Enantioface-selective complexation of prochiral dienes on planar-chiral cyclopentadienylruthenium complexes bearing an anchor phosphine ligand

Article abstract of DOI:10.1246/cl.2000.760

Planar-chiral cyclopentadienylruthenium complex 1 [{η⁵:η1-C₅H₂-2-Me-4-Ph-1-CO₂ CH₂CH₂PPh₂}Ru(CH₃CN)₂][PF ₆] bearing an anchor phosphine ligand exhibits a high enantioface selectivity in the reaction with prochiral dienes 2 to give planarchiral η⁴-diene complex 3.

Full text of DOI:10.1246/cl.2000.760

760
Chemistry Letters 2000
Enantioface-Selective Complexation of Prochiral Dienes on Planar-Chiral
Cyclopentadienylruthenium Complexes Bearing an Anchor Phosphine Ligand
Yuji Matsushima, Kiyotaka Onitsuka, and Shigetoshi Takahashi*
The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047
(Received March 3, 2000; CL-000303)
and angles as those of an analog, [{η5:η1-C5H2-2-Me-4-Ph-1-
Planar-chiral cyclopentadienylruthenium complex 1
[{η5:η1-C5H2-2-Me-4-Ph-1-CO2CH2CH2PPh2}Ru(CH3CN)2][PF6]
bearing an anchor phosphine ligand exhibits a high enantioface
selectivity in the reaction with prochiral dienes 2 to give planar-
chiral η4-diene complex 3.
CO2CH2CH2PPh2}Ru(S2CNMe2)][PF6], which previously we
crystallographically characterized.6 The ORTEP diagram
(Figure 1) of 3a-1 reveals that the isoprene ligand coordinates
to the ruthenium center in a prone fashion11 and bond distances
and angles are in the range of those for the known η4-diene-
ruthenium complexes.7b,12 The methyl group of isoprene ligand
is located under the hydrogen at 3-position of Cp' ligand, and
H3 is under the phenyl group at 4-position (Figure 2).
Planar chirality arises from the enantioface selection of
prochiral π-ligands with no C2 axis of symmetry on coordination
to a metal and thus is characteristic of organometallic π-complex-
es.1,2 In the transition metal-mediated and -catalyzed asymmetric
reactions of prochiral unsaturated substrates, the enantioselectivi-
ty of reactions essentially depends on enantioface selection by
chiral metal species toward the prochiral substrates such as
olefins and carbonyl compounds.3 For the development of new
asymmetric reactions fundamental information on enantioface
selection by metal complexes is of prime importance. The enan-
tioface selection is governed by an asymmetric environment
around a metal center which lies in a metal-centered, planar or
ligand chirality. Enantioselective π-complexation of unsaturated
substrates has extensively been studied on metal complexes with
a metal-centered1b and ligand chirality,1b,4 however few studies
have been made so far on effective enantioface selection by pla-
nar-chiral metal complexes.5 Recently we have reported the syn-
thesis of planar-chiral (η5:η1-cyclopentadienyl-phosphine)ruthe-
nium complex 1 which bears a phosphine ligand as an anchor.6 It
is well-known that cationic (π-cyclopentadienyl)ruthenium com-
plexes [π-CpRuL(CH3CN)2]+ (L = CO, PR3 etc.) undergo easy
replacement of the acetonitrile ligand by various dienes to give
η4-diene–ruthenium complexes [π-CpRuL(η4-diene)]+.7 Now
we have attempted to appraise the ability of our planar-chiral Ru
complex [(Cp'-P)Ru(CH3CN)2]+ 16 for enanitoface selection on
diene complexation (Eq 1).
Since 3a is a product from the reaction of a racemic mix-
ture of (SCp)- and (RCp)-1, and has three chiral centers (planar
chiralities of Cp' and isoprene, and Ru-centered chirality), there
should be eight possible stereoisomeres, i.e., four each for two
conformers in which isoprene coordinates on Ru in a prone and
supine fashion.11 The spectral and X-ray analyses showed 3a to
adopt a prone coordination of isoprene; therefore possible iso-
mers for 3a are (SRR), (RSS), (SSS), and (RRR).10 Enantioface
selectivity of 1 toward isoprene may be appraised based on the
ratio of diastereomers (SRR,RSS)-3a-1/(SSS,RRR)-3a-2 in prod-
uct 3a (Figure 3). Thus, the P-NMR of 3a showed two singlets
at δ 52.95 and 51.52 with an integral ratio of 12.5 : 1. H-NMR
of 3a also showed two sets of signals, indicating the formation
of two diastereoisomers 3a-1 and 3a-2. The integral ratios of
the two isomers in the P- and H-NMR spectra suggest a high
As a prochiral diene we chose isoprene and evaluated the
diastereoselectivity of the reaction with complex 1. Thus, to a
dichloromethane solution of 1 was added 10 equiv of isoprene
and refluxed for 12 h to give isoprene complex 3a8 as a mixture
of diastereomers 3a-1 (major) and 3a-2 (minor) in a good yield.
3a-1 was fully characterized by spectral analyses including H-
NOE spectra (vide infra), and finally the stereostructure has
been established by an X-ray crystallographic analysis9 to be
(SCp,RRu,Rdiene) or (RCp,SRu,Sdiene).10 The parent moiety {(η5:η1-
C5H2-2-Me-4-Ph-1-CO2CH2CH2PPh2)Ru} possesses essentially
the same configuration with the almost same bond distances
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
761
selectivity of 44% d.e. on treatment with isoprene (Eq 2), indi-
cating an important role of the anchor phosphine ligand for
enantioface-selective complexation of dienes.
Reference and Notes
1
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2
3
T. Katayama, Y. Morimoto, M. Yuge, M. Uno, and S. Takahashi,
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1152 (1995).
enantioface selection of 12.5/1 (86% d.e.) on the coordination
of isoprene to 1.
H-NOE spectra for 3a clearly revealed an interaction of the
methyl group of isoprene with the methyl at 2-position as well
as with the proton at 3-position of Cp' ligand, suggesting a
prone coordination of the isoprene ligand in 3a. This stereo-
chemistry accords with that established by the X-ray analysis,
indicating the same stereostructure both in solids and in a solu-
tion. The NMR study also indicated that even in a solution the
coordination of isoprene is fairly strong enough to restrict its
dissociation and rotation leading to epimerization at least at
ambient temperature.
Taking account of the steric effects of substituents on the
planar-chiral cyclopentadienyl and isoprene ligands, the enan-
tioface selection toward isoprene by 1 is likely to be controlled
by the asymmetric environment constructed by the planar chi-
rality of Cp'. Difference in steric bulkiness between hydrogen
and phenyl group on the Cp' ligand seems to be suitable for dis-
criminating the enantioface of isoprene which results in the pre-
dominant formation of complex 3a-1 with a configuration of
(SRR) or (RSS).
4
5
a) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, and R. M.
Waymouth, Angew. Chem., Int. Ed. Engl., 34, 1143 (1995). b) A. H.
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6
7
a) E. Rüba, W. Simanko, K. Mauthner, K. M. Soldouzi, C. Slugovc,
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IR (KBr) 1727 (C=O) cm–1. MS (FAB) m/z 581. Found: C, 52.74;
H, 4.25%. Calcd for C32H32F6O2P2Ru: C, 52.97; H, 4.45%. 3a-1:
1H-NMR (acetone-d6, 400 MHz) δ 7.78–7.57 (m, 10H, Ph),
7.41–7.32 (m, 3H, Ph), 7.07 (br, 2H, Ph), 6.30 (d, 1H, J = 1.0 Hz,
CpH), 5.82–5.76 (m, 2H, H3, CpH), 5.33–5.22 (m, 1H, OCH2), 4.49
(ddt, 1H, J = 2.2, 5.6, 11.7 Hz, OCH2), 3.53–3.36 (m, 3H, PCH2,
H1), 3.32 (s, 1H, H1), 2.58 (s, 3H, Me), 2.30 (d, 3H, J = 1.5 Hz,
Me), –0.17 (dd, 1H, J = 2.0, 16.1 Hz, H2), –0.29 (ddd, 1H, J = 2.2,
9.5, 15.9 Hz, H2) ppm. 31P-NMR (acetone-d6, 160 MHz) δ 52.95.
3a-2: 31P-NMR (acetone-d6, 160 MHz) δ 51.52.
8
9
Crystal data for 3a-1.CH3COCH3: C38H44F6O4P2Ru, FW = 841.77,
−
triclinic, space group P1(#2), a = 12.548(3), b = 14.590(3), c =
The enantioface selectivity of complex 1 was also exam-
ined toward other dienes, and 1 was allowed to react with sev-
eral prochiral dienes 2 under the same conditions (Table 1).
Complex 1 exhibited a selectivity toward dienes such as trans-
1,3-pentadiene and trans-1,3-hexadiene, but the selectivity is
lower than that for isoprene. A low selectivity was observed
toward 2,4-hexadien-1-ol. These observations suggest that
large difference in the steric bulkiness of substituents on prochi-
ral dienes should be required for high enantioface selection by
1. Particularly the substituent at 2-position on dienes is likely
recognized by the planar-chiral Cp' ligand with an assist from
the two phenyl groups of the anchor phosphine ligand,13 since
complex 4 bearing no anchor ligand, thus the Cp' ligand is
allowed to rotate around the bonding axis, showed a lower
11.140(2) Å, α = 107.23(1), β = 111.85(1), γ = 70.55(2)°, V =
1750.1(6) Å3, Z = 2, dcalcd = 1.597 g.cm–3, µ = 6.13 cm–1, R (Rw) =
0.067 (0.110) for 460 parameters against 7370 reflections with I >
3.0 σ(I) out of 8423 unique reflections by full-matrix least-squares
method, GOF = 1.14. The structure was solved and refined with
teXsan program package.
10 G. Paiaro and A. Panunzi, J. Am. Chem. Soc., 86, 5148 (1964).
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