Highly efficient chiral metal cluster systems derived from Ru3(CO)12 and chiral diiminodiphosphines for the asymmetric transfer hydrogenation of ketones

Article abstract of DOI:10.1039/b209974h

The chiral Ru cluster-based catalyst systems generated in situ from Ru₃(CO)₁₂ and chiral diiminodiphosphine tetradentate ligands effected asymmetric transfer hydrogenation of propiophenone in 2-propanol, leading to 1-phenyl-1-propano

Full text of DOI:10.1039/b209974h

Highly efficient chiral metal cluster systems derived from Ru3(CO)12 and
chiral diiminodiphosphines for the asymmetric transfer hydrogenation of
ketones
Hui Zhang,a Chuan-Bo Yang,a Yan-Yun Li,a Zhen-Rong Donga,a Jing-Xing Gao,*a Hideaki
Nakamura,b Kunihiko Muratab and Takao Ikariya*b
a Department of Chemistry, Institute of Physical Chemistry, State Key Laboratory for Physical Chemistry of
Solid Surfaces, Xiamen University, Xiamen, 361005 Fujian, China
b Graduate School of Science and Engineering, Tokyo Institute of Technology & CREST, 2-12-1 O-okayama,
Meguro-ku, Tokyo 152-8552, Japan. E-mail: tikariya@o.cc.titech.ac.jp
Received (in Cambridge, UK) 11th October 2002, Accepted 12th November 2002
First published as an Advance Article on the web 2nd December 2002
4
The chiral Ru cluster-based catalyst systems generated in
situ from Ru3(CO)12 and chiral diiminodiphosphine tetra-
dentate ligands effected asymmetric transfer hydrogenation
of propiophenone in 2-propanol, leading to 1-phenyl-
1-propanol in 94% yield and with 96% ee.
C6P2(NH)2 did not exhibit any acceleration effect on the
reaction under the conditions to those described in Table 1.
Neither chiral diphosphine ligands, BINAP or DIOP nor chiral
diimines, N,NA-bisbenzylidene-1,2-cyclohexanediamine (5b)
gave any reduction product at all. The Trost ligand (6)6
combined with 1 did not produce an active catalyst for this
transfer hydrogenation. The reaction of 2b with a binary system,
Enantioselective reduction of carbonyl compounds catalysed by
well-defined transition metal complexes has been extensively
studied as a practical synthetic tool to produce optically active
alcohols during the last decade.1 In particular, asymmetric
transfer hydrogenation with 2-propanol or formic acid catalysed
by chiral molecular catalysts including lanthanide metal or
group 8–10 metal complexes has been developed as a synthetic
method complementary to asymmetric hydrogenation.2 Al-
though an excellent catalyst performance in terms of reactivity
and selectivity has been attained in the transfer hydrogenation
of ketones by the mononuclear chiral metal complexes,2–4
neither efficient chiral metal clusters nor cluster-based chiral
catalyst systems which give good results for the reaction have
been reported.5 Herein we describe the first successful example
of the enantioselective transfer hydrogenation of aromatic
ketones catalysed by a cluster-based catalyst system generated
in situ from commercially available Ru3(CO)12 (1) and chiral
tetradentate diiminodiphosphine ligands.4
1
and (R,R)-1,2-cyclohexanediamine (CYDN) or (S,S)-
1,2-diphenylethylenediamine (DPEN) proceeded very slowly
and then stopped leading to the reduction product in low yield
and low enantioselectivity. These results indicate that the
structure of the PNNP tetradentate diiminodiphosphines is a
crucial factor for ligand acceleration.
Our earlier results indicated that the diaminodiphosphine
ligands, 4, combined with a mononuclear Ru complex (7)
effects the asymmetric transfer hydrogenation of ketones
leading to optically active alcohols with an excellent ee,4 while
the mononuclear Ru complexes (8) bearing the diiminodiphos-
phines, 3, were completely inert to the same reaction. The NH
moiety of the ligand in the mononuclear metal complex possibly
participates in the formation of a six-membered cyclic transition
We first examined the effect of several kinds of chiral ligands
combined with the trinuclear ruthenium carbonyl cluster 1 on
the rate of reduction of aromatic ketones 2 with 2-propanol at 45
°C as shown in Scheme 1. In the absence of any added
auxiliaries, the carbonyl cluster 1 was almost inert to the
reduction of the ketones. However, the screening tests revealed
that the asymmetric reduction of acetophenone 2a (substrate/
Ru3 molar ratio of 200+1) in 2-propanol containing a combined
catalyst system generated from the reaction of 1 with the chiral
tetradentate ligand N,NA-bis[o-(diphenylphosphino)benzyli-
dene]-1,2-diphenylethylenediamine, Ph2P2N2 (3a), proceeded
smoothly to give the corresponding optically active alcohol in
91% yield and with 81% ee. Addition of a strong base,
K[OCH(CH3)2] (Ru3+base = 1+10) led to a slight improvement
in the reactivity without any serious decrease in the enantiose-
lectivity. Table 1 lists some experimental results. The Ph2P2N2-
based complex provides better catalyst performance in terms of
the reactivity and selectivity than those attained with the
catalyst bearing N,NA-bis[o-(diphenylphosphino)benzylidene]-
cyclohexane-1,2-diamine, C6P2N2 (3b). Most notably, as the
bulkiness of the alkyl substituents increases, the ee increases in
the substituent order of CH3 < C2H5 < CH(CH3)2. The
reaction of the more congested isobutyrophenone (2c) which is
inert with mononuclear Ru catalyst systems,2–4 gave the
corresponding alcohols in good to excellent yields and with up
to 99% ee.
In sharp contrast to the diiminodiphosphine ligands, diamino-
diphosphines such as N,NA-bis[o-(diphenylphosphino)benzyl]-
1,2-diphenylethylenediamine (4a), Ph2P2(NH)2 or N,NA-bis[o-
(diphenylphosphino)benzyl]cyclohexane-1,2-diamine
(4b),
Scheme 1
142
CHEM. COMMUN., 2003, 142–143
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state through hydrogen bonding to accelerate the reaction.2a,3a
Contray to the mononuclear systems, in these Ru cluster
systems, the diiminodiphosphines 3 were effective for the
reduction of ketones with high enantioselectivity, suggesting
that the reaction mode with the cluster system may be different
from that proposed in the mononuclear Ru catalyst systems.7
Bhaduri and Sharma demonstrated that the transfer hydro-
genation of carbonyl compounds with 2-propanol catalysed by
a ruthenium carbonyl hydride cluster proceeded via a concerted
Meerwein–Ponndorf–Verley (MPV)8a mechanism, not by way
of the metal hydride species formed by b-hydride elimination in
the metal alkoxide intermediate.2a,8b During the reaction the
nuclearity of the catalyst does not change.8b A combined system
of Ru4H4(CO)12 and 3a gave no reduction product under the
reaction condition (Table 1). Based on the results obtained here
as well as reported results,4,8b the hydrogen transfer between
ketones and alcohols with these cluster-based catalyst systems
possibly proceeds by way of the direct mode like an MPV
mechanism. In fact, the more congested ketonic substrates
provide better enantioselectivity although lower reactivity as
shown in Table 1.
Although the nature of the active catalyst remains unknown,
IR and 31P NMR spectroscopies of the catalytic systems
provided us with further information about the real catalyst and
the reaction mechanism as well as the nuclearity of the catalyst.
The 31P NMR spectrum of the red solution obtained from the
reaction of 1 and (S,S)-3b in a 1+1 molar ratio in 2-propanol at
45 °C showed two major singlets around 31.5 and 213 ppm
possibly due to the coordinated- and free-ligand, respectively.
An addition of 10 equiv. of K[OCH(CH3)2] to the red solution
caused an increase in the intensity of the singlet peak at 31.5
ppm and a decrease in the intensity of the free ligand peak,
suggesting that the red complex generated in the solution phase
could be related to the active species for the reaction. Although
attempts to isolate the active catalysts from this reaction mixture
failed because of their thermal instability, a relevant cluster
complex bearing the chiral ligand, [(C2H5)4N][HRu3-
(CO)10{(S,S)-C6P2N2}],9 could be obtained as a red complex
from the reaction of 1 and (S,S)-C6P2N2 (3b) in 2-propanol
containing K[OCH(CH3)2] followed by an addition of
[(C2H5)4N]I.† The isolable complex effected the transfer
hydrogenation of acetophenone in 2-propanol containing no
additional strong base to give (R)-1-phenylethanol in excellent
yield albeit with lower enantioselectivity (96% yield, 47% ee,
5.5 h). These results suggest that the trinuclear cluster 1
combined with chiral ligand system most probably exists as the
catalytically active species under the catalytic reaction condi-
tions. This idea is possibly supported by the reaction rate, which
shows a first order dependence on the concentration of 1 and by
the fact that the mononuclear carbonyl complex, Ru-
(CO)3(PPh3)2 combined with 3b had no reactivity.
This study has demonstrated that chiral metal cluster
complexes may offer new and unique opportunities in asym-
metric catalysis and will stimulate further study on the
development of efficient chiral cluster catalyst systems.
We are grateful to the National Natural Science Foundation
of China (20073034), the Fujian Provincial Science and
Technology Commission (2002F016), and Xiamen Science and
Technology Commission (3502Z, 20021044) for financial
support, and to Professor Ryoji Noyori (Nagoya University) and
Professor Khirui Tsai (Xiamen University) for their valuable
assistance.
Notes and references
† Isolation and identification of catalytically active intermediate com-
plexes: a solution of Ru3(CO)12 (96 mg, 0.15 mmol) in 2-propanol (18 ml)
was added a 0.4 M solution of (CH3)2CHOK in 2-propanol (2.25 ml, 0.9
mmol). Then the chiral ligand (S,S)-C6P2N2 (99 mg, 0.15 mmol) was added
to the deep red solution. After stirring the reaction mixture at 45 °C for 45
min, a twofold excess of solid [(C2H5)4N]I (77 mg, 0.3 mmol) and water
were added to effect precipitation of a brown-red solid, which was collected
by filtration, and washed with water, and then with Et2O. Recrystallisation
from CH2Cl2–Et2O gave brick-red crystals (117 mg, 57% yield). The crude
compound was further purified by silica gel column chromatography with
acetone as eluent to give an orange crystalline complex. IR (KBr) (nCO in
the region 1900–2100 cm21): 2077vw, 2015s, 1974vs, 1949s and 1911w
cm21 31P NMR (CDCl3): d +31.47. Anal. Calc. for [(C2H5)4N][HRu-
;
3(CO)9{(S,S)-C6P2N2}]·2H2O, C61H61N3O9P2Ru3·2H2O: C, 53.05; H,
4.75; N, 3.04. Found: C, 52.66; H, 4.15; N, 3.24%.
Table 1 Asymmetric transfer hydrogenation of aromatic ketones catalysed
by a combined catalyst system of Ru3(CO)12 and chiral ligand
1 (a) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New
York, 1994; (b) Catalytic Asymmetric Synthesis, ed. I. Ojima, Wiley,
New York, 2nd edn., 2000.
Time/ Yield
Ee
(%)
Ketone
Chiral catalyst
Base
h
(%)
2 (a) Recent reviews for asymmetric transfer hydrogenation: R. Noyori and
S. Hashiguchi, Acc. Chem. Res., 1997, 30, 97; (b) M. J. Palmer and M.
Wills, Tetrahedron: Asymmetry, 1999, 10, 2045.
3 (a) S. Hashiguchi, A. Fuji, J. Takehara, T. Ikariya and R. Noyori, J. Am.
Chem. Soc., 1995, 117, 7562; (b) K. Murata, T. Ikariya and R. Noyori, J.
Org. Chem., 1999, 64, 2186; (c) M. Watanabe, K. Murata and T. Ikariya,
J. Org. Chem., 2002, 67, 1712.
4 J.-X. Gao, T. Ikariya and R. Noyori, Organometallics, 1996, 15, 1087.
5 (a) G. Sœss-Fink and M. Jahncke, in Catalysis by Di- and Polynuclear
Metal Cluster Complexes, ed. R. Adams and F. A. Cotton, Wiley-VCH,
New York, 1998, and references therein (b) G. Gervasio, R. Gobetto, P.
J. King, D. Marabello and E. Sappa, Polyhedron, 1998, 17, 2937; (c) F.
Prestopino, R. Persson, M. Monare, N. Focci and E. Nordlander, Inorg.
Chem. Commun., 1998, 1, 302; (d) S. Bhadouri, V. S. Darshane, K.
Sharma and D. Mukesh, J. Chem. Soc., Chem. Commun., 1992, 23, 1738;
(e) U. Mattedi, V. Beghetto and A. Scrivanti, J. Mol. Catal A: Chem.,
1996, 109, 45.
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
2d
2d
2b
2b
2b
2b
2b
2b
Ru3(CO)12/3a
Ru3(CO)12/3b
Ru3(CO)12/3a
Ru3(CO)12/3b
Ru3(CO)12/3a
Ru3(CO)12/3b
Ru3(CO)12/3b
Ru3(CO)12/3a
Ru3(CO)12/3a
Ru3(CO)12/3b
Ru3(CO)12/3b
Ru3(CO)12/3a
Ru3(CO)12/3b
Ru3(CO)12/4a
No
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
No
No
No
No
No
No
No
5
5
4
5
4
5
4
5
4
5
4
5
5
5
5
5
5
5
5
91
11
96
94
98
72
93
48
79
30
66
6
5
0
25
16
0
81
83
82
95
96
92
92
> 99
> 99
94
90
79
58
—
59
40
—
—
Ru3(CO)12/(S,S)-DPEN
Ru3(CO)12/(R,R)-CyDN
Ru3(CO)12/BINAP
Ru3(CO)12/DIOP
H4Ru4(CO)12/3a
6 B. M. Trost, D. L. Van Vranken and C. Bingle, J. Am. Chem. Soc., 1992,
114, 9327.
7 During the preparation of catalyst from the reaction of 1 and
0
0
—
diiminodiphosphine 3a in 2-propanol containing the base, the reduction
of 3a to 4a did not occur.
8 (a) J. D. Morrison and H. S. Mosher, in Asymmetric Organic Reactions,
Prentice-Hall, New Jersey, 1971, ch. 5; (b) S. Bhaduri and K. Sharma, J.
Chem. Soc., Chem. Commun., 1988, 173.
The reaction was carried out at 40 °C using a 0.1 mol dm–3 solution (5.0
mmol) in 2-propanol. Ketone+Ru3 molar ratio of 200+1. Added base:
Ru3+base
= 1+10. Yield was determined by GLC and NMR. The
enantiomeric excess was determined by GLC using a chiral column
(Chrompack CP-cyclodextrin-236-M-19 column).
9 See Notes and References.
CHEM. COMMUN., 2003, 142–143
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