Reaction of N-aryl- and N-alkyl-benzimidoyl chlorides with silver nitrate

Abstract

N-Arylbenzimidoyl chlorides, in which the N-aryl group is unsubstituted at the ortho- and para-positions, react with AgNO₃ to yield N-(nitroaryl)benzamides in which the NO₂ group resides in the ortho- or para-position. N-Arylbenzimidoyl chlorides, in which the N-aryl ring is 2,4,6-trisubstituted, react with AgNO₃ to yield the corresponding N-aryl-N-nitrobenzamides. The formation of both types of product can be explained by the intermediacy of an O-nitro imidate. Spectroscopic and chemical evidence is presented for the formation of this intermediate in the reaction of N-(2,4,6-trisubstituted phenyl)benzimidoyl chlorides with AgNO₃. Rearrangement of the O-nitro imidate is unimolecular and intramolecular. The rate of rearrangement is independent of the substituent in the C-aryl ring, but increases with the electron-withdrawing ability of the substituents in the N-aryl ring. A mechanism is proposed in which the imidoyl chloride reacts with AgNO₃ to produce first a nitrilium ion which goes on to form an O-nitro imidate that subsequently rearranges via a homolytic cleavage of the O–NO₂ bond. The ortho:para ratios of N-(nitroaryl)benzamides obtained in the present work indicate that O-nitro imidates are not responsible for the high ½ortho:para ratios sometimes observed in the nitration of anilides.

N-Alkylbenzimidoyl chlorides react with AgNO₃ to form the corresponding N-nitro- and N-nitrosobenzamides. The mechanism of formation of the N-alkyl-N-nitrobenzamide arises from a pathway analogous to that for N-aryl-N-nitrobenzamides, involving a nitrilium ion that gives rise to an O-nitro imidate. The evidence for the formation of the N-nitrosobenzamide points to an alternative reaction of the imidoyl chloride with AgNO₃. One possible mechanism for this reaction is described.