Chemistry of sulphines. Part XXVII. Conformational analysis of aryl- and alkyl-thio, aryl- and alkyl-sulphinyl, and aryl- and alkyl-sulphonyl sulphines by means of nuclear magnetic resonance spectra and dipole moments
The preferred conformations of mesityl aryl- and alkyl-thio E- and Z-sulphines (II) and (III), mesityl aryl- and alkyl-sulphinyl E- and Z-sulphines (IV) and (V), and mesityl aryl- and alkyl-sulphonyl E- and Z-sulphines (VI) and (VII) have been determined by means of n.m.r. spectra (chemical shifts, ASIS, and LIS) and dipole moments. A comparison has been made with the corresponding 2,4,6-trimethyldithiobenzoates (I) which possess a linear s-trans-conformation. In none of the sulphines has such a linear conformation been found. Folded and gauche conformations dominate. The preferred conformations are in most cases rather rigid and are discussed in terms of electrostatic interactions of the sulphide, sulphoxide, or sulphone functions either with the sulphine oxygen in the Z-sulphines or with the sulphine lone pair in the E-isomers on the one hand and in terms of steric interactions on the other. In the crowded sulphines (IIe)–(VIIe), where R2= 2,4,6-Me3C6H2, steric interactions between the ortho-methyl groups of both mesityl rings have been taken into account. In these sulphines the preferred conformations are sometimes different from those of the sulphines with R2= Ph. The sulphoxide E- and Z-sulphines exist as an equilibrium between two half-folded conformations. One conformation dominates the other. The thermodynamic parameters of these equilibria have been calculated. In the equilibrium of the sulphoxide E-sulphines remarkable effect on the equilibrium composition is observed upon addition of a shift reagent.