Structures and Reactivities of N-Alkenyl-Substituted Anilides: The "Magic" Methyl Effect on Alkene

Methyl substitution at the double bond of N-alkenyl anilides influences both the preferred conformation and the susceptibility to acidic hydrolysis. The R¹-substituted amide favors the trans conformation, whereas amides substituted at R² or R³ favor the cis conformation. Substitution at the R¹ and R³ positions increases the ratio of the trans conformer. DFT study indicated that these conformational preferences can be explained in terms of substituent-induced torsion twisting of the N-alkenyl moiety relative to the amide plane. R¹ substitution enhances the susceptibility to acidic hydrolysis, whereas R² or R³ substitution increases the stability. The effect of the double bond on the conformational effect was showcased by contrasting the preferred conformation of R¹-substituted anilide (trans) and hydrogenated N-isopropyl amide (cis).