CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XV. α-(ARYLSULFINYL)-p-SUBSTITUTED ACETOPHENONES

Abstract

The v_CO IR analysis of α-(p-phenylsulfinyl)-p-substituted acetophenones X-φC(O)CH₂S(O)φ-Y 1–8, being X and Y = NO₂, H and OMe substituents, supported by ab initio calculations of the α-methylsulfinyl/acetophenone (model compound) along with the X-ray geometrical data for 1,7 and 8, indicates 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 reduced to ca. 50% for 2,3,5-7 and to ca. 20% for 1 and 8. This behavior is discussed in terms of O^δ- _(CO—S^δ+ _(SO) charge transfer and Coulombic interactions, which stabilize the cis ₁ rotamer, and the π*_CO/δ_C-S, π_CO/n_s and π*_CO/δ_C-S orbital interactions, which stabilize the gauche rotamers. The progressive more negative carbonyl cis ₂ shifts (Δv_c), when X varies from NO₂ to H and to OMe for the same Y, along with the unexpected NAE values of the α-methylene carbon chemical shifts for compounds 1–8 give further support for the existence of a strong intramolecular complex betwen C=O and S=O dipoles which stabilizes the cis ₂ rotamer. The progressive more negative carbonyl gauche shifts (Δv_g), when X varies from NO₂ to H and to OMe for the same Y, is in line with the higher contibution of the interaction π_CO/δ*_C-S, which stabilizes the gauche rotamer of the title compounds.

Key Words:

• Conformational studies
• electronic interactions
• IR and ¹³C NMR spectroscopies
• α-(p-phenylsulfinyl)-p-substituted acetophenones