Density functional theory studies on the electrophilic versus electron transfer mechanisms of aryl vinyl ethers

The ab initio SCF MO and density functional theory (DFT) studies are carrie d out on the electrophilic (1a) and electron transfer (1b) addition reactio ns to the vinyl double bond of aryl vinyl sulfides and ethers, In the elect rophilic addition processes, a double bond shift from C-3 = C-4 to X = C3 o ccurs With occupation number (1.97) close to the normal two. Due to this sh ift direct conjugation between the cationic center, X = S or O, and the par a electron-donor substituent becomes impossible so that the reaction energi es (or log K) are correlated with sigma rather than sigma(+). By contrast, radical cation formation leads to delocalization of the SOMO, a lone-pair p i orbital on X, with four major resonance structures in which cationic char ge as well as spin density is delocalized over C-4, X and C-7 atoms. As a r esult, partial pi bonds are formed over C-1 X and C-3-C-4 With occupation n umbers (0.82) lower than one. In two of the cannonical structures, III(X)ov er dot and III(X+), direct conjugation between the cationic center, X, and the para substituent is achieved so that a better correlation with sigma(+) rather than sigma is obtained. The SCF MO energies at the HF/3-21G* and HF /6-31G* levels lead to very much inferior Hammett correlations in the sigma /sigma(+) diagnostic criterion. In contrast, the p values evaluated with th e DFT energies can give reliable diagnostic distinction between the two add ition mechanisms.