Studies of heterocyclic compounds. Part XVI. Mechanism of electrophilic substitution of 6a-thiathiophthens and related compounds: nitrosation with rearrangement

Abstract

Nitrosation of 6a-thiathiophthens and related compounds occurs with rearrangement. 6a-Thiathiophthens, 1-oxa-6,6a-dithiapentalenes, and 1,6a-dithia-6-azapentalenes rearrange into 1-oxa-6,6a-dithia-2-azapentalenes, and 1,6-dioxa-6a-thiapentalenes into 1,6-dioxa-6a-thia-2-azapentalenes. 6a-Thiathiophthens react with nitrous acid with difficulty unless activated by strongly electron-releasing substituents. 2-t-Butyl-6a-thiathiophthen reacted at position 4 to give 3-formyl-5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene in low yield. 2-Methylthio-5-t-butyl-and 2-dimethylamino-5-t-butyl-6a-thiathiophthen reacted readily at position 3 to give the methyl 3-dithiocarboxylate and the 3-NN-dimethylthiocarboxamide of 5-t-butyl-1-oxa-6,6a-dithia-2-azapentalene, respectively, selective desulphurisation of which afforded the corresponding S-methyl thioester and NN-dimethylcarboxamide. 5-Phenyl-, 5-t-butyl-, and 2,5-dimethyl-1-oxa-6,6a-dithiapentalene reacted smoothly at position 3 to give the corresponding 3-formyl(acetyl)-1-oxa-6,6a-dithia-2-azapentalenes. 6-Methyl-2-phenyl- and 6-methyl-2-t-butyl-1,6a-dithia-6-azapentalene also gave 3-formyl-1-oxa-6,6a-dithia-2-azapentalenes by hydrolysis in situ of the intermediate 3-methyliminomethyl-1-oxa-6,6a-dithia-2-azapentalenes. 1-Oxa-6,6a-dithiapentalenes and 1,6a-dithia-6-azapentalenes in which the reactive position 3(4) is blocked, reacted at position 3(4) with elimination of the formyl or methyliminomethyl group to yield 1-oxa-6,6a-dithia-2-azapentalenes. Nitrosation of 1,6-dioxa-6a-thiapentalene with nitrosyl hexafluorophosphate gave 3-formyl-1,6-dioxa-6a-thia-2-azapentalene, the first reported derivative of the 1,6-dioxa-6a-thia-2-azapentalene system. A mechanism is proposed to account for the various features of the electrophilic substitution of 6a-thiathiophthens and related hypervalent heterocyclic systems. It is proposed that reaction proceeds by way of stable 6π-electron monocyclic cations, such as 1,2-dithiolium and 1,2-oxathiolium.