Hydrogen bonds involving polar CH groups. Part 9. Optimum structural parameters, and unequivocal demonstration of such intramolecular interactions in 2-substituted 1,3-dithian 1,1,3,3-tetraoxides

Abstract

A series of 2-monosubstituted 1,3-dithian 1,1,3,3-tetraoxides have been prepared, having side chains with 2–6 carbon atoms, and bearing alkyl, aryl, methoxy, dialkylamino, and pyridyl terminal substituents, as models for intramolecular C–H ⋯ X hydrogen bonds. Methoxy- and amino-substituents, in compounds having two or three carbon atoms in the side chain, show significant intramolecular interactions with the disulphone methine hydrogen in [²H₂]dichloromethane, as evidenced by a downfield shift of the ¹H n.m.r. signals; shifts of up to 2.0 p.p.m. have been observed. Intramolecular H-bonds are disrupted in [²H₃]acetonitrile and in pyridine owing to competing intermolecular interactions with the solvents, and in trifluoroacetic acid owing to protonation of the donor atom.