Highly tunable anionic tethered N-heterocyclic carbene of Pd(II) complexes for asymmetric allylic alkylation reaction

Article abstract of DOI:10.1016/j.jorganchem.2012.12.015

The versatility of the anionic tethered NHC ligand system for the Pd-catalyzed allylic asymmetric alkylation (AAA) reaction was investigated. A well defined anionic amidate/NHC-Pd(II) complex catalyzed the AAA reaction of 1,3-diphenylprop-3-en-1-yl acetat

Full text of DOI:10.1016/j.jorganchem.2012.12.015

Journal of Organometallic Chemistry 726 (2013) 46e55
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Journal of Organometallic Chemistry
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Highly tunable anionic tethered N-heterocyclic carbene of Pd(II)
complexes for asymmetric allylic alkylation reaction
*
Hiroshi Shirasaki, Miaki Kawakami, Haruka Yamada, Ryuichi Arakawa, Satoshi Sakaguchi
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, 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:
The versatility of the anionic tethered NHC ligand system for the Pd-catalyzed allylic asymmetric
alkylation (AAA) reaction was investigated. A well defined anionic amidate/NHCePd(II) complex cata-
lyzed the AAA reaction of 1,3-diphenylprop-3-en-1-yl acetate (5) with NaCH(CO2Me)2. Similarly, the AAA
reaction occurred by employing an NHCePd(II) complex that was generated in situ. Screening of a wide
variety of NHC ligand precursors revealed that the combination of [Pd(allyl)Cl]2 and the azolium salt 9,
containing a 3-pentyl group at N(3) position in the NHC ring, efficiently promoted the AAA reaction to
afford the corresponding alkylated product 7 with 67% ee. Furthermore, it was found that the Pd/NHC
ratio was an important factor; the AAA reaction with a Pd/NHC ratio of 1:1 took place smoothly, whereas
utilization of a Pd/NHC ratio of 1:2 resulted in low conversion of the substrates probably by the formation
of the catalytically inert bis(NHC)ePd(II) complex. In fact, an independent experiment showed that the
bis(NHC)ePd(II) complex was formed by treating [Pd(allyl)Cl]2 with two equivalents of NHCeAg
complex. Further investigations on the catalytic AAA reaction revealed significant improvements in
both the product yield (99%) and the enantioselectivity (81% ee) when cyclopentyl methyl ether was used
as a solvent. A plausible reaction intermediate was discussed.
Received 23 October 2012
Received in revised form
7 December 2012
Accepted 13 December 2012
Keywords:
N-Heterocyclic carbene
Anionic tethered NHC ligand
NHCePd complex
Asymmetric allylic alkylation
Ligand design
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
donor-functionalized NHC contains an anionic or neutral 2e donor
atom (e.g., C, N, O, S or P), which acts as a polydentate ligand upon
Since the first formative studies by Wanzlick, Öfele, and Lappert
in the 1960s and 1970s focused on the synthesis of N-heterocyclic
carbene (NHC) complexes, investigations on their applications in
catalysis have been initiated [1]. The decisive breakthrough was
achieved in 1995 by Herrmann and coworkers, who reported that
NHC-containing well defined palladium(0) and palladium(II)
complexes efficiently catalyzed the Heck reaction [2]. Because of
the strong bonds between NHCs and transition metals, the interest
in these ligands for applications in catalysis has grown rapidly [3].
Another feature that makes NHCs a very interesting class of ligands
is the easy access to their respective precursors (mostly azolium
salts). This allows the design of a wide variety of NHC ligands and
the rapid elaboration of NHC ligand libraries.
An important step in the development of functionalized NHC
ligands is their chiral modification for asymmetric reactions [3,4].
For design of an efficient chiral NHC ligand, the introduction of
donor functionalities at the NHC ligand provides relatively inflex-
ible chelating NHC-based ligand, which is expected to offer a key
structure for the construction of efficient stereodirecting group. The
coordination to a metal center [3c]. Therefore, one obvious way for
achieving asymmetric reactions would be the use of the anionic or
neutral donor-functionalized chiral NHC ligand.
The Pd-catalyzed asymmetric allylic alkylation (AAA) reaction
has become an extremely useful synthetic methodology for ster-
eoselective CeC bond formation [5]. In comparison to reports on
oxazoline- or phosphine-based ligands, fewer studies on the use of
the NHC-based chiral ligand have appeared dealing with this
transformation (Scheme 1) [4]. Douthwaite et al. introduced the
imine-functionalized NHC containing a neutral donor N atom [6].
By employing the neutral, bidentate CN ligand system, 92% ee was
achieved in the Pd-catalyzed AAA reaction of 1,3-diphenylprop-3-
en-1-yl acetate with dimethyl malonate. Fernández et al. devel-
oped a Pd complex with a chelating neutral thioether/NHC ligand,
and tested the complex in the AAA reaction affording good yields of
product and moderate to good ee values [7]. Buono et al. reported
a study on synergistic effects between NHC and P-ligand [8]. The
combination of NHC and chiral P-donor ligands led to an
enhancement in the enantioselectivity. However, the reported
attempts to apply NHC in AAA found limited success, despite the
variety of monodentate or bidentate (CN, CP, or CS) NHC families
explored [9]. In addition, little attention has been paid to anionic
* Corresponding author. Tel.: þ81 6 6368 0867; fax: þ81 6 6339 4026.
E-mail address: satoshi@kansai-u.ac.jp (S. Sakaguchi).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.12.015

H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
47
O
Ph
O
Ph
N
N
N
i) Ag2O
R1
R1
N
N
HN
HO
Ph
HN
N
N
N
N
ii) PdCl2(CH3CN)2
Cl
Pd
R2
R2
Cl
Ar
O
N
H
Amidate/NHC-Pd
i
Pr
Azolium salt
[Pd(allyl)Cl]2/Ag2O
1a-4a
[Pd(allyl)Cl]2/Ag2O
R1 R2
Douthwaite (2003)
Roland (2007)
i
Pr Me
Bu Me
Bu Me
1
i
2
3
4
t
Ph Me
N
N
N
N
O P
S
Mes
N
N
Pd
X-ray structure of 3a
Ph
[Pd(allyl)Cl]2/BuLi
Scheme 2. One-pot synthesis of well defined anionic amidate/NHCePd(II) complexes
1ae4a from chiral hydroxy-amide functionalized azolium salts 1e4.
Fernández (2007)
Buono (2008)
carbene complexes, and these materials were used as a carbene
transfer agent. Reaction of the resulting NHCeAg complexes with
PdCl2(CH3CN)2 yielded the tridentate NHCePd(II) complexes 1ae
4a (Scheme 2). Notably, these complexes were air- and moisture-
stable products easily handled under air atmosphere and stored
as solids without any special precaution. In preceding papers, we
reported X-ray diffraction studies of the NHCePd complexes 1a, 2a,
and 4a [10a,c]. Herein, a new complex 3a was synthesized and
characterized by 1H and 13C NMR spectroscopies and elemental
analysis. In addition, 3a was successfully obtained as single crystals,
and X-ray diffraction studies were performed.
Scheme 1. Representative donor-functionalized NHC containing neutral 2e donor
atom (N, S, P) chelating ligand for Pd-catalyzed AAA reaction.
tethered NHCePd complexes, although the anionic tightly coordi-
nating polydentate NHC ligand system is expected to enhance the
catalyst stability and to offer a key structure for the construction of
efficient stereodirecting elements.
Recently, we synthesized and fully characterized new anionic
amidate/NHCePd(II) and dianionic amidate/alkoxy/NHCePd(II)
complexes [10]. Importantly, the well defined and highly purified
NHCePd complex catalyzed Heck-type reactions of arylboronic
acids with alkenes with high enantioselectivities. In addition, NHC
ligand precursors such as hydroxy-amide functionalized azolium
salts were tested for the Cu-catalyzed asymmetric conjugate
addition of enones with dialkylzincs [11]. In contrast to the Pd-
catalyzed Heck-type reaction, the Cu-catalyzed asymmetric 1,4-
addition reaction performed in situ generated the catalytic
system; enones were allowed to react with R2Zn in the presence of
catalytic amounts of a Cu salt combined with a chiral azolium salt to
afford the corresponding conjugate adducts with excellent
enantioselectivities.
In this study, we investigated the versatility of the anionic
tethered NHC ligand system for the Pd-catalyzed AAA reaction. The
most attractive feature of the ligand is that the NHC with an anionic
amidate group induces the generation of a strongly coordinating
polydentate ligand able to lock stereodirecting functionalized
groups in a fixed conformation. Moreover, the hydroxy-amide
functionalized azolium ligand precursors are easily accessible in
2.2. Catalytic studies
The potential application of the amidate/NHCePd(II) complexes
was explored in the Pd-catalyzed AAA reaction (Table 1). First, (E)-
1,3-diphenylprop-2-en-1-yl acetate (5) was allowed to react with
dimethyl malonate (6) under the influence of a well defined
amidate/NHCePd(II) complex 1a (R1 ¼ iPr, R2 ¼ Me) in the presence
of NaH in 1,2-dichloroethane at 80 ꢀC for 15 h. Unfortunately, the
corresponding desired product 7 was obtained with a poor yield
Table 1
Initial studies on AAA reaction.a
O
O
NHC-Pd
(5 mol%)
NaH, (CH2Cl)2
OAc
O
O
+
O
O
Ph
Ph
O
O
Ph
Ph
two steps from enantiopure
b-amino alcohols. Therefore, we
5
6
7
reasoned that a suitable ligand could be produced from the
screening of a large library through variation of the substituents at
the NHC ligand. Here, we report the AAA reaction of 1,3-
diphenylprop-3-en-1-yl acetate with dimethyl malonate cata-
lyzed by the well defined Pd(II) complex showing a highly tunable
anionic amidate/NHC ligand. We also explore the AAA reaction
under the influence of a NHCePd catalyst generated in situ from the
combination of an appropriate palladium catalyst precursor and
a chiral azolium ligand precursor.
Entry
Catalyst
Temp. (ꢀC)
Yield (%)
ee (%)
1
2
3
4
1a
2a
3a
4a
80
80
80
80
80
80
80
70
95
3
4
5
3
5
12
9
3
5
9
20
7
19
21
20
24
16
5b
6b,c
7b,c
8b,c
9b,c
3; PdCl2(CH3CN)2
3; [Pd(allyl)Cl]2
3; [Pd(cinnamyl)Cl]2
3; [Pd(allyl)Cl]2
3; [Pd(allyl)Cl]2
48
2. Results and discussion
a
Reaction conditions: see Experimental section for detail.
5 (0.2 mmol), 6
(0.6 mmol), NaH (0.57 mmol), NHCePd(II) complex (0.01 mmol), (CH2Cl)2 (3 mL),
2.1. Synthesis of anionic amidate/NHCePd(II) complexes
15 h.
b
Reaction conditions: see Experimental section for detail. Pd precursor
The hydroxy-amide functionalized azolium compounds were
reacted with Ag2O to cleanly afford the corresponding silver
(0.01 mmol), azolium salt 3 (0.01 mmol), Ag2O (0.005 mmol).
c
Pd precursor (0.005 mmol).

48
H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
(3%) and enantioselectivity (5% ee) (Table 1, entry 1). This result is in
contrast to the previously reported Pd(II)-catalyzed asymmetric
Heck-type reaction, where highly enantioselective cross-coupling
reaction was successfully achieved by employing the NHCePd(II)
complex derived from azolium salt 1 (R1 ¼ iPr, R2 ¼ Me) [10a,b].
Several well defined NHCePd(II) complexes 1ae4a were screened
(entries 1e4). Replacement of the alkyl substituent at the chiral
carbon center of the ligand (R1 group) from isopropyl group
(complex 1a) to more sterically hindered tert-butyl group (complex
3a) led to slight improvements in the AAA stereoselectivity (20% ee)
(entry 3).
Previously, we have reported on asymmetric transfer hydroge-
nation of acetophenone with isopropyl alcohol (IPA) catalyzed by
well defined NHCeIr complex prepared from an azolium NHC ligand
precursor 3 through an Ag2O method [12]. In addition, we have
shown that the enantioselective reduction of acetophenonewith IPA
by employing the NHCeIr catalyst that was generated in situ took
placewithout the loss of enantioselectivity. Therefore, we decided to
study the ability of the NHCePd complex that was generated in situ
to promote the present AAA reaction (Table 1, entries 5e9). After the
treatment of the relevant azolium salt 3 (R1 ¼ tBu, R2 ¼ Me) with
Ag2O in CH2Cl2 at room temperature, PdCl2(CH3CN)2 was added to
the reaction vessel. The resulting mixture was stirred at room
temperature, filtered with a membrane filter, and evaporated to
dryness under vacuum. Subsequently, 5 was treated with NaCH(-
CO2Me)2 in the presence of the unpurified NHCePd catalyst in
(CH2Cl)2 at 80 ꢀC for 15 h. By means of this procedure, the reaction
produced 7 in 5% yield and 19% ee (Table 1, entry 5). The advantage of
this procedure lies in the operational simplicity, which allows easy
ligand screening tests (vide infra, Table 2).
under the combined catalytic system of [Pd(allyl)Cl]2 and azolium
salt 3 to give the desired product 7 in the product yield (12%)
(Table 1, entry 6). In addition, the AAA reaction catalyzed by
[Pd(cinnamyl)Cl]2 combined with 3 proceeded in a similar manner
as compared to the reaction using PdCl2(CH3CN)2 (entry 5 vs. entry
7). Decreasing the reaction temperature to 70 ꢀC led to slower
reaction rates, while raising it to 90 ꢀC resulted in the formation of 7
in moderate yield (48%) (entries 8 and 9).
As mentioned in the Introduction, the easy tuning of both N-
anionic functional groups and N-alkyl groups of the hydroxy-amide
functionalized azolium NHC precursor would allow the develop-
ment of a huge variety of polydentate chiral ligands [10c,13].
Scheme 3 and Table 2 summarize results for the AAA reaction of 5
and 6 under the influence of catalytic amounts of [Pd(allyl)Cl]2
combined with various chiral azolium precursors, 3 and 8e29,
containing a tert-butyl substituent at the stereogenic center
(R1
¼
tBu). The use of an azolium salt 3, prepared from 1-
methylbenzimidazole, resulted in the formation of 7 with low
yield and ee (entry 1). Conversely, it was found that the AAA
reaction under the combination of [Pd(allyl)Cl]2 and 8 or 9, con-
taining isopropyl or 3-pentyl group at N(3) position at the NHC ring,
afforded 7 in 42% or 67% ee, respectively (entries 2 and 3). We
performed the AAA reaction without filtration of the resulting of
AgCl in the preparation of NHCePd catalyst (entry 4). However, the
reaction resulted in the formation of 7 with lower yield.
Since significant improvements in the stereoselectivity of the
AAA reaction were achieved by changing the R2 group at NHC ring
(Table 2, entries 1e3), further systematic studies were performed.
Treatment of 5 with NaCH(CO2Me)2 in the presence of a palladium
catalyst generated in situ from [Pd(allyl)Cl]2 and 10 (R2
¼
nBu)
As shown in Scheme 1, most of the previously reported AAA
reactions were performed by using a palladium catalyst generated
in situ from [Pd(allyl)Cl]2 and chiral NHC [6e9]. Thus, we assumed
that a metal complex formed from [Pd(allyl)Cl]2 would differ from
complex 3a formed from PdCl2(CH3CN)2. In fact, 5 was alkylated
furnished 7 with low enantioselectivity (38% ee) (entry 5). These
results might indicate that azolium salt possessing a secondary
alkyl group at R2 substituent would be suitable for the present
catalytic system (entries 1e5). Hence, the azolium salt 11
(R2 ¼ CH(C3H7)2) was synthesized from 1-(4-heptyl)benzimidazole
and subsequently evaluated. As expected, 5 was alkylated upon the
palladium catalyst derived from [Pd(allyl)Cl]2 and 11 to yield 7 in
64% ee (entry 6).
Table 2
Influence of azolium ligand precursor on AAA reaction by in situ generated palla-
dium catalyst.a
Moreover, we were interested in studying the influence of the
conformational rigidity of cycloalkyl substituent (secondary alkyl
group) at the R2 position of the ligand, on the stereoselectivity of
the AAA reaction. With this aim, a series of azolium ligand
precursors 12e15 bearing cyclopentyl, cyclohexyl, cycloheptyl, or
cyclooctyl substituent were evaluated (Table 2, entries 7e10).
Entry
Azolium salt
Yield (%)
ee (%)
1
2
3
8
9
9
12
38
65
16
21
44
9
32
22
12
6
21
42
67
69
38
64
64
28
28
2
3
4b
5
Although the enantioselectivity with 12 (R2
¼
cycloC5H9) was
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
comparable to that afforded with 11 (R2 ¼ CH(C3H7)2), the product
yield was very low (entry 7). In contrast to 12, AAA reaction under
13 (R2 ¼ cycloC6H11) took place with lower stereoselectivity (28% ee)
probably as a result of the conformational rigidness of the cyclo-
hexyl group (entry 8). The utilization of 15 (R2 ¼ cycloC8H15) resulted
in a racemic mixture of 7 (entry 10). In addition to these ligand
precursors such as 8, 9, and 11e15, bearing secondary alkyl group at
the R2 position, we prepared and evaluated tertiary alkyl groups
such as tert-butyl and 1-adamantyl groups (azolium salts 16 and
17). However, reaction was hardly produced under 16 or 17 (entries
11 and 12). This might be explained by the steric bulkiness of the
tertiary alkyl group that hinders the approach of the substances to
the palladium center.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
5
4
8
7
ꢁ4
5
24
43
45
15
24
24
49
59
ꢁ41
ꢁ10
32
20
12
41
25
26
27
48
22
6
While azolium ligand precursors possessing allyl and propargyl
substituents such as 18 and 19 were inert (Table 2, entries 13 and
14), replacement of these substituents with a benzyl group (azo-
lium salt 20) led to a marked increase in the enantioselectivity of
the AAA reaction of 5 with 6 (entry 15). Based on these results,
several azolium salts 21e26 were screened (entries 16e21).
Moderate ee (59%) was observed when 5 was treated with
7
a
Reaction conditions: see Experimental section for detail.
5
(0.2 mmol), 6
(0.6 mmol), NaH (0.57 mmol), [Pd(allyl)Cl]2 (0.005 mmol), azolium salt (0.01 mmol),
Ag2O (0.005 mmol), (CH2Cl)2 (3 mL), 80 ꢀC, 15 h.
b
In the preparation of NHCePd complex, no filtration of the resulting AgCl was
performed.

H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
49
1:1 affording 7 with a 65% yield and 67% ee (Table 2, entry 3;
Scheme 4, entry 1). On the other hand, most of the substrate was
recovered unchanged when the AAA reaction was conducted with
a Pd/NHC ratio of 1:2 (Scheme 4, entry 2). These results might be
explained by the formation of a bis(NHC)ePd complex A in the later
case (vide infra, Scheme 5). We assumed that the difficulty of the
reactant to approach the sterically hindered p-allyl palladium site
on bis(NHC)ePd complex A might cause the lower yield of the
product in the AAA reaction.
O
N
t
HN
HO
Bu
N
Cl
R2
R2 =
Indeed, an independent experiment revealed that the reaction
of [Pd(allyl)Cl]2 with two equivalents of NHCeAg complex yielded
bis(NHC)ePd complex A (Scheme 5). Treatment of 0.05 mmol of
[Pd(allyl)Cl]2 with 0.2 mmol of NHCeAg in CH2Cl2 at room
temperature afforded [Pd(allyl)(NHC)2]Cl 30 in 67% yield [14]. This
new complex 30 was isolated as a white and air stable solid. In the
literature, several types of well defined NHCePd(II) complexes such
H3C
3
8
9
10
as
coordinated
vent)]X have been reported [15]. However, the appearance of
bis(NHC)e -allyl palladium complexes [Pd(allyl)(NHC)2]X is rare.
Recently, Chen and coworkers reported the synthesis and charac-
terization of neutral -allyl palladium(II) complexes containing
p
-allyl palladium complexes [Pd(allyl)X(NHC)] and solvent-
p-allyl palladium complexes [Pd(allyl)(NHC)(sol-
11
12
13
14
p
p
bis(1,2,4-tirazol-5-ylidene-1-yl)borate ligands [15a].
The complex 30 was characterized by 1H and 13C NMR spec-
troscopies and elemental analysis (Fig. 1). The formation of the
15
16
17
18
carbeneemetal complex was evident from the distinctive Pde
13
C
peak, appearing at 191 ppm of 30. Three 1H NMR
carbene
signals with a ratio of 2:2:1 and two 13C NMR signals were observed
for the allylic groups in 30. The assignment of the Hsyn and Hanti
atoms in 30 was made based on the comparison with those re-
ported for NHCep-allyl palladium complexes [15a]. The peak of the
syn H atoms is downfield from that the anti atoms. The JHeH
Ph
19
20
21
22
couplings between the CH and Hsyn and Hanti atoms were 7.3 and
13.3 Hz, respectively. In 13C NMR, the signals at
d 125 and 62 ppm
corresponding to the allylic groups in 30 were observed.
The thus-obtained well defined [Pd(allyl)(NHC)2]Cl complex 30
did not show any catalytic activity in the AAA reaction of 5 and 6.
These results strongly suggest the formation of a bis(NHC)ePd
species A when the reaction takes place under the influence of
the Pd-catalyst generated in situ from [Pd(allyl)Cl]2 and chiral
azolium 9 with a Pd/NHC ratio of 1:2. Consequently, the occurrence
of the alkylation reaction would be difficult as shown in Scheme 4,
entry 2.
23
24
25
i) Ag2O
ii) [Pd(allyl)Cl]2
O2N
Azolium salt
9
26
29
27
28
filtration,
CH2Cl2
then evaporation
Scheme 3. List of chiral NHC precursors.
"NHC-Pd"
NaH, (CH2Cl)2
80 °C, 15 h
+
7
5
6
NaCH(CO2Me)2 under the influence of the NHCePd complex that
was generated in situ from [Pd(allyl)Cl]2 and 26 (R2 ¼ 9-fluorenyl)
(entry 21). Surprisingly, the enantioselectivity in the AAA reaction
was reversed by employing 27 (R2 ¼ Ph) (entry 22). While the
reaction under 28 (R2 ¼ 4-NO2C6H4) also led to the formation of
ent-7 (entry 23), the use of the azolium salt 29 (R2 ¼ mesityl) did
not produce the alkylated product with the opposite configuration
(e.g., ent-7) (entry 24). AAA reaction under these chiral ligand
precursors containing aryl substituent at the R2 position is
currently under investigation.
Entry
Yield (%) Ee (%)
1a
65
10
67
75
Pd:NHC = 1:1
Pd:NHC = 1:2
2b
a
Reaction conditions: 5 (0.2 mmol), 6 (0.6 mmol), NaH (0.57
mmol), [Pd(allyl)Cl]2 (0.005 mmol), 9 (0.01 mmol), Ag2O
b
During the course of this study, the ratio of the palladium
precursor and the NHC ligand precursor was found to be an
important factor (Scheme 4). As described above, the alkylation of 5
with NaCH(CO2Me)2 proceeded smoothly with a Pd/NHC ratio of
(0.005 mmol), (CH2Cl)2 (3 mL), 80 ºC, 15 h. 9 (0.02 mmol),
Ag2O (0.01 mmol).
Scheme 4. Influence of the Pd/NHC ratio.

50
H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
AAA reaction by the NHCePd(II) catalyst generated in situ with
a Pd/NHC ratio of 1:1 under the influence of an ethereal solvent
(Table 3).
Cl
N
N
i) Ag2O
ii) [Pd(allyl)Cl]2
In contrast to the reaction in (CH2Cl)2, the use of an ethereal
solvent such as THF led to an increase of the product yield even at
a lower reaction temperature (70 ꢀC), although a moderate enan-
tioselectivity was obtained (Table 3, entry 1 vs. entry 2). Encour-
aged by this success, we examined various ethereal solvents for
further optimizations (entries 3e7). The first promising result was
obtained by employing diisopropyl ether (DIPE) at 70 ꢀC, furnishing
7 in almost quantitative yield (99%) without loss of stereoselectivity
(66% ee) (entry 1 vs. entry 5). Although the reaction in DIPE was
performed at lower temperature (60 ꢀC), almost the same ee value
N
N
Pd
1
CH2Cl2, r.t.
O
O
NH
HN
[Pd(allyl)Cl]2 : 1 : Ag2O
= 0.05 : 0.2 : 0.1 mmol
30
HO
OH
67% yield
Scheme 5. Reaction of [Pd(allyl)Cl]2 with NHCeAg complex derived from azolium salt
1 with a Pd/NHC ratio of 1:2.
(68% ee) was obtained (entry 6). Fortunately,
a significant
improvement (79% ee) was achieved when the reaction was con-
ducted in cyclopentyl methyl ether (CPME) (entry 7). Further
optimization of the reaction conditions revealed that 5 was effi-
ciently alkylated with NaCH(CO2Me)2 (5 equiv. with respect to 5) in
CPME at 80 ꢀC for 15 h to produce 7 in 99% yield with 81% ee (entry
10). The reaction took place completely within 5 h under these
reaction conditions (entry 11). Similar result was obtained in the
reaction using [Pd(cinnamyl)Cl]2 in place of [Pd(allyl)Cl]2 (entry
12). In addition, the AAA reaction under the influence of 27
(R2 ¼ Ph) in place of 9 (R2 ¼ CH(C2H5)2) in CPME solvent led to
formation of the alkylated product with the opposite configuration
(e.g., ent-7) with a slight increase in the product yield (Table 2,
entry 22 vs. Table 3, entry 13).
It could be seen that a part of palladium precursor [Pd(allyl)Cl]2
would be converted into [Pd(allyl)(NHC)2]Cl A even under a Pd/
NHC ratio of 1:1 (Scheme 4, entry 1). Consequently, the formation
of the catalytically inert bis(NHC)ePd species A would lead to
a moderate yield (65%) of the desired product 7. According to this
assumption, we hypothesized that the use of an ethereal solvent in
place of a halogenated solvent such as (CH2Cl)2 could prevent the
formation of bis(NHC)ePd species A via coordination of palladium
metal with the ether oxygen of the solvent. We assumed that in an
ethereal solvent, [Pd(allyl)Cl(NHC)] and/or [Pd(allyl)(NHC)(sol-
vent)]Cl might be formed instead of [Pd(allyl)(NHC)2]Cl. The donor
numbers of ethereal solvents were reported to be much higher than
those of halogenated solvents [10d,16]. Hence, we investigated the
Fig. 1. 1H NMR (upper) and 13C NMR (lower) spectra of [Pd(allyl)(NHC)2]Cl 30.

H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
51
Table 3
Table 4
Solvent effect in AAA reaction by in situ generated palladium catalyst with a Pd/NHC
AAA reaction by in situ generated palladium catalyst from [Pd(allyl)Cl]2 and 31e35.a
ratio of 1:1.a
Entry
Azolium salt
7
Entry
Solvent
Temp. (ꢀC)
Yield (%)
ee (%)
Yield (%)
ee (%)
31
1
2
3
4
5
6
7
8
(CH2Cl)2
THF
80
70
80
50
70
60
80
90
70
80
80
80
80
65
96
58
90
99
70
83
90
52
99
97
99
33
67
37
47
57
66
68
79
75
75
81
81
80
ꢁ41
2-MeTHFb
TBMEc
DIPEd
O
O
N
1
19
HN
HO
N
Bn
Cl
DIPE
CPMEe
CPME
CPME
CPME
CPME
CPME
CPME
31
9
10f
11f,g
12f,h
13i
N
2
5
14
HN
N
Bn
Cl
32
a
Reaction conditions: see Experimental section for detail.
5
(0.2 mmol), 6
(0.6 mmol), NaH (0.57 mmol), [Pd(allyl)Cl]2 (0.005 mmol), 9 (0.01 mmol), Ag2O
(0.005 mmol), solvent (3 mL), 15 h.
O
N
b
3b
29 (65)
3 (8)
2-MeTHF: 2-methyltetrahydrofuran.
TBME: tert-butyl methyl ether.
DIPE: diisopropyl ether.
CPME: cyclopentyl methyl ether.
6 (1.0 mmol), NaH (0.95 mmol).
Reaction was run for 5 h.
[Pd(cinnamyl)Cl]2 (0.005 mmol) in place of [Pd(allyl)Cl]2 was used.
27 (0.01 mmol) in place of 9 was used.
HN
Me
N
Bn
c
Cl
d
e
33
HO
f
g
h
O
N
i
4b
3 (26)
4 (23)
2 (8)
HN
Me
N
Cl
Bn
2.3. Mechanistic aspects
34
At this stage, we had no experimental evidence to explain the
reaction mechanism. A plausible reaction model involves the
formation of an anionic amidate/palladium species B to lock
a stereodirecting group (R1) in a fixed conformation, as shown in
Scheme 6. It is known that in the absence of overriding steric
factors, the addition of soft nucleophiles to allyl complexes would
occur with regioselective trans to the ligand, which is the strongest
O
N
5b
5 (5)
HN
Me
N
Bn
Cl
35
MeO
a
Reaction conditions: see Experimental section for detail.
(0.6 mmol), NaH (0.57 mmol), [Pd(allyl)Cl]2 (0.005 mmol), azolium salt (0.01 mmol),
5
(0.2 mmol), 6
Ag2O (0.005 mmol), (CH2Cl)2 (3 mL), 80 ꢀC, 15 h.
s
-donor [6,17]. In the reaction model B, the nucleophilic attack
b
The numbers in parentheses showed the yield and ee in the AAA reaction using
would therefore take place trans to the anionic amidate group. For
stereoselective reaction, the orientation of the allyl moiety in the
intermediate B is required, which is in agreement with the
increasing bulkiness of both R1 (tBu) and R2 (CH(C2H5)2) groups at
the NHC ligand. Additionally, it seems likely that the hydroxy group
in the NHC ligand is also an important factor. Thus, the hydroxy
group would be converted into sodium alcoholate upon AAA
conditions involving NaCH(CO2Me)2. The interaction between the
sodium alcoholate and nucleophile would assist the approach of
6 (1.0 mmol), NaH (0.95 mmol), and CPME (3 mL).
1), most of the substrate 5 was recovered unchanged when the
reaction was carried out under the influence of 32 (entry 2). These
results strongly suggested that the hydroxy-amide functionality on
the NHC ligand not only induces the stereoselectivity of the AAA
reaction but also enhances the reaction rate.
In addition, azolium compounds 33e35 were synthesized from
commercially available L-alaninol, (S)-sec-butylamine and (S)-1-
the nucleophile to the p-allyl palladium center [18].
In order to obtain further information of the reaction pathway,
two amide-functionalized azolium salts such as 31 and 32 were
prepared (Table 4). While AAA of 5 with NaCH(CO2Me)2 by using
the palladium catalyst generated in situ from [Pd(allyl)Cl]2 and 31
(R1 ¼ cycloC6H11, R2 ¼ Bn) afforded 7 in 19% yield and 31% ee (entry
methoxy-2-propanamine, respectively. The AAA reaction of 5
with NaCH(CO2Me)2 under the influence of the hydroxy-amide
functionalized azolium salt 33 (R1 ¼ Me, R2 ¼ Bn) in (CH2Cl)2 or
CPME yielded the alkylated product 7 in 29% or 65% yield, respec-
tively, with low enantioselectivity (Table 4, entry 3). This is in
contrast to the reaction using 20 (R1 ¼ tBu, R2 ¼ Bn), in which 7 was
obtained with 43% ee (Table 2, entry 15). These results indicated
that the choice of sterically hindered substituent at the R1 position
on the chiral ligand is an important factor. On the other hand,
replacement of OH group (azolium 33) by a CH3 group or OCH3
group (azolium 34 or 35) in the chiral ligand led to a marked
decrease in the product yield (Table 4, entries 4 and 5). This sug-
gested again that the hydroxy functionality of the chiral ligand is of
critical importance for AAA reaction.
O
N
N
R1
N
Pd
R2
Ph
O
Na
Nu
3. Conclusion
Ph
B
We described the versatility of an anionic amidate/NHCePd
Scheme 6. Proposed allyl complex intermediate.
complex for the AAA reaction. This AAA reaction catalyzed by the

52
H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
NHCePd(II) complex that was generated in situ has been fully
investigated. Changing the R2 substituent at the NHC ring was
found to have a drastic effect on the outcome. The ligand screening
tests revealed the combination of [Pd(allyl)Cl]2 and chiral ligand 9
(R1 ¼ tBu, R2 ¼ CH(C2H5)2) as the best catalytic system. In addition,
the Pd precursor/ligand precursor ratio was found to be an
stirring the reaction mixture at 110 ꢀC for 24 h, the solvent was
removed under reduced pressure. The residue was dissolved in
methanol, and then activated carbon (ca. 1 g) was added. After 16 h,
the activated carbon was removed by filtration. After removing
methanol in vacuo from the filtrate, the crude residue was purified
by column chromatography (SiO2, CH2Cl2/MeOH: 9/1) to yield the
corresponding azolium salt. Azolium salts 1e4, 8e10, 16, 20, 23, 25
and 26 were reported in our preceding papers [10e12]. Because of
the hydroscopic character, elemental analysis of 11e15, 27 and 29
were not performed.
important factor determining the efficiency of AAA. A new
p-allyl
palladium(II) complex containing a bis(NHC) ligand was obtained
and characterized. According to the coordination chemistry, the
AAA reaction was found to be favored in CPME, producing the
desired product 7 with 81% ee. We believe that the present catalytic
system involving an anionic amidate tethered NHCePd(II) complex
would provide an alternative method for the Pd-catalyzed AAA
reaction. Further applications of the chiral chelating polydentate
ligand based on an NHC scaffold for efficient catalytic enantiose-
lective transformations are the subject of ongoing research in our
laboratory.
4.3.1. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-(1-propylbutyl)-1H-benzimidazolium chloride (11)
White solid (yield: 80%); 1H NMR ((CD3)2SO):
d 10.18 (s,1H), 8.59
(d, J ¼ 9.2 Hz, 1H), 8.22e8.19 (m, 1H), 8.04e8.01 (m, 1H), 7.69e7.64
(m, 2H), 5.47 (d, J ¼ 16.5 Hz, 1H), 5.38 (d, J ¼ 16.5 Hz, 1H), 4.94e4.87
(m, 1H), 4.67 (t, J ¼ 6.0 Hz, 1H), 3.62e3.55 (m, 2H), 3.43e3.37 (m,
1H), 2.07e1.89 (m, 4H),1.27e1.13 (m, 2H),1.12e1.02 (m, 2H), 0.86 (s,
4. Experimental
9H), 0.83 (t, J ¼ 7.3 Hz, 6H); 13C NMR ((CD3)2SO):
d 164.7, 142.6,
131.5, 130.8, 126.7, 126.6, 113.9, 113.7, 60.2, 59.8, 58.8, 48.9, 36.0,
33.6, 26.8, 18.4,13.4. HRMS (ESIþ) calcd for C22H36N3O2þ: 374.2802,
found 374.2794.
4.1. General procedures
All chemicals were obtained from commercial sources and were
used as received. 1H and 13C NMR spectra were recorded on spec-
trometers at 400 and 100 MHz, respectively. Chemical shifts were
reported in ppm relative to TMS for 1H and 13C NMR spectra.
(CD3)2SO, CD3OD or CDCl3 was used as the NMR solvent. Thin-layer
chromatography (TLC) analysis was performed with glass-backed
plates pre-coated with silica gel and examined under UV
(254 nm) irradiation. Flash column chromatography was executed
on silica gel 60 (230e400; particle size: 0.040e0.063 nm).
4.3.2. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-cyclopentyl-1H-benzimidazolium chloride (12)
White solid (yield: 84%); 1H NMR ((CD3)2SO):
d 10.14 (s,1H), 8.60
(d, J ¼ 8.7 Hz, 1H), 8.12e8.09 (m, 1H), 8.00e7.98 (m, 1H), 7.69e7.64
(m, 2H), 5.48 (d, J ¼ 16.5 Hz, 1H), 5.39 (d, J ¼ 16.5 Hz, 1H), 5.25e5.18
(m, 1H), 4.70 (br, 1H), 3.62e3.56 (m, 2H), 3.43e3.37 (m, 1H), 2.38e
2.30 (m, 2H), 2.06e1.98 (m, 2H), 1.93e1.83 (m, 2H), 1.80e1.71 (m,
2H), 0.87 (s, 9H); 13C NMR ((CD3)2SO):
d 164.8, 141.8, 131.8, 130.7,
126.7, 126.4, 114.0, 113.8, 60.3, 59.8, 58.9, 48.8, 33.6, 31.6, 26.9, 23.2.
4.2. General procedure for preparation of amidate/NHCePd(II)
complexes 1ae4a
HRMS (ESIþ) calcd for C20H30N3O2þ: 344.2332, found 344.2325.
4.3.3. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
A suspension of azolium salt (0.12 mmol) and silver(I) oxide
(0.07 mmol) in CH2Cl2 (5 mL) was stirred for 2 h in the dark at
refluxing temperature. The reaction mixture was concentrated
under reduced pressure to give a white solid. PdCl2(CH3CN)2
(0.1 mmol) was added to a suspension of the resulting silver
complex in CH3CN (5 mL) in the dark. Then, the resulting suspen-
sion was stirred for 2 h in the dark at room temperature and filtered
through a plug of glass fiber filter paper. The filtrate was evaporated
to dryness in vacuo, and the Pd complexes were purified by rep-
recipitation from ethyl acetate and hexane. These complexes are
very stable under air and could be stored for at least one month at
room temperature. 1a, 2a and 4a were reported in the preceding
2-oxoethyl]-1-cyclohexyl-1H-benzimidazolium chloride (13)
White solid (yield: 85%); 1H NMR ((CD3)2SO):
d 10.02 (s, 1H),
8.48 (d, J ¼ 8.7 Hz, 1H), 8.17e8.13 (m, 1H), 7.93e7.91 (m, 1H), 7.64e
7.61 (m, 2H), 5.40 (d, J ¼ 16.5 Hz, 1H), 5.31 (d, J ¼ 16.5 Hz, 1H), 4.77e
4.71 (m, 1H), 4.63 (br, 1H), 3.58e3.52 (m, 2H), 3.39e3.35 (m, 1H),
2.17e2.14 (m, 2H), 1.87e1.78 (m, 4H), 1.75e1.67 (m, 1H), 1.52e1.46
(m, 2H), 1.29e1.18 (m, 1H), 0.83 (s, 9H); 13C NMR ((CD3)2SO):
d
164.8, 141.8, 131.6, 130.3, 126.7, 126.4, 114.1, 113.8, 60.3, 59.9, 56.8,
48.9, 33.6, 32.0, 26.9, 24.7, 24.6. HRMS (ESIþ) calcd for
C21H32N3O2þ: 358.2489, found 358.2482.
4.3.4. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
paper [10c]. 3a: 1H NMR (CD3OD):
d
7.62 (br, 1H), 7.56e7.55 (m, 2H),
2-oxoethyl]-1-cycloheptyl-1H-benzimidazolium chloride (14)
7.38e7.34 (m, 2H), 5.98 (d, J ¼ 16.6 Hz, 1H), 5.42 (d, J ¼ 16.6 Hz, 1H),
White solid (yield: 42%); 1H NMR (CDCl3):
d 10.41 (s,1H), 8.92 (d,
4.35 (s, 3H), 3.82e3.47 (m, 2H), 3.50e3.45 (m, 1H), 0.79 (s, 9H); 13C
J ¼ 9.2 Hz, 1H), 8.10e8.07 (m, 1H), 7.74e7.70 (m, 1H), 7.59e7.57 (m,
2H), 5.91 (d, J ¼ 16.0 Hz, 1H), 5.84 (d, J ¼ 16.0 Hz, 1H), 4.92 (t,
J ¼ 6.0 Hz, 1H), 4.77e4.70 (m, 1H), 3.85e3.79 (m, 1H), 3.75e3.70 (m,
2H), 2.29e2.14 (m, 4H), 1.89e1.86 (m, 2H), 1.70e1.63 (m, 6H), 0.93
NMR (CD3CD):
d 169.0, 158.5, 136.2, 135.8, 125.0, 124.8, 112.3,
111.4, 62.2, 61.6, 52.6, 35.2, 34.6, 27.2. Anal. Calc. for
C16H22ClN3O2Pd$0.6CH2Cl2: C, 41.43; H, 4.86; N, 8.73. Found: C,
41.36; H, 4.62; N, 8.98%. Crystallization of 3a by slow evaporation
from methanol solution gave colorless crystals suitable for study by
X-ray diffraction. Crystal data: C16H22O2N3ClPd, M ¼ 430.23,
(s, 9H); 13C NMR (CDCl3):
d 165.6, 141.5, 131.8, 129.7, 126.8, 126.3,
114.0, 112.6, 60.8, 60.7, 60.4, 49.5, 49.1, 34.1, þ34.1, 33.3, 26.7, 26.7,
24.0. HRMS (ESIþ) calcd for C22H34N3O2
372.2638.
: 372.2645, found
ꢀ
orthorhombic, a ¼ 10.0266(3), b ¼ 12.7397(5), c ¼ 14.1604(5) A,
3
ꢀ
U ¼ 1808.79(12) A , T ¼ 123(2) K, space group P212121 (no.19), Z ¼ 4,
m
(Mo-K
a
) ¼ 11.859 cmꢁ1. The final Rw was 0.1249 (I > 2
s(I)).
4.3.5. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-cyclooctyl-1H-benzimidazolium chloride (15)
4.3. General procedure of preparation of azolium ligand precursors
White solid (yield: 74%); 1H NMR ((CD3)2SO):
d 10.17 (s, 1H), 8.61
(d, J ¼ 9.2 Hz, 1H), 8.14e8.10 (m, 1H), 8.02e7.98 (m, 1H), 7.68e7.64
(m, 2H), 5.48 (d, J ¼ 16.0 Hz, 1H), 5.39 (d, J ¼ 16.5 Hz, 1H), 5.06e5.01
(m, 1H), 4.71 (br, 1H), 3.61e3.55 (m, 2H), 3.43e3.39 (m, 1H), 2.23e
2.16 (m, 2H), 2.10e2.04 (m, 2H), 1.77e1.60 (m, 10H), 0.87 (s, 9H); 13C
To a flask were added N-alkylbenzimidazole (3 mmol), 1,4-
dioxane (12 mL) and
a-chloroacetoaminde derivative (3 mmol)
derived from chloroacetyl chloride and (S)-tert-leucinol. After

H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
53
NMR ((CD3)2SO):
d
164.8,142.1,131.6,129.9, 126.7,126.4,114.1,113.8,
4.3.11. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-(2-naphthalenylmethyl)-1H-benzimidazolium
60.3, 59.8, 58.5, 48.8, 33.6, 31.2, 26.9, 26.2, 24.8, 23.2. HRMS (ESIþ)
calcd for C23H36N3O2þ: 386.2802, found 386.2794.
chloride (24)
White solid (yield: 85%); 1H NMR ((CD3)2SO):
d 10.04 (s, 1H),
4.3.6. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
8.48 (d, J ¼ 9.1 Hz, 1H), 8.09e7.88 (m, 6H), 7.68e7.53 (m, 5H), 6.02
(s, 2H), 5.49 (d, J ¼ 16.3 Hz, 1H), 5.38 (d, J ¼ 16.3 Hz, 1H), 4.65 (t,
J ¼ 5.7 Hz, 1H), 3.63e3.57 (m, 2H), 3.42e3.37 (m, 1H), 0.86 (s, 9H);
2-oxoethyl]-1-(1-adamantyl)-1H-benzimidazolium chloride (17)
White solid (yield: 73%); 1H NMR ((CD3)2SO):
d 10.01 (s, 1H),
8.54 (d, J ¼ 9.2 Hz, 1H), 8.41 (d, J ¼ 8.2 Hz, 1H), 7.97e7.94 (m, 1H),
7.67e7.60 (m, 2H), 5.47 (d, J ¼ 16.0 Hz, 1H), 5.38 (d, J ¼ 16.0 Hz, 1H),
4.71 (t, J ¼ 5.5 Hz, 1H), 3.62e3.55 (m, 2H), 3.43e3.99 (m, 1H), 2.37-
2.28 (br, 7H), 1.87 (d, J ¼ 12.4 Hz, 3H), 1.76 (d, J ¼ 12.4 Hz, 3H), 0.87
13C NMR ((CD3)2SO):
d 164.8, 143.7, 132.7, 132.7, 131.8, 131.4, 130.6,
128.9, 127.9, 127.7, 127.6, 126.8, 126.7, 125.6, 114.0, 113.8, 60.3, 59.8,
50.1, 48.8, 33.6, 26.8. Anal. Calc. for C26H30ClN3O2$CH3
-
CO2C2H5$2H2O: C, 62.54; H, 7.35; N, 7.29. Found: C, 62.55; H, 6.87;
(s, 9H); 13C NMR ((CD3)2SO):
d 164.8, 141.8, 132.6, 129.1, 126.3, 126.0,
N, 7.45%. M.p. 134.2e134.8 ꢀC.
116.8, 114.2, 61.4, 60.3, 59.9, 48.7, 40.4, 34.9, 33.6, 29.1, 26.9. Anal.
Calc. for C25H36ClN3O2$1.8H2O: C, 62.76; H, 8.34; N, 8.78. Found: C,
62.93; H, 8.26; N, 8.73%. M.p. 215.2e215.5 ꢀC.
4.3.12. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]
amino}-2-oxoethyl]-1-phenyl-1H-benzimidazolium chloride (27)
White solid (yield: 65%); 1H NMR ((CD3)2SO):
d 10.43 (s, 1H),
4.3.7. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
8.62 (br, 1H), 8.14e8.09 (m, 1H), 7.88e7.84 (m, 3H), 7.78e7.69 (m,
5H), 5.60 (d, J ¼ 16.5 Hz, 1H), 5.52 (d, J ¼ 16.5 Hz, 1H), 3.65e3.60 (m,
2-oxoethyl]-1-(2-propenyl)-1H-benzimidazolium chloride (18)
White solid (yield: 33%); 1H NMR ((CD3)2SO):
d
9.88 (s, 1H), 8.52
2H), 3.46e3,41 (m, 1H), 0.90 (s, 9H); 13C NMR ((CD3)2SO):
d 164.5,
(br, 1H), 8.02e7.98 (m, 2H), 7.69e7.67 (m, 2H), 6.17e6.07 (m, 1H),
5.56e5.33 (m, 4H), 5.25 (d, J ¼ 5.4 Hz, 2H), 4.56 (t, J ¼ 5.7 Hz, 1H),
3.63e3.56 (m, 2H), 3.42e3.37 (m, 1H), 0.87 (s, 9H); 13C NMR
143.5, 133.0, 131.6, 130.7, 130.6, 130.5, 127.5, 127.1, 125.1, 114.2, 113.þ6,
60.3, 60.0, 49.1, 33.6, 26.9. HRMS (ESIþ) calcd for C21H26N3O2
352.2019, found 352.2012.
:
((CD3)2SO):
d 164.7, 143.4, 131.6, 131.0, 130.6, 126.7, 126.6, 120.2,
113.8, 113.6, 60.2, 59.8, 48.7, 35.0, 33.6, 26.8. Anal. Calc. for
C18H26ClN3O2$0.2CHCl3: C, 58.18; H, 7.03; N, 11.18. Found: C, 58.17;
H, 6.86; N, 11.35%. M.p. 220.8e221.0 ꢀC.
4.3.13. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-(4-nitrophenyl)-1H-benzimidazolium chloride (28)
Light yellow solid (yield: 17%); 1H NMR ((CD3)2SO):
d 10.55 (s,
1H), 8.63 (d, J ¼ 9.6 Hz, 1H), 8.58 (d, J ¼ 9.2 Hz, 2H), 8.20 (d,
J ¼ 9.2 Hz, 2H), 8.14 (d, J ¼ 7.3 Hz, 1H), 7.96e7.94 (m, 1H), 7.80e7.73
(m, 2H), 5.62 (d, J ¼ 16.0 Hz, 1H), 5.53 (d, J ¼ 16.0 Hz, 1H), 4.70 (t,
J ¼ 5.7 Hz, 1H), 3.66e3.59 (m, 2H), 3.46e3.38 (m, 1H), 0.90 (s, 9H);
4.3.8. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-(2-propynyl)-1H-benzimidazolium chloride (19)
White solid (yield: 38%); 1H NMR ((CD3)2SO):
d 9.93 (s, 1H), 8.46
(d, J ¼ 9.2 Hz, 1H), 8.06e8.04 (m, 1H), 8.02e7.97 (m, 1H), 7.75e7.69
(m, 2H), 5.61 (d, J ¼ 2.3 Hz, 2H), 5.50 (d, J ¼ 16.0 Hz, 1H), 5.39 (d,
J ¼ 16.0 Hz, 1H), 4.63 (t, J ¼ 5.3 Hz, 1H), 3.88 (t, J ¼ 2.3 Hz, 1H), 3.63e
3.57 (m, 2H), 3.42e3.37 (m, 1H), 0.87 (s, 9H); 13C NMR ((CD3)2SO):
13C NMR ((CD3)2SO):
d 164.3, 148.1, 144.0, 138.0, 131.7, 130.4, 127.7,
127.3,126.6,125.7,114.4,113.6, 60.3, 59.9, 49.3, 33.6, 26.9. Anal. Calc.
for C21H25ClN4O4$0.5H2O$0.5CH2Cl2: C, 53.31; H, 5.62; N, 11.57.
Found: C, 52.99; H, 5.53; N, 11.86%. M.p. 230.7e230.9 ꢀC.
d
164.6, 143.1, 131.6, 130.1, 126.9, 126.8, 113.8, 79.3, 75.5, 60.2, 59.8,
48.8, 36.7, 33.6, 26.8. Anal. Calc. for C18H24ClN3O2$0.2H2O: C, 61.16;
H, 6.96; N, 11.89. Found: C, 61.03; H, 6.77; N, 11.86%. M.p. 225.4e
225.9 ꢀC.
4.3.14. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-(2,4,6-trimethylphenyl)-1H-benzimidazolium
chloride (29)
White solid (yield: 44%); 1H NMR (CDCl3):
d 10.24 (s,1H), 8.91 (d,
4.3.9. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]amino}-
2-oxoethyl]-1-[(2-methylphenyl)methyl]-1H-benzimidazolium
chloride (21)
J ¼ 8.7 Hz, 1H), 8.16 (d, J ¼ 8.2 Hz, 1H), 7.68 (t, J ¼ 7.8 Hz, 1H), 7.56 (t,
J ¼ 7.8 Hz, 1H), 7.21 (d, J ¼ 8.2 Hz, 1H), 7.07 (s, 2H), 6.12 (d,
J ¼ 16.0 Hz,1H), 5.87 (d, J ¼ 16.0 Hz,1H), 4.56 (br,1H), 3.79e3.73 (m,
1H), 3.71e3.70 (m, 2H), 2.39 (s, 3H), 2.02 (s, 3H),1.99 (s, 3H), 0.92 (s,
White solid (yield: 88%); 1H NMR ((CD3)2SO):
d 9.75 (s, 1H), 8.42
(d, J ¼ 9.2 Hz, 1H), 7.98e7.93 (m, 2H), 7.71e7.63 (m, 2H), 7.31e7.30
(m, 2H), 7.24e7.20 (m, 1H), 7.14 (d, J ¼ 7.8 Hz, 1H), 5.85 (s, 2H), 5.46
(d, J ¼ 16.5 Hz, 1H), 5.36 (d, J ¼ 16.5 Hz, 1H), 4.66 (t, J ¼ 5.5 Hz, 1H),
3.63e3.56 (m, 2H), 3.41e3.37 (m, 1H), 2.33 (s, 3H), 0.86 (s, 9H); 13C
9H); 13C NMR (CDCl3):
d 165.6, 143.8, 141.5, 135.6, 135.4, 131.8, 131.2,
129.9, 129.9, 128.0, 127.6, 127.6, 114.5, 112.6, 61.1, 60.9, 50.0, 33.7,
26.9, 21.1, 17.5, 17.4. HRMS (ESIþ) calcd for C24H32N3O2þ: 394.2489,
found 394.2493.
NMR ((CD3)2SO):
d 164.8, 143.6, 138.3, 134.0, 131.7, 130.5, 129.3,
128.9, 128.7, 126.8, 126.7, 125.2, 113.9, 113.8, 60.3, 60.0, 49.8, 48.9,
33.6, 26.9, 21.0. Anal. Calc. for C23H30ClN3O2$0.3CH3CO2C2H5: C,
65.70; H, 7.38; N, 9.50. Found: C, 65.68; H, 7.18; N, 9.38%. M.p.
150.3e150.8 ꢀC.
4.3.15. 3-[2-{[(1S)-1-Cyclohexyl-2-hydroxyethyl]amino}-2-
oxoethyl]-1-phenylmethyl-1H-benzimidazolium chloride (31)
White solid (yield: 74%); 1H NMR ((CD3)2SO):
d 10.05 (s, 1H),
8.59 (br, 1H), 7.99e7.95 (m, 2H), 7.68e7.61 (m, 2H), 7.51e7.49 (m,
2H), 7.42e7.34 (m, 3H), 5.85 (s, 2H), 5.44 (d, J ¼ 16.0 Hz,1H), 5.36 (d,
J ¼ 16.0 Hz,1H), 4.77 (br, 1H), 3.60e3.53 (m, 1H), 3.46e3.43 (m, 2H),
4.3.10. 3-[2-{[(1S)-1-(Hydroxymethyl)-2,2-dimethylpropyl]
amino}-2-oxoethyl]-1-(4-biphenylmethyl)-1H-benzimidazolium
chloride (22)
1.70e1.47 (m, 6H), 1.21e0.89 (m, 5H); 13C NMR ((CD3)2SO):
d 164.3,
143.6, 134.0,131.7,130.5, 129.0,128.7,128.2,126.8,126.7,113.9,113.7,
60.8, 56.0, 49.8, 48.7, 37.9, 29.4, 28.2, 26.0, 25.8, 25.7. Anal. Calc. for
C24H30ClN3O2$0.3H2O$0.3CH2Cl2: C, 63.61; H, 6.85; N, 9.16. Found:
C, 64.00; H, 6.45; N, 9.38%. M.p. 119.7e120.0 ꢀC.
White solid (yield: 33%); 1H NMR ((CD3)2SO):
d 10.09 (s, 1H),
8.53 (d, J ¼ 9.2 Hz, 1H), 8.07e7.93 (m, 2H), 7.71e7.58 (m, 8H), 7.46e
7.42 (m, 2H), 7.37e7.33 (m,1H), 5.90 (s, 2H), 5.51 (d, J ¼ 16.5 Hz,1H),
5.41 (d, J ¼ 16.5 Hz, 1H), 4.66 (t, J ¼ 5.5 Hz, 1H), 3.62e3.57 (m, 2H),
3.41e3.39 (m, 1H), 0.87 (s, 9H); 13C NMR ((CD3)2SO):
d
164.8, 143.6,
4.3.16. 3-{2-[(1S)-1-Cyclohexylethylamino]-2-oxoethyl}-1-
140.6, 140.5, 139.3, 133.2, 131.7, 131.2, 130.5, 129.0, 128.9, 128.9,
127.8, 127.3, 126.8, 126.7, 60.3, 59.9, 49.5, 48.8, 33.6, 26.9. Anal. Calc.
for C28H32ClN3O2$2.5H2O: C, 64.29; H, 7.13; N, 8.03. Found: C,
64.56; H, 7.20; N, 7.59%. M.p. 227.6e227.9 ꢀC.
phenylmethyl-1H-benzimidazolium chloride (32)
White solid (yield: 65%); 1H NMR ((CD3)2SO):
d 10.02 (s, 1H),
8.69 (d, J ¼ 8.6 Hz, 1H), 8.00e7.93 (m, 2H), 7.69e7.62 (m, 2H), 7.51e
7.49 (m, 2H), 7.43e7.34 (m, 3H), 5.85 (s, 2H), 5.40 (d, J ¼ 16.3 Hz,

54
H. Shirasaki et al. / Journal of Organometallic Chemistry 726 (2013) 46e55
1H), 5.33 (d, J ¼ 16.3 Hz, 1H), 3.66e3.57 (m, 1H), 1.72e1.58 (m, 5H),
1.34e1.26 (m, 1H), 1.17e1.08 (m, 3H), 1.04 (d, J ¼ 6.8 Hz, 3H), 0.96e
temperature for an additional 16 h in the dark, filtered through
a membrane filter, and evaporated to dryness in vacuo. To the
resulting flask containing yellow solid of unpurified NHC-Pd(II)
complex, a solution of 5 (0.2 mmol, 50 mg) in CPME (2 mL), and
then subsequently a solution of NaCH(COOMe)2 (1 mmol) in CPME
(1 mL) were added. After stirring at 80 ꢀC for 15 h, the reaction was
quenched with water (ca. 2 mL) and extracted with diisopropyl
ether (ca. 2 mL).
0.87 (m, 2H); 13C NMR ((CD3)2SO):
d 163.7, 143.6, 134.1, 131.7, 130.5,
129.0, 128.8, 120.2, 126.8, 126.7, 114.0, 113.7, 49.8, 49.5, 48.6, 42.4,
28.7, 28.7, 26.0, 25.8, 25.7, 17.5. Anal. Calc. for C24H30ClN3O$H2O: C,
67.04; H, 7.50; N, 9.77. Found: C, 66.95; H, 7.24; N, 9.81%. M.p.
144.4e144.8 ꢀC.
4.3.17. 3-[2-{[(1S)-2-Hydroxy-1-methylethyl]amino}-2-oxoethyl]-
1-phenylmethyl-1H-benzimidazolium chloride (33)
4.6. One-pot synthesis of [Pd(allyl)(NHC)2]Cl 30 from azolium salt 1
and [Pd(allyl)Cl]2
White solid (yield: 48%); 1H NMR ((CD3)2SO):
d 10.06 (s, 1H),
8.80 (br, 1H), 8.03e7.92 (m, 2H), 7.66e7.35 (m, 7H), 5.85 (s, 2H),
5.37 (s, 2H), 4.90 (br, 1H), 3.79e3.76 (m, 1H), 3.38e3.36 (m, 2H),
A suspension of azolium salt 1 (0.2 mmol, 62 mg) and silver(I)
oxide (0.1 mmol, 23 mg) in CH2Cl2 (10 mL) was stirred in the dark at
room temperature. After 16 h, [Pd(allyl)Cl]2 (0.05 mmol, 18 mg) was
added to a suspension of the resulting silver complex in the dark at
room temperature. Then, the resulting suspension was stirred for
24 h and filtered through a plug of glass fiber filter paper. The
filtrate was evaporated to dryness in vacuo. The crude residue was
purified by column chromatography (SiO2, CH2Cl2/MeOH: 9/1) to
yield [Pd(allyl)(NHC)2]Cl 30 as white solid (50 mg, 67% yield). 30:
1.07 (d, J ¼ 7.6 Hz, 3H); 13C NMR ((CD3)2SO):
d 164.0, 143.6, 134.0,
131.8, 130.4, 129.0, 128.7, 128.2, 126.8, 126.7, 113.9, 113.8, 64.1, 59.8,
49.8, 48.7, 16.9. Anal. Calc. for C19H22ClN3O2$0.5H2O: C, 61.87; H,
6.28; N, 11.39. Found: C, 61.48; H, 6.03; N, 11.21%. M.p. 205.1e
205.4 ꢀC.
4.3.18. 3-[2-{[(1S)-1-Methylpropyl]amino}-2-oxoethyl]-1-
phenylmethyl-1H-benzimidazolium chloride (34)
White solid (yield: 26%); 1H NMR ((CD3)2SO):
d 10.11 (s,1H), 8.90
1H NMR (CD3OD):
d
7.59 (d, J ¼ 7.8 Hz, 2H), 7.44e7.31 (m, 6H), 5.69e
(d, J ¼ 7.8 Hz, 1H), 7.99e7.97 (m, 2H), 7.67e7.60 (m, 2H), 7.52e7.50
(m, 2H), 7.42e7.34 (m, 3H), 5.86 (s, 2H), 5.43 (d, J ¼ 16.5 Hz, 1H),
5.37 (d, J ¼ 16.5 Hz, 1H), 3.71e3.64 (m, 1H), 1.45e1.40 (m, 2H), 1.07
(d, J ¼ 6.9 Hz, 3H), 0.83 (t, J ¼ 7.3 Hz, 3H); 13C NMR ((CD3)2SO):
5.59 (m, 1H), 5.20 (d, J ¼ 16.9 Hz, 2H), 5.15 (d, J ¼ 16.9 Hz, 2H), 4.17
(d, J ¼ 7.3 Hz, 2H), 4.01 (s, 6H), 3.51e3.43 (m, 4H), 3.39e3.34 (m,
2H), 3.00 (d, J ¼ 13.3 Hz, 2H), 1.78e1.70 (m, 2H), 0.86 (d, J ¼ 6.9 Hz,
6H), 0.83 (d, J ¼ 6.9 Hz, 6H); 13C NMR (CD3OD):
d 191.2, 168.5, 136.7,
d
163.7, 143.6, 134.0, 131.7, 130.4, 129.0, 128.7, 128.2, 126.8, 126.6,
136.2, 124.7, 122.0, 111.9, 111.8, 62.8, 62.4, 58.6, 51.9, 35.8, 30.2, 19.9,
19.1. Anal. Calc. for C33H47ClN6O4Pd$0.3CHCl3$2H2O: C, 49.65; H,
6.42; N, 10.43. Found: C, 49.25; H, 6.10; N, 10.83%.
113.9, 113.6, 49.8, 48.6, 46.6, 28.7, 20.0, 10.4. HRMS (ESIþ) calcd for
C20H24N3O2þ: 322.1919, found 322.1920.
4.3.19. 3-[2-{[(1S)–2-Methoxyl-1-methylethyl]amino}-2-oxoethyl]-
1-phenylmethyl-1H-benzimidazolium chloride (35)
Acknowledgments
White solid (yield: 42%); 1H NMR ((CD3)2SO):
d 10.21 (br, 1H),
This work was financially supported by a Grant-in-Aid for
Scientific Research (C) (23550128) from Japan Society for the
Promotion of Science (JSPS).
9.14 (br, 1H), 8.00e7.96 (m, 2H), 7.68e7.62 (m, 2H), 7.53e7.51 (m,
2H), 7.42e7.36 (m, 3H), 5.88e5.87 (br, 2H), 5.42e5.38 (br, 2H),
3.98e3.92 (m, 1H), 3.40e3.23 (m, 5H), 1.09 (d, J ¼ 6.9 Hz, 3H); 13C
NMR ((CD3)2SO):
d 163.9, 143.6, 134.0, 131.6, 130.4, 128.9, 128.7,
Appendix A. Supplementary data
128.2, 126.7, 126.6, 113.9, 113.7, 74.7, 58.2, 49.7, 48.6, 44.7,17.1. HRMS
(ESIþ) calcd for C20H24N3O2þ: 338.1868, found 338.1863.
CCDC 739455 contains the supplementary crystallographic data
for 3c. This data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
4.4. General procedure for the AAA reaction catalyzed by well
defined amidate/NHCePd(II) complex
Dimethyl malonate (6) (0.6 mmol, 79 mg) was added slowly to
a stirring suspension of NaH 60% w/w dispersion in mineral oil
(0.57 mmol, 22.8 mg) in 1,2-dichloroethane (1 mL). Once gas
evolution had caused (5 min), this solution was added to the
solution of 1,3-diphenylprop-3-en-1-yl acetate (5) (0.2 mmol,
50 mg) and amidate/NHCePd(II) complex (5 mol %, 0.01 mol) in 1,2-
dichloroethane (2 mL). The resulting reaction mixture was stirred
at 80 ꢀC. After 15 h, the reaction was quenched with water (ca.
2 mL) and extracted with diisopropyl ether (ca. 2 mL). The product
yield was determined by GLC using an internal standard method.
Enantiomeric excess of the product was measured by HPLC using
OD chiral stationary phase and 99:1 hexaneeiPrOH as eluent,
flow ¼ 1 mL/min.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2012.12.015.
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