A
4-L, three-necked, round-bottomed flask equipped with a
mechanical stirrer,
500-mL pressure-equalizing dropping funnel,
thermometer, and
reflux condenser provided with a
calcium chloride drying tube is charged with
202.6 g (1.0 mol) of 1-chloro-2,4-dinitrobenzene (Note
1) and
1.2 L of dry pyridine (Note
2). The mixture is heated while it is stirred in a
water bath to 80–90°C for 4 hr, during which time a thick yellow precipitate of
N-(2,4-dinitrophenyl)pyridinium chloride is formed (Note
3). After cooling to 0°C a solution of
100.0 g (2.22 mol) of dimethylamine in
300 mL of dry pyridine was prechilled to 0°C and added dropwise over a period of about 30 min with stirring. The resulting brownish-red liquid reaction mixture is allowed to warm to room temperature and stirring is continued for 12 hr. The drying tube is replaced by a gas inlet and the system is flushed with dry
nitrogen in a
hood. Under
nitrogen,
70.0 g (1.06 mol) of ice-cold, freshly distilled cyclopentadiene (Note
4) is added, and subsequently
400 mL of 2.5 M sodium methoxide solution (Note
5) is slowly added dropwise to the stirred reaction mixture. During addition of the
sodium methoxide, the temperature rises to 35–40°C. After the addition is completed, stirring is continued for another 4 hr. The reaction vessel is immersed in an
oil bath, the dropping funnel removed, and the flask is fitted with a distillation head. The stirred mixture is cautiously heated under
nitrogen (Note
6), and a mixture of
pyridine and
methanol is distilled off until the temperature of the reaction mixture has increased to 105–110°C (Note
7). After the distillation head is removed and
1 L of dry pyridine added, the black mixture is heated with stirring under a
nitrogen atmosphere for 4 days with a bath temperature of 125°C. It is then cooled to 60°C, the reflux condenser is replaced by a distillation head, and
pyridine is removed under reduced pressure (Note
8). The gummy black solid residue is removed by a
spatula and rinsed with hexanes. It is extracted in a Soxhlet apparatus with
1.5 L of hexanes in several batches. To remove the remaining
pyridine, the combined blue
hexane rinse and the extraction solutions are carefully washed with two
150-L portions of 10% aqueous hydrochloric acid, then water (Note
9). The organic layer is dried with anhydrous
sodium sulfate, the drying agent is removed by filtration, and the solvent is distilled through a
50-cm vacuum-jacketed Vigreux column. The crude
azulene is purified by chromatography on activity II
alumina (Note
10) with
hexane and yields
azulene as blue plates, mp
96–97°C, yield
65–75 g (
51–59%) (Note
11).