| Abstract
| - High-level ab initio calculations were carried out in both gas phase and solvent (ε = 35.9) to estabilishthat the amination of ketenimine proceeds via amine addition across the CN bond rather thanthe CC bond, followed by tautomerization to form amidine product. The HOMO of ketenimine isperpendicular to its molecular plane with the largest coefficient on Cβ, while the LUMO is in itsmolecular plane with the largest coefficient on Cα. Amination of ketenimine involves in-plane attackof amine nucleophile on Cα (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 stepwiseand proceeds via n-butylamine addition across the CN bond of ketenimine 10 rather than theCC bond, followed by a slower tautomerization of vinylidenediamine trans-11 to amidine 12. Eventhough the second step is much slower, the first step involving amine addition across the CNbond is kinetic control. Surprisingly, in the reaction of 10 with n-BuNH2, attack of n-BuNH2 synto the phenyl group on Cβ of 10 is preferred, even though this produces a less stable product (trans-11); attack of n-BuNH2 anti to phenyl group on Cβ of 10 is lacking and results in serious nonbondinginteractions between the two phenyls of the ketenimine, as they are pushed together in thistransition state.
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