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 1 hour and then heated at reflux for 1 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.