N-Bromosuccinimide and Lithium Bromide: An Efficient Combination for the Dibromination of Carbon-Carbon Unsaturated Bonds

Abstract

Compounds possessing unsaturated bonds such as ­alkenes, alkynes, allenes, and methylenecyclopropanes (MCPs) can be dibrominated within minutes by NBS and lithium bromide in THF at room temperature in good to excellent yields under mild conditions.

Key words

NBS - lithium bromide - unsaturated compounds - di­bromination reaction

References and Notes

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9

Alkene 1a (0.2 mmol), LiBr (0.4 mmol), NBS (0.4 mmol), and THF (2.0 mL) were added to a Schlenk tube successively under an ambient atmosphere. The mixture was stirred at r.t. for the appropriate time. Sat. aq Na₂S₂O₃ was added to quench the reaction, the organic layer was washed, then dried over anhyd Na₂SO₄. Flash column chromatogra-phy gave the pure product 2a as a white solid. ¹H NMR (CDCl₃, 300 MHz, TMS): δ = 3.98-4.11 (m, 2 H), 5.14 (dd, 1 H, J = 5.4, 9.9 Hz), 7.34-7.42 (m, 5 H, Ar). ¹³C NMR (CDCl₃, 75 MHz, TMS): δ = 34.99, 50.84, 127.62, 128.83, 129.15, 138.56. MS: m/z (%) = 262 (M⁺, 1), 185 (86), 183 (90), 104 (100).

12

MCP 1a (0.2 mmol), LiBr (0.4 mmol), and THF (2.0 mL) were placed in a Schlenk tube under an ambient atmosphere and the mixture was stirred for about 10 min at r.t. Then NBS (0.4 mmol) was added and the mixture was stirred at r.t. for the appropriate time. Sat. aq Na₂S₂O₃ was added to quench the reaction. The mixture was washed and dried over anhyd Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by flash column chromatogra-phy to give product 8a as a colorless liquid. IR (CH₂Cl₂): 3933, 3054, 2985, 1486, 1443, 1265, 896, 746 cm^-1.
¹H NMR (CDCl₃, 300 MHz, TMS): δ = 3.07 (t, 2 H, J = 6.9 Hz), 3.62 (t, 2 H, J = 6.9 Hz), 7.20-7.32 (m, 10 H, Ar). ¹³C NMR (CDCl₃, 75 MHz, TMS): δ = 31.04, 40.76, 123.20, 127.42, 127.57, 128.06, 128.51, 128.72, 128.83, 140.31, 142.68, 144.87. MS: m/z (%) = 368 (29), 366 (60), 364 (M⁺, 34), 192 (100). HRMS: m/z calcd for C₁₆H₁₄Br₂: 363.9457; found: 363.9470.

14

We found that the reaction mixture quickly changed from colorless to deep orange as soon as NBS was added. After the reaction was complete, the reaction solution changed to pale yellow. When the reaction was quenched with a sat. solution of aq Na₂S₂O₃, the reaction mixture immediately changed from pale yellow to colorless. These phenomena suggest that molecular Br₂ is involved in the equilibrium.

15

We were unsuccessful in obtaining a crystal structure, however, the white solid obtained from the reaction of NBS and LiBr in THF decomposed to pyrrolidine-2,5-dione, which was confirmed by X-ray diffraction.

16

The yields in the presence of TEMPO and BHT were 99% and 96%, respectively.