Simple procedures for ethynylmagnesium bromide, ethynyltrialkylsilanes and ethynyltrialkylstannanes

Article abstract of DOI:10.1055/s-1999-3588

1 Molar solutions of ethynylmagnesium bromide in tetrahydrofuran can be successfully prepared by introducing acetylene into a cooled solution of ethylmagnesium bromide. Subsequent reaction with trialkylsilyl or trialkylstannyl chloride gives the expected ethynyl derivatives in excellent yields.

Full text of DOI:10.1055/s-1999-3588

SHORT PAPER
1727
Simple Procedures for Ethynylmagnesium Bromide, Ethynyltrialkylsilanes
and Ethynyltrialkylstannanes
L. Brandsma,* H.D. Verkruijsse
Department of Preparative Organic Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
E-mail: l.brandsma@chem.uu.nl
Received 25 May 1999
THF
Abstract: 1 Molar solutions of ethynylmagnesium bromide in tet-
rahydrofuran can be successfully prepared by introducing acetylene
into a cooled solution of ethylmagnesium bromide. Subsequent re-
action with trialkylsilyl or trialkylstannyl chloride gives the expect-
ed ethynyl derivatives in excellent yields.
C2H5MgBr
+
HC CH
HC CMgBr
+
C2H6
5-15 °C
Scheme 1
Key words: alkynes, disproportionation, ethynyl di-magnesium ha-
lides, Grignard reactions, magnesium, silicon, tin, organometallic
reagents
Although formation of the di-Grignard derivative
BrMgC∫CMgBr is in principle possible, we strongly sus-
pect that the formation of the mono-Grignard reagent will
occur to a significant extent if the acetylene is introduced
quickly and the concentration of the solution of ethylmag-
nesium bromide is not higher than 1 mol/L. Reactions of
the thus obtained suspension with chlorotrimethylsilane,
chlorotrimethylstannane and chlorotributylstannane gave
the expected ethynyl derivatives in 70% or higher yields
Ethynylmagnesium halides are extremely versatile inter-
mediates for the preparation of monosubstituted acety-
lenes. Traditionally, solutions of ethynylmagnesium
halides are prepared by a procedure of inverse addition,
i.e. slow addition of a solution of alkylmagnesium halide
in THF to a saturated solution of acetylene in this sol-
(
Scheme 2).
1
-4
vent.
R3XCl
The inverse addition seems to be essential, since the
mono-Grignard derivative is thought to be capable of re-
acting with alkylmagnesium halide to form the di-Grig-
nard derivative. A serious drawback of this procedure is
that its correct performance requires considerable experi-
mental skill and continuous alertness throughout the intro-
duction of acetylene over three hours. During this period
the temperature is kept below 30 °C in order to avoid for-
mation of the di-Grignard derivative by a disproportion-
ation reaction, while the rate of addition of the solution of
alkylmagnesium halide has to be continuously readjusted
due to the formation of solid material in the lower part of
the dropping funnel. In a recent Organic Syntheses
HC CMgBr
HC C–XR3
X = Si, R = CH3 : 75-80%
X = Sn, CH3 : 70%
X = Sn, C H : 85%
4 9
Scheme 2
It is essential to add the chloro compounds quickly with
efficient stirring. Slow addition may favour the formation
of disubstituted acetylenes, according to us by a process of
proton abstraction and subsequent substitution (compare
Ref. 2, note 7) (Scheme 3).
2
procedure a hot (60 °C) solution of butylmagnesium
chloride is added from the dropping funnel, but the advan-
tage of easier addition is relatively small when it is real-
ized that butylmagnesium chloride is much less smoothly
prepared than ethylmagnesium bromide. As a conse-
quence, the synthesis of mono-substituted acetylenes via
ethynylmagnesium halide is not highly attractive, espe-
cially when the acetylenes are only needed incidentally.
HC CMgBr
R3XCl
R3X C CH
R3X C C MgBr
R3X C C XR3
Scheme 3
Also higher concentrations of ethylmagnesium bromide
have to be avoided as the reaction mixtures become diffi-
cult to stir. As a result, the distribution of the chloro com-
pounds through the suspension becomes less efficient and
disubstituted acetylenes may form by the process men-
tioned above.
In this communication we present a procedure for ethy-
nylmagnesium bromide that should be preferred to the tra-
ditional ones. Its success is demonstrated by the
preparation of the useful synthetic intermediates, ethynyl-
trialkylsilanes and -stannanes, in excellent yields. In our
procedure acetylene is simply introduced into a 1 M solu-
tion of ethylmagnesium bromide in THF at temperatures
around 10 °C (Scheme 1).
From our results one may conclude that the reluctance to
apply the normal addition procedure for ethynylmagne-
sium halides is not fully justified. It has not been proved
that the formation of significant amounts of disubstituted
Synthesis 1999, No. 10, 1727–1728 ISSN 0039-7881 © Thieme Stuttgart · New York

1
728
L. Brandsma, H.D. Verkruijsse
SHORT PAPER
acetylenes is due to conversion of HC∫CMgX into Ethynyltrimethylsilane
Ethynyltrimethylsilane was distilled off at atmospheric pressure
from the petroleum through a 25 cm Vigreux column. The fraction
passing over between 50 and 110 °C still contained some THF. This
was removed almost completely by washing with cold (0 °C) 3 M
HCl (5 x 50 mL) in a small (200 mL) separating funnel. After dry-
XMgC∫CMgX by a disproportionation reaction at tem-
peratures below 30 °C. Our earlier described synthesis of
3
Me SiC∫CSiMe with an excellent yield by introducing
3
3
acetylene at ~50 °C into a solution of C H MgBr and sub-
2
5
sequent silylation suggests that this disproportionation ac-
ing over a small amount of MgSO , the ethynyltrimethylsilane was
4
tually can occur at temperatures above 40 °C.
distilled at atmospheric pressure; bp ~52 °C; yield:75–80%.
Ethynyltrimethylsilane, Ethynyltimethylstannane and Ethynyl-
tributylstannane; Typical Procedures
In a 2-L round-bottomed, three-necked flask equipped with a com-
Ethynyltrimethylstannane
The isolation of ethynyltrimethylstannane was carried out by sub-
jecting the petroleum solution to a vacuum distillation (15 Torr), us-
ing a 40-cm Vigreux column and a receiver cooled at ~ –75 °C. The
distillation was stopped when the petroleum began to reflux in the
upper part of the column. Careful redistillation through a 30-cm
spinning band column gave, after a first fraction mainly consisting
bination of a dropping funnel and a N inlet, a mechanical stirrer and
2
a combination of a thermometer and outlet were placed Mg turnings
(33.6 g, 1.4 mol) and anhyd THF (150 mL). After activation of the
Mg with 1,2-dibromoethane or I the flask was filled with N and
2
,
2
2
0
THF (1.2 L) was added. EtBr (141.7 g, 1.3 mol) was added drop-
wise over 1 h, while keeping the temperature between 30 and 40 °C.
After an additional hour (~35 °C) the solution was transferred into
of THF and ethynyltrimethylstannane; bp 98 °C/760 Torr; nD
1
1.461 in 70% yield. The H NMR spectrum (CCl ) showed signals
4
at d = 2.0 and 0.32.
another 2-L three-necked flask (previously filled with N , Note 1),
2
equipped with an inlet tube, a very efficient mechanical stirrer and a
thermometer-outlet-combination. The excess of Mg was rinsed with
THF (50 mL). The solution was cooled to about 5 °C (with tempo-
Ethynyltributylstannane
Ethynyl tributylstannane; bp 92 °C/1 Torr; nD 1.4758 (yield:85%)
was obtained by removal of the solvents under reduced pressure fol-
lowed by distillation through a 20 cm Vigreux column. The H
2
0
1
rary increase of the flow of N ). Acetylene (freed from acetone by
2
passing through two traps cooled at ~ –75 °C) was introduced with
vigorous stirring (high turbulence!) at a rate of ~2 L/min during 10
min, while keeping the temperature of the suspension between 5 and
NMR spectrum shows inter alia the signal at d = 2.0 for the ethynyl
proton. There was only about 4 g of higher boiling residue. The
preparation of this ethynyl compound in only 31% yield from lithi-
um acetylide-ethylene diamine complex and chlorotributylstannane
1
0 °C (during this period the heating effect, partly due to dissolution
5
of acetylene in the THF is rather strong requiring efficient cooling).
Subsequently the gas was introduced during 30 min at a rate of ~300
mL/min and the temperature of the suspension was maintained
closely around 10 °C. The rate of stirring was then diminished and
the suspension was stirred for an additional period of 1 h with slow
is described in Organic Syntheses. This modest yield may be due
to metallation of HC∫CSnBu by LiC∫CH and subsequent conver-
3
sion to Bu SnC∫CSnBu .
3
3
(
~100 mL/min) introduction of acetylene. External cooling was no
Notes
. It is not absolutely necessary to separate the solution of EtMgBr
from the excess of Mg.
. In the Organic Syntheses procedure for the preparation for ethy-
nyltrimethylsilane, no solvent for the extraction of the product was
used. This involves the risk of losing part of the volatile product
longer carried out. After an half-hour interval (temperature of the
suspension ~15 °C) the suspension was cooled to 5 °C and chlorot-
rimethylsilane (108.6 g, 1. 0 mol, distilled from ~10% of N,N-dieth-
ylaniline) was added in two to three portions during 2 min with
vigorous stirring and keeping the temperatrure below 20 °C (the
heating effect was moderate). The reaction mixture was stirred for
1
2
2
during repeated washing with H O.
3
0 min at 15–20 °C, subsequently for 30 min at 35 °C.
A similar procedure was carried out with chlorotrimethylstannane
60.0g, 0.30 mol) and chlorotributylstannane (97.8 g, 0.30 mol) in a References
-L round-bottomed flask (volume of THF ~450 mL, amounts of
2
(
1
(
(
(
1) Skattebøl, L.; Jones, E. R. H.; Whiting, M. C. Org. Synth.
Coll. Vol. 4 1992, 792.
2) Holmes, A. B.; Sporikou, C. N.Org. Synth., Coll. Vol. 8 1993,
606.
3) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes,
Allenes and Cumulenes, A Laboratory Manual, Elsevier:
Amsterdam, 1981.
Mg and EtBr 12 and 50 g, respectively).
For the workup the following operations were carried out. To the
mixture from the reactions with Me SiCl and Me SnCl was added
high boiling petroleum (bp ≥180 °C/760 Torr, 200 mL, Note 2) and
pentane (200 mL) in the case of the reaction with Bu SnCl. The
slurries were then poured with manual swirling into a cold (~5 °C)
solution of NH Cl (150, 50 and 50 g, respectively) in H O (2, 1 and
1
may escape!), the layers were separated. The upper layers were re-
peatedly (10 to 15 times) washed with H O (500 mL portions of ice
water in the case of Me SiC∫CH and Me SnC∫CH) in order to re-
move the THF as completely as possible. After drying (MgSO ), the
3
3
3
nd
4
2
L. Brandsma, Preparative Acetylenic Chemistry, 2 ed.,
L, respectively). After shaking (caution, some dissolved acetylene
Elsevier, Amsterdam, 1988.
(4) Fieser; L. F.; Fieser, M. Reagents for Organic Synthesis, Vol
1, Wiley: New York, 1967.
(5) Renaldo, A. F.; Labadie, J. F.; Stille, J. K. Org. Synth. Coll.
Vol. 8 1993, 268.
2
3
3
4
ethynyl derivatives were isolated.
Article Identifier:
1
437-210X,E;1999,0,10,1727,1728,ftx,en;H02999SS.pdf
Synthesis 1999, No. 10, 1727–1728 ISSN 0039-7881 © Thieme Stuttgart · New York