Hydroxy-group participation in the chlorination of thian 1-oxides

Abstract

Chlorination of 4-hydroxythian 1-oxides with hydroxy- and S-oxide groups cis to each other gives 4-chlorosulphones. The reaction proceeds via a cyclic alkoxyoxosulphonium ion formed by nucleophilic displacement of chlorine by the 4-hydroxy-group in an initially formed chloro-oxosulphonium ion in which the S-oxide group is axial. The equatorial chlorine atom is displaced with inversion at sulphur. Nucleophilic attack by chloride ion at C-4 of the cyclic alkoxyoxosulphonium ion with inversion at C-4 causes ring opening with transfer of the C-4 oxygen atom to sulphur. Chlorination of a thian 1-oxide containing a 4-chloro- or a 4-tosyloxy-group, or a 4-hydroxy-group trans to the S-oxide group, proceeds without participation to give the α-chloro-sulphoxide.