Checked by Daniel V. Paone and Amos B. Smith, III.
1. Procedure
A.
Allylic bromination of 1,5-cyclooctadiene. A
2-L, three-necked, round-bottomed flask, equipped with a
mechanical stirrer,
reflux condenser, and a
heating mantle, is charged with
216.4 g (2.0 mol) of 1,5-cyclooctadiene (Note
1),
44.5 g (0.25 mol) of N-bromosuccinimide (NBS),
0.5 g of benzoyl peroxide, and
700 mL of carbon tetrachloride. The mixture is heated to gentle reflux with stirring. When the reaction starts, a rapid reflux is observed. Three more
44.5-g portions (0.25 mol) of NBS are added at 30-min intervals (total 178 g, 1.0 mol). Heating is continued for 1.5 hr after addition of the final portion of NBS. The mixture is cooled to room temperature and suction filtered and the filter cake is washed with
150 mL of carbon tetrachloride (Note
2). The filtrate is washed once with 150 mL of water, dried over
calcium chloride, and filtered with suction. A vacuum distillation apparatus consisting of a
500-mL, two-necked, round-bottomed flask, a
stoppered pressure-equalizing dropping funnel, a
distilling head (25 cm long) packed with Pyrex glass tips or helices, a condenser, and
receivers is assembled (Note
3). The dried
carbon tetrachloride solution is transferred to the dropping funnel, the system is evacuated to 150 mm, and the solution is introduced continuously from the dropping funnel resulting in removal of the bulk of the solvent (Note
4). The pale yellow residue is then fractionally distilled first at 30 mm to remove the unreacted
1,5-cyclooctadiene (Note
5), and then at 5 mm to distill the bromocyclooctadienes to give
113–121 g (
60–65%) of a mixture of
3-bromo-1,5-cyclooctadiene and 6-bromo-1,4-cyclooctadiene,
2 bp
66–69°C at 5 mm (Note
6),(Note
7),(Note
8).
B.
1,3,5-Cyclooctatriene. A
1-L, three-necked, round-bottomed flask, equipped with a
magnetic stirring bar, pressure-equalizing dropping funnel,
immersion thermometer, and a condenser bearing a
gas inlet vented through a mineral oil bubbler, is charged with
25.9 g (0.35 mol) of lithium carbonate,
2.0 g (0.047 mol) of lithium chloride (Note
9), and
400 mL of dry dimethylformamide (DMF) (Note
10). The magnetically stirred mixture is heated to 90°C in an
oil bath (Note
11) and
113.5 g (0.607 mol) of the bromocyclooctadiene mixture (Part A) is added dropwise via the dropping funnel over 50 min. During the addition, rapid evolution of gas (
carbon dioxide) is observed via the bubbler. After completion of the addition, heating is continued for 1 hr at 90–95°C. The mixture is cooled to room temperature, diluted with 1 L of ice water, and the mixture extracted twice with
200-mL portions of pentane. The combined organic phase is washed twice with 100-mL portions of water, dried over
sodium sulfate, and filtered. The filtrate is distilled at atmospheric pressure to remove the
pentane, and the residue is distilled under reduced pressure, employing a
short (12 cm) Vigreux column, to give
54–58 g (
84–90%) of almost pure
1,3,5-cyclooctatriene, bp
63–65°C at 48 mm (Note
12).
2. Notes
1. All reagents and solvents are commercially available and are used without further purification.
2. The solids consisted of
94.0 g of succinimide (0.95 mol, 95% of theoretical).
3. The distillation system was connected to a closed-tube manometer and a
Cartesian diver-type pressure regulator employed to control the pressure.
4. When a
rotary evaporator was used for the concentration, the recovery of
1,5-cyclooctadiene decreased substantially.
5. Approximately
88.5 g (0.82 mol,
82% of theoretical) of
1,5-cyclooctadiene, bp
55–57°C at 30 mm, is recovered and can be recycled.
6. Care must be taken during fractionation of
1,5-cyclooctadiene and the bromocyclooctadienes, because contamination of the
bromide with
1,5-cyclooctadiene leads to contamination of
1,3,5-cyclooctatriene with the diene. A 1–2-mL intermediate fraction effects clean separation. The distillation took the checkers 6–8 hr. The bromides are extremely light sensitive, turning yellow to red-brown quickly. To avoid product coloration all product receiving flasks were wrapped in
aluminum foil.
7. When
NBS was added in two portions instead of four, the yield of bromocyclooctadienes decreased slightly to
60%. A preparation using
500 g (2.80 mol) of NBS (five-portion addition) gave a higher yield (
78%).
8. The spectral data for the mixture of bromocyclooctadienes is as follows:
1H NMR (500 MHz, CDCl
3) δ: 1.7–3.4 (m, 12 H), 4.6–5.25 (m, 2 H), 5.3–5.9 (m, 8 H);
13C NMR (125 MHz, CDCl
3) δ: 25.0, 27.7, 28.3, 28.7, 34.3, 36.8, 48.0, 49.9, 124.7, 127.0, 128.3, 128.8, 129.2, 130.4, 130.6, 131.7; IR (thin film) cm
−1: 3000, 2940, 2890, 2820, 1640, 1480, 1450, 1440, 1420, 1220, 1150, 1140, 990, 910, 860, 805, 670.
10. DMF dried azeotropically with
benzene is sufficient for the present reaction.
11. The checkers employed a heating mantle.
12.
1,3,5-Cyclooctatriene exists in equilibrium with
bicyclo[4.2.0]octa-2,4-diene, its valence isomer (ratio =

7:1).
3 4 5 6 1,3,5-Cyclooctatriene exhibits the following spectral data:
1H NMR (500 MHz, CDCl
3) δ: 2.43 (s, 4 H), 5.50–6.00 (m, 6 H);
13C NMR (125 MHz, CDCl
3) δ: 28.0, 125.9, 126.7, 135.5; IR (thin film) cm
−1: 3000, 2920, 2875, 2830, 1635, 1605, 1445, 1425, 1220, 690, 635. Signals for the minor valence isomer may be observed. No signals for
1,3,6-cyclooctatriene, another possible isomer, are observed.
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Mixtures of
1,3,5- and 1,3,6-cyclooctatriene were obtained by partial reduction of
cyclooctatetraene in ways such as protonation of cyclooctatetraene dianion
6,7 and reduction with zinc-alkali.
2,8 1,3,6-Cyclooctatriene is the major product in these reductions. However, since
1,3,6-cyclooctatriene isomerizes to
1,3,5-cyclooctatriene on treatment with base, quenching cyclooctatetraene dianion with
methanol and subsequent heating affords
1,3,5-cyclooctatriene in an
80% yield.
6 Reduction of
cyclooctatetraene with
sodium hydrazide and hydrazine also produces
1,3,5-cyclooctatriene.
9 Therefore, when
cyclooctatetraene is available in quantity, these procedures are the methods of choice.
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
NBS
1,3,5- and 1,3,6-cyclooctatriene
LiCl
Li2CO3
calcium chloride (10043-52-4)
Benzene (71-43-2)
methanol (67-56-1)
bromide (24959-67-9)
sodium sulfate (7757-82-6)
carbon tetrachloride (56-23-5)
aluminum (7429-90-5)
carbon dioxide (124-38-9)
Pentane (109-66-0)
hydrazine (302-01-2)
benzoyl peroxide (94-36-0)
Succinimide (123-56-8)
dimethylformamide,
DMF (68-12-2)
N-bromosuccinimide (128-08-5)
lithium carbonate (554-13-2)
Lithium chloride (7447-41-8)
1,5-cyclooctadiene
sodium hydrazide
1,3,5-Cyclooctatriene (1871-52-9)
cyclooctatetraene
3-Bromo-1,5-cyclooctadiene (23346-40-9)
6-Bromo-1,4-cyclooctadiene (23359-89-9)
bromocyclooctadiene
bicyclo[4.2.0]octa-2,4-diene
1,3,6-cyclooctatriene
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