Lipophilic N-Hydroxyimide Derivatives: Design, Synthesis, and Application to Aerobic Alkane Oxidation Under Neat Conditions.

This work reports the design and synthesis of a series of 5-membered and 6-membered N-hydroxyimides flanked by lipophilic alkyl groups adjacent to carbonyl groups, aiming to endow lipophilicity and tolerance toward ring opening degradation. Their solubility in cyclohexane was 5-20 times higher than those of unsubstituted N-hydroxyphthalimide (NHPI) and even 3-10 times higher than a formerly known lipophilic NHPI analog bearing a long alkyl ester. UV-Vis spectroscopy determined the rate constants of first-order decay of N-oxyls, which revealed enhanced stability of 3,5-di-tert-butyl-NHPI and N-hydroxyglutarimide flanked by spiro-cyclohexanes compared with that of NHPI. Their bond dissociation energies were assessed by EPR spectroscopy and DFT calculation, suggesting these lipophilic N-hydroxyimides catalyzed aerobic cyclohexane oxidation under neat conditions with up to 60 turnovers, which is the highest among hitherto reported reactions under neat conditions.