Reaction of essentially free benzyl cations with acetonitrile; Synthesis of ethanimidic carboxylic anhydrides and unsymmetrical diacylamines

Benzyl cations were generated via the thermal decomposition of N-benzyl-N-n itrosopivalamide in acetonitrile and acetonitrile-water mixtures at 25 degr ees C. The first-formed (primary) benzylating agent, the benzyl cation, was scavenged competitively by pivalate (trimethyl acetate) ion to yield benzy l pivalate, by acetonitrile to yield the corresponding N-benzylnitrilium io n, and by water (when present) to yield benzyl alcohol. The nitrilium ion u nderwent a cascade of reactions to yield an array of products whose identit ies and relative yields as a function of time were used to elucidate the me chanistic paths involved. Thus, the N-benzylnitrilium ion reacted with piva late ion to yield the Z-isomer of N-benzylethanimidic pivalic anhydride, fo llowed by its conversion into the E-isomer. This article appears to be the first to document compounds of this type. The E-isomer is labile under the reaction conditions, rearranging into N-acetyl-N-pivalylbenzylamine. In the presence of water as a diluent, a significant fraction of the nitrosoamide was hydrolyzed to benzyl alcohol; hydrolysis of the nitrilium ion yielded N-benzyl acetamide. The yield of hydrosylates was directly proportional to the mole fraction of water in the medium.