Meerwein's Catalytic Beckmann Rearrangement

Article abstract of DOI:10.1002/prac.19963380188

Meerwein recommended nitrilium salts (2) as catalysts for the Beckmann rearrangement of oximes (1) under neutral and water-free conditions. For the first time, primary adducts (Z)-(3) of oximes to nitrilium salts have been isolated. Reported are activation energies for Beckmann rearrangements of these adducts, and an X-ray structural analysis of (Z)-3a. Rearrangement of 3 produces mixtures of up to four different N-acylamidinium salts 5-8, which arise from fast reactions of primary-formed secondary amides (4) with nitrilium salts (2). Because of their ambident electrophilic character the N-acylamidinium salts react with excess of oxime not only to amides (4) but to mixtures of products. It is shown that nitrilium salts cannot be used as catalysts for the Beckmann rearrangement.