Studies of heterocyclic compounds. Part 21. Reactions of 1-aryl-6,6a-dithia-1,2-diazapentalenes with electrophiles

Abstract

1-Aryl-6,6a-dithia-1,2-diazapentalenes react with electrophiles at position 3. Bromination of 1-phenyl-, 1-p-bromophenyl-, 1-phenyl-5-t-butyl-, and 1-p-bromophenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene gave the 3-bromo-derivatives as the major products. Vilsmeier formylation of 1-phenyl-, 1-phenyl-5-t-butyl,- and 1-p-nitrophenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene took place with difficulty to give the corresponding 6,6a-dithia-1,2-diazapentalene-3-carbaldehydes. Nitration of 1-phenyl- and 1-phenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene gave the 3-nitro-derivatives as the main products. 3-Nitro-1-p-nitrophenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene, a minor product from the nitration of 1-phenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene, was also synthesised by the coupling of 3-nitromethylene-5-t-butyl-3H-1,2-dithiole with p-nitrobenzenediazonium tetrafluoroborate. Nitration of 3,4-dimethyl-1-phenyl-6,6a-dithia-1,2-diazapentalene, in which the reactive position 3 is blocked, gave as the major product 4-methyl-3-(1-nitroethylidine)-3H-1,2-dithiole by nitro-dediazoniation at position 3, together with 3,4-dimethyl-1-p-nitrophenyl-6,6a-dithia-1,2-diazapentalene. Nitrosation of 1-phenyl- and 1-phenyl-5-t-butyl-6,6a-dithia-1,2-diazapentalene took place with rearrangement to give 3-phenylazo- and 3-phenylazo-5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene, respectively, as the main products. Nitrosation of the blocked 3,4-dimethyl-1-phenyl-6,6a-dithia-1,2-diazapentalene gave mainly 3,4-dimethyl-1-oxa-6,6a-dithia-2-azapentalene by nitroso-dediazoniation, some 4-methyl-3-(1-nitroethylidene)-3H-1,2-dithiole by nitrodediazoniation, and a small amount of 3,4-dimethyl-1-p-nitrophenyl-6,6a-dithia-1,2-diazapentalene. Coupling of 1-aryl-5-t-butyl-6,6a-dithia-1,2-diazapentalenes with arenediazonium tetrafluoroborates took place in acetonitrile with exchange of the arylazo-units. These results are interpreted on the basis of a previously proposed mechanism of electrophilic substitution of 6a-thiathiophthens and related four-electron three-centre bonded structures.