Rhodium-Catalyzed Geminal Oxyfluorination and Oxytrifluoro-Methylation of Diazocarbonyl Compounds

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Diastereoselective Synthesis of 4-Hydroxy-2-quinolinones via Formal [2 + 4] Cycloaddition Reactions Using α-Diazo Pyrazoleamides as C2 Synthons

A rhodium-catalyzed highly stereoselective formal [2 + 4]-cycloaddition reaction of α-diazo pyrazoleamides and 2-aminophenyl ketones that produces 4-hydroxy-2-quinolinones in good yields with excellent diastereoselectivities has been developed. A pyrazolium ylide species that is generated from α-diazo pyrazoleamides is used as a C2 synthon for this cycloaddition. This protocol offers an efficient approach to a variety of 4-hydroxy-2-quinolinones featuring sequential quaternary centers.

Editing Tetrasubstituted Carbon: Dual C-O Bond Functionalization of Tertiary Alcohols Enabled by Palladium-Based Dyotropic Rearrangement

Many elegant asymmetric syntheses of enantioenriched tertiary alcohols have been developed, and both the transition metal-catalyzed and the radical-based peripheral functionalization of tertiary alcohols have attracted intensive research interest in recent years. However, directly editing tetrasubstituted carbons remains challenging. Herein, we report a Pd-catalyzed migratory fluoroarylation reaction that converts tertiary alcohols to α-fluorinated tertiary alkyl ethers in good to excellent yields. An unprecedented 1,2-aryl/PdIV dyotropic rearrangement along the C-O bond, integrated in a PdII-catalyzed domino process, is key to the dual functionalization of both the hydroxyl group and the tetrasubstituted carbon. This reaction, which is compatible with a broad range of functional groups, generates a tertiary alkyl fluoride and an alkyl-aryl ether functional group with inversion of the absolute configuration at the tetrasubstituted stereocenter.

Catalytic Asymmetric Difluoroalkylation Using In Situ Generated Difluoroenol Species as the Privileged Synthon

A robust and practical difluoroalkylation synthon, α,α-difluoroenol species, which generated in situ from trifluoromethyl diazo compounds and water in the presence of dirhodium complex, is disclosed. As compared to the presynthesized difluoroenoxysilane and in situ formed difluoroenolate under basic conditions, this difluoroenol intermediate displayed versatile reactivity, resulting in dramatically improved enantioselectivity under mild conditions. As demonstrated in catalytic asymmetric aldol reaction and Mannich reactions with ketones or imines in the presence of chiral organocatalysts, quinine-derived urea, and chiral phosphoric acid (CPA), respectively, this relay catalysis strategy provides an effective platform for applying asymmetric fluorination chemistry. Moreover, this method features a novel 1,2-difunctionalization process via installation of a carbonyl motif and an alkyl group on two vicinal carbons, which is a complementary protocol to the metal carbene gem-difunctionalization reaction.

Diazo-Transfer Reaction of Nonactivated Ketones with 2-Azido-1,3-bis(2,6-diisopropylphenyl)imidazolium Hexafluorophosphate (IPrAP)

The diazo-transfer reaction of nonactivated ketone under mild reaction conditions was developed. Various nonactivated ketones such as aryl methyl ketones, sec-alkyl methyl ketones, and cyclic ketones were transformed into their corresponding α-diazoketones in one step by treating 2-azido-1,3-bis(2,6-diisopropylphenyl)imidazolium hexafluorophosphate (IPrAP) in the presence of iPr2NH in ethylene glycol. In the reaction of IPrAP with prim-alkyl methyl ketone and prim-alkyl aryl ketones, migratory amidation proceeded under the reaction conditions to afford the corresponding amides.

A Strategic Synthesis of Fluoroethers via Ring-Opening Fluorinative Beckmann Fragmentation

An SN1-type fluorination method for monofluoroethers is developed. The key to this reaction is fluorinative C-C bond cleavage that is driven by oxygen-assisted Beckmann fragmentation. To enable this transformation, cyclic α-aryloxyoximes derived from 3-coumaranone and 1-indanones were investigated as substrates, using N,N-diethylaminosulfur trifluoride (DAST) as a dual-role reagent of an oxime activator and fluoride donor. This method features the synthesis of an underdeveloped chemical motif with simple and mild operating conditions.

Photochemical "Cut and Sew" Transformations of Ethynylbenziodoxolone Reagents and Diazo Compounds

Herein, we disclose a visible-light-induced oxy-alkynylation of diazo compounds with ethynylbenziodoxolones. The efficient protocol provides a mild and metal-free methodology to synthesize propargylic esters in moderate to good yields. Notably, this metal-free carbene transfer reaction appears to involve an oxonium ylide intermediate, followed by intramolecular ligand exchange and reductive elimination.

Visible Light-Driven α-Diazoketones as Denitrogenated Synthons: Synthesis of Fluorinated N-Heterocycles via Multicomponent Cyclization Reactions

We disclose herein an efficient photochemical formal [3+2+1] annulation strategy for the transformation of diazocarbonyl compounds into various fluorinated nitrogen-containing heterocycles. This transformation is characterized by reacting fluoroalkyl radicals with α-diazoketones, which are used as infrequent denitrogenated synthons under visible light. Moreover, a wide range of N-heterocycles containing precious CF₃ and perfluoroalkylated groups are constructed in moderate to good yields. Notably, this photochemical strategy may provide a fruitful path for the synthesis of complex organofluorides via diazo/fluorine/radical chemistry.

Electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles

With the fast development of synthetic chemistry, the introduction of functional group into organic molecules has attracted increasing attention. In these reactions, the difunctionalization of unsaturated bonds, traditionally with one nucleophile and one electrophile, is a powerful strategy for the chemical synthesis. In this work, we develop a different path of electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles. Under metal-free and external oxidant-free conditions, a series of structurally diverse heteroatom-containing compounds hardly synthesized by traditional methods (such as high-value alkoxy-substituted phenylthioacetates, α-thio, α-amino acid derivatives as well as α-amino, β-amino acid derivatives) are obtained in synthetically useful yields. In addition, the procedure exhibits mild reaction conditions, excellent functional-group tolerance and good efficiency on large-scale synthesis. Importantly, the protocol is also amenable to the key intermediate of bioactive molecules in a simple and practical process.

Three-component acyloxylation of diazo compounds with carboxylic acids and azadienes

Despite many studies on benzofuran-derived azadiene being reported, multi-component studies have scarcely been reported on this heterocyclic skeleton. The first cascade three-component acyloxylation of diazos with acids and azadienes has been reported under mild conditions. The reaction is applicable to various (A/A) diazo compounds, generating diverse complex benzofuran derivatives.

Metal-free aminofluorination of α-diazo 2H-benzopyran-4-one: convenient access to β-fluoramides

We have developed a convenient synthesis of a series of β-fluoramides in 65% yield. The process involved a tandem fluorination/Ritter reaction to synthesize β-fluoramides using α-diazo 2H-benzopyran-4-one compounds. Selectfluor was used as the electrophilic fluoride source in acetonitrile to build the β-fluorinated quaternary carbon center and amide derivatives of 2H-benzopyran-4-one in one step. The products N-(2-(2-fluoro-2,3-dihydro-3-oxobenzofuran-2-yl)propan-2-yl)acetamides were a series of bifunctional compounds with a 2-fluoro-2,3-dihydro-3-oxobenzofuran motif and amide groups.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

A diastereoselective three-component reaction for the assembly of succinimide and isatin hybrid molecules with potent anticancer activities

A Rh₂(OAc)₄ catalyzed three-component reaction of vinyl diazosuccinimides with alcohols and isatins has been reported, which provides a practical assess to the direct assembly of succinimide and isatin hybrid molecules in good-to-high yields with excellent stereoselectivity. The antiproliferation activity of these synthesized succinimide and isatin hybrid products has been tested via the CCK8 assay in different cancer cell lines, and compounds 4g (SJSA-1, IC₅₀ = 3.82 μM) and 4r (HCT-116, IC₅₀ = 9.02 μM) exhibit higher anticancer potency than other tested compounds.

Photoinduced Three-Component Difluoroalkylation of Quinoxalinones with Alkenes via Difluoroiodane(III) Reagents

An environmentally friendly strategy for the photocatalyzed three-component reaction between quinoxalinones, alkenes, and hypervalent iodine(III) reagents is disclosed. The new designed difluoroiodane(III) reagent shows excellent reactivity, providing a wide range of difluoroalkyl-substituted quinoxaline-2(1H)-ones in moderate to excellent yields under mild conditions. Experimental studies demonstrated that a difluoroalkyl radical intermediate was involved in this reaction.

H-F bond insertions into α-diazo carbonyl compounds

A reaction for H-F bond insertion into α-diazo carbonyl compounds is reported. The protocol describes a simple reaction setup employing commercially available HF·pyr (Olah reagent) as the fluorine source. The method is rapid and practical, and allows access to a broad range of α-fluorinated carbonyl compounds in generally good yields.

Preparation and Synthetic Applications of Five-to-Seven-Membered Cyclic α-Diazo Monocarbonyl Compounds

The reactivity of cyclic α-diazo monocarbonyl compounds differs from that of their acyclic counterparts. In this review, we summarize the current literature available on the synthesis and synthetic applications of three major classes of cyclic α-diazo monocarbonyl compounds: α-diazo ketones, α-diazo lactones and α-diazo lactams.

Asymmetric synthesis of isochromanone derivatives via trapping carboxylic oxonium ylides and aldol cascade

An efficient asymmetric synthesis of isochromanone derivatives was realized through Z-selective-1,3-OH insertion/aldol cyclization reaction involving acyclic carboxylic oxonium ylides. The combination of achiral dirhodium salts and chiral N,N′-dioxide–metal complexes, along with the use of α-diazoketones instead of α-diazoesters, enables the cascade reaction efficiently. A variety of benzo-fused δ-lactones bearing vicinal quaternary stereocenters were obtained with good to excellent enantioselectivity, bypassing the competitive 1,1-OH insertion and racemic background aldol reaction.

A highly enantioselective cascade Z-selective-1,3-OH insertion/aldol cyclization of ketoacids with diazoketones involving carboxylic oxonium ylides was achieved by using a bimetallic Rh(ii)/chiral N,N′-dioxide-Fe(iii) or Sc(iii) complex catalyst.

Water-promoted regio-selective trifluoromethylation of vinyl conjugated diazoacetates

An efficient trifluoromethylation reaction of vinyldiazoacetates under mild reaction conditions was depicted.

Asymmetric three-component reaction of diazo compound with alcohol and seven-membered imine

A dirhodium and chiral phosphoric acid co-catalyzed asymmetric three-component reaction of diazo compound with alcohol and seven-membered imine has been developed via Mannich-type interception of transient oxonium ylide. This reaction...

Ternary Catalysis Enabled Three-Component Asymmetric Allylic Alkylation as a Concise Track to Chiral α,α-Disubstituted Ketones

Multicomponent reactions that involve interception of onium ylides through Aldol, Mannich, and Michael addition with corresponding bench-stable acceptors have demonstrated broad applications in synthetic chemistry. However, because of the high reactivity and transient survival of these in situ generated intermediates, the substitution-type interception process, especially the asymmetric catalytic version, remains hitherto unknown. Herein, a three-component asymmetric allylation of α-diazo carbonyl compounds with alcohols and allyl carbonates is disclosed by employing a ternary cooperative catalysis of achiral Pd-complex, Rh₂(OAc)₄, and chiral phosphoric acid CPA. This method represents the first example of three-component asymmetric allylic alkylation through an S_N1-type trapping process, which involves a convergent assembly of two active intermediates, Pd-allyl species, and enol derived from onium ylides, providing an expeditious access to chiral α,α-disubstituted ketones in good to high yields with high to excellent enantioselectivity. Combined experimental and computational studies have shed light on the mechanism of this novel three-component reaction, including the critical role of Xantphos ligand and the origin of enantioselectivity.

Divergent Synthesis of α-Fluorinated Carbonyl and Carboxyl Derivatives by Double Electrophilic Activation of Amides

A straightforward and divergent entry to α-fluorinated carbonyl and carboxyl derivatives is reported. Upon activation of amides with triflic anhydride and a 2-halo-pyridine and subsequent trapping of the resulting keteniminium ions with nucleophiles followed by a second electrophilic activation with NFSI and final hydrolysis, a range of amides can be transformed to α-fluorinated ketones, esters, and amides under mild conditions. Moreover, this reaction can be performed to yield enantioenriched products with a traceless chiral auxiliary.

Ring Expansion Fluorination of Unactivated Cyclopropanes Mediated by a New Monofluoroiodane(III) Reagent

Herein, we report a new strategy for carbon-carbon bond scission and intramolecular ring expansion fluorination of unactivated cyclopropanes, which was accomplished with a new hypervalent fluoroiodane(III) reagent 1. This novel method delivers medicinally relevant 4-fully substituted fluoropiperidines in moderate to high yields with excellent regio- and diastereoselectivity. Reagent 1, which has an N-acetylbenziodazole framework, was readily synthesized via three steps in 76 % overall yield and was characterized by NMR spectroscopy and X-ray crystallography. Owing to the presence of a secondary I⋅⋅⋅O bonding interaction between the λ³ -iodane atom and the carbonyl oxygen of the acetyl group of the N-acetylbenziodazole framework, 1 has excellent stability and can be stored at ambient temperature for 6 months without any detectable decomposition. Density functional theory calculations and experimental studies showed that the reaction proceeds via a carbocation intermediate that readily combines with a fluoride ion to generate the product.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

We have synthesized the first isolable geminal bisenolates L2 K2 Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

We have synthesized the first isolable geminal bisenolates L2K2Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Synthesis of Dihydroxazines and Fluorinated Oxazepanes Using a Hypervalent Fluoroiodine Reagent

Application of a hypervalent fluoroiodane for the regiodivergent synthesis of dihydroxazines and fluorinated oxazepanes from allylaminoethanol was investigated. The reaction was carried out under mild conditions and gave the products in moderate to good yields. The selectivity of this transformation is controlled by the substituents of the allylaminoethanol.

Enantioselective Oxidative Multi-Functionalization of Terminal Alkynes with Nitrones and Alcohols for Expeditious Assembly of Chiral α-Alkoxy-β-amino-ketones

Catalytic oxidative functionalization of alkynes has emerged as an effective method in synthetic chemistry in recent decades. However, enantioselective transformations via metal carbene intermediates are quite rare due to the lack of robust chiral catalysts, especially in the intermolecular versions. Herein, we report the first asymmetric three-component reaction of commercially available alkynes with nitrones and alcohols, which affords α-alkoxy-β-amino-ketones in good yields with high to excellent enantioselectivity using combined catalysis by an achiral gold complex and a chiral spiro phosphoric acid (CPA). Mechanistically, this atom-economic reaction involves a catalytic alkyne oxidation/ylide formation/Mannich-type addition sequence that uses nitrone as the oxidant and the leaving fragment imine as the electrophile, providing a novel method for multi-functionalization of commercially available terminal alkynes.

Base-catalysed 18F-labelling of trifluoromethyl ketones. Application to the synthesis of 18F-labelled neutrophil elastase inhibitors

A new method for the fluorine-18 labelling of trifluoromethyl ketones has been developed. This method is based on the conversion of a-COCF₃ functional group to a difluoro enol silyl ether followed by halogenation and fluorine-18 labelling. The utility of this new method was demonstrated by the synthesis of fluorine-18 labelled neutrophil elastase inhibitors, which are potentially useful for detection of inflammatory disorders.

Copper-Mediated Cross-Coupling of Diazo Compounds with Sulfinates

A copper-mediated cross-coupling reaction between a diazo compound and a sodium alkane(arene)sulfinate gives a sulfone as the product. This reaction proceeds under mild conditions and features excellent functional group compatibility. A wide range of sodium alkane(arene)sulfinates were successfully applied in this chemistry. Mechanistic studies revealed that the overall reaction efficiency of the sulfinates was in line with their nucleophilicity in this reaction.

Enantioselective assembly of 3,3-disubstituted succinimides via three-component reaction of vinyl diazosuccinimides with alcohols and imines

An enantioselective three-component reaction of vinyl diazosuccinimdes with alcohols and imines has been realized by a cooperative catalysis of Rh2(OAc)4 and a chiral phosphoric acid, leading to chiral 3,3-disubstituted succinimides in good to high yields with high to excellent enantioselectivity. The generated product with an alkenyl species could be converted to the chiral tricyclic structure under mild conditions.

Cu(I)-Catalyzed gem-Aminoalkynylation of Diazo Compounds: Synthesis of Fluorinated Propargylic Amines

The gem-aminoalkynylation of fluorinated diazo compounds catalyzed by a simple Cu(I) salt is described. This three-component reaction allows the synthesis of propargylic amines with broad functional group tolerance. Both electron-rich and electron-poor anilines can be used as nucleophiles and alkyl-, aryl-, and silyl-substituted EthynylBenziodoXoles (EBX) as electrophiles.

Dirhodium(II)/Xantphos-Catalyzed Relay Carbene Insertion and Allylic Alkylation Process: Reaction Development and Mechanistic Insights

Although dirhodium-catalyzed multicomponent reactions of diazo compounds, nucleophiles and electrophiles have achieved great advance in organic synthesis, the introduction of allylic moiety as the third component via allylic metal intermediate remains a formidable challenge in this area. Herein, an attractive three-component reaction of readily accessible amines, diazo compounds, and allylic compounds enabled by a novel dirhodium(II)/Xantphos catalysis is disclosed, affording various architecturally complex and functionally diverse α-quaternary α-amino acid derivatives in good yields with high atom and step economy. Mechanistic studies indicate that the transformation is achieved through a relay dirhodium(II)-catalyzed carbene insertion and allylic alkylation process, in which the catalytic properties of dirhodium are effectively modified by the coordination with Xantphos, leading to good activity in the catalytic allylic alkylation process.

Accessing novel fluorinated heterocycles with the hypervalent fluoroiodane reagent by solution and mechanochemical synthesis

A new and efficient strategy for the rapid formation of novel fluorinated tetrahydropyridazines and dihydrooxazines has been developed by fluorocyclisation of β,γ-unsaturated hydrazones and oximes with the fluoroiodane reagent. Mechanochemical synthesis delivered fluorinated tetrahydropyridazines in similar excellent yields to conventional solution synthesis, whereas fluorinated dihydrooxazines were prepared in much better yields by ball-milling.

Diazocarbonyl Compounds in Organofluorine Chemistry

Diazocarbonyl compounds are useful substrates in di- and trifunctionalization reactions based on F/CF3/SCF3 introduction. In the presented reactions, various electrophilic F/CF3/SCF3-transfer reagents were used. The majority of the reactions were based on rhodium catalysis and the application of various oxygen nucleophiles, such as alcohols, cyclic/acyclic ethers, and carboxylic acids. The oxyfluorination reactions were further developed to provide a new fluorine-18 labeling method. Density functional theory (DFT) modeling studies were performed to get a deeper mechanistic understanding of these reactions. These DFT modeling studies indicated that the catalytic reactions proceed through formation of rhodium carbene and oxonium ylide intermediates. The oxonium ylides undergo tautomerization to enol ether type species that subsequently react with the electrophilic F/CF3/SCF3-transfer reagents. We also present an arylation–trifluoromethylthiolation reaction for simultaneous introduction of C–SCF3 and C–C bonds into diazocarbonyl compounds. This reaction does not proceed by rhodium catalysis, but follows a Hooz-type mechanism.

1 Introduction

2 Diazocarbonyl Compounds: Versatile Substrates in Organic ­Synthesis

3 Fluorination, Trifluoromethylation, and Trifluoromethylthiolation of Diazo Substrates

3.1 Metal-Free Reactions

3.2 Metal-Catalyzed Reactions with Nucleophilic Reagents

3.3 Metal-Catalyzed Reactions with Electrophilic Reagents

4 Oxyfluorination Reactions

4.1 Fluorobenziodoxole as a Fluorine Source

4.2 [18F]Fluorobenziodoxole as a Fluorine Source for Radiolabeling

4.3 Oxyfluorination with NFSI

5 Oxytrifluoromethylation

6 Oxytrifluoromethylthiolation

7 Arylation–Trifluoromethylthiolation Reaction

8 Conclusions and Outlook

Synthetic Routes Towards the Synthesis of Geminal α-Difunctionalized Ketones

The importance of gem-difunctionalized ketones is represented by their broad applications across chemical boundaries over recent years. The interesting reactivities that this class of compounds possess have made them ideal building blocks to access high-value organic molecules. Furthermore, the gem-difunctionalized ketone moiety has featured in numerous bioactive molecules. For these reasons, a plethora of routes to access such significant molecules have been developed by research groups worldwide - this account looks at delineating the synthesis of gem-difunctionalized ketones from carbonyl substrates, diazo compounds, sulfur ylides and alkynyl reactants.

Gold(i)-catalyzed redox transformation of o-nitroalkynes with indoles for the synthesis of 2,3′-biindole derivatives

A gold(i)-catalyzed cascade reaction of o-nitroalkynes with indoles has been reported for the rapid assembly of 2-indolyl indolone N-oxides, which exhibit high anticancer potency against SCLC cells.

Blue Light-Promoted Formal [4+1]-Annulation of Diazoacetates with o-Aminoacetophenones: Synthesis of Polysubstituted Indolines and Computational Study

A blue light-promoted formal [4+1]-annulation of diazoacetates with o-aminoacetophenones has been reported, which provides an environmentally friendly method for the synthesis of polysubstituted indoline derivatives in moderate to good yields with excellent diastereoselectivities. Detailed mechanistic studies through density functional theory calculations reveal that the (E)-enol species is the key intermediate in this transformation, and the excellent diastereoselectivity is enabled via H-bonding in the intramolecular Aldol-type addition.

Visible light-promoted reactions with diazo compounds: a mild and practical strategy towards free carbene intermediates

Carbenes are important intermediates in organic chemistry and have been widely applied in various types of organic reactions, ranging from cycloaddition reactions and sigmatropic rearrangements to C-H functionalizations, thus allowing the rapid construction of densely functionalized molecules. Over the past decades, remarkable progress has been achieved in metal-catalyzed carbene transfer reactions. Nevertheless, realizing these transformations under milder and/or greener conditions is still highly desirable. Only recently, visible light-promoted carbene transfer reactions of diazo compounds via free carbene intermediates have emerged as a practical, mild and powerful tool. In this tutorial review, we summarize the latest advances in the area, aiming at providing a clear overview on reaction design, mechanistic scenarios and potential future developments.