Thionyl Chloride → trans 3-Bromocinnamoyl chloride

Trans m-bromocinnamic acid (11.4 g) in dry benzene (75 ml) was heated to reflux and then a mixture of thionyl chloride (12 g) in dry benzene (50 ml) was added at such a rate as to maintain constant reflux. The reaction mixture was heated at reflux for an additional 2 hr after the addition of thionyl chloride. The solvent and excess thionyl chloride were then removed under reduced pressure to give trans 3-bromocinnamoyl chloride (ca. 12.3 g). A solution of the 3-bromocinnamoyl chloride in toluene (150 ml) was added dropwise with stirring to a solution of isopropylamine (10 g) in ether (200 ml). The reaction mixture was stirred at room temperature for one hour and then heated at reflux for one hour. The solvent and excess amine were removed under reduced pressure. The crude residue was triturated with water, filtered, and recrystallized from ethanol-water to give white, crystalline trans 3-bromo-N-isopropylcinnamamide, m.p. 85°-86° C. Elemental analysis, NMR and IR spectra all confirmed the identity of the product.