Theoretical design of multi-nitroxyl organocatalysts with enhanced reactivity for aerobic oxidation

Higher catalytic performances of N,N',N''-trihydroxyisocyanuric acid (THICA), N,N-dihydroxypyromellitimide (NDHPI), and N-hydroxynaphthalimide (NHNI) than that of N-hydroxyphthalimide (NHPI) have been demonstrated recently in aerobic oxidation. Herein, the rational design of reactive multi-nitroxyl organocatalysts has been addressed theoretically by using systematic analysis of some important properties and catalytic activities of yet-to-be-synthesized catalysts. Our results show that 1) NHNI and its analogue, similar to THICA, unlike NHPI and others, are unsuitable for solvent- or mediator-free catalysis due to their strong intramolecular hydrogen-bonding interactions; 2) increasing the reactive hydroxyimide groups on the same aromatic ring, or doped N atoms or ionic-pair groups onto the aromatic ring, can improve catalytic reactivity, whereas appropriate enlargement of conjugated aromatic systems results in unchanged activity; 3) the newly designed catalysts are more active than NHPI and NHNI and have catalytic activities comparable to NDHPI and THICA; 4) the ionic-pair supported case is suggested to be a very active catalyst, even towards inert propane, and can be used as a novel model catalyst for further improvements. The present work will be helpful in designing reactive hydroxyimide organocatalysts.