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•+BF4-) added to 2-butyne, 3-hexyne, 4-octyne, and
5-decyne in MeCN to form trans bisadducts R(Th+)CC(Th+)R, where R = Me, Et, Pr, Bu (7a−d).
Phenoxathiin cation radical tetrafluoroborate (PO•+BF4-) added similarly to the last three alkynes
to form adducts R(PO+)CC(PO+)R, 8b−d. Cyclic monoadducts were not found. The trans structures
of 7 and 8 were deduced with X-ray crystallography (7c) and NMR 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.