NMR and ab initio studies of amination of ketenimine: Direct evidence for a mechanism involving a vinylidenediamine as an intermediate

High-level ab initio calculations were carried out in both gas phase and solvent (∈ = 35.9) to estabilish that the amination of ketenimine proceeds via amine addition across the C=N bond rather than the C=C bond, followed by tautomerization to form amidine product. The HOMO of ketenimine is perpendicular to its molecular plane with the largest coefficient on C_β, while the LUMO is in its molecular plane with the largest coefficient on C_α. Amination of ketenimine involves in-plane attack of amine nucleophile on C_α (LUMO) of ketenimine. The labile vinylidenediamine intermediate trans-11 for the reaction of ketenimine 10 with n-butylamine was directly observed by means of low-temperature proton NMR spectra. The evidence confirms that the amination reaction is stepwise and proceeds via n-butylamine addition across the C=N bond of ketenimine 10 rather than the C=C bond, followed by a slower tautomerization ofvinylidenediamine trans-11 to amidine 12. Even though the second step is much slower, the first step involving amine addition across the C=N bond is kinetic control. Surprisingly, in the reaction of 10 with n-BuNH₂, attack of n-BuNH₂ syn to the phenyl group on C_β of 10 is preferred, even though this produces a less stable product (trans- 11); attack of n-BuNH₂ anti to phenyl group on C_β of 10 is lacking and results in serious nonbonding interactions between the two phenyls of the ketenimine, as they are pushed together in this transition state.