On the in situ trimethylsilylation of zinc acetylides

Article abstract of DOI:10.1016/S0022-328X(00)00103-0

Reinvestigation of the recently published formation of silylated alkynes by reaction of 1-alkynes with zinc and trimethylchlorosilane in acetonitrile showed that appreciable amounts of 1-alkenes are formed as side products.

Full text of DOI:10.1016/S0022-328X(00)00103-0

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Journal of Organometallic Chemistry 601 (2000) 341–342
Note
On the in situ trimethylsilylation of zinc acetylides
Uno Ma¨eorg a , Sa¨de Viirlaid a, Hannes Hagu a, Hermann D. Verkruijsse b,
Lambert Brandsma b,*
a Institute of Organic Chemistry, Uni6ersity of Tartu, Jakobi Street 2, EE-2400 Tartu, Estonia
b Department of Preparati6e Organic Chemistry, Utrecht Uni6ersity, Padualaan 8, NL-3584 CH Utrecht, The Netherlands
Received 15 November 1999; received in revised form 31 January 2000
Abstract
Reinvestigation of the recently published formation of silylated alkynes by reaction of 1-alkynes with zinc and
trimethylchlorosilane in acetonitrile showed that appreciable amounts of 1-alkenes are formed as side products. © 2000 Elsevier
Science S.A. All rights reserved.
Keywords: Trimethylacetylenes; Silylation; Reduction; Alkenes; Zinc powder
The versatility of 1-alkynes RCꢀCH as starting
reagents in organic syntheses has led to the develop-
ment of efficient functionalization methods. The first
step mostly involves metallation with a Grignard
reagent or a strongly basic reagent, e.g. n-butyllithium
[1,2]. Recently, Japanese chemists [3,4] reported the
formation of silylated alkynes in good to excellent
yields by heating a mixture of the 1-alkyne, excess of
trialkylchlorosilane and zinc powder in acetonitrile.
They claim superiority of their method to the existing
ones. Being interested in efficient synthetic procedures
(compare Ref. [2]), we decided to repeat the published
procedure on a preparative (0.10 M) scale taking the
silylation of PhCꢀCH as a model. This reaction was
carried out under atmospheric pressure.
After about 10 h no further increase of the temperature
in the boiling mixture (moderate, constant reflux) was
observed (ꢀ74“80°C) while GLC had shown com-
plete consumption of PhCꢁCH. The excess of
Me3SiCl was distilled off as completely as possible from
the stirred mixture. When CH3CN began to pass over,
the mixture was cooled to 20°C and the clear superna-
tant liquid was decanted from the excess of zinc and
poured into 200 ml of water. The remaining slurry of
zinc and CH3CN was washed repeatedly with pentane
(total volume ꢀ150 ml). The combined organic solu-
tions were washed four times with water in order to
remove the CH3CN. After drying over MgSO4, the
pentane was distilled off at atmospheric pressure. Care-
ful vacuum distillation (30 cm Vigreux column) gave a
volatile first fraction (4.0 g, consisting of ꢀ30% of
Me3SiOSiMe3 and 70% of styrene), and a main fraction
of PhꢀCC–SiMe3 (b.p. 90 KC/15 mmHg) in 63% yield.
There was no residue after distillation. The products
A magnetically stirred mixture of 30 g of zinc powder
(Merck, analar grade, particle size B60 mm), 21.6 g
(0.20 mol) of trimethylchlorosilane (distilled from 5%
(w/w) of N,N-diethylaniline) and 60 ml of acetonitrile
(analar grade, distilled from calcium hydride) was
heated under reflux under an atmosphere of nitrogen.
1
were analysed with H-NMR and GLC (identity with
authentic samples).
The reaction time could be shortened to about 5 h by
using activated zinc powder: a mixture of 35 g Zn, 50
ml of dry THF and 5 g of 1,2-dibromoethane was
heated under reflux (ꢀ30 min) until evolution of
* Corresponding author.
E-mail addresses: uno@chem.ur.ee (U. Ma¨eorg), l.brandsma@
chem.uu.nl (L. Brandsma)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00103-0

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U. Ma¨eorg et al. / Journal of Organometallic Chemistry 601 (2000) 341–342
ethene had stopped. After cooling, the THF was de-
canted and the remaining slurry rinsed several times
with CH3CN in order to remove the THF.
Reactions with HCꢀC(CH2)9OH and HCꢀC–n-
C10H21 were also carried out with zinc activated by
treatment with 1,2-dibromoethane. The rates of con-
version of these acetylenes were much lower than that
of PhCꢀCH. After 5 h refluxing work-up was carried
out and the product mixture analysed with GLC and
1H-NMR spectroscopy, 45% (rel.) of undecynol and
22% of dodecyne still being present. In addition the
reduced alkynes H2CꢁCH(CH2)9OH (11%) and
H2CꢁCH–C10H21 (22%) had formed, together with
the TMS-derivatives Me3SiCꢀC(CH2)9OH (44%) and
Me3SiCꢀCC10H21 (61%). Also, Sugita et al. [3] found
reduction to be a significant side reaction in several
cases when a Zn/Cu couple was used, whereas with
the same kind of activated zinc they obtained
PhCꢀCSiMe3 in an excellent yield and no styrene was
formed. The formation of olefinic compounds may be
explained by an electron transfer–deprotonation
mechanism, similar to the sequence involved in the
preparation of alkali acetylides from alkali metals and
acetylene in liquid ammonia [5].
According to this mechanism the maximally attainable
yield of silylated acetylenes is 66%, which compares with
the yield obtained by us from the reaction with PhCꢀCH.
It seems to us that a more thorough investigation is
necessary to reveal the cause of the discrepancies ob-
served.
References
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von Alkine, in: Houben-Weyl, Methoden der Organischen Chemie,
Band V-2a, Alkine, Di- un Polyine, Allene, Kumulene, Georg
Thieme Verlag, Stuttgart, 1977.
[2] L. Brandsma, Preparative Acetylenic Chemistry, Rev. Ed, Elsevier,
Amsterdam, 1988.
[3] H. Sugita, Y. Hatanaka, T. Hiyama, Tetrahedron Lett. 36 (1995)
2769.
[4] H. Sugita, Y. Hatanaka, T. Hiyama, Synlett (1996) 637.
[5] A.L. Henne, K.W. Greenlee, J. Am. Chem. Soc. 65 (1943) 2020.
(b) G.W. Watt, Chem. Revs. 46 (1950) 317.
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