Highly selective induction of metal-centered chirality in the ligand exchange reaction of planar-chiral cyclopentadienyl-ruthenium complex bearing an anchor phosphine ligand

Article abstract of DOI:10.1039/b009412i

Treatment of planar-chiral cyclopentadienyl-phosphine ruthenium complexes with phosphine and phosphite induces metal-centered chirality with a high stereoselectivity (?99 % de).

Full text of DOI:10.1039/b009412i

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Highly selective induction of metal-centered chirality in the ligand exchange
reaction of planar-chiral cyclopentadienyl–ruthenium complex bearing an
anchor phosphine ligand†
Kiyotaka Onitsuka, Noriko Dodo, Yuji Matsushima and Shigetoshi Takahashi*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047,
Japan. E-mail: takahashi@sanken.osaka-u.ac.jp
Received (in Cambridge, UK) 23rd November 2000, Accepted 6th February 2001
First published as an Advance Article on the web 26th February 2001
Treatment of planar-chiral cyclopentadienyl–phosphine ru-
thenium complexes with phosphine and phosphite induces
metal-centered chirality with a high stereoselectivity (699 %
de).
Planar-chiral cyclopentadienyl (Cp) complexes generated by
coordination of unsymmetrically substituted Cp ligands to
metal atoms have attracted much attention as novel asymmetric
catalysts.1 Although planar-chiral Cp complexes of early
transition metals have been applied to asymmetric reactions
with high selectivities, limited numbers of studies on such types
of complex involving late transition metals are found in the
literature. We have been investigating the synthesis, optical
resolution and properties of planar-chiral organometallic com-
plexes of Group 8 metals with trisubstituted Cp ligands.2
Recently we prepared planar-chiral Cp–phosphine ruthenium
complexes 1,3 which may serve as a good asymmetric
environment around the ruthenium atom to prevent the rotation
of the trisubstituted Cp ring by an anchor phosphine.4 Now we
examined the efficiency of the planar-chiral Cp–phosphine
ligand on the induction of metal-centered chirality since the
control of stereochemistry at a metal center is very important to
Scheme 1 Reagents and conditions: i, acetone, room temp., 3 h.
develop highly enantioselective reaction using three-legged
piano-stool complexes.5
Significant numbers of attempts to induce metal-centered
chirality in three-legged piano-stool complexes have been
found in the literature.6 However, successive examples of the
control of metal-centered chirality with a high selectivity are
still limited. Very recently, with planar-chiral Cp and indenyl
Rh and Ir complexes having anchor phosphine ligands it has
been shown that the metal-centered chirality is induced by
oxidative addition to the metal with high stereoselectivities.7
Herein we report the thermodynamically or kinetically con-
trolled induction of metal-centered chirality in the ligand
exchange reactions of planar-chiral Cp–phosphine Ru com-
plexes with various phosphines and phosphites.
Treatment of complex 1a with 1.1 equiv. of PPh3 in acetone
at room temperature resulted in the replacement of one of the
Fig. 1 Molecular structure of complex 2a-1·2CH2Cl2. Hydrogen atoms, a
two MeCN ligands to give complex 2a in quantitative yield.
counter anion and a solvent molecule are omitted for clarity.
Resulting complex 2a contained two diastereomers, each of
which consists of a pair of enantiomers when racemic 1a was
found that the selectivity depends upon the steric bulkiness of
used as the starting material (Scheme 1). Thus, the stereo-
incoming phosphines and phosphites, and of the substituents on
selectivity at a chiral metal center was appraised by the ratio of
the Cp group. In the reactions with bulky phosphines, such as
PPh3 and PBu3, the selectivities were higher than those with
smaller ones such as PMe3. The reaction of complex 1c having
a t-butyl group on the Cp ring gave products with a high
selectivity relative to those for complexes 1a and 1b. Thus, the
reactions of 1c with bulky phosphines and phosphites (entries
11, 12 and 14) produced single diastereomers exclusively
( > 99% de). The X-ray analyses of major products 2d-1, 2f-1,
2h-1, 2m-1, 2n-1 and 2o-1 revealed that all of them have the
same stereochemistry as that of 2a-1.†‡§ Similar reactions of
planar-chiral Ru complexes 3 involving a P(OMe)3 ligand
instead of an anchor phosphine one were performed (Scheme 2,
Table 2). Although the yields of products 4 were high, the
the major diastereomer (2a-1) to the minor one (2a-2). 1H and
31P NMR spectra of 2a clearly showed that the diastereo-
selectivity was 52% de. Fortunately, single crystals of 2a-1
were able to be grown in a CH2Cl2–ether solution of a mixture
of the two diastereomers. The stereochemistry of 2a-1 was
unequivocally identified by X-ray crystallography to be S1CSRu
1CRRu as shown in Fig. 1.‡
The stereoselectivity was then examined for the reactions of
/
R
1a–1c with phosphines and phosphites (Table 1), and it was
† Electronic supplementary information (ESI) available: crystal data and
ORTEP diagrams for complexes 1a, 2d-1, 2f-1, 2h-1, 2m-1, 2n-1 and 2o-1.
See http://www.rsc.org/suppdata/cc/b0/b009412i/
DOI: 10.1039/b009412i
Chem. Commun., 2001, 521–522
This journal is © The Royal Society of Chemistry 2001
521

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Table 1 Reactions of complexes 1a–1c with phosphines and phosphites
Yield
(%)a
Entry
Complex
PRA3
Product
% dea
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1a (R = Me) PPh3
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
100
100
100
100
100
100
96
100
100
100
91
52b
92
1a
1a
1a
1a
PBu3
PMe3
P(OPh)3
P(OMe)3
64
38b
40
1b (R = Ph) PPh3
80b
86
1b
1b
1b
1b
PBu3
PMe3
P(OPh)3
P(OMe)3
46b
42
44
Scheme 3
1c (R = But) PPh3
> 99
> 99
70b
> 99b
82b
1c
1c
1c
1c
PBu3
PMe3
P(OPh)3
P(OMe)3
100
100
100
96
(b), resulting in the selective formation of (S1CSRu/R1CRRu)-2.
This explanation is in good agreement with the steric influence
of incoming phosphines and the substituents on the Cp group
upon the selectivity of the reactions (vide supra).
a Determined by 1H and 31P NMR spectroscopy. b The structures of major
products were determined by X-ray crystallography.
In summary, we have disclosed here the first example of the
kinetic control of metal-centered chirality by planar-chirality of
the Cp group in the three-legged piano-stool complexes. Since
the resulting complexes are conformationally stable, they may
be applicable to novel asymmetric catalysts.
Notes and references
‡ Crystal data for complex 2a-1·2CH2Cl2: C44H44Cl4F6NO2P3Ru, M =
1068.63, triclinic, P1 (no. 2), a = 11.480(3), b = 19.232(5), c = 11.334(3)
Scheme 2 Reagents and conditions: i, 1.1 PPh3, acetone, room temp.,
3 h.
¯
Å, a = 98.40(2), b = 107.45(2), g = 80.66(2)°, V = 2342(1) Å3, Z = 2,
Dc = 1.515 cm23, m(Mo-Ka) = 7.26 cm21, 2qmax = 55°, 250 °C, R (Rw)
= 0.075 (0.137) for 550 parameters against 9502 reflections with I >
3.0s(I) out of 11 004 unique reflections (Rint = 0.019), GOF = 1.53.
CCDC 147977–84. See http://www.rsc.org/suppdata/cc/b0/b009412i/ for
crystallographic files in .cif or other electronic format.
Table 2 Reactions of complexes 3a–3c with phosphines
Entry
Complex
Product
Yield (%)a
% dea
§ In the reaction with PBu3 and PMe3, the assignments of configuration at
a metal center (RRu or SRu) in resulting complexes 2b, 2c, 2g, 2h, 21 and 2m
are reversed relative to those of other complexes with the same
conformation due to the change of the priorities of the anchor phosphine
ligand and the incoming P ligands. Thus, the stereochemistries of 2b-1, 2c-
1, 2g-1, 2h-1, 2l-1 and 2m-1 are S1CRRu/R1CSRu whereas those of other
1
2
3
3a (R = Me)
3b (R = Ph)
3c (R = But)
4a
4b
4c
92
100
100
28
2
34
a Determined by 1H and 31P NMR spectroscopy.
major products are S1CSRu/R1CRRu
.
diastereoselectivities were very low ( < 34% de) relative to
those of 2, suggesting that the anchor phosphine ligand has an
important role in control of the stereochemistry at the metal
center.
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It should be noted that complexes 2a-1 and 2f-1 slowly
isomerized into minor isomers 2a-2 and 2f-2, respectively, in
acetone at room temperature. For example, diastereomerically
pure complex 2a-1 was converted into a mixture of complexes
2a-1 and 2a-2 in a 74:26 ratio (48% de), which is almost the
same diastereoexcess as that observed for the reaction of 1a
with PPh3. Since easy replacement of the MeCN ligands in 2a-1
and 2f-1 with CD3CN was confirmed by means of 1H NMR, the
selectivities in these cases must be determined by the thermo-
dynamic stability of the products. On the other hand, no
epimerization at a metal center as well as no exchange of MeCN
with CD3CN were observed in other major products 2-1 in
solution. These results strongly suggest that the selectivities of
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The kinetic preference of S1CSRu/R1CRRu isomers to S1CRRu
/
R
1CSRu ones can be reasonably explained considering the
stereochemistry of intermediate 5 as shown in Scheme 3.
Ligand exchange reactions would proceed via unsaturated
species 5, which is generated by dissociation of MeCN from 1.
Coordination of incoming phosphines or phosphites from side
(a) gives (S1CSRu/R1CRRu)-2, while coordination from side (b)
produces (S1CRRu/R1CSRu)-2. The structures of planar-chiral
ruthenium complexes with Cp–phosphine ligands including
starting complex 1a‡ clearly show that the attack of phosphines
to the ruthenium center receives steric hindrance caused by the
equatorial phenyl group and the substituent (R) on the Cp group.
Thus, the attack from side (a) must be faster than that from side
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522
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