A computational study (DFT, MP2, and GIAO-DFT) of substituent effects on protonation regioselectivity in ƒÒ,ƒÒ-disubstituted vinyldiazonium cations; formation of highly delocalized carbenium-diazonium dications.

BOROSKY, GABRIELA L.; LAALI, KENNETH K.

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

Wiley

Año: 2010 vol. 23 p. 115 - 125

0894-3230

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-, (H3C)3Si-, as well as halogens F, Cl). Taking into account the various mesomeric forms that these species can represent, protonations at Ca, at the b-substituent, and at Nb were considered. The energetically most favored pathway in all cases was Cα 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.