STRUCTURE OF THE RADICALS FORMED IN THERMAL AND PHOTOCHEMICAL-REACTIONS OF 3-ALKYLTHIOPHENES UNDER ACIDIC CONDITIONS

It is shown that thallium(III) tris (trifluoroacetate) (denoted Tl-III ) in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) oxidizes 2-alkylthiophene s 3 to give the corresponding persistent 5,5'-dialkyl-2,2'-bithiophene radical cations, and that similar oxidation of 3-methylthiophene give s the EPR spectrum of the short-lived radical cation of 3,3'-dimethyl- 2,2'-bithiophene. The photolysis of 3-methylthiophene in HFP containin g a small amount of methanesulfonic acid gave an EPR spectrum which wa s assigned the structure of the 5,5'-bisprotonated radical cation form ally derived from the one-electron reduction of 5,5'-bis-protonated 4, 4'-dimethyl-2,2'-bithiophene. Calculated (UB3LYP/6-31G//UHF/3-21G*) E PR spectral parameters agreed well with this structure. Analogous spec ies were detected from the thermal treatment of 3-ethyl-, 3-isopropyl- and 3-tert-butylthiophene by HFP-methanesulfonic acid. In HFP-sulfuri c acid (0.7%) all four 3-alkylthiophenes gave these radicals in a ther mal reaction. The oxidation of 2,5-dimethylthiophene by Tl-III in HFP- trifluoroacetic acid (7%) gave solutions displaying EPR spectra which were assigned to cis- and trans-2,2', 5,5'-tetramethyl-3, 3'-bithiophe ne radical cations. Similar reactions occurred in HFP-sulfuric acid (0 .7%).