Repulsive gauche conformational effect. Analysis of seven-membered rings containing endo and exo sulfur atoms

Abstract

The so-called repulsive gauche conformational effect associated with sulfur is investigated through a study of 3-methylthio-1,5-benzodioxepin (1) and 3-methylthio-1-benzoxepin (2), together with 3-fluoro- and 3-chloro-1,5-benzodithiepins (3 and 4), by 13C and 1H dynamic nuclear magnetic resonance methods. The results show that, while compounds 2, 3, and 4 exhibit a large amount of the Ce conformation, the TB form predominates for compound 1. Intramolecular stereoelectronic σ–σ* orbital interactions are analyzed, together with several other factors (steric, electrostatic, and solvation), to explain the observed conformational behaviour. Keywords: gauche effect, seven-membered rings, 1,5-benzodithiepins, variable temperature NMR, orbital interactions.