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This article describes synthetic and structural studies of heterocycles based on hypervalent sulfur and selenium, in particular 1,6,6aλ4-triheterapentalenes, their monocyclic precursors and related compounds. What may be termed classical 1,6,6aλ4-triheterapentalenes conform to structure (1), in which Y = S, Se or Te and X and Z are O, S, Se or NR [1]. Many aza analogues of (1) are known in which one or more ring carbon atoms at positions 2, 3, 4 and 5 are replaced by nitrogen atoms. In the majority of known 1,6,6aλ4-triheterapentalenes, Y = S. Relatively few analogues are known in which Y = Se, and still fewer are known in which Y = Te. Analogues in which X and/or Ze is Te or PR have not hitherto been prepared.

The oxidation of methylene-3,3′- and -1,1′-di-indolizines with quinones has been studied. 1,1′,2,2′-Tetramethylmethylene-3,3′-di-indolizine gives products of hydride-transfer and cyclodehydrogenation whose structures have been elucidated.

Dimethylthioformamide condenses with 3-methyl(ene)-1,2-dithiolium salts in acetic anhydride to give 3-(2-dimethylaminovinyl)-1,2-dithiolium (Vilsmeier) salts. These salts react with sodium hydrogen sulphide to give thiothiophthenes, and with sodium hydroxide to form 3-acylmethylene-3H-1,2-dithioles. H N.m.r. data for these compounds are discussed. An improved procedure for the synthesis of 1,2-dithiolium salts is described.

4H-Thiopyran-4-thiones are ring-opened by sulphide or hydroxide in dimethyl sulphoxide or NN-dimethylformamide, forming anions which undergo intramolecular oxidative coupling to give 6a-thiathiophthens or 3-acylmethylene-3H-1,2-dithioles, respectively. The procedure provides a useful synthesis of 6a-thiathiophthen.

6a-Thiathiophthen and 2,5-dideuterio-6a-thiathiophthen undergo formylation with NN-dimethylthioformamidephosphoryl chloride at position 3. Formylation of 2-t-butyl-6a-thiathiophthen and 5-deuterio-2-t-butyl-6a-thiathiophthen occurs at position 4. The position of formylation was established by comparative 1H n.m.r. and mass spectral studies. 2-Methyl- and 2,5-dimethyl-6a-thiathiophthen condensed with NN-dimethylthioformamide at their reactive methyl groups, giving enamines. The i.r. spectra of formyl-6a-thiathiophthenes indicate that the 6a-thiathiophthen nucleus is electron-releasing at the 3(4)-position. Formyl-6a-thiathiophthens were reduced by sodium borohydride to the corresponding hydroxymethyl-6a-thiathiophthens, and were partially desulphurised by mercuric acetate to 3-diformylmethylene-3H-1,2-dithioles.

4-Seleno-4H-thiopyran-4-ones are formed by treatment of 4H-thiopyran-4-ones with phosphoryl chloride in NN-dimethylformamide and subsequent addition of aqueous potassium selenosulphate. 3,5-Dimethyl-4-seleno-4H-thiopyran-4-one was isolated and characterised, but 4-seleno-4H-thiopyran-4-one and 2,6-diphenyl-4-seleno-4H-thiopyran-4-one decomposed on attempted isolation. 4-Seleno-4H-thiopyran-4-ones are ring-opened by sulphide in dimethyl sulphoxide, forming anions which undergo oxidative coupling to give 6a-selenathiophthens, a new class of heterocyclic compounds. The structure of 6a-selenathiophthens is discussed in relation to their 1H n.m.r. spectra.

The structure previously suggested for the indeno[2,1 -a]phenalenium cation, and later supported by M.O. calculations, is confirmed by proton magnetic resonance studies. The spectrum of 1,4,7-trimethylphenalenium perchlorate is recorded.

N-Substituted thioamides react with 1-bromoindan-2-one to give thiolimidate ester salts which are cyclised to 4H-indeno[2,1-d]thiazolium salts by thionyl chloride in dimethylformamide or phosphorus pentachloride in nitromethane. Primary thioamides lead to 4H-indeno[2,1-d]thiazoles.

1- and 3-Thioformylindolizines, a class of stable heterocyclic thioaldehydes, have been synthesised by a novel variation of the Vilsmeier reaction. They have also been obtained, in lower yield, by thionation of 1- and 3-formyl-indolizines, and by solvolysis of 3-ethoxymethylene-3H-indolizinium salts with sodium hydrogen sulphide. U.v. and 1H n.m.r. spectral data of the thioaldehydes are discussed.

The electrophilic substitution of 6-methylpyrrolo[2,1-b]thiazole has been studied quantitatively. Acetylation, formylation, trifluoroacetylation, and nitrosation occur exclusively at position 5. Further reaction gives 5,7-disubstitution products. Tritylation gives mainly 5,7-ditrityl- and 5-trityl- together with some 7-trityl-6-methyl-pyrrolo[2,1-b]thiazole.

Nitrosation of 3-alkyl-1,2-dithiolium salts gives 1-oxa-6,6a-dithia-2-azapentalenes (3-nitrosomethylene-3H-1,2-dithioles) in excellent yield. 3-Alkyl-1,2-diselenolium salts likewise afford 1-oxa-6,6a-diselena-2-azapentalenes (3-nitrosomethylene-3H-1,2-diselenoles), a new class of heterocyclic compounds. Attempted nitration of 2-t-butyl-6a-thiathiophthen with tetranitromethane gave 3-formyl-5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene and a small amount of 5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene. 2-Phenyl-6a-thiathiophthen behaved analogously. 3,4-Dimethyl-6a-thiathiophthen and 4,5-dihydro-3H-benzo[cd][6a]thiathiophthen gave products of partial desulphurisation, nitration, and nitrosation in low yield. The significance of 1H n.m.r. spectral data is discussed in relation to structure of the reaction products.

The protonation of indolizines under reversible conditions has been studied. Indolizines unsubstituted at position 3 give 3H-indolizinium cations alone in measurable amount. 3-Substituted indolizines give mixtures of 3H- and 1H-indolizinium cations in proportions depending on the nature and position of substitution. Exceptionally 3,5-dimethylindolizine gives the 3H-cation alone, owing to intramolecular overcrowding.The protonation of indolizine at position 3 agrees with Huckel M.O. predictions of reactivity in terms of charge and frontier electron densities, but not of electrophilic localisation energies.