COMMUNICATION
DOI: 10.1002/chem.201201938
A Versatile Ruthenium(II)–NNC Complex Catalyst for Transfer
Hydrogenation of Ketones and Oppenauer-Type Oxidation of Alcohols
[
a]
[a]
[a]
[a, b]
Wangming Du, Liandi Wang, Ping Wu, and Zhengkun Yu*
Pyridyl-based ligands have been utilized as potential
frameworks for useful organometallic materials and active
[1]
complex catalysts. Pincer complexes of nonphosphorus tri-
[
2]
dentate NNN ligands, such as Pybox,
2,6-bis-
[3]
[4]
ACHTUNGTRENNUNG( imino)pyridines, terpyridines, and other symmetrical
polydentate ligands, have recently become more attractive
targets due to their tunable properties and powerful applica-
[5]
[6,7]
À1 [9e]
tions.
Although symmetrical ligands have been used to
(TOF)=125 h ). The NHC ligand has been known to be
a strong s-donor ligand, which may render transition-metal
complexes containing this ligand less labile during a catalytic
cycle.
construct diverse transition-metal complexes, catalyst com-
plexes containing unsymmetrical polydentate ligands usually
exhibit unexpectedly high catalytic activity due to the pre-
[8–10]
cise control of their electronic and geometric properties.
During the ongoing investigation into transition-metal-
complex catalysts, we envisioned that by using a carbanion
to replace the NHC coordinating arm in the ligand, the re-
Thus, unsymmetrical polydentate ligands are highly desira-
ble for complex catalyst design and functional materials fab-
rication.
II
sultant Ru complex containing an unsymmetrical NNC
The transfer hydrogenation of ketones and Oppenauer-
type dehydrogenative oxidation of alcohols have been ex-
ligand may exhibit decent catalytic activity. Herein, we
report a refined NNC-ligand-based Ru -complex catalyst
for the transfer hydrogenation of ketones and the Oppena-
uer-type oxidation of alcohols.
II
[11]
tensively studied and have become reliable synthetic pro-
tocols beyond conventional reduction and oxidation reac-
[12]
tions.
Ruthenium(II) N-tosylethylenediamine complexes
Compound 2 was readily prepared in 86% yield by a pal-
ladium-catalyzed Suzuki coupling of 1 with para-tolylboron-
developed by Noyori and co-workers have been proven to
be very efficient catalysts for asymmetric transfer hydroge-
II
ic acid. In the presence of triethylamine, Ru complex 3 was
[13]
nation of ketones.
Cyclometalated complexes containing
obtained from the reaction of 2 and an equimolar amount of
unsymmetrical NNC ligands have generally been utilized as
phosphorescent materials in organic light-emitting devices
[Ru ACHTUNGTNERUNG( PPh ) Cl ] heated at reflux in 2-propanol in 65% yield
(Scheme 1). Recrystallization of 3 from toluene/n-hexane
3 3 2
[14]
and chemosensors;
only a few examples have been ap-
plied as catalysts. Baratta et al. have documented the use of
highly active ruthenium(II) NNC-complex catalysts
(
type A), with an NH group as the accelerating functionali-
2
ty in 2-(aminomethyl)pyridine (ampy) ligands, for the trans-
[10, 15]
II
fer hydrogenation of ketones.
However, Ru complexes
(
type B) that contain a pyridyl-based pyrazolyl-N-heterocy-
II
clic carbene (NHC) ligand only exhibited a poor catalytic
activity for the same reactions (final turnover frequency
Scheme 1. Synthesis of Ru –NNC complex 3. i) Pd
A
H
U
T
E
N
N
2
, para-tolylbor-
, 2-propanol,
onic acid, DMF, 1008C, 10 h, 86%; ii) [Ru
28C, 4 h, 65%.
A
H
U
G
R
N
U
G
3
)
3
2
3
8
[
a] W. M. Du, L. D. Wang, P. Wu, Prof. Dr. Z. K. Yu
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
provided air-stable red-brown crystals. NMR spectroscopic
analysis of 3 in solution supported the predicted structure.
4
57 Zhongshan Road, Dalian, Liaoning 116023 (P.R. China)
3
1
1
Fax : (+86)411-8437-9227
The P{ H} NMR spectrum revealed a singlet at d=
E-mail: zkyu@dicp.ac.cn
26.4 ppm, suggesting that two identical PPh ligands were
3
[
b] Prof. Dr. Z. K. Yu
positioned trans to each other to reduce steric hinderance
around the metal center. The molecular structure of 3 was
further confirmed by X-ray crystallographic determination
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
(
Figure 1). In the solid state, complex 3 exhibits a neutral
3
54 Fenglin Road, Shanghai 200032 (P.R. China)
molecular structure with a nearly planar tridendate NNC
ligand, which is generated from the deprotonation of 2; the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201938.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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