A new coupling reaction for the synthesis of ruthenium half-sandwich complexes with sterically demanding cyclopentadienyl ligands

Article abstract of DOI:10.1039/b618712a

The reaction of RuCl₃(solv.)_n with tert-butylacetylene in methanol or ethanol leads to the formation of chloro-bridged half-sandwich complexes with sterically demanding cyclopentadienyl ligands, which are of high interest as starting materials for the synthesis of novel Ru catalysts. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618712a

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A new coupling reaction for the synthesis of ruthenium half-sandwich
complexes with sterically demanding cyclopentadienyl ligands{
S e´ bastien Gauthier, Euro Solari, Barnali Dutta, Rosario Scopelliti and Kay Severin*
Received (in Cambridge, UK) 22nd December 2006, Accepted 19th January 2007
First published as an Advance Article on the web 9th February 2007
DOI: 10.1039/b618712a
3 n
The reaction of RuCl (solv.) with tert-butylacetylene in
methanol or ethanol leads to the formation of chloro-bridged
half-sandwich complexes with sterically demanding cyclopen-
tadienyl ligands, which are of high interest as starting materials
for the synthesis of novel Ru catalysts.
Ruthenium half-sandwich complexes with cyclopentadienyl
1
ligands constitute a very important class of catalysts. The
2
transformations catalyzed by these complexes include allylic and
3
propargylic substitutions, cycloadditions, isomerizations, alkane
4
5
Scheme 1 Synthesis of the complexes 3 and 4.
6
7
borylations, and atom transfer radical addition and polymeriza-
8
1
structural information by NMR spectroscopy were not successful,
III
indicating the presence of paramagnetic Ru . This was confirmed
tion reactions, among many others. In order to optimize the
catalytic performance for a given reaction, a plethora of co-ligands
have been employed, such as phosphines, olefins, halides, nitriles
and thiolates. Structural modifications of the cyclopentadienyl
ligand, on the other hand, are not very common, and most
investigations have focused on Cp and Cp* complexes.{ The likely
explanation for the dominance of Cp and Cp* ligands is the
fact that versatile and easily accessible starting materials are
available for these complexes. The cationic acetonitrile
complex [{CpRu(CH CN) }(PF )] (1) and chloro-bridged dimer
by a single crystal X-ray analysis." The overall structure of
5
complex 3 is similar to that of 2: two (g -cyclopentadienyl)RuCl
2
fragments are connected by two chloro bridges to form a
centrosymmetric dimer (Fig. 1). However, the cyclopentadienyl
ligands show a unique substitution pattern, with two tert-butyl,
one neopentyl and one methoxy group attached to the aromatic
ring. Due to the presence of different side chains, the (p-ligand)Ru
fragment displays planar chirality. There are two independent
3
3
6
(
p-ligand)RuCl complexes in the asymmetric unit; the dimers are
2
[
Cp*RuCl
2
]
2
(2) have turned out to be particularly useful. The
one-step procedure from
3 2 n
RuCl (H O) ], and convenient syntheses have been developed
obtained by the symmetry operation 2x, 1 2 y, z and 1 – x, 1 – y,
latter can be obtained in
a
9
1 – z, respectively. The Ru atoms in these dimers are 3.684(1) and
3.743(1) s apart from each other, and therefore not bonded to
[
1
0
for the former. Meanwhile, both complexes are also commer-
cially available.
1
1
each other.
When [RuCl
ethanol instead of methanol, the ethoxy complex 4 was obtained in
0% yield (see ESI{). The structure of 4 is analogous to that of
3 n
(solv.) ] was reacted with tert-butylacetylene in
In the following communication, we describe a simple procedure
III
for the synthesis of two dimeric Ru complexes, 3 and 4, which
4
show overall structures analogous to 2, but have very distinctive
cyclopentadienyl ligands. These complexes are obtained from
3 n
[RuCl (solv.) ] in an unprecedented Ru-mediated coupling reac-
tion of three alkynes and an alcohol. They are expected to become
useful starting materials for the synthesis of novel Ru catalysts, as
evidenced by a first application in a racemization reaction.
3 n
When a solution of [RuCl (solv.) ]§ and 4.2 equivalents of tert-
butylacetylene in methanol were heated to 55 uC, a brown powder
began to precipitate after 2 h (Scheme 1). After 24 h, the mixture
was cooled to 220 uC and complex 3 was isolated in 51% yield
(see ESI{).
Elemental analysis of complex 3 showed that a carbon-rich
compound had formed (C = 50.47%). Attempts to obtain further
Fig. 1 Graphical representation of the molecular structure of complex 3
´
Institut des Sciences et Ing e´ nierie Chimiques, Ecole Polytechnique
in the crystal. Selected bond lengths [s] and angles [u]: Ru1A–Cl1A
F e´ d e´ rale de Lausanne (EPFL), Lausanne, Switzerland.
E-mail: kay.severin@epfl.ch; Fax: +41 (0)21 6939305;
Tel: +41 (0)21 6939302
…
.437(3), Ru1A–Cl1C 2.443(2), Ru1A–Cl2A 2.358(3), Ru1A Ru1C
2
3.684(1); Cl1A–Ru1A–Cl2A 88.89(10), Cl1A–Ru1A–Cl1C 99.0(3). Only
one of the two independent dimers is shown. The letter C stands for the
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b618712a
symmetry operation: 2x, 1 2 y, 2z.
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Chem. Commun., 2007, 1837–1839 | 1837

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complex 3, as evidenced by elemental analysis and by subsequent
reduction to diamagnetic complexes, which can be analyzed by
NMR spectroscopy (see below).
The formation of complexes 3 and 4 can be explained by a Ru-
mediated coupling reaction of three tert-butylacetylenes with
methanol or ethanol and with the elimination of HCl. Transition
metal-mediated [2 + 2 + 1] cyclotrimerizations of alkynes have
12,13
been observed in some cases,
but they are very rare compared
1,4
to the more common [2 + 2 + 2] cyclotrimerizations. A coupling
reaction of three alkynes and an alcohol, giving a cyclopentadienyl
ligand with an alkoxy-substituent directly attached to the ring, is,
to best of our knowledge, unprecedented. The five-membered ring
might be formed by an intramolecular reaction of a metallacyclo-
pentadiene with a vinylidene ligand, as described for other
12a
systems.
Fulvene p-complexes have been suggested as inter-
mediates in the [2 + 2 + 1] cyclotrimerization of alkynes. In our
case, such an intermediate seems unlikely because the nucleophilic
attack of an alcohol would be expected to occur at the exocyclic
13
carbon atom.
When phenylacetylene, cyclohexylacetylene or trimethylsilylace-
tylene were used instead of tert-butylacetylene, a mixture of
unidentified products was obtained. The difficulty in obtaining
coupling products analogous to 3 and 4 with other alkynes was not
unexpected. A complicated multi-component reaction of this kind
is likely to depend strongly on the size and reactivity of the alkyne.
Furthermore, it is known that (cyclopentadienyl)Ru half-sandwich
Scheme 2 Dimers 3 and 4 are versatile starting materials for the
II
synthesis of diamagnetic Ru complexes with sterically demanding
cyclopentadienyl ligands. For each product, only one of the two
enantiomers is shown.
by flash chromatography (see ESI{). Compounds of this kind are
of interest because (cyclopentadienyl)Ru(CO) X complexes are
2
14
complexes (including dimer 2) can react further with alkynes to
frequently used as robust catalysts for the racemization of
18
secondary alcohols. It had been reported that the substituents
1,4
give cycloaddition products or polymers. The importance of
such consecutive reactions will again depend on the nature of the
alkyne. Nevertheless, it is conceivable that complexes of type 3 and
on the cyclopentadienyl ligand are of importance for the
18,19
racemization reaction.
This finding was the motivation to test
4
can be obtained for other alkyne/alcohol combinations if a
the catalytic activity of our new complexes 8 and 9.
careful optimization of the reaction conditions is carried out.
The racemization of (S)-1-phenylethanol was examined using
0.25 mol% of complex 8 and 9, respectively. The reactions were
First investigations showed that complexes 3 and 4 could be
II
easily transformed into diamagnetic Ru complexes with various
performed without a protective inert atmosphere, and K PO was
3 4
co-ligands (see ESI{). The reaction of 3 with norbornadiene
used as a basic additive. Both complexes turned out to be highly
active catalysts, with the methoxy complex 8 (complete racemiza-
tion after 1 h) being slightly more active than the ethoxy complex 9
(complete racemization after 1.5 h). Other secondary alcohols such
as (S)-4-phenyl-2-butanol, (S)-1-(2-naphthyl)ethanol and (S)-1-
indanol were also racemized within 1.0–1.5 h using 0.5 mol% of
(NBD) in EtOH at 55 uC gave complex 5 in 60% yield (Scheme 2).
The accessibility of 5 is of interest in view of the fact that
the analogous Cp* complexes [Cp*RuCl(NBD)] and
[
Cp*RuCl(COD)] have been used extensively as catalysts for
1
various organic transformations.
When 3 was reacted with PCy or PPh in THF in the presence
2
3
3
of Zn, deep purple solutions were obtained, from which the 16e
complexes 6 and 7 were isolated. Apart from NMR spectroscopy
and elemental analysis (see ESI{), complex 7 was characterized by
single-crystal X-ray analysis (Fig. 2)." It is interesting to note that
II
structurally related Cp*Ru complexes have been isolated with
sterically demanding phosphine ligands such as PCy and PiPr ,
3
3
1
5
but not with PPh
mononuclear [Cp*RuCl(PPh
insoluble material, which was suggested to have a polymeric
3
.
On the contrary, attempts to make
3
)] failed and gave instead an
16
structure. This demonstrates that the sterically very demanding
,4-bis-tert-butyl-1-methoxy-3-neopentylcyclopentadienyl ligand is
2
able to stabilize complexes that are not accessible with the standard
Cp* ligand.
II
The synthesis of the Ru -carbonyl complexes 8 and 9 was
accomplished by the carbonylation (1 bar, RT) of cationic
tris(acetonitrile) complexes, which were prepared in situ by
Fig. 2 Graphic representation of the molecular structure of complex 7 in
the crystal. Selected bond lengths [s] and angles [u]: Ru1–Cl1 2.3705(8),
Ru1–P1 2.3786(9); Cl1–Ru1–P1 92.93(3).
17
reduction of 3 or 4 with Zn (Scheme 2). The air stable CO
complexes 8 and 9 were obtained in good yield after purification
1
838 | Chem. Commun., 2007, 1837–1839
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5
H. Suzuki and T. Takao, ‘‘Isomerization of Organic Substrates
Catalyzed by Ruthenium Complexes’’, in Ruthenium in Organic
Synthesis, ed. S.-I. Murahashi, Wiley-VCH, Weinheim, 2004. pp. 309.
complex 8 (see ESI{). These values are comparable to those
reported for the most active Ru-catalysts described so far.
18,19
In summary, we have described the syntheses and structures of
chloro-bridged half-sandwich complexes 3 and 4. They were
obtained in a single step by a new type of coupling reaction using
6 J. M. Murphy, J. D. Lawrence, K. Kawamura, C. Incarvito and
J. F. Hartwig, J. Am. Chem. Soc., 2006, 128, 13684.
7
8
(a) K. Severin, Curr. Org. Chem., 2006, 10, 217; (b) L. Delaude,
A. Demonceau and A. F. Noels, Top. Organomet. Chem., 2004, 11, 155.
(a) M. Kamigaito, T. Ando and M. Sawamoto, Chem. Rec., 2004, 4,
159; (b) M. Kamigaito, T. Ando and M. Sawamoto, Chem. Rev., 2001,
3 n
[RuCl (solv.) ], tert-butylacetylene and methanol or ethanol. The
complexes are expected to find high interest as starting materials
1
01, 3689.
for the synthesis of novel Ru catalysts. The basic structure of 3 and
9
(a) U. K o¨ lle, J. Kossakowski, D. Grumbine and T. D. Tilley, Inorg.
Synth., 1992, 29, 225; (b) T. D. Tilley, R. H. Grubbs and J. E. Bercaw,
Organometallics, 1984, 3, 274; (c) N. Oshima, H. Suzuki and Y. Moro-
oka, Chem. Lett., 1984, 1161.
4
are similar to that of complex 2, which represents one of the key
entry points for the synthesis of (cyclopentadienyl)Ru catalysts.
The p-ligands of 3 and 4, on the other hand, are quite different
from Cp* because of the sterically demanding tert-butyl and
neopentyl groups, and the alkoxy substituent. This is evidenced by
1
0 (a) E. P. K u¨ ndig and F. R. Monnier, Adv. Synth. Catal., 2004, 346, 901;
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T. P. Gilla and K. R. Mann, Organometallics, 1982, 1, 485.
(
2
the synthesis of 7, a 16e complex which is not accessible with the
11 For a discussion about metal–metal interactions in dimer 2, see: (a)
J. E. McGrady, Angew. Chem., Int. Ed., 2000, 39, 3077; (b) U. K o¨ lle,
K. Lueken, K. Handrick, H. Schilder, J. K. Burdett and S. Balleza,
Inorg. Chem., 1995, 34, 6273; (c) U. K o¨ lle, J. Kossakowski, N. Klaff,
L. Wesemann, U. Englert and G. E. Heberich, Angew. Chem., Int. Ed.
Engl., 1991, 30, 690.
standard Cp* ligand. The facile transformation into mononuclear
II
Ru complexes and the application of 8 and 9 as highly efficient
racemization catalysts under mild conditions represents further
evidence for the utility of 3 and 4.
1
2 (a) C. S. Chin and H. Lee, Chem.–Eur. J., 2004, 10, 4518; (b) E. Becker,
K. Mereitner, M. Puchberger, R. Schmid, K. Kirchner, A. Doppiu and
A. Salzer, Organometallics, 2003, 22, 3164; (c) U. Radhakrishnan,
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67; (e) A. D. Burrows, M. Green, J. C. Jeffery, J. M. Lynam and
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250, C15.
Notes and references
5
g -Indenyl and tris(pyrazolyl)borate ligands have shown considerable
{
success as substitutes for Cp and Cp* ligands (see refs. 1–8).
§
reaction, the commercial [RuCl
An excess of water was found to reduce the overall yield. Prior to the
3
(H
2
O) ] was therefore treated with THF, as
n
detailed in the ESI.{
"
Crystal data for complex 3: C38
space group P2 /c, a = 23.624(2), b = 14.0305(10), c = 12.2973(8) s, b =
0.572(6)u, V = 4075.8(5) s , Z = 4, rcalc = 1.465 g cm , m = 1.034 mm
F(000) = 1864, crystal dimensions 0.24 6 0.20 6 0.14 mm, T = 140(2) K,
Mo-K
radiation, l = 0.71073 s, h = 2.59–25.03u, 228 ¡ h ¡ 28, 213 ¡
k ¡ 16, 214 ¡ l ¡ 14, 23827 reflections collected, 7185 independent
reflections, Rint = 0.0751, R [I . 2s(I)] = 0.0586, wR (all data) = 0.1557,
largest difference peak 1.199 e s , largest difference minimum
H
66Cl
4 2 2 r
O Ru , M = 898.85, monoclinic,
1
3
23
21
,
9
13 (a) M. S. Lim, J. Y. Baeg and S. W. Lee, J. Organomet. Chem., 2006,
691, 4100; (b) W. S. Han and S. W. Lee, Organometallics, 2005, 24, 997.
14 For the reaction of complex 2 with alkynes, see: (a) T. Fukuyama,
R. Yamaura, Y. Higashibeppu, T. Okamura, I. Ryu, T. Kondo and
T.-A. Mitsudo, Org. Lett., 2005, 7, 5781; (b) I. Yamaguchi, K. Osakada
and T. Yamamoto, Inorg. Chim. Acta, 1994, 220, 35.
a
1
2
2
3
2
3
2
C
1
0.987
48ClOPRu, M
7.879(3), b = 9.4034(7), c = 20.2160(16) s, b = 92.158(8)u, V =
e
s
.
CCDC 609454. Crystal data for complex 7:
1
5 (a) J. Huang, E. D. Stevens, S. P. Nolan and J. L. Petersen, J. Am.
37
H
r
= 676.24, monoclinic, space group P2 /c, a =
1
Chem. Soc., 1999, 121, 2674; (b) J. Huang, E. D. Stevens, S. P. Nolan
and J. L. Petersen, Organometallics, 1999, 121, 2674; (c) L. Luo and
S. P. Nolan, Organometallics, 1994, 13, 4781; (d) T. J. Johnson,
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B. K. Campion, R. H. Heyn and T. D. Tilley, J. Chem. Soc., Chem.
Commun., 1988, 278.
6 T. Braun, G. M u¨ nch, B. Windm u¨ ller, O. Gevert, M. Laubender and
H. Werner, Chem.–Eur. J., 2003, 9, 2516.
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W. A. Schenk, Organometallics, 1999, 18, 943; (b) U. K o¨ lle, B.-S. Kang
and U. Englert, J. Organomet. Chem., 1991, 420, 227.
3
23
21
3
396.3(6) s , Z = 4, rcalc = 1.323 g cm , m = 0.614 mm , F(000) = 1416,
crystal dimensions 0.61 6 0.34 6 0.15 mm, T = 100(2) K, Mo-K
radiation, l = 0.71073 s, h = 3.10–25.03u, 221 ¡ h ¡ 21, 211 ¡ k ¡ 11,
24 ¡ l ¡ 23, 60255 reflections collected, 5984 independent reflections,
int = 0.0744, R [I . 2s(I)] = 0.0361, wR (all data) = 0.0814, largest
difference peak 0.581 e s , largest difference minimum 20.686 e s
a
2
R
1
2
23
23
.
1
CCDC 631864. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b618712a
1
1
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This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 1837–1839 | 1839