Selective benzylic C-H monooxygenation mediated by iodine oxides

A method for the selective monooxdiation of secondary benzylic C-H bonds is described using an N-oxyl catalyst and a hypervalent iodine species as a terminal oxidant. Combinations of ammonium iodate and catalytic N-hydroxyphthalimide (NHPI) were shown to be effective in the selective oxidation of n-butylbenzene directly to 1-phenylbutyl acetate in high yield (86%). This method shows moderate substrate tolerance in the oxygenation of substrates containing secondary benzylic C-H bonds, yielding the corresponding benzylic acetates in good to moderate yield. Tertiary benzylic C-H bonds were shown to be unreactive under similar conditions, despite the weaker C-H bond. A preliminary mechanistic analysis suggests that this NHPI-iodate system is functioning by a radical-based mechanism where iodine generated in situ captures formed benzylic radicals. The benzylic iodide intermediate then solvolyzes to yield the product ester.