Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.10.011

The synthesis of a series of azolium salts such as azolium iodides and chlorides having both N-anionic functional group and N-alkyl group have been developed. Reaction of azolium iodides or chlorides with Ag₂O gave the corresponding NHC-Ag complexes. It was found that the resulting NHC-Ag complexes derived from azolium iodides or chlorides differ in their physical properties. The azolium chlorides as well as azolium iodides were successfully converted into the NHC-Ag complexes, which subsequently reacted with PdCl₂(CH₃CN)₂ to give the anionic amidate/NHC-Pd complexes. Thus, a variety of the NHC-Pd complexes could be obtained from benzimidazolium and imidazolium salts.

Full text of DOI:10.1016/j.jorganchem.2009.10.011

Journal of Organometallic Chemistry 695 (2010) 195–200
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Tridentate, anionic tethered N-heterocyclic carbene of Pd(II) complexes
a
b
b
b,
*
Satoshi Sakaguchi a, , Miaki Kawakami , Justin O’Neill , Kyung Soo Yoo , Kyung Woon Jung
*
a Department of Chemistry and Materials Engineering and High Technology Research Center, Faculty of Chemistry, Materials and Bioengineering, Kansai University,
Suita, Osaka 564-8680, Japan
b Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a series of azolium salts such as azolium iodides and chlorides having both N-anionic
functional group and N-alkyl group have been developed. Reaction of azolium iodides or chlorides with
Ag2O gave the corresponding NHC–Ag complexes. It was found that the resulting NHC–Ag complexes
derived from azolium iodides or chlorides differ in their physical properties. The azolium chlorides as
well as azolium iodides were successfully converted into the NHC–Ag complexes, which subsequently
reacted with PdCl2(CH3CN)2 to give the anionic amidate/NHC–Pd complexes. Thus, a variety of the
NHC–Pd complexes could be obtained from benzimidazolium and imidazolium salts.
Published by Elsevier B.V.
Received 10 July 2009
Received in revised form 30 September
2009
Accepted 13 October 2009
Available online 10 November 2009
Keywords:
N-heterocyclic carbene
Anionic tridentate ligand
NHC–Ag complex
NHC–Pd complex
Ligand-transfer reaction
1. Introduction
In the last decade, heteroatom-functionalized NHC–metal com-
plexes having stereodirecting groups have been developed. These
Control of stereoselectivity in transition–metal-catalyzed or-
ganic transformation reactions depends on development of a ver-
satile ligand that would strongly coordinate with the metal
center [1]. Polydentate ligands having stereodirecting groups could
help control the stability and reactivity of metals efficiently in a
variety of homogeneous catalysis reactions. Thus, a ligand that
combines a strongly coordinating unit with a functional group hav-
ing great influence on the electronic and steric properties of the
metal center has considerable potential.
are classified as NHC-based chelate ligands that incorporate a neu-
tral or an anionic functional group. For the neutral-functionalized
NHC, a breakthrough has been achieved by Burgess, who reports
highly efficient asymmetric hydrogenation catalysis based on car-
bene/oxazoline iridium complexes [4]. Other important chiral
bidentate NHC complexes have been developed by Gade et al. [5]
and Douthwaite and coworker [6] for enantioselective hydrosilyla-
tion and alkylation, respectively. For the anionic-functionalized
NHC, anionic aryloxo- or alkoxo-tethered NHC has been designed
and successfully applied in catalytic asymmetric transformations.
Pioneering work was done by Hoveyda and coworkers [7], where
metathesis and alkylation reactions proceeded with high enanti-
oselectivity by the use of chiral NHC–Ag complexes as ligand pre-
cursors. Arnold et al. [8] as well as Mauduit and coworkers [9]
independently introduced chelating alkoxy NHC–Cu complexes
for asymmetric alkylations. Thus, the anionic, tightly coordinating
polydentate NHC-ligand system is expected to enhance catalyst
stability and to offer a key structure for the construction of efficient
stereodirecting elements.
In recent years, there has been growing interest in using N-het-
erocyclic carbene (NHC) as a ligand for homogenous catalysis [2].
An attractive feature of NHC is not only its strong
r-donating capa-
bility to metals but also the possibility of varying the substituents
on the nitrogen atom. It is possible that introduction of chiral sub-
stituents into NHC would result in enantioselective transforma-
tions in asymmetric catalysis. However, the standard chiral NHC
obtained by this strategy often leads to low chiral inductions,
mainly because of rapid internal rotation of the chiral substituents
around the C–N axis. Therefore, to lock the N substituents in fixed
conformation, a proposed strategy employs an NHC that bears both
a chiral center and a hard chelating functional group on the N sub-
stituents resulting in generation of polydentate ligands [1,3].
Recently, we developed a novel chiral tridentate NHC-ligand
and their Pd(II) complexes (Scheme 1) [10]. Importantly, the Pd(II)
complex derived from chiral b-amino alcohol catalyzes an asym-
metric oxidative Heck-type reaction with excellent enantioselec-
tivity (up to 98% ee). However, only two kinds of ligands which
are derived from (S)-valinol and (S)-2-phenylglycinol have been
appeared in a previous paper. In a continuation of this study, we
* Corresponding authors. Tel.: +81 6 6368 0867; fax: +81 6 6339 4026 (S.
Sakaguchi).
E-mail address: satoshi@ipcku.kansai-u.ac.jp (S. Sakaguchi).
0022-328X/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2009.10.011

196
S. Sakaguchi et al. / Journal of Organometallic Chemistry 695 (2010) 195–200
3.73–3.67 (m, 1H), 3.67–3.51 (m, 2H), 1.93–1.80 (m, 1H), 0.93 (d,
J = 6.6 Hz, 3H), 0.90 (d, J = 6.6 Hz, 3H); 13C NMR: d 169.3, 139.4,
128.9, 121.7, 62.9, 58.3, 50.3, 30.0, 19.9, 18.7.
O
N
R1
N
O
O
N
N
I
N
Pd
R2
R1
2.2.2. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-
oxoethyl]benzimidazole (5)
R1
HN
HO
N
N
R2
O
N
O
R2
Pd
R2
Pd
N
1H NMR (CD3OD): d 8.16 (s, 1H), 7.68–7.65 (m, 1H), 7.51–7.47
(m, 1H), 7.34–7.23 (m, 2H), 5.05 (d, J = 16.3 Hz, 1H), 4.98 (d,
J = 16.3 Hz, 1H), 3.75–3.67 (m, 1H), 3.67–3.52 (m, 2H), 1.94–1.80
(m, 1H), 0.93 (d, J = 4.7 Hz, 3H), 0.90 (d, J = 4.7 Hz, 3H); 13C NMR:
d 169.1, 145.7, 124.4, 123.7, 120.0, 111.3, 63.0, 58.4, 48.2, 30.0,
20.0, 18.8.
Cl
N
O
R1
N
R1, R2 = iPr, Me
H
O
R1, R2 = Ph, Me
Scheme 1. Previously reported benzimidazolium iodides and their Pd(II)
complexes.
2.3. General procedure for preparation of 3 and 6
assume that easy tuning of both N-anionic functional groups and
N-alkyl groups of the ligand would allow the development of a
huge variety of tridentate NHC-ligands. Here, we wish to report
the preparation of a series of NHC proligands such as azolium io-
dides and chlorides. In addition, the conversion of these azolium
salts into the anionic amidate/NHC–Pd(II) complexes via NHC–Ag
complexes, that has not been mentioned in detail in a previous pa-
per, will be discussed.
To a 300 mL round-bottom flask 2 or 5 (7.5 mmol), iodometh-
ane (22.5 mmol), and THF (120 mL) were added. The reaction mix-
ture was stirred under refluxing for 15 h. After cooling the solution
at room temperature, a white solid, which is the desired product,
was filtrated and then washed with THF. These compounds were
reported previously [10].
2.3.1. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
methylimidazolium iodide (3)
1H NMR (DMSO): d 9.07 (s, 1H), 8.14 (br, 1H), 7.67 (d, J = 1.7 Hz,
1H), 7.66 (d, J = 1.7 Hz, 1H), 5.02 (d, J = 16.3 Hz, 1H), 4.93 (d,
J = 16.3 Hz, 1H), 4.63 (br, 1H), 3.87 (s, 3H), 3.61–3.51 (m, 1H),
3.42–3.32 (m, 2H), 1.88–1.75 (m, 1H), 0.85 (d, J = 4.5 Hz, 3H),
0.83 (d, J = 4.5 Hz, 3H); 13C NMR: d 164.6, 137.6, 123.6, 122.9,
61.0, 56.4, 50.6, 35.8, 28.2, 19.5, 18.1.
2. Experimental
2.1. 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 270 or 400 and 67.5 or 100 MHz, respectively. 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 (Merck, mesh: 230–400; particle size: 0.040–
0.063 nm). Elemental analyses were performed at Osaka Univer-
sity. Because most of the imidazole compounds are highly hydro-
scopic, collection of some analytical data was failed. X-ray
diffraction measurements were made on a Rigaku RAXIS RAPID
imaging plate area detector with graphite monochromated Mo
2.3.2. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
methylbenzimidazolium iodide (6)
1H NMR (CD3OD): d 9.55 (s, 1H), 8.01–7.95 (m, 1H), 7.93–7.86
(m, 1H), 7.75–7.67 (m, 2H), 5.43 (d, J = 16.3 Hz, 1H), 5.35 (d,
J = 16.3 Hz, 1H), 4.18 (s, 3H), 3.78–3.69 (m, 1H), 3.69–3.55 (m,
2H), 1.97–1.83 (m, 1H), 0.96-0.94 (m, 6H); 13C NMR: d 167.2,
144.7, 128.3, 128.2, 114.4, 114.2, 63.0, 58.9, 49.7, 34.0, 30.0, 20.0,
18.9.
Ka radiation. The structures were solved by direct methods and re-
2.4. General procedure for preparation of 7–16
fined by full-matrix least-squares. The crystal data collection and
refinement parameters are summarized in Supplementary
material.
To a flask were added N-alkylated azole (4.4 mmol), 1,4-dioxane
(15 mL), and
a-chloroacetoaminde (4 mmol). 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 (ca. 1 g) was added. After 16 h, the activated car-
bon was removed by filtration. The filtrate was concentrated under
reduced pressure to obtain a solid, which was purified by re-pre-
cipitation using ethyl acetate and methanol to afford the corre-
sponding coupling product as a white solid.
2.2. General procedure for preparation of 2 and 5
To a 100 mL three-necked flask was added ZnCl2 (2.9 mmol),
and then the flask was dried by heating with heat-gun in vacuo.
After cooling at room temperature, 1 or 4 (7.5 mmol), PhCl
(10 mL), and (S)-valinol (7 mmol) were successively added. After
stirring the reaction mixture at refluxing temperature for 15 h un-
der N2 atmosphere, the solvent was removed under reduced pres-
sure. The resulting reaction mixture was dissolved in methanol (ca.
20 mL), and then silica gel (ca. 5 g) was added. The solution was
stirred at refluxing temperature for 2 h. Subsequently, the silica
gel was removed by filtration. The filtrate was concentrated under
reduced pressure, which was purified by column chromatography
on silica gel using AcOEt followed by MeOH as an eluent to afford 2
or 5 as a white solid. These compounds were reported previously
[10].
2.4.1. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
methylbenzimidazolium chloride (7)
1H NMR (CD3OD): d 9.55 (s, 1H), 7.97–7.89 (m, 2H), 7.73–7.68
(m, 2H), 5.42 (d, J = 16.8 Hz, 1H), 5.36 (d, J = 16.8 Hz, 1H), 4.18 (s,
3H), 3.76–3.57 (m, 3H), 1.94–1.85 (m, 1H), 0.98–0.94 (m, 6H);
13C NMR: d 166.8, 133.3, 133.1, 128.3, 128.2, 114.3, 114.2, 63.0,
58.9, 33.9, 30.2, 20.1, 18.8. Anal. Calc. for C15H23ClN3O2ꢀ1.5H2O:
C, 53.17; H, 7.44; N, 12.40. Found: C, 53.73; H, 7.29; N, 12.33%.
½
a 2D5
ꢁ
¼ ꢂ9:9 (c = 1.0 in CH3OH). M.p. 179.4–180.0 °C.
2.2.1. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-
oxoethyl]imidazole (2)
2.4.2. 1-[2-((S)-1-hydroxy-4-methyl-2-pentanylamino)-2-oxoethyl]-
3-methylbenzimidazolium chloride (8)
1H NMR (CD3OD): d 7.66 (s, 1H), 7.11 (t, J = 1.3 Hz, 1H), 6.97 (t,
J = 1.3 Hz, 1H), 4.79 (d, J = 16.2 Hz, 1H), 4.73 (d, J = 16.2 Hz, 1H),
1H NMR (DMSO): d 9.80 (s, 1H), 8.60 (d, J = 8.7 Hz, 1H), 8.03–
8.00 (m, 1H), 7.93–7.90 (m, 1H), 7.71–7.66 (m, 2H), 5.36 (d,

S. Sakaguchi et al. / Journal of Organometallic Chemistry 695 (2010) 195–200
197
J = 16.3 Hz, 1H), 5.29 (d, J = 16.3 Hz, 1H), 4.83 (t, J = 5.7 Hz, 1H),
2.4.9. 1-[2-((S)-1-hydroxy-4-methyl-2-pentanylamino)-2-oxoethyl]-
3-butylimidazolium chloride (15)
4.13 (s, 3H), 3.85–3.78 (m, 1H), 3.35–3.31 (m, 2H), 1.65–1.55 (m,
1H), 1.38–1.26 (m, 2H), 0.86 (d, J = 6.4 Hz, 3H), 0.79 (d, J = 6.4 Hz,
3H); 13C NMR: d 164.1, 143.6, 131.8, 126.4, 113.6, 113.5, 63.5,
49.6, 48.8, 39.8, 33.3, 24.2, 23.5, 21.8. Anal. Calc. for
C16H24ClN3O2ꢀ2H2O: C, 53.11; H, 7.80; N, 11.61. Found: C, 53.09;
1H NMR (DMSO): d 9.29 (s, 1H), 8.50 (d, J = 8.5 Hz, 1H), 7.80 (s,
1H), 7.73 (s, 1H), 5.02 (d, J = 16.3 Hz, 1H), 4.98 (d, J = 16.3 Hz, 1H),
4.83 (t, J = 12.0 Hz, 1H), 4.21 (t, J = = 14.1 Hz, 2H), 3.80–3.72 (m,
1H), 3.33–3.29 (m, 2H), 1.79–1.72 (m, 2H), 1.61–1.55 (m, 1H),
1.32–1.21 (m, 4H), 0.90–0.80 (m, 9H); 13C NMR: d 164.3, 137.2,
123.7, 121.6, 63.3, 50.6, 49.5, 48.4, 31.3, 24.1, 23.1, 21.9, 18.6,
13.2. Anal. Calc. for C15H28ClN3O2ꢀH2O: C, 53.64; H, 9.00; N,
H, 7.28; N, 11.62%. ½a D25
¼ ꢂ10:0 (c = 1.0 in CH3OH). M.p. 180.0–
ꢁ
180.4 °C.
2.4.3. 1-[2-((S)-1-hydroxy-3,3-dimethyl-2-butanylamino)-2-
oxoethyl]-3-methylbenzimidazolium chloride (9)
12.51. Found: C, 53.95; H, 8.66; N, 12.64%. ½a D25
¼ þ5:9 (c = 1.0 in
ꢁ
CH3OH). M.p. 52.8–53.1 °C.
1H NMR (DMSO): d 9.89 (s, 1H), 8.63 (d, J = 9.0 Hz, 1H), 8.03–
8.00 (m, 2H), 7.67–7.65 (m, 2H), 5.53 (d, J = 16.3 Hz, 1H), 5.43 (d,
J = 16.3 Hz, 1H), 4.70 (t, J = 5.7 Hz, 1H), 4.13 (s, 3H), 3.60–3.57 (m,
2H), 3.43–3.40 (m, 1H), 0.86 (s, 9H); 13C NMR: d 164.8, 143.6,
131.4, 131.3, 126.5, 126.3, 113.5, 113.4, 60.2, 59.8, 48.6, 33.5,
33.2, 26.8. Anal. Calc. for C16H24ClN3O2ꢀ2.5H2O: C, 51.82; H, 7.88;
2.4.10. 1-[2-((S)-1-hydroxy-4-methyl-2-pentanylamino)-2-oxoethyl]-
3-benzylimidazolium chloride (16)
1H NMR (DMSO): d 9.34 (s, 1H), 8.37 (br, 1H), 7.81 (s, 1H), 7.72
(s, 1H), 7.42–7.38 (m, 5H), 5.50 (s, 2H), 5.04 (d, J = 16.3 Hz, 1H),
4.99 (d, J = 16.3 Hz, 1H), 4.77 (t, J = 5.9 Hz, 1H), 3.81–3.73 (m,
1H), 3.34–3.26 (m, 2H), 1.62–1.55 (m, 1H), 1.32–1.28 (m, 2H),
0.86 (d, J = 6.6 Hz, 3H), 0.82 (d, J = 6.6 Hz, 3H); 13C NMR: d 164.3,
137.4, 134.9, 129.0, 128.7, 128.2, 124.1, 121.8, 63.4, 51.8, 50.7,
49.6, 39.8, 24.1, 23.2, 21.9. Anal. Calc. for C18H26ClN3O2: C, 61.44;
H, 7.45; N, 11.94. Found: C, 61.26; H, 7.25; N, 11.92%.
N, 11.33. Found: C, 51.50; H, 7.59; N, 11.32%. ½a D25
¼ þ15:9
ꢁ
(c = 1.0 in CH3OH). M.p. 53.4–53.8 °C.
2.4.4. 1-[2-((S)-1-hydroxy-2-phenyl-2-ethanylamino)-2-oxoethyl]-3-
methylbenzimidazolium chloride (10)
1H NMR (CD3OD): d 7.95–7.93 (m, 1H), 7.86–7.84 (m, 1H), 7.73–
7.65 (m, 2H), 7.37–7.31 (m, 4H), 7.28–7.24 (m, 1H), 5.42 (d,
J = 16.6 Hz, 1H), 5.38 (d, J = 16.6 Hz, 1H), 5.03–5.00 (m, 1H), 4.14
(s, 3H), 3.83–3.74 (m, 2H); 13C NMR: d 166.4, 140.5, 133.3, 133.1,
129.7, 128.7, 128.3, 128.2, 128.0, 114.3, 114.2, 66.0, 57.7, 49.6,
33.8. Anal. Calc. for C18H20ClN3O2ꢀH2O: C, 59.42; H, 6.09; N,
½
a 2D5
ꢁ
¼ ꢂ17:0 (c = 1.0 in CH3OH). M.p. 178.0–178.3 °C.
2.5. General procedure for preparation of 3a and 7a
A suspension of azolium salt 3 or 7 (0.5 mmol) and silver(I)
oxide (0.25 mmol) in CH2Cl2 (35 mL) was stirred for 1.5 h at reflux-
ing temperature. After the reaction, a white precipitate was
formed, which was filtered with suction. The resulting white solid
was dissolved in DMSO-d6, and then the NMR measurement was
performed.
11.55. Found: C, 59.39; H, 5.88; N, 11.57%. ½a D25
¼ þ105:7 (c = 1.0
ꢁ
in CH3OH). M.p. 162.5–163.0 °C.
2.4.5. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
methylimidazolium chloride (11)
1H NMR (CD3OD): d 8.96 (s, 1H), 7.60 (d, J = 1.8 Hz, 1H), 7.59 (d,
J = 1.8 Hz, 1H), 5.08 (d, J = 15.8 Hz, 1H), 5.03 (d, J = 15.8 Hz, 1H),
3.95 (s, 3H), 3.75–3.55 (m, 3H), 1.92–1.83 (m, 1H), 0.94 (m, 6H);
13C NMR: d 167.0, 139.3, 125.0, 124.4, 63.0, 58.8, 51.9, 36.6, 30.2,
2.5.1. NHC–Ag complex 3a
1H NMR (DMSO): d 7.96 (br, 1H), 7.38 (d, J = 1.7 Hz, 1H), 7.36 (d,
J = 1.7 Hz, 1H), 4.91 (d, J = 15.7 Hz, 1H), 4.80 (d, J = 15.7 Hz, 1H),
4.63 (br, 1H), 3.80 (s, 3H), 3.61–3.51 (m, 1H), 3.42–3.32 (m, 2H),
1.86–1.72 (m, 1H), 0.81 (d, J = 7.2 Hz, 3H), 0.78 (d, J = 7.2 Hz, 3H);
13C NMR: d 181.6, 166.5, 123.3, 122.4, 61.1, 55.9, 53.0, 38.0, 28.2,
19.6, 18.1.
19.9, 18.8. ½a 2D5
¼ ꢂ10:9 (c = 1.0 in CH3OH).
ꢁ
2.4.6. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
butylimidazolium chloride (12)
1H NMR (CD3OD): d 9.04 (s, 1H), 7.67 (t, J =3.3 Hz, 1H), 7.61 (t,
J = 3.3 Hz, 1H), 5.10 (d, J= 16.5 Hz, 1H), 5.04 (d, J = 16.5 Hz, 1H),
4.25 (t, J = 14.5 Hz, 2H), 3.73–3.52 (m, 3H), 1.94–1.83 (m, 3H),
1.42–1.34 (m, 2H), 1.01–0.92 (m, 9H); 13C NMR: d 167.0, 125.0,
123.1, 63.0, 58.8, 51.9, 50.7, 33.0, 30.2, 20.4, 19.7, 18.8, 13.7.
2.5.2. NHC–Ag complex 7a
1H NMR (DMSO): d 8.20 (br, 1H), 7.78–7.76 (m, 1H), 7.66–7.63
(m, 1H), 7.47–7.43 (m, 2H), 5.23 (d, J = 15.9 Hz, 1H), 5.14 (d,
J = 15.4 Hz, 1H), 4.67 (br, 1H), 4.05 (s, 3H), 3.60–3.56 (m, 1H),
3.44–3.43 (m, 2H), 1.87–1.82 (m, 1H), 0.86–0.84 (m, 6H); 13C
NMR: d 166.0, 133.8, 133.7, 123.9, 123.7, 112.0, 111.9, 61.2, 56.1,
50.9, 35.6, 28.2, 19.7, 18.2, the carbene 13C NMR resonance was
not observed.
½
a 2D5
ꢁ
¼ ꢂ11:0 (c = 1.0 in CH3OH).
2.4.7. 1-[2-((S)-1-hydroxy-3-methyl-2-butanylamino)-2-oxoethyl]-3-
benzylimidazolium chloride (13)
1H NMR (CD3OD): d 9.10 (s, 1H), 7.63–7.61 (m, 2H), 7.44–7.40
(m, 5H), 5.45 (s, 2H), 5.09 (d, J = 16.3 Hz, 1H), 5.04 (d, J = 16.3 Hz,
1H), 3.75–3.53 (m, 3H), 1.91–1.82 (m, 1H), 0.95 (d, J = 6.9 Hz,
3H), 0.93 (d, J = 6.9 Hz, 3H); 13C NMR: d 166.9, 138.8, 135.1,
130.4, 130.3, 129.6, 125.4, 123.2, 63.0, 58.8, 54.1, 52.1, 30.2, 19.9,
2.6. General procedure for preparation of 8b, 10b, 12b and 13b
A suspension of azolium compound (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 com-
plex in CH3CN (5 mL) in the dark at room temperature. Then, the
resulting suspension was stirred for 2 h and filtered through a plug
of glass fiber filter paper. The filtrate was evaporated to dryness in
vacuo, and the Pd complexes were crystallized from ethyl acetate
or methanol. The yield of 8b, 10b, 12b and 13b were 79%, 52%,
11% and 17%, respectively. These complexes are very stable under
air and could be stored for at least 1 month at room temperature.
18.8. ½a 2D5
¼ ꢂ13:8 (c = 1.0 in CH3OH). M.p. 156.1–156.5 °C.
ꢁ
2.4.8. 1-[2-((S)-1-hydroxy-4-methyl-2-pentanylamino)-2-oxoethyl]-
3-methylimidazolium chloride (14)
1H NMR (DMSO):d 9.14 (s, 1H), 8.38 (d, J = 8.2 Hz, 1H), 7.69 (s,
1H), 7.68 (s, 1H), 4.99 (d, J = 16.5 Hz, 1H), 4.94 (d, J = 16.5 Hz,
1H), 4.81 (t, J = 11.9 Hz, 1H), 3.87 (s, 3H), 3.80–3.73 (m, 1H),
3.32–3.28 (m, 2H), 1.63–1.51 (m, 1H), 1.32–1.27 (m, 2H), 0.86 (d,
J = 6.6 Hz, 3H), 0.82 (d, J = 6.3 Hz, 3H); 13C NMR: d 164.3, 137.7,
123.7, 122.9, 63.4, 50.6, 49.5, 39.9, 35.8, 24.2, 23.2, 21.9.
½
a 2D5
ꢁ
¼ ꢂ12:0 (c = 1.0 in CH3OH). M.p. 176.8–177.1 °C.

198
S. Sakaguchi et al. / Journal of Organometallic Chemistry 695 (2010) 195–200
2.6.1. Amidate/NHC–Pd(II) complex 8b
responding imidazolium salt 3 in 80% yield. Similarly, the benz-
imidazole 5 and the benzimidazolium salt 6 were prepared in
68% and 94% yields, respectively, from 1-(cyanomethyl)benzimid-
azole (4) and (S)-valinol as starting materials.
Next, we tried to introduce several N-alkyl groups instead of N-
methyl group into the NHC proligand. However, it is more difficult
to alkylate with ethyl iodide than CH3I. Therefore, we proposed an-
other synthetic route to the NHC proligand (Scheme 3). Reaction of
1H NMR (CDCl3): d 7.80 (br, 1H), 7.42–7.32 (m, 4H), 5.31 (d,
J = 16.8 Hz, 1H), 5.04 (d, J = 16.8 Hz, 1H), 4.47–4.36 (m, 1H), 4.27
(s, 3H), 3.76–3.73 (m, 1H), 3.60–3.58 (m, 1H), 1.88–1.83 (m, 1H),
1.48–1.37 (m, 2H), 0.89 (d, J = 6.3 Hz, 3H), 0.86 (d, J = 6.3 Hz, 3H);
13C NMR: d 165.6, 158.4, 134.3, 133.3, 124.2, 123.7, 111.8, 110.3,
69.5, 55.7, 52.7, 42.7, 35.4, 24.8, 23.1, 22.4. Anal. Calc. for
C16H22ClN3O2Pd: C, 44.67; H, 5.15; N, 9.77. Found: C, 44.92; H,
5.04; N, 9.62%.
chloroacetyl chloride with b-amino alcohol afforded a-chloroacet-
amide in almost quantitative yield [11], which subsequently cou-
pled with N-alkylated azole to yield the corresponding azolium
chloride. Since some N-alkylated imidazoles are commercially
available, a variety of the desired azolium compounds can be syn-
thesized using this route. According to the route shown in Scheme
3, an azolium chloride should be generated, whereas the previous
route (Scheme 2) generates an azolium iodide.
2.6.2. Amidate/NHC–Pd(II) complex 10b
1H NMR (DMSO): d 9.11 (br, 1H), 7.87–7.85 (m, 1H), 7.73–7.71
(m, 1H), 7.47–7.39 (m, 4H), 7.29–7.18 (m, 3H), 5.26 (d, J = 3.7 Hz,
1H), 5.05 (d, J = 16.9 Hz, 1H), 4.94 (d, J = 16.8 Hz, 1H), 4.19 (s,
3H), 3.88–3.86 (m, 1H), 3.72–3.69 (m, 1H), 3.34–3.30 (m, 1H);
13C NMR: d 165.1, 141.8, 133.9, 132.8, 128.0, 126.8, 126.4, 124.0,
123.7, 111.4, 111.3, 70.6, 59.2, 52.0, 35.0. Anal. Calc. for
C18H18ClN3O2Pd: C, 48.02.19; H, 4.03; N, 9.33. Found: C, 48.13; H,
3.74; N, 9.34%.
Table 1 summarizes the results of the coupling reaction of
a-
chloroacetamides with N-methyl-, N-butyl-, or N-benzylazole
derivatives. For example, 2-chloro-N-[(S)-1-(hydroxymethyl)-2-
methylpropyl]acetamide derived from (S)-valinol was allowed to
react with 1-methylbenzimidazole in 1,4-dioxane at refluxing tem-
perature to give benzimidazolium chloride 7 (Run 1). NMR and IR
spectra of 7 are consistent with those of 6. The compounds 11–14
are obtained as hygroscopic white-colored powders; the other azo-
lium salts are stable in air. Thus, we succeeded in preparing of a
wide variety of tridentate NHC-ligand precursors bearing both N-
anionic functional groups and N-alkyl groups.
2.6.3. Amidate/NHC–Pd(II) complex 12b
1H NMR (CD3OD): d 7.31 (d, J = 2.0 Hz, 1H), 7.22 (d, J = 2.0 Hz,
1H), 4.96–4.86 (m, 1H), 4.91 (d, J = 16.4 Hz, 1H), 4.44 (d,
J = 16.4 Hz, 1H), 4.08–4.02 (m, 1H), 3.74 (d, J = 10.0 Hz, 1H), 3.61
(dd, J = 3.6 and 10.0 Hz, 1H), 3.36 (dd, J = 3.6 and 10.0 Hz, 1H),
2.19–2.09 (m, 1H), 2.01–1.86 (m, 2H), 1.42–1.32 (m, 2H), 1.01 (t,
J = 7.3 Hz, 3H), 0.96 (d, J = 6.8 Hz, 3H), 0.71 (d, J = 6.8 Hz, 3H); 13C
NMR: d 167.9, 146.0, 123.6, 123.4, 67.1, 65.2, 56.8, 51.1, 34.8,
31.1, 20.5, 19.8, 19.8, 14.1. Anal. Calc. for C14H24ClN3O2Pd: C,
41.19; H, 5.93; N, 10.29. Found: C, 41.01; H, 5.71; N, 10.24%.
3.2. Synthesis of tridentate anionic amidate/NHC–Pd complexes
through NHC–Ag complexes
2.6.4. Amidate/NHC–Pd(II)ꢀCH3OH complex 13bꢀCH3OH
1H NMR (DMSO): d 8.73 (s, 1H), 7.50–7.29 (m, 7H), 6.31 (d,
J = 15.1 Hz, 1H), 5.20 (d, J = 14.7 Hz, 1H), 4.84 (d, J = 16.0 Hz, 1H),
4.41 (d, J = 16.0 Hz, 1H), 4.13–4.09 (m, 1H), 3.58–3.48 (m, 2H),
3.33 (br, 1H), 3.17–3.16 (m, 3H),1.93–1.84 (m, 1H), 0.84 (d,
J = 6.4 Hz, 3H), 0.53 (d, J = 6.9 Hz, 3H); 13C NMR: d 164.7, 145.8,
137.7,128.4, 128.3, 127.7, 127.3, 122.9, 122.5, 65.8, 62.6, 55.6,
52.2, 48.6, 29.5, 19.4, 19.3. Anal. Calc. for C17H22ClN3O2PdꢀH2O: C,
44.36; H, 5.26; N, 9.13. Found: C, 44.54; H, 4.92; N, 9.05%.
For coordination of azolium salt as NHC to palladium, a strategy
based on ligand transfer with the aid of an NHC–Ag complex has
been employed [12]. The reaction is driven by precipitation of sil-
ver halide salts. Compound 3 was dissolved in dichloromethane at
refluxing temperature, followed by addition of 0.5 equiv. of Ag2O to
the reaction vessel. Upon continuous heating for approximately
1.5 h, a white precipitate, which is expected to be the correspond-
ing NHC–Ag complex 3a, was formed (Scheme 4). In a previous pa-
per, we showed the similar reaction of benzimidazolium iodide 6
with Ag2O [10]. Although the NHC–Ag complex 6a obtained from
6 had poor solubility in all solvents, the complex 3a was found
to be dissolved in DMSO.
3. Results and discussion
3.1. Synthesis of azolium iodides and azolium chlorides
Fig. 1 shows 1H NMR spectra of 3a in DMSO-d6. The signal at d
9.1 ppm attributed to the imidazolium proton is observed in 3, but
not in the NHC–Ag complex 3a. The signals at around d 8.2 and
4.6 ppm corresponding to N–H and O–H protons, respectively, in
3 still exist in the spectrum of 3a, indicating that no anionic ami-
date- and alkoxy-tethered NHC–Ag complex was formed. Arnold
synthesized a chelating alkoxy NHC–Ag complex by the reaction
of an imidazolium salt derived from epichlorohydrin and imidazole
with Ag2O [13]. Upon deprotonation, the signal of the carbene C
The NHC-ligand precursors, azolium iodides, have been success-
fully synthesized as shown in Scheme 2. This route slightly differs
from the previous one [10]. Reaction of 1-(cyanomethyl)imidazole
(1) with (S)-valinol catalyzed by ZnCl2 in PhCl at refluxing temper-
ature, followed by treatment with methanol in the presence of sil-
ica gel, afforded 2 in 44% yield. The purified 2 was allowed to react
with methyl iodide in THF at refluxing temperature, giving the cor-
O
O
O
R1
N
N
I
(i)
(ii)
+
HN
HO
iPr
HN
HO
iPr
Cl
OH
N
CN
N
N
Me
Cl
H2N
N
O
R1
Cl
OH
1 (imidazole derivative)
4 (benzimidazole derivative)
2 (44%)
5 (68%)
3 (80%)
6 (94%)
O
R2X
N
H
N
N
N
HN
HO
R1
N
R2 Cl
N
HN
(i) ( S)-valinol, cat.ZnCl2, PhCl, rf, 15 h, followed by cat.SiO2,
MeOH, rf, 2 h. (ii) MeI, THF, rf, 15 h.
R2
Scheme 2. Synthesis of azolium iodides 3 and 6.
Scheme 3. Another route to azolium chloride.

S. Sakaguchi et al. / Journal of Organometallic Chemistry 695 (2010) 195–200
199
Table 1
Coupling of a
-chloroacetamides with N-alkylated azoles to azolium chlorides.a
O
N
i) 1/2 Ag2O, CH2Cl2, rf, 2 h
ii) PdCl2(CH3CN)2, CH3CN, r.t., 1 h
Run
Substrate
Product (Yield%b)
iPr
N
N
Me
Pd
7
R1
80% yield
O
Cl
O
O
N
N
Cl
OH
H
HN
N
R1
N
H
N
6b (=7b)
R2 Cl
R2
Scheme 5. Synthesis of Amidate/NHC–Pd(II) Complex 6b (= 7b) from 7 via NHC–Ag
complex 7a.
HO
1
2
3
4
R1 = iPr
iBu
R2 = Me
Me
Me
7
8
9
10
(70)
(85)
(48)
(82)
iBu
Ph
Me
chloride complex 7a is light sensitive, unlike the silver iodide com-
plex 6a. In fact, the color of the compound 7a immediately changes
from white to brown during the isolation procedure. It is well-
known that NHC–AgCl complexes are light sensitive and that prep-
aration of these complexes should be carried out under exclusion
of light [15]. In addition, the chloride 7a is easily dissolved in
DMSO, while the corresponding iodide 6a has poor solubility, as de-
scribed above. Other azolium compounds 8–16 can also be trans-
formed into the corresponding NHC–Ag complexes by allowing
them to react with Ag2O in CH2Cl2.
Based on these results, synthesis of the NHC–Pd(II) complexes
was examined. In a previous communication, we showed a one-
pot procedure for preparation of the NHC–Pd complex 6b without
isolation of the NHC–Ag complex 6a that was generated by the
reaction of benzimdazolium iodide 6 with Ag2O [10]. Now, we
are interested in the synthesis of 6b from the corresponding azo-
lium chloride 7. Treatment of 7 with 0.5 equiv. of Ag2O gave a white
precipitate. After removal of the solvent under reduced pressure,
PdCl2(CH3CN)2 in CH3CN solution was added to the reaction vessel.
Stirring the reaction mixture at room temperature for 1 h gave the
desired anionic amidate/NHC–Pd complex 6b (= 7b) in 80% yield
(Scheme 5).
O
N
O
R1
N
R1
Cl
OH
N
HN
N
Cl
N
H
R2
R2
HO
5
6
7
8
9
10
R1 = iPr
iPr
R2 = Me
nBu
Bn
11
12
13
14
15
16
(57)
(66)
(50)
(58)
(69)
(92)
iPr
iBu
Me
iBu
nBu
Bn
iBu
a
Experimental procedure: N-Alkylated azole (4.4 mmol) was allowed to react
-chloroacetamide (4 mmol) in 1,4-dioxane (15 mL) at 110 °C for 16 h.
with
a
b
Isolated yield.
O
O
N
N
N
Ag2O (0.5 equiv)
CH2Cl2, rf, 1.5 h
93%
HN
iPr
HN
iPr
N
AgI
I
Me
Me
HO
3
HO
3a
Scheme 4. Reaction of 3 with Ag2O.
In the same manner, the conversion of various benzimidazoli-
um and imidazolium chlorides 8–16 into the corresponding
NHC–Pd complexes has been examined, and several amidate/
NHC–Pd complexes 8b, 10b, 12b, and 13b from 8, 10, 12, and 13,
respectively, were successfully obtained as single crystals. X-ray
diffraction studies were performed (Fig. 2).
Table 2 shows the bond lengths around Pd metal in these
complexes. Attention should be given to the Pd–N bond length.
Fig. 1. 1H NMR of 3 and 3a in DMSO-d6.
O
N
i) 1/2 Ag2O, CH2Cl2, rf, 2 h
ii) PdCl2(CH3CN)2, CH3CN, r.t., 1 h
iPr
N
N
Me
Pd
7
shifts to higher frequency by about 40 ppm [14]. The characteristic
carbene C value of 182 ppm is consequently observed.
Next, we examined the reaction of the benzimidazolim chloride
7 with Ag2O to form the corresponding NHC–Ag complex 7a.
Although 7a has the same skeleton as the NHC–AgI complex 6a,
it was found that they differ in their physical properties. The silver
80% yield
Cl
O
H
6b (=7b)
Fig. 2. X-ray structures of several amidate/NHC–Pd(II) complexes.

200
S. Sakaguchi et al. / Journal of Organometallic Chemistry 695 (2010) 195–200
Table 2
Appendix A. Supplementary material
Bond lengths around Pd in several amidate/NHC–Pd complexes.
Product
Pd–C
Pd–N
Pd–O
Pd–Cl
CCDC 739454, 739456, 739457, and 739458 contain the supple-
mentary crystallographic data for 8b, 10b, 12b and 13b. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via http://www.ccdc.cam.ac.uk/data_request/
cif. Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.jorganchem.2009.10.011.
6b (= 7b)
8b
10b
12b
13b
1.937(2)
1.948 (5)
1.968(4)
1.938(4)
1.946(3)
1.980(2)
2.010(5)
2.012(3)
1.992(3)
1.988(2)
2.110(18)
2.116(4)
2.089(3)
2.103(3)
2.102(2)
2.313(15)
2.303(18)
2.320(10)
2.308(11)
2.330(10)
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