Organic Syntheses, CV 9, 129
Checked by Paul N. Devine and Ichiro Shinkai.
1. Procedure
CAUTION! Propiolates and their bromo derivatives are lachrymators and must be handled under an
efficient hood. Distillation of bromopropiolates should be carried out behind a safety shield (Note
1).
Methyl 3-bromopropiolate. A
250-mL, one-necked, round-bottomed flask equipped with a
magnetic stirring bar is charged with
100 mL of acetone (Note
2) and
4.0 g (47.6 mmol) of methyl propiolate (Note
3). To the stirred solution at room temperature is added
0.8 g (4.7 mmol) of silver nitrate. After 5 min,
9.8 g of N-bromosuccinimide (55 mmol) is added at once. The homogeneous mixture becomes cloudy and a grayish precipitate develops. Stirring is continued for 2 hr (Note
4). The solids are filtered through a pad of Celite, which is rinsed with
acetone (30–50 mL). After careful rotary evaporation of the
acetone at ≈20°C under 20 mm, the oily residue is bulb-to-bulb distilled at room temperature under reduced pressure (≈0.1 mm), affording
methyl 3-bromopropiolate as a colorless liquid (
7.0–7.5 g, 42.9–46.0 mmol,
90–97%) solidifying in the
refrigerator (mp
≈20°C) (Note
5).
tert-Butyl 3-bromopropiolate.
tert-Butyl propiolate (Note
6) (5.42 g, 42.8 mmol) dissolved in
150 mL of acetone is treated as above with
0.8 g of silver nitrate (4.7 mmol) and, after 5 min of stirring at room temperature, with
9.8 g (55 mmol) of N-bromosuccinimide introduced at once into the suspension. Stirring is continued for 90 min. The solids are filtered through a pad of Celite, rinsed with
acetone (30–50 mL) and the filtrate is concentrated at 20–25°C (20 mm) to give a white pasty solid. Water (80 mL) is added and the mixture is extracted with
ether (3 × 80 mL). The ethereal layer is dried over anhydrous
magnesium sulfate and the solvent is evaporated, leaving a semi-solid residue which is bulb-to-bulb distilled in an oil-bath at
45–50°C (≈0.1 mm) to give
tert-butyl 3-bromopropiolate as a white semi-solid (
8.51 g, 41.5 mmol,
97%) (Note
7).
2. Notes
1. Explosions during distillation of certain bromoalkynes have been reported.
2 Although
methyl 3-bromopropiolate was not specifically cited, precautionary measures are recommended. The tert-butyl ester is a new compound
3 and must be handled like the methyl ester.
2.
Acetone may be redistilled before use to remove the eventual autocondensation product,
4-hydroxy-4-methyl-2-pentanone (diacetone alcohol, bp 166°C). The submitter used, as received, fresh
99% pure acetone from Prolabo.
4. One hour of stirring is usually sufficient for completion of the reaction, but this time can be exceeded; longer reaction times improve the succinimide particle size, which aids the subsequent filtration. The product seems to adhere to the succinimide.
5. Depending on the conditions of the preliminary evaporation this product is contaminated with up to 5% of residual
acetone. Nevertheless, it can be considered as pure enough for certain uses. Complete removal of
acetone may be obtained by distillation in a
small Vigreux column, although a yellowing of the distillate is observed: bp
86–88°C/100 mm (lit.
2 88°C/100 mm and lit.
4 40–45°C/5 mm). The spectral and analytical properties of
methyl 3-bromopropiolate are as follows:
1H NMR (200 MHz, CDCl
3) d: 3.73 (s, CH
3);
13C NMR (50 MHz, CDCl
3) δ: 52.8 (C-3), 52.9 (CH
3), 72.4 (C-2), 152.8 (C-1). Anal. Calcd for C
4H
3BrO
2 (undistilled sample): C, 29.48; H, 1.86; Br, 49.03. Found: C, 29.11; H, 1.92; Br 48.79.
7. Although pure enough to be used as obtained,
tert-butyl 3-bromopropiolate may be distilled in a Vigreux column at
75–77°C (15 mm) to give a colorless oil crystallizing as plates: mp
25–27°C. The spectral and analytical properties of
tert-butyl 3-bromopropiolate are as follows:
1H NMR (200 MHz, CDCl
3) δ: 1.46 (s, CH
3);
13C NMR (50 MHz, CDCl
3) δ: 27.8 (CH
3), 50.0 (C-3), 73.9 (C-2), 84.0 (CMe
3), 151.3 (C-1). Anal. Calcd for C
7H
9BrO
2 (undistilled sample): C, 41.00; H, 4.42; Br, 38.48. Found: C, 40.48, H, 4.40; Br, 38.10.
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The present procedure provides ready access to 3-bromopropiolic esters, the methyl ester requiring adapted work up, because of its low boiling point. Less volatile esters, like tert-butyl, can be conveniently isolated by a standard aqueous-extraction work up.
Methyl 3-bromopropiolate has been used in Diels-Alder reactions either as a methoxycarbonyl ketene equivalent
9 10 or for the synthesis of functionalized naphthalenes.
11
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