Organic Syntheses, CV 5, 929
Submitted by R. J. Petersen and P. S. Skell
1.
Checked by Thomas R. Lynch and Peter Yates.
1. Procedure
Caution! This reaction should be carried out behind a safety screen.
A
1-l. three-necked flask is fitted with a
reflux condenser, an
addition funnel, and a
thermometer. It is charged with
450 ml. of 95% ethanol,
230 ml. (236 g.) of 85% hydrazine hydrate (Note
1), and several porcelain boiling chips. The solution is brought to reflux with a
heating mantle.
Cinnamaldehyde (200 g., 1.51 moles) (Note
2) is added dropwise over a period of 45 minutes to the refluxing solution, while the mixture turns orange because of the formation of
cinnamalazine in a side reaction (Note
3). After an additional 30 minutes at reflux, the flask is fitted with a simple takeoff head, and
ethanol, water, and
hydrazine hydrate are slowly removed by distillation. After approximately 3 hours the pot temperature rises to 200°, and
phenylcyclopropane begins to codistil with the last of the
hydrazine hydrate (Note
4). The distillate from this point is collected in a
250-ml. receiver, the main fraction coming over at 170-80°. When the pot temperature exceeds 250°, the decomposition is essentially complete (Note
5), (Note
6).
The crude, cloudy distillate (110–130 g.) is washed twice with 100-ml. portions of water and dried over anhydrous
potassium carbonate. Distillation at reduced pressure, b.p.
60° (13 mm.),
79–80° (37 mm.), through a
12-in. Vigreux column gives
phenylcyclopropane pure enough for most purposes; yield
80–100 g. (
45–56%),
n25D 1.5309.
2. Notes
3. Reversal of the addition procedure results in formation of
cinnamalazine as a major product.
5. The checkers found in a full-scale run that the pot temperature had to be raised to close to 250° before the onset of reaction, which was then very vigorous.
6.
Pyrazoline vapors are known to be rather flammable. It is advisable, therefore, to cool the pyrolysis flask somewhat before dismantling the apparatus. The syrupy residue in the flask sets to a hard mass on cooling; it can be removed by heating under
dimethylformamide on a
steam bath overnight.
3. Discussion
4. Merits of the Preparation
The procedure outlined is much quicker and simpler than previous methods. Starting materials are readily available, and the preparation can be run on any scale in the length of a day. Because exclusion of a basic catalyst eliminates the Wolff-Kishner reduction of the
cinnamalhydrazone, separation of the
5-phenylpyrazoline from
cinnamalhydrazone, or of
phenylcyclopropane from
propenylbenzene, does not have to be effected. The present procedure can also be used to convert other ring-substituted cinnamaldehydes to the corresponding arylcyclopropanes.
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