.10 mmol), NaOAc (134 mg, 1.60 mmol), and KI (66 mg,
0.40 mmol) was stirred in CH3CN (12 mL) under reflux
for 24 h. After cooling, volatiles were removed under re-
duced pressure and the residue was purified by silica gel
flash column chromatography (eluent: petroleum ether/ethyl
acetate 1:1) to give an orange solid, bis-NHC–Rh(III) com-
plex 6. Yield: 137 mg (83%). A single crystal suitable for X-
ray crystal analysis was obtained by recrystallization from
a saturated solution of petroleum ether/ethyl acetate (1:1).
Mp>250 ꢀC (dec); IR (CH2Cl2): n 3063, 2949, 2376, 1464,
3.1.8. Synthesis of NHC–Ir(I) complex 9. A mixture of 5
(134 mg, 0.20 mmol), [Ir(COD)Cl]2 (67 mg, 0.10 mmol),
KOtBu (50 mg, 0.45 mmol), and KI (66 mg, 0.40 mmol)
was stirred in THF (12 mL) under reflux for 24 h. After cool-
ing, volatiles were removed under reduced pressure and the
residue was purified by silica gel flash column chromato-
graphy (eluent: petroleum ether/ethyl acetate 4:1–3:1) to
1332, 1220, 1095, 949, 743 cmꢁ1 1H NMR (300 MHz,
;

T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
4879
Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004,
33, 619–636.
give a yellow solid, Ir(I) complex 9. Yield: 21 mg (12%). A
single crystal suitable for X-ray crystal analysis was ob-
tained by recrystallization from a saturated solution of petro-
leum ether/ethyl acetate (4:1). Mp>250 ꢀC (dec); IR
(CH2Cl2): n 3057, 2955, 2926, 2030, 1721, 1494, 1386,
5. (a) Herrmann, W. A.; Bohm, V. P. W.; Gstottmayr, C. W. K.;
Grosche, M.; Reisinger, C.-P.; Weskamp, T. J. Organomet.
Chem. 2001, 617 and 618, 616–628; (b) Grasa, G. A.; Viciu,
M. S.; Huang, J.; Zhang, C.; Trudell, M. L.; Nolan, S. P.
Organometallics 2002, 21, 2866–2873; (c) Yang, C.; Lee,
H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511–1514.
6. (a) Shi, M.; Qian, H.-X. Tetrahedron 2005, 61, 4949–4955; (b)
Xu, Q.; Duan, W.-L.; Lei, Z.-Y.; Zhu, Z.-B.; Shi, M.
Tetrahedron 2005, 61, 11225–11229; (c) Chen, T.; Gao, J.;
Shi, M. Tetrahedron 2006, 62, 6289–6294.
7. (a) Cheng, J.; Trudell, M. L. Org. Lett. 2001, 3, 1371–1374; (b)
Stauffer, S. R.; Lee, S.; Stambuli, J. P.; Hauck, S. I.; Hartwig,
J. F. Org. Lett. 2000, 2, 1423–1426.
8. (a) Enders, D.; Gielen, H. J. Organomet. Chem. 2001, 617 and
618, 70–80; (b) Duan, W.-L.; Shi, M.; Rong, G.-B. Chem.
Commun. 2003, 2916–2917; (c) Faller, J. W.; Fontaine, P. P.
Organometallics 2006, 25, 5887–5893.
9. (a) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H.
Tetrahedron Lett. 1999, 40, 2247–2250; (b) Sanford, M. S.;
Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123,
6543–6554.
10. Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem.,
Int. Ed. 2002, 41, 1745–1747.
11. For some recent reviews, see: (a) Peris, E.; Crabtree, R. H.
Coord. Chem. Rev. 2004, 248, 2239–2246; (b) Mata, J. A.;
Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841–859.
12. (a) Hermann, W. A.; Reisinger, C. P.; Spiegler, M.
J. Organomet. Chem. 1998, 557, 93–96; (b) Zhang, M.;
Trudell, M. L. Tetrahedron Lett. 2000, 41, 595–598; (c)
Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem.
Commun. 2002, 32–33; (d) Perry, M. C.; Cui, X.; Burgess, K.
Tetrahedron: Asymmetry 2002, 13, 1969–1972.
1
1205, 1081, 966, 746 cmꢁ1; H NMR (300 MHz, CDCl3,
TMS): d 0.85–0.93 (m, 2H, CH2), 1.07–1.15 (m, 1H,
CH2), 1.38–1.53 (m, 3H, CH2), 1.64–1.70 (m, 1H, CH2),
1.81–1.86 (m, 1H, CH2), 2.11–2.22 (m, 1H, CH), 2.55–
2.61 (m, 1H, CH), 3.33 (s, 3H, Me), 4.19 (s, 3H, Me),
4.78–4.79 (m, 1H, CH), 4.88–4.92 (m, 1H, CH), 6.42–6.48
(m, 1H, ArH), 6.87–6.93 (m, 2H, ArH), 7.05–7.09 (m, 4H,
ArH), 7.16–7.23 (m, 5H, ArH), 7.46–7.51 (m, 2H, ArH),
8.03–8.23 (m, 1H, ArH), 8.74–8.77 (m, 1H, ArH); MS
(EI) m/z (%): 858 (M+, 0.8), 729 (M+ꢁ129, 46), 623
(M+ꢁ235, 100); HRMS (ESI) calcd for C36H34IrN4O
(M+ꢁI) requires 731.2362, found: 731.2325.
3.2. Representative procedure for bis-NHC–Rh(III)-
catalyzed hydrosilylation of ketones
In a flame-dried Schlenk flask was placed bis-NHC–Rh(III)
(0.005 mmol, 1 mol %) under argon. Anhydrous toluene
(2 mL) and then acetophenone (0.5 mmol) were added by
syringe through a septum, diphenylsilane (1 mmol) was
added slowly, and the reaction mixture was stirred at room
temperature for 48 h. After 48 h, hydrolysis was performed
by the addition of MeOH (2 mL) and 1 N HCl (2 mL). The
resulting aqueous solution was stirred for 2 h at room tem-
perature and extracted with Et2O (3ꢂ10 mL) and dried
over anhydrous Na2SO4. The solvent was removed under
reduced pressure and the residue was purified by silica
gel flash column chromatography (eluent: petroleum ether/
ethyl acetate 15:1–10:1) to give the corresponding 1-phenyl-
ethanol.
13. Clyne, D. S.; Jin, S.; Genest, E.; Gallucci, J. C.; RajanBabu,
T. V. Org. Lett. 2000, 2, 1125–1128.
14. Chianese, A. R.; Crabtree, R. H. Organometallics 2005, 24,
4432–4436.
Acknowledgements
15. Scarborough, C. C.; Grady, M. J. W.; Guzei, I. A.; Gandhi,
B. A.; Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005,
44, 5269–5272.
16. (a) Chen, T.; Jiang, J.-J.; Xu, Q.; Shi, M. Org. Lett. 2007, 9,
865–868; (b) Xu, Q.; Gu, X.-X.; Liu, S.-J.; Du, Q.-Y.; Shi,
M. J. Org. Chem. 2007, 72, 2240–2242.
Financial support from the Shanghai Municipal Committee
of Science and Technology (04JC14083, 06XD14005),
the National Natural Science Foundation of China
(203900502, 20472096, and 20672127), and the Cheung
Kong Scholar Program is greatly appreciated.
17. Shi, M.; Duan, W.-L. Appl. Organomet. Chem. 2005, 19, 40–44.
18. The crystal data of 6 have been deposited in CCDC with num-
ber 632167. Empirical formula: C32H27Cl6I2N4O2Rh; formula
weight: 1068.99; crystal color, habit: colorless, prismatic; crys-
tal system: monoclinic; lattice type: primitive; lattice para-
Supplementary data
Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2007.
03.150.
˚
˚
˚
meters: a¼13.9748(11) A, b¼23.3683(18) A, c¼11.7901(9) A,
3
a¼90ꢀ, b¼95.0670(10)ꢀ, g¼90ꢀ, V¼3835.2(5) A ; space
˚
group: C2/c; Z¼4; Dcalcd¼1.851 g/cm3; F000¼2064; diffrac-
tometer: Rigaku AFC7R; residuals: R; Rw: 0.0482, 0.1340.
19. The crystal data of 7 have been deposited in CCDC with num-
ber 632169. Empirical formula: C33H32I2IrN4O2; formula
weight: 962.63; crystal color, habit: colorless, prismatic; crys-
tal system: monoclinic; lattice type: primitive; lattice para-
References and notes
¨
1. Ofele, K. J. Organomet. Chem. 1968, 12, 42–43.
€
2. Wanzlick, H. W.; Schonherr, H. J. Angew. Chem., Int. Ed. Engl.
1968, 7, 141–142.
3. Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361–363.
4. (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G.
Chem. Rev. 2000, 100, 39–91; (b) Herrmann, W. A. Angew.
˚
˚
˚
meters: a¼8.8851(7) A, b¼21.4149(18) A, c¼17.3292(15) A,
3
a¼90ꢀ, b¼100.698(2)ꢀ, g¼90ꢀ, V¼3240.0(5) A ; space
˚
group: P2(1)/c; Z¼4; Dcalcd¼1.973 g/cm3; F000¼1828; diffrac-
tometer: Rigaku AFC7R; residuals: R; Rw: 0.0537, 0.1188.
20. For some examples of oxidizing NHC with O2, see: (a) Enders,
D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder,
ꢀ
Chem., Int. Ed. 2002, 40, 1290–1309; (c) Cesar, V.;

4880
T. Chen et al. / Tetrahedron 63 (2007) 4874–4880
J. P.; Ebel, K.; Brode, S. Angew. Chem., Int. Ed. Engl. 1995, 34,
22. The crystal data of 9 has been deposited in CCDC with number
632170. Empirical formula: C36H34IIrN4O; formula weight:
857.77; crystal color, habit: colorless, prismatic; crystal sys-
tem: monoclinic; lattice type: primitive; lattice parameters:
1021–1023; (b) Shi, Z.; Thummel, R. P. J. Org. Chem. 1995,
60, 5935–5945; (c) Khramov, D. M.; Boydston, A. J.;
Bielawski, C. W. Angew. Chem., Int. Ed. 2006, 45, 6186–6189.
˚
˚
˚
21. The crystal data of 8 have been deposited in CCDC with num-
ber 632168. Empirical formula: C36H34IN4ORh; formula
weight: 768.48; crystal color, habit: colorless, prismatic; crys-
tal system: monoclinic; lattice type: primitive; lattice para-
a¼14.7020(9) A,
b¼13.2513(9) A,
c¼16.7290(11) A,
3
ꢀ
ꢀ
ꢀ
˚
a¼90 , b¼101.0960(10) , g¼90 , V¼3198.2(4) A ; space
group: P2(1)/n; Z¼4; Dcalcd¼1.781 g/cm3; F000¼1664; dif-
fractometer: Rigaku AFC7R; residuals: R; Rw: 0.0491, 0.1040.
23. Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218–
220.
˚
˚
˚
meters: a¼14.7269(10) A, b¼13.2489(9) A, c¼16.7079(12) A,
3
a¼90ꢀ, b¼101.136(2)ꢀ, g¼90ꢀ, V¼3198.6(4) A ; space
˚
group: P2(1)/n; Z¼4; Dcalcd¼1.596 g/cm3; F000¼1536; dif-
24. Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974,
15, 18–20.
fractometer: Rigaku AFC7R; residuals: R; Rw: 0.0482, 0.1095.