Photochemical O–H Functionalization of Aryldiazoacetates with Phenols via Proton Transfer

Photosensitizer-Free Photoinduced Ground-State Triplet Carbene-Assisted Persistent Aryloxy Radical Generation via Hydrogen Atom Transfer

The traditional intermolecular O-H insertion strategy is typically associated with the reactivity exhibited by the singlet spin state, or it can alter the spin state from triplet to singlet by hydrogen bonding. Herein, we report diazoarylidene succinimide that generates a persistent ground-state triplet carbene under visible light (Blue LED, 456 nm) without a photosensitizer. This triplet carbene undergoes an intramolecular O-H insertion via hydrogen atom transfer, forming a persistent aryloxy radical without altering its spin state and leading to biologically relevant 2H-chromenes.

Visible Light Induced B-H Bond Insertion Reaction with Diazo Compounds

A protocol induced by visible light for the direct insertion of α-carbonyl carbenes into the B-H bond of amine-borane adducts has been developed under conditions that are free of metal and photocatalyst. This approach provides a straightforward route to various organoboron compounds from diazo compounds and amine-borane adducts with moderate to good yields. Mechanistic investigations reveal that this photoinduced reaction proceeds through concerted carbene insertion into the B-H bond, and the photoinduced generation of free carbene from α-diazo esters may be the rate-determining step.

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.

Visible-light-induced three-component reactions of α-diazoesters, quinazolinones and cyclic ethers toward quinazoline-based hybrids

An effective approach for the construction of 4-short-chain ether attached carbonyl group-substituted quinazolines was developed. Visible-light-induced three-component reactions of α-diazoesters, quinazolinones, and cyclic ethers, with a broad substrate scope and excellent functional group tolerance, under extremely mild conditions without the need for any additional additives and catalysts, selectively led to quinazoline-based hybrids in good to excellent yields. The synthesized hybrids, which are a conglomeration of a quinazoline, a short-chain ether, and a carbonyl group in one molecular skeleton, have potential for application in the development of new drugs or drug candidates.

Rhodium(I)-Catalyzed O-H Insertions on O-Protected α-Diazo-β-Hydroxyesters

The X-H insertion reaction constitutes a powerful tool to create diversity through the diazo decomposition of diazocarbonyl compounds. However, until now, X-H insertion on α-diazo-β-aryl-β-hydroxyester scaffolds, readily prepared by aldol-type addition, remained a challenge for the organic chemist. We report herein the first O-H insertions on O-protected α-diazo-β-aryl-β-hydroxyesters, providing straightforward access to a wide range of α,β-dioxygenated esters through modulation of the alcohol and of the aryl substituent. The key feature to achieving this transformation is the use of Rh(I) catalysts, which proved to be crucial to favor the targeted O-H insertion product over the competing 1,2-H and 1,2-Ar migration products. Overall, 32 O-H insertion products have been prepared, in moderate to good yields, with a diastereoisomeric ratio up to 7.5:1 in favor of the syn diastereoisomer.

O-H Insertion of Hydrogenphosphate Derivatives and α-Diazo Compounds

An efficient O-H insertion of hydrogenphosphate derivatives and α-diazo compounds has been developed to construct α-phosphoryloxy scaffolds. Diverse α-phosphoryloxy skeletons could be obtained under mild and catalyst-free conditions in good yields. The control experiments suggest a protonation and nucleophilic addition process of α-diazo compounds via a diazonium ion pair for this transformation.

Rhodium-Catalyzed C(sp2)-O Cross Couplings of Diazo Quinones with Phenols to Construct Diaryl Ethers

The diaryl ether represents a prevalent structural motif found in numerous biologically active molecules. Herein, we describe a dirhodium-catalyzed C(sp2)-O cross coupling reaction between diazo quinones and phenols for the construction of diaryl ethers in moderate to high yields. The reaction proceeds under mild and neutral conditions and is tolerant of various functional groups. The synthetic method has been successfully applied to the concise synthesis of a Navl.7 inhibitor.

Unveiling catalyst-free electro-photochemical reactivity of aryl diazoesters and facile synthesis of oxazoles, imide-fused pyrroles and tetrahydro-epoxy-pyridines via carbene radical anions

Herein, we report a reagent-less (devoid of catalyst, supporting electrolyte, oxidant and reductant) electro-photochemical (EPC) reaction [electricity (50 μA) and blue LED (5 W)] of aryl diazoesters to generate radical anions which are subsequently reacted with acetonitrile or propionitrile and maleimides to generate diversely substituted oxazoles, diastereo-selective imide-fused pyrroles and tetrahydroepoxy-pyridines in good to excellent yield. Thorough mechanistic investigation including a 'biphasic e-cell' experiment supports the reaction mechanism involving a carbene radical anion. The tetrahydroepoxy-pyridines could be fluently converted to fused pyridines resembling vitamin B₆ derivatives. The source of the electric current in the EPC reaction could be a simple cell phone charger. The reaction was efficiently scaled up to the gram level. Crystal structure, 1D, 2D NMRs and HRMS data confirmed the product structures. This report demonstrates a unique generation of radical anions via electro-photochemistry and their direct applications in the synthesis of important heterocycles.

Visible-Light-Induced Trifluoromethylsulfonylation Reaction of Diazo Compounds Enabled by Manganese Catalysis

A visible-light-induced trifluoromethylsulfonylation reaction of diazo compounds is herein reported. This developed synthetic method captures the relatively rare trifluoromethyl sulfone radicals via coordination to the Mn(acac)₃ catalyst, delivering the corresponding α-trifluoromethyl sulfone esters in good to moderate yields (up to 82%). This protocol exhibits broad substrate scope and is easily carried out under mild reaction conditions. Furthermore, a plausible mechanism of the reaction was investigated through DFT calculations.

Photochemical Intermolecular Cyclopropanation Reactions of Allylic Alcohols for the Synthesis of [3.1.0]-Bicyclohexanes

Cyclopropane-fused lactones are highly desirable in drug and natural products synthesis. Herein, we report on a photochemical, chemoselective reaction of aryldiazoacetates with allylic alcohols that furnishes cyclopropane-fused lactone skeletons efficiently in one step. The diastereoselectivity of the protocol was precisely controlled, and chemoselective cyclopropanation of allylic alcohols via free carbene intermediate followed by transesterification constitutes a series of bicyclic lactones in high yield without the formation of ether byproducts via typical O-H insertion reactions.

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.

Application of Aromatic Substituted 2,2,2-Trifluoro Diazoethanes in Organic Reactions

Abstract:

This review provides an overview of metal-, nonmetal-, light-, or catalyst free-promoting reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with organic molecules for the synthesis of trifluoromethyl-substituted compounds. Several approaches will be reviewed and divided into (i) copper-, iron-, Trop(BF4)-, B(C6F5)3-, light-, or rhodium-promoted reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with silanes, amines, mercaptans, phosphonates, p-cyanophenol, benzoic acid, diphenylphosphinic acid, boranes and nBu3SnH, (ii) rhodium-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with amides and phenylhydroxylamine, (iii) copper-, rhodium-, silver-, and light-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with alkynes, (iv) palladium-, copper-, rhodium- and iron-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with alkenes, (v) BF3·OEt2-, copper-, tin- or TBAB-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with HF·Py, (difluoroiodo)toluene (p-TolIF2), TMSCF3, AgSCF3, TMSCF2Br or 1,3-dicarbonyl compounds, (vi) palladium-, copper-, gold/silver- or rhodium-catalyzed reactions of aromatic substituted 2,2,2-trifluoro diazoethanes with indoles, benzene compounds or pyridines, and (vii) palladium-catalyzed reaction of aromatic substituted 2,2,2-trifluoro diazoethanes with benzyl or allyl bromides.

A Computational Study on the Photochemical O-H Functionalization of Alcohols with Diazoacetates

In this computational study, we provide a detailed analysis of the underlying reaction mechanism and show that a singlet carbene is initially formed. Depending on the pK_A of the alcohol, this singlet carbene can engage in direct protonation or enol formation to yield the O-H functionalization product. On the contrary, propargylic alcohols take up a dual role and form a complex with the carbene intermediate that leads to facile cyclopropenation reactions.

A Visible Light-Mediated Three-Component Strategy Based on the Ring-Opening of Cyclic Ethers with Aryldiazoacetates and Nucleophiles

A blue light-promoted reaction between aryldiazoacetates and different nucleophiles has been developed in the presence of THF (and other cyclic ethers) as solvent, allowing the incorporation of these three elements...

Unlocking novel reaction pathways of diazoalkanes with visible light

Photochemistry has recently attracted the interest of synthetic chemists to conduct photolysis reactions of diazoalkanes. In this feature article, we provide a concise overview on this field, starting with discoveries...

Dearomative Cyclopropanation of Naphthols via Cyclopropene Ring-Opening

The dearomatization of 2-naphthols represents a simple method for the construction of complex 3D structures from simple planar starting materials. We describe a cyclopropanation of 2-naphthols that proceeds via cyclopropene ring-opening using rhodium and acid catalysis under mild conditions. The vinyl cyclopropane molecules were formed with high chemoselectivity and scalability, which could be further functionalized at different sites. Both computational and experimental evidence were used to elucidate the reaction mechanism.

Visible-Light-Mediated Strategies for the Preparation of Oxime Ethers Derived from O-H Insertions of Oximes into Aryldiazoacetates

Two visible-light-mediated O-H insertion protocols involving oximes and aryldiazoacetates leading to different products depending on the solvent employed are reported. In DCM, direct O-H insertion takes place. In THF, there is the additional incorporation of the ring-opened form of this solvent into the structure of the product. These metal-free protocols are mild and tolerant to air and moisture. The preparation of an acaricide has been developed as an example of synthetic application.

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.

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

Chemodivergent reactions of oximes and diazo esters involving Rh-catalyzed [3+2] annulation and photodriven O-H insertion have been developed to generate oxazoles and oxime ethers. A range of aldehyde and ketone oximes reacted with α-diazocarbonyl compounds in a controllable manner in which functional groups, including ketone, ester, amide, ether, thiol ether, silane, alkene, allene, and alkyne groups, were well tolerated.

B(C6F5)3-catalyzed O-H insertion reactions of diazoalkanes with phosphinic acids

A highly efficient base-, metal-, and oxidant-free catalytic O-H insertion reaction of diazoalkanes and phosphinic acids in the presence of B(C6F5)3 has been developed. This powerful methodology provides a green approach towards the synthesis of a broad spectrum of α-phosphoryloxy carbonyl compounds with good to excellent yields (up to 99% yield). The protocol features the advantages of operational simplicity, high atom economy, practicality, easy scalability and environmental friendliness.

Visible-light-promoted selective O-alkylation of 2-pyridones with α-aryldiazoacetates

A visible-light-promoted O-H insertion reaction between 2-pyridones and α-aryldiazoacetates has been developed. Upon visible light irradiation, the reaction proceeds smoothly under mild and catalyst-free conditions. A wide scope of 2-pyridones and α-aryldiazoacetates are well tolerated, and various O-alkylated 2-pyridones are obtained with perfect selectivity and good functional group tolerance. A photoinduced radical process is probably responsible for the excellent selectivity.

Synthesis of trisubstituted hydroxylamines by a visible light-promoted multicomponent reaction

A green and efficient route for the synthesis of trisubstituted hydroxylamines from β-keto ester, 2-nitrosopyridine and aryldiazoacetates has been reported. This multicomponent reaction occurred under mild conditions without catalysts or additives.

Brønsted acid-catalyzed homogeneous O–H and S–H insertion reactions under metal- and ligand-free conditions

The economical and accessible CF₃SO₃H successfully catalyzed homogeneous O–H and S–H bond insertion reactions between hydroxyl compounds, thiols and diazo compounds under metal- and ligand-free conditions.

Regiospecific and site-selective C–H allylation of phenols with vinyldiazo compounds catalyzed by In(iii)

An In(OTf)3-catalyzed regiospecific, site-selective, and C–H chemoselective insertion reaction of phenols with vinyldiazoacetates was developed. The reactions of aryl or alkyl substituted vinyldiazoacetates exhibited different selective manners.