Chemodivergent Synthesis of Oxazoles and Oxime Ethers Initiated by Selective C-N/C-O Formation of Oximes and Diazo Esters

α-Carbonyl Rh-Carbenoid Initiated Cascade Assembly of Diazobarbiturates with Alkylidene Pyrazolones for Synthesis of Spirofuropyrimidines

Catalyzed by Rh2(esp)2 (10 mol%) and (±)-BINAP (20 mol%) in DCE at 80 °C, the cascade assembly between diazobarbiturates and alkylidene pyrazolones proceeded readily and produced spiro-furopyrimidines in 38-96% chemical yields. The chemical structure of the prepared spirofuro-pyrimidines was firmly confirmed by X-ray diffraction analysis.

Chemodivergent phosphonylation of diazocarboxylates: light-on vs. light-off reactions

By tapping into the divergent reactivity of diazocarboxylates under thermal and photocatalytic conditions, we could develop chemodivergent phosphonylation protocols for α-diazocarboxylates with trialkyl phosphites. While the thermal reaction led to N-P bond formation affording phosphonylated hydrazones, the visible light-mediated reaction furnished phosphonylated aryl carboxylates through C-P bond formation. Both reactions are notable for their operational simplicity and mild conditions affording products in good yields without the requirement of a metal, base or photocatalyst.

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.

Generation of Carbonyl Compounds from Oximes through Electrooxidative Deoximation

An electrochemical protocol was developed to generate carbonyl compounds from oximes with excellent functional group compatibility. Mechanistic experimental studies support the idea that an electrooxidative pathway of oximes might be involved, in which the water may serve as an oxygen nucleophile and the oxygen source for the final carbonyl compounds.

Access to Chiral O,O-Acetals Enabled by Palladium-Catalyzed Asymmetric Addition of Oximes to Alkoxyallenes

Enantiomerically pure acyclic O,O-acetal compounds (up to 97 % ee) have been accessed through chemo-, regio- and enantioselective palladium-catalyzed addition of oximes to alkoxyallenes. DFT calculations support that a protonative hydropalladation pathway is favourable, in which the hydrogen bonding interaction between the amide group of the diphosphine ligand and the alkoxyallene is critical for the highly stereoselective formation of the dioxygenated stereogenic center.

Oxygen-Promoted 6-endo-trig Cyclization of β,γ-Unsaturated Hydrazones/Ketoximes with Diazonium Tetrafluoroborates for Pyridazin-4(1H)-ones/Oxazin-4(1H)-ones

An oxidative 6-endo-trig cyclization and [2 + 2] cycloaddition of β,γ-unsaturated hydrazones/ketoximes and diazonium tetrafluoroborates for a synthetic strategy to pyridazin-4(1H)-ones/oxazin-4(1H)-ones under metal-free conditions is presented in a one-pot procedure. This protocol features excellent functional group tolerance and remarkable regioselectivity. A mechanistic study has been verified via 18O labeling and the H218O labeling method, in which O2 acts as both a reaction component and an oxidant.

Copper-Catalyzed Chemodivergent Synthesis of Oxazoles and Imidazolidones by Selective C-O/C-N Cyclization

Efficient synthesis of phenylalanine-derived oxazoles and imidazolidones can be achieved by copper-catalyzed reactions that are controlled by directing groups and proceed by selective C-O or C-N coupling. This strategy employs inexpensive commercial copper catalysts and readily available starting materials. It uses a convenient reaction procedure and provides a reliable approach to the versatile and flexible assembly of heterocyclic building blocks.

Photocatalytic Benzylic C-H Oxidation/Cyclization of Enaminones to the Synthesis of Polysubstituted Oxazoles

Photocatalytic benzylic C-H oxidation/cyclization of enaminones was efficiently achieved to afford oxazole derivatives under mild conditions. The oxygen in the oxazole ring originated from green oxidant molecular oxygen. The synthetic protocol features broad substrate scopes and good functional group tolerance. The combined experimental and theoretical studies reveal that in suit benzylic C-H oxidation/cyclization is involved in the reaction transformations.

Chemodivergent Photocatalyzed Heterocyclization of Hydrazones and Isothiocyanates for the Selectivity Synthesis of 2-Amino-1,3,4-thiadiazoles and 1,2,4-Triazole-3-thiones

A photocatalytic chemodivergent reaction for the selectivity formation of C-S and C-N bonds in a controlled manner was proposed. The reaction medium, either neutral or acidic, is critical to dictate the formation of 2-amino-1,3,4-thiadiazoles and 1,2,4-triazole-3-thiones from isothiocyanates and hydrazones. This is a practical protocol to achieve the chemoselectivity under mild and metal-free conditions.

Recent Advances in N-O Bond Cleavage of Oximes and Hydroxylamines to Construct N-Heterocycle

Oximes and hydroxylamines are a very important class of skeletons that not only widely exist in natural products and drug molecules, but also a class of synthon, which have been widely used in industrial production. Due to weak N-O σ bonds of oximes and hydroxylamines, they can be easily transformed into other functional groups by N-O bond cleavage. Therefore, the synthesis of N-heterocycle by using oximes and hydroxylamines as nitrogen sources has attracted wide attention. Recent advances for the synthesis of N-heterocycle through transition-metal-catalyzed and radical-mediated cyclization classified by the type of nitrogen sources and rings are summarized. In this paper, the recent advances in the N-O bond cleavage of oximes and hydroxylamines are reviewed. We hope that this review provides a new perspective on this field, and also provides a reference to develop environmentally friendly and sustainable methods.

Visible-Light-Induced Imide Synthesis through a Nitrile Ylide Formation/Trapping Cascade

A visible-light-promoted three component reaction of diazo compounds, nitriles, and carboxylic acids is reported. The reaction utilizes acceptor-only diazo compounds as carbene precursors and nitriles as carbene-trapping reagents to form the key nitrile ylides. Under the optimal reaction conditions, a wide range of imide products were obtained in good to excellent yields. The gram-scale synthesis and synthetic application of the imide products to form isoquinoline-1,3(2H,4H)-dione derivatives further proved the value of this method.

Gold-catalyzed synthesis of oxazoles from alkynyl triazenes and dioxazoles

A gold-catalyzed regioselective [3 + 2] cycloaddition of alkynyl triazenes with 1,2,4-dioxazoles was developed.

Visible-Light-Mediated Strategies to Assemble Alkyl 2-Carboxylate-2,3,3-Trisubstituted β-Lactams and 5-Alkoxy-2,2,4-Trisubstituted Furan-3(2H)-ones Using Aryldiazoacetates and Aryldiazoketones

Two new visible-light-mediated strategies are described starting from aryldiazoacetates. The first approach describes their reaction with azides to afford the corresponding imines, and then reaction with aryldiazoketones produces alkyl 2-carboxylate-2,3,3-trisubstituted β-lactams. The second approach describes the reaction with sulfoxides to afford the corresponding sulfoxonium ylides, followed by reaction with aryldiazoketones to produce 5-alkoxy-2,2,4-trisubstituted furan-3(2H)-ones. These protocols take advantage of the photolysis of aryldiazoacetates and the photochemically promoted Wolff rearrangement of aryldiazoketones.

A Selective C-C Bond Cleavage Strategy Promoted by Visible Light

A new visible-light-promoted reaction between aryldiazoacetates and 1,3-diketones allows good yields and selectivities for C-C bond insertions, leading to the corresponding 1,4-dicarbonyl compounds. This transformation is straightforward and highly practical. It tolerates air and moisture and does not require the use of any metals. Mechanistic investigations support the involvement of a key cyclopropanol intermediate derived from an intramolecular rearrangement.