Studies of heterocyclic compounds. Part XV. Synthesis and structure of 6a-thia-1,6-diazapentalenes (isothiazolo[5,1-e]isothiazoles)

Abstract

Methylation of 6-methyl-1,6a-dithia-6-azapentalenes with methyl iodide occurs exclusively at sulphur (S-1). The resulting 2-methyl-5-(2-methylthiovinyl)isothiazolium iodides react with methylamine to give symmetrical 6a-thia-1,6-diazapentalenes, a new class of hypervalent heterocyclic compound. 4,5-Dihydro-1,7-dimethyl-3H-benzo[cd]-6a-thia-1,6-diazapentalene and 1,2,5,6-tetramethyl-6a-thia-1,6-diazapentalene were accompanied by small quantities of thiones which result from reductive cleavage of the S–N bond in the 2-methyl-5-(2-methylthiovinyl)isothiazolium cations. 1,6-Dimethyl- and 1,3,4,6-tetramethyl-6a-thia-1,6-diazapentalene were accompanied by the corresponding 6-methyl-1,6a-dithia-6-azapentalenes fromed by S-demethylation of the 2-methyl-5-(2-methylthiovinyl)isothiazolium cations. ¹H N.m.r. spectral studies show that symmetrically substituted 6a-thia-1,6-diazapentalenes in solution possess real or time-averaged C_2v symmetry. The pattern of the spectrum of 1,3,4,6-tetramethyl-6a-thia-1,6-diazapentalene was unaffected by change of solvent [CDCl₃, CS₂, or (CD₃)₂SO] or by lowering the temperature of a solution in CS₂ to –70°. The significance of the ¹H n.m.r. spectra of 6a-thia-1,6-diazapentalenes and analogues of 6a-thiathiophthens is discussed in relation to structure. 6a-Thia-1,6-diazapentalenes form stable charge-transfer complexes with 1,3,5-trinitrobenzene.