Formation of N-benzylbenzamide from benzyl phenyl ketone oxime in a system C2H2-KOH-DMSO. Key role of acetylene

Abstract

Oximes rearrange into amides under the action of strong acids (Beckmann rearrangement) [1]. Only seldom this rearrangement occurs in the presence of bases, for instance, that of benzoquinosylidone Eand Z-oximes [2]. The preparation of benzamide from benzaldoxime in the system KOH–DMSO was described through hydration of the intermediate nitrile [3]. In the course of the synthesis of 1-vinyl-2,3-diphenylpyrrole (II) from benzyl phenyl ketone oxime (I) and acetylene by Trofi mov reaction [4–6] in the system KOH–DMSO at the atmospheric pressure (130°C, 7 h) alongside the expected pyrrole II (yield 14–34%) N-benzylbenzamide (III) was obtained (14–24%) (Scheme1). Besides after acidifying the reaction mixture benzoic acid was isolated (up to 56%). Special experiment showed that amide III was not a product of oxime I rearrangement under the action of KOH–DMSO (at heating oxime I under the same conditions without acetylene it was nearly completely recovered, to 95–97%). Obviously the key role in the formation of amide III from oxime I belonged to acetylene. Apparently O-vinyloxime IV, the intermediate in the synthesis of pyrroles from oximes and acetylene, was deprotonated by the superbase KOH–DMSO forming carbanion V that further replaced intramolecularly the vinyloxy anion with the closure of the azirine ring VI (Scheme 2). Its hydration into hydroxyaziridine VII followed by a rearrangement resulted in amide III. The stage of azirine formation from O-vinyloxime under the action of the superbase was suggested recently for the understanding of the assembling of 4-methylene-3oxa-1-azabicyclo[3.1.0]hexanes from ketoximes, ketones, and acetylene [7]. In essence this stage resembles the azirine synthesis from oximes or their ethers by Hoch– Campbell reaction [1, 8]. This reaction fundamentally supplements the common Scheme 1. Dedicated to Full Member of the Russian Academy of Sciences N.S. Zefi rov on occasion of his 75th anniversary