A computational study (DFT, MP2, and GIAO-DFT) of substituent effects on protonation regioselectivity in β,β-disubstituted vinyldiazonium cations: Formation of highly delocalized carbenium/diazonium dications

Abstract

Protonation reactions were studied by quantum-chemical theoretical methods (DFT and MP2) for a series of β,β-disubstituted vinyldiazonium cations (1+-14+), bearing stabilizing electron-releasing groups (H3CO-, (H3C)2N-, H3C-, (H 3C)3Si-, as well as halogens F, Cl). Taking into account the various mesomeric forms that these species can represent, protonations at Cα, at the β-substituent, and at Nβ were considered. The energetically most favored pathway in all cases was C α protonation, which formally corresponds to trapping of the mesomeric diazonium ylid. Based on the computed properties (optimized geometries, NPA-charge densities, and multinuclear GIAO-NMR chemical shifts), the resulting dications can best be viewed as carbenium/diazonium dications, in which the carbocation is further delocalized into the β-substituent. For the α-nitro derivative 15, protonation of the nitro group was predicted to be the most favored reaction, while Cα-and Nβ-protonation resulted in the loss of the nitronium ion.