Silanol-molybdenum hexacarbonyl as a new efficient catalyst for metathesis of functionalised alkynes under microwave irradiation

Article abstract of DOI:10.1016/S0040-4039(01)00549-4

The catalytic mixture based on Mo(CO)₆ was re-examined in the metathesis of 4-methoxytolan. Silanols (4) or (5) were found to be very active as co-catalysts. The metathesis of functional alkynes was convenient and rapidly achieved in octane as solvent under microwave irradiation in a resonance cavity.

Full text of DOI:10.1016/S0040-4039(01)00549-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3701–3703
Silanol–molybdenum hexacarbonyl as a new efficient
catalyst for metathesis of functionalised alkynes
under microwave irradiation
Didier Villemin,* Michael He´roux and Virginie Blot
Ecole Nationale Supe´rieure d’Inge´nieurs de Caen, ISMRA, Universite´ de Caen, e´quipe associe´e au CNRS, F-14050 Caen, France
Received 30 March 2001; revised 2 April 2001
Abstract—The catalytic mixture based on Mo(CO)6 was re-examined in the metathesis of 4-methoxytolan. Silanols (4) or (5) were
found to be very active as co-catalysts. The metathesis of functional alkynes was convenient and rapidly achieved in octane as
solvent under microwave irradiation in a resonance cavity. © 2001 Elsevier Science Ltd. All rights reserved.
The metathesis of alkynes has received less attention
than that of alkenes because alkynes and active alkyne
metathesis catalysts are less common than their alkene
counterparts. Metathesis of alkenes generally results in
a mixture of Z and E isomers and the stereochemistry
cannot at present be properly controlled. However,
alkynes are readily available and are precursors of
stereospecific alkenes. Ring opening of strained
cycloalkynes catalysed by well defined carbyne was first
described by Schrock1 using defined metallocarbynes.
Recently these carbyne complexes have been used in
ring formation of functional cycloalkynes;2 however,
these carbynes are not readily available and are air
sensitive.
metathesis (Scheme 1). An equilibrium was observed
(1)/(2)/(3)=2/1/1 when metathesis took place.
First we tried using 4-chlorophenol with a metal com-
plex other than molybdenum hexacarbonyl. The substi-
tution of Mo(CO)6 by [Rh(CO)2Cl]2, Ru3(CO)12,
Re2(CO)10 or W(CO)6 did not undergo metathesis. We
studied the influence of temperature in the formation of
the catalyst: the activity of Mo(CO)6/chlorophenol
began at 110°C, and the solution obtained after thermal
activation did not catalyse the metathesis of methoxy-
tolan at room temperature. Some form of thermal
activation seems necessary for observing the metathesis
of methoxytolan.
The first homogeneous catalyst (phenol+Mo(CO)6) for
alkyne metathesis was described by Mortreux3 and we
have described the use of such a catalyst [4-chlorophe-
nol+Mo(CO)6] in the metathesis of functional alkynes.4
Although this catalyst has been use recently in metathe-
sis of functional alkynes,5 we decided to re-examine the
potential of the metathesis of acyclic functional alkynes
in order to find a more active and more convenient
catalyst. Initially, we chose the cross-metathesis of
methoxytolan as a model of funtionalised alkyne
Next we explored the use of co-catalysts other than
4-chlorophenol. Under the same conditions, 4-tri-
fluromethylphenol,5 2,4-dichlorophenol, 2,4,6-tribromo-
phenol, 2,4-dimethylphenol are also active as co-cata-
lysts. It is interesting to note that a hindered phenol like
2,4,6-tribromophenol is active because hindered phe-
nols were used in designing selective and tolerant car-
bene catalysts in olefin metathesis. In all cases the
equilibrium was obtained with a ratio of alkyne/Mo/
phenol=20/1/20. It is possible to diminish
Scheme 1. Metathesis of 4-methoxytolan.
Keywords: alkyne metathesis; microwave; silanol.
* Corresponding author. Fax: (33) 2 31 45 28 40; e-mail: villemin@ismra.unicaen.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00549-4

3702
D. Villemin et al. / Tetrahedron Letters 42 (2001) 3701–3703
of microwave by the mixture. We obtained better
results using propylene carbonate instead of octane.
Propylene carbonate is a more polar solvent (m=69
Debyes) than octane (m=2 Debyes) and needed a power-
ful irradiation (5X1 min, 300 W), the temperature
reached was 126°C.
the alkyne/phenol ratio and determine the activity.
We found that the activity of the catalyst increased
when the acidity of phenol was increased: 2,4,6-
tribromophenol (pKa 6.2)–2,4-dichlorophenol (pKa
7.7)>4-trifluromethylphenol (pKa=8.67)>4-chlorophe-
nol (pKa=9.4)>phenol (pKa=9.9)–2,4-dimethylphenol
(pKa=10.6).
With silanols in octane the equilibrium was observed
after only 10 min of irradiation9 instead of the 16 h
under reflux. With trisilanol (6) the equilibrium was
observed after 20 min of irradiation instead of 20 h
under reflux. We tested the catalyst [(4)+Mo(CO)6]
under microwaves with varying functionalised alkynes4
in cross metathesis.8 The equilibrium was observed in
all cases after less than 15 min of irradiation. With the
very sensitive enone (7g) the equilibrium was observed
with [(4)+Mo(CO)6], but we did not observe metathesis
with the phenol catalyst [Mo(CO)6+ClC6HOH].
Dimolydenum tetraacetate (Mo2(OAc)4) in presence of
4-chlorophenol catalyses the metathesis of 4-methoxy-
tolan. Dimolybdenum tetraacetate is formed by the
reaction of acetic acid on molybdenum hexacarbonyl;
however, we did not observe metathesis of 4-methoxy-
tolan in acetic acid in the presence of Mo(CO)6 when
heated. We think that a complex with a molybde-
numꢀmolybdenum triple bond was formed by protona-
tion by the phenol of the tetrabond of dimolybdenum
tetraacetate. However, (Mo2(OAc)4) in the presence of
4-chlorophenol is less active as
Mo(CO)6.
a catalyst than
With (7f) we observed the shift of equilibrium, and a
quantitative yield of (2) was obtained due to the elimi-
nation of volatile hexamethyldisilylacetylene (8f)
(Scheme 3).
Finally, we discovered that phenol can be advanta-
geously substituted by silanols to form an efficient
co-catalyst in alkyne metathesis with Mo(CO)6. With
triphenylsilanol (4) or diphenylsilanediol (5) [(1)/
Mo(CO)6/silanol=20/1/2] the metathesis equilibrium
was obtained after 16 h of reflux in octane under argon.
In the case of the trisilanol-POSS (6),6 the equilibrium
was observed only after 20 h of reflux and needed more
octane due to the poor solubility of (6) (Scheme 2).
In conclusion silanol [triphenylsilanol (4) and diphenyl-
silanol (5)] with Mo(CO)6 is readily available, particu-
larly efficient and functionally tolerant catalyst for
alkyne metathesis. The metathesis is very rapid under
microwave irradiation.
Recently many reactions with transition metal com-
plexes have been accelerated by microwave irradiation.7
We tried to accelerate the metathesis of alkynes by
microwave irradiation at 2450 MHz in a resonance
cavity.8 With phenol as cocatalyst, we did not obtain
good results using octane due to the small absorption
Acknowledgements
We thank Dr. P.-A. Jaffre`s for the preparation of a
sample of (6).
Scheme 2. Silanols used as cocatalyst with Mo(CO)6.
Scheme 3. Metathesis of functionalised alkynes under microwave irradiation.

D. Villemin et al. / Tetrahedron Letters 42 (2001) 3701–3703
3703
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