Dynamic Path Bifurcation in the Beckmann Reaction: Support from Kinetic Analyses

The reactions of oximes to amides, known as the Beckmann rearrangement, may undergo fragmentation to form carbocations + nitriles when the migrating groups have reasonable stability as cations. The reactions of oxime sulfonates of 1-substituted-phenyl-2-propanone derivatives (<b>7</b>-X) and related substrates (<b>8</b>-X, <b>9a</b>-X) in aqueous CH<sub>3</sub>CN gave both rearrangement products (amides) and fragmentation products (alcohols), the ratio of which depends on the system; the reactions of <b>7</b>-X gave amides predominantly, whereas <b>9a</b>-X yielded alcohols as the major product. The log<i>k</i>–log<i>k</i> plots between the systems gave excellent linear correlations with slopes of near unity. The results support the occurrence of path bifurcation after the rate-determining TS of the Beckmann rearrangement/fragmentation reaction, which has previously been proposed on the basis of molecular dynamics simulations. It was concluded that path-bifurcation phenomenon could be more common than thought and that a reactivity-selectivity argument based on the traditional TS theory may not always be applicable even to a well-known textbook organic reaction.