Electrophilicity of aliphatic nitrilium closo-decaborate clusters: Hyperconjugation provides an unexpected inverse reactivity order

Abstract

The nucleophilic addition of 2-pyridyl carbamidoxime to the C≡N group of the aliphatic nitrilium closo-decaborate clusters (PhCH2PPh3)[B10H9NCR] (R = Me, Et, and iPr) proceeds to give 2-iminium closo-decaborates (68–88% isolated yields). A systematic kinetic and theoretical study of the nucleophilic addition indicates that the reactivity of the nitrile ligand at the boron cluster unexpectedly increases in the order of R: Me < Et < iPr. It was shown theoretically, that the electrophilicity of the C≡N group is primarily determined by hyperconjugation with the neighboring aliphatic moiety, which induced by substantial (ca. +0.6 e) positive charge at the nitrile C atom.