The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1+ and 2+ undergo CαH deprotonation as the exclusive reaction with k=4.6×104 and 3.2×104 s−1, respectively. In basic solution 1+ and 2+ behave as oxygen acids undergoing −OH-induced αOH deprotonation in a diffusion controlled process (k−OH≈1010 M−1 s−1). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1+) or as the exclusive pathway (with 2+). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.