Checked by Dallas D. Crotts, Bruce E. Eaton, and Clayton H. Heathcock.
1. Procedure
A.
Butylmagnesium chloride. A dry,
1-L, three-necked, round-bottomed flask is equipped with a
sealed mechanical stirrer (Note
1), a
250-mL pressure-equalizing dropping funnel and a
reflux condenser to the top of which is attached a T-piece connected at one end to a supply of dry
nitrogen, and at the other to an
oil or mercury bubbler. At the same time a dry,
2-L, three-necked, round-bottomed flask, fitted with a sealed mechanical stirrer and
two swan-neck adapters, is prepared, with one adapter holding a
gas inlet and a
calcium chloride drying tube, and the other supporting a
thermometer and a
1-L pressure-equalizing dropping funnel.
The 1-L flask is charged with
magnesium turnings (39.6 g, 1.65 g-atom) (Note
2) and dry
tetrahydrofuran (THF) (150 mL). The mixture is heated to reflux temperature under an atmosphere of dry
nitrogen, and a crystal of
iodine is added. The dropping funnel is filled with
1-chlorobutane (173 mL, 152.5 g, 1.65 mol) (Note 3) and a portion (15 mL) is added to the boiling THF mixture. The source of heat is removed. After the reaction has commenced, the THF begins to boil more vigorously and a further volume
(400 mL) of THF is added to the reaction mixture. Then the remainder of the
chlorobutane is added slowly at a rate sufficient to maintain the reaction under reflux. Finally, the reaction mixture is stirred and heated under reflux until all the
magnesium has been consumed (0.5–1 hr).
B.
Ethynylmagnesium chloride. While the
butylmagnesium chloride is being prepared, the 2-L flask is filled with dry
THF (500 mL), which is saturated by bubbling
acetylene through it for 0.5–1 hr (Note
4). The warm (ca. 60°C)
butylmagnesium chloride is rapidly poured under a liberal blanket of
nitrogen into the 1-L dropping funnel, which is then flushed with
nitrogen before being stoppered. The rapid flow of
acetylene is maintained. The 2-L flask is cooled to −5°C in a
dry ice–acetone bath, the
acetylene is bubbled rapidly through the THF (Note
4), and the
butylmagnesium chloride is added dropwise to the stirred reaction mixture at a rate sufficient to maintain the temperature below 20°C (Note
5). This addition requires 1 hr; then
acetylene is bubbled through the reaction mixture for a further 0.5 hr. (The mixture cools to about 5°C during this period.) The
acetylene supply is disconnected and replaced by dry
nitrogen.
C.
Trimethylsilylacetylene. A solution of
chlorotrimethylsilane (152 mL, 130 g, 1.197 mol) (Note
6) in dry
THF (100 mL) is placed in the 1-L dropping funnel and is added (20 min) to the cooled and stirred solution of
ethynylmagnesium chloride at a rate sufficient to maintain a reaction temperature of about 15–20°C (Note
7). Finally, the dropping funnel is replaced by an
efficient double-surface condenser and calcium chloride drying tube, and the reaction mixture is heated under reflux for 1 hr (Note
8). The reflux condenser is replaced by a distillation head and a double surface condenser is connected to a
receiver flask which is cooled in an
ice bath (Note
9). The reaction mixture is distilled under
nitrogen with stirring until all the azeotrope of
trimethylsilylacetylene and
THF (700–800 mL, bp ca. 66°C) has distilled (Note
10) and (Note
11). The distillate is washed with ice–water portions (10 × 500 mL) to remove the THF. Washing is continued until the organic layer stays constant in volume (Note
12). Distillation (Note
10) of the organic layer under an atmosphere of
nitrogen through a short Vigreux column (Note
13) gives
trimethylsilylacetylene, bp
50–52°C/760mm,
nD20 1.391; (lit.
2,3,4 53.5°C at 762 mm,
nD20 1.3900;
52°C at 760 mm,
nD20 1.3900;
52°C at 760 mm,
nD20 1.3935) in yields ranging from
72.5 g (
62%) to
87.5 g (
75%) (Note
14) and (Note
15).
2. Notes
1. The checkers used an
efficient magnetic stirrer.
2. The
magnesium turnings for Grignard reactions were supplied by Fisons Scientific Apparatus, Loughborough. If less than
1.65 g-atom of magnesium is used, the final product will be contaminated with
1-chlorobutane.
3. It is essential that the
1-chlorobutane be free of
1-butanol.
1-Chlorobutane (Aldrich Chemical Company, Inc.) was purified by rapid passage through basic alumina (activity 1) before use.
5. The temperature reaches 15°C after 0.5 hr, and 20°C after 0.8 hr. Ethynylmagnesium halides can rapidly disproportionate to
bis(chloromagnesium)acetylene and
acetylene at higher temperatures.
2 It is important to maintain the reaction mixture at or below 20°C and to have an excess of
acetylene in order to prevent formation of the
bis(magnesium chloride). The checkers found that the
ethynylmagnesium chloride can be formed at 10–15°C, thus minimizing the problem.
7. It is important that the addition be done fairly rapidly. The checkers found that slow addition (2 hr at 20°C) resulted in significant disproportionation of the
ethynylmagnesium chloride.
8. It is essential to have an efficient condenser during the reflux and distillation stages because the product,
trimethylsilylacetylene, is extremely volatile.
9. The receiver must be cooled to avoid serious loss of volatile product.
10. The hot distillation apparatus should be allowed to cool under a
nitrogen atmosphere before being dismantled.
11. The submitters distilled the azeotrope immediately after heating the mixture to reflux. If it is allowed to stand and cool, it sets solid with magnesium chloride and subsequent distillation results in appreciably lower yields.
12. The presence of residual THF in the final product is easily detected by the characteristic
1H NMR signals at δ 1.85 (4 H) and 3.75 (4 H). The checkers found GLC to be more convenient than
1H NMR for analysis at this point. Either a 6-m × 2-mm glass column of 3% OV-101 on WHP 80/100 with 30 mL/min He or a 15-m × 0.25-mm fused-silica capillary column of DB-5 (cross linked phenylsilicone) with 1 mL/min H
2 proved sufficient to resolve butane,
trimethylsilylacetylene,
butyltrimethylsilane, THF,
1-chlorobutane, and
bis(trimethylsilyl)acetylene.
13. The checkers used an
18-in. Vigreux column, equipped with an adjustable reflux ratio takeoff. The final product can be contaminated with up to several percent butane unless careful distillation is carried out.
14.
Trimethylsilylacetylene displayed the following spectroscopic properties: IR (CCl
4) cm
−1: 3280 (s) and 2050 (s);
1H NMR: (CDCl
3, 250 MHz) δ: 0.18 (s, 9 H, CH
3), 2.36 (s, 1 H,

CH).
15. The submitters have carried out the preparation on twice the above scale with no reduction in yield.
3. Discussion
Trimethylsilylacetylene has been prepared by silylation of a variety of ethynyl metal derivatives.
2,3,4,5,6,7,8,9,10 The most useful methods are the silylation of
ethynylmagnesium bromide3,4,5 and chloride.
2,7,10 The use of
ethynylmagnesium bromide has been reported to suffer from complicating side reactions,
2 and the results obtained in our hands were unreliable.
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