Organic Syntheses, CV 5, 249
Submitted by John P. Schaefer, J. G. Higgins, and P. K. Shenoy
1.
Checked by R. Breslow and J. T. Groves.
1. Procedure
A
1-l., three-necked, round-bottomed flask equipped with a
Trubore stirrer, a
pressure-equalizing dropping funnel, and a
reflux condenser with a
drying tube is charged with
350 ml. of acetonitrile (Note
1) and
106.4 g. (0.41 mole) of triphenylphosphine (Note
2). The flask is cooled in an
ice-water bath (Note
3), and
64 g. (0.40 mole) of bromine is added dropwise over a period of
ca. 15–20 minutes (Note
4) and (Note
5). The ice-water bath is removed, and a solution of
54 g. (0.40 mole) of cinnamyl alcohol in
50 ml. of acetonitrile is added in portions over a period of 5–10 minutes with continued stirring (Note
6). The solvent is removed by distillation with the use of a
water aspirator (30–40 mm.) and an
oil bath until the bath temperature reaches 120°. The water aspirator is replaced by a
vacuum pump and the
water-cooled condenser with an
air condenser, and the distillation is continued with rapid stirring (Note
7), (Note
8), and (Note
9). Most of the product (Note
10) distills at
91–98° (2–4 mm.), and about
59 g. of product crystallizes in the receiving flask (
63–75% yield) (Note
11).
The product is dissolved in
200 ml. of ether, and the solution is washed with
75 ml. of saturated aqueous sodium carbonate, dried over anhydrous
magnesium sulfate, and distilled to give
47–56 g. (
60–71%) of product, b.p.
66–68° (0.07 mm.),
84–86° (0.8 mm.); m.p.
29°.
2. Notes
2.
Triphenylphosphine was obtained from M and T Chemicals, Inc., and used without further purification.
5. The solid
triphenylphosphine disappears, but at the same time the adduct, (C
6H
5)
3PBr
2, precipitates as a white solid.
6. This addition is mildly exothermic, and the temperature rises to 50–60°. All the precipitate should dissolve at this point; warming by external heat may be necessary.
7. To protect the vacuum pump from damage a
dry ice-acetone trap and two
liquid nitrogen traps are necessary to condense and solidify the
hydrogen bromide evolved.
8. The receiving flask is placed in an ice-water bath.
9. The distillation is continued until the
triphenylphosphine oxide solidifies and no more product distills. The oil bath is maintained at 130–140° during the distillation.
10. Some product is carried over by the
hydrogen bromide in the initial stages of the distillation.
11. When this distillation was replaced by a procedure in which the
acetonitrile was removed with a
rotary evaporator and
steam bath, and the product was extracted from the
triphenylphosphine oxide with small portions of
acetonitrile totaling ca. 250 ml., the checkers obtained an improved yield (
79%) of
cinnamyl bromide.
3. Discussion
4. Merits of the Preparation
The method described is general for converting alcohols to alkyl halides and is stereospecific.
6
This preparation is referenced from:
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