Highly Brominated Norbornanes by Photobromination as Precursors for the Convenient Synthesis of 2,3,5,6-Tetrabromo- and 2,3,5,6-Tetramethoxy-substituted Norbornadienes

Abstract

An efficient synthesis is described for hexabromonorbornane by photobromination of norbornadiene. Double dehydrobromination of the hexabromide by t-BuOK affords in high yield 2,3,5,6-tetrabromonorbornadiene, its copper-assisted treatment with sodium methoxide leads to the 2,3,5,6-tetramethoxy derivative. The tetrabromide constitutes a valuable precursor for the preparation of functionalized tetrasubstituted norbornadienes.

Key words

bromine - halides - photobromination - bromonorbornane - bromonorbornadiene

References

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8

2,2,3,5,5,6-Hexabromonorbornane(5): A solution of bromine (14.4 g, 90.0 mmol) in CCl₄ (15 mL) was added at reflux within 10 min to a magnetically stirred solution of norbornadiene (1.38 g, 15 mmol) in CCl₄ (35 mL), in the photochemical apparatus, which was supplied with an addition funnel, a condenser and a 250-W projector lamp. The reaction mixture was irradiated for 50 h at reflux, the solvent and excess bromine were removed (20 °C, 10 Torr), and the residue was passed through a short Al₂O₃ (20 g) column by eluting with hexane. Recrystallization from ether-petroleum ether by allowing the sample to stand overnight in the refrigerator (ca. 5 °C) afforded 5.39 g (63%) of hexabromide 5 as colorless needles, mp 143-144 °C. Anal. Calcd for C₇H₆Br₆: C, 14.76; H, 1.06. Found: C, 14.84; H, 1.37.

9

2,3,5,6-Tetrabromonorbornadiene(9): To a stirred solution of the hexabromide 5 (5.71 g, 10.0 mmol) in dry and freshly distilled THF (80 mL) was added potassium tert-butoxide (2.46 g, 22.0 mmol) in dry and freshly distilled THF (40 mL). The resulting reaction mixture was magnetically stirred for 12 h at ca. 20 °C, diluted with water (70 mL), the phases separated. The aq solution was extracted with ether (50 × 3 mL), the combined organic layers were washed with water (2 × 40 mL), and dried over anhyd MgSO₄. After evaporation of the solvent (20 °C, 10 Torr), the residue was placed on a short silica-gel (10 g) column and eluted with petroleum ether. Recrystallization from CH₂Cl₂-petroleum ether by allowing the sample to stand in the refrigerator (ca. 5 °C) afforded 3.67 g (90%) of the pure tetrabromide 9 as colorless needles, mp 125-126 °C. Anal. Calcd for C₇H₄Br₄: C, 20.62; H, 0.99. Found: C, 20.54; H, 0.92.

11

2,3,5,6-Tetramethoxynorbornadiene(10): Freshly cut sodium metal (1.38 g, 60.0 mmol) was dissolved in dry methanol (30 mL) and after complete dissolution, dry dimethylformamide (DMF, 30 mL) was added, followed by vacuum-dried cuprous iodide (480 mg, 2.5 mmol) and tetrabromide 9 (2.04 g, 5.0 mmol) in dry DMF (30 mL). The reaction mixture was magnetically stirred for 15 h under an argon-gas atmosphere at reflux (ca. 100 °C), the reaction progress was monitored by TLC. After cooling to r.t., the solvent (20 °C, 10 Torr) was removed until a final volume of ca. 20 mL. In order to prevent autooxidation of the air-sensitive product, the concentrate was diluted with 1:1 ether-pentane (50 mL) and passed through basic Al₂O₃ (activity III, 20 g) column under an argon-gas atmosphere. Removal of the solvent (25 °C, 20 Torr) gave 636 mg (60%) of the pure 2,3,5,6-tetramethoxynorbornadiene(10) as an oil, which was kept under an argon-gas atmosphere in the freezer (ca. -20 °C).