Gas-phase SN2 reactivity of dicoordinated borinium cations using pentaquadrupole mass spectrometry

We report gas phase ion/molecule reactions between dialkoxyborinium cations (RO-B+-OR) and small organic amides, such as N,N-dimethylformamide and N,N-diethylpropionamide. Besides direct addition at boron, the results show efficient SN2 methyl transfer from the borinium ion to the amide. Isotopic labeling and collision-induced dissociation (CID) of the methyl transfer products demonstrate O-methylation of the amide. Methyl substitution at the alpha-carbon of the amide affects the degree of alkylation and adduct formation. Direct proton abstraction via beta-elimination is a major competitive reaction for substituents other than methyl. Ab initio calculations at the B3LYP/6-31G(d) level indicate that SN2 transmethylation is highly exothermic with O-methylation favored over N-methylation by 14.8 kcal/mol.