Structural effects on the OH--promoted fragmentation of methoxy-substitued1-arylalkanol radical cations in aqueous solution: The role of oxygen acidity

A kinetic and product study of the OH -induced decay in H2O of the radical cations generated from some di- and tri-methoxy-substituted 1-arylalkanols (ArCH(OH)R.+) and 2- and 3-(3,4-dimethoxyphenyl)alkanols has been carried o ut by using pulse- and gamma -radiolysis techniques. In the 1-arylalkanol s ystem, the radical cation 3,4-(MeO)(2)C6H3CH2OH.+ decay at a rate more than two orders of magnitude higher than that of its methyl ether; this indicat es the key role of the side-chain OH group in the decay process (oxygen aci dity). However, quite a large deuterium kinetic isotope effect (3.7) is pre sent for this radical cation compared with its alpha -dideuterated counterp art. A mechanism is suggested in which a fast OH deprotonation leads to a r adical zwitterion which then undergoes a rate-determining 1,2-H shift, coup led to a side-chain-to-ring intramolecular electron transfer (ET) step. Thi s concept also attributes an important role to the energy barrierfor this E T, which should depend on the stability of the positive charge in the ring and, hence, on the number and position of methoxy groups. On a similar expe rimental basis, the same mechanism is suggested for 2,5-(MeO)(2)C6H3CH2OH. as for 3,4-(MeO)(2)C6H3CH2OH.+ in which some contribution from direct C-H deprotonation (carbon acidity) is possible. In fact, the latter process dom inates the decay of the trimethoxylated system 2,4,5-(MeO)(3)C6H2CH2-OH.+ w hich, accordingly, reacts with OH- at the same rate as that of its methyl e ther. Thus, a shift from oxygen to carbon acidity is observed as the positi ve charge is increasingly stabilized in the ring: this is attributed to a c orresponding increase in the energy barrier for the intramolecular ET. When R-tBu, the OH- -promoted decay of the radical cation ArCH(ON)R.+ leads to products of C-C bond cleavage. With both Ar=3,4- and 2,5-dimethoxyphenyl th e reactivity is three orders of magnitude higher than that of the correspon ding cumyl alcohol radical cations; this suggests a mechanism in which a ke y role is played by the oxygen acidity as well as by the strength of the sc issile C-C bond: a radical zwitterion is formed which undergoes a rate-dete rmining C-C bond cleavage, coupled with the intramolecular ET Finally, oxyg en acidity also determines the reactivity of the radical cations of 2-(3,4- dimethoxyphenyl)ethanol and 3-(3,4-dimethoxyphenyl)propanol. In the former the decay involves C-C bond cleavage, in the latter it leads to 3-(3,4-dime thoxyphenyl)propanal. In both cases no products of C-H deprotonation were o bserved. possible mechanisms, again involving the initial formation of a ra dical zwitterion, are discussed.