ANODIC-OXIDATION OF ALPHA-SUBSTITUTED PARA-XYLENES - ELECTRONIC AND STEREOELECTRONIC EFFECTS OF ALPHA-SUBSTITUENTS IN THE DEPROTONATION OF ALKYLAROMATIC RADICAL CATIONS

The effect of alpha-substituents on the deprotonation rate from the benzylic position of alkylaromatic radical cations, k(CH2Z)/k(CH3), has been investigated by determining the intramolecular selectivity in the anodic oxidation in acetic acid of alpha-Z-substituted p-xylenes 1 (Z = H, OMe, OH, Me, tert-butyl, OAc, COOMe, CN), 5,6-dimethylindan 4 (R = H), and 2,2,5,6-tetramethylindan 4 (R = Me). Some oxidations induced by CAN have also been carried out. It has been found that, with the exception of when Z = tert-butyl, the deprotonation rate of I.+ is always faster from the CH2Z group than from the CH3 group, independently of the electron-donating or electron-withdrawing nature of Z. The electron-donating groups (OH, OMe, Me), however, exert a larger effect than the electron-withdrawing ones (COOMe, CN). The negligible deprotonation rate from CH2-t-Bu has been ascribed to stereoelectronic effects (the bulky tert-butyl group does not allow the C-H bonds to be collinear with the pi-system), the suggestion being nicely confirmed by the observation that the deprotonation rate from the position 1(3), relative to that from the 5(6)-methyl group, is almost identical in the radical cations of 4 (R = H and Me). The effect of the other alpha-substituents is mainly of electronic nature and has been rationalized on the basis of a variable transition-state structure for the deprotonation process. It is suggested that with +R groups most of the charge, in the transition state, has been transferred to the C(alpha)-H bond where it can be stabilized by the alpha-substituent. With electron-withdrawing groups less charge transfer has taken place and the rate-enhancing effects of these groups is ascribed to their capability to significantly decrease the strength of the C(alpha)-H bond.