Oxidation of adamantane with 1 atm molecular oxygen by vanadium-substituted polyoxometalates

The oxidation of adamantane with molecular oxygen as a sole oxidant was efficiently promoted by catalyst precursors of vanadium-substituted Keggin-type phosphomolybdates such as H4PVMo11O40, H5PV2Mo10O40, and H6PV3Mo9O40 in butyronitrile. The major product was a tertiary CH bond oxygenated product of 1-adamantanol, and secondary CH bond oxygenated products were also formed. The total yield of oxygenated products for the oxidation of adamantane in the presence of H5PV2Mo10O40 reached 84%. NMR and IR data show that the vanadium-substituted phosphomolybdates, such as H4PVMo11O40 and H5PV2Mo10O40, decompose to form the monomeric vanadium species (VVO2+ (main) and VIVO2+) and PMo12O403− Keggin anion. The reaction mechanism involving a radical species was proposed from ESR and kinetic data. The catalysts initially abstract the hydrogen of adamantane to form the adamantyl radical and reduced catalysts. This step would be promoted mainly by the vanadium species, such as VVO2+, and the phosphomolybdates, PMo12O40n−, enhance the activity. The adamantyl radical formed promotes the successive formation of the key intermediates, such as adamantyl radical and hydroperoxide species.