Si-C bond activation in the reaction of first generation Grubbs' catalyst with alkynylsilanes - Formation of [Cl2{P(C6H 11)3}2Ru(CHCHCHPh)] and disiloxanes

Article abstract of DOI:10.1039/b924945c

The first generation Grubbs catalyst [Cl₂{P(C₆H ₁₁)₃}₂RuC(H)Ph] reacts efficiently with alkynylsilanes in the presence of water to give the styryl carbene complex [Cl₂{P(C₆H

Full text of DOI:10.1039/b924945c

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Si–C bond activation in the reaction of first generation Grubbs’ catalyst with
=
=
alkynylsilanes - formation of [Cl2{P(C6H11)3}2Ru( CHCH CHPh)] and
disiloxanes†
Beata Powała,a Helmut Fischerb and Cezary Pietraszuk*a
Received 26th November 2009, Accepted 9th December 2009
First published as an Advance Article on the web 5th January 2010
DOI: 10.1039/b924945c
and water led to the complete conversion of 1 within 5 h. An orange
The first generation Grubbs catalyst [Cl2{P(C6H11)3}2-
=
solid precipitated when acetone was added to the concentrated
Ru C(H)Ph] reacts efficiently with alkynylsilanes in the
presence of water to give the styryl carbene complex
post-reaction mixture. Analysis of the purified complex by 1H, 13
C
and 31P NMR spectroscopy revealed the formation of the styryl
carbene complex (3) (isolated yield 48%). An alternative way of
synthesising 3 is described in literature.15 No signal that could be
attributed to the silyl group was observed in the 1H NMR spectrum
of complex 3. The GC-MS analysis of the post-reaction mixture
indicated disiloxane to be the only silicon containing product. The
reaction is summarized in Scheme 1.
=
=
[Cl2{P(C6H11)}2 Ru C(CH CHPh)H] and disiloxane.
The activity of the family of Grubbs catalysts in metathesis
transformations of carbon–carbon triple bonds such as in enyne
metathesis,1 alkyne2 and diyne polymerization3 is well known.
Until now, only a few reports have appeared on the reactivity
of alkynes in equimolar reactions with Grubbs type ruthenium
carbene complexes. Grubbs reported the reaction of electron-rich
=
disubstituted alkynes with [Cl2{P(C6H11)3}(H2IMes)Ru C(H)Ph]
3
giving phosphane-free h -vinylcarbene complexes.4 The reactivity
of alkynes bearing a silyl group directly attached to the carbon–
carbon triple bond towards Grubbs catalysts has not been studied
in detail. Only a few examples of the activity of Grubbs catalysts
in enyne ring-closing metathesis (RCM)5,6 and in cross metathesis
(CM) of alkynylsilanes with olefins are known.5,7,8
Scheme 1
In this communication we report on the cleavage of the
carbon–silicon bond accompanying the reaction of a first gen-
eration Grubbs catalyst (1) with ethynylsilanes in the presence
of water. The known examples of the cleavage of the Si–C
bond in silylacetylenes by transition metal complexes include
1,2-silyl migration accompanying the formation of vinylidene
complexes of various transition metals,9 palladium catalysed cross-
coupling silylacetylenes with organic halides,10 s-bond metathesis
of silylalkynes by uranium metallocene11 as well as some other
platinum,12 rhodium13 or nickel14 catalysed reactions.
The reactions of
1 with other silylalkynes instead of
ethynyltrimethylsilane proceeded analogously, giving 3 and a
corresponding disiloxane. The yields of complex 3 measured on the
basis of 1H NMR spectra were found to be nearly independent of
the substituents at silicon (43, 40 and 38%, respectively), however,
they increased significantly (85% for reaction with the use of 2a)
when instead of benzene, methylene chloride was used as the
solvent. No reaction was observed when internal silylacetylenes
≡
of the type R-C C-SiMe3, where R = Me, Ph, SiMe3 were used
as reagents.
When we treated a first generation Grubbs catalyst (1) with
stoichiometric amounts of ethynyltrimethylsilane in benzene at
40 ◦C in the presence of water, a gradual change in the colour of the
solution from violet to red was observed. GC-MS analysis of the
reaction mixture indicated the conversion of silylacetylene and the
formation of hexamethyldisiloxane. Monitoring the process by 1H
NMR spectroscopy indicated the disappearance of the signal d =
20.63 ppm assigned to the hydrogen atom at the carbene carbon in
A dramatic decrease in the yield of 3 was observed upon
addition of three equivalents of phosphine to the reaction mixture.
Therefore, we believe that dissociation of phosphine precedes the
coordination of acetylene and the subsequent transformations.
In order to elucidate the mechanism of formation of the vinyl-
carbene complex, experiments with deuterium labelled reagents
were performed and monitored by H NMR spectroscopy. The
reaction of deuterium labelled alkylidene complex (1-d6) with
ethynyltrimethylsilane (2a) afforded complex 3-d6 (Scheme 2).
1
=
[Ru] CHPh (1) and the simultaneous formation of a new signal,
a doublet at d = 19.75 ppm (J = 10.4 Hz). After 5 h 55% of 1 was
consumed. The reaction of 1 with a five-fold excess of ethynylsilane
aFaculty of Chemistry, Adam Mickiewicz University, 60-780, Poznan´,
Poland. E-mail: pietrasz@amu.edu.pl; Fax: +48 61 8291508; Tel: +48
8291328
bFachbereich Chemie, Universita¨t Konstanz, Fach 727, 78457, Konstanz,
Germany
Scheme 2
† Electronic supplementary information (ESI) available: Experimental
details including optimized synthesis procedure for complex 3. See DOI:
10.1039/b924945c
Moreover, reaction of 1 with 2a-d1 leads to selective formation
of 3-d1 bearing a deuterium label at the b-carbon (Scheme 3).
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by P(C6H11)3 present in the reaction mixture and proceeds
immediately after mixing the reagents (as confirmed in a separate
experiment). In the last stage of the sequence (Scheme 5) the
alkylidyne complex immediately reacts with hydrochloride to form
complex 3. The formation of a ruthenium alkylidene complex via
addition of HCl to an alkylidyne complex was recently reported.19
When monitoring the reaction (eq. 1) we never observed
any signal which could be assigned to an alkylidyne complex.
However, such an alkylidyne complex, once formed, is expected
to immediately accept HCl to form 3. Therefore, the alkylidyne
complex will not accumulate in the mixture to a detectable level.
This assumption was confirmed in a separate experiment. When
Scheme 3
The positions occupied by the deuterium atoms in 3-d6 and 3-d1
suggested that the reactions proceed via the carbene mechanism.
To clarify the role of water in the process, the reaction of 1 with
ethynyltrimethylsilane was performed under rigorous exclusion of
water but with the controlled addition of a fivefold excess of D2O.
Deuterium was incorporated into the styrylcarbene complex and
3¢-d1 was formed (Scheme 4) confirming the participation of water
in the reaction.
the alkylidyne complex [Cl{P(C6H11)3}2Ru C(C6H4t-Bu-4)]19 was
≡
treated in C6D6 with an equimolar amount of chlorotrimethyl-
silane in the presence of an equimolar amount of water the
=
immediate formation of [Cl2{P(C6H11)3}2Ru CH(C6H4t-Bu-4)]
was observed.
In conclusion, unprecedented activation of alkynylsilanes by
first generation Grubbs’ catalyst is described. The reaction re-
ported constitutes an alternative and easy route to ruthenium
styrylcarbene complexes.22
Scheme 4
1H NMR spectroscopy indicated that the deuterium label
was attached exclusively to the a-carbon atom. The analogous
reaction of 1 with monodeuterated methanol likewise gave com-
plex 3¢-d1 and methoxytrimethylsilane. On the basis of these
results it is reasonable to suppose that the reaction of 1 with
ethynylsilane initially leads to an 1-(silyl)benzylidene complex
Acknowledgements
Financial support from the Ministry of Science and Higher
Education (Poland), (project No. N 204 1935 33) is gratefully
acknowledged.
=
=
[Cl2{P(C6H11)3}2Ru C(SiR3)CH CHPh)] (4) via stoichiometric
metathesis (Scheme 5). Although ruthenium alkylidene complexes
bearing a silyl group at the a-carbon atom are very rare,16,17
such complexes must be involved in the catalytic cycles of the
cross metathesis of silylacetylene with olefins7 and the homo-
metathesis of vinylsilanes18 proceeding by the carbene mecha-
nism. Presumably, the putative complex 4 immediately eliminates
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1924 | Dalton Trans., 2010, 39, 1923–1925
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This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 1923–1925 | 1925
©