Checked by Nobuyoshi Nomura and Hisashi Yamamoto.
1. Procedure
For flash vacuum pyrolysis experiments on a preparative scale the apparatus shown in
f.htmigure 1 is used.
2 It consists of a vertically-mounted assembly of fused silica and
Pyrex tubing 3.2 cm in diameter. The high temperature section
A, 45 cm in length, is made from fused silica. It carries inner 29/32 ground glass joints. The upper evaporation section
B, 25 cm in length, is made of Pyrex. It is fitted with two outer 29/32 ground glass joints (Note
1). Both tubes are loosely filled with Raschig rings cut from thin-walled, fused silica tubing 1 cm in diameter. The evaporation zone is heated by glass fiber heating tape (180 × 1 cm) controlled by a variac transformer. Heat for the pyrolysis section is provided by an electric tube furnace (Note
2). The temperature is measured with a thermocouple fixed on the surface of the silica tube in the middle of the hot reaction zone. The substance to be pyrolyzed is added to the evaporator under reduced pressure and at a constant drop rate from a
250-mL, pressure-equalizing dropping funnel C (Note
3). A
250-mL, long-necked, round-bottomed flask D cooled to −78°C serves as a receiver. (
Caution! Flask D (Note 4) must be checked by visual inspection at regular intervals. If condensed product accumulates in the entrance neck, the cooling bath level must be lowered to prevent clogging. Flask D must always be cooled to the neck of the flask. Otherwise material might be transferred from the receiver into trap E, which might then become plugged). Flask
D is connected by a wide glass tube (Note
4) to a cooling trap
E kept at liquid
nitrogen temperature (Note
5).
Figure 1

At a pyrolysis
oven temperature of 630°C, an
evaporator temperature of 200°C, and a pressure of 12 mm,
2-toluic acid chloride (240 g, 1.55 mol) (Note
6) is added in about 2.5 hr at a constant rate of 40–60 drops/min (Note
7). The pressure should be monitored in the
addition funnel C during the pyrolysis. A sudden rise in pressure indicates that a restriction is building up between the pyrolysis reactor and the pump. In this case addition of reactant must be stopped. Ten minutes after the addition is complete the apparatus is flushed with
nitrogen. The dark brown, oily condensate in the receiver is dissolved in
500 mL of acetone and diluted with 250 mL of cold water (Note
8) and (Note
9). The homogeneous solution is stirred at room temperature for 1 hr. It then is separated into two phases by the addition of
500 mL of 2 N sodium carbonate solution and
250 mL of ether. The aqueous layers are extracted twice with
250-mL portions of ether. The
ether extracts are washed with 2 N
sodium carbonate solution and with saturated
sodium chloride solution. The combined
ether layers are dried over
sodium sulfate. Removal of the solvent and distillation of the residue yields
65 g of a yellow oil, bp
87–90°C/13 mm, with the following composition:
Benzocyclobutenone (
97%),
benzyl chloride (
3%) (Note
10). Fractionation at
12 mm through a 25-cm spinning band column (Note
11) gives
58 g of
benzocyclobutenone (>99%) as a colorless oil, bp
84–86°C/12 mm (Note
12) and (Note
13). The product crystallizes at 0°C and then melts at
10–15°C (Note
14).
Most of the
acetone is removed from the combined aqueous layers by distillation. Upon acidification with concentrated
hydrochloric acid a thick precipitate forms. Filtration and drying leads to
106 g of almost colorless 2-toluic acid, mp
103–107°C. This material is of sufficient purity for conversion into the acid chloride and for subsequent flash vacuum pyrolysis.
2. Notes
1. Teflon sleeves are used to assure the vacuum tight connection between the ground joints subjected to heat. The checkers used clear-seal joints in all of the glass connections.
2. Electric tube furnaces of appropriate dimensions are available from various manufacturers. A model RO 4/25 by Heraeus GmbH, Hanau, FRG is suitable. However, a very satisfactory furnace can be built by any well equipped laboratory workshop at little cost and effort. The material required consists of thin walled ceramic tubing, 3.5 cm i.d., nichrome resistance wire, heat resistant insulation, and ordinary hardware material. A technical drawing will be provided by the submitters upon request. The temperature of the furnace can be adjusted by an electronic temperature controller using a thermocouple sensor. A 1.5 kW-Variac transformer and any high temperature
thermometer would do as well for the budget-minded chemist.
3. A
dropping funnel with a needle valve, NORMAG type 8056, manufactured by NORMAG GmbH, D-6238 Hofheim, FRG, has been used by the submitters. The checkers used a standard pressure-equalizing dropping funnel with a Teflon stopcock.
4. Receiver
D is a
250-mL, round-bottomed flask with two, 15-cm long necks with a diameter of 3.5 to 5 cm (entrance neck) and 2.5 cm (exit neck) (see
f.htmigure 1). If a receiver
D with a narrower entrance neck than indicated is used the neck can become plugged by condensate. A wide-bore connection is used between receiver
D and the cold
trap E to prevent a pressure drop during pyrolysis, which might be caused by a restriction in the HCl-gas flow.
5.
Caution! Hydrogen chloride (bp −84.9°C) is condensed in trap
E. Exercise care in its disposal.
7. The pyrolysis temperature and the rate of addition are chosen such that about 50% of the acid chloride is recovered as
2-toluic acid after hydrolysis. Under these conditions only a small amount of
benzyl chloride and polymeric material is formed in addition to
benzocyclobutenone. The percentage of reactant conversion depends not only on the pyrolysis temperature, but also on the pressure in the reactor and on the rate of reactant addition. It is advisable, therefore, to optimize the pyrolysis temperature in trial runs keeping the other variables constant.
In the runs carried out by the checkers, conversion of reactant was slightly lower than
50% as shown by the recovery of
2-toluic acid in
63–74% yield. In these runs
benzocyclobutenone was obtained in 17–20% yield only. However, the yield calculated with respect to reactant consumed and not recovered as
2-toluic acid amounted to 56–64% and thus did not differ significantly from the value given in the procedure.
8. If separation into two phases occurs,
acetone is added until a homogeneous solution results.
9. Hydrolysis of unreacted acid chloride is carried out without neutralization of the acid generated. In the presence of a base such as
sodium carbonate or
sodium bicarbonate the anhydride of
2-toluic acid would be formed, in addition to the sodium salt of the free acid. This anhydride is much less reactive toward hydrolysis than the acid chloride.
10. VPC analyses were performed using a
PEG column of 25 m × 0.25 mm.
11. A
25-cm spinning band column of 12 theoretical plates, manufactured by NORMAG GmbH, D-6238 Hofheim, FRG, has been used by the submitters. The checkers found a
30-cm Vigreux distillation column just as suitable for the separation.
14. The spectral properties are as follows:
1H NMR (CDCl
3, 300 MHz) δ: 4.00 (s, 2 H), 7.34–7.45 (m, 2 H), 7.50–7.57 (m, 2 H); IR (liquid film) cm
−1: 3060, 2920, 1780, 1760, 1585, 955, and 755.
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The present pyrolytic method is based on a communication by the authors
13 with some modifications of the pyrolysis apparatus for preparative scale.
14 This method is applicable to the preparation of benzocyclobutenones carrying various substituents on the aromatic ring (see Table). Its merit lies in its simplicity for generating the strained bicyclic ring system from inexpensive precursors. Benzocyclobutenones in general are thermolabile, losing
carbon monoxide at higher temperature.
11 It therefore is advisable to keep thermolytic conversion below 100%. Unconverted starting material can be recovered after hydrolysis of the crude reaction mixture as carboxylic acid and reused for the conversion to acid
chloride without further purification. Gas phase pyrolysis has been available as a preparative method in organic chemistry for a long time.
15 Various procedures using this method have been described in previous volumes of
Organic Syntheses.
16 17 18 19 20 21 22 23 Several reviews describing applications of the pyrolytic method in preparative organic chemistry have appeared in recent years.
24 25
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