Intramolecular Cyclization of 1-Benzyl-2-(nitromethylene)pyrrolidines in Triflic Acid

Reactions of 3,3,3-Trihalogeno-1-nitropropenes with Arenes in the Superacid CF3SO3H: Synthesis of ( Z)-3,3,3-Trihalogeno-1,2-diarylpropan-1-one Oximes and Study on the Reaction Mechanism

3,3,3-Trihalogeno-1-nitropropenes C(Hal₃)CH═CH(NO₂) (Hal = F, Cl, Br) in reaction with arenes in the superacid CF₃SO₃H (TfOH) at room temperature in 1 h afford 3,3,3-trihalogeno-1,2-diarylpropan-1-one oximes C(Hal₃)CH(Ar)-C(Ar)═NOH (CHal₃-oximes) in yields of 23-99%. Such CHal₃-oximes having one ortho-substituent in the aryl ring exist as atropoisomers in solutions at room temperature. Several cationic intermediates of this reaction were studied by means of NMR and DFT calculations, which proves the detailed reaction mechanism of the formation of CHal₃-oximes in TfOH. CHal₃-oximes (for Hal = Cl, Br) with DBU in DMF at microwave or thermal activation are cyclized into 5-halogeno-3,4-diarylisoxazoles in yields of 37-59%. CHal₃-oximes under the conditions of Beckmann rearrangement with PCl₅ in benzene at room temperature in 24 h are turned at first into imidoyl chlorides (yields of 94-96%), which undergo transformation into the corresponding benzamides PhCONHCHPh(CHal₃) on silica gel (yields of 46-47%).