Using a recyclable hypervalent iodine reagent 1, the authors have constructed versatile and green methods for the hypervalent iodine and nitroxyl radical-mediated selective oxidation of alcohols to aldehydes and ketones. The recyclable reagent 1 having a unique tetraphenyladamantane structure exhibited almost the same reactivity as the ordinary reagent, phenyliodine diacetate (PIDA), in the hypervalent iodine(III)/2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated oxidation. For recycling, the reagent 1 could be nearly quantitatively recovered as the reduced tetraiodide 2 by filtration after reaction completion, utilizing the insolubility of the formed 2 in a polar solvent, specifically, methanol. Based on the confirmed reactivity and excellent recycling operation of the reagent 1, the methodology has been further extended to a greener dual recycling strategy. By combining the recyclable iodine reagent 1 and silica-supported TEPMO catalyst, a variety of alcohols were effectively oxidized to the desired aldehydes, and the two types of used reagent and catalyst, the iodine 1 and immobilized TEMPO 5, could be separately recovered by an easy workup, and both repeatedly used without any loss of their original activities for at least four cycles.
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Dohi et al. (2012) studied this question.