A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

Article abstract of DOI:10.1016/j.jorganchem.2010.07.038

A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C₂-symmetric bis(hydroxyamide)- functionalized benzimidazolium salts from chiral β-amino alcohols was developed. The combination of Cu(OTf)

Full text of DOI:10.1016/j.jorganchem.2010.07.038

Journal of Organometallic Chemistry 696 (2011) 61e67
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A new C2-symmetric azolium compound for Cu-catalyzed asymmetric
conjugate addition of R2Zn to cyclic enone
*
Ayako Harano, Satoshi Sakaguchi
Department of Chemistry and Materials Engineering & High Technology Research Center, Faculty of Chemistry, Materials and Bioengineering,
Kansai University, Suita, Osaka 564-8680, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C2-
symmetric bis(hydroxyamide)-functionalized benzimidazolium salts from chiral b-amino alcohols was
Received 15 July 2010
Received in revised form
29 July 2010
Accepted 30 July 2010
Available online 7 August 2010
developed. The combination of Cu(OTf)2 and the chiral azolium compound efficiently promoted the
conjugate addition reaction of cyclic enone with dialkylzinc to give the corresponding adduct in good
yield. Among a series of chiral NHC proligands, the functionalized benzimidazolium chloride possessing
a tert-butyl group as a stereodirecting group was found to be the best choice of ligand. Under optimized
reaction conditions, an excellent enantioselectivity (96% ee) was realized by allowing 2-cyclohepten-1-
one to react with Bu2Zn at room temperature.
Keywords:
Asymmetric catalysis
N-Heterocyclic carbene
Conjugate addition
Polydentate ligand
Ligand design
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Arduengo-type diaminocarbene ligands afforded moderate enan-
tioselectivity in the 1,4-addition of Et2Zn to cyclic enones [4,5]. The
The conjugate addition (1,4-addition) of carbon nucleophiles to
a,b-unsaturated carbonyl compounds is one of the most widely
introduction of a second coordination site at the NHC ligand
provided a tightly coordinating polydentate NHC ligand system
that is expected to enhance catalyst stability and to offer a key
structure for the construction of efficient stereodirecting elements
[6e10]. Pioneering work was performed by Arnold et al. in 2004 [6]
in which the Cu-catalyzed enantioselective conjugate addition
reaction proceeded with moderate enantioselectivity using a chiral
anionic tethered bidentate NHC ligand. A breakthrough was ach-
ieved by Mauduit and Alexakis et al. who developed various
bidentate hydroxyalkyl-NHC precursors for the ECA reaction of
cyclic enones with dialkylzincs as well as the 1,4-addition reaction
used methods for the construction of CeC bonds. Since the reaction
often yields a stereogenic center at the -carbon, considerable
b
efforts have been devoted to the development of its asymmetric
versions [1]. The control of stereoselectivity in transition-metal-
catalyzed reactions depends on the design of a versatile ligand that
would strongly coordinate with the metal center. Chiral N-hetero-
cyclic carbene (NHC) ligands have received considerable interest in
recent years because of their strong s-donating capability to metals
and the possibility of varying the substituents on the nitrogen atom
[2]. Since NHCs are readily accessible, easily tunable, and form
highly stable and efficient NHCemetal catalysts, a rapid elaboration
of NHC ligand libraries based on the same scaffold can be envisaged
and would enhance the chances of discovering an ideal chiral
ligand for a given metal-catalyzed asymmetric transformation [3].
Over the past decade, much attention has been paid to the
enantioselective conjugate addition (ECA) reaction catalyzed by
a copper salt combined with a functionalized NHC [1]. In 2001,
Alexakis and Roland independently reported that monodentate
of b-substituted cyclic enones with Grignard reagents [7]. More-
over, Hoveyda et al. introduced chelating anionic hydroxyaryl-NHC
ligands which were successfully used not only in asymmetric
catalytic allylic alkylations but also in ECA reactions [8,9]. Thus,
anionic tethered polydentate NHC chemistry has received consid-
erable attention [11].
Recently, we succeeded in the development of a novel chiral
polydentate hydroxyamide-functionalized NHC ligand and its Pd(II)
complexes [12]. Note that the Pd(II) complex derived from a chiral
b-amino alcohol catalyzes an asymmetric oxidative Heck-type
reaction with excellent enantioselectivity. Encouraged by this
success, we developed an efficient route to a huge variety of chiral
NHC precursors, such as azolium salts and the tridentate, anionic
* Corresponding author. Tel.: þ81 6 6368 0867; fax: þ81 6 6339 4026.
E-mail address: satoshi@ipcku.kansai-u.ac.jp (S. Sakaguchi).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.07.038

62
A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
tethered NHCePd(II) complexes [13]. Ligand precursors, such as
azolium chlorides, have also been tested in the Cu-catalyzed
asymmetric conjugate addition of dialkylzincs to cyclic enones. We
discovered that reversal of enantioselectivity with the same ligand
could be achieved by changing the Cu precatalyst from Cu(OTf)2 to
Cu(acac)2 [14].
As part of our continuing efforts to design new catalyst systems
for enantioselective CeC bond formation, our interests turned to
the design and development of C2-symmetrical, anionic chelating
NHC ligand precursors such as bis(hydroxyamide)-functionalized
azolium compounds. It is known that the pincer-type ligands act as
tightly coordinating polydentate ligands and that they provide a C2-
symmetric and meridional environment around the metal center.
Hence, it is expected that the metal complex having of the pincer
ligand becomes a suitable catalyst to distinguish the prochiral face
of a substrate [15,16]. Herein, we describe an efficient Cu-catalyzed
asymmetric conjugate addition of dialkylzinc to cyclic enones via
the new class of chiral C2-symmetrical ligands based on an NHC
scaffold.
(iii) the products in each steps could be purified via a simple
procedure without employing column chromatographic tech-
niques; (iv) the yields are moderate to good; (v) the azolium
compounds 5ae5g are air-stable and easy to handle.
To test the possibility of achieving an asymmetric trans-
formation reaction using the chiral benzimidazolium chlorides
5ae5g, we chose the Cu-catalyzed conjugate addition of Et2Zn to 2-
cyclohexen-1-one (6) as a model reaction. First, we screened azo-
lium salts and copper salts for the ECA reaction (Table 1). Treatment
of 6 (1 mmol) with Et2Zn (3 mmol) in THF in the presence of Cu
(OTf)2 (2 mol %) and 5a (3 mol %) at room temperature for 3 h
produced the desired adduct, such as 3-ethylcyclohexanone (7) in
almost quantitative yield and 44% ee (Run 1). The NHC ligand
precursor, such as 5b or 5c derived from (S)-leucinol or (S)-iso-
leucinol, respectively, showed almost the same results (Runs 2 and
3). A sterically hindered substituent such as a tert-butyl group in
the ligand proved efficient to produce 7 in 62% ee; however, the
yield decreased, probably owing to the steric hindrance of the
ligand (Run 4). Both 5e and 5f resulted in poor enantioselectivities
(Runs 5 and 6). When the reaction was carried out using Cu(OTf)2
2. Results and discussion
Table 1
A series of benzimidazolium chlorides have been successfully
ECA reaction of 6 with Et2Zn catalyzed by Cu salt combined with benzimidazolium
synthesized using commercially available
chiral source (Scheme 1). Reaction of (S)-valinol with bromoacetyl
bromide or chloroacetyl chloride produced the corresponding
bromo- or -chloroacetamide derivative, 2a or 4a, in almost
quantitative yield, respectively [17]. The hydroxyamide group was
introduced into the benzimidazole ring by allowing 2a to react with
benzimidazole (1) in the presence of KOH in CH3CN at room
temperature. The resulting N-hydroxyamide-functionalized benz-
imidazole 3a was coupled with 4a to yield the corresponding
benzimidazolium salt 5a in 71% yield. Several azolium compounds
b-amino alcohols as the
salt.a
a
-
O
O
a
cat.
Cu salt / Azolium salt
THF, r.t., 3 h
+ Et2Zn
Et
6
7
Run
Cu salt
Azolium salt
Yieldb (%)
Eeb (%)
1
2
3
4
5
6
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(NO3)2
Cu(II) saltd
CuCl2
5a
5b
5c
5d
5e
5f
5d
5d
5d
5d
5d
5d
5hf
>99
90
84
43
94
78
96
82
82
70
88
83
87
44
54
51
62
25
30
63
57
42
13
2
5be5g could be obtained via the same route from chiral b-amino
alcohols such as (S)-leucinol, iso-leucinol, tert-leucinol, phenyl-
alaninol, and phenylglycinol, respectively (Scheme 1). This
synthetic approach offers several advantages: (i) the starting
materials used, such as chiral b-amino alcohols and reagents are
readily available; (ii) all operations in the reaction steps are simple;
7c
8c
9c
10c
11c
12c
13
Cu(acac)2
Cu(II) salte
Cu(OTf)2
O
Br
O
5
N
R1
ent-52
HN
R1
+
NH
HN
N
KOH,
a
6 (1 mmol), Et2Zn (3 mmol), Cu salt (2 mol %), benzimidazolium salt (3 mol %),
CH3CN, r.t.
N
1
THF (3 mL), r.t., 3 h.
OH
b
OH
Yield (%)
2a-f
3
Yield and ee were determined by GLC analysis. Average of two runs.
Cu salt (4 mol %), 5d (6 mol %).
c
R1
R1
t
Yield (%)
CF3
F3C
i
3a
70
68
50
90
80
78
Pr
Bu
3d Bu
i
O
O
3b
3e
Bn
d
s
Cu
3c
3f
Bu
Ph
O
O
CF3
F3C
O
O
N
Cl
HN
R1
O
Cu
O
N
R2
N
e
HN
+
3
Cl
NH
1,4-dioxane,
rf.
N
OH
OH
4a-f
O
R2
OH
5
O
N
R1 R2 Yield (%)
R1 R2 Yield (%)
i
Bu
HN
i
i
t
t
N
5a
5b
5c
71
74
53
5d
5e
5f
53
45
84
58
Pr Pr
Bu Bu
Bn Bn
Ph Ph
f
Cl
i
i
Bu Bu
s
s
NH
OH
Bu Bu
i
i
5g
O
Pr Bu
i
OH
Bu
5h
Scheme 1. Synthesis of various benzimidazolium salts.

A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
63
(4 mol %) and 5d (6 mol %), the adduct was obtained in excellent
yield with 63% ee (Run 7).
Then, we examined the influence of the copper/ligand ratio as
well as catalyst loading on stereoselectivity in the reaction of 6 with
Et2Zn in DMA (Runs 14e17). The reaction with a Cu(OTf)2/5d ratio
of 1/1.5, 1/1, and 1/0.5 took place in a similar fashion, affording the
corresponding adduct in moderate to good yield and enantiose-
lectivity (Runs 12, 14 and 15). Decreasing the amount of Cu(OTf)2
only slightly reduced the product yield (Runs 16 and 17). In addi-
tion, the reaction catalyzed by Cu(OTf)2 (4 mol %) combined with
5d (2 mol %) in ethyl acetate proceeded in a comparable way to the
reaction in THF, giving 7 in 92% yield and 62% ee (Run 1 vs. Run 18).
Next, we were interested in the influence of N-functional groups
at the azolium ring on the stereoselectivity in the ECA reaction
(Table 3). A series of non-C2-symmetric azolium compounds 8e11
having the hydroxyamide-functionality at the N(1) substituent
could be synthesized in a route similar to that shown in Scheme 1.
These examinations revealed that a bis(hydroxyamide)-substituted
azolium compound such as 5d works more efficiently than azolium
salts possessing an alkylamide group or a hydroxyamide group
without any stereodirecting elements at the N(3) substituent, such
as 8 or 9 (Runs 1e3). The azolium salt 10 that was prepared by the
coupling of N-methylbenzimidazole with 4d provided the conju-
gate addition product 7 in 71% yield and 62% ee (Run 4), while
a slightly better result was obtained using the chiral NHC precursor
11 bearing an N-benzyl group at the azolium ring (Run 5). The ECA
reaction using 10 or 11 as a chiral ligand is currently under inves-
tigation [14b].
Having established the catalytic activity of Cu(OTf)2 combined
with the C2-symmetrical azolium salt 5d, we subsequently turned
our attention to the effect of copper salt other than Cu(OTf)2 on
reactivity and enantioselectivity (Runs 8e12). The result with Cu
(NO3)2 was similar to that with Cu(OTf)2 (Run 8). CuCl2, Cu(acac)2,
and bis(8-quinolinolato)copper(II) led to racemic 7, although
moderate to good yields were obtained in each case (Runs 10e12).
In the preceding paper, we showed that the conjugate addition of
Et2Zn to 6 catalyzed by Cu(OTf)2 combined with a chiral azolium
chloride having both a hydroxyamide group at the N(1) substituent
and a methyl group at the N(3) substituent of the azolium ring gave
(S)-7, while use of Cu(acac)2 in combination with the same ligand
afforded (R)-7 as the major product [14]. However, the reversal of
enantioselectivity by changing the Cu species from Cu(OTf)2 to Cu
(acac)2 was not observed in the case of 5d as a chiral ligand
precursor (Run 7 vs. Run 11). (R)-6 was obtained as the major
product using 5h derived from (R)-iso-leucinol (Run 2 vs. Run 13).
Numerous variations of the reaction conditions were explored
for the Cu(OTf)2-catalyzed addition of Et2Zn to 6 in the presence of
5d (Table 2). For further reaction optimization, the solvent effect
was investigated. The first promising result was achieved with THF,
which gave 63% ee and 96% product yield in the ECA reaction by the
Cu(OTf)2/5d catalytic system (Run 1). Almost the same result was
observed for the reaction in 1,4-dioxane, giving 7 in 96% yield and
57% ee (Run 2). Other ethereal solvents such as Et2O, TBME, and
CPME as well as non-polar solvents such as CH2Cl2 and toluene did
not improve the enantioselectivity, although the conjugate addition
took place smoothly to produce 7 in good to excellent yield (Runs
3e7). The reaction in polar solvents such as CH3CN, DMSO and DMF
gave 7 with difficulty (Runs 8e10). However, the stereoselectivity
of the conjugate adduct in the reaction using DMA as a solvent was
found to be comparable to or better than that obtained using THF
(Run 11). The quantity of Et2Zn in this ECA reaction has a significant
effect on the yield of product (Runs 11e13).
Moreover, the easy access to N-functionalized NHC precursors
allowed us to investigate the synergetic effect of the stereodirecting
groups on the ligands (Scheme 2). With 5a and 5b, 6 reacted with
Table 3
Influence of N(3) substituent at benzimidazolium ring on the ECA reaction of 6 with
Et2Zn catalyzed by Cu(OTf)2.a
Run
Azolium salt
Yieldb (%)
Eeb (%)
O
N
t
HN Bu
N
Cl
R3
Table 2
OH
Solvent effect on ECA reaction of 6 with Et2Zn catalyzed by Cu(OTf)2 combined with
5d.a
Run
Cu(OTf)2
(mol %)
5d (mol %)
Solvent
Yield (%)b
Eeb (%)
t
O
Bu
R3 =
OH
N
H
1
2
3
4
5
6
7
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2
2
4
6
6
6
6
6
6
6
6
6
6
6
6
6
4
2
5
3
2
THF
1,4-dioxane
Et2O
96
96
74
95
86
>99
70
46
43
30
77
83
27
82
72
73
69
92
63
57
25
20
22
15
9
43
45
55
67
70
53
66
68
64
64
62
1
2
5d
8
72
95
63
68
46
35
TBMEc
CPMEd
CH2Cl2
toulene
CH3CN
DMSO
DMF
O
N
H
9
10
11
12f
13g
14f
15f
16f
17f
18f
O
DMAe
DMA
DMA
DMA
DMA
DMA
DMA
AcOEt
OH
N
3
4
9
H
Me
Bn
10
11
71
83
62
70
a
6 (1 mmol), Et2Zn (3 mmol), solvent (3 mL), r.t., 3 h.
Yield and ee were determined by GLC analysis. Average of two runs.
tert-Butyl methyl ether.
Cyclopentyl methyl ether.
N,N-Dimethyl acetoamide.
b
c
d
e
f
5
a
6 (1 mmol), Et2Zn (4 mmol), DMA (3 mL), Cu(OTf)2 (4 mol %), benzimidazolium
Et2Zn (4 mmol).
Et2Zn (1.5 mmol).
salt (2 mol %), r.t., 3 h.
g
b
Yield and ee were determined by GLC analysis. Average of two runs.

64
A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
DMA gave (S)-12 with a somewhat lower ee value (Run 2). Reaction
Azolium salt
Yield (%) Ee (%)
of a cyclic enone consisting of a seven-membered ring such as 2-
cyclohepten-1-one (13) increased the stereoselectivity (Runs 3e6).
For example, cyclic enone 13 efficiently underwent 1,4-addition
with Et2Zn in DMA to afford (S)-3-ethylcycloheptanone (14) in 96%
yield and 85% ee (Run 4). An excellent ee value of 96% was obtained
by allowing 13 to react with Bu2Zn in DMA to produce (S)-3-
butylcycloheptanone (15) (Run 6). 4,4-Dimethyl-2-cyclohexen-1-
one (16) also reacted smoothly with Et2Zn to form (R)-3-ethyl-4,
4-dimethylcyclohexanone (17) in good yield (Runs 7e8). A better
result was obtained by using DMA as a solvent than that obtained
using THF.
O
i
5a (R1, R2 = Pr)
N
36
59
80
42
47
53
HN
R1
N
i
5b (R1, R2 = Bu)
Cl
NH
i
i
5g (R1, R2 = Pr, Bu)
OH
O
R2
OH
Reaction contions: 6 (1 mmol), Et2Zn (4 mmol), Cu(OTf)2 (4 mol %),
benzimidazolium salt (2 mol %), DMA (3 mL), r.t., 3 h.
Scheme 2. Synergetic effect of the stereodirecting groups on the benzimiazolium salt
in the ECA reaction of 6 with Et2Zn catalyzed by Cu(OTf)2.
3. Conclusion
We have demonstrated that a C2-symmetric bis(hydroxyamide)-
functionalized benzimidazolium salt efficiently performs a copper-
catalyzed asymmetric conjugate addition of dialkylzinc reagents to
cyclic enones, with up to 96% ee. These air-stable, chiral ligands are
especially useful because of the operational simplicity of their
preparation. In addition, the ECA reaction could be carried out at
ambient temperature without controlling the temperature of the
solution. The applications of the chiral chelating polydentate NHC
catalysts as well as the design and development of a new class of
chiral ligand based on an NHC scaffold for efficient catalytic enan-
tioselective transformations is the subject of ongoing research in
our laboratory.
Et2Zn in DMA under the influence of Cu(OTf)2 to produce the
conjugate adduct 7 in low yield and enantioselectivity, respectively.
However, it was found that the reaction with non-C2-symmetric bis
(hydroxyamide)-functionalized benzimidazolium salt 5g was more
efficient (80% yield) as well as slightly more enantioselective (53%
ee) than that with the C2-symmetric azolium salts 5a or 5b. The
reason for this synergetic effect is unclear, but one possibility is that
a suitable site for coordination of the enone 6 might be generated in
the catalytic-active species derived from Cu(OTf)2 and 5g having
both iso-propyl and iso-butyl groups.
With the optimal catalyst combination and reaction conditions
in hand, we finally investigated the scope of the ECA reaction of
different cyclic enones with dialkylzincs (Table 4). These reactions
were performed in the presence of catalytic amounts of Cu(OTf)2
and 5d at room temperature for 3 h with either THF or DMA as
a solvent. Replacement of Et2Zn by Bu2Zn in the reaction of 6 in THF
led to a slight improvement in the enantioselectivity (76% ee) of the
corresponding adducts (S)-12 (Run 1). Treatment of 6 with Bu2Zn in
4. Experimental
4.1. General procedure for the preparation of 3 and 5
To CH3CN were added KOH (0.48 M), benzimidazole (1) (0.32 M)
and
a
-bromoacetamide derivative 2 (0.32 M) derived from bro-
-amino alcohol. After stirring the reaction
moacetyl bromide and
b
mixture at room temperature for 16 h, H2O was added to form
a white precipitate. The white product was separated from the
solution by filtration under reduced pressure, and then the solid
was dissolved in methanol. The resulting solution was dried over
Na2SO4. After filtration, the filtrate was concentrated under
reduced pressure to obtain the desired product such as N-hydrox-
yamide substituted benzimidazole derivative 3. The product 3
could be used without further purification in the next step.
Table 4
ECA reaction of several cyclic enones with R2Zn catalyzed by Cu(OTf)2 combined
with 5d.a
Run
Substrate
Solvent
Product
Yieldb (%)
Eec (%)
O
1
2
THF
DMA
78
84
76
66
6þBu2Zn
Then, to 1,4-dioxane were added 3 (0.293 M) and
oacetamide 4 (0.267 M) derived from chloroacetyl chloride and
a-chlor-
Bu
b
-
12
amino alcohol. After stirring the reaction mixture at 110 ꢀC for 16 h,
the solvent was removed under reduced pressure. The residue was
dissolved in methanol, and then activated carbon was added. After
16 h, the activated carbon was removed by filtration. The filtrate
was concentrated under reduced pressure to obtain a solid, which
was purified by reprecipitation using ethyl acetate and methanol to
afford the corresponding coupling product 5 as a solid.
O
O
3
4
THF
DMA
65
96
74
85
Et2Zn
Et
13
14
O
13þBu2Zn
5
6
THF
DMA
91
83
84
96
Bu
4.2. Analytical data
15
4.2.1. Compound 3a
1H-NMR (CD3OD):
d 8.16 (s,1H), 7.68e7.65 (m,1H), 7.51e7.47 (m,
O
O
7
8
THF
DMA
86
78
58
71
1H), 7.34e7.23 (m, 2H), 5.05 (d, J ¼ 16.3 Hz, 1H), 4.98 (d, J ¼ 16.3 Hz,
1H), 3.75e3.67 (m, 1H), 3.67e3.52 (m, 2H), 1.94e1.80 (m, 1H), 0.93
Et2Zn
(d, J ¼ 4.7 Hz, 3H), 0.90 (d, J ¼ 4.7 Hz, 3H); 13C-NMR:
d 169.1, 145.7,
Et
124.4, 123.7, 120.0, 111.3, 63.0, 58.4, 48.2, 30.0, 20.0, 18.8.
16
17
a
Enone (1 mmol), R2Zn (3 mmol for THF or 4 mmol for DMA), solvent (3 mL), Cu
4.2.2. Compound 3b
(OTf)2 (4 mol %), 5d (6 mol %), r.t., 3 h.
1H-NMR ((CD3)2SO):
d
8.15 (s, 1H), 8.13 (d, J ¼ 12.0 Hz, 1H),
b
Isolated yield. Average of two runs.
c
Ee was determined by GLC analysis.
7.65e7.63 (m, 1H), 7.42e7.41 (m, 1H), 7.24e7.17 (m, 2H), 4.93 (d,

A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
65
J ¼ 16.0 Hz,1H), 4.87 (d, J ¼ 16.0 Hz,1H), 4.75 (br,1H), 3.84e3.75 (m,
4.2.10. Compound 5d
1H), 3.37e3.26 (m, 2H), 1.64e1.54 (m, 1H), 1.33e1.30 (m, 2H), 0.87
White solid; mp 116.2e117.1 ꢀC. 1H-NMR ((CD3)2SO):
d
9.86 (s,
(d, J ¼ 4.0 Hz, 3H), 0.80 (d, J ¼ 4.0 Hz, 3H); 13C-NMR:
d
166.1, 144.9,
1H), 8.45 (d, J ¼ 9.2 Hz, 2H), 7.98e7.95 (m, 2H), 7.70e7.67 (m, 2H),
5.52 (d, J ¼ 16.4 Hz, 2H), 5.42 (d, J ¼ 16.4 Hz, 2H), 4.64 (t, J ¼ 5.6 Hz,
2H), 3.78e3.60 (m, 4H), 3.43e3.40 (m, 2H), 0.89 (s, 18H); 13C-NMR:
143.3, 134.2, 122.3, 121.5, 119.4, 110.2, 63.7, 49.2, 48.6, 47.0, 24.3,
23.4, 21.8.
d
164.7, 144.3, 131.2, 126.7, 113.6, 60.3, 59.8, 48.7, 33.6, 26.8. Anal.
4.2.3. Compound 3c
Calc. for C23H37ClN4O4$3H2O: C, 52.81; H, 8.29; N, 10.71. Found: C,
52.42; H, 7.76; N, 10.57%.
1H-NMR ((CD3)2SO):
d
8.15 (s, 1H), 8.12 (d, J ¼ 9.0 Hz, 1H),
7.65e7.63 (m, 1H), 7.45e7.43 (m, 1H), 7.24e7.17 (m, 2H), 4.96 (d,
J ¼ 16.1 Hz, 1H), 4.89 (d, J ¼ 16.1 Hz, 1H), 4.64 (br, 1H), 3.64e3.57 (m,
1H), 3.48e3.41 (m, 2H), 1.66e1.55 (m, 1H), 1.46e1.40 (m, 1H),
4.2.11. Compound 5e
White solid; mp 215.6e216.6 ꢀC. 1H-NMR ((CD3)2SO):
d 9.63 (s,
1.10e1.03 (m, 1H), 0.83e0.78 (m, 6H); 13C-NMR:
d
166.2, 144.9,
1H), 8.76 (d, J ¼ 8.0 Hz, 2H), 7.58e7.51 (m, 4H), 7.24e7.18 (m, 10H),
5.31 (d, J ¼ 16.0 Hz, 2H), 5.22 (d, J ¼ 16.0 Hz, 2H), 5.00 (br, 2H),
3.97e3.89 (m, 2H), 3.47e3.33 (m, 4H), 2.89e2.84 (m, 2H),
143.2, 134.2, 122.2, 121.5, 119.3, 110.2, 60.9, 55.0, 47.0, 34.8, 24.7,
15.5, 11.2.
2.68e2.63 (m, 2H); 13C-NMR:
d 163.9, 144.1, 138.9, 131.0, 129.2,
4.2.4. Compound 3d
128.2, 126.7, 126.1, 113.4, 62.6, 53.3, 48.6, 36.6. Anal. Calc. for
C29H33ClN4O4$2.25H2O: C, 60.30; H, 6.54; N, 9.70. Found: C, 60.31;
H, 6.16; N, 9.74%.
1H-NMR ((CD3)2SO):
d 8.16 (s, 1H), 8.02 (br, 1H), 7.65e7.63 (m,
1H), 7.47e7.45 (m, 1H), 7.24e7.16 (m, 2H), 5.00 (d, J ¼ 16.4 Hz, 1H),
4.92 (d, J ¼ 16.4 Hz, 1H), 4.52 (br, 1H), 3.62e3.57 (m, 2H), 3.38e3.32
(m, 1H), 0.85 (s, 9H); 13C-NMR:
d
166.9, 144.9, 143.2, 134.2, 122.2,
4.2.12. Compound 5f
121.4, 119.3, 110.3, 60.4, 59.2, 47.0, 33.7, 26.9.
Light yellow solid; mp 215.3e216.0 ꢀC. 1H-NMR ((CD3)2SO):
d
9.81 (s, 1H), 9.38 (d, J ¼ 8.1 Hz, 2H), 7.96e7.93 (m, 2H), 7.67e7.63
(m, 2H), 7.38e7.21 (m, 10H), 5.49 (s, 4H), 5.12 (t, J ¼ 5.7 Hz, 2H),
4.89e4.81 (m, 2H), 3.64e3.56 (m, 4H); 13C-NMR:
164.2, 144.3,
4.2.5. Compound 3e
1H-NMR ((CD3)2SO):
7.62e7.60 (m, 1H), 7.26e7.14 (m, 8H), 4.94 (t, J ¼ 6.0 Hz, 1H), 4.89 (d,
J ¼ 16.0 Hz, 1H), 4.79 (d, J ¼ 16.0 Hz, 1H), 3.97e3.89 (m, 1H),
3.47e3.33 (m, 2H), 2.89e2.85 (m, 1H), 2.66e2.49 (m, 1H); 13C-
d
8.34 (d, J ¼ 8.0 Hz, 1H), 8.08 (s, 1H),
d
140.4, 131.2, 128.1, 127.0, 126.9, 126.6, 113.6, 64.6, 55.9, 48.6. Anal.
Calc. for C27H29ClN4O4$2.3H2O: C, 58.92; H, 6.15; N, 10.18. Found: C,
58.83; H, 5.37; N, 10.18%.
NMR:
d 166.0, 144.9, 143.2, 139.0, 134.1, 129.2, 128.2, 126.1, 122.4,
121.5, 119.3, 110.3, 62.7, 52.8, 46.9, 36.6.
4.2.13. Compound 5g
White solid; mp 198.7e199.3 ꢀC. 1H-NMR ((CD3)2SO):
d 9.85 (s,
4.2.6. Compound 3f
1H), 8.59 (d, J ¼ 8.4 Hz, 1H), 8.55 (d, J ¼ 8.8 Hz, 1H), 7.97e7.91 (m,
2H), 7.70e7.68 (m, 2H), 5.48 (d, J ¼ 16.4 Hz, 1H), 5.43 (d,
J ¼ 11.2 Hz, 1H), 5.39 (d, J ¼ 11.2 Hz, 1H), 5.36 (d, J ¼ 16.4 Hz, 1H),
4.84 (t, J ¼ 10.0 Hz, 1H), 4.78 (t, J ¼ 10.0 Hz, 1H), 3.82e3.80 (m,
1H), 3.61e3.57 (m, 1H), 3.46e3.44 (m, 4H), 1.88e1.83 (m, 1H),
1.64e1.61 (m, 1H), 1.39e1.32 (m, 2H), 0.88e0.81 (m, 12H); 13C-
1H-NMR ((CD3)2SO):
d
8.85 (d, J ¼ 8.2 Hz, 1H), 8.15 (s, 1H), 7.63
(d, J ¼ 7.6 Hz, 1H), 7.44 (d, J ¼ 7.6 Hz, 1H), 7.32e7.16 (m, 7H),
5.01e4.98 (m, 3H), 4.86e4.84 (m, 1H), 3.64e3.54 (m, 2H); 13C-
NMR:
d 166.3, 144.9, 143.2, 140.7, 134.2, 128.1, 126.9, 122.2, 121.5,
119.3, 110.3, 64.6, 55.4, 46.9.
NMR:
d 164.3, 163.9, 144.3, 131.2, 126.7, 126.6, 113.6, 63.5, 61.1,
4.2.7. Compound 5a
56.6, 49.6, 48.6, 28.3, 24.2, 23.2, 21.8, 19.6, 18.2. Anal. Calc. for
C22H35ClN4O4$H2O: C, 55.86; H, 7.88; N, 11.84. Found: C, 55.69; H,
7.51; N, 11.76%.
White solid; mp 212.7e214.5 ꢀC. 1H-NMR (CD3OD):
d 9.62 (s,
1H), 7.93e7.87 (m, 2H), 7.74e7.67 (m, 2H), 5.42 (d, J ¼ 16.4 Hz, 2H),
5.35 (d, J ¼ 16.4 Hz, 2H), 3.78e3.55 (m, 6H), 1.96e1.81 (m, 2H), 0.96
(d, J ¼ 6.4 Hz, 6H), 0.94 (d, J ¼ 6.4 Hz, 6H); 13C-NMR:
d
166.6, 145.0,
4.2.14. Compound 5h
133.0, 128.5, 114.4, 63.0, 59.0, 49.8, 30.2, 20.0, 18.9. Anal. Calc. for
C21H33ClN4O4$0.75H2O: C, 55.50; H, 7.65; N, 12.33. Found: C, 55.14;
H, 7.24; N, 12.18%.
White solid; mp 172.5e173.3 ꢀC. 1H-NMR ((CD3)2SO):
d 9.79 (s,
1H), 8.52 (d, J ¼ 8.0 Hz, 2H), 7.90e7.88 (m, 2H), 7.69e7.67 (m, 2H),
5.40 (d, J ¼ 16.0 Hz, 2H), 5.33 (d, J ¼ 16.0 Hz, 2H), 4.83 (t, J ¼ 8.0 Hz,
2H), 3.84e3.76 (m, 2H), 3.46e3.41 (m, 4H), 1.66e1.56 (m, 2H),
1.38e1.27 (m, 4H), 0.87 (d, J ¼ 6.8 Hz, 6H), 0.80 (d, J ¼ 6.8 Hz, 6H);
4.2.8. Compound 5b
White solid; mp 167.4e168.3 ꢀC. 1H-NMR ((CD3)2SO):
d
9.82 (s,
13C-NMR:
d 164.0, 144.3, 131.2, 126.7, 113.6, 63.5, 49.6, 48.7, 24.2,
1H), 8.57 (d, J ¼ 8.8 Hz, 2H), 7.93e7.90 (m, 2H), 7.71e7.67 (m, 2H),
5.42 (d, J ¼ 16.4 Hz, 2H), 5.35 (d, J ¼ 16.4 Hz, 2H), 4.84 (t, J ¼ 5.6 Hz,
2H), 3.84e3.78 (m, 2H), 3.41e3.46 (m, 4H), 1.71e1.58 (m, 2H),
1.34e1.28 (m, 4H), 0.88 (d, J ¼ 6.8 Hz, 6H), 0.81 (d, J ¼ 6.8 Hz, 6H);
23.3, 21.8. Anal. Calc. for C23H37ClN4O4$1.125H2O: C, 56.46; H, 8.09;
N, 11.45. Found: C, 56.13; H, 7.89; N, 11.33%.
4.2.15. Compound 8
13C-NMR:
d
164.0, 144.3, 131.2, 126.7, 113.6, 63.5, 49.6, 48.7, 24.3,
White solid; mp 119.2e121.3 ꢀC. 1H-NMR ((CD3)2SO):
d 9.78 (s,
23.3, 21.9. Anal. Calc. for C23H37ClN4O4$1.5H2O: C, 55.69; H, 8.13; N,
11.29. Found: C, 55.37; H, 7.65; N, 11.22%.
1H), 8.62 (br, 1H), 8.31 (br, 1H), 7.92e7.89 (m, 2H), 7.69e7.67 (m,
2H), 5.47 (d, J ¼ 16.0 Hz, 1H), 5.37 (s, 2H), 5.36 (d, J ¼ 16.0 Hz, 1H),
4.59 (t, J ¼ 4.0 Hz, 1H), 3.64e3.58 (m, 2H), 3.46e3.45 (m, 1H),
3.11e3.07 (m, 2H), 1.49e1.41 (m, 2H), 0.88 (s, 9H), 0.87 (t, J ¼ 8.0 Hz,
4.2.9. Compound 5c
White solid; mp 222.7e223.1 ꢀC. 1H-NMR ((CD3)2SO):
d
9.81 (s,
3H); 13C-NMR:
d 164.6, 164.3, 144.3, 131.3, 131.2, 126.7, 126.7, 113.7,
1H), 8.52 (d, J ¼ 8.5 Hz, 2H), 7.93e7.90 (m, 2H), 7.71e7.68 (m, 2H),
5.44 (d, J ¼ 16.1 Hz, 2H), 5.36 (d, J ¼ 16.1 Hz, 2H), 4.73 (t, J ¼ 5.4 Hz,
2H), 3.68e3.60 (m, 2H), 3.48e3.46 (m, 4H), 1.65e1.58 (m, 2H),
1.49e1.43 (m, 2H), 1.19e1.06 (m, 2H), 0.86e0.81 (m, 12H); 13C-
113.6, 60.3, 59.8, 48.8, 48.5, 40.8, 33.7, 26.8, 22.2, 11.4. Anal. Calc. for
C20H31ClN4O3$1.125H2O: C, 55.71; H, 7.77; N,12.99. Found: C, 55.32;
H, 7.46; N, 12.82%.
NMR:
d
164.1, 144.3, 131.2, 126.7, 113.5, 60.8, 55.5, 48.7, 34.9, 24.7,
4.2.16. Compound 9
15.5, 11.2. Anal. Calc. for C23H37ClN4O4$0.5H2O: C, 57.79; H, 8.01; N,
White solid; mp 160.2e160.9 ꢀC. 1H-NMR ((CD3)2SO):
d
9.80 (s,
11.72. Found: C, 57.49; H, 7.84; N, 11.73%.
1H), 8.75 (br,1H), 8.40 (br,1H), 7.94e7.91 (m, 2H), 7.69e7.66 (m, 2H),

66
A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
(k) S.-J. Li, J.-H. Zhong, Y.-G. Wang, Tetrahedron: Asymmetry 17 (2006)
1650e1654.
[4] (a) F. Guillen, C.L. Winn, A. Alexakis, Tetrahedron: Asymmetry 12 (2001)
2083e2086;
5.48 (d, J ¼ 16.0 Hz, 1H), 5.37 (s, 2H), 5.37 (d, J ¼ 16.0 Hz, 1H), 4.85 (t,
J ¼ 8.0 Hz, 1H), 4.63 (t, J ¼ 8.0 Hz, 1H), 3.69e3.58 (m, 2H), 3.46e3.45
(m, 1H), 3.21e3.19 (m, 2H), 0.87 (s, 9H); 13C-NMR:
d 164.7, 164.6,
(b) J. Pytkowicz, S. Roland, P. Mangeney, Tetrahedron: Asymmetry 12 (2001)
2087e2089.
[5] (a) K.-S. Lee, A.H. Hoveyda, J. Org. Chem. 74 (2009) 4455e4462;
(b) C.L. Winn, F. Guillen, J. Pytkowicz, S. Roland, P. Mangeney, A. Alexakis,
J. Organomet. Chem. 690 (2005) 5672e5695;
144.3,131.3,131.2,126.7,126.7,113.8,113.7, 60.3, 59.9, 59.5, 48.8, 48.6,
42.0, 33.7, 26.9. Anal. Calc. for C19H29ClN4O4$0.5CH2Cl2: C, 51.43; H,
6.64; N, 12.30. Found: C, 51.29; H, 6.69; N, 12.57%.
(c) A. Alexakis, C.L. Winn, F. Guillen, J. Pytkowicz, S. Roland, P. Mangeney, Adv.
Synth. Catal. 345 (2003) 345e348.
[6] P.L. Arnold, M. Rodden, K.M. Davis, A.C. Scarisbrick, A.J. Blake, C. Wilson, Chem.
Commun. (2004) 1612e1613.
4.3. General procedure for the Cu(OTf)2-catalyzed ECA of enone
with R2Zn
[7] (a) J. Wencel, M. Mauduit, H. Hénon, S. Kehrli, A. Alexakis, Aldrichim. Acta 42
(2009) 43e50;
To a solution of azolium salt (0.04 mmol) in THF (3 mL) were
added Cu(OTf)2 (0.06 mmol) and enone (1 mmol). After the
mixture was cooled to 0 ꢀC, Et2Zn (3 mmol, 1 mol/L in hexanes) was
added to the reaction vessel. The color immediately changed from
yellow to dark brown. After stirring at room temperature for 3 h,
the reaction was quenched with 10% HCl aq. The resulting mixture
was extracted with diisopropyl ether and dried over Na2SO4. The
product was purified by silica gel column chromatography (hexane/
EtOAc). The ee was measured by chiral GLC.
(b) D. Rix, S. Labat, L. Toupet, C. Crévisy, M. Mauduit, Eur. J. Inorg. Chem.
(2009) 1989e1999;
(c) H. Hénon, M. Mauduit, A. Alexakis, Angew. Chem., Int. Ed. 47 (2008)
9122e9124;
(d) H. Clavier, J.-C. Guillemin, M. Mauduit, Chirality 19 (2007) 471e476;
(e) D. Martin, S. Kehrli, M. d’Augustin, H. Clavier, M. Mauduit, A. Alexakis,
J. Am. Chem. Soc. 128 (2006) 8416e8417;
(f) H. Clavier, L. Coutable, L. Toupet, J.-C. Guillemin, M. Mauduit, J. Organomet.
Chem. 690 (2005) 5237e5254;
(g) H. Clavier, L. Coutable, J.-C. Guillemin, M. Mauduit, Tetrahedron: Asym-
metry 16 (2005) 921e924.
[8] (a) Y. Lee, H. Jang, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 18234e18235;
(b) Y. Lee, B. Li, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 11625e11633;
(c) Y. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 131 (2009) 3160e3161;
(d) M.A. Kacprzynski, T.L. May, S.A. Kazane, A.H. Hoveyda, Angew. Chem., Int.
Ed. 46 (2007) 4554e4558;
Acknowledgments
This work was financially supported by a Grant-in-Aid for
Scientific Research (C) (20550103) from Japan Society for the
Promotion of Science (JSPS) and the Kansai University Grant-in-Aid
for progress of research in graduate course, 2010.
(e) Y. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 128 (2006) 15604e15605;
(f) A.O. Larsen, W. Leu, C.N. Oberhuber, J.E. Campbell, A.H. Hoveyda, J. Am.
Chem. Soc. 126 (2004) 11130e11131.
[9] (a) M.K. Brown, A.H. Hoveyda, J. Am. Chem. Soc. 130 (2008) 12904e12906;
(b) T.L. May, M.K. Brown, A.H. Hoveyda, Angew. Chem., Int. Ed. 47 (2008)
7358e7362;
Appendix. Supplementary data
(c) M.K. Brown, T.L. May, C.A. Baxter, A.H. Hoveyda, Angew. Chem., Int. Ed. 46
(2007) 1097e1100;
(d) K.-S. Lee, M.K. Brown, A.W. Hird, A.H. Hoveyda, J. Am. Chem. Soc. 128
(2006) 7182e7184.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.jorganchem.2010.07.038.
[10] (a) T. Uchida, T. Katsuki, Tetrahedron Lett. 50 (2009) 4741e4743;
(b) Y. Matsumoto, K.-I. Yamada, K. Tomioka, J. Org. Chem. 73 (2008) 4578e4581;
(c) T. Moore, M. Merzouk, N. Williams, Synlett (2008) 21e24.
[11] (a) C. Romain, K. Miqueu, J.-M. Sotiropoulos, S. Bellemin-Laponnaz,
S. Dagorne, Angew. Chem., Int. Ed. 49 (2010) 2198e2201;
(b) S. Bellemin-Laponnaz, R. Welter, L. Brelot, S. Dagorne, J. Organomet. Chem.
694 (2009) 604e606;
References
[1] (a) T. Jerphagnon, M.G. Pizzuti, A.J. Minnaard, B.L. Feringa, Chem. Soc. Rev. 38
(2009) 1039e1075;
(c) P.L. Arnold, M.S. Sanford, S.M. Pearson, J. Am. Chem. Soc. 131 (2009)
13912e13913;
(d) S.T. Liddle, I.S. Edworthy, P.L. Arnold, Chem. Soc. Rev. 36 (2007)
1732e1744;
(e) O. Kühl, Chem. Soc. Rev. 36 (2007) 592e607;
(b) A. Alexakis, J.E. Bäckvall, N. Krause, O. Pàmies, M. Diéguez, Chem. Rev. 108
(2008) 2796e2823;
(c) A. Alexakis, C. Benhaim, Eur. J. Org. Chem. (2002) 3221e3236.
[2] For selected recent reviews: (a) O. Kühl, Funtionalised N-Heterocyclic Carbene
Complexes. Wiley-VCH, Weinheim, 2010;
(f) H. Ren, P. Yao, S. Xu, H. Song, B. Wang, J. Organomet. Chem. 692 (2007)
2092e2098;
(b) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009)
3612e3676;
(g) D. Pugh, A.A. Danopoulos, Coord. Chem. Rev. 251 (2007) 610e641;
(h) C.-Y. Liao, K.-T. Chan, J.-Y. Zeng, C.-H. Hu, C.-Y. Tu, H.M. Lee, Organome-
tallics 26 (2007) 1692e1702;
(i) A.J. Boydston, J.D. Rice, M.D. Sanderson, O.L. Dykhno, C.W. Bielawski,
Organometallics 25 (2006) 6087e6098;
(c) D.R. Snead, H. Seo, S. Hong, Curr. Org. Chem. 12 (2008) 1370e1387;
(d) F.E. Hahn, M.C. Jahnke, Angew. Chem., Int. Ed. 47 (2008) 3122e3172;
(e) L.H. Gade, S. Bellemin-Laponnaz, in: F. Glorius (Ed.), Topics in Organo-
metallic Chemistry, vol. 21, Springer, Berlin, 2007, pp. 117e157;
(f) L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718e725;
(g) E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem., Int. Ed. 46 (2007)
2768e2813;
(h) M.S. Sigman, D.R. Jensen, Acc. Chem. Res. 39 (2006) 221e229;
(i) E. Peris, R.H. Crabtree, Coord. Chem. Rev. 248 (2004) 2239e2246;
(j) V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004)
619e636;
(j) W.-F. Li, H.-M. Sun, Z.-G. Wang, M.-Z. Chen, Q. Shen, Y. Zhang, J. Organomet.
Chem. 690 (2005) 6227e6232.
[12] (a) S. Sakaguchi, K.S. Yoo, J. O’Neill, J.H. Lee, T. Stewart, K.W. Jung, Angew.
Chem., Int. Ed. 47 (2008) 9326e9329;
(b) K.S. Yoo, J. O’Neill, S. Sakaguchi, R. Giles, J.H. Lee, K.W. Jung, J. Org. Chem.
75 (2010) 95e101.
[13] S. Sakaguchi, M. Kawakami, J. O’Neill, K.S. Yoo, K.W. Jung, J. Organomet. Chem.
695 (2010) 195e200.
[14] (a) M. Okamoto, Y. Yamamoto, S. Sakaguchi, Chem. Commun. (2009)
7363e7365;
(b) N. Shibata, M. Okamoto, Y. Yamamoto, S. Sakaguchi, J. Org. Chem. 75
(2010) 5707e5715.
[15] (a) I. Moreno, R. SanMartin, B. Inés, M.T. Herrero, E. Domínguez, Curr. Org.
Chem. 13 (2009) 878e895;
(k) M.C. Perry, K. Burgess, Tetrahedron: Asymmetry 14 (2003) 951e961;
(l) W.A. Herrmann, Angew. Chem., Int. Ed. 41 (2002) 1290e1309;
(m) D. Bourissou, O. Guerret, F.P. Gabbaï, G. Bertrand, Chem. Rev. 100 (2000)
39e91.
[3] For selected recent publications on asymmetric synthesis using chiral NHC
ligands: (a) Z. Liu, M. Shi, Tetrahedron 66 (2010) 2619e2623;
(b) K.-S. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 132 (2010) 2898e2900;
(c) G. Dyson, J.-C. Frison, A.C. Whitwood, R.E. Douthwaite, Dalton Trans. (2009)
7141e7151;
(b) D. Morales-Morales, C.M. Jensen (Eds.), The Chemistry of Pincer
Compounds, Elsevier, Amsterdam, 2007;
(d) W. Wang, T. Zhang, M. Shi, Organometallics 28 (2009) 2640e2642;
(e) G.-N. Ma, T. Zhang, M. Shi, Org. Lett. 11 (2009) 875e878;
(f) Y.-X. Jia, J.M. Hillgren, E.L. Watson, S.P. Marsden, E.P. Kündig, Chem.
Commun. (2008) 4040e4042;
(g) P.-A. Fournier, S.K. Collins, Organometallics 26 (2007) 2945e2949;
(h) G.A. Cortez, R.R. Schrock, A.H. Hoveyda, Angew. Chem., Int. Ed. 46 (2007)
4534e4538;
(i) G.A. Cortez, C.A. Baxter, R.R. Schrock, A.H. Hoveyda, Org. Lett. 9 (2007)
2871e2874;
(j) S. Nanchen, A. Pfaltz, Chem.dEur. J. 12 (2006) 4550e4558;
(c) H. Nishiyama, J. Ito, Chem. Rec. 7 (2007) 159e166;
(d) M.E. van der Boom, D. Milstein, Chem. Rev. 103 (2003) 1759e1792;
(e) M. Albrecht, G. van Koten, Angew. Chem., Int. Ed. 40 (2001) 3750e3781.
[16] For selected publications on C2-symmetric chiral NHC-metal complexes used
in enantioselective catalysis: (a) W.A. Herrmann, L.J. Goossen, C. Köcher,
G.R.J. Artus, Angew. Chem., Int. Ed. Engl. 35 (1996) 2805e2807;
(b) T.J. Seiders, D.W. Ward, R.H. Grubbs, Org. Lett. 3 (2001) 3225e3228;
(c) Y. Ma, C. Song, C. Ma, Z. Sun, Q. Chai, M.B. Andrus, Angew. Chem., Int. Ed. 42
(2003) 5871e5874;

A. Harano, S. Sakaguchi / Journal of Organometallic Chemistry 696 (2011) 61e67
67
(d) D.R. Jensen, M.S. Sigman, Org. Lett. 5 (2003) 63e65;
(e) S. Tominaga, Y. Oi, T. Kato, D.K. An, S. Okamoto, Tetrahedron Lett. 45 (2004)
5585e5588;
(f) M.R. Chaulagain, G.J. Sormunen, J. Montgomery, J. Am. Chem. Soc. 129
(2007) 9568e9569;
(g) Y. Sato, Y. Hinata, R. Seki, Y. Oonishi, N. Saito, Org. Lett. 9 (2007) 5597e5599;
(h) R.D. Baxter, J. Montgomery, J. Am. Chem. Soc. 130 (2008) 9662e9663;
(i) L. Xu, Y. Shi, J. Org. Chem. 73 (2008) 749e751;
(j) V. Lillo, A. Prieto, A. Bonet, M.M. Díaz-Requejo, J. Ramírez, P.J. Pérez,
E. Fernández, Organometallics 28 (2009) 659e662.
[17] G. Guillemot, M. Neuburger, A. Pfaltz, Chem.dEur. J. 13 (2007) 8960e
8970.