Enantioselective catalytic hydrosilylation of propiophenone with a simple combination of a cationic iridium complex and a chiral azolium salt

Article abstract of DOI:10.1016/j.jorganchem.2018.09.001

This study aims to propose a simple procedure for the development of enantioselective hydrosilylation of a ketone using catalytic amounts of [Ir(cod)₂]BF₄ and chiral azolium salt. Previously, catalytic asymmetric hydrosilylation reac

Full text of DOI:10.1016/j.jorganchem.2018.09.001

Journal of Organometallic Chemistry 875 (2018) 52e58
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Enantioselective catalytic hydrosilylation of propiophenone with a
simple combination of a cationic iridium complex and a chiral azolium
salt
Hiro Teramoto, Satoshi Sakaguchi*
Department of Chemistry and Materials Engineering, 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:
This study aims to propose a simple procedure for the development of enantioselective hydrosilylation of
a ketone using catalytic amounts of [Ir(cod)2]BF4 and chiral azolium salt. Previously, catalytic asymmetric
hydrosilylation reactions have used well-defined metal-N-heterocyclic carbene (NHC) complexes. The
proposed method offers an important advantage of avoiding preparation of NHC-metal species. Several
reaction parameters including the amount of reductant, solvent, catalyst loading and ligand structure
were evaluated. In addition, the investigation of the reaction progress as a function of time revealed that
an iridium species, which was generated after 5 h of reaction time, catalyzed the stereoselective
reduction with almost perfect facial selection of the ketone. An attempt to obtain a catalytic active
species from the reaction of [Ir(cod)2]BF4 and chiral azolium salt has been made. The newly obtained
iridium species promoted the hydrosilylation of a ketone with high yield and enantioselectivity.
© 2018 Elsevier B.V. All rights reserved.
Received 24 July 2018
Received in revised form
28 August 2018
Accepted 1 September 2018
Available online 4 September 2018
Keywords:
Asymmetric catalysis
Enantioselective reduction
Hydrosilylation reaction
N-Heterocyclic carbene
Iridium
1. Introduction
on the successful enantioselective hydrosilylation using an in-situ
generated chiral metal-NHC complex. Andrus demonstrated
Controlling the stereoselectivity of asymmetric metal-mediated
catalytic processes depends on the design of a versatile ligand that
strongly coordinates to a metal center. In recent years, chiral N-
heterocyclic carbene ligands (NHCs) have attracted considerable
asymmetric hydrosilylation catalyzed by RuCl2(PPh3)2 combined
with a monodentate chiral NHC ligand that contained planar [2.2]
paracyclophane in the presence of AgOTf [6].
Previously, we reported the synthesis of hydroxyamide-
functionalized azolium salt 1, a precursor of an NHC, from
commercially available (S)-leucine (Scheme 1) [7]. From 1, the
synthesis of the well-defined monodentate IrCl(NHC)(cod) complex
3 was successfully achieved. This well-defined iridium complex
catalyzed the asymmetric hydrosilylation of ketones in the pres-
ence of AgBF4 to afford the corresponding optically active alcohols
with high enantioselectivities (Scheme 1).
In-situ generated chiral metal complexes offer several distinct
advantages over well-defined metal complexes [8]. Because of a
quick and easy synthesis of the hydroxyamide-functionalized
azolium salt, a large library of compounds can be easily obtained
by varying the substituents at the NHC ligand precursor. Therefore,
in-situ generated catalysts would allow for rapid screening and
tuning of diverse chiral NHC precursors. From this viewpoint, we
have investigated catalytic asymmetric hydrosilylation using an in-
situ generated IrCl(NHC)(cod) species 3′, and an efficient procedure
for this process has also been successfully developed (Scheme 2,
method A) [9]. A typical procedure includes the treatment of the
attention owing to their strong s-donating capability to metals and
the ability to vary the substituents on the nitrogen atom [1]. In
1996, Hermann reported the asymmetric hydrosilylation reaction
of acetophenone with diphenylsilane, which was the first catalytic
process where chiral induction was achieved using a chiral NHC [2].
The use of a well-defined metal complex seems to be ideal for the
strict stereocontrol in asymmetric catalysis. In 2003, Shi achieved a
breakthrough in asymmetric catalysis (98% ee) by developing a
bidentate axially chiral bis(NHC)-Rh(III) complex with a 1,10-
binaphthalenyl backbone [1j, 3]. Gade and co-workers introduced a
bidentate oxazoline/NHC-Rh(I) complex that was obtained through
the direct linkage of heterocycles [1l, 1r, 1v, 4]. In addition to these
two important works, many investigators have developed different
classes of well-defined metal-NHC over the past two decades [5]. On
the contrary, to the best of our knowledge, there is only one report
* Corresponding author.
E-mail address: satoshi@kansai-u.ac.jp (S. Sakaguchi).
https://doi.org/10.1016/j.jorganchem.2018.09.001
0022-328X/© 2018 Elsevier B.V. All rights reserved.

H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
53
2. Results and discussion
Representative results for the reaction of propiophenone (4)
with (EtO)2MeSiH in the presence of catalytic amounts of [Ir(cod)2]
BF4 and chiral ligand precursor 1 are summarized in Table 1. When
4 was allowed to react with 2 eq. of (EtO)2MeSiH in the presence of
4 mol % of [Ir(cod)2]BF4 and 1 in cyclopentyl methyl ether (CPME) at
room temperature for 20 h, (S)-1-phenyl-1-propanol ((S)-5) was
obtained in 49% yield with 88% ee (entry 1).
Several reaction parameters including amount of the reductant,
solvent and catalyst loading were evaluated (Table 1). Although the
use of 2 eq. of reductant with respect to ketone 4 resulted in a
moderate yield of (S)-5 (entry 1), 4 eq. of (EtO)2MeSiH was enough
to achieve optimum conversion (entries 2e5). Various solvents
were explored and THF, 2-MeTHF and diethylene glycol dimethyl
ether (diglyme) proved to be superior (entries 6, 7 and 10). In
contrast, the catalytic reaction in Et2O, CH2Cl2 and DMSO resulted
in lower yield of the reduced product (entries 9e13).
Scheme 1. Development of well-defined NHC-Ir complex for enantioselective hydro-
silylation (Previous work).
Increasing the amount of the azolium salt 1 (2 eq. with respect
to [Ir(cod)2]BF4) resulted in a slightly lower yield of the desired
product (S)-5 with 90% ee (entry 14). Using half the amount of 1
with respect to the iridium catalyst precursor led to a poor yield
and enantioselectivity of reduced product (entry 15). Moreover, a
decrease in the catalyst loading (Ir/1 ¼ 2/2 mol%) did not result in a
significant decrease in the product yield or ee (entry 6 vs. entry 16).
It is noteworthy that almost no reaction occurred in absence of the
azolium salt 1 (entry 18). This strong ligand-accelerated catalysis
(LAC) facilitated by the NHC ligand will be discussed later.
The higher performance of the combined catalytic system of an
Ir catalyst precursor and 1 motivated us to study catalysis using the
chiral NHC ligand with the opposite configuration. As expected,
when 4 was allowed to react with (EtO)2MeSiH in the presence of
catalytic amounts of [Ir(cod)2]BF4 and ent-1, the corresponding
alcohol such as (R)-5 was preferentially obtained in 92% yield and
90% ee (entry 19).
Scheme 2. Development of in-situ generated catalyst under operationally simple
conditions.
azolium salt 1 with Ag2O to produce the corresponding NHC-Ag
complex 2 through deprotonation of C-H bond at the C2 position
of 1. Subsequently, the resulting NHC-Ag complex 2 was allowed to
react with [IrCl(cod)]2 to afford the IrCl(NHC)(cod) complex 3’.
Next, propiophenone (4) was combined with (EtO)2MeSiH in the
presence of unpurified complex 3’ and AgBF4 to yield (S)-1-phenyl-
1-propanol ((S)-5) in 92% yield with 92% ee.
Table 1
Evaluation of several reaction parameters.a
Entry
Silane
[eq.]
Ir-cat./1
[mol %]
Solvent
Yield [%]b
ee [%]c
1
2
3
4
5
6
7
8
2
3
3.5
4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/4
4/8
4/2
2/2
1/1
4/0
4/4
CPME
CPME
CPME
CPME
CPME
THF
2-MeTHF
1,4-dioxane
Et2O
diglyme
toluene
CH2Cl2
DMSO
THF
49
83
93
98
97
97
98
34
9
87
57
16
<1
80
12
90
31
<1
92
88
88
91
93
90
93
93
81
12
92
79
59
The previous procedure employed stepwise addition of Ag2O,
[IrCl(cod)]2 and AgBF4 to a THF solution of azolium salt 1 to
generate IrCl(NHC)(cod) 3’ (Scheme 2, method A) [9]. We assumed
that these components could be added simultaneously to the re-
action vessel. During the course of these studies, we discovered
that a simple combination of [Ir(cod)2]BF4 and the chiral azolium
salt 1 promoted the catalytic asymmetric hydrosilylation (Scheme
2, method B). So far, it has been considered that Ag2O was needed
to deprotonate the C-H bond at the C2 position of the azolium salt
to afford a carbene species. Indeed, in some previous studies, the
well-defined NHC-M (M ¼ Rh or Ru) complexes for catalytic
asymmetric hydrosilylation have been synthesized through the
pretreatment of a chiral azolium salt with a base such as Ag2O and
tBuOK [3e5,7]. As such, we were surprised that pretreatment of
the azolium salt with Ag2O to form the corresponding NHC species
was not needed for hydrosilylation with the [Ir(cod)2]BF4/azolium
salt catalytic system (Scheme 2, method B). The proposed method
offers the additional important advantage of avoiding advance
preparation of NHC-metal species. In this study, we report an
asymmetric reduction of a ketone with the [Ir(cod)2]BF4/azolium
salt catalytic system.
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
9
10
11
12
13
14
15
16
17
18
19d
90
25
91
39
THF
THF
THF
THF
THF
90
a
4 (0.5 mmol), (EtO)2MeSiH, [Ir(cod)2]BF4, 1, solvent (2 mL) at room temperature
for 20 h under Ar. After the reaction, K2CO3 (2 mg) and MeOH (2 mL) were added,
and then the reaction mixture was stirred at room temperature for 2 h, affording (S)-
5.
b
Determined by GC using the internal standard method.
Determined by GC on a chiral stationary phase.
Ent-1 in place of 1 was used, affording (R)-5.
c
d

54
H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
Table 2
Next, easy access to N-functionalized NHC precursors allowed us
to investigate the effect of the substituents on the nitrogen atom
(Table 2). A schematic presentation is shown in Fig. 1.
Drastic effect of chiral ligand structure.a
Entry
Azolium salt
Yield [%]
ee [%]
Notably, the replacement of the methyl- with
a benzyl-
substituent at the azolium ring led to a marked increase in both
product yield and enantioselectivity (entries 1 and 2). While the
use of ligand 6 (R1 ¼ Me) resulted in the formation of a racemic
mixture of alcohol 5 in 20% yield (entry 1), an excellent ee value of
92% was observed by the use of ligand 7 (R1 ¼ Bn) (entry 2). The
stereoselectivity of hydrosilylation with azolium salt 1 with an N-
(1-naphthalenylmethyl) substituent was comparable to that ob-
tained with 8 having N-(2-naphthalenylmethyl) substituent (en-
tries 3 and 4). In stark contrast, the reduction of 4 under the
influence of 9 with an N-anthracenylmethyl group led to poor yield
(17%) and enantioselectivity (4%), likely caused by a steric effect
(entry 5).
1
2
R1
¼
20
79
0
92
Interestingly, a drastic effect was also observed in the investi-
i
gation of a series of stereodirecting alkyl groups (R2 ¼ Me, Et, Bu,
3
97
93
i
t
Bn, Pr, Cy, Bu) at the chiral ligand (Table 2, entries 6e12). Treat-
ment of 4 with (EtO)2MeSiH in the presence of [Ir(cod)2]BF4 and 10
(R2 ¼ Me), prepared from (S)-alaninol, yielded (S)-5 in moderate
yield (48%) and enantioselectivity (66%) (entry 6). Chiral ligands
containing a CH2Z (Z ¼ alkyl or phenyl) type substituent at the
asymmetric center work more efficiently to give the desired
product (entries 7e9). The azolium salt 7 (R2 ¼ iBu) was especially
useful because it can be prepared from a readily available natural
amino acid such as (S)-leucine (entry 8). In contrast, increasing the
bulkiness of the alkyl substituent, such as CHZ2 and CZ3 type sub-
stituents (13e15), at the R2 position of the ligand led to low
enantioselectivities (entries 10e12).
4
5
88
17
94
4
As shown in Table 1 entry 18, strong LAC using the NHC ligand
was observed. Further investigations revealed that reduction of 4
was difficult in the presence of [Ir(cod)2]BF4 in combination with
the NHC ligand precursor 16 derived from 2-aminoethanol (Table 2,
entry 13). These results confirm the importance of the alkyl sub-
stituent R2 on the hydroxyamide side-arm of the ligand for reaction
efficiency and stereoselectivity.
As mentioned above (Table 1, entries 6 and 19), the benzimi-
dazolium salt with both an isobutyl group at the stereogenic center
and a N-(1-naphthalenylmethyl) substituent at the azolium ring
was found to be the ligand that gave the best results. With both
enantiomers of chiral ligand (1 and ent-1) in hand, the relation
between catalyst ee (eecat) and product ee (eepro) in the asymmetric
hydrosilylation of 4 was investigated. A clear linear relation was
observed (Fig. 2). The same phenomenon has been previously re-
ported in asymmetric hydrosilylation catalyzed by a well-defined
IrCl(NHC)(cod) complex 3 [7a].
A reaction time-course study was conducted for the hydro-
silylation of 4 with the [Ir(cod)2]BF4/1 catalytic system to obtain an
insight into various aspects of the reduction. Fig. 3 shows the
changes in the total yield of alcohols ((S)-5 þ (R)-5), which depends
on the reaction time. Clearly, it appeared that the hydrosilylation
reaction occurred after an induction period.
6
substituent type: -CH3
R2
¼
48
66
7
8
-CH2Z
56
79
85
92
9
55
79
In addition, we investigated the time-dependence of the for-
mation of both (S)-5 and (R)-5 for this reduction (Fig. 4). Surpris-
ingly, the formation of (R)-5 stopped after it was generated at an
early stage of the reaction (within ca. 5 h). In contrast, (S)-5 was
gradually formed during the reaction and, as a result, the product ee
increases from 88% to 92%. These results may indicate that the
catalytically active iridium species, which are generated at the early
stages of the reaction (within ca. 5 h), differs from the iridium
species, which was formed after ca. 5 h. The later iridium species
catalyzes the stereoselective hydrosilylation reaction with almost
perfect facial selection of the ketone.
10
11
-CHZ2
32
30
42
33
Finally, we attempted to isolate the iridium complex from the

H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
55
Table 2 (continued )
Entry
12
Azolium salt
Yield [%]
32
ee [%]
-CZ3
45
13
-H
27
0
a
4 (0.5 mmol), (EtO)2MeSiH (2.25 mmol), [Ir(cod)2]BF4 (4 mol %), azolium salt
(4 mol %), THF (2 mL) at room temperature for 20 h under Ar. After the reaction,
K2CO3 (2 mg) and MeOH (2 mL) were added, and then the reaction mixture was
stirred at room temperature for 2 h, affording (S)-5.
reaction of [Ir(cod)2]BF4 with azolium salt 1. Thus, 1 was treated
with one equivalent of [Ir(cod)2]BF4 in THF at room temperature. A
preliminary examination of the reaction is shown in Scheme 3.
Although TLC analysis of the crude reaction mixture showed the
formation of several unidentified products, the most abundant
compound 17 was isolated by silica gel column chromatography.
Fig. 3. Time-dependence curves for the reduction of
Table 1, entry 5).
4
(Reaction conditions: see
Fig. 4. Reaction time vs. product yield ((S)-5 and (R)-5) in the reduction of 4 (Reaction
conditions: see Table 1, entry 5).
Fig. 1. A schematic presentation of the relationship between R1 or R2 substituent of the
ligand and product ee in the reduction of 4 (Data are shown in Table 2.).
Figs. 5 and 6 show the 1H and 13C NMR spectra, respectively, of 17 in
CDCl3. The spectra of azolium salt 1 are also shown to compare the
difference between 17 and the starting material 1. Notably, a signal
at
d
¼ 10.5 ppm in 1H NMR spectra of 1, represented by “a,” is
attributed to the proton in the C2 position of the azolium salt. In the
1H NMR spectra of 17, this signal has disappeared (Fig. 5), indicating
that deprotonation of 1 occurred in the presence of [Ir(cod)2]BF4.
Additionally, the signal patterns of diastereotopic methylene pro-
tons in the a-carbonyl and benzylic positions, represented by “b”
and “c,” respectively, have changed in 1H NMR spectra of 17
compared to 1 (Fig. 5).
In the 13C NMR of 17, the characteristic signal for the carbene
carbon, which is expected to be shifted to low field (typically ca.
200 ppm), was not observed (Fig. 6); the corresponding 13C NMR
signal of 17 appeared at 143 ppm.
Although the detailed structure of 17 is still unknown at this
stage, we speculate the tentative structure of 17 as shown in
Scheme 3. 17 might be formed through a deprotonation-metalation
sequence by treating 1 with [Ir(cod)2]BF4. 17 seems to be generated
by exchanging the proton at the C2 position of the azolium salt into
iridium cation. We assume the existence of resonance structures of
17, as shown in Scheme 4. The contribution of the two resonance
structures to the azolium form would be most significant in 17
Fig. 2. Relationship between the catalyst ee (eecat) and product ee (eepro
reduction of 4 (Reaction conditions: see Table 1, entry 6).
) in the

56
H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
Scheme 4. Plausible resonance structures of cationic Ir complex 17.
with a chiral ligand. Studies focusing on the identification of this
newly obtained iridium complex are currently ongoing.
Additionally, the enantioselective reduction of several ketones
has been evaluated using ‘the simple combination method’ (Table
3). The reduction of butyrophenone (18) smoothly afforded (S)-1-
phenyl-1-butanol ((S)-19) in 80% yield with 90% ee (entry 1). In
contrast, isopropyl phenyl ketone (20) was converted into the
corresponding optically active alcohol with slightly lower yield
under these reaction conditions (entry 2). Fortunately, cyclohexyl
phenyl ketone (22) was reduced with excellent yield (93%) and
enantioselectivity (95% ee) (entry 3). The reduction of p-methox-
yacetophenone (24) yielded (S)-1-(4-methoxyphenyl)ethanol ((S)-
25) in 95% yield with 88% ee (entry 4). Furthermore, we discovered
the enantioselective reduction reaction of propiophenone (4) effi-
ciently occurred with the following procedure. After stirring a
mixture of 1, [Ir(cod)2]BF4 and (EtO)2MeSiH in THF at room tem-
perature for 20 h under open-air conditions, successively addition
of ketone 4, K2CO3 and methanol to the reaction vessel yielded (S)-5
Scheme 3. Treatment of 1 with [Ir(cod)2]BF4.
(Scheme 4, left side). On the other hand, a cationic iridium complex
with a NHC ligand (NHC-Irþ) might be one of these resonance
structures (Scheme 4, right side). Recently, a similar observation
has been reported by Nomura et al., where (imido)vanadium(V)
complexes containing an anionic NHC ligand with a weakly coor-
dinating borate moiety were synthesized [10].
Our scope of interest is to study whether 17 acts as a catalyst for
the asymmetric hydrosilylation of ketone. Thus, 4 (0.5 mmol) was
treated with 4.5 eq. of (EtO)2MeSiH in the presence of a small
amount of 17 (12 wt% with respect to 4, 16 mg) in THF (2 mL) at
room temperature for 20 h. After protonolysis of the resulting
reduced product, (S)-5 was obtained in 94% yield with 94% ee. This
result strongly indicated that the 17 consists of an iridium species
Fig. 5. 1H NMR of 1 and 17 in CDCl3.
Fig. 6. 13C NMR of 1 and 17 in CDCl3.

H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
57
Table 3
Evaluation of several ketones.a
Entry
Ketone (R3COR4)
Yield [%]b
ee [%]c
Conf.d
R3
R4
1
2
3
4
C6H5
C6H5
C6H5
C6H4(4-OMe)
C3H7
18
20
22
24
80
54
93
95
90
88
95
88
S
S
S
S
i-C3H7
c-C6H11
CH3
a
Ketone (0.5 mmol), (EtO)2MeSiH (2.25 mmol), [Ir(cod)2]BF4 (4 mol %), 1 (4 mol %), THF (2 mL) at room temperature for 20 h under Ar. After the reaction, K2CO3 (2 mg) and
MeOH (2 mL) were added, and then the reaction mixture was stirred at room temperature for 2 h.
b
Isolated yield of the alcohol.
Determined by GC on a chiral stationary phase.
Absolute configuration of the alcohol formed.
c
d
in 93% yield with 93% ee. Studies focusing on the asymmetric
hydrosilylation with this ‘premixed’ method are also currently
ongoing.
(4 mL) was heated to 110 ꢀC and stirred for 24 h. After the reac-
tion, the solvent was removed under reduced pressure. The azolium
salt 1 was purified by reprecipitation using CH3OH and CH3CO2C2H5
affording white solid (yield: 472 mg, 72%). Other azolium salts
could be prepared by the similar reaction procedure. Compounds 1
[9], 6 [11], 7 [11], 8 [9], 9 [9], 10 [9], 11 [7a], 12 [7a], 14 [12] and 15
[7a] were reported in our previous publications (see also Appendix
A. Supplementary data).
3. Conclusion
We discovered that a simple combination of [Ir(cod)2]BF4 and
chiral azolium salt promoted the reduction of propiophenone using
(EtO)2MeSiH with an excellent enantioselectivity. This method of-
fers an additional important advantage of avoiding the preparation
of NHC-metal species in advance. Although the scope and limita-
tions of the reactions over a range of ketones have not been
demonstrated, we believe that the current preliminary results
prove the usefulness of our new findings.
The investigation of a series of chiral azolium ligands revealed
that the product yield and enantioselectivity are drastically affected
by the substituents on the azolium ring (R1) as well as at the ster-
eogenic center (R2) of the ligand. In addition, strong LAC was
confirmed. Furthermore, careful analysis of the reaction over time
showed that the iridium species, which was generated after 5 h of
reaction, catalyze the stereoselective reduction with almost perfect
facial selection of the ketone.
The detailed structure of iridium complex 17 that was obtained
from the reaction of [Ir(cod)2]BF4 and azolium salt 1 is still un-
known; however, it can be said that 17 is very stable, contains
almost the same organic molecule as 1, and facilitates the enan-
tioselective hydrosilylation reaction. Overall, the proposed work
provides invaluable information for the design of efficient catalyst
for asymmetric reduction.
4.2.1. Compound 13
1H NMR ((CD3)2SO):
d
10.10 (s, 1H), 8.64 (d, J ¼ 9.2 Hz, 1H), 7.98
(d, J ¼ 9.2 Hz, 2H), 7.67e7.62 (m, 2H), 7.49 (d, J ¼ 10.8 Hz, 2H),
7.42e7.33 (m, 3H), 5.86 (s, 2H), 5.47 (d, J ¼ 16.0 Hz, 1H), 5.40 (d,
J ¼ 16.0 Hz, 1H), 4.80 (t, J ¼ 5.6 Hz, 1H), 3.57 (br, 1H), 3.45e3.37 (m,
2H), 1.86e1.81 (m, 1H), 0.85 (d, J ¼ 6.8 Hz, 3H), 0.84 (d, J ¼ 6.8 Hz,
3H); 13C NMR:
d 164.4, 143.6, 134.0, 131.6, 130.4, 129.0, 128.9, 128.1,
126.8, 126.6, 113.9, 113.6, 61.1, 56.7, 49.7, 48.7, 28.2, 19.5, 18.2. Anal.
Calc. for C21H26ClN3O2ꢁ0.3H2O: C, 64.13; H, 6.82; N, 10.68. Found: C,
63.96; H, 6.48; N, 10.64%. M.p. 211.4e212.9 ꢀC.
4.2.2. Compound 16
1H NMR ((CD3)2SO):
d 10.08 (s, 1H), 8.95 (br, 1H), 7.97 (d,
J ¼ 9.6 Hz, 2H), 7.66e7.60 (m, 2H), 7.50 (d, J ¼ 6.8 Hz, 2H), 7.42e7.33
(m, 3H), 5.85 (s, 2H), 5.38 (s, 2H), 4.92 (t, J ¼ 5.6 Hz, 1H), 3.45 (t,
J ¼ 5.6 Hz, 2H), 3.20 (t, J ¼ 5.6 Hz, 2H); 13C NMR:
d 164.6, 143.6,
134.0, 131.8, 130.4, 129.0, 128.7, 128.2, 126.8, 126.6, 113.9, 113.8, 59.4,
49.8, 48.6, 42.0. Anal. Calc. for C18H20ClN3O2$2H2O: C, 56.62; H,
6.34; N, 11.00. Found: C, 56.65; H, 5.98; N, 10.85%. M.p.
194.1e194.4 ꢀC.
4. Experimental
4.1. General procedures
4.3. General procedure for asymmetric hydrosilane reduction
All other chemical reagents and solvents were obtained from
commercial sources. Column chromatography was performed with
To a THF (2 mL) solution of [Ir(cod)2]BF4 (0.02 mmol, 9.9 mg)
and azolium salt 1 (0.02 mmol, 9.1 mg) were added propiophenone
(4, 0.50 mmol, 67 mg), (EtO)2MeSiH (2.25 mmol, 302 mg). After
stirring at room temperature for 20 h under open-air conditions,
K2CO3 (2 mg) and MeOH (2 mL) were added. Then, the resulting
mixture was stirred at room temperature for 2 h. After evaporation
of the solvents, the residue obtained was purified by column
chromatography on silica gel (Et2O/n-hexane ¼ 3:7) to give (S)-1-
phenyl-1-propanol ((S)-5, 62 mg, 91% isolated yield). The ee was
measured by chiral GLC.
silica gel 60 (63e210 mm) purchased from Kanto Chemical Co., Inc.
1H NMR spectra were recorded on a JEOL ECA400 (400 MHz for 1H
NMR and 100 MHz for 13C NMR) spectrometer. Chemical shifts
were reported downfield from TMS (
d
¼ 0 ppm) for 1H NMR. For 13
C
NMR, chemical shifts were reported on the scale relative to the
solvent used as an internal reference. Elemental analyses were
performed at Osaka University. Enantiomeric excesses were
measured using gas chromatography.
4.2. General procedure of preparation of azolium ligand precursors
Appendix A. Supplementary data
The reaction mixture of 1-(1-naphthalenyl)-1H-benzimidazole
(373 mg, 1.44 mmol) and
derived from chloroacetyl chloride and L-luecinol, in 1,4-dioxane
a
-chloroacetamide (279 mg, 1.44 mmol),
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jorganchem.2018.09.001.

58
H. Teramoto, S. Sakaguchi / Journal of Organometallic Chemistry 875 (2018) 52e58
(1996) 2805e2807.
References
[3] (a) L. Liu, F. Wang, M. Shi, Organometallics 28 (2009) 4416e4420;
(b) Q. Xu, X. Gu, S. Liu, Q. Dou, M. Shi, J. Org. Chem. 72 (2007) 2240e2242;
(c) W.-L. Duan, M. Shi, G.-B. Rong, Chem. Commun. (2003) 2916e2917.
[4] (a) V. Cesar, S. Bellemin-Laponnaz, H. Wadepohl, L.H. Gade, Chem. Eur J. 11
[1] (For selected recent reviews, see) (a) M. Iglesias, L.A. Oro, Chem. Soc. Rev. 47
(2018) 2772e2808;
ꢀ
ꢀ
(b) S. Díez-Gonzalez (Ed.), N-Heterocyclic Carbenes, second ed., RSC-Publish-
(2005) 2862e2873;
ing, Cambridge, 2017;
(c) V.N. Khose, M.E. John, A.D. Pandey, A.V. Karnik, Tetrahedron: Asymmetry
28 (2017) 1233e1289;
(d) D. Wang, D. Astruc, Chem. Rev. 115 (2015) 6621e6686;
(e) F. Lazreg, F. Nahra, C.S.J. Cazin, Coord. Chem. Rev. (2015) 293e294, 48e79;
(f) C. Fliedel, P. Braunstein, J. Organomet. Chem. 751 (2015) 286e300;
(g) L.A. Schaper, S.J. Hock, W.A. Herrmann, F.E. Kühn, Angew. Chem. Int. Ed. 52
(2013) 270e289;
(h) F.B. Hamadd, T. Sun, S. Xiao, F. Verpoor, Coord. Chem. Rev. 257 (2013)
2274e2292;
(i) J. Tornatzky, A. Kannenberg, S. Blechert, Dalton Trans. 41 (2012)
8215e8225;
ꢀ
(b) L.H. Gade, V. Cesar, S. Bellemin-Laponnaz, Angew. Chem. Int. Ed. 43 (2004)
1014e1017.
[5] (For selected recent publications, see) (a) A.S. Henderson, J.F. Bower,
M.C. Galan, Org. Biomol. Chem. 12 (2014) 9180e9183;
(b) P. Gu, Q. Xu, M. Shi, Tetrahedron 70 (2014) 7886e7892;
(c) E. Spahn, A. Albright, M. Shevlin, L. Pauli, A. Pfaltz, R.E. Gawley, J. Org.
Chem. 78 (2013) 2731e2735;
(d) A. Albright, R.E. Gawley, J. Am. Chem. Soc. 133 (2011) 19680e19683;
(e) A. Albright, D. Eddings, R. Black, C.J. Welch, N.N. Gerasimchuk, R.E. Gawley,
J. Org. Chem. 76 (2011) 7341e7351;
(f) M. Steinbeck, G.D. Frey, W.W. Schoeller, W.A. Herrmann, J. Organomet.
Chem. 696 (2011) 3945e3954;
(g) H. Song, Y. Liu, D. Fan, G. Zi, J. Organomet. Chem. 696 (2011) 3714e3720;
(h) J.W. Faller, P.P. Fontaine, Organometallics 25 (2006) 5887e5893;
(i) A.R. Chianese, R.H. Crabtree, Organometallics 24 (2005) 4432e4436;
(j) L. Ren, A.C. Chen, A. Decken, C.M. Crudden, Can. J. Chem. 82 (2004)
1781e1787;
(k) D. Enders, H. Gielen, J. Organomet. Chem. (2001) 617e618, 70e80;
(l) D. Enders, H. Gielen, J. Runsink, K. Breuer, S. Brode, K. Boehn, Eur. J. Inorg.
Chem. (1998) 913e919;
(j) F. Wang, L.-j. Liu, W. Wang, S. Li, M. Shi, Coord. Chem. Rev. 256 (2012)
804e853;
ꢀ
(k) S. Díez-Gonzalez (Ed.), N-Heterocyclic Carbenes, RSC-Publishing, Cam-
bridge, 2011;
ꢀ
(l) L. Benhamou, E. Chardon, G. Lavigne, S. Bellemin-Laponnaz, V. Cesar, Chem.
Rev. 111 (2011) 2705e2733;
(m) O. Kühl, Funtionalised N-heterocyclic Carbene Complexes, Wiley-VCH,
Weinheim, 2010;
ꢀ
(n) S. Díez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009)
(m) D. Enders, H. Gielen, K. Breuer, Tetrahedron: Asymmetry
8 (1997)
3612e3676;
3571e3574.
ꢀ
(o) J. Wencel, M. Mauduit, H. Henon, S. Kehrli, A. Alexakis, Aldrichim Acta 42
[6] C. Song, C. Ma, Y. Ma, W. Feng, S. Ma, Q. Chaia, M.B. Andrus, Tetrahedron Lett.
46 (2005) 3241e3244.
[7] (a) K. Shinohara, S. Kawabata, H. Nakamura, Y. Manabe, S. Sakaguchi, Eur. J.
Org Chem. (2014) 5532e5539;
(b) S. Kawabata, H. Tokura, H. Chiyojima, M. Okamoto, S. Sakaguchi, Adv.
Synth. Catal. 354 (2012) 807e812.
[8] A.B. Mullick, A.S. Jeletic, A.R. Powers, I. Ghiviriga, I.K.A. Abboud, A.S. Veige,
Polyhedron 52 (2013) 810e819.
[9] Y. Manabe, K. Shinohara, H. Nakamura, H. Teramoto, S. Sakaguchi, J. Mol. Catal.
Chem. 421 (2016) 138e145.
[10] A. Igarashi, E.L. Kolychev, M. Tamm, K. Nomura, Organometallics 35 (2016)
1778e1784.
[11] M. Okamoto, Y. Yamamoto, S. Sakaguchi, Chem. Commun. (2009) 7363e7365.
[12] H. Shirasaki, M. Kawakami, H. Yamada, R. Arakawa, S. Sakaguchi,
J. Organomet. Chem. 726 (2013) 46e55.
(2009) 43e50;
(p) D.R. Snead, H. Seo, S. Hong, Curr. Org. Chem. 12 (2008) 1370e1387;
(q) L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718e725;
(r) F. Glorius (Ed.), N-Heterocyclic Carbenes in Transition Metal Catalysis,
Springer, Berlin, 2007;
(s) R.E. Douthwaite, Coord. Chem. Rev. 251 (2007) 702e717;
(t) E.A.B. Kantchev, C.J. O'Brien, M.G. Organ, Angew. Chem. Int. Ed. 46 (2007)
2768e2813;
(u) S.P. Nolan (Ed.), N-Heterocyclic Carbenes in Synthesis, Wiley-VCH, Wein-
heim, 2006;
ꢀ
(v) V. Cesar, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004)
619e636;
(w) E. Peris, R.H. Crabtree, Coord. Chem. Rev. 248 (2004) 2239e2246;
(x) M.C. Perry, K. Burgess, Tetrahedron: Asymmetry 14 (2003) 951e961;
(y) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290e1309.
€
[2] W.A. Herrmann, L.J. Goossen, C. Kocher, G.R.J. Artus, Angew. Chem. Int. Ed. 35