Sunlight Induced Oxidative Photoactivation of N-Hydroxyphthalimide Mediated by Naphthalene Imides

We report the aerobic photoactivation of N-hydroxyphthlimide (NHPI) to the phthalimido-N-oxyl (PINO) radical mediated by naphthalene monoimides (NI) for promoting the selective oxidation of alkylaromatics and allylic compounds to the corresponding hydroperoxides. In the absence of either NI or NHPI no oxidation was observed, meaning that the two molecules operate in a synergistic way. Sunlight as well as artificial UV-light irradiation was necessary in order to perform the process at low temperature (30-35 degrees C). EPR spectroscopy confirmed the role of NI and oxygen in promoting the formation of the superoxide radicals O-2(.-) which, in turn, increased the concentration of PINO radicals during the UV light irradiation of NI/NHPI mixtures in MeCN. The investigation was extended to NI bearing different substituents on the naphthalene moiety. Finally, the synthesis and application of a unique photocatalyst including the NI and NHPI moieties linked by a suitable spacer was also considered. In this case the photocatalyst showed a substrate-dependent behaviour with some peculiarities in comparison to the system where NI and NHPI are independent units in the same reacting system. This photocatalytic system paves the way to a non-thermal, metal-free approach for CH bond activation towards aerobic oxidation under very mild conditions.