Interception of Radicals by Molecular Oxygen and Diazo Compounds: Direct Synthesis of Oxalate Esters Using Visible-Light Catalysis

Exploring Diazo Compounds for the Divergent Electro-Cascade Sequence and S-H Insertion Reaction

We have explored the reactivity of diazo compounds under electrochemical conditions to establish a cascade sequence of thiolation/cyclization/reduction reactions. Electrolyzing styryl diazo imides and aryl thiols enables direct access to a single diastereoisomer of 2,5-pyrrolidine-dione-fused thiochromans in good yield under mild and metal-free conditions. Notably, a tunable reactivity has been achieved via S-H insertion at the diazo center in modified reaction conditions. Based on the experimental evidences, including the detection of key intermediates and computational studies, the mechanism for the electrochemical cascade reaction has also been established.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines

Herein, we report a visible-light-mediated palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines, affording unsaturated γ- and ε-amino acid derivatives with diverse structures. In this methodology, the diazo compound readily transforms into a hybrid α-ester alkylpalladium radical with the release of dinitrogen. The radical intermediate selectively adds to the double bond of a 1,3-diene or allene, followed by the allylpalladium radical-polar crossover path and selective allylic substitution with the amine substrate, thereby leading to a single unsaturated γ- or ε-amino acid derivative. This approach proceeds under mild and simple reaction conditions and shows high functional group tolerance, especially in the incorporation of various bioactive molecules. The studies on scale-up reactions and diverse derivatizations highlight the practical utility of this multicomponent reaction protocol.

Photodriven Radical-Polar Crossover Cyclization Strategy: Synthesis of Pyrazolo[1,5-a]pyridines from Diazo Compounds

This work demonstrates the synthesis of a variety of perfluoroalkyl heterocycles via a visible-light-driven radical-polar crossover cyclization strategy. In this process, single-electron reduction/SNV-type/cyclization sequences follow the radical addition reaction of a diazoester, which differs from the current role of diazoesters as radical precursors/acceptors. This transformation demonstrates excellent functional group compatibility and allows for the modification of many bioactive molecules with diazoesters. Such a reaction could represent a novel approach to the photochemical transformation of diazo compounds.

Copper-Catalyzed Three-Component Tandem Reaction of Alkynes, α-Diazo Esters, and TMSN3 to Access N-Substituted 1,2,3-Triazoles

An efficient copper-catalyzed three-component tandem reaction of alkynes, α-diazo esters, and TMSN3 to construct triazoles has been developed. Through this strategy, a number of diverse N-substituted 1,2,3-triazoles were conveniently obtained in moderate to good yields from simple and readily available starting materials using K2CO3 as the base. The mechanism of the tandem Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and Cu-carbenoid-participated C-N coupling reaction has been investigated.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Efficient and practical synthesis of monoalkyl oxalates under green conditions

Monoalkyl oxalates are among the most important building blocks being applied to the synthesis of a variety of significant classes of compounds or applied to various cutting-edge reactions. However, their commercial availability is limited. Their synthetic methods are also limited because of the difficulty to synthesize them, and those hitherto reported are carried out in organic solvents often with the use of toxic reagents with mostly low to modest yields. Here we have developed practical synthesis of monoalkyl oxalates in aqueous media by applying the highly efficient selective monohydrolysis reactions of symmetric diesters which we reported previously. The best conditions apply an aqueous NaOH solution with relatively nontoxic THF or acetonitrile as a co-solvent at around 0–5 °C. The procedures are simple and environmentally friendly without requiring toxic or expensive reagents, yet yielding the corresponding half-esters in high yields with high purities. All the half-esters prepared here are stable over a long period of time. Therefore, our studies are expected to offer practical green methods for the synthesis of monoalkyl oxalates.

Simple and environmentally-friendly methods have been developed for the synthesis of monoalkyl oxalates without requiring toxic or expensive reagents.

Molecular oxygen-mediated oxygenation reactions involving radicals

Molecular oxygen as a green, non-toxic and inexpensive oxidant has displayed lots of advantages compared with other oxidants towards more selective, sustainable, and environmentally benign organic transformations. The oxygenation reactions which employ molecular oxygen or ambient air as both an oxidant and an oxygen source provide an efficient route to the synthesis of oxygen-containing compounds, and have been demonstrated in practical applications such as pharmaceutical synthesis and late-stage functionalization of complex molecules. This review article introduces the recent advances of radical processes in molecular oxygen-mediated oxygenation reactions. Reaction scopes, limitations and mechanisms are discussed based on reaction types and catalytic systems. Conclusions and perspectives are also given in the end.

Synthesis of Isoxazolines and Oxazines by Electrochemical Intermolecular [2 + 1 + n] Annulation: Diazo Compounds Act as Radical Acceptors

Reported herein is an unprecedented synthesis of isoxazolines and oxazines through electrochemical intermolecular annulation of alkenes with tert-butyl nitrite, in which diazo compounds serve as radical acceptors. Notably, [2 + 1 + 2] and [2 + 1 + 3] annulations occur when styrenes and allylbenzenes are used as substrates, respectively. The latter reaction undergoes group migration to form more stable radical, manifesting radical route instead of conventional 1,3-dipolar cycloaddition occurs. Moreover, scale-up experiments suggest the potential application value of these transformations in industry.

Recent developments in photochemical reactions of diazo compounds

Chemistry of diazo compounds is dominated by transition metal catalysis but recently, photoinitiated reactions of diazo compounds have attracted a lot of attention. This mini-review describes recent discoveries on the reactivity of diazo compounds under light irradiation.