Structure of the Radicals Formed in Thermal and Photochemical Reactions of 3-Alkylthiophenes under Acidic Conditions.
作者:Frederique Barbosa、Lennart Eberson、Georg Gescheidt、Salo Gronowitz、Anna-Britta Hörnfeldt、Luis Juliá、Ola Persson
DOI:10.3891/acta.chem.scand.52-1275
日期:——
It is shown that thallium(III) tris (trifluoroacetate) (denoted Tl-III) in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) oxidizes 2-alkylthiophenes 3 to give the corresponding persistent 5,5'-dialkyl-2,2'-bithiophene radical cations, and that similar oxidation of 3-methylthiophene gives the EPR spectrum of the short-lived radical cation of 3,3'-dimethyl-2,2'-bithiophene.The photolysis of 3-methylthiophene in HFP containing a small amount of methanesulfonic acid gave an EPR spectrum which was assigned the structure of the 5,5'-bisprotonated radical cation formally derived from the one-electron reduction of 5,5'-bis-protonated 4,4'-dimethyl-2,2'-bithiophene. Calculated (UB3LYP/6-31G*//UHF/3-21G*) EPR spectral parameters agreed well with this structure. Analogous species were detected from the thermal treatment of 3-ethyl-, 3-isopropyl- and 3-tert-butylthiophene by HFP-methanesulfonic acid. In HFP-sulfuric acid (0.7%) all four 3-alkylthiophenes gave these radicals in a thermal 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'-bithiophene radical cations. Similar reactions occurred in HFP-sulfuric acid (0.7%).