Cyclic sulphones. Part X. Kinetic evidence for the aromatic character of anions derived from benzo- and dibenzothiopyran SS-dioxide systems

Kinetic acidities of benzo- and dibenzothiopyran SS-dioxides of some methyl derivatives, and of the corresponding open-chain analogues have been determined in [2H5]pyridine–heavy water; it is found that the two pairs of isomers show similar kinetic acidities and that their deuterium exchange rates exceed those of the open-chain analogues by a factor of 103–105. Other stabilising features being common in the two series, the greater stability of the cyclic anions must be associated with their cyclic unsaturated nature: to account for the magnitude of the effect, it is suggested that the conjugative stabilisation developing in the anions is aromatic in character.