Experimental and theoretical study of the intramolecular interactions determining the conformation of β-carbonyl sulfoxides

Abstract

Information on the geometrical and electronic structures of α-methylsulfinylacetophenone, C₆H₅C(O)CH₂S(O)CH₃ 2, have been obtained from X-ray diffraction analysis, UV photoelectron spectroscopy and ab initio 6-31G** calculations. A comparison of the results with those obtained from the spectra and the computations on α-methylthioacetophenone, C₆H₅C(O)CH₂SCH₃ 1 and α-methylsulfonylacetophenone, C₆H₅C(O)CH₂SO₂CH₃ 3, together with previous results on β-keto sulfides and β-keto sulfones indicates that the CH₂–S(O) bond in 2 is quasi-cis to the carbonyl group in the gas and solid phase, at variance with the other β-carbonyl thioderivatives which adopt a gauche conformation. Eigenvector analysis, electron charge distribution at various atoms and/or groups and geometric parameters indicate that the cis conformation of 2 is stabilized by a strong non-bonded interaction between the negatively charged carbonyl oxygen and the positively charged sulfur atom from which it is separated by a distance (2.8–2.9 Å) much shorter than the sum of the van der Waals radii. The predominant charge transfer interaction in 3 and related sulfones occurs in the opposite direction (O_SO2→ C_CO). The inversion of the direction of the charge transfer (and the change of the cis/gauche orientation of the thio group) from sulfone to sulfoxide is associated with an increase of electron affinity of the thio group in the latter, and could explain its smaller thermal stability.

Ab initio 3-21G* calculations on several conformations of the bis-thioderivatives C₆H₅C(O)CH(SCH₃)S(O)CH₃ 4, C₆H₅C(O)CH(SR)SO₂R (R = Me 5 and Ph 6) and C₆H₅C(O)CH(SOCH₃)SO₂CH₃ 7, together with X-ray diffraction (4, 6 and 7) and photoelectron spectroscopy (4) analyses confirmed the cis(SOR) and gauche(SR and SO₂R) preferred orientation of the thio groups with respect to the carbonyl group as observed in the monosubstituted derivatives. In 4 and 7 the S_SO atom is about 30° out of the cis plane [O(1)–C(2)–C(3)].