On the Homoconjugation of Two Acceptor Groups

Cyclovoltammetric investigations have been carried out on 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetrone (4), bicyclo[3.2.2]nonane-6,7,8,9-tetrone (5), and several of their congeners, such as 6,13-dihydro-6,6,13,13-tetramethylquinoxalino[2,3-b]phenazine (13), 5,10-dihydro-5,5,10,10-tetramethylpyrazino[2,3-g]quinoxaline (17), and 6,13-dihydro-6,13-propanoquinoxalino[2,3-b]phenazine (20). For 5 and 20 a large difference (Delta E degrees greater than or equal to 400 mV) between the first and second reduction potentials was found, The ESR results of the radical anion 20(.-) are in support of a strong homoconjugation. ESR studies of 4(.-), 13(.-), and 17(.-) also reveal a symmetrical displacement of the unpaired electron over both acceptor groups on the ESR time scale which, however, based on the small potential difference Delta E degrees less than or equal to 200 mV can be most likely described to a fast electron exchange.