Convenient syntheses of novel ruthenium catalysts bearing N-heterocyclic carbenes

Article abstract of DOI:10.1016/S0022-328X(99)00435-0

The 16-electron ruthenium(II) complexes Cp*Ru[C(R)N(H)C=C(H)N(R)]Cl (Cp=η⁵-C₅Me₅; R=Cy (ICy), 1a; Mes (IMes), 1b) containing N-heterocyclic carbenes are easily accessible in quantitative yields from [Cp*Ru(OMe)]₂ (Me=CH₃) and the corresponding 1,3-diorganylimidazolium chloride by methanol elimination. Compounds 1a-b can also be prepared in 75-80% yield by treating the commercially available polymeric ruthenium(III) compound [Cp*RuCl₂]_n with the free 1,3-diorganylimidazolin-2-ylidenes in 1 to 1.5 molar amounts. 1a reacts with CO, PPh₃, pyridine and ethyl diazoacetate (EDA) affording the 18-electron derivatives Cp*Ru(ICy)(L)Cl (L=CO, 2; PPh₃, 3; py, 4; CHCO₂Et, 5). The mixed dicarbene complex 5 is the first isolable ruthenium cyclopentadienyl species bearing a CHCO₂Et moiety. Compounds 1a-b catalyze the carbon-carbon coupling of terminal alkynes HC?CR (R=Ph, SiMe₃, ^tBu, p-Tol) under mild conditions, with the selectivity strongly depending on the substituent R.

Full text of DOI:10.1016/S0022-328X(99)00435-0

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 593–594 (2000) 489–493
Communication
Convenient syntheses of novel ruthenium catalysts bearing
ꢀ
N-heterocyclic carbenes
a,1
b,
a
b
Walter Baratta , Wolfgang A. Herrmann *, Pierluigi Rigo , J u¨ rgen Schwarz
a
Dipartimento di Scienze e Tecnologie Chimiche, Uni6ersit a` di Udine, 6ia Cotonificio 108, I-33100 Udine, Italy
b
Anorganisch-chemisches Institut, Technische Uni6ersit a¨ t M u¨ nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany
Received 9 August 1999; accepted 23 September 1999
Abstract
The 16-electron ruthenium(II) complexes Cp*Ru[C(R)N(H)CꢀC(H)N(R)]Cl (Cp*=h -C Me ; R=Cy (ICy), 1a; Mes (IMes),
¸¹¹¹¹¹¹¹¹¹º
5
5
5
1
b) containing N-heterocyclic carbenes are easily accessible in quantitative yields from [Cp*Ru(OMe)]2 (Me=CH ) and the
3
corresponding 1,3-diorganylimidazolium chloride by methanol elimination. Compounds 1a–b can also be prepared in 75–80%
yield by treating the commercially available polymeric ruthenium(III) compound [Cp*RuCl ] with the free 1,3-diorganylimida-
2
n
zolin-2-ylidenes in 1 to 1.5 molar amounts. 1a reacts with CO, PPh , pyridine and ethyl diazoacetate (EDA) affording the
3
1
8-electron derivatives Cp*Ru(ICy)(L)Cl (L=CO, 2; PPh , 3; py, 4; CHCO Et, 5). The mixed dicarbene complex 5 is the first
3 2
isolable ruthenium cyclopentadienyl species bearing a CHCO Et moiety. Compounds 1a–b catalyze the carbonꢁcarbon coupling
2
t
of terminal alkynes HCꢂCR (R=Ph, SiMe , Bu, p-Tol) under mild conditions, with the selectivity strongly depending on the
3
substituent R. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Ruthenium; Carbenes; Alkyne coupling; Homogeneous catalysis
vast number of publications on cyclopentadienyl ruthe-
1
. Introduction
5
nium complexes of general formula (h -C R )Ru(L) X
5
5
2
(
R=H, Me; L=phosphine, alkene; X=halogen),
Coordinatively unsaturated ruthenium complexes
there are very few reports dealing with their application
continue to attract a great deal of interest because of
their implications in manifold catalytic transformations
involving CꢁC and CꢁH bond formation [1]. Free
N-heterocyclic carbenes, having bulky substituents,
have recently been shown to be suitable ligands to
afford highly active catalysts with several transition
metals [2]. As regard to ruthenium chemistry, the com-
plexes RuCl (L) (ꢀCHPh) (L=imidazolin-2-ylidene,
5
in catalysis [4]. Recently we have shown that (h -
C H )Ru(PPh ) Cl catalyzes the stereoselective forma-
5
5
3 2
tion of cis-enediones from a-diazo carbonyl compounds
via displacement of one phosphine [5]. Although
Cp*Ru(L)X type 16-electron complexes have been iso-
lated using bulky phosphino ligands [6], the employ-
ment of sterically demanding N-heterocyclic carbenes
has not been considered until a very recent communica-
tion [3c].
2
2
phosphine) have been found to be very efficient cata-
lytic precursors for olefin metathesis [3]. In spite of the
We now describe two simple routes to prepare the
highly reactive, coordinatively unsaturated half-sand-
ꢀ
Communication 24 of the series: ‘N-Heterocyclic Carbenes’; Pre-
¸¹¹¹¹¹¹¹¹¹¹º
wich derivatives Cp*Ru[C(R)N(H)CꢀC(H)N(R)]Cl
(R=Cy, 1a; Mes, 1b) on a gram scale. Stoichiometric
reactions of 1a–b with several ligands as well as cata-
lytic transformations of alkynes promoted by 1a–b are
reported.
ceding paper: M. Prinz, M. Grosche, E. Herdtweck, W.A. Herrmann,
Organometallics (1999) submitted.
*
Corresponding author.
E-mail addresses: inorg@dstc.uniud.it (W. Baratta), her-
mann.jom@ch.tum.de (W.A. Herrmann)
1
Also corresponding author.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00435-0

490
W. Baratta et al. / Journal of Organometallic Chemistry 593–594 (2000) 489–493
2
. Results and discussion
at 93.5 ppm for the CO and Cp* ligands, respectively.
31
The P-NMR spectrum of 3 displays a resonance at
13
2.1. Preparation and reacti6ity of 1a–b
47.6 ppm and in the C-NMR spectrum the Cp* ligand
appears as a doublet at 84.7 ppm (J(C, P)=2.0 Hz) at
13
The 16-electron complexes 1a–b can be easily pre-
25°C. The C-NMR spectrum of 4 (C D , 25°C) shows
6
6
pared by two different synthetic pathways. Treatment
a single resonance for the two NCH carbons of the
of a THF solution of the di-m-methoxo derivative
imidazolin-2-ylidene based ring system at l 117.7, indi-
[
Cp*Ru(OMe)] [7] with the corresponding 1,3-dior-
cating that rotation around the RuꢁC
bond is
carbene
relatively fast on the C-NMR time scale. By way of
2
13
ganylimidazolium chloride affords compounds 1a–b, as
a result of methanol elimination. 1a–b are formed
quantitatively, as monitored by NMR, and were iso-
lated in \85% yields after extraction with diethyl ether
contrast, no reaction of 1a with ethylene (1 atm) and
dihydrogen (1 atm) was observed in C D6 at room
6
temperature. In line with our previous findings that
5
(
Scheme 1).
(h -C H )Ru(PPh )(ꢀCHCO Et)Cl is the key intermedi-
5
5
3
2
ate for the selective carbene transfer reactions from
EDA [5], we succeeded in preparing a mixed dicarbene
complex, which contains both one electrophilic and one
N-heterocyclic carbene ligand. Thus, reaction of 1a in
toluene at −10°C with EDA affords the complex
Cp*Ru(ICy)(ꢀCHCO Et)Cl) (5), which was isolated
2
and characterized as an analytically pure compound
(
Scheme 2).
Scheme 1. Syntheses of the coordinatively unsaturated complexes
5
1
a–b (Cp*=h -C Me , Cy=cyclohexyl, Mes=mesityl).
5
5
Alternatively, compounds 1a–b can also be prepared
5
by the reaction of commercially available [(h -
C Me )RuCl ] with 1.5 equivalents of the correspond-
5
5
2 n
ing free carbenes to give 1a–b in \75% yield. In this
case, the free carbenes act both as strong coordinating
ligands and reducing reagents [8]. The remarkable
1
3
highfield C-NMR resonances of the Cp* ring carbon
atoms (l=73.3, 1a; 73.0, 1b) are similar to those
reported for other coordinatively unsaturated half-
sandwich ruthenium complexes [3c,6], suggesting that
Scheme 2. Reactivity of 1a towards ethyl diazoacetate (EDA).
1
13
The H- and C-NMR spectra of 5 show resonances
1
1a–b are apparently monomeric in solution. Although
for the RuꢀCH moiety at l 16.0 ( H-NMR) and 260.6
13
complexes 1a–b exhibit relatively low air sensitivity in
the solid state, they are highly soluble in toluene,
affording deep blue solutions, which promptly turn
brown if air is admitted. According to the coordinative
unsaturation of 1a–b, these compounds promptly react
ppm ( C-NMR), respectively, while the ring carbon
atoms of the Cp* appear at 101.1 ppm. Whereas com-
pound 5 is indefinitely stable in solid state, it slowly
reacts in solution (C D ) to give 1a and 0.5 equiv. of
6 6
free diethyl maleate within 2 days at room temperature,
as a result of a stereoselective carbene degradation
process yielding the cis olefin only.
with CO, PPh and pyridine to give quantitatively the
3
corresponding 18-electron complexes Cp*Ru(ICy)(L)Cl
(
L=CO, 2; PPh , 3; Py, 4), which were isolated and
3
fully characterized (Eq. (1)) [9].
2.2. Catalytic alkyne dimerization
The transition metal mediated dimerization of termi-
nal alkynes is of considerable interest because it can
lead to
a wide variety of organic enyne and
oligoacetylene products, that are useful synthetic pre-
cursors for organic conducting polymers and other
carbon-rich derivatives [4,10]. However, its synthetic
application in organic synthesis has been limited due to
low selectivity on dimeric products [11]. It has been
reported that dimerization of alkynes and coupling
reactions of alkynes with alkenes occur in presence of
(
1)
The carbonyl derivative 2 exhibits a w
frequency at 1914 cm
stretching
(Nujol). The C-NMR spec-
trum of 2 in C D at 70°C reveals signals at l 298.5 and
CO
−
1
13
6
6

W. Baratta et al. / Journal of Organometallic Chemistry 593–594 (2000) 489–493
491
5
catalytic amounts of (h -C R )Ru(L) X (L=phos-
for phenylacetylene as the most active substrate. By
contrast, no catalytic reaction with 1a or 1b occurred in
5
5
n
m
phine, vinylidene; X=H, Cl) type complexes through
dissociation of one ligand [4f–i]. Furthermore,
ruthenacyclopentadienes are obtained from acetylene
with (h -C R )Ru(P Pr )Cl or PhCꢂCH with (h -
C H )Ru(COD)Cl as results of stoichiometric car-
bonꢁcarbon coupling reactions [12]. Therefore our
investigation on the catalytic activity of 1a–b with
alkynes was substantially mandated.
When a toluene solution of phenylacetylene is treated
with 1 mol% of 1a under an argon atmosphere at room
temperature, quantitative conversion occurs within 5
minutes in a very fast and exothermic reaction to give
the dimeric coupling products I–III, the derivative I
being the major product present in solution (Eq. (2)).
the case of primary alkynes such as PhCH CꢂCH. It is
2
noteworthy that when 1b, bearing two mesityl sub-
stituents, is used instead of 1a, an inverse selectivity was
observed for R=Ph and R=p-Tol (entries 7 and 8). In
all cases the head-to-tail coupling product III is formed
with very high selectivity, when complex 1b is em-
ployed. In addition, when catalysts 1a–b are employed
with terminal alkynes bearing electron withdrawing
groups, HCꢂCCOR (R=OMe, OEt, H) or with inter-
5
i
5
5
5
3
5
5
nal alkynes RO CCꢂCCO R (R=Me, Et), the
2
2
chemoselectivity completely changes and benzene
derivatives are obtained in almost quantitative yields.
Although the detailed mechanism and the nature of the
intermediate species have not been clearly elucidated, it
is likely that the carbonꢁcarbon coupling products form
via a ruthenacyclopentadiene complex similar to that
isolated by Yi and co-workers, from Cp*Ru(PPh ) Cl
3
2
and acetylene by displacement of one phosphine [4e].
In summary, employment of the easily accessible
[Cp*Ru(OMe)]2 and the corresponding imidazolium
chloride salts offers a general entry into the chemistry
of this new class of ruthenium cyclopentadienyl com-
plexes. Sterically demanding 1,3-disubstituted N-heter-
cocyclic carbenes as strong |- and p-donors stabilize
the 16-electron coordinatively unsaturated species by
preventing dimerization or solvent coordination. Com-
plexes 1a–b represent one of the most active catalyst
systems for alkyne dimerization reported to date. The
application of these highly reactive compounds in other
carbonꢁcarbon forming processes and carbene transfer
reactions from diazo compounds are currently under
investigation.
(
2)
To explore the scope of this catalytic reaction, the
dimerization of other terminal alkynes was investigated
under similar reaction conditions. Both conversion and
selectivity are strongly dependent on the alkyne as well
as the N-heterocyclic carbene substituents (Table 1).
For R=SiMe (1a, entry 4) the selectivity is reversed
3
t
with respect to R=Ph, p-Tol and Bu (entries 1–3, 5,
3. Experimental
6) giving III (92%) as the main coupling product and
only 8% of I. A maximum turn over frequency of 10320
All reactions were carried out with dried solvents
under an argon atmosphere using standard Schlenk
−
1
h
(turn over number=860) (entry 3) was achieved
Table 1
Catalytic dimerization of terminal alkynes
Entry
Catalyst (mol%)
R
Time
Product ratio I:II:III
Conversion a (%)
1
2
3
4
5
6
7
8
9
1a (1.0)
1a (0.5)
1a (0.1)
1a (1.0)
1a (1.0)
1a (1.0)
1b (1.0)
1b (1.0)
1b (1.0)
Ph
Ph
Ph
5 min
5 min
5 min
2 h
10 min
5 min
2 h
76:16:8
76:16:8
76:16:8
8:0:92
90:0:10
44:38:18
0:10:90
0:4:96
100
100
86
100
100
100
95
SiMe
3
t
Bu
p-Tol
Ph
p-Tol
SiMe3
2 h
24 h
95
22
0:0:100
a
Product yields were determined by GC–MS using diethylene glycol-n-butylether as an internal standard.

492
W. Baratta et al. / Journal of Organometallic Chemistry 593–594 (2000) 489–493
1
techniques. The ruthenium complex [Cp*Ru(OMe)]2
13], 1,3-diorganylimidazolium chlorides and free 1,3-
Yield: 1.92 g (3.33 mmol, 89%); H-NMR (200.1
[
MHz, C D , 25°C): l 6.75 (s, 2H, Mes), 6.20 (s, 2H,
6
6
diorganylimidazolin-2-ylidenes were prepared accord-
ing to the literature procedures [14]. [Cp*RuCl ] and
NCH), 2.27 (br, 12H, o-CH3 of Mes), 2.12 (s, 6H,
13
p-CH of Mes), 1.23 (s, 15H, C Me ). C-NMR (50.3
2
n
3
5
5
the other chemicals were purchased from Aldrich and
used without further purification. The NMR spectra
MHz, C D , 25°C): l 200.2 (NCN), 138.1, 137.5, 136.9
6 6
and 130.3 (4×Mes), 123.1 (NCH), 73.0 (C Me ), 21.0,
5
5
1
were recorded on a Bruker AC 200 instrument; the H
20.0 and 19.0 (CH of Mes), 10.8 (C Me ). Found: C,
3 5 5
1
3
and C chemical shifts, in ppm, are relative to TMS,
64.81; H, 6.78; N, 4.93. Anal. Calc. for C H N ClRu
31 39 2
(576.19): C, 64.62; H, 6.82; N, 4.86.
3
1
while H PO is used for P. Mass spectra were per-
3
4
formed at the TU M u¨ nchen Mass Spectrometry Labo-
ratory on a Finnigan MAT 90 spectrometer using the
CI (isobutane) technique. All elemental analyses were
carried out with a Carlo Erba 1106 elemental analyzer.
3.2.2. Method 2
A suspension of [Cp*RuCl ] (300 mg, 0.98 mmol) in
2
n
THF (6 ml) was treated with a solution of 1,3-dime-
sitylimidazolin-2-ylidene (1.46 mmol) in THF (0.70 ml)
and stirred at room temperature for 3 h, during this
time the reaction mixture turned deep blue from being
initially brown. The product was worked up as de-
scribed in method 1. Yields: 443 mg (0.77 mmol, 79%).
¸
¹¹¹¹¹¹¹¹¹¹º
3
.1. Synthesis of Cp*Ru[C(Cy)N(H)CꢀC(H)N(Cy)]Cl
(
1a)
3.1.1. Method 1
A suspension of [Cp*Ru(OMe)] (1.00 g, 1.87 mmol)
2
3
.3. Synthesis of
and 1,3-dicyclohexylimidazolium chloride (1.26 g, 4.67
mmol) in THF (20 ml) was stirred at 45°C for 1 h.
THF was removed in vacuo and the product was
extracted with 30 ml of diethyl ether. Further purifica-
tion could be achieved by recrystallization from hot
diethyl ether.
¸¹¹¹¹¹¹¹¹¹¹º
Cp*Ru[C(Cy)N(H)CꢀC(H)N(Cy)](=CHCO Et)Cl (5)
2
A toluene solution (4 ml) of 1a (200 mg, 0.26 mmol)
was cooled at −10°C and treated with EDA (89 mg,
0
vent was removed in vacuo. Pentane (8 ml) was added
to the oily residue to give the product as a brown solid.
Yield: 112 mg (0.19 mmol, 73%); H-NMR (C D ,
25°C, ppm): l 16.04 (s, 1H, CHCO
CH of NC H ), 6.73 (bs, 2H, NCH), 3.67 (q, 2H,
CH CH , J(H, H)=7.6 Hz), 1.84–0.73 (br, 23H, CH
2
of NC H and CH CH ), 1.25 (s, 15H, CH of
.78 mmol). After the N -evolution stopped, the sol-
2
1
Yield: 1.64 g (3.25 mmol, 87%); H-NMR (200.1
MHz, C D , 25°C): l 6.51 (s, 2H, NCH), 4.39 (br, 2H,
6
6
1
6
6
CH of NCy), 2.13–0.72 (br, 20H, CH of NCy), 1.65
2
1
5 5 6 6
3
2
Et), 7.45 (bs, 2H,
(
1
s, 15H, C Me ). C-NMR (50.3 MHz, C D , 25°C): l
6
11
96.6 (NCN), 116.9 (NCH), 73.4 (C Me ), 59.5 (NCH
5 5
3
2
3
of NCy), 34.8, 34.0, 26.3, 26.0 and 25.7 (CH of NCy),
2
6
11
13
2
3
3
1
1.8 (C Me ). Found: C, 59.84; H, 7.69; N, 5.71. Anal.
5 5
C Me ).
C-NMR (C D , 25°C, ppm):
l
260.6
5
5
6
6
Calc. for C H N ClRu (504.12): C, 59.56; H, 7.80; N,
5
5
39
2
(
CHCO Et), 181.7 (CO), 171.1 (NCN), 119.9 and
2
5
.56.
1
18.5 (NCH), 101.1 (C Me ), 60.9 and 58.8 (NCH of
5 5
NC H ), 59.9 (CH CH ). 35.3, 35.0, 33.8, 33.5, 33.9,
6
11
2
3
3
.1.2. Method 2
A suspension of [Cp*RuCl ] (300 mg, 0.98 mmol) in
2
6.3, 26.2, 25.9, 25.5 and 25.1 (CH of NC H ), 13.9
2 6 11
2
n
(
CH CH ), 10.0 (CH of C Me ). Found: C, 59.65; H,
2 3 3 5 5
THF (6 ml) was treated with a solution of 1,3-dicyclo-
hexylimidazolin-2-ylidene (1.46 mmol) in THF (0.70
ml) and stirred at room temperature for 3 h, during
this time the reaction mixture turned deep blue from
being initially brown. The product was worked up as
described in method 1. Yield: 375 mg (0.74 mmol,
7
.73; N, 4.97. Anal. Calc. for C H N O ClRu: C,
29 45 2 2
5
9.02; H, 7.68; N, 4.75.
3.4. General procedure for dimerization catalysis
The solutions for alkyne dimerization studies were
7
6%).
typically prepared as follows: the alkyne (0.1 mmol),
internal standard (diethylene glycol-n-butylether) and
catalyst were added to 3 ml of toluene under an argon
atmosphere. The reaction progress was monitored by
the removal of a small aliquot of the reaction mixture
which was analyzed by GC–MS. After the reaction
time (Table 1), the reaction was quenched by exposure
of the reaction mixture to air and a small aliquot was
taken for GC–MS analysis. Products were identified
by comparisons with authentic samples. In addition,
the structures of the dimeric products were unequivo-
cally established by spectroscopic methods.
3
.2. Synthesis of
¸
¹¹¹¹¹¹¹¹¹¹¹¹º
Cp*Ru[C(Mes)N(H)C=C(H)N(Mes)]Cl (1b)
3
.2.1. Method 1
A suspension of [Cp*Ru(OMe)] (1.00 g, 1.87 mmol)
,3-dimesitylimidazolium chloride (1.59 g, 4.67 mmol)
2
1
in THF (20 ml) was stirred at 45°C for 1 h. THF was
removed in vacuo and the product was extracted with
3
0 ml of diethylether. Further purification could be
achieved by recrystallization from hot heptane.

W. Baratta et al. / Journal of Organometallic Chemistry 593–594 (2000) 489–493
493
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Acknowledgements
We thank the Consorzio Universitario del Friuli in
sponsorship of a fellowship for J.S. and we gratefully
acknowledge the financial support from Ministero della
Ricerca Scientifica
Bayerische Forschungsstiftung (FORKAT) and the
Fonds der Chemischen Industrie.
e Tecnologica (MURST), the
[
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