Oxo-tethered ruthenium(II) complex as a bifunctional catalyst for asymmetric transfer hydrogenation and H2 hydrogenation

Article abstract of DOI:10.1021/ja207283t

Newly developed oxo-tethered Ru amido complexes (R,R)-1 and their HCl adducts (R,R)-2 exhibited excellent catalytic performance for both asymmetric transfer hydrogenation and the hydrogenation of ketonic substrates under neutral conditions without any cocatalysts to give chiral secondary alcohols with high levels of enantioselectivity.

Full text of DOI:10.1021/ja207283t

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pubs.acs.org/JACS
Oxo-Tethered Ruthenium(II) Complex as a Bifunctional Catalyst for
Asymmetric Transfer Hydrogenation and H2 Hydrogenation
Taichiro Touge,† Tomohiko Hakamata,† Hideki Nara,† Tohru Kobayashi,† Noboru Sayo,† Takao Saito,†
Yoshihito Kayaki,‡ and Takao Ikariya*,‡
†Corporate Research & Development Division, Takasago International Corporation, 4-11, Nishiyawata 1-chome, Hiratsuka City,
Kanagawa 254-0073, Japan
‡Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-E4-1
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S Supporting Information
b
Scheme 1. Synthesis of Chiral Amido Catalysts (R,R)-1
and Their HCl Adducts (R,R)-2 and an ORTEP Drawing of
(R,R)-2a
ABSTRACT: Newly developed oxo-tethered Ru amido
complexes (R,R)-1 and their HCl adducts (R,R)-2 exhib-
ited excellent catalytic performance for both asymmetric
transfer hydrogenation and the hydrogenation of ketonic
substrates under neutral conditions without any cocatalysts
to give chiral secondary alcohols with high levels of
enantioselectivity.
atalytic asymmetric reduction of carbonyl compounds is one
C
of the most sophisticated chiral multiplication methods in
organic synthesis.1 Significant progress has been made in the
asymmetric transfer hydrogenation of aromatic ketones using
2-propanol or formate salts through the use of conceptually new
chiral bifunctional hydrido-Ru(II) catalysts with monosulfony-
lated chiral diamines, RuH[(R,R)-Tsdpen](η6-arene) [TsDPEN
= TsNCH(C6H5)CH(C6H5)NH2],2 and an innovative reaction
protocol has been realized as a practical tool that is complemen-
tary to asymmetric hydrogenation for accessing chiral secondary
alcohols, even in chemical and pharmaceutical processes.3 Further
intense research efforts to refine the prototype of bifunctional
catalyst have led to the development of new catalytic systems
displaying greater efficiency in terms of both reactivity and
enantioselectivity.3,4 In particular, Wills’ tethered catalysts (I),
shown in Chart 1, are noteworthy because they show greater
activities and are more stable than the original catalysts.5
variants, serve as excellent catalysts for asymmetric hydrogena-
tion of aromatic ketones but unfortunately not for transfer
hydrogenation.8 We report here a new generation of hetero-
atom-tethered complexes, (R,R)-1 and their HCl adducts (R,R)-
2 (Scheme 1), that exhibit excellent catalytic performance in
terms of reactivity and selectivity for both asymmetric transfer
hydrogenation and H2 hydrogenation under neutral conditions
without any cocatalyst.
Newly developed oxo-tethered Ru amido complexes 1a,b
and their HCl adducts 2a,b (a = TsDPEN derivatives; b =
MsDPEN derivatives), which were conveniently prepared
by the one-pot reaction of a dimeric (η6-bromoxylene)Ru
complex with monosulfonylated DPEN derivatives hav-
ing a hydroxyalkyl chain, followed for 2 by treatment with
HCl (Scheme 1; also see the Supporting Information),
proved to be efficient chiral catalysts for the asymmetric
reduction of ketones. The structure of complex (R,R)-2a was
determined by X-ray crystallography to have a three-legged
piano-stool configuration around the central metal, as shown in
Scheme 1.
Recently, structural modification of the catalyst prototype
through changes in the chelating amine ligands or the formation
of cationic amine complexes has been shown to cause facile acti-
vation of H2, leading to practical asymmetric hydrogenation
catalysts.6,7 We previously reported that triflylamide-tethered
RuÀTsdpen complexes (II in Chart 1), as well as their Rh and Ir
Chart 1
Received: August 3, 2011
Published: August 26, 2011
r
2011 American Chemical Society
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|

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COMMUNICATION
Scheme 2. Asymmetric Reduction of Ketones
Table 1. Asymmetric Transfer Hydrogenation of Ketones
Catalyzed by (R,R)-1a, (R,R)-2a, or (S,S)-2ca
run
ketone
cat
S/C
time (h)
% yieldb
% eec,d
1
2
3a
2a
2b
2a
2a
1000
1000
30000
40000
40000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
200
3
3
>99
>99
95
97 (R)
95 (R)
97 (R)
97 (R)
96 (S)
94 (R)
93 (R)
98 (R)
>99 (R)
96 (S)e
97 (S)
95 (R)
84 (R)
97 (R)
98 (R)
96 (R)
3
96
72
72
24
24
5
4
75
5
(S,S)-2c
2a
15
6
3b
3c
3d
3e
3f
>99
>99
>99
>99
98
7
2a
8
2a
9
2a
5
10
11f
12
13
14
15
16
2a
5
3g
3h
3i
2a
5
>99
>99
94
2a
5
2a
24
24
24
6
2b
96
3l
2b
85
3m
2a
94
Asymmetric transfer hydrogenation of acetophenone (3a)
with tethered complexes (R,R)-2a,b in an azeotropic mixture
of HCO2H and N(C2H5)3 (Scheme 2) proceeded rapidly
[substrate/catalyst (S/C) = 1000] in 3 h at 60 °C, giving the
corresponding chiral (R)-alcohol in up to 97% ee (Table 1, runs 1
and 2). Catalyst (R,R)-2a maintained its catalytic ability after 4
days of reaction, showed an equal level of enantioselectivity at
loadings as low as S/C = 30 000, and provided the highest activity
among a series of (arene)RuÀTsdpen complexes (run 3). In
addition, the catalytic activity of 2a was much higher than that of
the Wills-type C4-tethered complex (S,S)-2c.5b,g
A range of ketones, including propiophenone (3b) and
o-methoxyacetophenone (3c), can be reduced to the correspond-
ing secondary alcohols in high chemical yields with excellent ee's
at relatively low catalyst concentration (S/C = 1000) (runs 6 and 7).
Notably, the reduction of oxygen-functionalized ketones 3d and
3e gave the corresponding alcohols within 5 h with almost perfect
enantioselectivities (runs 8 and 9). Other functionalized ketones,
including α-hydroxy-, α-chloro-, and α-cyanoacetophenone (3fÀh),
were also reduced in a highly enantioselective manner to give
quantitatively chiral products with 96, 97, and 95% ee, respec-
tively, without any byproducts (runs 10À12). In the reduction of
1-acetonaphthone, the complex (R,R)-2b provided a better result
than the Ts analogue (runs 13 and 14). (R,R)-2b also showed
excellent enantiodiscrimination ability in the reaction with a seven-
membered fused-ring aromatic ketone (3l; run 15), although the
rate was low. The reaction of cyclic enone 3m with S/C = 200
gave the corresponding allyl alcohol with 96% ee (run 16). 1,2-
Diketone 3n was also reducible at 60 °C with S/C = 10 000 in a
DMF solution of HCOOH/N(C2H5)3 containing (R,R)-2a to
give chiral (S,S)-hydrobenzoin (4n) almost quantitatively with
good diastereoselectivity (dl:meso = 97.2:2.8) and excellent ee
(>99% ee).9
a Standard reaction conditions (S/C = 1000): substrate (5 mmol),
catalyst (0.005 mmol), 5:2 formic acid/triethylamine azeotropic mixture
(2.5 mL). b Determined by GC or 1H NMR spectroscopy. c The ee was
determined by GC analysis using a CHIRALSIL-DEX-CB capillary
column, unless otherwise noted. d The major enantiomer was deter-
mined by the sign of optical rotation. e The ee was determined by HPLC
analysis using a TCI MB-S column. f EtOAc was added as a cosolvent.
and 3.0MPa H2 proceeded smoothly to yield (R)-1-phenylethanol
almost quantitatively with 95% ee, whereas the catalyst (R,R)-2a
gave the product in only moderate yield (Table 2, runs 1 and 2).
Various aromatic ketones can be hydrogenated with high chemi-
cal yield and satisfactory ee with S/C = 1000. A base-sensitive
substrate, 4-chromanone (3e), and other fused-ring aromatic
ketones, 1-indanone (3j) and 1-tetralone (3k), were also reduced
using (R,R)-1 or -2, and the amido catalyst (R,R)-1a showed
catalytic activities higher than and selectivities comparable to
those of (R,R)-2a (runs 3, 4, and 6À9). The hydrogenation of α-
hydroxyacetophenone 3f using (R,R)-2a was almost complete
within 18 h even with S/C = 5000, while the C4-tethered com-
plex (S,S)-2c gave an unsatisfactory result (runs 10 and 11).
The hydrogenation ability of the tethered complexes was suc-
cessfully applied to the reduction of lactone. The reaction of
γ-butyrolactone with pressurized H2 (5.0 MPa) was promoted
by (R,R)-2a (2 mol %) in 2-propanol containing tBuOK (20 mol %)
to yield 1,4-butanediol quantitatively after 48 h (Scheme 3).6g,11
The remarkable improvement in the catalytic performance of
the oxo-tethered Ru catalysts 1 and 2 in terms of reactivity,
stability, and diversity depending on the hydrogen source might
be explained by the delicate electronic tuning of cooperating
ligands in the present catalysts. Separate experiments with non-
tethered RuÀarene complexes for the transfer hydrogenation of
acetophenone (Scheme 4) revealed that a complex containing
electron-donating p-xylene exhibited a higher catalytic activity
than a complex with an electron-withdrawing ester group, which
is consistent with the results in preceding studies.3c,12 The intro-
duction of a CH2OH group on the η6-arene ligand resulted in
complete conversion of the ketone with excellent enantioselectivity.
In addition, the use of the secondary amino unit in the
TsDPEN ligand might enhance the electron density to give the
Encouraged by the marked increase in catalytic activity, which
was possibly due to the introduction of electron-donating groups
in the tether unit and the amino unit, we anticipated that in-
creased basicity of the amine might render the amido-Ru com-
plexes (R,R)-1 and chloro-Ru complexes (R,R)-2 catalytically
active for the hydrogenation reaction without any modification.10
In fact, the hydrogenation of acetophenone in methanol containing
the 16-electron amido complex (R,R)-1a (S/C = 500) at 60 °C
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Journal of the American Chemical Society
COMMUNICATION
Table 2. Asymmetric Hydrogenation of Ketones Catalyzed
Scheme 5. Formation of Hydrido-Ru Complex 5a by Treat-
ment of Tethered Amido-Ru Complex 1a with H2
by (R,R)-1a, (R,R)-2a, (R,R)-5a, or (S,S)-2ca
run
ketone
cat
S/C
time, h
% yieldb
% eec,d
1
2
3a
1a
2a
1a
2a
5a
1a
2a
1a
2a
2a
500
500
20
20
20
20
20
18
18
18
18
18
18
99
58
99
99
99
97
59
85
52
99
35
95 (R)
90 (R)
97 (R)
99 (R)
99 (R)
98 (R)
98 (R)
>99 (R)
>99 (R)
93 (S)e
89 (R)e
3
3e
1000
1000
1000
1000
1000
1000
1000
5000
5000
4
5
6
3j
7
further ligand modification, these two asymmetric reductions of
ketones can be used in a complementary manner in organic
synthesis. Rational design of the ligands that adjust the balance of
electronic factors on the M/NH units in the bifunctional catalysts
is crucially important for further exploitation of this unprecedented
catalyst performance.
8
3k
3f
9
10
11
(S,S)-2c
a Standard reaction conditions (S/C = 1000): substrate (5 mmol),
catalyst (0.005 mmol), MeOH (4.4 mL), H2 (3.0 MPa). b Determined
by GC or H NMR spectroscopy. c The ee was determined by GC
1
’ ASSOCIATED CONTENT
analysis using a CHIRALSIL-DEX-CB capillary column, unless
otherwise noted. d The major enantiomer was determined by the sign
of optical rotation. e The ee was determined by HPLC analysis using a
TCI MB-S column.
S
Supporting Information. Results of asymmetric transfer
b
hydrogenation using other substrates; results of an experiment
comparing oxo-tethered complex (R,R)-2a and C4-tethered
complex (S,S)-2c; preparative methods for Ru complexes; ex-
perimental procedures for asymmetric transfer hydrogenation
and H2 hydrogenation; NMR, GC, and HPLC data; [α]D values
for products; and X-ray crystallographic data for (R,R)-2a and
(S,S)-2c (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Scheme 3. Asymmetric Hydrogenation of Lactone Catalyzed
by Ruthenium Catalyst (R,R)-2a
’ AUTHOR INFORMATION
Corresponding Author
tikariya@apc.titech.ac.jp
Scheme 4. Asymmetric Hydrogenation of Acetophenone
Catalyzed by Nontethered Ru Complexesa
’ ACKNOWLEDGMENT
This research was financially supported by a Grant-in-Aid for
Scientific Research (S) (22225004) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and partially
supported by the GCOE Program. We also thank Messrs.
Yoshihiro Yaguchi, Daisuke Sugimoto, and Akihiro Kawaraya,
Ms. Kyoko Zaizen and Seika Abe, and the members of the
heteroatom-tethered catalyst research group at Takasago
International Corporation for the measurement of NMR and mass
spectra and experimental assistance.
a Conditions: S/C = 500, 5:2 HCO2H/N(C2H5)3, 2 M, 30 °C, 24 h
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the oxo-tethered Ru amido or chloride complexes exhibit ex-
cellent catalytic performance for both asymmetric transfer hydro-
genation and hydrogenation of ketones. Since the bifunctional
nature of the present tethered catalysts could be achieved without
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