Highly Enantioselective Synthesis of 1,3-Oxazolidin-2-imine Derivatives by Asymmetric Cycloaddition Reactions of Vinyloxiranes with Unsymmetrical Carbodiimides Catalyzed by Palladium(0) Complexes

4-Vinyl-1,3-oxazoilidin-2-imine derivatives have been synthesized by cycloaddition reactions of 2-vinyloxiranes with unsymmetrical carbodiimides catalyzed by palladium(0) complexes in excellent total isolated yields. After reaction two compounds were always formed, one of which was isolated as the major product. A bulky alkyl group on one of the nitrogen atoms of the carbodiimide enhanced the product ratio in favor of the N-aryl-3-alkyl-1,3-oxazolidin-2-imine. Highly enantioselective cycloadducts (up to >99% ee) were formed by using TolBINAP as the chiral phosphine ligand, in THF at ambient temperatures. The enantiodetermination is believed to be dependent on nucleophilic attack of the anionic nitrogen of the carbodiimide due to the steric interaction of the carbodiimide substituents with the chiral phosphine ligand.