A Convenient Route to 2-Bromo-3-chloronorbornadiene and 2,3-Dibromonorbornadiene

This paper is dedicated to Prof. K. P. C. Vollhardt on occasion of his 69^th birthday. Thank you for inspirational leadership and enlightening supervision.

Abstract

Substituted norbornadienes are useful in a wide range of applications, including molecular solar-thermal (MOST) energy storage systems. An important precursor for 2,3-substituted norbornadienes is 2-bromo-3-chloronorbornadiene, where the two halogen atoms can be substituted selectively through two consecutive Suzuki cross-coupling reactions. Previous routes to 2-bromo-3-chloronorbornadiene have used 1,2-dibromoethane as a brominating agent, a substance known to be carcinogenic and the use of which is restricted in certain countries. Herein is reported a one-pot route to 2-bromo-3-chloronorbornadiene in 50% yield using p-toluenesulfonyl bromide as a bromine source. In addition, the procedure has been adapted to allow synthesis of 2,3-dibromonorbornadiene in 37% yield.

Key words

bromine - chlorine - organometallic reagents - regioselectivity - alkenes

• References and Notes

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• 19 p-Toluenesulfonyl Bromide p-Toluenesulfonyl hydrazide (50 g, 0.27 mol) was mixed with CHCl₃ (500 mL) at 0 °C. Small amounts of ice were also added to the reaction mixture throughout the reaction. Bromine (27 mL, 0.54 mol) was added in small portions allowing the orange color to disappear between each addition. If, when the last portion of bromine was added, the color was still white, a small amount of extra bromine was added until a light orange colour persisted. The phases were separated and the organic phase was washed with sat. aq NaHCO₃ solution (200 mL) and 1% aq Na₂S₂O₃ solution (100 mL). The organic phase was dried over Na₂SO₄, filtered, and the solvent was removed on a rotary evaporator. The product was ground to powder and dried at 5·10^–2 mbar until NMR showed no traces of solvents or H₂O; yield: 59.6 g (94%).
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• 21 2-Bromo-3-chloronorbornadiene (3) Potassium tert-butoxide (11.2 g, 0.10 mol) was dissolved in THF (200 mL), and the solution was cooled to –84 °C. Norbornadiene (12.2 mL, 0.12 mol) was added, followed by n-BuLi (2.5 M in hexanes, 40 mL, 0.10 mol) over 60 min. The yellow solution was stirred for 5 min at –84 °C and 60 min at –41 °C. The solution was cooled again to –84 °C, and p-toluenesulfonyl chloride (19.0 g, 0.10 mol) was added. The mixture was stirred for 30 min and n-BuLi (2.5 M in hexanes, 40 mL, 0.10 mol) was added over 60 min. The mixture was stirred for 5 min at –84 °C and 60 min at –41 °C. The solution was cooled to –84 °C, and p-toluenesulfonyl bromide (23.4 g, 0.10 mol) was added. The mixture was stirred for 15 min and was then heated to ambient temperature on a room-tempered water bath. The reaction mixture was quenched with H₂O (50 mL), the phases were separated, and the aqueous phase was extracted with Et₂O (3 × 20 mL). The solvents from the combined organic phases were slowly removed on a rotary evaporator (40 °C, 300 mbar). The residue was dissolved in pentane (50 ml), washed with H₂O (10 × 10 mL), brine (20 mL), and dried over Na₂SO₄. The solvent was removed in vacuo (40 °C, 300 mbar) and the product distilled (2 × 10^–2 mbar) using a short Vigreux column, collecting the main fraction at 25–27 °C. This afforded 3 as a colorless liquid; yield 10.24 g (50%). Analytical data were consistent with previous reports.¹⁴ The product was stored at –20 °C and should be used within a few weeks to avoid decomposition.
• 22 2,3-Dibromonorbornadiene (8) Potassium tert-butoxide (11.2 g, 0.10 mol) was dissolved in THF (200 mL), and the solution was cooled to –84 °C. Norbornadiene (12.2 mL, 0.12 mol) was added followed by n-BuLi (2.5 M in hexanes, 40 mL, 0.10 mol) under 60 min. The yellow solution was stirred for 5 min at –84 °C and 60 min at –41 °C. The solution was cooled again to –84 °C, and p-toluenesulfonyl bromide (11.7 g, 0.050 mol) was added. The mixture was stirred for 15 min at –84 °C and 60 min at –41 °C. The solution was cooled to –84 °C, and p-toluenesulfonyl bromide (11.7 g, 0.05 mol) was added. The mixture was stirred for 15 min and was then heated to ambient temperature on a room-tempered water bath. The reaction mixture was quenched with H₂O (50 mL), the phases were separated, and the aqueous phase was extracted with Et₂O (3 × 20 mL). The solvents from the combined organic phases were slowly removed on a rotary evaporator (40 °C, 300 mbar). The residue was dissolved in pentane (50 mL), washed with H₂O (10 × 10 mL), brine (20 mL), and dried over Na₂SO₄. The solvent was removed in vacuo (40 °C, 300 mbar) and the product distilled (5 × 10^–2 mbar) using a short Vigreux column, collecting the main fraction at 29–31 °C. This afforded 8 as a colorless liquid; yield 4.63 g (37%). Analytical data were consistent with previous reports.¹⁴ The product was stored at –20 °C and should be used within a few weeks to avoid decomposition.