Reaction of thianthrene and phenoxathiin cation radicals with 2,3-dimethyl-2-butene. Chemical and electrochemical studies.

Thianthrene cation radical tetrafluoroborate (Th*+ BF4-) has been found to add to 2,3-dimethyl-2-butene (DMB) at 0 degrees C and -15 degrees C. The adduct, 2,3-dimethyl-2,3-(5,10-thianthreniumdiyl)butane ditetrafluoroborate (12), was isolated at -15 degrees C, and its 1H NMR spectrum was recorded at that temperature. The adduct was stable in CD3CN solution at -15 degrees C but decomposed slowly at 0 degrees C and quickly at 23 degrees C, forming the salt of 2,4,4,5,5-pentamethyl-2-oxazoline (8) with loss of thianthrene (Th). These results explain why earlier attempts to prepare 12 and detect its formation at room temperature with NMR spectroscopy were not successful. Reaction of Th*+ with DMB was followed with cyclic voltammetry and was found to exhibit redox catalysis in which Th was regenerated. With the faster scanning techniques of cyclic voltammetry, the formation of 12 was detectable, with a reduction potential of about -1.0 V at 25 degrees C and 3 degrees C. The observed reduction potential was in harmony with reduction potentials of a number of other, stable monoadducts. Thus, the redox catalysis involved the rapid formation of 12 and its rapid decomposition into 8 and Th, the newly formed Th being responsible for the observed enhanced oxidation currents. In contrast, 8 appears to be formed directly by oxidation of DMB by PO*+PF6-.