Organic Syntheses, CV 5, 263
Submitted by J. A. Skorcz and F. E. Kaminski
1.
Checked by V. Z. Williams and K. B. Wiberg.
1. Procedure
A
2-l. three-necked flask is thoroughly dried and fitted with a large
dry-ice condenser, a
mechanical stirrer, a
nitrogen inlet, and a powder funnel in an
efficient hood. With
nitrogen flowing through the system,
62.5 g. (1.60 moles) of commercial sodium amide (Note
1) is added rapidly.
(Caution! Sodium amide is corrosive and readily decomposes in the presence of moisture.) The funnel is replaced by a gas-inlet tube, the condenser is filled with a mixture of dry ice and
acetone, and
ca. 400 ml. of liquid ammonia is introduced from a cylinder. The gas-inlet tube is replaced by an addition funnel, stirring is commenced, and
66.3 g. (0.400 mole) of o-chlorohydrocinnamonitrile (Note
2) is added over a 10-minute period. The last traces of the nitrile are washed into the flask with small amounts of anhydrous
ether.
The dark green reaction mixture is stirred vigorously for 3 hours and then is treated carefully with
96 g. (1.2 moles) of solid ammonium nitrate (Note
3). All the fittings are removed from the flask, and the
ammonia is allowed to evaporate (Note
4). Water (300 ml.) is added cautiously to the residue.
(Caution! Traces of undecomposed sodium amide may adhere to the upper walls of the flask.) The organic layer is taken up in two
160-ml. portions of chloroform, and the solutions are combined and washed twice with
100 ml. of 5% hydrochloric acid and once with 100 ml. of water.
(Caution! The extraction procedure and subsequent chloroform distillation should be conducted in a hood because some hydrogen cyanide is usually evolved.) The
chloroform solution is dried over anhydrous
sodium sulfate, and the
chloroform is removed by distillation. The residual liquid is distilled under reduced pressure through an insulated,
5-in. Vigreux column. The forerun, b.p.
95–100° (3 mm.), weighs
ca. 1 g.; the product boils at
100–101° (3 mm.);
n25D 1.5451. The yield of
1-cyanobenzocyclobutene is
33–34 g. (
64–66%) (Note
5) and (Note
6).
2. Notes
1. The
sodium amide was obtained from Farchan Research Laboratories and was approximately 90% pure.
4. Overnight evaporation at room temperature is convenient.
5. The submitters carried out the reaction on 1-molar and 3-molar scales and obtained yields of
62–64% and
67%, respectively.
3. Discussion
4. Merits of the Preparation
Cyclization by addition of a side-chain carbanion to an aryne bond has been proposed as the method of choice for synthesis of the versatile 1-substituted
benzocyclobutene system.
7 This general procedure now has been modified to permit convenient large-scale preparations utilizing a commercially available base, a minimum amount of liquid
ammonia, and distillation for isolation of the product.
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