Orthogonally Protected 1,2-Diols from Electron-Rich Alkenes Using Metal-Free Olefin syn-Dihydroxylation

Elucidating the Mechanism of Electrooxidative Allene Dioxygenation: Dual Role of Tetramethylpiperidine N-Oxyl (TEMPO)

The cumulated π system of a nonsymmetric allene contains three distinct unsaturated carbons that imbue it with unique reactivity toward radicals as compared to its alkene and alkyne counterparts. Despite the synthetic potential of these versatile building blocks, electrochemical transformations of allenes have been historically underexplored. Myriad strategies for easy access to allenes, coupled with the resurgence of interest in sustainable oxidative transformations of hydrocarbons, prompted our efforts to conduct an in-depth investigation of a rare example of an electrochemical TEMPO-mediated allene dioxygenation. The resultant vinyl-TEMPO motif is readily postfunctionalized to install a heteroatom at each allene carbon. Mechanistic investigations, including cyclic voltammetry (CV) studies, computations, and monitoring by operando NMR (ReactNMR) were performed to lay the groundwork for future electrochemical allene functionalizations that deliver unique synthetic building blocks.

Three-Component 1,2-Dioxygenation of 1,3-Dienes Using Carboxylic Acids and TEMPO

A mild, metal-free 1,2-dioxygenation of 1,3-dienes using TEMPO and carboxylic acids is reported. This method includes examples for a variety of 1,3-dienes as well as aliphatic and aromatic carboxylic acids. This reaction also demonstrates complete site- and regioselectivity in the oxygen addition. Furthermore, extensive derivatization of the differential oxygen groups in the product has been demonstrated, including reduction of the remaining alkene to access alkyl, vicinally dioxygenated scaffolds. Finally, this reaction is shown both experimentally and computationally to occur through carboxylic acid-driven disproportionation of TEMPO.

K2S2O8-Mediated or Azobisisobutyronitrile-Catalyzed Regioselective Aerobic Oxidative Cleavage of 1-Arylbutadienes to Cinnamaldehydes

This work reveals the regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes mediated by K2S2O8 or catalyzed by azobisisobutyronitrile, a very common free radical initiator, in an easy to handle, simple procedure and free of transition metals. This approach demonstrates excellent regioselectivity, mild reaction conditions, and compatibility with a broad range of functional groups (45 examples).

Difunctionalization Processes Enabled by Hexafluoroisopropanol

In the past 5 years, hexafluoroisopropanol (HFIP) has been used as a unique solvent or additive to enable challenging transformations through substrate activation and stabilization of reactive intermediates. In this Review, we aim at describing difunctionalization processes which were unlocked when HFIP was involved. Specifically, we focus on cyclizations and additions to alkenes, alkynes, epoxides, and carbonyls that introduce a wide range of functional groups of interest.

Trifluoromethylarylation of alkenes using anilines

Nitrogen containing compounds, such as anilines, are some of the most widespread and useful chemical species, although their high and unselective reactivity has prevented their incorporation into many interesting transformations, such as the functionalization of alkenes. Herein we report a method that allows the trifluoromethylarylation of alkenes using anilines, for the first time, with no need for additives, transition metals, photocatalysts or an excess of reagents. An in-depth mechanistic study reveals the key role of hexafluoroisopropanol (HFIP) as a unique solvent, establishing a hydrogen bonding network with aniline and trifluoromethyl reagent, that is responsible for the altered reactivity and exquisite selectivity. This work uncovers a new mode of reactivity that involves the use of abundant anilines as a non-prefunctionalized aromatic source and the simultaneous activation of trifluoromethyl hypervalent iodine reagent.

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.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Updates on hypervalent-iodine reagents in metal-free organic synthesis

Hypervalent iodine (HVI) chemistry is a rapidly growing subdomain of contemporary organic chemistry because of its enormous synthetic applications. The high nucleofugality of the phenyliodonio group (I+Ph) and their radical...

HFIP promoted thio(hetero)arylation of imidazoheterocycles under metal- and base-free conditions

A new reaction methodology has been developed for HFIP promoted Thio(hetero)arylation of imidazoheterocycles under metal and base-free conditions. To the best of our knowledge, this is the first report that describes linking of imidazopyridines with electron deficient heteroarenes through a sulphur atom and also for the synthesis of most of these compounds. The reaction conditions are well tolerated by almost all kinds of 2-chloroheteroarenes and a wide range of imidazoheterocycles. The synthesized compounds can show significant biological properties.

Visible light-induced aerobic dioxygenation of α,β-unsaturated amides/alkenes toward selective synthesis of β-oxy alcohols using rose bengal as a photosensitizer

The first visible light-induced aerobic dioxygenation of alkenes for the selective synthesis of β-oxy alcohols was developed using non-toxic rose bengal as a photosensitizer.

Use of hydroxylamines, hydroxamic acids, oximes and amines as nucleophiles in the Zbiral oxidative deamination of N-acetyl neuraminic acid. Isolation and characterization of novel mono- and disubstitution products

The oxidative deamination of N-nitroso N-acetylneuraminic acid (NeuAc) derivatives is a useful reaction for the formation of 5-desamino-5-hydroxy NeuAc derivatives and their stereoisomers. We demonstrated previously that replacement of the classical nucleophile in these reactions, acetic acid, by phenols resulted in a novel double displacement process with substitution of the acetoxy group at the 4-position taking place in addition to that of the 5-acetamido group, for which we postulated a mechanism centered on the formation of a highly reactive vinyl diazonium ion. We now extend these studies to encompass the use of hydroxylamine-based systems and weakly basic amines as nucleophile. We find that the nature of the product depends significantly on the pKa of the nucleophile, with the more acidic species typically affording only substitution at the 5-position, while the less acidic species give mixtures of elimination products and disubstitution products. The use of aniline as nucleophile is of particular note as it affords a novel aziridine spanning positions 4- and 5- of the neuraminic acid skeleton.

Iron(III)/O2-Mediated Regioselective Oxidative Cleavage of 1-Arylbutadienes to Cinnamaldehydes

A simple, efficient, and environmentally benevolent regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes mediated by iron(III) sulfate/O₂ has been developed. The reaction offered good yields and excellent regioselectivity and showed good functional group tolerance (31 examples). The method is important, as few reports with limited substrate scope are available for such excellent oxidative cleavage of conjugated dienes.

Hypervalent iodine initiated intramolecular alkene dimerisation: a stereodivergent entry to cyclobutanes

The emergence of new methods for the stereoselective synthesis of strained carbocycles is a challenging but worthwhile endeavour. Cyclobutanes, in particular, have attracted the attention of both medicinal chemists and material scientists for their unique properties. Herein, we present a new method that allows access to highly functionalized cyclobutanes with complementary all-trans and trans-cis-trans relative stereochemistry, that could not be accessed before. This approach consists of an intramolecular dimerisation of non-conjugated dienes using an oxidative single electron transfer (SET) process, and is initiated by catalytic amounts of hypervalent iodine reagents. The potential uses of these cyclobutanes is demonstrated with selective functionalization, including the formation of diols and carboxylic acids.

Aerobic Metal-Free Dioxygenation of Alkenes with tert-Butyl Nitrite and N-Hydroxylamines

Metal-free dioxygenation of alkenes with tert-butyl nitrite and N-hydroxylamines (N-hydroxyphthalimide, N-hydroxybenzotriazole, and N-hydroxysuccinimide) is described to produce β-aminoxy nitrate esters using air as the oxidant. These organic nitrates can be readily converted into 1,2-diols and 1,2-diketone with broad substrate scope and functional group diversity.