Multiple ligand transfer to planar-chiral cyclopentadienylruthenium complexes inducing metal-centered chirality

Article abstract of DOI:10.1039/b006458k

Multiple ligand transfer reaction between planar-chiral cyclopentadienylruthenium complexes [Cp'Ru(AN)₃][PF₆] [(Cp' = 1-(CO₂Et)-2-Me-4-RC₅H₂, R = Me, Ph, Bu(t), AN = acetonitrile] and iron complexes CpFe(CO)(L)X (L = PMe₃, PPh₃; X = I, Br) results in formation of metal-centered chiral ruthenium complexes Cp'Ru(CO)(L)X with a diastereoselectivity (de) up to 68%.

Full text of DOI:10.1039/b006458k

Multiple ligand transfer to planar-chiral cyclopentadienylruthenium complexes
inducing metal-centered chirality
Taku Katayama, Yuji Matsushima, Kiyotaka Onitsuka 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) 7th August 2000, Accepted 11th October 2000
First published as an Advance Article on the web 10th November 2000
Multiple ligand transfer reaction between planar-chiral
cyclopentadienylruthenium complexes [CpARu(AN)3][PF6]
[(CpA = 1-(CO2Et)-2-Me-4-RC5H2, R = Me, Ph, But, AN =
acetonitrile] and iron complexes CpFe(CO)(L)X (L = PMe3,
PPh3; X = I, Br) results in formation of metal-centered
chiral ruthenium complexes CpARu(CO)(L)X with a diaster-
eoselectivity (de) up to 68%.
Half-sandwich transition metal complexes CpML3 with a three-
legged piano stool structure are fascinating molecules owing to
their potential as catalytic or stoichiometric mediators in precise
organic syntheses and some of their chiral versions have
Scheme 1 Reagents and conditions: i, CH2Cl2, reflux, 3 h.
recently been applied in asymmetric organic synthesis.1 Al-
though most chiral organometallic complexes have chiral
5
organic groups on the ligands, chiral half-sandwich complexes
can be formed in the absence of chiral ligands. While
coordination of three different ligands to a metal generates a
stereogenic center at the metal atom,2 unsymmetrically substi-
tuted cyclopentadienyl ligands provide planar chirality.3 Stud-
ies on such chiral complexes give us fundamental and important
information that serves for the development of novel asym-
metric reactions.
4-PhC5H2) and 1c [CpA = h -1-(CO2Et)-2-Me-4-ButC5H2] also
produced the corresponding dicarbonylruthenium complexes
3d–h. The resulting Ru complex 3 was fully characterized by
spectral analyses and X-ray crystallography.† The geometry
around the metal atom is similar to those of analogous iron
5
complexes (R)- and (S)-CpAFe(CO)2I [CpA = h -1-(CO2Men)-
2-Me-4-PhC5H2; Men = (l)- or (d)-menthyl].7
When enantiomerically pure planar-chiral complexes (R)-
and (S)-[CpARu(AN)3][PF6] 1d [CpA
2,4-Me2C5H2; Adm = 2-adamantyl] were used as starting
materials in the reaction with 2a, the corresponding Ru
complexes (R)- and (S)-CpARu(CO)2I 3i were isolated, re-
5
We have been studying planar-chiral Ru complexes with
trisubstituted cyclopentadienyl ligands.4,5 Recently we also
reported a novel multiple ligand transfer reaction between
=
h -1-(CO2Adm)-
[CpRu(L)(AN)2][PF6] [L
= AN, CO, P(OMe)3; AN =
acetonitrile] and CpFe(CO)(LA)X [LA = CO, PMe3, PMe2Ph,
PMePh2, PPh3, P(OPh)3; X = I, Br, Cl].6 Now, we have
examined multiple ligand transfer reactions using planar-chiral
cyclopentadienylruthenium complexes and found the induction
of Ru-centered chirality by planar chirality on ligand transfer
from Fe complexes.
spectively, in an enantiomerically pure form [(R)-3i: [a]D =
+143° (c 0.100, CH2Cl2); (R)-3i: [a]D = 2142° (c 0.105,
1
CH2Cl2)]. H NMR experiments using a chiral shift reagent
Eu(hfc)3 unequivocally indicate that no racemization of the
planar-chiral cyclopentadienyl ligand occurred in the multiple
ligand transfer reactions.
Table 1 lists the results obtained from the reactions of the
trisubstituted cyclopentadienyl ruthenium tris(acetonitrile)
complex [CpARu(AN)3][PF6] 14 with Fe complex 2 of the type
Then, we investigated the influence of planar chirality of the
cyclopentadienyl group on the stereochemistry at the Ru center
of the triple ligand transfer product CpARu(CO)(L)X 7 from the
reaction with CpFe(CO)(L)X 6 (L = PMe3, PPh3; X = I, Br)
(Scheme 2). As illustrated in Scheme 3, complex 7 contains two
diastereomerically related pairs, each of which consists of
enantiomers. Thus, the diastereoselectivity of metal-centered
chirality affected by the planar chirality of cyclopentadienyl
group was appraised by the diastereomer ratio of 7 (Table 2).
Although other Ru complexes 3 and/or 4 were also produced as
5
CpFe(CO)2X. Thus, treatment of Ru complex 1a [CpA = h -
1-(CO2Et)-2,4-Me2C5H2] with an equimolar amount of CpFe-
(CO)2I 2a in refluxing CH2Cl2 for 3 h produced a triple ligand
transfer product, CpARu(CO)2I 3a, in 76% yield (Scheme 1).
Similar reactions of 1a with CpFe(CO)2Br 2b and CpFe-
(CO)2Cl 2c gave CpARu(CO)2Br 3b and CpARu(CO)2Cl 3c,
5
respectively. Ru complexes 1b [CpA = h -1-(CO2Et)-2-Me-
Table 1 Triple ligand transfer reactions between [CpARu(AN)3][PF6] 1 and CpFe(CO)2X 2
Substrate
Isolated yields of products (%)
Run
Ru complex
Fe complex
CpARu(CO)2X 3a
CpACpRu 4a
Cp2Fe 5b
Recovery of 2
1
2
3
4
5
6
7
8
1a (R = Me)
1a
1a
1b (R = Ph)
1b
2a (X = I)
2b (X = Br)
2c (X = Cl)
2a
2b
2c
2a
2b
76 (3a)
91 (3b)
47 (3c)
71 (3d)
70 (3e)
43 (3f)
72 (3g)
79 (3h)
21
9
9 (4a)
7 (4b)
43
13
26
33
6
26
24
1b
1c (R = But)
1c
a Yields are based on the starting Ru complex 1. b Yields are based on the starting Fe complex 2.
DOI: 10.1039/b006458k
Chem. Commun., 2000, 2337–2338
This journal is © The Royal Society of Chemistry 2000
2337

Table 2 Triple ligand transfer reactions between [CpARu(AN)3][PF6] 1 and CpFe(CO)(L)X 6
Substrate
Isolated yields of products (%)
Run
Ru complex
Fe complex
CpARu(CO)2X 7a
49 (68)c (7a)
CpARuCO)2X 3a CpACpRu 4a
Cp2Fe 5b
Recovery of 6
1
2
3
4
5
1a
1a
1a
1b
1c
6a (L = PMe3, X = I)
6b (L = PPh3, X = I)
6c (L = PPh3, X = Br)
6a
6a
12 (3a)
35 (3b)
31 (3c)
17 (3d)
18 (3g)
6 (4a)
2 (4b)
12 (4c)
2 (4a)
12
17
24
12 (28)c (7b)
36 (22)c (7c)
48 (40)c (7d)
1
20
10
23
9
a Yields are based on the starting Ru complex 1. b Yields are based on the starting Fe complex 6. c Parentheses indicate % de of 7 determined by 1H and 31
P
NMR spectroscopy.
Scheme 2 Reagents and conditions: i, CH2Cl2, reflux, 3 h.
Fig. 1 ORTEP diagram of (RC1,SRu)/(SC1,RRu)-7a (major diastereomer).
Hydrogen atoms are omitted for clarity.
chirality in ligand transfer reactions. Further investigation
focusing on the mechanism of asymmetric induction is now in
progress.
Scheme 3 Stereoisomers of 7a.
This work was partly supported by Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Science, Sports
and Culture. We are grateful to The Material Analysis Center,
ISIR, Osaka University, for technical support of spectral
measurements and X-ray analysis.
well as 7 in the reaction of 1 and 6, they were easily separated
by column chromatography on silica gel. Yields of products
depended both on the substituents on the cyclopentadienyl
group as well as the phosphine initially coordinated to Fe.
Reactions with 6a with PMe3 gave desired complex 7 in
moderate yields (runs 1, 4 and 5), while yields of 7 were low in
the reactions with 6b and 6c having PPh3 (runs 2 and 3). To our
surprise, the best result (68% de) in the diastereoselectivity of 7
was oberved in the reaction of 1a having a small substituent
(Me) on the cyclopentadienyl group with 6a having a small
phosphine ligand (PMe3) (run 1). Reactions of 1b and 1c having
a larger substituent (Ph or But) on the cyclopentadienyl group
gave complexes 7c and 7d in 22 and 40% de, respectively (runs
4 and 5). Although asymmetric induction at a Ru center by a
chiral organic group on the cyclopentadienyl ring has been
Notes and references
† Crystal data: for 3d: C17H15IO4Ru, M = 511.28, monoclinic, P1, a =
¯
9.631(3), b = 11.246(3), c = 8.313(2) Å, a = 98.76(2), b = 100.410(8),
g = 93.71(3)°, V = 868.7(4) Å3, Z = 2, Dc = 1.954 g cm23, m(Mo–Ka)
= 26.96 cm21, 6 < 2q < 55°, T = 250 °C, R (Rw) = 0.029 (0.069) for
181 parameters vs. 3889 reflections with I > 3.0s(I) out of 4122 unique
reflections (Rint
= 0.023), GOF = 1.35. For (RC1,SRu)/(SC1,RRu)-7a:
C
14H22IO3PRu, M = 497.27, monoclinic, P21/n, a = 10.067(2), b =
9.951(2), c = 18.322(1) Å, b = 100.410(8)°, V = 1805.2(4) Å3, Z = 4, Dc
= 1.830 g cm23, m(Mo–Ka) = 26.71 cm21, 6 < 2q < 55°, T = 275 °C,
R (Rw) = 0.037 (0.055) for 208 parameters vs. 3645 reflections with I >
3.0s(I) out of 3990 unique reflections (Rint = 0.016), GOF = 1.35.
CCDC 182/1818. See http://www.rsc.org/suppdata/cc/b0/b006458k/ for
crystallographic files in .cif format.
5
attempted in the ligand exchange reaction of (h -C5H4R*)Ru-
(CO)2X (R* = neomenthyl) with phosphine and phosphite, the
5
diastereoselectivities of products (h -C5H4R*)Ru(CO)(PR3)X
were fairly low (up to 19% de).8
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2000, 610, 31.
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Organometallics, 1999, 18, 3087.
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Fortunately single crystals of the major diastereomer se-
lectively grew on recrystallization of a diastereomeric mixture
of 7a from Et2O–hexane. As seen in Fig. 1 the molecular
structure of the major diastereomer of 7a was established by X-
ray analysis to possess the configuration (RC1,SRu)/(SC1,RRu).†
Previously we also found the induction of metal-centered
chirality by CO insertion into the Fe–C bond of planar-chiral Fe
complexes giving CpAFe(CO)(PPh3)(COMe) 8.9 Facile iso-
merization of complex 8 around the metal center under the
employed reaction conditions suggested that the selectivity of
the resulting complex is controlled by thermodynamic factors.
In contrast, no isomerization at a Ru center was observed for a
CH2Cl2 solution of the major diastereomer of 7a, isolated by
recrystallization (vide supra), under reflux for 3 h. The reactions
presented here provide the first induction of metal-centered
2338
Chem. Commun., 2000, 2337–2338