The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

Enantioselective [5 + 1] cycloaddition of sulfur ylides and vinylethylene carbonates via synergistic palladium/chiral phosphonic acid catalysis

An effective method for the synthesis of dihydropyrans through synergistic palladium and chiral phosphonic acid catalysis was reported. This protocol proceeded under mild reactions and provided dihydropyrans in up to 87% yield and up to 97% ee. Meanwhile, various derivations such as oxidation, Wittig-reaction, reductions, nucleophilic substitution, and Baeyer-Villiger were accomplished to furnish interesting compounds. To gain insight into the reaction mechanism, nonlinear relationship experiments and Hammett plot experiments were carried out. In addition, a range of products (3i, 4b, 4f, 4g, and 4j) accessible from this method exhibit various anti-inflammatory activities on NO and ROS inhibition.

Imidazole-catalyzed construction of bridged bicyclo [3.3.1] ketals via formal [3 + 3]-cycloaddition of naphthols and 2-hydroxyl chromene derivatives

Bridged bicyclo[3.3.1]nonanes having the O,O-ketal subunit are widely found in medicinal compounds. While several acid-catalyzed approaches have been reported for the construction of ketals, this study discloses an imidazole-catalyzed modular construction of bicyclo[3.3.1]nonanes from naphthols/phenols and 3-acyl-2-hydroxyl chromenes. The proposed reaction pathway follows a formal [3 + 3] cycloaddition of phenols and an oxonium dipolar intermediate. The reaction is efficient regarding scalability, environmentally benign conditions, and resulting in the products in good to excellent yields.

Dirhodium-Catalyzed Asymmetric Transformations of Alkynes via Carbene Intermediates

ConspectusFunctionalization of alkynes is an established cornerstone of organic synthesis. While numerous transition metals exhibit promising activities in the transformations of alkynes via π-insertion or oxidative cyclometalation, Lewis π-acids offer a different approach. By coordinating with alkynes through π-bonding, Lewis π-acids facilitate nucleophilic addition, leading to the formation of alkenyl metal species. These species can undergo electron rearrangement to generate metal carbenes, which are crucial intermediates for subsequent carbene transfer reactions. This reaction pathway provides a versatile route for alkyne functionalization, especially in an asymmetric manner. Although the Lewis π-acid, gold(I), pioneered this reaction mode, the development of asymmetric variants remains challenging due to the linear coordination of gold(I). Therefore, expanding the range of catalysts beyond gold(I) complexes to other metal catalysts would facilitate further advances in chiral molecule construction and the exploration of novel reaction modes.In this Account, we present a concise review of alkyne multifunctionalization via dirhodium-catalyzed asymmetric transformations, providing the development of the modulation strategies and substrates and plausible reaction mechanisms. In the aromatization-driven strategy, the furanyl dirhodium carbene is generated from an enynone, which is terminated by enantioselective intramolecular C-H insertion, cyclopropanation, aromatic substitution, or the Büchner reaction, giving chiral dihydroindoles, dihydrobenzofurans, cyclopropane-fused tetrahydroquinolines, fluorenes, or cyclohepta[b]benzofurans. The cap-tether modulation strategy was developed in a subsequent study to balance the reactivity and selectivity of an azo-enyne. This strategy gave the first catalytic asymmetric cycloisomerization of azo-enyne, affording centrally and axially chiral isoindazole derivatives. The synergistic activation strategy, i.e., EWG activation and C-H···O interaction, was introduced to achieve the first dirhodium-catalyzed asymmetric cycloisomerization of enynes, providing a range of chiral cyclopropane-annulated bicyclic systems from enynals. Benefiting from these successes, difluoromethyl-substituted enynes were designed and proven to be effective substrates. With the corresponding benzo-1,6-enynes as the substrates, the enantioselective biscyclopropanation and the cascaded cyclopropanation/cyclopropenation were achieved using alkynes as dicarbene equivalents. Additionally, benzo-1,5-enynal generated vinyl dirhodium carbene, which reacted with a variety of alkenes via [2 + 1] cycloaddition, [4 + 3] cycloaddition, or formal allylation, giving spiro and fused polycyclic heterocycles. Coupling the synergistic activation strategy with desymmetrization, we further successfully achieved the asymmetric cycloisomerization of diynals, constructing furan-fused dihydropiperidines with an alkyne-substituted aza-quaternary stereocenter. Notably, by analyzing X-ray structures of several dirhodium-alkyne π-complexes, together with the results of DFT calculations and control experiments, we obtained evidence supporting the synergistic activation mode, making the well-defined paddlewheel-like dirhodium(II) stand out among the other metal complexes. We anticipate that our research will significantly advance the fields of dirhodium, alkyne, and carbene chemistry.

Lewis-Acid-Catalyzed Diastereoselective [4 + 2] Cycloaddition of Vinyldiazo Compounds with N-Acyliminium Cations

An FeCl3-catalyzed [4 + 2] cycloaddition of vinyldiazo compounds with N-acyliminium cations generated from 3-hydroxyisoindolinones is described. A series of diazo-containing isoindolo[2,1-a]quinolinones were constructed in good yields with excellent diastereoselectivities, including those with three contiguous stereogenic centers. The resultant products were readily converted into various isoindolo[2,1-a]quinolinone derivatives based on the rich chemistry of the remaining diazo functionality.

Ring Expansion toward Fused Diazabicyclo[3.1.1]heptanes through Lewis Acid Catalyzed Highly Selective C-C/C-N Bond Cross-Exchange Reaction between Bicyclobutanes and Diaziridines

The synthesis of bicyclic scaffolds has garnered considerable interest in drug discovery because of their ability to mimic benzene bioisosteres. Herein, we introduce a new approach that utilizes a Lewis acid (Sc(OTf)3)-catalyzed σ-bond cross-exchange reaction between the C-C bond of bicyclobutanes and the C-N bond of diaziridines to produce multifunctionalized and medicinally interesting azabicyclo[3.1.1]heptane derivatives. The reaction proceeds well with different bicyclobutanes and a broad range of aryl- as well as alkenyl-, but also alkyl-substituted diaziridines (up to 98 % yield). Conducting a scale-up experiment and exploring the synthetic transformations of the cycloadducts emphasized the practical application of the synthesis. Furthermore, a zinc-based chiral Lewis acid catalytic system was developed for the enantioselective version of this reaction (up to 96 % ee).

Visible-Light-Induced Copper-Catalyzed Radical Reactions of Diazo Arylidene Succinimides to Access the Pyromellitic Diimide (PMDI) Core

The synthesis of pyromellitic diimides (PMDIs) through visible-light-promoted copper-catalyzed reaction of diazo arylidene succinimides has been accomplished without the use of external oxidants. This transformation involves a carbon radical from diazo arylidene succinimides with a copper catalyst or photocatalyst via the proton-coupled electron transfer (PCET) process. This approach successfully challenges a long-standing paradigm in the synthesis of PMDIs. Notably, copper complex (CuNCS) formed in situ proved to be playing a pivotal role to drive the reaction via photoinitiation. Additionally, we synthesized a PMDI molecule known for its prominent aggregation-induced emission (AIE) property. For the very first time, we have synthesized unsymmetrical PMDIs by employing the developed protocol.

Enantioselective dearomative formal (3+3) cycloadditions of bicyclobutanes with aromatic azomethine imines: access to fused 2,3-diazabicyclo[3.1.1]heptanes

Although cycloadditions of bicyclobutanes (BCBs) have emerged as a reliable approach for producing bicyclo[n.1.1]alkanes such as azabicyclo[3.1.1]heptanes (aza-BCHeps), serving as saturated bioisosteres of arenes, the catalytic asymmetric variant remains underdeveloped and presents challenges. Herein, we developed several Lewis acid-catalyzed systems for the challenging dearomative (3+3) cycloaddition of BCBs and aromatic azomethine imines. This resulted in fused 2,3-diazabicyclo[3.1.1]heptanes, introducing a novel chemical space for the caged hydrocarbons. Moreover, an asymmetric Lewis acid catalysis strategy was devised for the (3+3) cycloadditions of BCBs and N-iminoisoquinolinium ylides, forming chiral diaza-BCHeps with up to 99% yield and 97% ee. This study showcases a unique instance of asymmetric (3+3) cycloaddition facilitated by the creation of a chiral environment via the activation of BCBs.

Theoretical exploration of siloxy carbenes: photogeneration and [2+1] photocyclization mechanisms

Carbenes are highly reactive intermediates central to various organic transformations, particularly within photochemistry. This study investigates siloxy carbenes generated from acyl silanes via a 1,2-silyl shift, focusing on their generation and reactivity in excited states, using the multiconfiguration perturbation theory (CASPT2//CASSCF/PCM). Our findings reveal that the presence of an aryl group conjugated with the carbonyl moiety substantially lowers the excitation energy of the singlet 1nπ* state, enabling the 1,2-Brook rearrangement to proceed directly on the singlet hypersurface. This direct pathway, mediated by singlet SΣP(σ1π1) and S0(σ2π0) carbenes, bypasses the need for intersystem crossing (ISC) to the triplet 3nπ* state, which is the rate-determining step in the stepwise triplet pathway involving a triplet TΣP(σ1π1) carbene, thereby enhancing reaction rates and stereoselectivity by preventing undesired bond rotations. This contribution deepens the understanding of siloxy carbene reactivity and lays the groundwork for their future applications.

Oxy-pyridinium Ylides Mediated 1,4-Pyridyl/Aryl Translocation

Molecular rearrangement via carbene transfer is a powerful tool to access molecular diversity. Herein, we describe an efficient approach to selective pyridyl/aryl relocation via a rhodium-catalyzed aminoarylation of diazo compounds, providing a promising strategy to access ortho-pyridyl N-alkylated pyridone scaffolds in a single operation. This reaction features the novel reactivity of oxy-pyridinium ylide, rhodium-associated five-membered transition state, and 1,4-pyridyl/aryl relocation. A computational study discloses the initial oxy-pyridinium ylide formation, keto-enol tautomerization, and 1,4-pyridyl migration to complete the whole rearrangement.

Copper-catalyzed strain-enabled reaction of bicyclobutanes with diazo compounds to synthesize penta-1,4-dienes

Herein we report an interesting copper-catalyzed transformation of BCBs with diazo compounds. This reaction leads to the synthesis of substituted skipped penta-1,4-dienes in good to excellent yields with only one diastomer obtained, and the reaction can also be performed on a gram scale. The transformation is compatible with many different functional groups attached to the BCBs and the diazo compounds.

Photoinduced [4 + 2]-cycloaddition reactions of vinyldiazo compounds for the construction of heterocyclic and bicyclic rings

Highly selective formal [4 + 2]-cycloaddition of vinyldiazoacetates with azoalkenes from α-halohydrazones, as well as with cyclopentadiene and furan, occurs with light irradiation at room temperature, producing highly functionalized heterocyclic and bicyclic compounds in good yields and excellent diastereoseletivity. Under blue light these vinyldiazoacetate reagents selectively form unstable cyclopropenes that undergo intermolecular cycloaddition reactions at a faster rate than their competitive ene dimerization. [4 + 2]-cycloaddition of vinyldiazoacetates with in situ formed azoalkenes produces bicyclo[4.1.0]tetrahydropyridazine derivatives and, together with their cycloaddition using cyclopentadiene and furan that form tricyclic compounds, they occur with high chemoselectivity and diastereocontrol, good functional group tolerance, and excellent scalability. Subsequent transformations portray the synthetic versatility of these structures.

Photocatalytic (3 + 3) Annnulation of Vinyldiazo Compounds and Aminocyclopropanes

A (3 + 3) annulation of aminocyclopropanes and vinyldiazo compounds enabled by organo-photocatalysis is described. The reaction allows the regioselective synthesis of cyclohexenes bearing adjacent amino and carbonyl groups with broad functional group tolerance. In a departure from previous reports, our work demonstrated that a distonic radical cation can be preferentially intercepted by weakly nucleophilic vinyldiazo compounds, followed by an exclusive 6-endo radical cyclization for ring closure. Based on the interaction between adjacent amino and ester groups, the products can be further converted to cyclohexene-fused 1,3-oxazinane and azetidine.

A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Earth-abundant-metal catalyzed double bond transposition offers a sustainable and atom-economical route toward the synthesis of internal alkenes. With an emphasis specifically on internal olefins and ethers, the isomerization of allylic amines has been particularly under represented in the literature. Herein, we report an efficient methodology for the selective isomerization of N-allylic organic compounds, including amines, amides, and imines. The reaction is catalyzed by a neutral PCNHCP cobalt(I) pincer complex and proceeds via a π-allyl mechanism. The isomerization occurs readily at 80-90 °C, and it is compatible with a wide variety of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction to furnish a series of diversely substituted heterobiaryls, which is further discussed in this report.

Photocatalytic Boryl Radicals Triggered Sequential B─N/C─N Bond Formation to Assemble Boron-Handled Pyrazoles

Vinyldiazo compounds are one of the most important synthons in the construction of a cyclic ring. Most photochemical transformations of vinyldiazo compounds are mainly focusing on utilization of their C═C bond site, while reactions taking place at terminal nitrogen atom are largely unexplored. Herein, a photocatalytic cascade radical cyclization of LBRs with vinyldiazo reagents through sequential B─N/C─N bond formation is described. The reaction starts with the addition of LBRs (Lewis base-boryl radicals) at diazo site, followed by intramolecular radical cyclization to access a wide range of important boron-handled pyrazoles in good to excellent yields. Control experiments, together with detailed mechanism studies well explain the observed reactivity. Further studies demonstrate the utility of this approach for applications in pharmaceutical and agrochemical research.

Synthesis and Reactivity of Aluminum Disilacyclopropenes. Cyclic AlSi2 Delocalized 2π Systems

The synthesis, structures, and reactivity of the first unsaturated AlSi2 three-membered ring systems were described. Reactions of dilithiodisilene [(NHB)LiSi═SiLi(NHB)] (1, NHB = diazaborolyl) with aluminum halides AlCl3, Ar(SiMe3)NAlCl2 (Ar = 2,6-iPr2C6H3), Cp*AlBr2 (Cp* = C5Me5), and TipAlBr2·Et2O (Tip = 2,4,6-iPr3C6H2) led to the formation of AlSi2 three-membered ring species, solvated (NHBSi)2AlCl(OEt2) (2) and solvent-free (NHBSi)2AlN(SiMe3) Ar (3), (NHBSi)2AlCp* (4), and (NHBSi)2AlTip (5), in good yields. X-ray diffraction studies and DFT calculations disclosed delocalized AlSi2 2π electron systems. Methanolysis of 4a resulted in cleavage of the Al-Si σ and Si-Si π bonds, giving trihydrodisilane (NHB)H(MeO)SiSiH2 (NHB) (6). Reaction of 4b with 4 equiv of N2O and H2C═CH2 resulted in the insertion of four oxygen atoms and four H2C═CH2 π bonds into all of the Al-Si and Si-Si bonds, yielding the O- and CH2CH2-bridged polycyclic species 7 and 8, demonstrating the synergistic reactivity of the Al-Si and Si-Si bonds in the AlSi2 ring system.

Pd-catalyzed oxa-[4 + n] dipolar cycloaddition using 1,4-O/C dipole synthons for the synthesis of O-heterocycles

Transition metal-catalyzed dipolar cycloaddition is one of the most efficient and powerful synthetic strategies to produce diverse heterocycles. In particular, for the construction of oxygen-containing heterocycles, which are valuable structural motifs found in pharmaceuticals and natural compounds, transition metal-catalyzed oxa-dipolar cycloaddition using an oxygen-containing dipole has emerged as a promising method. In recent years, the 1,4-O/C dipole synthons 2-alkylidenetrimethylene carbonate and 2-hydroxymethylallyl carbonate have been developed and successfully applied to palladium-catalyzed oxa-[4 + n] dipolar cycloadditions with diverse dipolarophiles. In this review, we summarize recent advances in palladium-catalyzed oxa-[4 + n] dipolar cycloadditions using 1,4-O/C dipoles including asymmetric catalysis and divergent catalysis toward five- to nine-membered O-heterocycles.

gem-Difluoro-Masked o-Quinone Methides Generated by Photocatalytic Radical (3+3) Annulation and Their (4+1) Cycloaddition with Sulfur Ylides

A visible light-promoted radical (3+3) annulation of vinyldiazo compounds and bromodifluoromethyl alkynyl ketones for the construction of gem-difluoro-masked o-quinone methides (o-QMs) is described. The reactivity of this new type of o-QM precursor is demonstrated by its (4+1) cycloaddition with sulfur ylides, affording monofluorinated aromatic benzofurans by the elimination of HBr without external oxidants.

Asymmetric Acyclic 1,3-Difunctionalization of Vinyl Carbenes via Site-Selective Vinylogous Mannich-Type Interception of Oxonium Ylides

A novel and highly stereoselective acyclic 1,3-difunctionalization of vinyl metal carbene species has been developed via Rh(II)/chiral phosphoric acid co-catalyzed three-component reactions of vinyldiazoacetates with alcohols and imines. This innovative approach features excellent regio-, diastereo-, and enantioselectivities, demonstrating a broad scope and functional group compatibility. Notably, this is the first example of three-component asymmetric acyclic 1,3-difunctionalization with in situ-formed vinyl metal carbenes.

Chemodivergent 1,3-Dipolar Cycloadditions of C,N-Cyclic Azomethine Imines with γ-Sulfonamido-α,β-Unsaturated Ketones to Synthesize Tricyclic Dinitrogen-Fused Heterocycles

1,3-Dipolar cycloadditions of C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones were developed to synthesize tricyclic dinitrogen-fused heterocycles. Highly functionalized tricyclic tetrahydroisoquinolines were readily obtained in good to high yields in the [3 + 2]-cycloaddition reaction of N-Bz-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones under mild reaction conditions. Moreover, DABCO-catalyzed cycloaddition of N-Ts-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones followed by cleavage of the tosyl group is a convenient route to synthesize tetrahydropyrazolo [5,1-a]isoquinolines in good yields with excellent diastereoselectivities.

Piperidine Derivatives: Recent Advances in Synthesis and Pharmacological Applications

Piperidines are among the most important synthetic fragments for designing drugs and play a significant role in the pharmaceutical industry. Their derivatives are present in more than twenty classes of pharmaceuticals, as well as alkaloids. The current review summarizes recent scientific literature on intra- and intermolecular reactions leading to the formation of various piperidine derivatives: substituted piperidines, spiropiperidines, condensed piperidines, and piperidinones. Moreover, the pharmaceutical applications of synthetic and natural piperidines were covered, as well as the latest scientific advances in the discovery and biological evaluation of potential drugs containing piperidine moiety. This review is designed to help both novice researchers taking their first steps in this field and experienced scientists looking for suitable substrates for the synthesis of biologically active piperidines.

Divergent Construction of N-Doped Polycyclic Aromatic Hydrocarbons with Indole as the Nitrogen Source Building Block

An Ag/Au-catalyzed divergent cascade reaction of alkyne embedded diazoketones with indoles has been described. Preliminary mechanistic studies indicate that the reaction goes through a [4+2]-cycloaddition of an in situ formed isobenzopyrylium intermediate with indole, followed by a sequential retro-Michael addition/carbene N-H insertion process to give the benzo[i]phenanthridines products with gold catalysis; whereas a dearomatization/rearomatization sequence occurs favourably when the reaction is catalyzed by a silver catalyst, delivering benzo[b]carbazoles in generally high to excellent yields. Notably, this is a rare example of using indole as the dienophile for cycloaddition with the isobenzopyrylium species, providing a concise and practical approach for the selective construction of N-doped polycyclic aromatic hydrocarbons (PAHs) with structural diversity and broad functional-group compatibility.

Synthesis of 1,2,4-Oxadiazolines through Deoxygenative Cyclization of N-Vinyl-α,β-Unsaturated Nitrones with in Situ Generated Nitrile Oxides from Hydroxamoyl Chlorides

A variety of 1,2,4-oxadiazoline derivatives were synthesized in moderate to good yields through a deoxygenative cyclization cascade reaction of N-vinyl-α,β-unsaturated nitrones and hydroxamoyl chlorides. Mechanistic studies revealed that the reaction underwent double additions of nitrile oxides to N-vinyl-α,β-unsaturated nitrones, sequential elimination, and intramolecular cyclization to afford 1,2,4-oxadiazolines. Alternatively, 1,2,5-oxadiazolines were also obtained as major products in i-PrOH solvent through [3 + 3] cycloaddition and selective [3,3]-rearrangement. Moreover, the prepared 1,2,4-oxadiazolines were easily converted to polysubstituted pyrroles under thermal conditions.

Efficient Synthesis of Diaryl Quaternary Centers by Rh(II)/Xantphos Catalyzed Relay C-H Functionalization and Allylic Alkylation

A three-component reaction of N, N-disubstituted aniline, α-diazo ester, and an allylic electrophile has been realized by [Rh(II)]₂ /Xantphos catalysis, providing a direct access to various aniline derivatives bearing diaryl allylic quaternary centers in good yields. The synthetic utility of this protocol was demonstrated by facile derivatization of the products for preparation of biologically relevant molecules and structural scaffolds, which offers a high potential for increasing the molecular diversity. Mechanistic studies identified α, α-diarylacetate species as an active intermediate, thereby revealing the presence of a C(sp² )-H functionalization of aniline derivatives/allylic alkylation cascade in this attractive catalytic transformation.

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.

Palladium-Catalyzed [2+3] Cycloaddition/Cross-Coupling Reaction: Z/E and Diastereoselective Synthesis of Dendralene-Functionalized Dihydrofurans

Herein, we describe a Pd-catalyzed [2+3] cycloaddition/cross-coupling reaction of allenyl acetates for the Z/E selective and diastereoselective synthesis of dendralene-functionalized dihydrofurans. Remarkably, mechanistic studies show the formation of an epoxide from a carbonyl bond via cycloaddition, which is practically and mechanistically significant for the construction of other bioactive heterocyclic epoxides. This research also revealed the utility and potential of allenic esters as C2 synthons and 1,2-biselectrophiles in cycloaddition reactions.

Construction of heterocyclic rings from cyclopropenes

Heterocyclic rings are the fundamental building blocks of biological systems and have wide applications in synthetic chemistry and medicinal science. The development of novel synthetic methodology for heterocyclic skeletons from a variety of starting materials has made great progress in the past decades. Meanwhile, highly strained cyclopropenes as reactive reagents in organic transformations have drawn much attention from chemists. The rich chemical reactivity and reaction routes have been well investigated, and some review articles related to the reactivity of cyclopropenes and the construction of carbocycles and acyclic compounds have appeared in these years. Thus, this review mainly focuses on the progress in the construction of heterocyclic rings starting from various cyclopropenes including the reactions of commonly available stable cyclopropenes, in situ generated reactive cyclopropenes and cyclopropene precursors during this decade. Firstly, the transformations of common cyclopropenes into donor-type vinyl metal carbenes via transition metal induced ring opening, direct metalation of the CC bond of metal complexes, and cycloaddition reactions with 1,3-dipoles are described. Next, the annulation reactions of reactive cyclopropenes generated in situ with donor-acceptor reagents, intramolecular nucleophilic addition, and the cycloaddition reactions with 1,3-dipoles are introduced. Then, the transformation of cyclopropene precursors such as alkyl 1-chloro- or 1-alkoxy-2-aroylcyclopropanecarboxylates into five-membered heteroaromatic compounds is also mentioned. In addition, a brief outlook of the opportunity and challenges in the field of bio-orthogonal reactions related to cyclopropenes is given.

High-order dipolar annulations with metal-containing reactive dipoles

Medium-sized heterocycles are widespread among a spectrum of structurally intriguing and biologically significant natural products and synthetic pharmaceuticals. Metal-catalyzed high-order dipolar annulations resembling reactions of metal-containing reactive dipoles with dipolarophiles constitute a highly efficient and flexible strategy for constructing medium-sized heterocycles. Mechanistically, these annulation reactions usually proceeding through stepwise pathways are different from the classic high-order pericyclic reactions that follow the Woodward-Hoffman rules. More significantly, asymmetric high-order dipolar annulations using chiral organometallic catalysts have been proven successful for constructing chiral medium-sized heterocycles with high enantio- and diastereoselectivity. This review highlights the impressive advances in this area and is focused on the reactivity, scope, mechanisms and applications of high-order dipolar annulation reactions.

Radical differentiation of two ester groups in unsymmetrical diazomalonates for highly asymmetric olefin cyclopropanation

Diazomalonates have been demonstrated as effective metalloradicophiles for asymmetric radical olefin cyclopropanation via Co(II)-metalloradical catalysis (MRC). Supported by D ₂-symmetric chiral amidoporphyrin ligand, Co(II)-based metalloradical system can efficiently activate unsymmetrical methyl phenyl diazomalonate (MPDM) with effective differentiation of the two ester groups for asymmetric cyclopropanation, enabling stereoselective construction of 1,1-cyclopropanediesters bearing two contiguous chiral centers, including all-carbon quaternary stereogenic center. The Co(II)-catalyzed asymmetric cyclopropanation, which operates at room temperature without slow addition of the diazo compound, is generally applicable to broad-ranging olefins and tolerates various functionalities, providing a streamlined synthesis of chiral 1,1-cyclopropanediesters in high yields with both high diastereoselectivity and enantioselectivity. Combined computational and experimental studies support the underlying stepwise radical mechanism for Co(II)-catalyzed cyclopropanation. In addition to functioning as 1,3-dipoles for forming five-membered structures, enantioenriched (E)-1,1-cyclopropanediesters serve as useful building blocks for stereoselective synthesis of different cyclopropane derivatives. In addition, the enantioenriched (E)-1,1-cyclopropanediesters can be stereoselectively converted to (Z)-diastereomers.

Synthesis, crystal structure and Hirshfeld surface analysis of dimethyl 3-(3-bromo-phen-yl)-6-methyl-7-oxo-3,5,6,7-tetra-hydro-pyrazolo-[1,2-a]pyrazole-1,2-di-carboxyl-ate

The title compound, C17H17BrN2O5, resulted from the 1,3-dipolar cyclo-addition reaction between dimethyl acetyl-enedi-carboxyl-ate and (3-bromo-benzyl-idene)-4-methyl-5-oxopyrazolidin-2-ium-1-ide in CHCl3. The dihedral angle between the pyrazole rings (all atoms) is 32.91 (10)°; the oxo-pyrazole ring displays an envelope conformation whereas the other pyrazole ring adopts a twisted conformation. The bromo-phenyl ring subtends a dihedral angle of 88.95 (9)° with the mean plane of its attached pyrazole ring. In the crystal, the mol-ecules are linked by C-H⋯O hydrogen bonds and aromatic π-π inter-actions with an inter-centroid distance of 3.8369 (10) Å. The Hirshfeld surface analysis and fingerprint plots reveal that the mol-ecular packing is governed by H⋯H (37.1%), O⋯H/H⋯O (31.3%), Br⋯H/H⋯Br (13.5%) and C⋯H/H⋯C (10.6%) contacts. The energy framework indicates that dispersion energy is the major contributor to the mol-ecular packing.

Theoretical insights into the mechanism and origin of chemoselectivity in the catalyst- and directing group-dependent oxidative cyclization of diynes with pyridine N-oxides

The mechanisms and origin of the chemoselectivity of Cu(i)- and Au(i)-catalyzed oxidation of diynes for the divergent syntheses of two different N-heterocycles, substituted pyrroles and dihydroindeno[1,2-c]pyrrol-3(2H)-ones, were elucidated using density functional theory.

Copper-Catalyzed Asymmetric Diyne Cyclization via [1,2]-Stevens-Type Rearrangement for the Synthesis of Chiral Chromeno[3,4-c]pyrroles

Here, we report a copper-catalyzed asymmetric cascade cyclization/[1,2]-Stevens-type rearrangement via a non-diazo approach, leading to the practical and atom-economic assembly of various valuable chiral chromeno[3,4-c]pyrroles bearing a quaternary carbon stereocenter in generally moderate to good yields with wide substrate scope and excellent enantioselectivities (up to 99 % ee). Importantly, this protocol not only represents the first example of catalytic asymmetric [1,2]-Stevens-type rearrangement based on alkynes but also constitutes the first asymmetric formal carbene insertion into the Si-O bond.

An asymmetric oxidative cyclization/Mannich-type addition cascade reaction for direct access to chiral pyrrolidin-3-ones

An efficient gold and chiral phosphoric acid cooperatively catalyzed enantioselective oxidative cyclization/Mannich-type addition reaction of homopropargyl amides with nitrones has been developed, which provides chiral pyrrolidin-3-ones in high yields with excellent enantioselectivities under mild conditions. This reaction employed stable and readily available alkynes as non-diazo carbene precursors, which provides a 100% atom economy method with high bond formation efficiency.

Assembling Complex Structures through Cascade and Cycloaddition Processes via Non-Acceptor Gold or Rhodium Carbenes

The ability of highly energetic metal–carbene intermediates to engage in complex cascade or formal cycloaddition processes is one of the most powerful tools for building intricate molecular architectures in a straightforward manner. Among this type of organometallic intermediates, non-acceptor metal carbenes are particularly challenging to access and, therefore, have experienced slower development. In this regard, our group has exploited the use of electrophilic gold(I) complexes to selectively activate certain classes of substrates for the generation of this type of intermediate. Thus, very different types of molecules, such as enynes or 7-substituted cycloheptatrienes, lead to the formation of carbenes under gold(I) catalysis. Related rhodium(II) carbenes can also be generated from cycloheptatrienes. In this account, we aim to summarize our efforts towards the in situ generation of such highly versatile organometallic species as well as studies on their reactivity through formal cycloadditions or complex cascade reactions.

1 Introduction

2 Generation of Au(I)-Vinylcarbenes via a Cycloisomerization/1,5-Alkoxy Migration Cascade

2.1 Intramolecular Trapping of Au(I) Vinylcarbenes

2.1.1 Applications in Total Synthesis

2.2 Intermolecular Trapping of Au(I) Vinylcarbenes

2.2.1 Total Synthesis of Schisanwilsonene A

2.2.2 Trapping with Furans, 1,3-Dicarbonyl Compounds and Cyclic Alkenes

2.2.3 Mechanism of the Cycloisomerization/1,5-Migration Sequence and the Role of the OR Migrating Group

2.2.4 (4+3) Cycloadditions from Enynes

3 Formal Cycloadditions of Simple Donor Metal Carbenes

3.1 The Metal-Catalyzed Retro-Buchner Reaction

3.2 Formal Cycloadditions with Non-Acceptor Carbenes via Metal-Catalyzed Aromative Decarbenations

3.2.1 (4+1) Cycloadditions of Au(I) Carbenes

3.2.2 (3+2) Cycloadditions of Au(I) Carbenes

3.2.3 (4+3) Cycloadditions of Rh(II) Carbenes

4 Concluding Remarks