Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Copper- and Chiral Nitroxide-Catalyzed Oxidative Kinetic Resolution of Axially Chiral N-Arylpyrroles

A readily prepared C₂-symmetric, α-hydrogen-substituted chiral hydroxylamine serves as a precatalyst to generate a chiral nitroxide in situ. This chiral nitroxide catalyst in combination with a copper co-catalyst functions as an oxidant for an unprecedented enantioselective oxidative kinetic resolution (OKR) of racemic axially chiral N-arylpyrrole alcohols using atmospheric oxygen as an environmentally friendly terminal oxidant. The OKR process provides the axially chiral N-arylpyrroles in er up to 3.5:96.5 and with s factors up to 24.

Synthesis, crystal structures and magnetic properties of a P-stereogenic ortho-(4-amino-tempo)phosphinic amide radical and its CuII complex

The synthesis of phosphinic amides containing one 4-amino-TEMPO substituent at the ortho position has been achieved through copper(i) catalyzed cross-coupling reactions of ortho-iodophosphinic amides with 4-amino-TEMPO. The method has been extended to the preparation of the first example of a P-stereogenic ortho-(4-amino-tempo)phosphinic amide radical 10. The reaction of 10 with Cu(hfac)2 afforded the P-stereogenic CuII complex 19. The crystal structure of both chiral compounds is reported. The molecular structure of 10 consists of a supramolecular zig-zag chain formed by intermolecular hydrogen bonds between the NH group of the phosphinic amide moiety and the nitroxide oxygen atom. In complex 19, the ligand acts as a bridge between two CuII ions coordinated to the oxygen atoms of the P[double bond, length as m-dash]O and N-O· groups leading to the formation of a polymeric helicate chain in which the metal ions exist in a distorted octahedral geometry. The magnetic behavior of ligand 10 is characterized by very weak intermolecular antiferromagnetic interactions, whereas ferro- and anti-ferromagnetic interactions are present in complex 19.

Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV-Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions

Operando EPR, XANES/EXAFS, UV‐Vis and ATR‐IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6‐tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least‐squares fitting (MCR‐ALS) of simultaneously recorded XAS and UV‐Vis data sets, it was found that an initially formed (bpy)(NMI)Cu^I‐ complex (bpy=2,2′‐bipyridine, NMI=N‐methylimidazole ) is converted to two different Cu^II species, a mononuclear (bpy)(NMI)(CH₃CN)Cu^II‐OOH species detectable by EPR and ESI‐MS, and an EPR‐silent dinuclear (CH₃CN)(bpy)(NMI)Cu^II(μ‐OH)₂⋅Cu^II (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)Cu^II‐TEMPO monomers that are also EPR‐silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non‐reducible Cu^II leads to slight deactivation at extended reaction times.

Rogersonins A and B, Imidazolone N-Oxide-Incorporating Indole Alkaloids from a verG Disruption Mutant of Clonostachys rogersoniana

Rogersonins A (1) and B (2), two new indole alkaloid-polyketide hybrids, have been isolated from cultures of a verG disruption mutant of the Cordyceps-colonizing fungus Clonostachys rogersoniana; their structures were elucidated primarily by NMR experiments. The absolute configurations of 1 and 2 were assigned using the modified Mosher method and via electronic circular dichroism and NMR calculations. Compounds 1 and 2 incorporate an imidazolone N-oxide moiety that has not been reported in any natural products.

Understanding the axial chirality control of quinidine-derived ammonium cation-directed O-alkylation: a computational study

As a privileged chiral scaffold, cinchona alkaloid and its derivatives have reached remarkable success in catalytic asymmetric organic synthesis. In addition to the wide applications of point chirality control, Smith and co-workers recently discovered a quinidine-derived ammonium cation-catalyzed O-alkylation of tetralones, providing an effective approach for the synthesis of axially chiral biaryls. Using density functional theory (DFT) calculations, we studied the reaction mechanism and origins of enantioselectivity of this novel transformation. A stepwise strategy is adopted to ensure efficient and thorough exploration of the massive conformational space of transition state. Our computations suggested that enolate oxygen forms two hydrogen bonds with the chiral ammonium catalyst, and the non-covalent interactions between the cationic benzylic fragment and the methoxy group of enolate plays a critical role in determining the enantioselectivity.