Direct access to furan and cyclopropane derivatives via palladium-catalyzed C-H activation/alkene insertion/annulation

A practical and effective palladium-catalyzed C-H activation/alkene insertion/annulation has been reported for the synthesis of furans and cyclopropanes from cyclic 1,3-diketones or 1,3-indandione and diverse alkenes, resulting in moderate to good yields. This protocol demonstrates excellent selectivity and is well-compatible with a wide range of alkene substrates, exhibiting exceptional regioselectivities, high efficiency, and good functional group tolerance.

Understanding the Remarkable Stability of Well-Defined Dinuclear Copper(I) Carbene Complexes

The synthesis of a well-defined dicopper carbene complex supported by the PNNP (2,7-bis(di-tert-butylphosphaneyl)methyl-1,8-naphthyridine) expanded pincer ligand is reported. This carbene complex is remarkably stable, even in the presence of air and water. The reactivity of this complex was explored towards typical carbene transfer substrates and its electronic structure was investigated. Using a combination of experiments and DFT calculations, the principles that underly the stability of dinuclear carbene complexes are probed.

Light Induced Cobalt(III) Carbene Radical Formation from Dimethyl Malonate As Carbene Precursor

Radical-type carbene transfer catalysis is an efficient method for the direct functionalization of C-H and C=C bonds. However, carbene radical complexes are currently formed via high-energy carbene precursors, such as diazo compounds or iodonium ylides. Many of these carbene precursors require additional synthetic steps, have an explosive nature, or generate halogenated waste. Consequently, the utilization of carbene radical catalysis is limited by specific carbene precursors that access the carbene radical intermediate. In this study, we generate a cobalt(III) carbene radical complex from dimethyl malonate, which is commercially available and bench-stable. EPR and NMR spectroscopy were used to identify the intermediates and showed that the cobalt(III) carbene radical complex is formed upon light irradiation. In the presence of styrene, carbene transfer occurred, forming cyclopropane as the product. With this photochemical method, we demonstrate that dimethyl malonate can be used as an alternative carbene precursor in the formation of a cobalt(III) carbene radical complex.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Skeletal Editing: Ring Insertion for Direct Access to Heterocycles

Skeleton editing has rapidly advanced as a synthetic methodology in recent years, significantly streamlining the synthesis process and gaining widespread acceptance in drug synthesis and development. This field encompasses diverse ring reactions, many of which exhibit immense potential in skeleton editing, facilitating the generation of novel ring skeletons. Notably, reactions that involve the cleavage of two distinct rings followed by the reformation of new rings through ring insertion play a pivotal role in the construction of novel ring skeletons. This article aims to compile and systematize this category of reactions, emphasizing the two primary reaction types and offering a thorough exploration of their associated complexities and challenges. Our endeavor is to furnish readers with comprehensive reaction strategies, igniting research interest and injecting fresh impetus into the advancement of this domain.

Rh-Catalyzed Chemodivergent [3+3] Annulations of Diazoenals and α-Aminoketones: Direct Synthesis of Functionalized 1,2-Dihydropyridines and Fused 1,4-Oxazines

Novel rhodium-catalyzed [3+3] annulations of diazoenals and α-amino ketones has been disclosed here. The reactivity of diazoenals has been switched from carbenoid to vinylogous NH-insertion by altering acyclic to cyclic α-amino ketones. In this direction, we report an efficient strategy to synthesize 1,2-dihydropyridines (DHPs) and fused 1,4-oxazines. Mechanistic investigation revealed that the formyl group is necessary for carbenoid [3+3] annulation and the cyclohexyl group is the dictating factor for vinylogous NH- insertion. The synthetic utility of 1,2-dihydropyridines was demonstrated by synthesizing piperidine, pyrido[1,2-a]indole, and 2-pyridone scaffolds. Further, structural diversification of fused 1,4-oxazines resulted in the short synthesis of hexahydroquinolin-2(1H)-ones, hexahydro quinolines and tetrahydroquinolinones via ring opening rearrangement and a new oxidative deformylation, respectively.

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.

Aminoquinoline-Based Tridentate (NNN)-Copper Catalyst for C-N Bond-Forming Reactions from Aniline and Diazo Compounds

A new tridentate Cu2+ complex based on (E)-1-(pyridin-2-yl)-N-(quinolin-8-yl)methanimine (PQM) was generated and characterized to support the activation of diazo compounds for the formation of new C-N bonds. This neutral Schiff base ligand was structurally characterized to coordinate with copper(II) in an equatorial fashion, yielding a distorted octahedral complex. Upon characterization, this copper(II) complex was used to catalyze an efficient and cost-effective protocol for C-N bond formation between N-nucleophiles and copper carbene complexes arising from the activation of diazo carbonyl compounds. A substrate scope of approximately 15 different amine-based substrates was screened, yielding 2° or 3° amine products with acceptable to good yields under mild reaction conditions. Reactivity towards phenol and thiophenol were also screened, showing relatively weak C-O or C-S bond formation under optimized 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.

"Sandwich" Diimine-Copper Catalyzed Trifluoroethylation and Pentafluoropropylation of Unactivated C(sp3 )-H Bonds by Carbene Insertion

We report here "sandwich"-diimine copper complex-catalyzed trifluoroethylation and pentafluoropropylation of unactivated C(sp3 )-H bonds in alkyl esters, halides, and protected amines by employing CF3 CHN2 and CF3 CF2 CHN2 reagents. Reactions proceed in dichloromethane solvent at room temperature. Identical C-H functionalization conditions and stoichiometries are employed for generality and convenience. Selectivities for C-H insertions are higher for compounds possessing stronger electron-withdrawing substituents. Preliminary mechanistic studies point to a mechanism involving a pre-equilibrium forming a "sandwich"-diimine copper-CF3 CHN2 complex followed by rate-determining loss of nitrogen affording the reactive copper carbene. It reacts with trifluoromethyldiazomethane about 6.5 times faster than with 1-fluoroadamantane explaining the need for slow addition of the diazo compound.

Diruthenium Tetracarboxylate-Catalyzed Enantioselective Cyclopropanation with Aryldiazoacetates

A series of chiral bowl-shaped diruthenium(II,III) tetracarboxylate catalysts were prepared and evaluated in asymmetric cyclopropanations with donor/acceptor carbenes derived from aryldiazoacetates. The diruthenium catalysts self-assembled to generate C4-symmetric bowl-shaped structures in an analogous manner to their dirhodium counterparts. The optimum catalyst was found to be Ru2(S-TPPTTL)4·BArF [S-TPPTTL = (S)-2-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate, BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate], which resulted in the cyclopropanation of a range of substrates in up to 94% ee. Synthesis and evaluation of first-row transition-metal congeners [Cu(II/II) and Co(II/II)] invariably resulted in catalysts that afforded little to no asymmetric induction. Computational studies indicate that the carbene complexes of these dicopper and dicobalt complexes, unlike the dirhodium and diruthenium systems, are prone to the loss of carboxylate ligands, which would destroy the bowl-shaped structure critical for asymmetric induction.

Carbene Radicals in Transition-Metal-Catalyzed Reactions

Discovered as organometallic curiosities in the 1970s, carbene radicals have become a staple in modern-day homogeneous catalysis. Carbene radicals exhibit nucleophilic radical-type reactivity orthogonal to classical electrophilic diamagnetic Fischer carbenes. Their successful catalytic application has led to the synthesis of a myriad of carbo- and heterocycles, ranging from simple cyclopropanes to more challenging eight-membered rings. The field has matured to employ densely functionalized chiral porphyrin-based platforms that exhibit high enantio-, regio-, and stereoselectivity. Thus far the focus has largely been on cobalt-based systems, but interest has been growing for the past few years to expand the application of carbene radicals to other transition metals. This Perspective covers the advances made since 2011 and gives an overview on the coordination chemistry, reactivity, and catalytic application of carbene radical species using transition metal complexes and catalysts.

Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Tunable Key [3 + 2] and [2 + 1] Cycloaddition of Enaminones and α-Diazo Compounds for the Synthesis of Isomeric Isoxazoles: Metal-Controlled Selectivity

The three-component reactions of enaminones, α-diazo esters/ketones, and t-butyl nitrite (TBN) for the switchable synthesis of isomeric isoxazoles have been realized. The catalysis with Cu(II) salt provides 3,4-disubsituted isoxazoles via [3 + 2] cycloaddition. On the other hand, the catalysis of Ag(I) with identical substrates leads to isomeric isoxazoles with reversed C3 and C4 substitution based on a key [2 + 1] cycloaddition.

Copper iodide nanoparticles (CuI NPs): an efficient catalyst for the synthesis of alkynyl esters

An environmentally benign protocol for the synthesis of alkynyl esters, by the cross-coupling of diazoacetate with various substituted alkynes under neat reaction conditions, has been described. Copper iodide nanoparticles (CuI NPs) were found to promote the Sonogashira-type coupling to afford the corresponding alkynyl esters in good yields. The CuI nanoparticles were characterized by PXRD, FESEM, EDAX, and Raman techniques. The developed methodology has several advantages such as a broad substrate scope, less reaction time, atom economy, avoidance of an additive/base/solvent, and enhanced values of green chemistry. The catalyst was recycled up to threefold without the loss of its catalytic activity.

Theoretical Study on the Copper-Catalyzed ortho-Selective C-H Functionalization of Naphthols with α-Phenyl-α-Diazoesters

The aromatic C(sp²)-H functionalization of unprotected naphthols with α-phenyl-α-diazoesters under mild conditions catalyzed by CuCl and CuCl₂ exhibits high efficiency and unique ortho-selectivity. In this study, the combination of density functional theory (DFT) calculations and experiments is employed to investigate the mechanism of C-H functionalization, which reveals the fundamental origin of the site-selectivity. It explains that CuCl-catalyzed ortho-selective C-H functionlization is due to the bimetallic carbene, which differs from the reaction catalyzed by CuCl₂ via monometallic carbene. The results demonstrate the function of favourable H-bond interactions on the site- and chemo-selectivity of reaction through stabilizing the rate-determining transition states in proton (1,3)-migration.

α-Aminocarbene-Mediated Si-H Insertion: Deoxygenative Silylation of Aromatic Amides with Silanes

While metal carbene-mediated Si-H insertion reactions have become a powerful strategy to build new C-Si bonds, the utilization of α-aminocarbene intermediates generated from readily available precursors in the Si-H insertion reaction remains a longstanding challenge. Herein, we develop a practical and general strategy to synthesize α-aminosilanes through a deoxygenative cross-coupling of amides and silanes mediated by Sm/SmI₂. Given the simplicity and versatility, this methodology represents a fascinating example for the effective utilization of inert amides as α-aminocarbene precursors in organic synthesis.

Green carbon-carbon homocoupling of terminal alkynes by a silica supported Cu(II)-hydrazone coordination compound

A Cu(II) complex, [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH (1), was obtained by the reaction of Cu(NO₃)₂·3H₂O and H2L in methanol solvent (H2L is (E)-4-amino-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide). H2L and compound 1 were characterized by various spectroscopic analyses and the molecular structure of [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH was determined by single-crystal X-ray analysis. The results indicated the product is a mononuclear Cu(II) complex and contains a free NH₂ functional group on the structure of the ligand. [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH was used for the preparation of a heterogeneous catalyst by supporting it on functionalized silica gel. The heterogeneous catalyst (Si-Cu) was prepared by an amidification reaction of [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH with functionalized silica gel. The resulting silica-supported catalyst (Si-Cu) was characterized by TGA, FT-IR, EPR, DRS, EDS, XRD, SEM and XPS analyses. Si-Cu was employed in a carbon-carbon coupling reaction and the effects of the amount of Si-Cu and temperature were investigated in the catalytic coupling. The structure of one of the products of the catalytic reactions (C₁₆H₂₂O₂, CP1) was determined by single-crystal X-ray analysis, which proved the formation of a C-C bond and the production of di-acetylene by homocoupling of terminal alkyne. This catalytic system is stable and it can be reused for a coupling reaction without a significant change in its catalytic activity.

Photo‐Induced Homologation of Carbonyl Compounds for Iterative Syntheses

We describe a photo-induced reaction for the in situ generation of highly reactive alkyl diazo species from carbonyl precursors via photo-excitation of N-tosylhydrazone anions. The diazo intermediates undergo efficient C-H insertion of aldehydes, leading to the productive synthesis of aldehydes and ketones. The method is applicable to the iterative synthesis of densely functionalized carbonyl compounds through sequential trapping of the diazo species with various aldehydes. The reaction proceeds without the need of any catalyst by light irradiation and features high functional group tolerance. More than 70 examples, some performed on a gram-scale, demonstrate the broad applicability of this reaction sequence in synthesis.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Synthesis of functionalized S-benzyl dithiocarbamates from diazo-compounds via multi-component reactions with carbon disulfide and secondary amines

Triflic acid promoted multi-component synthesis of functionalized S-benzyl dithiocarbamates from diazo compounds, carbon disulfide and secondary amines is reported. The reactions proceeded at room temperature and gave the desired dithiocarbamates in good yields. Wide-substrate scope and easy operation are the important features of this methodology.

Copper-Catalyzed Cycloadditions of Diazo Compounds with Imidazolidines/Hexahydropyrimidines for the Syntheses of N-Heterocycles

Reported herein are the unprecedented copper-catalyzed formal [n + 1]/[n + 3] (n = 5, 6) cycloadditions of diazo compounds with imidazolidines/hexahydropyrimidines, thus providing a general, economical, and efficient route to construct different sized (six- to nine-membered) diaza-heterocycles in moderate to excellent yields under mild reaction conditions. This strategy features the use of copper catalyst to accomplish such diverse annulations and the utilization of imidazolidines/hexahydropyrimidines as stable 1,5-/1,6-dipoles.

Visible Light-Promoted Diazoacetates and Nitriles Generating Nitrilium Ions Trapped by Benzotriazoles and Carboxylic Acids

A visible light-promoted generation of nitrilium ions from diazoacetates and nitriles has been developed. The reaction utilized visible light transformation of diazoacetates to the free carbene that could be trapped by nitriles to generate nitrilium ions, followed by nucleophilic attack on the benzotriazoles and carboxylic acids. This protocol provides an efficient and practical approach to N-imidoylbenzotriazoles and diacylglycine esters in good to excellent yields.

“Sandwich” Diimine‐Copper Catalysts for C−H Functionalization by Carbene Insertion

We report here "sandwich" diimine-copper(I) catalysts for C(sp³ )-H bond functionalization. Reactions of alkanes and ethers with trimethylsilyldiazomethane, ethyl diazoacetate, and trifluoromethyl-diazomethane have been demonstrated. We also report C(sp³ )-H bond methylation, benzylation, and diphenylmethylation by diazomethane, aryldiazomethanes, and diphenyldiazomethane. These reactions are rare examples of base-metal catalyzed, intermolecular C(sp³ )-H functionalizations by employing unactivated diazo compounds. Electrophilicity and unique steric environment of "sandwich"-copper catalysts are likely reasons for their catalytic efficiency.

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.

Cobalt(II)-tetraphenylporphyrin-catalysed carbene transfer from acceptor-acceptor iodonium ylides via N-enolate-carbene radicals

Square-planar cobalt(II) systems have emerged as powerful carbene transfer catalysts for the synthesis of numerous (hetero)cyclic compounds via cobalt(III)-carbene radical intermediates. Spectroscopic detection and characterization of reactive carbene radical intermediates is limited to a few scattered experiments, centered around monosubstituted carbenes. Here, we reveal the formation of disubstituted cobalt(III)-carbene radicals derived from a cobalt(II)-tetraphenylporphyrin complex and acceptor-acceptor λ³-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application. Iodonium ylides generate biscarbenoid species via reversible ligand modification of the paramagnetic cobalt(II)-tetraphenylporphyrin complex catalyst. Two interconnected catalytic cycles are involved in the overall mechanism, with a monocarbene radical and an N-enolate-carbene radical intermediate at the heart of each respective cycle. Notably, N-enolate formation is not a deactivation pathway but a reversible process, enabling transfer of two carbene moieties from a single N-enolate-carbene radical intermediate. The findings are supported by extensive experimental and computational studies.

Copper-Catalyzed [5 + 1] Cyclization of o-Pyrrolo Anilines and Heterocyclic N-Tosylhydrazones for Access to Spiro-dihydropyrrolo[1,2-a]quinoxaline Derivatives

An inexpensive copper-catalyzed sequential reaction process, proceeding via a nucleophilic attack of amine to Cu-carbene generated in situ from heterocyclic N-tosylhydrazone precursors followed by a 1,2-H shift/oxidative cyclization cascade of N-ylides, has been described, smoothly generating the corresponding structurally various spiro-dihydropyrrolo[1,2-a]quinoxaline derivatives. Furthermore, the significance of this protocol can be also highlighted by its diverse conversions of the synthetic compounds to the potentially bioactive molecules such as the 2-substituted pyrrolo[1,2-a]quinoxalins.

A Rh(II)-catalyzed highly stereoselective [3 + 2] annulation of vinyl diazoacetates with indole-2-carbaldehyde for the synthesis of indolyl dihydrofurans

A highly stereoselective Rh₂(Oct)₄-catalyzed [3 + 2] cycloaddition of vinyl diazoacetates with indolyl aldehyde has been developed. This protocol provides an efficient access to both cis and trans indolyl dihydrofurans with high yields and diastereoselectivities under mild conditions without or with Lewis acid as additive, respectively. Moreover, these generated functionalized dihydrofurans exhibit potent antiproliferation activity in three different cancer cell lines.

Copper-catalyzed cascade reaction of tryptamines with diazo compounds to access hexahydropyrroloindoline derivatives

A Cu-catalyzed cyclopropanation/ring-opening/iminium cyclization of tryptamine derivatives with donor–acceptor diazo compounds is developed to furnish pyrroloindolines, creating three consecutive stereogenic centers.

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.

Copper-Catalyzed Annulation of Indolyl α-Diazocarbonyl Compounds Leads to Structurally Rearranged Carbazoles

Indolyl α-diazocarbonyl compounds have proven to be effective starting materials for the construction of various 2,3-ring fused indole frameworks. Activation of the diazo functional group under metal catalysis generates a spiro-cyclic indolenine-type intermediate which rearranges to provide two distinct carbazoles upon oxidation. The current study investigates the effects of the catalyst as well as the substituents on the migratory group involved in controlling the selectivity of the rearrangement.

Catalyst-Controlled Regiodivergence in Rearrangements of Indole-Based Onium Ylides

We have developed catalyst-controlled regiodivergent rearrangements of onium-ylides derived from indole substrates. Oxonium ylides formed in situ from substituted indoles selectively undergo [2,3]- and [1,2]-rearrangements in the presence of a rhodium and a copper catalyst, respectively. The combined experimental and density functional theory (DFT) computational studies indicate divergent mechanistic pathways involving a metal-free ylide in the rhodium catalyzed reaction favoring [2,3]-rearrangement, and a metal-coordinated ion-pair in the copper catalyzed [1,2]-rearrangement that recombines in the solvent-cage. The application of our methodology was demonstrated in the first total synthesis of the indole alkaloid (±)-sorazolon B, which enabled the stereochemical reassignment of the natural product. Further functional group transformations of the rearrangement products to generate valuable synthetic intermediates were also demonstrated.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Catalyst- and Substituent-Controlled Regio- and Stereoselective Synthesis of Indolyl Acrylates by Lewis-Acid-Catalyzed Direct Functionalization of 3-Formylindoles with Diazo Esters

A facile and efficient In(OTf)₃- and BF₃·OEt₂-catalyzed direct transformation of 3-formylindoles with diazo esters has been developed for synthesizing diverse and functionalized indolyl acrylates. This one-pot protocol furnishes various (Z)-α-hydroxy-β-indolyl acrylates, (E)-β-(2-alkoxy-2-oxoethoxy)-α-indolyl acrylates, and (Z)-3-hydroxy-2-indolyl acrylates by a catalyst- and substituent-controlled, regio- and stereoselective cascade reaction. The protocol has several advantages, including low loading of the catalyst, mild reaction conditions, broad scope, and high functional group tolerance. The synthesized compounds can be further converted into diversely functionalized materials.