A Remarkably Simple One-Step Procedure for the Preparation of α-Bromo-α,β-Unsaturated Carbonyl Compounds

Abstract

An easy and convenient one-step procedure for the conversion of α,β-unsaturated carbonyl compounds into their corresponding bromo-enones using NBS-Et₃N˙3HBr in the presence of potassium carbonate in dichloromethane at 0 ˚C to room temperature under very mild conditions in high yields and significantly shorter times, is reported.

Key words

NBS-Et₃N˙3HBr - α-bromo-α,β-unsaturated carbonyl compounds - bromination - mild conditions

References and Notes

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22

Typical Procedure: To a stirred solution of 3-methylcyclo-hex-2-en-1-one (1a; 2.5 g, 13.23 mmol) in anhydrous CH₂Cl₂ (25 mL) were added NBS (14 mmol), Et₃N˙3HBr (23 mmol) and K₂CO₃ (40 mmol) at 0 ˚C. After 10 min the reaction mixture was slowly brought to r.t. and stirring was continued. The reaction was complete within 25 min as monitored by GC [SE-30 SS; 2m × 1/8′′ column on Shimadzu 14-B/Mayura 9800 gas chromatographs]. The residual solid was filtered off and the solid residue was washed with anhydrous CH₂Cl₂ (15 mL). The combined filtrate was concentrated under vacuum and the crude product was purified by chromatography (silica gel; EtOAc-hexane, 1:4) to afford, after concentration, the product 2a as light-yellow oil in 88% yield. IR (neat): 2920 (S), 1695 (S), 1605 (S), 1255 (S). ^¹H NMR (400 MHz, CDCl₃): δ = 2.59 (t, J = 6.7 Hz, 2 H), 2.52 (t, J = 6 Hz, 2 H), 2.18 (s, 3 H), 2.05-1.97 (m, 2 H). ^¹³C NMR (400 MHz, CDCl₃): δ = 190.9 (C=O), 160.3 (C), 122.7 (CH), 37.6 (CH₂), 25.9 (CH₃), 21.8 (CH₂). GCMS (Shimadzu GC-17A coupled to Shimadzu GC-MS QP 5050 A with capillary column: BP 5, length: 30 m): m/z (%) = 190 (10), 188 (15), 162 (25), 160 (30), 132 (5), 127 (7), 190 (12), 93 (4), 83 (4), 81 (30), 79 (25), 76 (5), 66 (10), 52 (10), 67 (8), 63 (19), 56 (5), 55 (13), 53 (80), 51 (47), 50 (32), 43 (13), 42 (100), 41 (88), 40 (60).