Spectroscopic and theoretical studies of some 2-(2′-ethylsulfanyl)acetyl-5-substituted furans and thiophenes

The conformational equilibrium of 2-(2' -ethylsulfanyl)acetyl-5-substituted furans and thiophenes [EtSCH2C(O)(C4H2X)Y] (1 –5; Scheme 1) was determined through the infrared analysis of the carbonyl stretching band (νCO ) supported by DFT (B3LYP/aug-cc-pVTZ) and molecular dynamics simulations (OPLS- AA). Six stable conformations, syn (1–3) and anti (1–3), were obtained through the optimisation of the iso- lated systems, where the syn / anti nomenclature refers to the dihedral angle between the carbonyl and the aromatic heteroatom (O = C–C–X; X = O, S) and 1, 2 , and 3 refer to the ethyl rotation (Scheme 2) (torsion around the γ dihedral angle). The three ethyl conformers resemble six- ( 1 and 2 ) or seven-( 3 ) member pseudo-rings with an electrostatic intramolecular hydrogen bond (O δ- ···H δ+ ). The IR spectra in solution show νCO doublets for all compounds in all solvents ( n -C 6 H 14 , CCl 4 , CHCl 3 , CH 2 Cl 2 and CH 3 CN). The polarisable continuum model and molecular dynamics data allow the lowest wavenumber and more intense νCO IR component to be ascribed to the three anti conformers of the furan derivatives and to the three syn ones of the thiophene compounds. The conformational preferences of all compounds are governed by a balance between orbital and coulombic interactions, as estimated by means of natural bond orbital, natural coulomb electrostatic, pairwise steric exchange energy, quantum theory of atoms in molecules, non-covalent interaction and short-contact analyses. While the conformational isomerism of the ethylthioacetyl moiety is determined by both quantum and columbic effects, the equilibrium involv- ing the carbonyl and heterocyclic moiety is controlled largely by electrostatic interactions.