Aerobic oxidation catalysis with stable radicals

Selective oxidation reactions are challenging when carried out on an industrial scale. Many traditional methods are undesirable from an environmental or safety point of view. There is a need to develop sustainable catalytic approaches that use molecular oxygen as the terminal oxidant. This review will discuss the use of stable radicals (primarily nitroxyl radicals) in aerobic oxidation catalysis. We will discuss the important advances that have occurred in recent years, highlighting the catalytic performance, mechanistic insights and the expanding synthetic utility of these catalytic systems.

Efficient and selective Cu/nitroxyl-catalyzed methods for aerobic oxidative lactonization of diols

Cu/nitroxyl catalysts have been identified that promote highly efficient and selective aerobic oxidative lactonization of diols under mild reaction conditions using ambient air as the oxidant. The chemo- and regioselectivity of the reaction may be tuned by changing the identity of the nitroxyl cocatalyst. A Cu/ABNO catalyst system (ABNO = 9-azabicyclo[3.3.1]nonan-N-oxyl) shows excellent reactivity with symmetrical diols and hindered unsymmetrical diols, whereas a Cu/TEMPO catalyst system (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl) displays excellent chemo- and regioselectivity for the oxidation of less hindered unsymmetrical diols. These catalyst systems are compatible with all classes of alcohols (benzylic, allylic, aliphatic), mediate efficient lactonization of 1,4-, 1,5-, and some 1,6-diols, and tolerate diverse functional groups, including alkenes, heterocycles, and other heteroatom-containing groups.

Oxidative cleavage of allyl ethers by an oxoammonium salt

A method to oxidatively cleave allyl ethers to their corresponding aldehydes mediated by an oxoammonium salt is described. Using a biphasic solvent system and mild heating, cleavage proceeds readily, furnishing a variety of α,β-unsaturated aldehydes and ketones.

Mechanistic insight into aerobic alcohol oxidation using NOx-nitroxide catalysis based on catalyst structure-activity relationships

The mechanism of an NOx-assisted, nitroxide(nitroxyl radical)-catalyzed aerobic oxidation of alcohols was investigated using a set of sterically and electronically modified nitroxides (i.e., TEMPO, AZADO (1), 5-F-AZADO (2), 5,7-DiF-AZADO (3), 5-MeO-AZADO (4), 5,7-DiMeO-AZADO (5), oxa-AZADO (6), TsN-AZADO (7), and DiAZADO (8)). The motivation for the present study stemmed from our previous observation that the introduction of an F atom at a remote position from the nitroxyl radical moiety on the azaadamantane nucleus effectively enhanced the catalytic activity under typical NOx-mediated aerobic-oxidation conditions. The kinetic profiles of the azaadamantane-N-oxyl-[AZADO (1)-, 5-F-AZADO (2)-, and 5,7-DiF-AZADO (3)]-catalyzed aerobic oxidations were closely investigated, revealing that AZADO (1) showed a high initial reaction rate compared to 5-F-AZADO (2) and 5,7-DiF-AZADO (3); however, AZADO-catalyzed oxidation exhibited a marked slowdown, resulting in ∼90% conversion, whereas 5-F-AZADO-catalyzed oxidation smoothly reached completion without a marked slowdown. The reasons for the marked slowdown and the role of the fluoro group are discussed. Oxa-AZADO (6), TsN-AZADO (7), and DiAZADO (8) were designed and synthesized to confirm their comparable catalytic efficiency to that of 5-F-AZADO (2), providing supporting evidence for the electronic effect on the catalytic efficiency of the heteroatoms under NOx-assisted aerobic-oxidation conditions.

Practical aerobic oxidations of alcohols and amines with homogeneous copper/TEMPO and related catalyst systems

Oxidations of alcohols and amines are common reactions in the synthesis of organic molecules in the laboratory and industry. Aerobic oxidation methods have long been sought for these transformations, but few practical methods exist that offer advantages over traditional oxidation methods. Recently developed homogeneous Cu/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-N-oxyl) and related catalyst systems appear to fill this void. The reactions exhibit high levels of chemoselectivity and broad functional-group tolerance, and they often operate efficiently at room temperature with ambient air as the oxidant. These advances, together with their historical context and recent applications, are highlighted in this Minireview.

Mechanism of alcohol oxidation mediated by copper(II) and nitroxyl radicals

2,2'-Bipyridine-ligated copper complexes, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are highly effective catalysts for aerobic alcohol oxidation. Considerable uncertainty and debate exist over the mechanism of alcohol oxidation mediated by Cu(II) and TEMPO. Here, we report experimental and density functional theory (DFT) computational studies that distinguish among numerous previously proposed mechanistic pathways. Oxidation of various classes of radical-probe substrates shows that long-lived radicals are not formed in the reaction. DFT computational studies support this conclusion. A bimolecular pathway involving hydrogen-atom-transfer from a Cu(II)-alkoxide to a nitroxyl radical is higher in energy than hydrogen transfer from a Cu(II)-alkoxide to a coordinated nitroxyl species. The data presented here reconcile a collection of diverse and seemingly contradictory experimental and computational data reported previously in the literature. The resulting Oppenauer-like reaction pathway further explains experimental trends in the relative reactivity of different classes of alcohols (benzylic versus aliphatic and primary versus secondary), as well as the different reactivity observed between TEMPO and bicyclic nitroxyls, such as ABNO (ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl).

Practical organic solvent-free Cu(OAc)2/DMAP/TEMPO-catalyzed aldehyde and imine formation from alcohols under air atmosphere

A highly efficient and practical organic solvent-free Cu(OAc)₂/DMAP/TEMPO catalyst system for the selective aerobic oxidation of benzylic and allylic alcohols to aldehydes and phenones under an ambient air atmosphere was reported.

Oxidation of allylic and benzylic alcohols to aldehydes and carboxylic acids

The oxidation of allylic and benzylic alcohols to the corresponding carboxylic acids is effected by merging a Cu-catalyzed oxidation using O₂ as a terminal oxidant with a subsequent chlorite oxidation (Lindgren oxidation).

N,O-ligated Pd(ii) complexes for catalytic alcohol oxidation

The (HSA)Pd(OAc)₂ complex (where HSA = 8-hydroxyquinoline-2-sulfonic acid) is a highly active catalyst for the oxidation of a range of secondary alcohols in 4–6 hours at a low loading of 0.5 mol%. The crystal structure has been obtained and the influence of reaction conditions on catalyst degradation was also examined.