Studies of heterocyclic compounds. Part XVII. Synthesis of 1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes

Abstract

1,6-Dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes, two new classes of hypervalent heterocyclic compound, have been synthesised by the reaction of pyran-4-thiones and -4-selones, respectively, with thallium(III) trifluoroacetate, and treatment of the resulting labile thallium-containing pyrylium trifluoroacetates with water 1,6-Dioxa-6a-thiapentalene was also formed in low yield by ring-opening of pyran-4-thione with sodium hydroxide in aqueous dimethyl sulphoxide, and intramolecular oxidative coupling of the resulting anion with potassium ferricyanide. ¹H N.m.r. spectra of symmetrically substituted 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes show real or time averaged C_2v symmetry. The pattern of the spectrum of 1,6-dioxa-6a-thiapentalene was unaffected by change of solvent (CDCl₃, CS₂, or CD₃CN) or by lowering the temperature of a solution in CDCl₃(to –50°) or CS₂(to –90°). The i.r. spectra of 1,6-dioxa-6a-thiapentalene, 1,6-dioxa-6a-selenapentalene, and their 2,5-dimethyl derivatives show no absorption in the double bond region above 1515 cm^–1. The significance of the ¹H n.m.r. and i.r. spectra of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes is discussed in relation to structure. Thionation of 1,6-dioxa-6a-thiapentalene with phosphorus pentasulphide gave 6a-thiathiophthen. 1,6-Dioxa-6a-thiapentalene underwent sulphur–oxygen exchange in boiling thioacetic acid to give a mixture of 6a-thiathiophthen and 1-oxa-6,6a-dithiapentalene. Thionation of 1,6-dioxa-6a-selenapentalene with phosphorus pentasulphide afforded 6a-selenathiophthen.