The effect of substituents on the electrochemical oxidation potentials of 1,1,2,2-tetraphenylcyclopropane and 1,1,3,3-tetraphenylpropenyl radical

The effects of multiple substitution by methoxy and cyano in the 4-position(s) of 1,1,2,2-tetraphenylcyclopropane (1) and 1,1,3,3-tetraphenylpropenyl radical (2•) on the oxidation potentials have been measured. The results indicate that the oxidation process for the cyclopropanes (1a–m) is irreversible. Nevertheless, the substituent effect on the potential is essentially additive and correlates reasonably well with Σσ+. A slow electron transfer process (an overall conversion of 1 to 2+), with a transition state resembling the ring-opened radical cation, is consistent with these observations. The oxidation of the radical 2• is quasi-reversible in dichloromethane. In this case also, the substituent effect on the oxidation potential is additive and correlates with Σσ+. Any deviation from planarity of 2• is not sufficient to prevent substituents from exerting normal (additive) behaviour.