Stereochemical and electronic interaction studies of some meta-substituted and para-substituted alpha-methylsulfinyl-alpha-diethoxyphosphorylacetophenones

The IR and 1H NMR spectra analyses of some meta- (YZOMe 1, Me 2, F 3, Cl 4, Br 5 and NO2 6) and para- (YZOMe 7, H 8, F 9, Cl 10, Br 11 and NO2 12) substituted a-methylsulfinyl-a-diethoxyphosphorylacetophenones m,p-YPhC(O)C*H[S*(O)Me][P(O)(OEt)2], along with theoretical calculations at the B3LYP/6-31Cg(d,p) level, for compound 8, have shown, in solution of solvents of increasing polarity and in the gas phase, the existence of two enantiomeric pairs (racemic mixtures). The more stable and less polar (CSSS; CRSR) enantiomeric pair (conformer pair I) display the [MeS(O)] group in a quasi-periplanar (quasi-cis) geometry and the [P(O)(OEt)2] group in a anti-clinal (gauche) geometry relative to the carbonyl group. The less stable and more polar (CRSS; CSSR) enantiomeric pair (conformer pair II) presents both substituents in a syn-clinal (gauche) geometry. The observed solvent effect on the IR nCO doublet components and on the two 1H NMR methyl signals relative intensities, for the whole series along with the complete deuterium exchange of the a-methine hydrogen atom of 1, strongly indicate that both pairs of diastereomers [(CSSS; CRSR) and (CRSS; CSSR)] are equilibrated in solution through an intermediate enolic form. The more stable (CSSS; CRSR) enantiomeric pair [conformer pair (I)] is strongly stabilised by O(CO) dK /S(SO) dC charge transfer and electrostatic interaction, while the remaining electronic interactions are almost counterbalanced among themselves, and are important in the stabilisation of the less stable (CRSS; CSSR) enantiomeric pair [conformer pair (II)]. Compound 1, in the solid state, occurs as the (CRSS; CSSR) enantiomeric pair [conformer pair (II0)], as determined by X-ray diffraction analysis, whose geometry is similar to conformer pair (II), found in gas phase. The O(CO) dK /S(SO) dC , O(POEt) dK /C(CO) dC , O(POEt) dK /S(SO) dC and H(o-Ph) dC /O(CO) dK intramolecular interactions contribute for the stabilisation of conformer pair (II0) in the solid. Further stabilisation derives from dipole moment couplings and by an intricate network of electrostatic interactions (hydrogen bonds)