Adducts of thianthrene- and phenoxathiin cation radical salts with symmetrical alkynes. Structure and formation of cumulenes on alumina leading to α-diketones, α-hydroxyalkynes, and α-acetamidoalkynes

Thianthrene cation radical tetrafluoroborate (Th^.+BF ₄^-) added to 2-butyne, 3-hexyne, 4-octyne, and 5-decyne in MeCN to form trans bisadducts E(Th⁺)C=C(Th⁺)R, where E = Me, Et, Pr, Bu (7a-d). Phenoxathiin cation radical tetrafluoroborate (PO ^.+BF₄^-) added similarly to the last three alkynes to form adducts E(PO⁺)C=C(PO⁺)E, 8b-d. Cyclic monoadducts were not found. The trans structures of 7 and 8 were deduced with X-ray crystallography (7c) and NME spectroscopy. When solutions of adducts in CHCl₃ and MeCN were deposited on activated alumina, elimination of thianthrene (Th) and phenoxathiin (PO) occurred almost quantitatively. Detailed studies with (7b-d) indicated that a cumulene (15) was formed by the elimination of Th and that 15 was subsequently converted into small amounts of other products. In CHCl₃, these products were the respective alkyne, thianthrene 5-oxide, an α-diketone (11), an α-hydroxyalkyne (12), and hydrogen. The same products were formed in MeCN along with an α-acetamidoalkyne (13). The formation of 15 and products derived from it is explained and was confirmed by preparation and reactions of 2,3,4-hexatriene.