Conformational analysis and electronic interactions of some 2-[2’-(4′-substituted-phenylsulfinyl)-acetyl]-5-methylfurans

Abstract

A conformational study of some 2-[2’-(4′-substituted-phenylsulfinyl)-acetyl]-5-methylfurans 1–4 (OMe1, Me 2, H 3, Cl 4) was performed using IR carbonyl stretching and analysis supported by Natural Bond Orbital (NBO) and Polarisable Continuum Model (PCM) calculations at the B3LYP-D3/aug-cc-pVDZ level, and X-ray diffraction (for 2 and 4). The computational results indicated the existence of four stable conformations with variable degrees of folding (stacking) between the phenyl and the furanyl rings, in the following order of stability: sc(anti)1≫ac(syn)>sc(anti)2 >sc(syn).The PCM data showed that the mean abundance of the most stable lowest frequency sc(anti)1 conformer in vacuum decreases progressively as the solvent permittivity increases, and, concomitantly, the population of the highest νCO frequency sc(anti)2 conformer increases in the same direction. Close inspection to the PCM νCO frequencies and populations of the sc(syn) and ac(syn) conformers suggests that their IR lines should be missed due to their low intensities or hidden under the intense lowest νCO component of the sc(anti)1 conformer. The IR experimental trend observed for the components of the doublet carbonyl band in all solvents just matches the computational results and allows ascribing the most intense lowest νCO frequency and the least intense highest νCO frequency components in CCl4, to the sc(anti)1 and sc(anti)2 conformers, respectively.The appropriate Sδ+(4) … Oδ–(1) short contacts stabilise the four conformers to different extents, while the quasi-parallel Cδ+ = Oδ– and Sδ+ = Oδ– dipole orientations strongly destabilise electrostatically only the sc(anti)2 conformer. This Repulsive Field Effect is also responsible for the increase in its νCO stretching frequency with respect to the other conformers. The suitable β and γ dihedral angles of the anti conformers favour the close proximity of the centroids of the furan and the phenyl rings and the stabilising O(9)[furan] … C(10) [phenyl] short contacts. In contrast, the appropriate geometry adopted by the ac(syn) conformer gives rise to the Oδ–(26) … Hδ+(18) short contact and the LPO(26)→σ*C6–H18 orbital delocalisation (hydrogen bond), which significantly stabilise this conformer with respect to the sc(syn) form.The sum of the delocalisation energies in the NBO analysis does not match the calculated stability of the four conformers for series 1–4. The closeness of the mean delocalisation energy values found for all the conformers suggests that the computed order of stability is controlled mainly by the relevant short contacts and the coulombic repulsions between the negatively charged carbonyl, sulfinyl and furyl oxygen atoms.In particular, the strong electrostatic short contact Oδ–(CO) … Oδ–(SO) repulsion and the half as intense Oδ–(CO) … Oδ–(fur) repulsion destabilise to different extents the sc(anti)2 and the pair of syn conformers, respectively. Therefore, the calculated order of stability of the four conformers for 1–4 results from the overall balance of the orbital and electrostatic interactions. X-ray single crystal analysis of 2 and 4 revealed that the molecules assume an unfolded (stretched) geometry with a sc(syn) conformation, and that they are stabilised through centrosymmetrical C–H(o-Ph) …. O(co) and C–H(m-Ph) …. Cg1(fur) interactions (for 2) and C–H(o-Ph) … O(co) and CPh–Cl … Cg2(Ph) interactions (for 4).