Mechanism of nitrile-forming fragmentation reactions

Abstract

Treatment of the 2,4-dinitrophenyl ethers of the anti-form of the α-keto-oximes PhCO·C(Ph)NOH (I) and 2,4,6-Me₃C₆H₂CO·C(Ph)NOH (II) with the nucleophiles OH^–, OMe^–, OPr^i– leads to a Beckmann fragmentation with the formation of benzonitrile and the corresponding benzoic or 2,4,6-trimethylbenzoic acids. Similar reactions occur between the benziloxime ether and BuO^t– and NH₃, whereas, these nucleophiles substitute the 2,4-dinitrophenyl ring of the ether of (II). The reactions of the 2,4-dinitrophenyl ether of (II) with OH^–, OMe^– and OPr^i– follow second-order kinetics ruling out a mechanism involving the rate-determining formation of 2,4,6-trimethylbenzoylium cation. Examination of the oxygen-18 content of recovered 2,4-dinitrophenyl ether of (II) labelled at the carbonyl group after the ether had been subjected to solvolytic conditions, i.e. sodium hydroxide in dioxan–water, showed no loss of oxygen-18. The mechanism of the Beckmann fragmentation reaction is discussed in the light of these observations. When the 2,4-dinitrophenyl ethers of (I) and (II) are heated in acetonitrile or benzene, benzonitrile and the 2,4-dinitrophenyl ester benzoic or 2,4,6-trimethylbenzoic acid are formed. Kinetic measurements suggest that reaction involve an initial Beckmann rearrangement followed by collapse of the resulting nitrilium salt.