Selective terminal alkyne metathesis: Synthesis and use of a unique triple bonded dinuclear tungsten alkoxy complex containing a hemilabile ligand

Article abstract of DOI:10.1002/adsc.200700104

The in situ synthesis of new alkyne metathesis catalysts is described, with particular emphasis on the search for tris-alkoxytungsten-based terminal alkyne metathesis. In that context, hemilabile, ether-containing alkoxy ligands have proved to be suitable and have led to the design and use of a sterically hindered hemilable ligand for the synthesis of a well-defined binuclear, triple-bonded W ≡ W complex. This complex is shown to be a highly active and selective catalyst precursor for terminal alkyne metathesis, and allows the unprecedented metathesis of phenylacetylene.

Full text of DOI:10.1002/adsc.200700104

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DOI: 10.1002/adsc.200700104
Selective Terminal Alkyne Metathesis: Synthesis and Use of a
Unique Triple Bonded Dinuclear Tungsten Alkoxy Complex
Containing a Hemilabile Ligand
a
a
a
a,
Olivier Coutelier, Guy Nowogrocki, Jean-FranÅois Paul, and AndrØ Mortreux *
a
UnitØ de Catalyse et Chimie du Solide, UMR CNRS 8181, ENSCL, BP 90108, 59652 Villeneuve d’Ascq CØdex, France
Fax : (+33)-3-2043-6585; e-mail: andre.mortreux@ensc-lille.fr
Received: February 21, 2007; Revised: July 13, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The in situ synthesis of new alkyne meta-
thesis catalysts is described, with particular empha-
sis on the search for tris-alkoxytungsten-based ter-
minal alkyne metathesis. In that context, hemilabile,
ether-containing alkoxy ligands have proved to be
suitable and have led to the design and use of a
sterically hindered hemilable ligand for the synthe-
sis of a well-defined binuclear, triple-bonded Wꢀ
W complex. This complex is shown to be a highly
active and selective catalyst precursor for terminal
alkyne metathesis, and allows the unprecedented
metathesis of phenylacetylene.
stituted compounds via a two-step procedure involv-
ing first a triple bond metathesis reaction then a ste-
reoselective hydrogenation. At the same time the syn-
thesis of aromatic polyynes has been realized by the
polymerization
diynes.
of
a,w-aromatic
disubstituted
[
5,6]
However, the extension of this chemistry to termi-
nal alkynes has been far less studied: early work by
Schrock using his well-defined, highly active, (t-
BuO) WꢀC-t-Bu catalyst 1 has shown that phenyl-
3
[
7]
acetylene only polymerized. Using the same catalyst
we have demonstrated some promising results on
alkyl-monosubstituted alkynes using diethyl ether as
[
8]
Keywords: alkynes; catalysis; metathesis; terminal
solvent.
alkynes; tungsten
More recent results in our group have shown that
the addition of an external ligand such as quinuclidine
to complex 1 could improve the selectivity: an 80%
metathesis selectivity was attained using hept-1-yne as
substrate.
Based on the idea that coordination at the metal
[
9]
Metathesis, in particular alkene metathesis, has re-
ceived intense interest in recent years and has now centre was responsible for this improvement in selec-
become a powerful tool in organic and polymer syn- tivity, we decided to look at the synthesis of new cata-
[1]
thesis. Extensive searches for new active carbene lysts where ligand modification would give rise to var-
complexes have resulted in the discovery of well-de- iable coordination within the first coordination sphere
fined catalysts which now have application in diverse of the metal by using a hemilabile ligand.
fields of chemistry such as total organic synthesis and
polymerization.
First experiments were made using a procedure by
which ligand variation has been screened via the in
[2]
The related area of alkyne metathesis has been situ synthesis of tungstenacarbyne species, followed
[
3]
known for more than three decades and has re- by catalytic reactions using hept-1-yne as substrate:
[
4]
ceived renewed interest during the last decade. In the metathetic reaction of 3 equivalents of alcoholates
organic syntheses, this reaction has been used mostly with Cl
(DME)WꢀC-t-Bu (Scheme 1) was used for
AHCTREUNG
3
for the controlled synthesis of E- or Z-olefinic disub- these screening experiments (Table 1).
Scheme 1. In situ synthesis of tungsten carbyne for hept-1-yne metathesis.
Adv. Synth. Catal. 2007, 349, 2259 – 2263
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Olivier Coutelier et al.
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Table 1. Screening of alcoholates as ligands for hept-1-yne
[a]
metathesis.
Entry Alcoholate
Metathesis Polymerization
Yield [%]
of 4
Yield [%]
[
b]
1
2
3
t-BuONa
MeO(CH ) ONa
40
12
56
44
71
ACHTREUNG
2
2
c
MeOCH CH(Me)ONa 24
2
[
a]
Reactions were carried out using hept-1-yne (190 mL,
,45 mmol), decane (100 mL), solvent toluene (5 mL), 3.5
1
mol% catalyst, T 808C, reaction time 1 min.
Determined by GC using n-decane as internal standard.
Reaction time 5 min.
[
[
b]
c]
Figure 1. ORTEP view of the structure of complex 2, ellip-
soids at 30% probability, hydrogens are omitted for clarity
The best selectivity was obtained with the tert-
butoxy ligand corresponding to the in situ synthesis of
complex 1 (entry 1).
(
tungsten in black, oxygen in white, carbon in grey).
However, whereas sterically unhindered alkoxy- complex
substituted tungstenacarbyne complexes are unknown (Scheme 2).
as alkyne metathesis catalysts, a metathetic reaction is Crystallization of the product by slow evaporation
W
A
T
E
N
(MMPO)
2
virtually quantitatively
2
6
observed upon using b-methoxy-substituted alcohol- of a concentrated toluene solution gives analytically
ate ligands: the hemilability of the ether function is pure black crystals, suitable for X-ray diffraction anal-
[
13]
undoubtedly one of the major factors governing this ysis (Figure 1).
selectivity improvement.
The length of the WꢀW triple bond, 2.430(8) Š, is
This result can be related to others obtained in comparable to that in similar complexes in the litera-
[
14]
alkene metathesis, where the introduction of ether ture. The ether function of one of the ligands coor-
functions in the coordination sphere of the ligand in dinates the other tungsten atom with a WÀO length
well-defined ruthenacarbene species has resulted in of 2.257(3) Š, vs. 1.957(3) Š, 1.897(3) Š, and
[
10]
improved activity and stability of the catalyst.
1.920(3) Š for the three WÀO bonds to non-bridging
All attempts to isolate pure tungsten complexes ligands. Each tungsten atom adopts a square pyrami-
using the above salt metathesis procedure failed: dal geometry imposed by the topology of the ligand,
therefore we next decided to change the in situ proce- which differs from the classic tetrahedral geometry
dure for catalyst synthesis. Using the tungsten com- usually observed in X WꢀWX species.
3
3
plex W
A
C
H
T
R
E
U
N
G
(NMe ) as starting material, dinuclear com-
Complex 2 catalyzes internal alkyne metathesis,
2
2 6
plexes (RO) WꢀW(OR) can be synthesized via since non-4-yne is readily transformed at 258C into
3
3
[
11]
direct protonolysis using an excess of alcohol. The metathesis compounds at equilibrium (Table 2). 1-
interest in this reaction relies on the fact that not only Phenyl-1-propyne requires a higher temperature to be
does the synthesis yield volatile dimethylamine as by- metathesized and a benzylidyne carbyne could be
1
3
product, but also that such WꢀW complexes are characterized by C NMR spectroscopy with a chemi-
known to react with disubstituted alkynes, readily cal shift at 273.89 ppm (JC,W =277.17 Hz) for the car-
giving the mononuclear carbyne propagating spe- byne and 148.53 ppm (JC,W =49.25 Hz) for the ipso
[12]
cies.
Reacting W
carbon of the phenyl ring.
A
C
H
T
R
E
U
N
G
(NMe ) in toluene with an excess of
This catalyst was then further used for hept-1-yne
2
2 6
the sterically hindered 1-methoxy-2-methylpropan-2- metathesis and compared with the well-defined metal-
ol (MMPOH) leads to the formation of the bimetallic lacarbyne 1 and the dinuclear complex 3 generated in
Scheme 2. Reaction scheme for the preparation of the bimetallic complex 2.
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Adv. Synth. Catal. 2007, 349, 2259 – 2263

Synthesis and Use of a Unique Triple Bonded Dinuclear Tungsten Alkoxy Complex
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[a]
Table 2. Metathesis of internal and terminal alkynes using different catalyst precursors.
Entry
Catalyst
Alkyne
T [8C]
Conversion [%]
Metathesis [%]
[
[
[
b]
c]
c]
1
2
3
4
5
6
7
8
9
1
1
2
2
1
1
3
2
2
2
non-4-yne
1-phenyl-1-propyne
hept-1-yne
hept-1-yne
hept-1-yne
hept-1-yne
hept-1-yne
hept-1-yne
hept-1-yne
25
80
80
25
80
25
80
80
80
80
80
50
70
89
93
78
25
87
86
50
100
22
50
70
33
23
33
25
65
77
47
0
[
d]
2+quinuclidine
1
2
0
1
phenylacetylene
phenylacetylene
10
[
a]
[
Alkyne]/[W]=28.5; solvent toluene; temperature 808C; reaction time 1 hour; the difference between conversion and
metathesis is assumed to correspond to polymerization.
Same conditions but at 258C.
Same conditions but reaction time 1 min.
Same conditions but under static vacuum.
[
[
[
b]
c]
d]
situ from W
A
C
H
T
R
E
U
N
G
(NMe ) and tert-butyl alcohol. Remarka-
In our case, using hemilabile alkoxy ligands, the
2
2 6
bly, a strong enhancement of the metathesis selectivi- presence of the extra coordinating moiety in the coor-
ty was found: at 258C, no polymerization was ob- dination sphere undoubtedly plays a major role in the
served (100% selectivity, run 6), but the conversion selectivity enhancement: DFT calculations on these
was reduced to 25%. Increasing the temperature to metallacyclic intermediates and transition states are
8
08 with complex 2 improved the activity, and con- currently underway to determine the precise role of
ducting the reaction under a static vacuum (to allow these ligands in the deprotonation process.
the acetylene to be rapidly removed from the reaction The activity and selectivity obtained with this new
mixture) led to a further increase in the metathesis catalyst allow this reaction to be applied to alkyl-sub-
yield, up to 77%. The combination of catalyst 2 and stituted terminal alkynes for applications in organic
one equivalent of quinuclidine was even more selec- synthesis. Although some interesting results have
[
17]
tive (94% selectivity, run 9), but resulted in a drop in been obtained using phenylacetylene,
progress
conversion. needs to be made in that context to improve the yield
Phenylacetylene, which previously has never been of metathesis compound for this substrate. Using the
metathesized, still gave a low, but definite, catalytic hemilability concept with other ligands would help in
metathetic reaction using 2, and, in contrast to com- that context: further work is in progress in that direc-
plex 1, did not give further polymerisation. (run 11).
As far as selectivity is concerned, mechanistic in-
vestigations have already established that the major
reason why the reaction did not follow a metathetic
course derives from the fact that the metallacyclobu-
tadiene intermediate is prone to an irreversible de-
protonation process, leading to a deprotiometallacy-
tion in our laboratory.
Experimental Section
General Remarks
clobutadiene
responsible
Accordingly,such species synthesized
for
polymerization
All experiments were carried out under argon in a glove
box or using Schlenk techniques. Solvents were purchased
[7]
(
Scheme 3).
from complex 1 and tert-butylacetylene have been from SDS or the Scharlau Company. Toluene and decane
found to be extremely reactive towards phenylacety- (Aldrich) were dried over sodium and distilled under argon.
[
8]
lene polymerization at room temperature.
Triethylamine and pyridine were dried over KOH and dis-
tilled under argon. Alkynes (Aldrich or GFS Chemicals)
were dried over CaH and distilled under argon. Benzene-d6
2
(
Eurisotop) was dried over sodium and distilled under
argon. All solvents were degassed by three freeze-pump-
[
15]
thaw cycles prior to use. Cl
A
T
E
N
(DME)WꢀC-t-Bu,
and (t-
3
[16]
BuO) WꢀC-t-Bu were synthesized as described in the lit-
3
erature. Alcohols were obtained from commercial sources,
dried over CaH , and distilled under argon. Alcoholates
2
Scheme 3. General scheme for deprotonation of metallacy-
clobutadiene.
were obtained from commercial sources and used without
further purification or synthezised by mixing the corre-
Adv. Synth. Catal. 2007, 349, 2259 – 2263
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Olivier Coutelier et al.
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sponding alcohol in excess with sodium in diethyl ether until General Procedure for Alkyne Metathesis Reaction
all sodium has disappeared, and then dried overnight under
In a glove box, a round-bottom flask was charged with the
vacuum.
alkyne (1.45 mmol), toluene (5 mL), n-decane (0.1 mL) and
a magnetic stirring bar. The round-bottom flask was then
connected to an argon/vacuum line, placed under argon and
General Instrumentation
GC analyses were performed on a CHROMPACK CP-9002
equipped with a CPSil-8CB column. H NMR spectra were
stirred at 808C. In a glove box, the catalytic solution was
prepared in Schlenk flask with (MMPO)
6
1
a
W
A
H
R
U
G
2
measured on a Brucker AVANCE 300.13 MHz spectrome-
ter. H chemical shifts are expressed in parts per million
(0.0254 mmol), toluene (1 mL) and a magnetic stir bar. The
flask containing the solution was then connected to an
argon/vacuum line, placed under argon and stirred at room
temperature for 15 min, following which the catalytic solu-
tion was injected by syringe into the alkyne solution. After 1
hour, the reaction was quenched with methanol and the
products analyzed by GC.
1
downfield from tetramethylsilane (d=0 ppm). The residual
1
H signal of benzene-d was used as reference. The amount
6
of dodec-6-yne was calculated on the basis of a calibration
curve made with pure dodec-6-yne and n-decane.
Metathesis yields were calculated on the basis of dodec-6-
yne formed as follows: %metathesis=(2ndodec-6-yne
/
Note: For the reaction under reduced pressure, the alkyne
solution in the round-bottom flask was cooled at 08C and
then put under staticva cu um (2 mmHg) . The catalytic solu-
tion was then added via an addition funnel at normal pres-
sure already connected to the round-bottom flask in the
glove box. The internal standard (n-decane) was added after
quenching the reaction with methanol and opening to air.
nihept-1-yne)”100, where ndodec-6-yne refers to the amount of
dodec-6-yne produced (in mol) and nihept-1-yne refers to the in-
itial amount of hept-1-yne used for the reaction (in mol).
The amount of dodec-6-yne is calculated using n-decane as
internal standard. The same method was used to determine
the amount of tolane.
Synthesis of W (MMPO)
A
C
H
T
R
E
U
N
G
2 6
In a glove box, a Schlenk flask was charged with 0.1 g Acknowledgements
(
0.16 mmol) of W (NMe ) , 10 mL of toluene and a magnet-
A
C
H
T
R
E
U
N
G
2 2 6
ic stir bar. A solution of methoxy-methylpropan-2-ol (1 g,
.6 mmol) in 5 mL of toluene was then added and the reac-
We gratefully acknowledge Prof. A. J. Welch from Heriot
Watt University in Edinburgh for helpful suggestions, the
CNRS, the Ministry of Research and Technology for their fi-
nancial support, and the Institut Universitaire de France for a
PhD grant to O.C.
9
tion mixture is stirred at room temperature for 12 h. After
evaporation of the solvent and excess alcohol under
vacuum, the product was obtained quantitatively as an
orange-yellow solid that can be used without further purifi-
cation.
Crystallization of the compound from a highly concentrat-
ed solution in toluene at room temperature afforded black
References
crystals suitable for X-ray diffraction analysis and elemental
[
1] a) Y. Chauvin, Angew. Chem. Int. Ed. 2006, 45, 3741;
b) R. R. Schrock, Angew. Chem. Int. Ed. 2006, 45,
3748; c) R. H. Grubbs, Angew. Chem. Int. Ed. 2006, 45,
3760.
1
analysis (yield after crystallization: 50%). H NMR (C D ):
6
6
d=1.71 [s, 6H, OC(Me) CH ], 3.22 (s, 3H, CH OMe), 3.59
2
2
2
1
3
1
(
s, 2H, CH OMe); C{ H} NMR (C D ): d=29.09 [s, OC-
2
6
6
A
C
H
T
R
E
U
N
G
(CH ) CH ], 59.80 (s, CH OCH ), 79.04 [s, OC(CH ) CH ],
A
H
E
N
3
2
2
2
3
3 2 2
[2] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34,
8
3.20 (s, CH OCH ); anal. calcd.: C 36.52%, H 6.74%;
2 3
18.
found: C 36.62%, H 6.82%.
[
3] a) A. Mortreux, M. Blanchard, J. Chem. Soc., Chem.
Commun. 1974, 786; b) A. Mortreux, N. Dy, M. Blan-
chard, J. Mol. Catal. 1976, 1, 101; c) A. Mortreux, J. C.
Delgrange, M. Blanchard, B. J. Lubochinsky, Mol.
Catal. 1977, 73.
General Procedure for the Screening in Alcoholate
In a glove box, a round-bottom flask was charged with the
alkyne (1.45 mmol), toluene (5 mL), n-decane (0.1 mL) and
a magnetic stirring bar. The flask was then connected to an
argon/vacuum line, placed under argon and stirred at 808C.
In a glove box, the catalytic solution was prepared in a
[
4] a) A. Furstner, P. W. Davies, Chem. Commun. 2005,
2
307; b) A. Fürstner, O. Guth, A. Rumbo, G. Seindel,
J. Am. Chem. Soc. 1999, 121, 11108; c) A. Fürstner, A.
Rumbo, J. Org. Chem. 2000, 65, 2608; d) A. Fürstner,
K. Radkowski, J. Grabowski, C. Wirtz, R. Mynott, J.
Org. Chem. 2000, 65, 8758; e) A. Fürstner, A.-S. Casta-
net, K. Radkowski, C. Lehmann, J. Org. Chem. 2003,
Schlenk flask by mixing Cl3
.101 mmol) and the alcoholate (0.303) mmol in toluene
2 mL) with a magnetic stirring bar at room temperature.
A
H
R
U
G
0
(
The flask containing the solution was then connected to an
argon/vacuum line, placed under argon and stirred at room
temperature for 15 min. Stirring was stopped and 1 mL of
the supernatant was withdrawn with a syringe and injected
into the alkyne solution. After 1 min, the reaction was
quenched with methanol and the products analyzed by GC.
6
3, 1521; f) N. Ghalit, A. J. Poot, A. Fürstner, D. T. S.
Rijkers, R. M. Liskamp, Org. Lett. 2005, 7, 2961; g) A.
Fürstner, F. Stelzer, A. Rumbo, H. Krause, Chem. Eur.
J. 2002, 8, 1856; h) S. A. Krouse, R. R. Schrock, Macro-
molecules 1989, 22, 2569; i) X. P. Zhang, G. Bazan,
Macromolecules 1994, 27, 4627; j) W. Steffen, U. H. F.
Bunz, Macromolecules 2000, 33, 9518; k) U. H. F. Bunz,
Acc. Chem. Res. 2001, 34, 998; l) W. Zhang, J. S.
Moore, Macromolecules 2004, 37, 3973.
2262
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 2259 – 2263

Synthesis and Use of a Unique Triple Bonded Dinuclear Tungsten Alkoxy Complex
COMMUNICATIONS
[
5] a) A. Fürstner, O. Guth, A. Rumbo, G. Seindel, J. Am.
Chem. Soc. 1999, 121, 11108; b) K. Grela, I. Jolanta,
Org. Lett. 2002, 4, 3747.
[11] M. Akiyama, M. H. Chisholm, F. A. Cotton, M. W.
Extine, D. A. Haitko, D. Little, P. E. Fanwick, Inorg.
Chem. 1979, 18, 2266.
[
6] A. Mortreux, O. Coutelier, J. Mol. Catal. A. 2006, 254,
[12] R. R. Schrock, M. L. Listeman, L. G. Sturgeoff, J. Am.
9
6.
Chem. Soc. 1982, 104, 4291.
[13] For the X-ray crystal structure analysis of W
see the Supporting Information.
[
7] L. G. McCullough, M. L. Listemann, R. R. Schrock,
A
H
R
U
G
M. R. Churchill, J. W. Ziller, J. Am. Chem. Soc. 1983,
[
14] J. H. Freudenberger, R. R. Schrock, Organometallics
986, 5, 1411.
15] InorganicSynthesis 1989, 26, 44.
1
05, 6729.
1
[
8] a) A. Bray, A. Mortreux, F. Petit, M. Petit, T. Szyman-
ska-Buzar, J. Chem. Soc., Chem. Commun. 1993, 197;
b) A. Mortreux, F. Petit, M. Petit, T. Szymanska-Buzar,
J. Mol. Catal. 1995, 96, 95.
[
[
16] J. H. Wengrovius, J. Sancho, R. R. Schrock, J. Am.
Chem. Soc. 1981, 103, 3932.
[
17] Disubstituted aromatic alkynes are much less prone to
metathesis than their dialkyl homologues (see Table 2,
entries 1 and 2). This might explain why such a low se-
lectivity is obtained with phenylacetylene. Other details
concerning the analysis of the polymers are available in
the Supporting Information.
[
9] O. Coutelier, A. Mortreux, Adv. Synth. Catal. 2006,
3
48, 2038.
[
10] A. H. Hoveyda, D. G. Gillingham, J. J. Van Veldhuizen,
O. Kataoka, S. B. Garber, J. S. Kingsbury, J. P. A. Harri-
ty, Org. Biomol. Chem. 2004, 2, 8.
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