Some new aspects of the side-chain reactions of aromatic radical cations

The side-chain reactivity of 1-arylalkanol radical cations, ArCH(OH)R.+, has been studied in water by the pulse radiolysis technique. It is shown that these radical cations exhibit both carbon and oxygen acidity, depending on pH. In acid media (pH < 5) the radical cations undergo the expected deprotonation reaction at the alpha-C-H bond (carbon acidity), water being the proton abstracting base. However, in basic media (pH = 10-11), the -OH and phosphate-induced deprotonation involves the O-H bond, leading to the formation of a benzyloxyl radical. This radical can then undergo C-C bond cleavage to form a benzaldehyde (R = tBu), or a 1,2-H atom shift to give a carbon centered radical (R = H, Me). In a study with 4-methoxycumyl alcohol radical cation direct evidence of the formation of the cumyloxyl radical was obtained. The same dichotomy was also observed in the case of 2-arylalkanol radical cations (carbon acidity in acid media and oxygen acidity in the presence of -OH).