Studies of heterocyclic compounds. Part XIV. 1,6a-Dithia-6-azapentalenes

Abstract

6-Methyl-1,6a-dithia-6-azapentalenes have been synthesised by the reaction of 3-(2-dimethylaminovinyl)-1,2-dithiolium (Vilsmeier) salts or 6a-thiathiophthens with methylamine. 1,6a-Dithia-6-azapentalenes form stable charge-transfer complexes with 1,3,5-trinitrobenzene. The significance of the ¹H n.m.r. spectra of 1,6a-dithia-6-azapentalenes is discussed in relation to their structure. 1,6a-Dithia-6-azapentalenes are basic; they form stable 3-(2-methylaminovinyl)-1,2-dithiolium salts with perchloric acid. ¹H N.m.r. spectral studies show that protonation of 1,6a-dithia-6-azapentalenes in trifluoroacetic acid occurs predominantly at the nitrogen atom, to form the 3-(2-methylaminovinyl)-1,2-dithiolium cations. The co-existence of low equilibrium concentrations of the corresponding C(4)-protonation products with the major products was inferred from the occurrence of H–D exchange in deuteriotrifluoroacetic acid. Exceptionally, 2,5,6-trimethyl-1,6a-dithia-6-azapentalene gave the N-and theC(4)-protonation product in comparable amounts. Variable temperature ¹H n.m.r. spectral studies show that 5-t-butyl- and 5-methyl-3-(2-dimethylaminovinyl)-1,2-dithiolium perchlorate in CD₃CN exist at low temperatures (–20 to –50°) as mixtures of two of two trans-isomers, owing to restricted rotation at the 3,1′-and C(2′), N-bonds. At +60° rapid interconversion of the isomers gives a time-averaged spectrum corresponding to the presence of a single trans species. Similar temperature-dependent behaviour was shown by the structurally related 5-t-butyl-and 5-methyl-3-(2-methylaminovinyl)-1,2-dithiolium perchlorates.