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Neighboring group participation is one of the fundamental interactions in organic reactions and can influence the reaction rate, stereoselectivity, and reaction pathway through transient carbon-carbon or carbon-heteroatom bond formation. The latter category includes cyclic three- and five-membered bromonium ions, wherein lone-pair electrons of the monovalent bromine atom stabilize a trigonal carbocation. Although similar nucleophilic interactions of monovalent halogen atoms with non–carbon atom-centered cations have long been predicted, we know of no experimental evidence of such an interaction. Here, we demonstrate a nucleophilic interaction of neighboring monovalent halogen to stabilize an imino sp2 nitrogen cation. This interaction has an overwhelming impact on the reaction pathway, completely altering the migratory preference under acid-catalyzed Beckmann rearrangement conditions. In sharp contrast to the general case ofanti -migration,peri -chloro– andperi -bromo–substituted O-tosyl oximes of 1-tetralone substructures and their derivatives undergosyn -migration under Beckmann rearrangement conditions (i.e., migration of the group on thesyn side of the leaving group). Theperi -chloro orperi -bromo neighboring group turned out to provide strong anchimeric assistance forsyn -migration via transient formation of a cyclic five-membered imino-halonium cation with dissociation of tosylic acid. Thus, formation of thesyn -migration products can be attributed to a reaction mechanism that is different from the conventional Beckmann rearrangement mechanism. That is, the positively charged imino nitrogen atom can be stabilized by, or interact with, a chloro or bromo group in close spatial proximity, and this interaction dramatically changes the reaction pathway, selectively affording regioisomeric lactams from closely related starting materials.

Stereochemical evidence for stabilization of a nitrogen cation by neighboring chlorine or bromine