The Isonitrile-Nitrile Rearrangement. A Reaction without a Structure-Reactivity Relationship

Article abstract of DOI:10.1021/jo00380a028

Reproducible rates of isomerisation of aliphatic isonitriles to nitriles in solution can be measured by GLC or IR when free-radical inhibitors are added to suppress a competing radical chain reaction.The reactivities of 19 primary, secondary, tertiary, cyclic, bicyclic, bridgehead, benzyl, substituted-benzyl, α-carbomethoxymethyl, and triphenylmethyl isocyanides in this rearrangement reaction vary by only a factor of 67 in rate or by +/- 2 kcal mol^-1 in ΔG(excit.).This is explained by a tight hypervalent three-membered cyclic transition state in agreement with a previous prediction by ab initio calculation.The slower rate of 9-triptycyl isocyanide is proposed to be due to steric hindrance by the three peri hydrogens.Aromatic isocyanides isomerize about 10 times faster independent of polar para substituents and bulky ortho substituents.A hypervalent orthogonal transition state with retention of the aromatic sextet is proposed in contrast to the popular phenonium-type transition states for aryl migration in other 1,2-rearrangements.The reactivity data and the transition-state structures are discussed in context with other cationotropic 1,2-shifts.