TEMPO-Mediated Synthesis of Tetrahydropyridinofullerenes: Reaction of [60]Fullerene with α-Methyl-Substituted Arylmethanamines and Aldehydes in the Presence of 4-Dimethylaminopyridine

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.

TEMPO-Mediated Synthesis of Indolyl/Imidazo[1,2-a]pyridinyl-Substituted para-Quinone Methides from Butylated Hydroxytoluene

A series of indolyl or imidazo[1,2-a]pyridinyl-substituted para-quinone methides (p-QMs) is prepared by a metal-free, TEMPO-mediated cross-dehydrogenative coupling of butylated hydroxytoluene (BHT) with indoles or imidazo[1,2-a]pyridines in good to high yields. Broad substrate scope with respect to indoles and imidazo[1,2-a]pyridines, good functional group tolerance, and acid/base-free conditions are advantageous feature of the developed protocol. The method was amenable for scale-up on the gram scale. Based on control experiments, a reaction mechanism is proposed to describe this transformation.

Manganese(III) acetate-mediated synthesis of N-substituted fulleropyrrolines via the reaction of [60]fullerene with α-monosubstituted acetaldehydes and primary amines

The simple one-step reaction of [60]fullerene with α-monosubstituted acetaldehydes and primary amines in the presence of Mn(OAc)₃·2H₂O under air conditions afforded a series of novel N-substituted fulleropyrrolines with trisubstituted CC bonds in moderate to good yields. The addition of Mn(OAc)₃·2H₂O played a crucial role in the successful synthesis of N-aryl fulleropyrrolines with trisubstituted CC bonds, which would be extremely difficult to prepare by known methods as a result of the decreased nucleophilicity of arylamines due to the p-π conjugation effect. Intriguingly, arylamines displayed abnormally higher reactivity as compared with non-arylamines in the current reaction system by the observation of obviously decreased equivalent of Mn(OAc)₃·2H₂O, higher product yields, and lower reaction temperature probably due to the radical reaction mechanism initiated by Mn(OAc)₃·2H₂O. On the basis of experimental observations, a plausible formation pathway for N-substituted fulleropyrrolines with trisubstituted CC bonds was proposed to elucidate the above-mentioned reaction process.

Selective formation of dihydrofuran fused [60] fullerene derivatives by TEMPO mediated [3 + 2] cycloaddition of medium chain β-keto esters to C60

In this study, β-keto esters as readily available bio-based building blocks were used to decorate the C₆₀ sphere. Generally, cyclopropanated fullerene derivatives are obtained by the standard Bingel–Hirsch procedure. Herein, omitting the iodine from the reaction mixture and adding TEMPO afforded dihydrofuran fused C₆₀ fullerene derivatives. The mechanism of the reaction shifted from nucleophilic aliphatic substitution to oxidative [3 + 2] cycloaddition via fullerenyl cations as an intermediate. This mechanism is proposed based on a series of control experiments with radical scavengers. Therefore, dihydrofuran-fused C₆₀ derivatives were selectively obtained in good yields and their structures were established based on UV-Vis, IR, NMR spectroscopy and mass spectrometry. The electrochemical properties of the synthesized compounds were investigated by cyclic voltammetry. DFT calculations were performed in order to investigate the difference in stability, electronic properties and π-electron delocalization between methano and furano fullerenes.

In this study, β-keto esters as readily available bio-based building blocks were used to decorate the C₆₀ sphere.

Facile access to amino-substituted cyclopentafullerenes: novel reaction of [60]fullerene with β-substituted propionaldehydes and secondary amines in the absence/presence of magnesium perchlorate

A series of scarce amino-substituted cyclopentafullerenes instead of the expected N-alkyl-2,5-disubstituted fulleropyrrolidines were synthesized in moderate to excellent yields via the simple one-step reaction of [60]fullerene with cheap and easily available β-substituted propionaldehydes and secondary amines in the absence/presence of magnesium perchlorate. The in situ generation of allylic amines from β-substituted propionaldehydes and secondary amines played a crucial role in the successful preparation of amino-substituted cyclopentafullerenes without additional carbons. With the addition of magnesium perchlorate, secondary amines containing ethyl group(s) could produce novel amino-substituted cyclopentafullerenes with two additional carbons. All the obtained cyclopentafullerenes displayed high stereoselectivity with cis isomers as the exclusive or major products. Plausible reaction mechanisms are proposed to elucidate the above-mentioned reaction process.

Stereoselective synthesis of amino-substituted cyclopentafullerenes promoted by magnesium perchlorate/ferric perchlorate

A facile one-step reaction of [60]fullerene with cinnamaldehydes and amines promoted by magnesium perchlorate/ferric perchlorate under air conditions afforded a series of rare amino-substituted cyclopentafullerenes in moderate to good yields. Stereoselectivity was readily achieved. Secondary amines exclusively produced N,N-disubstituted cyclopentafullerenes as cis isomers, while arylamines gave N-monosubstituted cyclopentafullerenes with a preference of cis isomers as major products. N-Monosubstituted cyclopentafullerenes could be further converted into other scarce cyclopentafullerenes in the presence of acid chloride or paraformaldehyde. A possible reaction pathway was proposed to elucidate the formation of amino-substituted cyclopentafullerenes.

Metal-Free Synthesis of N-Alkyl-2,5-Unsubstituted/Monosubstituted Fulleropyrrolidines: Reaction of [60]Fullerene with Paraformaldehyde and Amines

A series of scarce N-alkyl-2,5-unsubstituted/monosubstituted fulleropyrrolidines were synthesized in moderate to excellent yields by the simple one-step thermal reaction of [60]fullerene with primary/secondary amines in the presence of paraformaldehyde without the addition of valuable metal salts. Intriguingly, the reaction with primary amines unexpectedly afforded N-alkyl-2,5-unsubstituted fulleropyrrolidines instead of the anticipated 2,5-monosubstituted fulleropyrrolidines. A plausible reaction pathway is proposed to elucidate the above-mentioned reaction process based on the experimental results.

Selective synthesis of pyrrolidin-2-ones and 3-iodopyrroles via the ring contraction and deformylative functionalization of piperidine derivatives

A selective synthesis of pyrrolidin-2-ones and 3-iodopyrroles through the oxidative ring contraction and deformylative functionalization of piperidine derivatives is presented.

Stereoselective synthesis of cyclopentafullerenes: the reaction of [60]fullerene with aldehydes and triethylamine promoted by magnesium perchlorate

The magnesium perchlorate-mediated reaction of [60]fullerene with aldehydes and triethylamine generated a series of rare amino-substituted cyclopentafullerenes with high stereoselectivity.

Metal-free synthesis of fulleropyrrolidin-2-ols: a novel reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes

The reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes in the absence of metal salts afforded a series of novel fulleropyrrolidin-2-ols in moderate yields.

Stereoselective synthesis of N-ethyl-2-arylvinyl-5-methyl fulleropyrrolidines: reaction of [60]fullerene with aromatic aldehydes and triethylamine/diethylamine in the absence or presence of manganese(iii) acetate

Reaction of [60]fullerene with aromatic aldehydes and triethylamine/diethylamine afforded a series of novel fulleropyrrolidines.