Synthesis of novel planar chiral Ag and Rh N-heterocyclic carbene complexes derived from [2.2]paracyclophane and their application in ultrasound assisted asymmetric addition reactions of organoboronic acids to aldehydes

Article abstract of DOI:10.1016/j.tetasy.2013.01.017

Three novel planar chiral N-heterocyclic carbene silver and rhodium complexes based on [2.2]paracyclophane have been prepared. These could be used as catalysts/precatalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes. We optimized the reaction conditions and have applied ultrasonic irradiation in the asymmetric arylation for the first time. Under ultrasound irradiation, the combination of planar chiral NHC-Ag complex 5 and RhCl ₃ can achieve higher catalytic activities in the asymmetric addition of organoboronic acids to aldehydes.

Full text of DOI:10.1016/j.tetasy.2013.01.017

Tetrahedron: Asymmetry 24 (2013) 241–248
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of novel planar chiral Ag and Rh N-heterocyclic carbene
complexes derived from [2.2]paracyclophane and their application
in ultrasound assisted asymmetric addition reactions of organoboronic
acids to aldehydes
⇑
⇑
Wenzeng Duan, Yudao Ma , Fuyan He, Lei Zhao, Jianqiang Chen, Chun Song
Department of Chemistry, Shandong University, Shanda South Road No. 27, Jinan 250100, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel planar chiral N-heterocyclic carbene silver and rhodium complexes based on [2.2]paracyclo-
phane have been prepared. These could be used as catalysts/precatalysts for the asymmetric 1,2-addition
of organoboronic acids to aldehydes. We optimized the reaction conditions and have applied ultrasonic
irradiation in the asymmetric arylation for the first time. Under ultrasound irradiation, the combination
of planar chiral NHC–Ag complex 5 and RhCl3 can achieve higher catalytic activities in the asymmetric
addition of organoboronic acids to aldehydes.
Received 9 November 2012
Accepted 22 January 2013
Available online 27 February 2013
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
arylation of aldehydes and ketones.9 Shi has also shown that when
the reactions were performed at a lower temperature, higher
Since N-heterocyclic carbenes (NHCs) are excellent
r
-donors
enantioselectivities were obtained.9 Inspired by these results, we
turned our attention to the preparation of planar chiral NHC–metal
complexes and an investigation of new catalyst systems for the
asymmetric addition reaction of organoboronic acids to aldehydes.
Using chiral NHC–Rh complexes at lower reaction temperatures
may be an efficient method to improve the enantioselectivity.8,9
As a chiral source, planar chiral [2.2]paracyclophane derivatives
have been successfully applied in asymmetric catalysis.10
However, the application of structurally well-defined chiral
[2.2]paracyclophane-based NHC–Rh complexes in catalytic
asymmetric addition of organoboronic acids to aldehydes has not
been explored to date.
Recently, our group has exploited the use of planar chiral
[2.2]paracyclophane-based NHCs as ligands for the in situ Rh-
catalyzed asymmetric arylation of aldehydes.11 However, there
are three drawbacks in our previous system:11b (1) the reaction
temperature could not be lowered below 80 °C; (2) the scope of
the aldehydes that were used in the catalysis reaction is somewhat
limited; and (3) the structure of the NHC–Rh complex is not clear.
The planar chiral [2.2]paracyclophane-based NHCs not only pro-
duced transition metal catalysts in situ, but also led to the isolation
of metal complexes, which would allow further fine-tuning of
catalyst properties. Few reports have considered the NHC–Rh
complex catalyzed asymmetric addition of organoboronic acids
to aldehydes. To the best of our knowledge, the reactions did not
proceed well at room temperature even at elevated temperatures.
It is worth noting that lowering the temperature may be a key
strategy to achieve highly stereocontrolled reactions.9
and form rather strong metal–carbon bonds, NHC–metal com-
plexes exhibit higher air and thermal stability than those contain-
ing phosphines.1 These factors have made NHC a promising
alternative ligand to the currently used phosphines.2 The success-
ful applications of chiral phosphines in asymmetric catalysis are
currently applicable to NHCs and their metal complexes.3 Despite
the considerable effort that has been devoted to this field, the de-
sign and synthesis of novel chiral NHCs to enhance their enantiose-
lectivity is still a challenge.4 Moreover, a better understanding of
NHC design strategies and NHC applications in metal-based asym-
metric catalysis would promote future development in this field.
Chiral diarylmethanols are important intermediates for the syn-
thesis of biologically and pharmaceutically active compounds.5
A
general method for the synthesis of optically active diarylmetha-
nols is the Rh-catalyzed asymmetric addition of organoboronic
acids to aldehydes. Since Miyaura first reported the Rh-catalyzed
asymmetric addition of organoboronic acids to aldehydes in
1998,6 much attention has been shifted to a practical strategy:
either to improve enantioselectivity or accelerate the reaction by
using new catalyst systems.7–9 Among these catalyst systems,
Hayashi has developed several novel and effective chiral NHC–Cu
complexes for the asymmetric addition of arylboronates to isatins
in 2010.8 Shi has recently reported other chiral NHC–Pd complexes
and their successful application in the catalytic enantioselective
⇑
Corresponding authors. Tel.: +86 531 88361869; fax: +86 531 88565211 (Y.M.).
E-mail addresses: ydma@sdu.edu.cn (Y. Ma), chunsong@sdu.edu.cn (C. Song).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.01.017

242
W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
Recently, ultrasound has been used widely as a tool to enhance
NHC–Ag 5.16 Accordingly, the bromo[N,N0-bis[(Rp)-(+)-12-meth-
oxy-4-[2.2]paracyclophanyl]imidazol-2-ylidene]silver 5 was also
synthesized by treating N,N-bis[(Rp)-(ꢀ)-12-methoxy-4-[2.2]para-
cyclophanyl]imidazolium triflate with NaBr and Ag2O in CH2Cl2.
NHC–Ag 2 underwent a facile reaction with [Rh(COD)Cl]2 to form
an NHC–Rh complex 3. The complexes chloro[N,N0-bis[(Rp)-(+)-4-
[2.2]paracyclophanyl]imidazol-2-ylidene]silver 2, bromo(N,N0-
bis[(Rp)-(+)-12-methoxy-4-[2.2]paracyclophanyl]imidazolium)sil-
the yields and rates of many chemical reactions.12 The use of ultra-
sound in chemical reactions provides specific activation based on a
physical phenomenon, known as acoustic cavitation.13 We
wondered whether under sonication conditions the asymmetric
addition reaction would provide higher yields and enantioselectiv-
ities. Herein, we report the synthesis of novel planar chiral
imidazol-2-ylidene complexes of silver and rhodium and their
application in the ultrasound-assisted rhodium-catalyzed asym-
metric addition of organoboronic acids to aldehydes.
ver 5, and chloro(g2
,
g2-1,5-cyclooctadiene)[N,N0-bis[(Rp)-(+)-4-
were
[2.2]paracyclophanyl]imidazole-2-ylidene]rhodium
3
characterized by 1H NMR, 13C NMR, high-resolution mass spec-
trometry (HRMS), and elemental analysis. The absence of an
N–CH–N resonance in the 1H NMR spectra confirmed the forma-
tion of the carbene complexes. The 13C NMR, high-resolution mass
spectrometry, or elemental analysis further confirmed the molecu-
lar structures. The structure of the novel planar chiral NHC–Rh
complex 3 was determined by X-ray diffraction (Fig. 1).
With the planar chiral NHC–metal complexes in hand, we
examined their application in the asymmetric addition of organo-
boronic acids to aldehydes.
2. Results and discussion
We designed and synthesized three new planar chiral NHC–Ag
and NHC–Rh complexes based on the [2.2]paracyclophane skele-
ton. The synthetic route to the new NHC–metal complexes is
shown in Schemes 1 and 2. The NHC complexes were synthesized
from enantiomerically pure N,N0-bis[(Rp)-(ꢀ)-4-[2.2]paracyclopha-
nyl]imidazolium triflate 1 and N,N0-bis[(Rp)-(ꢀ)-12-methoxy-4-
[2.2]paracyclophanyl]imidazolium triflate 4, which can be ob-
tained by following the previous literature.11 All of our attempts
to prepare free carbene from the imidazolium salt using standard
deprotonating agents such as tert-BuOK or NaH failed. Deprotona-
tion using silver oxide as base has been widely used to synthesize
NHC–Ag complexes which can transfer the carbene ligands to a
variety of other metals.2e,14 According to this strategy, the imidazo-
lium triflate 1 was treated with an excess of Ag2O in anhydrous
CH2Cl2, but no reaction occurred. By anion exchange of the imi-
dazolium triflate 1 with tetrabutylammonium chloride in the pres-
ence of Ag2O in anhydrous CH2Cl2, the NHC–Ag complex 2 could be
obtained in moderate yield.15 However, the ion exchange product
was contaminated by tetrabutylammonium chloride, although it
could be purified by repeated recrystallization from dichlorometh-
ane and hexane. As an alternative procedure, NaBr was used in-
stead of tetrabutylammonium chloride for the preparation of
Our study began with structurally well-defined chloro(g2 2-1,
,g
5-cyclooctadiene)[N,N0-bis[(Rp)-(+)-4-[2.2]paracyclophanyl]imid-
azole-2-ylidene]rhodium 3 under the analogous conditions as used
in our previous studies of the rhodium-catalyzed asymmetric aryla-
tion of aldehydes11 (Table 1). The reaction of phenylboronic acid and
1-naphthaldehyde was performed with 3.0 mol % of catalyst in
MeOH/DME (5:1) or t-BuOH/MeOH (5:1) at 80 °C for 3 h. Compared
to our previous work, the enantioselectivities of the reaction were
lower (Table 1, entries 1, 2). By screening bases in MeOH/DME
(5:1) or t-BuOH/MeOH (5:1), we found that the addition of an excess
of KF (6.0 equiv) significantly improved the enantioselectivities (Ta-
ble 1, entries 3, 4). We further optimized the reaction conditions by
tuning the solvent effect. Variation of the solvent showed that the t-
BuOH/MeOH (5:1) was the best choice of solvent (Table 1, entries 5–
10). Upon loweringthe reaction temperatureto rt almostno reaction
Cl
(n-Bu)4NCl
Ag2O
CH2Cl2
Ag
OTf
N
N
N
N
1
2
Cl
[Rh(COD)Cl]2
Rh
N
N
3
Scheme 1. Synthesis of planar chiral NHC–Rh 3.
OMe
OTf
MeO
N
OMe Br
Ag
MeO
N
NaBr
Ag2O
CH2Cl2
N
N
5
4
Scheme 2. Synthesis of planar chiral NHC–Ag 5.

W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
243
Figure 1. ORTEP drawing of NHC–Rh complex 3 showing atomic numbering scheme at 50% probability ellipsoids. Hydrogen atoms and H2O molecules are omitted for clarity.
Selected bond distance (Å) and angles (°): Rh(1)–C(1) = 2.019(5), Rh(1)–C(36) = 2.089(6), Rh(1)–C(37) = 2.119(5), Rh(1)–C(40) = 2.179(6), Rh(1)–C(41) = 2.206(5), Rh(1)–
Cl(1) = 2.3862(14), C(1)–Rh(1)–C(36) = 93.5(2), C(1)–Rh(1)–C(37) = 94.4(2), C(36)–Rh(1)–C(37) = 39.2(3), C(1)–Rh(1)–C(40) = 158.0(2), C(36)–Rh(1)–C(40) = 96.5(3), C(37)–
Rh(1)–C(40) = 81.3(2), C(1)–Rh(1)–C(41) = 166.2(2), C(36)–Rh(1)–C(41) = 80.5(3), C(37)–Rh(1)–C(41) = 88.8(2), C(40)–Rh(1)–C(41) = 35.8(3), C(1)–Rh(1)–Cl(1) = 89.04(14),
C(36)–Rh(1)–Cl(1) = 155.9(2), C(37)–Rh(1)–Cl(1) = 164.26(18), C(40)–Rh(1)–Cl(1) = 89.82(17), C(41)–Rh(1)–Cl(1) = 91.53(18).
occurred, however, a 47% conversion and 36% ee were observed at
50 °C (Table 1, entries 11, 12). Significantly, lower temperature can
afford the desired product with higher enantioselectivity.
Next, we screened different rhodium compounds as metal
sources and NHC–Ag 2 as a precatalyst for the asymmetric addition
reaction. [Rh(COD)Cl]2 and [Rh(COD)OH]2 were found to be less
efficient in this reaction in terms of poor enantioselectivities (6–
12% ee) (Table 2, entries 1, 2), and other rhodium compounds
tested showed similar enantioselectivities (32–37% ee) (Table 2,
entries 3–5), whereas RhCl3 gave the highest enantioselectivity
(Table 2, entry 6). It is known that the methoxy group on the
[2.2]paracyclophanyl ligand backbone plays an important role in
the catalytic process,6,7a,b,11b so the bromo[N,N0-bis[(Rp)-(+)-12-
methoxy-4-[2.2]paracyclophanyl]imidazol-2-ylidene]silver 5 was
then used as a precatalyst and RhCl3 used as a rhodium source
for this reaction. The reaction gave a moderate yield (51%) and
enantioselectivity (52% ee). Reducing the reaction temperature to
40 °C, the combination of NHC–Ag 2 and RhCl3 showed no catalytic
activity (Table 2, entry 8), however, NHC–Ag 5 combined with
RhCl3 catalyzed this reaction to provide the desired product (33%
yield, 58% ee) (Table 2, entry 9). In order to improve the yield, an
ultrasonic oscillator was used as an alternative energy source to
facilitate the chemical reactions. Under sonication (the reaction
flask was clamped in an ultrasonic bath of 40 kHz frequency at
40 °C for the purpose of irradiation), using NHC–Ag 5 as precatalyst
exhibited an astonishingly high catalytic activity (73% yield)
(Table 2, entry 11), but the combination of NHC–Ag 2 and RhCl3
showed a lower catalytic activity (14% yield; Table 2, entry 10).
Encouraged by these results we embarked on the use of ultrasonic
energy to assist the asymmetric addition of organoboronic acids to
aldehydes.
Having optimized the reaction conditions, we finally evaluated
the substrate scope with various aldehydes and organoboronic
acids. As shown in Table 3, the optimized protocol was compatible
with a wide variety of functional groups on both reaction partners,
and in most cases, the reaction could proceed with notable
efficiency (up to 94% yield) with 3 mol % catalyst loading. The elec-
tronic property of the arylaldehyde has an important effect on the
enantioselectivity of the catalytic reaction. With electron-deficient
arylaldehydes, the catalyst was more enantioselective (Table 3, en-
tries 1–3, 5–10). However, the electron-rich aryladehydes a5 and
a6 reacted with aromatic boronic acids b2 and b1 and afforded
the corresponding adducts c52 and c61 in 41% and 39% ee, respec-
tively (Table 3, entries 11, 12). Unfortunately, the reaction condi-
tions above were not suitable for the heterocyclic substrates. The
Table 1
Evaluation of solvent and basea
HO
CHO
B(OH)2
*
3 (3 mol%)
+
base / solvent
a1
Entry
b2
c12
Yieldb (%)
Solvent
Base
eec (%)
1
2
3
4
5
6
7
8
9
MeOH/DME (5:1)
t-BuOH/MeOH (5:1)
MeOH/DME (5:1)
t-BuOH/MeOH (5:1)
MeOH
t-BuOK
t-BuOK
KF
KF
KF
KF
KF
KF
KF
85
84
86
88
64
57
67
51
84
72
0
8 (R)
12 (R)
26 (R)
28 (R)
21 (R)
21 (R)
4 (R)
2 (R)
6 (R)
10 (R)
—
36 (R)
i-PrOH
DME/H2O (10:1)
Toluene/H2O (5:1)
ClCH2CH2Cl/H2O (1:1)
t-BuOH/EtOH (5:1)
t-BuOH/MeOH (5:1)
MeOH/DME (5:1)
10
11d
12d
KF
KF
KF
47
a
Reaction conditions: chloro(g2 g2-1,5-cyclo-octadiene)-[N,N0-bis[(Rp)-(+)-4-
,
[2.2] paracyclophanyl]imidazole-2-ylidene]rhodium 3 (3 mol %), t-BuOK (1 equiv),
or KF (6 equiv), phenylboronic acid (2 equiv), 1-naphthaldehyde (1 equiv), N2, 80 °C,
3 h.
b
Isolated yield.
c
Determined by chiral HPLC (CHIRALPAK IA Column) analysis.
Reaction temperature: 50 °C.
d

244
W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
Table 2
reactions of 2-furylaldehyde and 2-thienylaldehyde with various
Evaluation of the rhodium source and precatalysta
aromatic boronic acids b1, b2, b3, and b4 afforded the correspond-
ing products in moderate to high yields (59–91%) with low enanti-
oselectivities (5–24% ee) (Table 3, entries 13–20). We also
examined heteroaromatic boronic acids, very low activities were
observed using phenylaldehyde as a substrate (Table 3, entries
22, 23). When an aromatic aldehyde with an electron-withdrawing
group a4 was employed, better activities were achieved (82% and
94% yield), but only low enantioselectivities were obtained (Ta-
ble 3, entries 24, 25).
HO
CHO
B(OH)2
*
NHC-Ag (3 mol%)
Rhodium source(3 mol%)
+
KF
t-BuOH/MeOH=5/1
c12
a1
Entry
b2
Rhodium source
Ag-NHC
Yieldb (%)
78
eec (%)
3. Conclusion
1
2
3
4
5
[Rh(COD)Cl]2
[Rh(COD)OH]2
[Rh(OAc)2]2
[Rh(NBD)Cl]2
[Rh(C2H4)2Cl]2
RhCl3
RhCl3
RhCl3
RhCl3
RhCl3
2
2
2
2
2
2
5
2
5
2
5
6 (R)
12 (R)
37 (R)
36 (R)
32 (R)
48 (S)
52 (S)
—
58
83
81
79
87
51
0
33
14
73
Three novel planar chiral N-heterocyclic carbene silver and rho-
dium complexes based on [2.2]paracyclophane have been synthe-
sized and applied in the asymmetric addition of organoboronic
acids to aldehydes. All of these complexes were fully characterized
by 1H and 13C NMR, high-resolution mass spectrometry (HRMS),
and elemental analysis. Single-crystal X-ray diffraction result fur-
ther confirmed the molecular structure of NHC–Rh complex 3.
We optimized the reaction conditions and applied ultrasonic irra-
diation in the asymmetric arylation for the first time. Under ultra-
sound irradiation, the planar chiral NHC–Rh complexes can achieve
higher catalytic activities in the asymmetric addition of aromatic
boronic acids to arylaldehydes, and the best results are observed
with a combination of NHC–Ag 5 and RhCl3. However, the reaction
conditions above were not suitable for the heterocyclic substrates,
only low enantioselectivities could be obtained. Further modifica-
tions of [2.2]paracyclophane-based carbene–metal complexes as
well as applications in asymmetric catalysis are currently under-
way in our group.
6
7
8e
9e
58 (S)
61 (S)
58 (S)
10d,e
11d,e
RhCl3
a
Reaction conditions: Ag–NHC 2 or 5 (3 mol %), KF (6 equiv), phenylboronic acid
(2 equiv), 1-naphthaldehyde (1 equiv), N2, 80 °C, 3 h.
b
Isolated yield.
c
Determined by chiral HPLC (CHIRALPAK IA Column) analysis.
Ultrasound irradiation.
Reaction temperature: 40 °C.
d
e
Table 3
Scope of the methodologya
5
3 mol%
OH
O
RhCl3 3 mol%
4. Experimental section
4.1. General
R2 B(OH)2
+
R1
R2
R1
H
KF
t-BuOH/MeOH=5/1
))))
a
b
c
Commercially available reagents were used without further
purification unless otherwise noted. Solvents were of reagent
grade and purified by standard techniques. The planar chiral imi-
dazolium salts 1 and 4 were obtained by following the previous lit-
erature.11 Melting points were recorded on a melting point
apparatus and were uncorrected. 1H and 13C NMR spectra were
recorded on a Bruker AVANCE 300 at 298 K. HRMS spectra were
recorded on an Agilent Technologies 6510 Q-Tof LC/MS. Enantio-
meric excess was determined using HPLC on a Chiralpak IA chiral
column. Optical rotations were taken on a polarimeter WZZ-2B
equipped with a sodium lamp (k = 589 nm). The concentration ‘c’
has units of g/100 mL (or 10 mg/mL) unless otherwise noted.
Entry
Ar1
Ar2
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1-Naphthyl a1
1-Naphthyl a1
1-Naphthyl a1
Phenyl a2
4-ClC6H4 a3
4-ClC6H4 a3
Phenyl b2
73 c12
72 c13
52 c14
92 c21
57 c32
87 c33
67 c34
94 c42
87 c43
76 c44
72 c52
58 c61
91 c71
82 c72
74 c73
71 c74
82 c81
59 c82
81 c83
71 c84
0
58 (S)
67 (+)
52 (ꢀ)
44 (R)
58 (S)
48 (+)
54 (S)
61 (R)
58 (ꢀ)
62 (+)
41 (ꢀ)
39 (+)
24 (ꢀ)
20 (S)
14 (+)
24 (ꢀ)
20 (ꢀ)
19 (R)
5 (ꢀ)
18 (ꢀ)
—
2-MeOC6H4 b3
3-MeOC6H4 b4
1-Naphthyl b1
Phenyl b2
2-MeOC6H4 b3
3-MeOC6H4 b4
Phenyl b2
2-MeOC6H4 b3
3-MeOC6H4 b4
Phenyl b2
1-Naphthyl b1
1-Naphthyl b1
Phenyl b2
2-MeOC6H4 b3
3-MeOC6H4 b4
1-Naphthyl b1
Phenyl b2
2-MeOC6H4 b3
3-MeOC6H4 b4
Phenyl b2
2-Furyl b5
2-Thienyl b6
2-Furyl b5
4-ClC6H4 a3
4-MeOOCC6H4 a4
4-MeOOCC6H4 a4
4-MeOOCC6H4 a4
3-MeOC6H4 a5
2,4,6-Trimethyl C6H2 a6
2-Furyl a7
2-Furyl a7
2-Furyl a7
2-Furyl a7
2-Thienyl a8
2-Thienyl a8
2-Thienyl a8
2-Thienyl a8
3-Pyridyl
Phenyl a2
Phenyl a2
4-MeOOCC6H4 a4
4-MeOOCC6H4 a4
4.2. Chloro[N,N0-bis[(Rp)-(+)-4-[2.2]paracyclophanyl]imidazol-
2-ylidene]silver 2
A dry Schlenk tube flask was charged with N,N0-bis[(Rp)-(ꢀ)-4-
[2.2]paracyclophanyl] imidazolium triflate 1 (63 mg, 0.1 mmol),
tetrabutylammonium chloride (83 mg, 0.3 mmol), and Ag2O
(14 mg, 0.06 mmol). After backfilling with N2 of the reaction tube,
CH2Cl2 (2.0 mL) was added. The mixture was stirred in the absence
of light for 24 h at room temperature. The resulting off-white pre-
cipitate was filtered and the solvent was reduced until the solution
became cloudy. Subsequently hexane was added to precipitate the
product. Pure product 2 was obtained as a white solid by repeated
recrystallization from dichloromethane and hexane (29.4 mg, 47%
Trace
Trace
94 c45
82 c46
—
—
27 (ꢀ)
2-Thienyl b6
10 (ꢀ)
a
Reaction conditions: Ag–NHC 5 (3 mol %), RhCl3 (3 mol %), KF (6 equiv), orga-
noboronic acids (2 equiv), aldehydes (1 equiv), N2, 40 °C, Ultrasound irradiation:))))
3 h.
yield). Mp >260 °C, Rf 0.2 (CH2Cl2/ethanol = 30:1); ½a D20
ꢁ
¼ þ73:3
b
Isolated yield.
c
(c 0.21, CH2Cl2); 1H NMR (300 MHz, CDCl3, rt):
d
7.57 (d,
Determined by chiral HPLC (CHIRALPAK IA Column or Chiralcel OD-H column)
analysis.
J = 1.5 Hz, 2H), 6.80–6.62 (m, 14H), 3.25–2.84 (m, 16H); 13C NMR

W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
245
(75 MHz, CDCl3, rt): d 141.9, 139.6, 139.2, 138.5, 137.5, 134.6,
133.4, 133.3, 132.8, 129.4, 126.5, 122.5, 122.4, 35.3, 35.1, 34.9,
32.8. Anal. Calcd for C35H33AgClN2ꢂH2O: C, 65.38; H, 5.49; N, 4.36.
Found: C, 65.34; H, 5.36; N, 4.74.
concentrated in vacuo and the residue was purified by chromatog-
raphy (hexane/ethyl acetate = 15:1), to give the desired diaryl-
methanol as a slightly yellow oil at room temperature.
4.6. General procedure for the rhodium source evaluation
(Table 2)
4.3. Chloro(g2 g2-1,5-cyclo-octadiene)-[N,N0-bis[(Rp)-(+)-4-
,
[2.2]paracyclophanyl]imidazole-2-ylidene]rhodium 3
The NHC–Ag complex (3.8 ꢃ 10ꢀ3 mmol, 3 mol%) and rhodium
source (3.8 ꢃ 10ꢀ3 mmol, 3 mol %) were added to 0.5 mL of anhy-
drous methanol in a dried tube equipped with a condenser under
a nitrogen atmosphere. The mixture was stirred at room tempera-
ture overnight to give a yellow solution of the rhodium-complex.
Then the solvent was removed in vacuum and t-BuOH/MeOH
(5:1) (1.2 mL) was added and the suspension was stirred at room
temperature for 5 min. Then, phenylboronic acid (30.5 mg,
0.25 mmol), KF (43.5 mg, 0.75 mmol), and 1-naphthaldehyde
(19.5 mg, 0.125 mmol) were added successively. The resulting
mixture was clamped in an ultrasonic bath of 40 kHz frequency
at 40 °C for 3 h. The reaction mixture was concentrated in vacuo
and the residue was purified by chromatography (hexane/ethyl
acetate = 15:1), to give the desired diarylmethanol as a slightly yel-
low oil at room temperature.
Under an atmosphere of nitrogen, a methanol (1 mL) solution of
2 (29.4 mg, 0.047 mmol) and [Rh(COD)Cl]2 (13.0 mg, 0.024 mmol)
was stirred at 60 °C for 12 h. The solvent was removed in vacuo,
and the remaining yellow precipitate was then dissolved in
dichloromethane (2.0 mL). The product was purified by column
chromatography on silica gel (CH2Cl2/acetone = 20:1), the target
compound 3 was obtained as a yellow solid (29.5 mg, 83% yield).
The solid was recrystallized from hexane–CH2Cl2 to afford yellow
crystals of the product for X-ray diffraction analysis. Mp: 186–
188 °C, Rf 0.4 (CH2Cl2/ethyl acetate = 50:1); ½a D20
¼ þ47 (c 0.2,
ꢁ
CH2Cl2); 1H NMR (300 MHz, CDCl3, rt): d 7.77 (d, J = 1.9 Hz, 1H),
7.63 (d, J = 1.9 Hz, 1H), 6.91 (s, 1H), 6.79–6.62 (m, 10H), 6.56–
6.49 (m, 2H), 6.41 (d, J = 1.7 Hz, 1H), 4.51–4.44 (m, 1H), 4.33
(s, 1H), 3.67–3.57 (m, 1H); 3.34–2.81 (m, 16H); 2.20 (s, 1H),
1.82–1.74 (m, 3H), 1.54–1.11 (m, 7H). 13C NMR (75 MHz, CDCl3,
rt): d 187.4, 186.8, 141.1, 140.4, 140.3, 140.1, 139.0, 138.5, 137.9,
137.6, 136.5, 135.1, 135.0, 134.4, 134.3, 133.8, 133.6, 132.8,
132.6, 132.4, 132.3, 132.0, 129.8, 129.6, 128.1, 121.8, 121.2, 96.6
(d, J = 7.3 Hz), 96.1 (d, J = 7.3 Hz), 68.8 (d, J = 14.7 Hz), 66.5
(d, J = 14.6 Hz), 35.8, 35.6, 35.5, 35.4, 35.0, 34.3, 32.6, 32.0, 29.7,
28.4, 27.9. Anal. Calcd for C43H44ClN2Rhꢂ0.5H2Oꢂ0.5CH2Cl2: C,
67.10; H, 5.95; N, 3.60. Found: C, 67.06; H, 5.84; N, 3.22.
4.7. General procedure for the scope of the methodology
(Table 3)
The NHC–Ag complex 5 (2.73 mg, 3.8 ꢃ 10ꢀ3 mmol) and RhCl3
(0.78 mg, 3.8 ꢃ 10ꢀ3 mmol) were added to 0.5 mL of anhydrous
methanol in a dried tube equipped with a condenser under a
nitrogen atmosphere. The mixture was stirred at room temperature
overnight to give a yellow solution of the rhodium-complex. Then
the solvent was removed in vacuum and t-BuOH/MeOH (5:1)
(1.2 mL) was added and the suspension was stirred at room temper-
ature for 5 min. Then, organoboronic acid (0.25 mmol), KF (43.5 mg,
0.75 mmol), and aldehyde (0.125 mmol) were added successively.
The resulting mixture was clamped in an ultrasonic bath of 40 kHz
frequency at 40 °C for 3 h. The reaction mixture was concentrated
in vacuo and the residue was purified by column chromatography
on silica gel (hexane/ethyl acetate = 15:1), giving the desired diaryl-
methanols. The following diarylmethanol products were identified
by comparison to data reported in the literature.11,17
4.4. Bromo[N,N0-bis[(Rp)-(+)-12-methoxy-4-[2.2]paracyclo-
phanyl]imidazol-2-ylidene]silver 5
Under an atmosphere of nitrogen, N,N0-bis[(Rp)-(ꢀ)-12-meth-
oxy-4-[2.2]paracyclophanyl]imidazolium
triflate
(69 mg,
0.1 mmol) was dissolved in CH2Cl2 (5.0 mL), Ag2O (14 mg,
0.06 mmol) and NaBr (51 mg, 0.5 mmol) were added. The mixture
was stirred in the absence of light for 3 days at room temperature.
The resulting off-white precipitate was filtered and washed with
three 25 mL portions of dichloromethane. The combined filtrates
were collected, and the solvent was reduced until the solution be-
came cloudy. Subsequently hexane was added to precipitate prod-
4.7.1. (1-Naphthyl) phenylmethanols c12 and c21
uct
4 as a white solid (41.4 mg, 57% yield). Mp >186 °C
(S)-(ꢀ)-c12: 73% yield; ½a D20
¼ ꢀ24:7 (c 0.15, CHCl3) with 58%
ꢁ
(decomposition), Rf 0.2 (CH2Cl2/ethanol = 30:1); ½a D20
¼ þ125:2 (c
ꢁ
0.27, CH2Cl2); 1H NMR (300 MHz, CDCl3, rt): d 7.90 (s, 2H), 6.91
(d, J = 1.7 Hz, 2H), 6.72 (d, J = 7.9 Hz, 2H), 6.64 (d, J = 7.8 Hz, 2H),
6.53–6.45 (m, 4H), 6.00 (d, J = 1.6 Hz, 2H), 3.81 (s, 6H), 3.47–3.39
(m, 2H), 3.14–3.04 (m, 8H), 2.85–2.73 (m, 6H); 13C NMR
(75 MHz, CDCl3, rt): d 158.0, 141.8, 140.8, 138.3, 136.0, 134.8,
134.0, 132.6, 129.5, 127.6, 123.8, 121.6, 119.5, 59.3, 33.9, 33.1,
ee; the ee value was determined by HPLC analysis using a chiral
column (Chiralpak IA column, n-hexane/ethanol = 50:1, flow
0.5 ml/min, detection at 254 nm), retention times 67.7 min (major)
and 74.0 min (minor).
(R)-(+)-c21: 92% yield; ½a D20
¼ þ15:1 (c 0.25, CHCl3) with 44%
ꢁ
ee; the ee value was determined by HPLC analysis using a chiral
column (Chiralpak IA column, n-hexane/ethanol = 100:1, flow
0.5 ml/min, detection at 254 nm), retention times 100.4 min (min-
or) and 108.3 min (major).
31.8, 30.1. HRMS (ESI): calcd for
647.1828, found 647.1858.
C
37H36Ag N2O2 (MꢀBr)+
4.5. General procedure for the optimization of the base and
solvent for the arylation of aldehyde (Table 1)
4.7.2. (1-Naphthyl) (2-methoxyphenyl) methanol c13
(+)-c13: 72% yield; ½a D20
¼ þ31:1 (c 0.26, CHCl3) with 67% ee;
ꢁ
chloro(g2 g2-1,5-cyclo-octadiene)[N,N0-bis[(Rp)-(+)-4-
,
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 50:1, flow 0.5 ml/
min, detection at 254 nm), retention times 43.9 min (minor) and
52.3 min (major).
First,
[2.2]paracyclophanyl] imidazole-2-ylidene]rhodium 3 (2.84 mg,
3.8 ꢃ 10ꢀ3 mmol, 3 mol %) was weighed into a dried tube equipped
with a condenser under a nitrogen atmosphere. The solvent
(1.0 mL) was added and the suspension was stirred at room tem-
perature for 5 min. Then, t-BuOK (1 equiv) or KF (2–7 equiv), phen-
ylboronic acid (30.5 mg, 0.25 mmol), and 1-naphthaldehyde
(19.5 mg, 0.125 mmol) were added successively. The resulting
mixture was stirred at 80 °C for 3 h. The reaction mixture was
4.7.3. (1-Naphthyl) (3-methoxyphenyl) methanol c14
(ꢀ)-c14: 52% yield; ½a D20
¼ ꢀ35:6 (c 0.1, CHCl3) with 52% ee; the
ꢁ
ee value was determined by HPLC analysis using a chiral column
(Chiralpak IA column, n-hexane/ethanol = 50:1, flow 0.5 ml/min,

246
W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
detection at 254 nm), retention times 74.1 min (major) and
100.8 min (minor).
4.7.12. (2-Furyl) (1-naphthyl) methanol c71
(ꢀ)-c71: 91% yield; ½a D20
¼ ꢀ4:0 (c 0.18, CH2Cl2) with 24% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 20:1, flow 0.5 ml/
min, detection at 254 nm), retention times 26.5 min (major) and
28.5 min (minor). 1H NMR (300 MHz, CDCl3, rt): d 7.97–8.00 (m,
1H), 7.81–7.88 (m, 2H), 7.69 (d, J = 7.2 Hz, 1H), 7.39–7.51 (m,
4H), 6.53 (s, 1H), 6.27–6.28 (m, 1H), 6.02 (d, J = 3.3 Hz, 1H), 2.59
(s, 1H); 13C NMR (75 MHz, CDCl3, rt): d 155.2, 142.0, 135.8, 133.3,
130.1, 128.3, 125.7, 125.2, 124.9, 123.8, 123.1, 109.9, 107.5, 67.1.
4.7.4. (4-Chlorophenyl) phenylmethanol c32
(S)-(+)-c32: 57% yield; ½a D20
¼ þ25:6 (c 0.13, CHCl3) with 58% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 10:1, flow 0.5 ml/
min, detection at 254 nm), retention times 14.9 min (minor) and
15.7 min (major).
4.7.5. (4-Chlorophenyl) (2-methoxyphenyl) methanol c33
HRMS (ESI): calcd for
207.0814.
C
15H11
O
(MꢀOH)+ 207.0804, found
(+)-c33: 87% yield; ½a D20
¼ þ15:9 (c 0.25, CHCl3) with 48% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 10:1, flow 0.5 ml/
min, detection at 254 nm), retention times 15.7 min (minor) and
17.4 min (major).
4.7.13. (2-Furyl) phenylmethanol c72
(S)-(ꢀ)-c72: 82% yield; ½a D20
¼ ꢀ1:4 (c 0.15, CHCl3) with 20% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral
column (Chiralpak IA column, n-hexane/ethanol = 100:1, flow
0.5 ml/min, detection at 254 nm), retention times 71.5 min (major)
and 75.2 min (minor).
4.7.6. (4-Chlorophenyl) (3-methoxyphenyl) methanol c34
(S)-(+)-c34: 67% yield; ½a D20
¼ þ3:3 (c 0.37, CHCl3) with 54% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 10:1, flow 0.5 ml/
min, detection at 254 nm), retention times 32.4 min (minor) and
38.9 min (major).
4.7.14. (2-Furyl) (2-methoxyphenyl) methanol c73
(+)-c73: 74% yield; ½a D20
¼ þ5:1 (c 0.15, CH2Cl2) with 14% ee; the
ꢁ
ee value was determined by HPLC analysis using a chiral column
(Chiralpak IA column, n-hexane/i-propanol = 20:1, flow 0.5 ml/
min, detection at 254 nm), retention times 35.5 min (major) and
39.2 min (minor).
4.7.7. [4-(Methoxycarbonyl)phenyl] phenylmethanol c42
(R)-(ꢀ)-c42: 94% yield; ½a D20
¼ ꢀ20:0 (c 0.35, CHCl3) with 61%
ꢁ
ee; the ee value was determined by HPLC analysis using a chiral
column (Chiralpak IA column, n-hexane/ethanol = 10:1, flow
1.0 ml/min, detection at 254 nm), retention times 13.9 min (minor)
and 22.0 min (major).
4.7.15. (2-Furyl) (3-methoxyphenyl) methanol c74
(ꢀ)-c74: 71% yield; ½a D20
¼ ꢀ7:8 (c 0.14, CH2Cl2) with 24% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralcel OD-H column, n-hexane/i-propanol = 95:5, flow
0.5 ml/min, detection at 254 nm), retention times 49.4 min (major)
and 54.8 min (minor).
4.7.8. [4-(Methoxycarbonyl)phenyl] (2-methoxyphenyl)
methanol c43
(ꢀ)-c43: 87% yield; ½a D20
¼ ꢀ17:6 (c 0.31, CHCl3) with 58% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 10:1, flow 1.0 ml/
min, detection at 254 nm), retention times 12.5 min (minor) and
19.6 min (major).
4.7.16. (2-Thienyl) (1-naphthyl) methanol c81
(ꢀ)-c81: 82% yield; ½a D20
¼ ꢀ8:2 (c 0.15, CH2Cl2) with 20% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/i-propanol = 20:1, flow
0.5 ml/min, detection at 254 nm), retention times 33.9 min (minor)
and 36.8 min (major).
4.7.9. [4-(Methoxycarbonyl)phenyl] (3-methoxyphenyl)
methanol c44
(+)-c44: 76% yield; ½a D20
¼ þ27:9 (c 0.40, CHCl3) with 62% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/i-propanol = 10:1, flow
1.0 ml/min, detection at 254 nm), retention times 17.0 min (minor)
and 29.4 min (major).
4.7.17. (2-Thienyl) phenylmethanol c82
(R)-(ꢀ)-c82: 59% yield; ½a D20
¼ ꢀ1:7 (c 0.3, CHCl3) with 19% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/i-propanol = 100:1, flow
0.5 ml/min, detection at 254 nm), retention times 96.3 min (major)
and 99.4 min (minor).
4.7.10. (3-Methoxyphenyl) phenylmethanol c52
(R)-(ꢀ)-c52: 72% yield; ½a D20
¼ ꢀ12:2 (c 0.15, CHCl3) with 41%
ꢁ
ee; the ee value was determined by HPLC analysis using a chiral
column (Chiralpak IA column, n-hexane/ethanol = 50:1, flow
0.5 ml/min, detection at 254 nm), retention times 71.1 min (major)
and 87.2 min (minor).
4.7.18. (2-Thienyl) (2-methoxyphenyl) methanol c83
(ꢀ)-c83: 81% yield; ½a D20
¼ ꢀ1:3 (c 0.18, CH2Cl2) with 5% ee; the
ꢁ
ee value was determined by HPLC analysis using a chiral column
(Chiralpak IA column, n-hexane/i-propanol = 50:1, flow 0.5 ml/
min, detection at 254 nm), retention times 70.5 min (minor) and
75.7 min (major).
4.7.11. (2,4,6-Trimethylphenyl) (1-naphthyl) methanol c61
(+)-c61: 58% yield; ½a D20
¼ þ11:2 (c 0.21, CHCl3) with 39% ee;
ꢁ
The ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/i-propanol = 100:1, flow
0.5 mL/min, detection at 254 nm), retention times 64.8 min (ma-
jor) and 73.5 min (minor). 1H NMR (300 MHz, CDCl3, rt): d 8.22
(d, J = 3.3 Hz, 1H), 7.89 (d, J = 5.2 Hz, 1H), 7.80–7.86 (m, 7H), 6.53
(s, 1H), 2.30 (s, 9H), 2.15 (s, 1H); 13C NMR (75 MHz, CDCl3, rt): d
137.2, 137.1, 137.1, 135.4, 134.1, 131.5, 130.4, 128.8, 128.5,
126.2, 125.6, 125.1, 125.0, 124.4, 70.9, 21.2, 20.9. HRMS (ESI): calcd
for C20H19 (MꢀOH)+ 259.1487, found 259.1493.
4.7.19. (2-Thienyl) (3-methoxyphenyl) methanol c84
(ꢀ)-c84: 71% yield; ½a D20
¼ ꢀ1:7 (c 0.12, CH2Cl2) with 18% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/ethanol = 20:1, flow 0.5 ml/
min, detection at 254 nm), retention times 41.0 min (minor) and
44.3 min (major).

W. Duan et al. / Tetrahedron: Asymmetry 24 (2013) 241–248
247
4.7.20. [4-(Methoxycarbonyl)phenyl] (2-furyl) methanol c45
References
(ꢀ)-c45: 94% yield; ½a D20
¼ ꢀ8:4 (c 0.15, CH2Cl2) with 27% ee;
ꢁ
1. (a) Huang, J.; Schanz, H. J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18,
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the ee value was determined by HPLC analysis using a chiral col-
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min, detection at 254 nm), retention times 67.9 min (minor) and
75.5 min (major).
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4.7.21. [4-(Methoxycarbonyl)phenyl] (2-thienyl) methanol c46
(ꢀ)-c46: 82% yield; ½a D20
¼ ꢀ10:4 (c 0.21, CH2Cl2) with 10% ee;
ꢁ
the ee value was determined by HPLC analysis using a chiral col-
umn (Chiralpak IA column, n-hexane/i-propanol = 10:1, flow
0.5 ml/min, detection at 254 nm), retention times 26.5 min (minor)
and 29.4 min (major). 1H NMR (300 MHz, CDCl3, rt): d 8.03 (d,
J = 8.4 Hz, 2H), 7.53 (d, J = 8.4 Hz, 2H), 7.26–7.29 (m, 1H), 6.93–
6.96 (m, 1H), 6.90–6.91 (m, 1H), 6.11 (s, 1H), 3.91 (s, 3H), 2.57 (s,
1H); 13C NMR (75 MHz, CDCl3, rt): d 166.4, 147.4, 146.8, 129.4,
129.2, 126.3, 125.7, 125.4, 124.8, 71.4, 51.7. HRMS (ESI): calcd for
C
13H11SO2 (MꢀOH)+ 231.0474, found 231.0485.
4.8. Crystallographic analysis of NHC-Rh 3
Single crystals of complex 3 could be obtained from hexane–
CH2Cl2 by slow evaporation of CH2Cl2. X-ray diffraction data were
collected on a Bruker Smart APEXIICCD area-detector diffractome-
ter equipped with a graphite-monochromated Mo K
a radiation
(k = 0.71073 Å) at 273(2) K. The structure was solved by direct
methods, and refined by full-matrix least-squares techniques using
the SHELXTL-9718 program. All of the non-hydrogen positions were
refined anisotropically. Hydrogen atoms were placed in calculated
positions and refined using a riding model.
The molecular structure of complex 3 in the solid state is
shown in Figure 1. In the structure of 3, the Rh–C(1) bond
length of 2.019(5) Å compares well with those reported for
other Rh–NHC complexes.19 Due to the influences of the NHC
and the chloro-ligand, a significantly longer bond (0.09 Å) in
the rhodium to carbon distances of the cyclooctadiene (COD)
is observed. While the Rh–C bond lengths for carbons C(36)
and C(37) are located at 2.089(6) and 2.119(5) Å, respectively
the Rh–C for carbons C(40) and C(41) are 2.179(6) and
2.206(5) Å, respectively. The Rh–Cl bond length (2.3862(14) Å)
in 3 is only slightly longer than other carbene complexes with
less steric hindrance.
8. Shintani, R.; Takatsu, K.; Hayashi, T. Chem. Commun. 2010, 46, 6822–6824.
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Am. Chem. Soc. 1997, 119, 6207–6208; (b) Zanotti-Gerosa, A.; Malan, C.;
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Soc. 2002, 124, 5940–5941; (d) Wu, X. W.; Yuan, K.; Sun, W.; Zhang, M. J.; Hou,
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Crystallographic data (excluding structure factors) for 3 have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 903887. Copies of
the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK [fax: +44 1223 336 033;
e-mail: deposit@ccdc.cam.ac.uk].
Crystal data for 3. C43H52ClN2O4Rh, M = 799.23, yellow, crystal
dimension: 0.15 ꢃ 0.12 ꢃ 0.10 mm3, orthorhombic, space group:
P2(1)2(1)2(1), a = 10.9922(9) Å, b = 12.3838(10) Å, c = 30.784(3) Å,
a
= 90.00°,
b = 90.00°,
c
= 90.00°,
V = 4190.5(6) Å3,
Z = 4,
Dcalcd = 1.267 g/cm3, F(000) = 1672, T = 273(2) K,
l
= 0.512 mmꢀ1
,
h Range: 2.11–25.05°, 21,808 measured reflections, 7368 unique
reflections (Rint = 0.1045), min/max transmission factors 0.9271/
0.9506, final agreement factors R1 = 0.0531 and wR2 = 0.1500,
7368/27/460 data/restraints/parameters, GOOF = S = 1.079, largest
peak and hole 1.660 and ꢀ0.447 e/Å3.
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Acknowledgments
Financial support from the National Natural Science
Foundation of China (Grant Nos. 20671059) and Shandong Prov-
ince Natural Science Foundation (ZR2011BM013) is gratefully
acknowledged.
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