Chiral sulfur-containing ligands for iridium(I)-catalyzed asymmetric transfer hydrogenation of aromatic ketones

Article abstract of DOI:10.1016/j.tetasy.2007.08.016

New efficient catalyst systems, coupled with IrCl(COD)PPh₃ and chiral [SNNS]-type ligands, were employed in the asymmetric transfer hydrogenation of aromatic ketones under mild reaction conditions. The corresponding optically active alcohols we

Full text of DOI:10.1016/j.tetasy.2007.08.016

Tetrahedron: Asymmetry 18 (2007) 2049–2054
Chiral sulfur-containing ligands for iridium(I)-catalyzed
asymmetric transfer hydrogenation of aromatic ketones
Xue-Qin Zhang, Yan-Yun Li, Hui Zhang and Jing-Xing Gao*
State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemical and Biochemical Engineering,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, PR China
Received 22 June 2007; accepted 10 August 2007
3
Abstract—New efficient catalyst systems, coupled with IrCl(COD)PPh and chiral [SNNS]-type ligands, were employed in the asymmetric
transfer hydrogenation of aromatic ketones under mild reaction conditions. The corresponding optically active alcohols were obtained in
high yield and good to excellent enantioselectivities (up to 96% ee). The chiral Ir(I) complexes with the ligands of [SNNS]-type were also
prepared and characterized, which showed good enantioselectivity and high activity. The reactions can be performed in air and the
catalytic experiments are greatly simplified.
Ó 2007 Published by Elsevier Ltd.
1. Introduction
the synthesis and application of chiral sulfur-containing
ligands in asymmetric catalysis have been published.
1
9–22
Transition metal complex catalysts based on ligands with
mixed functional groups have great potential application
in asymmetric catalysis. Over the past two decades, a large
Recently, chiral sulfur-containing ligands have been suc-
cessfully used as chiral auxiliaries for a wide range of reac-
1
23
24
26
tions, such as allylic substitution, vinylation of ketones,
2
5
number of the mixed chiral [PN], [NPN], [PNP], [PNNP],
asymmetric reduction, enantioselective ring opening,
1
4,27
[
ON], and [ONNO] ligands have been synthesized and
and asymmetric transfer hydrogenation (ATH).
serve as excellent chiral auxiliaries for catalytic asymmetric
2
–13
synthesis.
Compared to ligands with P, N or O as donor
Over the past 10 years, the ATH of prochiral ketones has
attracted considerable attention. Most research works in
this area were carried out by using the chirally modified
transition metals, such as Ru, Rh, and Ir, as catalyst pre-
atoms, sulfur-containing chiral mixed ligands have received
much less attention. A possible reason is that sulfur has a
tendency to poison transition metal catalysts.
2
8–34
cursors.
Indeed, sulfur possesses higher oxidation states available
and can form some compounds that have different func-
tional groups, such as thiol, sulfide, thioamide, sulfoxide,
sulfinyl, thiocarbonyl, and thiocarbamide. Furthermore,
its empty relatively low-energy d orbitals can accept back-
donation of p-electron density from the metal, leading to
stabilizing the metal–S bond. On complexation to a metal,
In our earlier studies, we synthesized a family of excellent
chiral [PNNP] ligands, and successfully employed them in
3
5–40
ATH of aromatic ketones with up to 99% ee.
To
extend our study, we developed the application of chiral
tetradentate [SNNS] ligands in iridium(I)-catalyzed ATH
of a series of aromatic ketones under mild reaction condi-
tions, giving the corresponding optically active alcohols
with high yield and up to 96% ee.
1
4,15
chirality can be induced at sulfur.
These key structural
features exemplify very rich coordination chemistry toward
transition metals, and they serve as powerful stereodirecting
1
6–18
ligands in asymmetric synthesis.
Over the last three dec-
ades, the study of sulfur-containing ligands has increased
considerably. Several excellent review articles regarding
2. Results and discussion
2.1. Preparation of chiral [SNNS]-type ligands
1
2
*
Corresponding author. Tel.: +86 592 2180116; fax: +86 592 2183047;
e-mail addresses: jxgao@xmu.edu.cn; cuihua@xmu.edu.cn
Chiral ligand (1R,2R)-N ,N -bis(thiophen-2-ylmethylene)-
cyclohexane-1,2-diamine, (R,R)-I, was prepared by literature
0957-4166/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2007.08.016

2050
X.-Q. Zhang et al. / Tetrahedron: Asymmetry 18 (2007) 2049–2054
procedure,41 except using (R,R)-1,2-diammoniumcyclo-
hexane mono-(+)-tartrate salt instead of (R,R)-1,2-cyclo-
hexanediamine (Scheme 1). The reduction of ligand
[Ir(COD)-(S,S)-II]Cl bear a mirror-image relationship with
Demax at 417 nm (Fig. 1), indicating that the metal center
chelated with the chiral ligands.
(
R,R)-I with excess NaBH was carried out in refluxing
4
1
2
ethanol to afford (1R,2R)-N ,N -bis(thiophen-2-ylmethyl)-
cyclohexane-1,2-diamine, (R,R)-II, in 99% yield.
2.3. ATH with [SNNS]-type ligands as chiral auxiliaries
Recently, K o¨ nig et al. employed chiral diimine ligand
R,R)-I to catalyze asymmetric addition of diethyl zinc to
(
benzaldehyde, while Jacobsen et al. used ligand (R,R)-I
CHO
S
N
N
as chiral auxiliary in asymmetric aziridination of alkenes,
+H3N
NH3+
a
41,42
.
but the results were unsatisfactory.
Umani-Ronchi
+
S
S
and Kim, respectively, reported the application of a chiral
diamine ligand (R,R)-II in the asymmetric allylic alkyl-
ation, but the molar ratio of substrate to ligand was very
HO
OH
(
R,R)-I
-OOC
COO-
4
3,44
low (S:L = 10:1).
In our study, we first employed chiral
b
[
SNNS]-type ligands in ATH of aromatic ketones under an
air atmosphere, giving the corresponding optically active
alcohols with high yield and enantioselectivity.
H
H
N
N
2.3.1. ATH of propiophenone. In the initial experiment,
the ATH of propiophenone was chosen as a model
reaction. Some Ru, Rh, and Ir complexes with ligand
S
S
(
R,R)-II have been tested. The catalyst systems were gener-
(
R,R)-II
ated in situ by mixing ligand (R,R)-II and metal complexes
in PrOH under an air atmosphere. Typical results are listed
i
Scheme 1. Synthesis of chiral [SNNS] ligands (R,R)-I and (R,R)-II.
Reagents and conditions: (a) K CO , CH CH OH/H O, reflux; (b)
NaBH , CH CH OH, reflux.
2
3
3
2
2
in Table 1.
4
3
2
Table 1. ATH of propiophenone with (R,R)-II and various metal
complexes as catalyst precursors
2.2. Synthesis and characterization of chiral [SNNS]-
iridium(I) complexes
a
Entry Metal complex
Time (h)
Alcohol
b
b
Yield (%) ee (%)
The interaction of [Ir(COD)Cl] with 2 equiv of (R,R)-II or
2
(
S,S)-II in CH Cl at room temperature gave chiral
1
2
3
4
5
6
7
8
9
Ru(DMSO) Cl
2
8
8
8
8
8
5
5
4
15
25
14
3
71
97
92
95
95
5
22
52
—
45
75
61
77
86
2
2
4
[SNNS]-Ir(I) complexes [Ir(COD)-(R,R)-II]Cl and [Ir(COD)-
CpRu(PPh ) Cl
3
3
(
S,S)-II]Cl, respectively. The CD spectra of chiral [SNNS]
Ru(PPh
trans-RhCl(CO)(PPh
[RhCl(COD)]
[IrCl(COD)]
[IrHCl (COD)]
IrH(CO)(PPh
3
)
3
Cl
2
3
)
3
ligands and corresponding chiral iridium complexes have
been measured in MeOH. Only weak absorbing peaks at
2
2
2
26 nm were found in the CD spectra of chiral [SNNS]
2
2
ligands, while the CD spectra of [Ir(COD)-(R,R)-II]Cl and
c
3
)
3
IrCl(COD)PPh
3
4.5
a
*
Reaction conditions: L = (R,R)-II; propiophenone, 0.5 mmol; Sub.–
[
i
M]–Ligand–KOH = 100:1:1:4; PrOH, 5 mL; temp, 45 °C; air
1
0
5
0
5
atmosphere.
b
c
Yield and ee were determined by GC analysis using a chiral G-TA
column.
Temp, 60 °C.
[Ir(COD)-(R, R)-II]Cl
It could be seen that Ru complexes showed low activity and
low to moderate enantioselectivities in the reactions (Table
, entries 1–3). The trans-RhCl(CO)(PPh ) /(R,R)-II sys-
tem showed very low activity (Table 1, entry 4), while
RhCl(COD)] was used, the activity improved but ee was
1
3 3
Δε
[
Ir(COD)-(S, S)-II]Cl
[
-
2
still unsatisfactory (71% yield and 45% ee, Table 1, entry
). When Ir complexes were employed as metallic precur-
5
sors, good conversion and moderate to good enantioselec-
tivities were obtained (Table 1, entries 6–9). The catalytic
-
10
250
300
350
400
450
system IrCl(COD)PPh /(R,R)-II gave the best result (95%
3
λ (nm)
yield and 86% ee). Therefore, IrCl(COD)PPh was chosen
3
Figure 1. The CD spectra of chiral iridium complexes.
as a metallic precursor for further study.

X.-Q. Zhang et al. / Tetrahedron: Asymmetry 18 (2007) 2049–2054
2051
a
Table 2. ATH of various ketones catalyzed by IrCl(COD)PPh
Entry Substrate Ligand
3
/[SNNS]-type ligand II
Time (h)
Alcohol
b
b
d
Yield (%)
ee (%)
Config
O
1
(R,R)-II
6
96
83
(S)
O
O
O
2
3
4
(R,R)-II
(R,R)-II
(R,R)-II
8
10
12
94
94
97
87
88
90
(S)
(S)
(S)
O
O
O
O
O
5
6
7
8
9
(R,R)-II
(S,S)-II
(S,S)-II
(R,R)-II
48
46
22
9
92
92
95
94
(S)
(R)
(R)
(S)
c
c
97
96
96
91
(S,S)-II
(R,R)-II
(S,S)-II
9
7
7
92
94
94
90
82
89
(R)
(S)
(R)
O
1
0
1
O
H CO
1
3
O
Cl
1
2
3
(R,R)-II
(R,R)-II
5
99
94
71
58
(S)
(S)
O
1
5
Cl
a
b
c
i
Reaction conditions: ketone, 0.5 mmol; Sub.–[M]–Ligand–KOH = 100:1:1:8; PrOH, 5 mL; temp, 25 °C; air atmosphere.
Yield and ee were determined by GC analysis using chiral G-TA column unless otherwise specified.
Yield and ee were determined by GC analysis using chiral CP-Chirasil-Dex CB column.
The configurations were determined by comparison of the retention times of the enantiomers on the GC traces with literature values.
d

2052
X.-Q. Zhang et al. / Tetrahedron: Asymmetry 18 (2007) 2049–2054
Chiral diimine ligand (R,R)-I was also used as a ligand with
preformed complex probably exists as the catalytically
active species in the course of the catalytic reaction.
IrCl(COD)PPh in the reduction reaction. But the reaction
3
proceeded very slowly, producing (S)-1-phenylpropan-1-ol
in only 27% yield and 5% ee (45 °C, 17 h). These results are
3
5,45
similar to those of earlier studies,
indicating that the
3. Conclusion
NH functions in the ligands are responsible for the high
activity and the NH linkage can possibly stabilize a cata-
lytic transition state.
We have found the new catalytic systems, coupled with
IrCl(COD)PPh3 and chiral [SNNS]-type ligands, which
efficiently catalyzed the ATH of a series of aromatic ke-
4
6–48
i
2
.3.2. ATH of various aromatic ketones. The catalytic sys-
tones with PrOH in air, giving the corresponding optically
tems IrCl(COD)PPh /II, which were generated in situ by
mixing chiral ligand II and IrCl(COD)PPh3 in PrOH
active alcohols with up to 99% yield and up to 96% ee. The
related chiral iridium complexes [Ir(COD)-(R,R)-II]Cl and
[Ir(COD)-(S,S)-II]Cl have been synthesized and fully char-
acterized. These complexes act as a catalytic precursor and
catalyze the ATH with good results. In order to reveal the
exact reaction mechanism, isolation, and characterization
of the catalytic active species is under further investigation.
This work will provide a valuable index to develop the
chemistry of chiral sulfur-containing ligands.
3
i
under an air atmosphere, have been further examined for
the ATH of various aromatic ketones. The results are sum-
marized in Table 2. A variety of aromatic ketones could be
reduced to the corresponding optically active secondary
alcohols with high chemical yield and good enantiomeric
excesses under mild reaction conditions. The reactivity
and enantioselectivity were affected by the steric property
of the substrates in the alkyl moiety. As the bulkiness of
the alkyl group increased from methyl, ethyl, propyl to n-
butyl, the enantioselectivity increased gradually, while the
reaction rate slowed down (Table 2, entries 1–4). The
reduction of isobutyrophenone proceeded smoothly with
4. Experimental
4.1. General methods
9
5% ee (Table 2, entry 5). Notably, for cyclohexyl phenyl
ketone the corresponding chiral alcohols could be obtained
with 97% yield and 96% ee (Table 2, entry 7). The elec-
tronic properties and position of the groups in the ring sub-
stituent also affected the enantioselectivity of reduction
reaction. ortho-Methyl acetophenone was reduced with
The preparation of ligands and chiral iridium complexes
was carried out under a nitrogen atmosphere. NMR spec-
tra were recorded on a Bruker AV 400 instrument. Mass
spectra were carried out on a Finnigan LCQ mass spec-
trometer. IR spectra were recorded on a Nicolet Avatar
360 FT-IR spectrometer. CD spectra were measured with
a JASCO J-810 spectrophotometer. The yields and ee val-
ues were determined by GC analysis with a chiral G-TA
column or CP-Chirasil-Dex CB column. Melting points
were measured using an X-4 digital apparatus and are
uncorrected. Optical rotations were measured with a Per-
kin–Elmer 341 polarimeter. Conductance was determined
by conducto metermodec DDS-11A. The solvents were
dried and purified according to standard methods.
9
1% ee (Table 2, entry 8), while a lower ee was obtained
for meta-methyl acetophenone (Table 2, entry 10). The
ketone having an electron-donating substituent, meta-
methoxyl acetophenone, achieved 94% chemical yield with
8
9% ee (Table 2, entry 11). Introduction of an electron-
withdrawing group such as a chloro substituent onto the
aromatic ring would decrease the enantioselectivity (Table
2
, entries 12 and 13).
To determine the role of sulfur in the thiophene group
of ligand II, we synthesized the chiral ligand, (1R,
4.2. Preparation of chiral [SNNS]-type ligands
1
2
2
R)-N ,N -bis(furan-2-ylmethyl)cyclohexane-1,2-diamine,
which used oxygen instead of sulfur. This ligand has also
been examined in the ATH of propiophenone, with lower
conversion and ee achieved (25% conv., 75% ee). This
result indicated that the sulfur in the thiophene group of
ligand II played an important role in ATH.
To a mixture of (R,R)-1,2-diammoniumcyclohexane mono-
(+)-tartrate salt (5.29 g, 20 mmol) and K CO3 (5.54 g,
2
40 mmol) were added H O (60 mL) and EtOH (120 mL).
2
The solution was refluxed with stirring for 5 h before
thiophene-2-carbaldehyde (3.82 mL, 40 mmol) was added.
Then the mixture was refluxed with stirring overnight.
The mixture solution was extracted with CH Cl . The com-
2.4. ATH with chiral [SNNS]-iridium(I) complexes as
catalysts
2
2
bined extracts were washed with H O and the organic layer
2
was dried over anhydrous Na SO , followed by filtration
2
4
The above results indicated that the chiral [SNNS]/Ir(I)
mixed systems were very effective for the ATH of aromatic
ketones under mild conditions. Although the nature of the
active catalyst remained unknown, the relevant Ir(I) com-
plexes [Ir(COD)-(R,R)-II]Cl and [Ir(COD)-(S,S)-II]Cl have
been synthesized and fully characterized by IR, NMR, MS,
and CD. These chiral iridium complexes served as a cata-
lyst for the ATH of propiophenone, giving 98% yield and
and concentration. The crude product was recrystallised
from hot ethanol to give (R,R)-I as a white solid (5.08 g,
84% yield).
A solution of (R,R)-I (2.70 g, 8.94 mmol) and NaBH4
(7.05 g, 178.8 mmol) in absolute ethanol (140 mL) was re-
fluxed with stirring for 2 days. The solution was cooled
to room temperature and H O was added to destroy excess
2
8
2% ee (45 °C, 4 h). When ortho-methyl acetophenone
NaBH . The mixture solution was extracted with CH Cl .
4
2
2
was used as a substrate, 99% yield and 87% ee were
The combined extracts were washed with saturated NH Cl
4
obtained (45 °C, 8 h). These results suggest that the
solution and H O. The organic layer was dried over anhy-
2

X.-Q. Zhang et al. / Tetrahedron: Asymmetry 18 (2007) 2049–2054
2053
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½
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5
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. Bellucci, C. M.; Bergamini, A.; Cozzi, P. G.; Papa, A.;
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1
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2
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1
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ꢁ1
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2
2
6
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4
.4. General procedure for ATH of ketones
i
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2
5
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3
Acknowledgements
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We thank the National Natural Science Foundation of
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Science and Technology Commission (No. 2005YZ1020)
for financial support.
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