Alkenylcarbene ruthenium arene complexes as initiators of alkene metathesis: An enyne creates a catalyst that promotes its selective transformation

Article abstract of DOI:10.1002/anie.200462865

(Chemical Equation Presented) A molecule makes its own catalyst: [RuCl(arene)(PCy₃)]OTf (e.g. 1) with allyl or propyl propargyl ethers gives alkenylcarbene derivatives 2 by a metallo-retroene reaction. The latter species are efficient initiators for catalytic ring-closing metathesis of allyl propargyl ethers into alkenylcycloalkenes and of dienes into cyclic alkenes (see scheme).

Full text of DOI:10.1002/anie.200462865

Communications
tolerance to functional groups.[1d] The moderate stability of
the coordinatively unsaturated ruthenium alkylidene catalytic
species[2] can be compensated by high efficiency of the
catalytic moiety, which allows transformation of a large
quantity of substrate under mild conditions in a short time.
Consequently, attempts are currently being made to generate
highly active catalytic species in situ.[3,4]
Ruthenium arene complexes are effective precursors of
catalytically active unsaturated alkylidene species.[3,5–8] Noels
et al. showed that activation of [{RuCl2(h6-p-cymene)}2] by
addition of a bulky phosphane and (trimethylsilyl)diazo-
=
methane gives the highly active catalyst [RuCl2( CHSiMe3)-
(PCy3)] for ring-opening metathesis polymerization (ROMP)
of cycloolefins.[5] In situ introduction of a more electron-
releasing N-heterocyclic carbene increases the catalytic
performance.[6] Allenylidene ruthenium arene complexes
promote alkene metathesis reactions,[1a,7] and their activity
has been tremendously improved by protonation to generate
a highly active indenylidene species.[8]
We now disclose the in situ generation of a new ruthenium
alkene metathesis catalyst, namely, an alkenylcarbene ruthe-
nium arene species generated from [RuCl(h6-arene)(PCy3)]X
on activation of enynes. We show that the catalyst results from
a metallo-retro-ene reaction involving a coordinated allyl or
alkyl propargylic ether and allows the ring-closing metathesis
(RCM) of enynes and dienes [Eq. (1)]. We present a
remarkable example of a smart substrate: a propargyl ether
that generates in situ a catalyst for its own selective trans-
formation.
Metathesis Catalysts
Alkenylcarbene Ruthenium Arene Complexes as
Initiators of Alkene Metathesis: An Enyne
Creates a Catalyst that Promotes Its Selective
Transformation**
The 16-electron cationic complex [RuCl(h6-p-cymene)-
(PCy3)]CF3SO3 (1), precursor of the allenylidene derivative
Ricardo Castarlenas, Matthieu Eckert, and
Pierre H. Dixneuf*
6
[7]
= = =
[RuCl(h -p-cymene)( C C CPh2)(PCy3)]CF3SO3 (2), does
not show significant activity for the RCM of diallyl tosylamide
(3; Table 1, entry 1). However, despite the fact that RCM of
enynes is usually more difficult to perform than that of dienes,
it was surprisingly found that complex 1 achieves excellent
activity in the transformation of enynes into the correspond-
ing alkenyl cycloalkenes under mild conditions [Eq. (1),
Table 1, entries 2–6].[9] Thus, when allyl propargyl ethers 5–9
were treated with 2 mol% of 1 in CH2Cl2 at room temper-
ature, alkenyl cycloalkenes 10–14 were selectively formed
with a remarkable turnover frequency (TOF) of 12–47 hÀ1.
These results are just in the range of those obtained with other
ruthenium-based olefin metathesis catalysts, as shown by
Fꢀrstner et al.[10] (1 mol% of catalyst, 808C, 80–85% yield,
0.3–1 h) and Grela et al.[11] (1 mol% of catalyst, 08C, 98%
yield, 1 h). However, the lower catalytic activity for allyl
propargyl tosylamide (15; Table 1, entry 7) confirms that allyl
propargyl ethers play an important role.
Alkene metathesis has become a powerful reaction in organic
synthesis and polymerization as it allows new approaches for
À
making crucial C C bonds and thus reduces the number of
steps in the synthesis to natural or biologically relevant
compounds.[1] This is due to the discovery of well-defined
alkylidene metal catalysts, of which those of ruthenium offer
an especially good compromise between efficiency and
[*] Dr. R. Castarlenas, M. Eckert, Prof. Dr. P. H. Dixneuf
Institut de chimie de Rennes
UMR 6509 CNRS-Universitꢀ de Rennes
Organomꢀtalliques et Catalyse
Campus de Beaulieu, 35042 Rennes (France)
Fax: (+33)2-2323-6939
E-mail: pierre.dixneuf@univ-rennes1.fr
[**] This work was supported by the CNRS, the Ministꢁre de la
Recherche, and the European Union through a Marie Curie
Fellowship to R.C.
It was first thought that the good catalytic activity
observed was due to the fact that terminal enynes can react
with 1 to form vinylidene species, as vinylidene ruthenium
intermediates are moderate olefin metathesis initiators.[12]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2576
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DOI: 10.1002/anie.200462865
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Angewandte
Chemie
Table 1: Diene and enyne RCM reactions promoted by 1 at room
Table 2: Diene RCM reactions with 2% mol catalyst at room temperature.
temperature.[a]
Entry Catalyst
Substrate
Product
t [h] Conv. [%][a] TOF [hÀ1
]
Entry
1
Substrate
Product
t [h] Conv. [%][b] TOF [hÀ1
]
1
2
3
4
5
6
17
18
20
3
3
3
5
3
3
4
4
4
10
4
4
20
20
20
1
0.25 99
0.25 95
40
35
22
99
1
0.87
0.55
49.50
198
190
20
1
3
0.07
21[b]
21[b]
24[b]
3
4
7
21[b]
4
2
99
99
12.37
2
3
4
95
47.5
5
6
7
10
11
12
25
26
28
29
8
21[b]
24.74
49.50
2.5 60
12.0
45.0
9
21[b]
1
99
1
2
90
87
27
30
[a] Determined by GC. [b] Catalyst prepared in situ from 1 and 22 or 23.
5
21.7
cannot explain the excellent catalytic activity displayed by
complex 1 in the RCM of enynes 5–9.
8
13
To elucidate the active species, a stoichiometric reaction
between complex 1 and enyne 5 was carried out. After 5 min
at room temperature, 5 was transformed into diene 10,
whereas cationic complex 1 apparently remained unchanged.
This suggested that only a small amount of complex 1 was
activated by 5 to form an undetectable highly active species
that promotes RCM of the enyne. To increase the chances of
observing the intermediate, the counteranion in 1 was
exchanged with tetrakis[3,5-bis(trifluoromethyl)phenyl]bo-
rate (BArF4À) to give the new cationic complex [RuCl(h6-p-
cymene)(PCy3)]BArF4 (19; Scheme 1). The reaction of 19 with
1 equivalent of 5 was monitored by NMR spectroscopy at
À308C. A new signal at d = 48.1 ppm appeared in the 31P{1H}
NMR spectrum showing the formation of a new complex
which, on the basis of 1H, 13C, 1H,1H COSY, and 1H,13C
6
7
3
93
70
15.5
1.5
9
14
16
24
15
[a] [Monomer]/[Ru]=50. [b] Determined by GC.
With the aim of confirming this hypothesis, new vinylidene
derivatives [RuCl(h -p-cymene){ C CH(R)}(PCy3)]CF3SO3
(R = Ph (17), CH2Ph (18)) were synthesized from 1 and
phenyl acetylene and benzyl acetylene, respectively [Eq. (2)].
6
= =
HMQC and HMBC correlation experiments, was identified
6
=
À
=
as alkenylcarbene complex [RuCl(h -p-cymene){ CH CH
C(C5H10)}(PCy3)]BArF4 (20). Accompanying the signals for 20,
three multiplets at d = 9.51, 6.25, and 6.10 ppm were observed
and shown to correspond to the presence of acrolein.
A rational mechanism for the formation of the alkenyl-
carbene complex is proposed in Scheme 1. From the initially
formed vinylidene species, a pericyclic retro-ene cleavage[13]
generates the alkenylcarbene complex and acrolein. It has
been shown that thermolysis of propargyl ethers at 350–
4508C leads to allenes and carbonyl compounds by a retro-
ene transformation.[14] Mediation of a transition metal is
expected to reduce the activation energy for the metallo-
retro-ene reaction[15] and thus allow it to occur at low
temperature (À308C).
Complexes 17 and 18 are unstable at room temperature
but were characterized at À308C. In the 13C{1H} NMR
spectrum, the most interesting feature is the low-field
doublets that appear at 358.3 (2JC,P = 20.1 Hz, 17) and
345.0 ppm (2JC,P = 19.3 Hz, 18), which correspond to the
The catalytic activity of the relatively stable BAr4FÀ
-
=
Ru C carbon atom of the vinylidene moiety. However, 17
containing complex 20 in the transformation of diene 3 into 4
remains moderate (Table 2, entry 3), likely owing to the
inhibition by BArF4À. Indeed, the nature of the escorting anion
and 18 only slightly promote the RCM of diallyl tosylamide
(TOF 0.87–1 hÀ1, Table 2, entries 1 and 2) and therefore
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Communications
Scheme 1. Formation of ruthenium alkenylcarbene complexes by a metallo-retro-ene reaction.
in ruthenium allenylidene complex 2 drastically influences its
complexes 21 and 24 could not be observed on reaction of 1
with enynes 5 or 6 owing to their high catalytic activity toward
these enynes.
catalytic activity in alkene metathesis according to the
[7]
sequence OTfÀ > PF6À > BPh4À > BArF4À
.
Analogously to the formation of 20 from 19, the reaction
of triflate-containing complex 1 with enynes 5–9 is thus
expected to generate ruthenium alkenylcarbene triflate
catalyst 21, which is much more active than the BAr4F
analogue 20 (Scheme 1). In fact, in this case the ruthenium
alkenylcarbene 21 could not be isolated or characterized from
the reaction of 1 with enyne 5. To prove that the vinylcarbene
intermediate can be generated by a retro-ene reaction
independent of the enyne substrate, stoichiometric activation
Addition of enyne 5 to a catalytic amount of 21, generated
in situ by reaction of 1 and 22, led over 1 h at room
temperature to complete formation of 10 (Table 2, entry 4)
and confirmed the formation of 21 from 1 and 5.
These in situ generated catalyst systems 21 and 24
tremendously increase olefin metathesis activity for forma-
tion of 4 from diene 3 in comparison with parent complex 1
(Table 1, entry 1 versus Table 2, entries 5 and 6). Moreover,
dienes 25–27, which are almost inert in the presence of 1, are
completely converted into 28–30 by addition of 2 mol% of 21
at room temperature (Table 2, entries 7–9).
ꢀ
of the propyl propargyl ethers of type CH3CH2CH2OCR2C
CH (R2 = C5H10 (22), R = Ph (23)), which cannot give rise to
enyne RCM reactions, was attempted with 1. At À308C in
dichloromethane the reaction led in this case to formation of
These new 18-electron, ionic alkenylcarbenes [RuCl(h6-p-
=
À
=
cymene){ CH CH CR2}(PCy3)]X strongly differ in their
propanal and the triflate-containing alkenylcarbene com-
nature and synthesis with the neutral, 16-electron
6
=
À
=
=
À
=
plexes [RuCl(h -p-cymene)( CH CH CR2)(PCy3)]CF3SO3
(R2 = C5H10 (21), R = Ph (24), Scheme 2). Both complexes
21 and 24 decompose at room temperature but could be
characterised at À308C by NMR experiments. Note that
[RuCl2( CH CH CPh2)(PCy3)2] complex arising from
diphenyl cyclopropene[16] and diphenyl propargylic chlo-
ride.[17]
The above simple reaction of ruthenium complex 1 with
allyl or propyl propargylic ethers is a simple and novel
method of preparing ruthenium alkenylcarbene complexes
and involves a metal-assisted retro-ene reaction. The catalytic
transformation of allyl propargylic ethers is a remarkable
example of a substrate that transforms in situ an inert
ruthenium complex into an active catalyst that is then able to
convert the parent enyne into a highly reorganized alkenyl-
cycloalkene. This new method of generating an active alkene
metathesis initiator from alkyl propargylic ethers has poten-
tial for the catalytic transformation of a variety of substrates
by simple variation of propargyl ethers and electron-donor
ligands in complex 1.
Scheme 2. Triflate alkenylcarbene catalysts generated from 1 and
propyl propargyl ethers.
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Angewandte
Chemie
B. L. He, Polym. Adv. Technol. 2003, 14, 226 – 231; d) R.
Experimental Section
Castarlenas, C. Fischmeister, P. H. Dixneuf, New J. Chem.
2003, 27, 215 – 217; e) C. van Schalkwyk, H. C. M. Vosloo, J. M.
Botha, J. Mol. Catal. A 2002, 190, 185 – 195.
Full experimental details and spectroscopic data are available in the
Supporting Information. Coupling constants are given in Hz.
17, 18: An orange solution of 1 (50 mg, 0.071 mmol) in CD2Cl2
(0.5 mL) in an NMR tube under argon atmosphere at 213 K was
treated with phenylacetylene (9 mL, 0.081 mmol) or benzylacetylene
(11 mL, 0.088 mmol). The color changed immediately to dark orange,
and, after sealing the NMR tube, measurements were made. Selected
data for 17: 1H NMR (300 MHz, CD2Cl2, 243 K): d = 5.93 ppm (s, 1H,
[5] A. Demonceau, A. W. Stumpf, E. Saive, A. F. Noels, Macro-
molecules 1997, 30, 3127 – 3136.
[6] D. Sꢂmeril, C. Bruneau, P. H. Dixneuf, Helv. Chim. Acta 2001,
84, 3335 – 3341.
[7] A. Fꢀrstner, M. Liebl, C. W. Lehmann, M. Picquet, R. Kunz, C.
Bruneau, D. Touchard, P. H. Dixneuf, Chem. Eur. J. 2000, 6,
1847 – 1857.
[8] R. Castarlenas, P. H. Dixneuf, Angew. Chem. 2003, 115, 4662 –
4665; Angew. Chem. Int. Ed. 2003, 42, 4524 – 4527.
[9] For recent reviews on enyne metathesis, see a) S. T. Diver, A. J.
Giessert, Chem. Rev. 2004, 104, 1317 – 1382; b) C. S. Poulsen, R.
Madsen, Synthesis 2003, 1 – 18; c) D. Sꢂmeril, C. Bruneau, P. H.
Dixneuf, Adv. Synth. Catal. 2002, 344, 585 – 595.
[10] A. Fꢀrstner, L. Ackermann, B. Gabor, R. Goddard, C. W.
Lehmann, R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J. 2001,
7, 3236 – 3253.
[11] K. Grela, S. Harutyunyan, A. Michrowska, Angew. Chem. 2002,
114, 4210 – 4212; Angew. Chem. Int. Ed. 2002, 41, 4038 – 4040.
[12] a) T. Opstal, F. Verpoort, J. Mol. Catal. A 2003, 200, 49 – 61; b) H.
Katayama, H. Urushima, F. Ozawa, J. Organomet. Chem. 2000,
606, 16 – 25; c) H. Katayama, T. Yoshida, F. Ozawa, J. Organo-
met. Chem. 1998, 562, 203 – 206.
[13] a) J. Dubac, A. Laporterie, Chem. Rev. 1987, 87, 319 – 334; b) W.
Oppolzer, V. Snieckus, Angew. Chem. 1978, 90, 506 – 516;
Angew. Chem. Int. Ed. Engl. 1978, 17, 476 – 486.
[14] a) P. Vꢅrnai, G. M. Keserꢀ, J. Org. Chem. 1996, 61, 5831 – 5836;
b) A. Viola, J. J. Collins, N. Filipp, Tetrahedron 1981, 37, 3765 –
3811.
C CH). 13C{1H} NMR (75.4 MHz, CD2Cl2, 243 K): d = 358.3 (d,
= =
=
= =
J
C,P = 20.1, Ru C), 118.4 ppm (s, Ru C C). Selected data for 18:
1H NMR (300 MHz, CD2Cl2, 243 K): d = 5.17 (dd, JH,H = 9.9, 6.8, 1H,
= =
=
C CH), 3.95 (dd, JH,H = 14.6, 6.8, 1H, C(CH2Ph)), 3.60 ppm (dd,
=
J
H,H = 14.6, JH,H = 9.9, 1H, C(CH2Ph)). 13C{1H} NMR (75.4 MHz,
=
CD2Cl2, 243 K): d = 345.0 (d, JC,P = 19.3 Hz, Ru C), 112.1 ppm (s,
= =
Ru C C).
20: Aviolet solution of 19 (300 mg, 0.21 mmol) in CH2Cl2 (10 mL)
at 243 K was treated with allyl 1-ethynylcyclohexyl ether (40 mg,
0.24 mmol), and the reaction mixture was stirred for 2 h. The resulting
orange solution was evaporated to dryness below 243 K, and the
residue was washed three times with cold pentane/diethyl ether (5/
1 mL). The yellow solid was dried under vacuum (270 mg, 84% yield).
1
Selected NMR data: H NMR (300 MHz, CD2Cl2, 243 K): d = 16.28
=
=
À
(d, JH,H = 14.1, 1H, Ru CH), 8.12 ppm (d, JH,H = 14.1, 1H, Ru CH
CH ). 13C{1H} NMR, (75.4 MHz, CD2Cl2, 243 K): d = 302.3 (d, JC,P
=
=
=
=
À
=
18.7, Ru C), 173.8 ppm (s, C(C5H10)), 146.3 ppm (s, C CH C).
21, 24: An orange solution of 1 (50 mg, 0.071 mmol) in CD2Cl2
(0.5 mL) in an NMR tube under argon atmosphere at 213 K was
treated with propyl 1-ethynylcyclohexyl ether (22; 12 mg,
0.072 mmol) or propyl 1,1-diphenylethynyl ether (23; 18 mg,
0.072 mmol). Measurements were made immediately. Selected data
for 21: 1H NMR (300 MHz, CD2Cl2, 243 K): d = 16.12 (d, JH,H = 13.8,
1H, Ru CH), 8.10 ppm (d, JH,H = 13.8, 1H, Ru CH CH ). 13C{1H}
[15] W. Oppolzer, Angew. Chem. 1989, 101, 39 – 53; Angew. Chem.
Int. Ed. Engl. 1989, 28, 38 – 52.
[16] S. T. Nguyen, L. K. Johnson, R. H. Grubbs, J. W. Ziller, J. Am.
Chem. Soc. 1992, 114, 3974 – 3975.
[17] T. E. Wilhelm, T. R. Belderrain, S. N. Brown, R. H. Grubbs,
Organometallics 1997, 16, 3867 – 3869.
=
=
À
=
=
NMR (75.4 MHz, CD2Cl2, 243 K): d = 299.9 (d, JC,P = 17.4, Ru C),
=
À
=
172.5 (s, C(C5H10)), 147.2 ppm (s, C CH C). Selected data for 24:
1H NMR (300 MHz, CD2Cl2, 243 K): d = 15.28 (d, JH,H = 13.7, 1H,
Ru CH), 8.56 ppm (d, JH,H = 13.7, 1H, Ru CH CH C). 13C{1H}
=
=
=
À
=
NMR (75.4 MHz, CD2Cl2, 243 K): d = 300.1 (d, JC,P = 19.5, Ru C),
=
À
=
159.2 (s, CPh2), 146.4 ppm (s, C CH C).
Received: December 9, 2004
Published online: March 22, 2005
Keywords: ene reaction · enynes · homogeneous catalysis ·
.
metathesis · ruthenium
[1] Recent reviews on alkene metathesis: a) R. Castarlenas, C.
Fischmeister, C. Bruneau, P. H. Dixneuf, J. Mol. Catal. A 2004,
213, 31 – 37; b) R. R. Schrock, A. H. Hoveyda, Angew. Chem.
2003, 115, 4740 – 4782; Angew. Chem. Int. Ed. 2003, 42, 4592 –
4633; c) S. J. Connon, S. Blechert, Angew. Chem. 2003, 115,
1944 – 1968; Angew. Chem. Int. Ed. 2003, 42, 1900 – 1923;
d) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18 – 29.
[2] M. S. Sanford, J. A. Love, R. H. Grubbs, J. Am. Chem. Soc. 2001,
123, 6543 – 6554.
[3] a) B. C¸ etinkaya, S. Demir, I. ꢁzdemir, L. Toupet, D. Sꢂmeril, C.
Bruneau, P. H. Dixneuf, Chem. Eur. J. 2003, 9, 2323 – 2330; b) R.
Castarlenas, I. Alaoui-Abdallaoui, D. Sꢂmeril, B. Mernari, S.
Guesmi, P. H. Dixneuf, New J. Chem. 2003, 27, 6 – 8; c) A.
Hafner, A. Mꢀhlebach, P. A. van der Schaaf, Angew. Chem.
1997, 109, 2213 – 2216; Angew. Chem. Int. Ed. Engl. 1997, 36,
2121 – 2124.
[4] a) B. Schmidt, Eur. J. Org. Chem. 2004, 1865 – 1880; b) J.
Le Nꢃtre, A. A. Martꢄnez, P. H. Dixneuf, C. Bruneau, Tetrahe-
dron 2003, 59, 9425 – 9432; c) H. Li, W. F. Zhang, Y. Q. Wang,
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