Ab Initio Density Functional Theory Optimized Geometries of Vinyl Trialkylammonium, Vinylpyridinium and Vinyldiazonium Tetrafluoroborate Salts with Natural Bond Orbital Charge Distributions in the Gas Phase

Abstract

N-Vinyltrialkylammonium and N-vinylimmonium tetrafluoroborate salts are novel classes of compounds discovered by Buszek and Jung. They were originally used as dienophiles and as ß-dialkylamino equivalents in Diels-Alder cycloadditions. More recently, they found application as electrophilic partners for Pd(0)-catalyzed Suzuki-Miyaura cross-coupling reactions, for Nozaki-Hiyama-Kishi reactions, and for the synthesis of symmetrical (2E,4E)-1,6-dioxo-2,4-dienes used to make 2,5-furans, -pyrroles, and -thiophenes. In an effort to expand the scope and synthetic utility of these valuable and versatile reagents and, more fundamentally, to understand better their electronic properties (and thus their reactivity), the first computational study of the title compounds was carried out. The study used the Gaussian 16 program to obtain density functional theory B3LYP geometries at the 6-311G++ (2dp,f) level of theory. The initial phase of this work focused on understanding the basis for the relatively high Diels-Alder reactivity of these compounds despite their large steric hindrance. It is concluded that the powerful inductive electron-withdrawing effects of the trialkylammonium and immonium groups substantially increase the net charge at the dienophilic carbons, making them more reactive in thermal cycloadditions and compensating for the otherwise large steric demand.