Conformational and electronic interaction studies of alpha-substituted carbonyl compounds. XV. alpha-(arylsulfinyl)-p-substituted acetophenones

The V-CO IR analysis of alpha-(p-phenylsulfinyl)-p-substituted acetophenone s X-phi C(O)CH2S(O) phi-Y 1-8, being X and Y = NO2, H and OMe substituents, supported by ab initio calculations of the alpha-methylsulfinyl/acetopheno ne (model compound) along with the X-ray geometrical data for 1, 7 and 8, i ndicates the existence of the cis(2) and gauche rotamers for compounds 1-4 and 6. Compounds 5, 7 and 8 present another less stable and more polar cis( 1) rotamer. The cis(2) rotamer concentration for 4 (ca. 97% in CCl4) is red uced to ca. 50% for 2, 3, 5-7 and to ca. 20% for 1 and 8. This behavior is discussed in terms of O-(CO)(delta-)-S-(SO)(delta+) charge transfer and Cou lombic interactions, which stabilize the cis(1) rotamer, and the pi(CO)/sig ma*(C-S), pi*(CO)/n(s) and pi*(CO)/sigma(C-S) orbital interactions, which s tabilize the gauche rotamers. The progressive more negative carbonyl cis(2) shifts (Delta v(c)), when X varies from NO2 to H and to OMe for the same Y , along with the unexpected NAE values of the alpha-methylene carbon chemic al shifts for compounds 1-8 give further support for the existence of a str ong intramolecular complex between C=O and S=O dipoles which stabilizes the cis(2) rotamer. The progressive more negative carbonyl gauche shifts (Delt a v(g)), when X varies from NO2 to H and to OMe for the same Y, is in line with the higher contribution of the interaction pi(CO)/sigma*(C-S), which s tabilizes the gauche rotamer of the title compounds.