A Useful Brominating Reagent

Bromodimethylsulfonium bromide (1) is a brominating reagent for various compounds and its utility has been reported.¹⁾　For example, β-keto esters and 1,3-diketones are brominated smoothly at 0 °C to room temperature to afford the corresponding α-bromo derivatives selectively in high yields.²⁾　The notable advantages of this protocol are use of reagent 1, which is less hazardous than molecular bromine, and no added base, Lewis acid, or other catalyst.
Moreover, 1 can be used as a catalyst for protection and de-protection of various functional groups.¹⁾　Recently, a mild Beckmann Rearrangement using 1 has been reported.³⁾　According to the report, a wide range of ketoximes are converted into the corresponding amides or lactams in high yields.

Typical Procedure: α-Bromination of β-keto esters and 1,3-diketones²⁾
Bromodimethylsulfonium bromide (0.278 g, 1.25 mmol) is added to a stirred solution of β-keto ester or 1,3-diketone (1 mmol) in CH₂Cl₂ (5 mL) at 0–5 °C or room temperature. After 20 min, the reaction mixture is washed with water (10 mL × 2) and extracted with CH₂Cl₂ (10 mL × 2). The organic layers are combined and dried over anhydrous Na₂SO₄, and solvents are removed by evaporation in a rotary evaporator to get the crude product.