Nitroxide-catalyzed transition-metal-free aerobic oxidation processes

Abstract

Advances in the field of transition-metal-free nitroxide-catalyzed aerobic oxidation processes are covered in this critical review. Along with the relevant physical properties of nitroxides, their application as oxidation catalysts by using dioxygen as a reagent or terminal oxidant will be discussed. The second part of the review focuses on the use of TEMPO-based nitroxides as catalysts for aerobic alcohol oxidation. Mechanistic studies will also be addressed (130 references).

Design concept for α-hydrogen-substituted nitroxides

“…This report constitutes a design concept for cyclic a-hydrogen nitroxides, that is, nitroxides that are not bicyclic like AZADO (5) or acyclic like TIPNO (7). The design principles outlined in this paper are likely not restricted to nitroxides 1 and 2, and may be applied to the preparation of other nitroxides.…”

“…Their stability is believed to be derived from the inability to form a nitrone C ¼ N double bond to the bridgehead-an elegant application of Bredt's rule 22 . AZADO (5) has recently been shown to be a powerful oxidation catalyst and is now commercially available because of the efforts of Iwabuchi and co-workers [23][24][25] , who also reported a number of other nitroxides inspired by the same design concept. Unfortunately, the bridged bicyclic design is not universally applicable.…”

“…P ersistent nitroxides have intriguing structural and chemical properties [1][2][3] and possess a multitude of highly important applications, such as oxidation catalysts [4][5][6][7] , reagents in living polymerization reactions 8 , spin probes in biology 9 , radical traps in mechanistic investigations, spin trapping agents, medicinal agents 10,11 and in materials chemistry 12 . Designing and preparing stable nitroxides, as well as predicting their properties, remains an important challenge that must be met through a combination of computational methods and creativity 12 .…”

“…TEMPO (3) is a shelf-stable compound and is the catalyst of choice in industry 14 as well as in academia for the oxidation of primary alcohols to aldehydes [1][2][3][4][5][6][7] . It has many applications in evaluating radical formation in biological tissues 15,16 and is also the trapping agent of choice in mechanistic investigations of chemical processes when a radical intermediate is suspected 17,18 .…”

Facile TEMPO Immobilization onto Poly(acrylic acid)-Modified Magnetic Nanoparticles: Preparation and Property

“…Typically, organocatalyst TEMPO can utilize harmless and friendly oxidants [25,26] such as molecular oxygen, hydrogen peroxide, or air for alcohols/aldehydes or ketones stoichiometric transformation. Immobilizing TEMPO on magnetic nanocomposites heightens catalyst-recycling efficiency; however magnetic nanoparticles are inorganic, and it is challenging to directly immobilize organic catalysts.…”