In order to develop a new bridge oxidation reagent, (diacetoxyiodo)benzene has been used to oxidize a variety of p-tert-butylcalix4arene derivatives. The oxidation experiments showed that bridge mono-oxidation with (diacetoxyiodo)benzene can be executed in moderate yields (up to 65%). The optimal solvent, (diacetoxyiodo)benzene amount, temperature, and time for bridge mono-oxidation of 1a are dioxane, 5 equiv, 80 °C, and 12 h, respectively. The substituents with strong electron-withdrawing capacity are detrimental to bridge mono-oxidation. p-tert-Butylcalix4arene derivatives in 1,3-alternate and 1,2-alternate conformations can be oxidized into p-tert-butyl-monoketocalix4arenes while those in cone and partial cone conformations cannot. A plausible mechanism including two processes was proposed for bridge mono-oxidation. The bridge mono-oxidation results confirm that (diacetoxyiodo)benzene can be used as an alternative and efficient metal-free oxidant for the synthesis of p-tert-butyl-monoketocalix4arene in 1,3-alternate and 1,2-alternate conformations. The further derivatizations show that the monobridge carbonyl group can impart a selective bridge-modification to the calix4arene skeleton.
Pang et al. (Tue,) studied this question.