A Mild Exchange Reaction of Xanthates with Bromine

Abstract

Secondary S-alkyl-O-ethyl (or O-neopentyl) xanthates can be converted into the corresponding bromides by heating with ethyl 2-bromo-2-methylpropionate and cumyl peroxide in refluxing chlorobenzene; this transformation can be coupled to a xanthate transfer radical addition to an olefin.

Key words

bromine atom transfer - xanthates - radical reactions - lactones

References

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• 5b

  Typical Procedure: 1 g (4.9 mmol) of xanthate 1a and 1.8 g (9.8 mmol) of olefin 2b dissolved in 6 mL of 1,2-dichloroethane were refluxed for a few minutes under nitrogen before addition of 98 mg (0.24 mmol) of commercially available lauroyl peroxide. The reflux is maintained for another 2 h. The solvent was then removed under reduced pressure and the residue was purified by chromatography on silica gel (PE/EtOAc 9/1-7/3) to give 1.8 g of a white solid (mp 107 °C) 3b (92%). ¹H NMR (CDCl₃, 200 MHz, ppm): δ = 7.92-7.83 (2 H, m), 7.80-7.70 (2 H, m), 4.30 (2 H, s), 4.29 (1 H, m), 4.03 (1 H, dd, J = 7-14 Hz), 3.97 (1 H, dd, J = 7-14 Hz), 2.74-2.48 (2H, m), 2.28-1.96 (2 H, m), 1.00 (9 H, s). ¹³C NMR (CDCl₃, 50 MHz, ppm): δ = 212.0 (C=S), 168.0 (C=O), 134.4 (CHAr), 131.9 (CqAr), 123.7 (CHAr), 118.7 (CN), 84.0 (CH₂O), 48.8 (CHS), 40.4 (CH₂), 32.0 (CMe₃), 28.3 (CH₂), 26.6 (CMe ₃), 15.0 (CH₂). IR (CCl₄, cm^-1) 2962, 2250, 1777, 1723, 1392, 1227, 1064. MS (CI): [MH]⁺ = 391, [MNH₄]⁺ = 408.

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6

0.20 g (0.5 mmol) of xanthate 3b and 0.49 g (2.5 mmol) of ethyl 2-bromo-2-methylpropionate dissolved in 7 mL of chlorobenzene were refluxed for a few minutes under nitrogen before addition of 68 mg of commercially available cumyl peroxide. While maintaining the reflux under nitrogen, the same amount of peroxide was added every 2 h until complete disappearence of the starting xanthate. The solvent was removed under reduced pressure and the residue was purified by chromatography on silica gel (PE/EtOAc 9/1-7/3) to give after recrystallisation from ethanol 105 mg of white crystals (mp 120 °C) 5b (70%). ¹H NMR (CDCl₃, 400 MHz, ppm): δ = 7.90-7.85 (2 H, m), 7.78-7.40 (2 H, m), 4.38 (1 H, m) 4.14 (1 H, dd, J = 7-14 Hz), 3.99 (1 H, dd, J = 7-14 Hz), 2.72 (1 H, ddd, J = 4.5-8-17 Hz), 2.59 (1 H, td, 8-17 Hz), 2.28 (1 H, m), 2.09 (1 H, m). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ = 167.9 (C=O), 134.5 (CHAr), 131.6 (CqAr), 123.8 (CHAr), 118.4 (CN), 49.0 (CHBr), 43.8 (CH₂), 31.8 (CH₂), 15.9 (CH₂). IR (CCl₄, cm^-1): 2250, 1776, 1724, 1392. MS (CI): [MNH₄]⁺ = 324 and 326.