Conformational and electronic interaction studies of some para-substituted S-phenyl α-ethylsulfonylthioacetates

Abstract The preferred conformations of some p -substituted S -phenyl α-ethylsulfonylthioacetates, p -X-Ph-SC(O)CH 2 SO 2 Et (X=NO 2 1 , Cl 2 , Br 3 , H 4 , Me 5 and OMe 6 ) are determined through ν CO infrared analysis and ab initio HF/6-31G** calculations, which show the occurrence of two stable conformations, i.e. the gauche 2 ( syn ) and the gauche 1 ( anti ), with the former being the most stable. The lower frequency and more intense component of the doublet for the carbonyl absorption is then attributed to gauche 2 ( syn ) while the higher frequency and less intense component is attributed to the gauche 1 ( anti ) conformer. Experimental and theoretical data of the corresponding p -substituted- S -phenyl thioacetates (X=NO 2 10 , Cl 11 , Br 12 , H 13 , Me 14 and OMe 15 ), not bearing an α-ethylsulfonyl group, lead to similar results, confirming the frequency assignments. The progressive increase of the gauche 2 ( syn )/ gauche 1 ( s - anti ) infrared population ratio for compounds 1 – 6 , on going from electron attracting to electron donating substituents at the para position, along with larger negative carbonyl frequency shifts (Δ ν ) for the gauche 2 ( syn ) conformer relative to the gauche 1 ( anti ) conformer, and the ab initio data for the corresponding thioacetate methylsulfones 7 – 9 are interpreted in terms of the predominance of the summing up of the O (CO) δ− ⋯S (SO 2 ) δ+ and H (SO 2 Me) δ+ ⋯O (CO) δ− interactions over the O (SO 2 ) δ− ⋯C (CO) δ+ interaction which stabilises the gauche 2 ( syn ) conformer into a larger extent than the gauche 1 ( anti ) conformer. X-ray single crystal diffraction analyses for the S -phenyl- ( 4 ) and p -methyl- S -phenyl α-ethylsulfonylthioacetates ( 5 ) indicate that these compounds exist in the solid state as a slightly distorted gauche 2 ( syn ) conformation, which is stabilised by dipole moment coupling along with intermolecular hydrogen bond interactions.