Adducts of Thianthrene- and Phenoxathiin Cation Radical Tetrafluoroborates to 1-Alkynes. Structures and Formation of 1-(5-Thianthreniumyl)- and 1-(10-Phenoxathiiniumyl)alkynes on Alumina Leading to α-Ketoylides and α-Ketols

Thianthrene cation radical tetrafluoroborate (Th•+BF4-) added to the terminal alkynes 1-pentyne,
1-hexyne, 1-heptyne, 1-octyne, 1-nonyne, and 1-decyne to form trans-1,2-bis(5-thianthreniumyl)alkene tetrafluoroborates (1−6). Similarly, addition of phenoxathiin cation radical tetrafluoroborate
(PO•+BF4-) to the same alkynes gave 1,2-bis(10-phenoxathiiniumyl)alkene tetrafluoroborates (7−12). The trans configuration of two of the adducts (1 and 4) was shown with X-ray crystallography.
When solutions of 1−6 in chloroform were stirred with activated alumina, cis elimination of a proton
and thianthrene (Th) occurred with the formation of 1-(5-thianthreniumyl)alkyne tetrafluoroborates
(1a−6a). Similar treatment of 8−12 caused elimination of a proton and phenoxathiin (PO) with
formation of 1-(10-phenoxathiiniumyl)alkene tetrafluoroborates (8a−12a). Stirring of 1a−6a with
alumina for short periods of time caused their conversion into 5-[(α-keto)alkyl]thianthrenium ylides
(1b−6b) and α-ketols, RC(O)CH2OH (1c−6c).