Reduction of Acetylated Tetraphenylethylenes: Electrochemical Behavior and Stability of the Related Reduced Anions

The synthesis and characterization of 1-(4-acetylphenyl)-1,2,2-triphenylethylene ( 2 ), 1,1-bis(4-acetylphenyl)-2,2-diphenylethylene ( 3a ), trans -1,2-bis(4-acetylphenyl)-1,2-diphenylethylene ( 3b ), cis- 1,2-bis(4-acetylphenyl)-1,2-diphenylethylene ( 3c ), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene ( 4 ), and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene ( 5 ) are described. The molecular orbital energies, charge densities, and electronic spectra of the neutral and reduced species of 2 and 3a − 3c were calculated using INDO/1 parameters for structures that had been optimized geometrically with AM1 parameters. The calculated electronic spectra for the neutral compounds compare favorably with the experimentally observed absorption spectra. Cyclic voltammetry of these compounds in deoxygenated THF containing 0.3 M [ n -Bu 4 N]PF 6 showed that 2 and 3a have irreversible one-electron first reduction waves, whereas 3b, 3c, 4, and 5 display quasi-reversible two-electron first reduction waves. Chemical reduction of 2 − 4 with Na/Hg in THF results in highly colored solutions, the absorbance spectra of which have been compared to the calculated electronic spectra for the radical anions and dianions of these compounds. Reduction of 2 results in dimerization to a pinacol product, whereas those of 3b and 4 result only in formation of the tetraarylethane product. 3a gives both pinacol and tetraarylethane upon reduction.