Transition state Beckmann rearrangement

Nitrilium ions are generally indicated as discrete intermediates in the Beckmann rearrangement of oximes (equation 37, Y = OH2) and in the Schmidt rearrangement (equation 37, Y = N2) of ketones and aldehydes with hydrogen azide. The first stage of these reactions has been defined as an ionization by rearrangement process through a transition state of type 183 (Smith, 1963). 

The Beckmann rearrangement proceeds through a transition state represented by structure (59) in equation (27). Thus, the organoaluminum-catalyzed protocol presents an opportunity to convert a ketone regiospecifically and stereospecifically into a chain-extended amino substrate. 

Beckmann rearrangement of oximes to amides can deviate to fragmentation to form nitriles and carbocations, if the latter possess reasonable stability. Both 1-substituted-phenyl-2-propanones and 3-substituted-phenyl-2-butanones in aqueous solvents give both products, and calculations have been used to probe the mechanisms. In borderline cases, a dynamic path bifurcation from a single transition state is claimed. 

Kinetic evidence for such a dynamic path bifurcation in the mechanism of the Beckmann rearrangement has also been presented for reactions of oxime sulfonates (43, R /R = H/Me). In addition to the rearrangement product (amide), these substrates can also fragment (to alcohols), with the bifurcation in the mechanism apparently occurring after the rate-determining transition state. 

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