1482
Chemistry Letters Vol.36, No.12 (2007)
Enantiofacial Control of Planar Chiral Arene Ru Complexes Bearing
Tropos Biphenyl Ligands
Kohsuke Aikawa, Isao Kaito, and Koichi Mikamiꢀ
Department of Applied Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552
(Received August 28, 2007; CL-070927; E-mail: mikami.k.ab@m.titech.ac.jp)
A variety of planar chiral Ru-complexes bearing tropos or-
tho-substituted biphenyl ligands were synthesized. The planar
chirality control of the Ru complexes by enantiopure (R)-H8–
DABN selectively gave the thermodynamically stable diaster-
eomers via association of solvents employed.
R
1a−1d
'
PR
2
Cl
Ru
Ru
Cl
Cl
(2 equiv.)
Cl
Cl
P
R
Ru
Cl
R
'
(CH2Cl)2
80 °C, >48 h
(CH2Cl)2
r.t., 2 h
2
Yield >99%
Asymmetric catalysis is the most powerful for the produc-
tion of a various enantio-enriched molecules.1 In asymmetric
catalysis, atropisomeric or planar chiral metal complexes bear-
ing particularly ferrocenyl–diphosphine ligands, for example
(R)-N,N-dimethyl-1-[(S)-10,2-bis(diphenylphosphino)ferroceny-
l]ethylamine [(R)-(S)-BPPEA]2 with appendages of stereogenic
centers, are generally used. These planar chiral metal com-
plexes3 have been synthesized in diastereofacial selective fash-
ion. There has been no example of enantiofacial selective syn-
thesis of planar chiral Ru complex without any appendage of
stereogenic center. We have already succeeded in axial chirality
control of Ru-complexes bearing chirally flexible (tropos)
bis(phosphanyl)biphenyl (BIPHEP) ligands.4 The axial chirality
can be controlled by enantiopure (R)-2,20-diamino-1,10-bi-
naphthyl ((R)-DABN). During the course of our study on the ax-
ial chirality control, we have found that the BIPHEP–Ru com-
plex bearing axial chirality could be transformed to BIPHEP–
monoxide–Ru complex bearing planar chirality (Scheme 1).5
We herein report the enantiofacial control of planar chiral Ru
complexes of tropos biphenyl ligands.6
Axial chiral BIPHEP–Ru complex7 was synthesized through
addition of 2 equiv. of BIPHEP to [RuCl2(benzene)]2 in DMF at
room temperature under argon atmosphere and then at 100 ꢁC
within 10 min. Heating over 24 h gave BIPHEP–monoxide–Ru
complex bearing planar chirality through oxidation of one
phosphine portion in 80% yield, by virtue of the tropos nature
(Scheme 1). In sharp contrast, when the atropos BINAP or SEG-
PHOS8 counterparts were used as a ligand, their planar Ru-com-
plexes were not obtained but the axial chiral complexes decom-
posed for their air sensitivity. It was thus clarified that the tropos
biphenyl backbone is the key in obtaining the planar complexes.
A variety of planar chiral Ru complexes 2a–2d bearing or-
tho-substituted tropos biphenyl ligands 1a–1d were thus pre-
pared (Scheme 2). The complexation of [RuCl2(benzene)]2
R
a: R = NMe2, R' = Ph
b: R = NMe2, R' = Cy10
c: R = OMe, R' = Ph
d: R = OMe,
R
+
Ru
Ru
Cl
Cl
R-2a−2d
Cl
Cl
S-2a−2d
P
P
R'2
R'2
R' = 3,5-Me2−C6H3
Yield >95%
Scheme 2. Synthesis of planar arene Ru complexes.
and two equimolar amounts of tropos biphenyl ligands 1a–1d
in dichloroethane at room temperature was examined to give
first benzene phosphine complexes quantitatively within 2 h.
Subsequently, intramolecular ligand exchange9 was observed
to give planar chiral Ru-complexes 2a–2d bearing tropos
biphenyl ligands over 48 h even at 80 ꢁC in dichloroethane.
Planar chirality control of complexes 3a and 3b was next at-
tempted to convert the less favorable diastereomer to the favor-
able diastereomer (Table 1). A planar chiral Ru-complex was
mixed with one equimolar amount of (R)-H8–DABN and an ex-
cess amount of AgOTf salt to give a 1:1 (R=aR:S=aR) mixture of
diastereomers quantitatively. No change was observed in the di-
astereomeric ratio at room temperature. However, the isomeriza-
tion of 3a proceeded in 2-propanol/(CH2Cl)2 at 80 ꢁC for 72 h to
give the S=aR-3a as the major diastereomer (Entry 1). On the
other hand, the complex 3b11 bearing PCy2 was decomposed in
2-propanol/(CH2Cl)2 (Entry 2). The use of chlorobenzene gave
the thermodynamically stable diastereomer S=aR-3b selectively
(85:1ꢂ5) without decomposition (Entry 3). Complex 3b with
SbF6 as a counter anion was decomposed under the same con-
ditions (Entry 4). The isomerization rate in chlorobenzene was
influenced by the concentration of 3b (See SI, Figure S1).12
Two possible mechanisms can thus be envisaged for isomeriza-
tion of 3b. One is intermolecular migration of Ru via the chlor-
obenzene-complex produced by dissociation of biphenyl ligand
2b. The other is intramolecular migration of Ru through direct
assistance of other Ru-complexes.
O
The configuration of the diastereomer R=aR-3b bearing
ꢂ
Ph2P
Ph2
SbF6 was determined by X-ray analysis of a single crystal ob-
tained in dichoromethane solution of 1:1 diastereomer mixture
(Figure 1).12
P
DMF, O2
RuCl2(dmf)n
100 °C, 24 h
Ru
P
Cl
Cl
( )-BIPHEP−monoxide−Ru
P
Ph2
The complexes 3c and 3d bearing OMe instead of NMe2
were next examined (Table 2). The isomerization of 3c proceed-
ed in 2-propanol/(CH2Cl)2 at 80 ꢁC to give the S=aR-3c as the
major diastereomer (Entry 1). Using 3d with sterically demand-
Yield 80%
Ph2
( )-BIPHEP Ru
Scheme 1. Transfer of axial chirality to planar chirality.
Copyright Ó 2007 The Chemical Society of Japan
Aikawa, Kohsuke
