Studies of heterocyclic compounds. Part XVIII. Protonation of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes: formation of 1,2-oxathiolium and 1,2-oxaselenolium cations

A 1H n.m.r. spectroscopic study of the protonation of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes shows that O- and C(3)-protonation occurs. Equilibrium mixtures are formed which contain the unprotonated heterocycle and the O- and the C(3)-protonated species in varying relative amounts, depending on the nature of the heterocycle and the acid medium. 1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalene remained largely unprotonated in trifluoroacetic acid, but the occurrence of H–D exchange at C-3 and C-4 in trifluoroacetic [2H]acid revealed the presence of the 3-formylmethyl-1,2-oxathiolium and -1,2-oxaselenolium cations. The major species in a solution of 2,5-dimethyl-1,6-dioxa-6a-thiapentalene in trifluoroacetic acid were the unprotonated heterocycle and two isomeric 3-(2-hydroxypropenyl)-5-methyl-1,2-oxathiolium cations in a 4 : 6 : 1 ratio. 2,5-Dimethyl-1,6-dioxa-6a-selenapentalene was largely unprotonated in trifluoroacetic acid. In trifluoroacetic acid containing 5%(v/v) perchloric acid 2,5-dimethyl-1,6-dioxa-6a-thia- and 2,5-dimethyl-1,6-dioxa-6a-selenapentalene gave the 3-acetonyl-5-methyl-1,2-oxathiolium and -1,2-oxaselenolium cations, respectively, as the only observable species. The O- and C(3)-protonated 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes are the first members of the 1,2-oxathiolium and the 1,2-oxaselenolium systems to be identified. The structure of the C(3)-protonated 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes is discussed in relation to the mechanism of electrophilic substitution of 1,6-dioxa-6a-thiapentalenes and related hypervalent heterocyclic compounds.