Challenges and opportunities for N-hydroxyphthalimide supported over heterogeneous solids for aerobic oxidations

Organocatalysis, in combination with, or as an alternative to the use of metals, is attracting increasing interest as a sustainable route to promote selective transformations. In this context, Nhydroxyphthalimide (NHPI) has found ample application as an effective organocatalyst capable to promote the aerobic oxidation of a wide range of organic substrates, including saturated and unsaturated hydrocarbons, alcohols, amines and sulphides among others. However, most of these transformations required the use of NHPI dissolved in solution, with consequent limits in the scale-up of the process. Leaving aside the reviews of NHPI as organocatalyst in homogeneous phase, the present review describes the strategies reported to support or anchor NHPI on a wide range of insoluble solids, thus converting this homogeneous organocatalyst into a heterogeneous system, with the consequent advantages related to the ease of separation and recovery of the catalyst from the reaction medium, its recyclability and the possibility to develop continuous flow reactions. After a brief introductory section, a detailed description of the catalytic mechanism is here presented, also providing a comparison between NHPI and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical, and by highlighting the main challenges associated to NHPI heterogenization. The critical description of the heterogeneous solutions is organized according to the nature of the support, focusing on metal–organic frameworks (MOFs), organic polymers, carbon-based materials, and inorganic supports, while the growing use of NHPI-containing photocatalysts is presented separately. In the final section, we outline the main achievements on the use of NHPI heterogeneous catalysis and provide our views for future developments in the area.