Conformational analysis and electronic interactions of some 2- [2′-(4′-sustituted-phenylsulfanyl)-acetyl]-5-substituted furans and 2- [2′-(phenylselanyl)-acetyl]-5-methylfuran

The conformational equilibrium of 2-[2′-(phenylselanyl)-acetyl]-5-methylfuran (1) and 2-[2′-(4′-sustituted-phenylsulfanyl)-acetyl]-5-substituted furans (2–7) was determined through the infrared (IR) analysis of the carbonyl stretching band (νCO) supported by M06–2X/aug-cc-pVDZ level of theory. Three stable conformations [sc(anti), ac(anti) and sc(syn)] were obtained in vacuum, with the sc(anti) the most stable for compound 1–6 and the ac(anti) for compound 7. The IR spectrain solution of n-C6H14, CCl4, CHCl3, CH2Cl2 and CH3CN show in general νCO doublets for compounds 2–6, with the exception of triplets in n-C6H14 for 2–4 and a symmetrical band in CHCl3 for 1, 3–6 and in CH2Cl2 and CH3CN for 1. The p-nitrophenyl compound 7 is insoluble in n-C6H14 and CCl4 and displays a doublet in all the other polar solvents. The PCM data allow to ascribe the sc(anti) conformer to the lowest frequency more intense νCO IR component and the sc(syn) one to the other doublet component for compounds 1–6, while the intermediary νCO frequency ac(anti) conformer, with negligible population, is assignedto the third triplet component predicted in n-C6H14 for compounds 2–4. Conversely, for compound 7, the more intense and lowest frequency νCOIR component was ascribed to the ac(anti), whereas the highest frequency one to the sum of the sc(anti) and sc(syn) populations. The conformational preferences of compounds 1–7 are governed by a balance between the orbital and the coulombic interactions estimated by means of natural bond orbitals (NBO), quantum theory of atoms in molecules (QTAIM), non covalent interaction (NCI) and short contacts analysis. While NBO delocalization energies indicate the ac(anti) conformer as the most stable for all compounds, NCI analysis reveals in the sc(anti) and sc(syn) conformers of compounds 1–6 an additional intramolecular stabilizing π‧‧‧π stacking interaction between the furyl and phenyl ring, which is counterbalanced in the sc(syn) conformer by the repulsive coulombic short contact between the carbonyl and furyl oxygen atoms. For compound 7, the ac(anti) conformer turns to be the most stable one as the electron withdrawing effect of the nitro substituent on the phenyl ring decreasesthe stabilizing π‧‧‧π stacking on the sc(anti) conformer.