Reactivity and selectivity of aryloxylium ions

The reactivity and selectivity of aryloxylium ions in acetonitrile–water mixtures are described. The 4-bromo-2,4,6-trialkylcyclohexa-2,5-dienones used as substrates were synthesised by electrophilic bromination in yields of 60% (3 R = Me) and 52% (7 R = Bu^t). Under solvolysis conditions 3 and 7 decompose cleanly via their respective aryloxylium cations to the 4-hydroxy-2,4,6-trialkylcyclohexa-2,5-dienones 4 and 12. As the polarity of the solvent increases, the observed rates of reaction increase by a factor of 36 (3) and 56 (7). A detailed study gave Grunwald–Winstein m_s values of 0.56 (3) and 0.63 (7). Addition of sodium bromide caused a common ion rate depression with α = 22.1 (3) and α = 33.3 (7). Competitive azide trapping gave k_az ∶ k_MeOH = 3610 and k_MeOH ∶ k_H2O = 0.755, while the lifetime of the cation 11 in water was estimated to be 0.55 µs. Sterically hindered alcohols (propan-2-ol, cyclohexanol) do not trap the aryloxylium ions, instead products of reduction (the corresponding phenols) were formed in quantitative yield. Following a careful search it was determined that these products did not arise from hydride transfer from these alcohols.