Mechanistic Studies of Transition-Metal-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Copper-Catalyzed Intermolecular [2 + 2 + 2] Annulation of Diynes with Alkynes: Construction of Carbazoles

Transition-metal-catalyzed [2 + 2 + 2] annulation of alkynes is an efficient pathway for the synthesis of aromatic compounds. However, most of the established methods require noble metal catalysts. Herein, we report a copper-catalyzed intermolecular [2 + 2 + 2] annulation of diynes with alkynes through vinyl cation intermediates, enabling the atom-economical preparation of biologically important carbazole skeletons. The reaction shows good regioselectivity in the reaction of aryl(alkyl)alkynes. Moreover, preliminary results have also been obtained for the related catalytic atroposelective transformation. This reaction represents a rare example of non-noble-metal-catalyzed intermolecular [2 + 2 + 2] annulation of ynamides through the vinyl cation pathway.

Stereoselective transition metal-catalyzed [(2+2)+1] and [(2+2)+2] carbocyclization reactions using 1,6-enynes with 1,1-disubstituted olefins: construction of quaternary centers

Transition metal-catalyzed carbocyclization reactions provide a powerful method for the stereoselective assembly of complex, highly substituted (poly)cyclic scaffolds. Although 1,6-enynes are common substrates for these transformations, using polysubstituted alkene derivatives to construct functionalized cyclic products remains challenging due to their significantly lower reactivity. This Perspective highlights key developments in stereoselective semi-intramolecular metal-catalyzed [(2+2)+1] and [(2+2)+2] carbocyclizations of 1,6-enynes containing 1,1-disubstituted alkenes, which produce cycloadducts with quaternary stereogenic centers. The insights gleaned from these examples provide a blueprint for developing more general carbocyclization strategies with challenging polysubstituted olefins.

Mo(0) carbonyl complexes bearing the bis(3,5-dimethylpyrazole) ligand: catalysis of the regioselective [2 + 2 + 2] cycloaddition of terminal alkynes to synthesize 1,3,5-isomers

The reaction between 1,3-bis(3,5-dimethylpyrazolylmethyl)hexahydropyrimidine L and Mo(CO)6 in CH3CN at 130 °C afforded a binuclear Mo(0) complex 1 containing a new macrocycle formed upon C-N bond cleavage in L in good yield. Conversely, a clean reaction takes place between L and [Mo(CO)4(COD)] in THF at 60 °C to give a new metalloligand complex [Mo(CO)4(κ2-N,N-L)] 2 containing a spectator pyrazole arm in 83% yield. Their structures were determined by X-ray diffraction methods, and a plausible mechanism is proposed for the C-N bond cleavage leading to complex 1. In view of a limited number of reports of catalysts for the synthesis of 1,3,5-trisubstituted benzene from the [2 + 2 + 2] cycloaddition of terminal alkynes, the catalytic application of complex 2 was explored. Interestingly, 14 terminal alkynes including those containing polar functional groups such as OH, NH2, and CHO among others were converted into their 1,3,5-isomers as the major products with a high regioselectivity ratio of >90 : 10 (1,3,5-/1,2,4-isomer) for most cases.

Dirhodium-Catalyzed [2 + 2 + 2] Cycloaddition of 1,6-Diynes and Alkynes

A facile method for the construction of fused arenes has been developed through dirhodium-catalyzed [2 + 2 + 2] cycloaddition, which represents a new application of dirhodium complexes. This protocol is convenient to handle without the addition of extra ligands and reductants and tolerates a broad range of functional groups. Mechanistic studies revealed that the two-electron oxidation process, carboxylate ligand departure, and heteroatom coordination-promoted [2 + 2 + 2] cycloaddition were possibly involved.

Radical 6-Endo Addition Enables Pyridine Synthesis under Metal-Free Conditions

Metal-free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to eliminating the need to remove trace metal catalysts from the reaction. We report a radical 6-endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal-free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with 57 examples. Mechanistic investigations and computational studies indicate the unprecedented 6-endo-trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5-exo-trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late-stage pyridine installation on complex molecules.

Selectivity in the Formal [2 + 2 + 2] Cycloaromatization of Enyne-Allenes Generated by the Alder-ene Reaction from Triynes

1,3-Diynyl propiolates undergo the Alder-ene reaction to generate enyne-allenes, which participate in the Diels-Alder reaction to provide products of a formal [2 + 2 + 2] cycloaromatization of three alkynes. Without an external alkyne, enyne-allene reacts with one of the alkyne moieties of 1,3-diynyl propiolate, whereas external alkynes can be used to trap enyne-allene to provide various arene products. The substituents on the dienophilic alkynes have a profound impact on their reactivity. In this Diels-Alder reaction, 1,3-diynes display higher reactivity than monoynes; thus, an excess amount (4-5 equiv) of external monoynes needs to be employed to get good product selectivity.

Cascade C-H Activation and Two C-C Bond Forming in Reaction of Azalutetacyclopentadiene with 2-Methylbenzonitriles

Azametallacyclopentadienes are an important class of metallacycles as the key intermediates in metal-promoted or catalyzed carbon-carbon coupling reaction of nitriles and alkynes. Rare-earth azametallacyclopentadienes have shown various reactivity toward nitriles, depending on the substituents of nitriles. The reaction of azalutetacyclopentadienes toward 2-methylbenzonitriles has been investigated in this work, which selectively affords the fused 7-5-6-membered azalutetacycles as products. Computational studies reveal that the reaction of azalutetacyclopentadienes toward 2-methylbenzonitriles selectively initiates with the remote activation of the benzylic C-H bond by the Lu-N bond, followed by the intramolecular nucleophilic attack from the deprotonated benzylic carbon to form a C-C bond. Subsequently, the high ring strain promoted the generation of the uncoordinated carbanion dissociated from the lutetium center, which then undergoes intramolecular nucleophilic attack toward C=N triple bond to give the final product containing fused 7-5-6-membered azalutetacycle. This work not only achieves highly selective three-step cascade reaction to form a unique class of rare-earth metallacycle, but also provides a new idea for the transformation of unsaturated substrates with C-H bonds that can be activated.

Enantioselective Construction of Tetrahydroindole Skeletons by Rh-Catalyzed [2+2+2] Cycloaddition of Homopropargyl Enamides with Alkynes

We have developed the Rh-catalyzed enantioselective [2+2+2] cycloaddition of homopropargyl enamides (tosylamide-tethered 1,6-enynes) with alkynes to construct tetrahydroindole skeletons found in natural alkaloids and pharmaceuticals. This cycloaddition proceeds at room temperature in high yields and regio- and enantioselectivity with a broad substrate scope. The preparative scale reaction followed by substituent conversion on the nitrogen atom and the diastereoselective [4+2] cycloaddition with singlet O2 affords hexahydroindole-diols bearing three stereogenic centers and variable substituents on the nitrogen. Mechanistic studies have revealed that the substituents of the enynes change the ratio of intramolecular and intermolecular rhodacycle formation when using terminal alkynes, varying the ee values of the cycloadducts.

Pd/Cu Dual Metal-Catalyzed Regioselective [2 + 2 + 2] Cycloaddition of Malononitriles with Alkynes to Densely Substituted Pyridines

Transition metal-catalyzed [2 + 2 + 2] cycloaddition of nitriles and two alkynes is an efficient method for assembling pyridines. However, examples employing palladium catalysis have rarely been disclosed, and the processes of reactivity and selectivity remain unclear. We report here a palladium/copper dual metal-catalyzed [2 + 2 + 2] cycloaddition of diynyl-tethered malononitriles and terminal alkynes to synthesize densely substituted pyridines. This method features a good substrate scope, synthetically useful yields, and perfect regioselectivity. The derivatization of the pyridine products demonstrates the potential application of this method in synthesizing heterocycles and as ligands in photocatalysis. Preliminary mechanistic studies suggest that the reaction undergoes aza-oxidative cycloaddition of Pd(0) with nitrile and alkyne, followed by alkyne insertion and reductive elimination. The presence of copper is crucial to its reactivity and regioselectivity.

Dehydrogenative [4 + 2] Annulation of 1-Indanones with Alkynes Enabled by In-Situ-Generated Nickel Hydride

A practical and effective nickel-catalyzed dehydrogenative [4 + 2] annulation of 1-indanones with alkynes was reported. In this protocol, nickel-catalyzed desaturation of 1-indanones and nickel hydride catalyzed coupling with alkynes were first incorporated. A cyclopentadiene-type nickel hydride species was generated in situ via β-H elimination, and they subsequently reacted with a wide variety of alkynes to afford various benzo[a]fluorenone derivatives in good yields and regioselectivity.

Iridium-Catalyzed [2 + 2 + 2] Cycloaddition of Bithiophen-Linked Diynes with Nitriles: Scope and Mechanistic Study with Quantum Chemical Calculation

We report a simple and atom-efficient method for the synthesis of bithiophene-fused isoquinolines by iridium-catalyzed [2 + 2 + 2] cycloaddition of bithiophene-linked diynes with nitriles. All three structural isomers of bithiophene-linked diynes underwent [2 + 2 + 2] cycloaddition, and the trend in the reactivity for cycloaddition was diyne 1 = diyne 3 > diyne 2. Dibenzothiophene-linked diyne also reacted with nitriles to form a variety of cycloadducts. Cycloaddition of bithiophene-linked diynes with alkynes and an isocyanate formed naphthodithiophenes and a 2-pyridone derivative, respectively. Cycloadducts bearing a 2-aminopyridine moiety and benzothiophene rings showed intense fluorescence at around 530 nm and gave a fluorescence quantum yield of 0.44. Furthermore, quantum chemical calculations provided insight into the origin of the difference in reactivity of three bithiophene-linked diynes. The different reactivities of the three diynes 1-3 are believed to originate from the step where an iridacyclopentadiene reacts with a coordinated nitrile to form azairidabicyclo[3.2.0]heptatriene. HOMOs of iridacyclopentadiene play a decisive role in this step.

Palladium catalyzed stereoselective intramolecular [3 + 2] cycloaddition reactions of (E) & (Z)-ene-vinylidenecyclopropanes

A palladium-catalyzed ring-opening cyclization of (E) & (Z)-ene-vinylidenecyclopropanes has been developed via an intramolecular [3 + 2] cycloaddition process in the presence of a sterically bulky biaryl phosphine ligand, stereoselectively affording fused cis- & trans-bicyclo[4.3.0] skeletal products in good yields with a broad substrate scope and good functional tolerance. A plausible reaction mechanism was proposed on the basis of previous work and the DFT calculations.

Mechanistic Investigation on the Regioselectivity of Electrochemical Co(II)-Catalyzed [2 + 2 + 2] Cycloaddition of Terminal Acetylenes

In this paper, the regioselectivity of electrochemical Co(II)-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes was investigated using density functional theory. We explored in detail the energy profiles for both 1,2,4- and 1,3,5-regioselectivity pathways and revealed the origin of the regioselectivity. Two kinds of conformational isomers derived from the different coordination modes of alkynes with cobaltacyclopentadiene have been found, which were formed through electrochemically mediated redox processes. The regioselectivity of the reaction depends on the two coordination modes. When the Co(II) center attacks α-C of the third alkyne, while β2-C in cyclopentadiene bonds to β-C of the alkyne, the reaction favors the formation of 1,2,4-products. In contrast, when the Co(II) center connects to β-C of the alkyne, it forms only the 1,3,5-products via [4 + 2] cycloaddition because of the steric repulsion between the bulky ligand on Co(II) and the phenyl group in the alkyne.

Regioselective [2 + 2 + 2] Alkyne Cyclotrimerizations to Hexasubstituted Benzenes: Syntheses of Fomajorin D and Fomajorin S

Hexasubstituted benzenoids are prepared by regioselective bimolecular [2 + 2 + 2] alkyne cyclotrimerizations of diynes with alkynes. These convergent and efficient benzannulations are directed toward and lead to the first total syntheses of the illudalane sequiterpenes fomajorin D and S, in 10 and 7 steps, respectively, from commercially available dimedone. Control experiments suggest that hydrogen bonding may play a role in preorganizing the diyne and alkyne coupling partners for establishing the desired regioselectivity, but other factors are likely involved in the selective formation of other regioisomers.

A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization

The synthesis, structure, and catalytic activity of a Ti(II)/Ti(III) inverted sandwich compound are presented in this study. Synthesis of the arene-bridged dititanium compound begins with the preparation of the titanium(IV) precursor [TiCl2(MesPDA)(thf)2] (MesPDA = N,N'-bis(2,4,6-trimethylphenyl)-o-phenylenediamide) (2). The reduction of 2 with sodium metal results in species [{Ti(MesPDA)(thf)}2(μ-Cl)3{Na}] (3) in oxidation state III. To achieve the lower oxidation state II, 2 undergoes reduction through alkylation with lithium cyclopentyl. This alkylation approach triggers a cascade of reactions, including β-hydride abstraction/elimination, hydrogen evolution, and chemical reduction, to generate the Ti(II)/Ti(III) compound [Li(thf)4][(TiMesPDA)2(μ-η6: η6-C6H6)] (4). X-ray and EPR characterization confirms the mixed-valence states of the titanium species. Compound 4 catalyzes a mild, efficient, and regiospecific cyclotrimerization of alkynes to form 1,3,5-substituted arenes. Kinetic data support a mechanism involving a binuclear titanium arene compound, similar to compound 4, as the resting state. The active catalyst promotes the oxidative coupling of two alkynes in the rate-limiting step, followed by a rapid [4 + 2] cycloaddition to form the arene product. Computational analysis of the resting state for the cycloaddition of trimethylsilylacetylene indicates a thermodynamic preference for stabilizing the 1,3,5-arene within the space between the two [TiMesPDA] fragments, consistent with the observed regioselectivity.

Ru(II)-Catalyzed Deoxygenative Formal [3 + 1 + 2] Benzannulation of Allyl Alcohols and Acetylenediesters via C-H Activation and Selective Carbon-Carbon Triple Bond Cleavage

An unprecedented Ru(II)-catalyzed deoxygenative, site-selective formal [3 + 1 + 2] benzannulation reaction for the efficient synthesis of highly substituted benzene molecules is reported. This reaction between allyl alcohols and acetylenedicarboxylate esters proceeds via cascade C-H activation, consecutive double migratory insertion with alkynes, and cycloaromatization followed by an unusual specific C-C triple bond cleavage.

Divergent Geminal Alkynylation-Allylation and Acylation-Allylation of Carbenes: Evolution and Roles of Two Transition-Metal Catalysts

Cooperative bimetallic catalysis to access novel reactivities is a powerful strategy for reaction development in transition-metal-catalyzed chemistry. Particularly, elucidation of the evolution of two transition-metal catalysts and understanding their roles in dual catalysis are among the most fundamental goals for bimetallic catalysis. Herein, a novel three-component reaction of a terminal alkyne, a diazo ester, and an allylic carbonate was successfully developed via cooperative Cu/Rh catalysis with Xantphos as the ligand, providing a highly efficient strategy to access 1,5-enynes with an all-carbon quaternary center that can be used as immediate synthetic precursors for complex cyclic molecules. Notably, a Meyer-Schuster rearrangement was involved in the reactions using propargylic alcohols, resulting in an unprecedented acylation-allylation of carbenes. Mechanistic studies suggested that in the course of the reaction Cu(I) species might aggregate to some types of Cu clusters and nanoparticles (NPs), while the Rh(II)2 precursor can dissociate to mono-Rh species, wherein Cu NPs are proposed to be responsible for the alkynylation of carbenes and work in cooperation with Xantphos-coordinated dirhodium(II) or Rh(I)-catalyzed allylic alkylation.

Ni0-Catalyzed Regioselective [2 + 2 + 2] Cyclotrimerization of 1,3-Diynes: An Expeditious Synthesis of Hexasubstituted Alkynyl Benzenes

In the present work, we demonstrate a regioselective [2 + 2 + 2] cyclotrimerization of 1,3-diynes catalyzed by Ni0 to provide hexasubstituted benzenes (HSBs). HSBs have significant applications as functional materials and pharmaceuticals. The present protocol exhibited remarkable versatility, transforming 1,3-diynes with diverse alkyl, aryl, and heterocyclic groups to the corresponding HSBs. With the help of control experiments and density functional theory (DFT), the mechanism of the reaction and the origin of regioselectivity were elucidated.

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Two-Fold Interpenetrated Binuclear Nickel Metal-Organic Framework as a Heterogeneous Catalyst for N-Heterocycle Synthesis

A binuclear Ni(II)-based metal-organic framework {[Ni2(btb)1.333(H2O)3.578(py)1.422]·(DMF)(H2O)3.25}n (Nibtb) was solvothermally synthesized (H3btb = 1,3,5-tri(4-carboxylphenyl)benzene, py = pyridine, DMF = N,N-dimethylformamide). Nibtb shows a rare 2-fold interpenetrating (3,4)-connected 3D network with a point symbol of (83)4(86)3 based on binuclear Ni(II) clusters. Nibtb as a heterogeneous catalyst combines the high stability of MOFs and excellent catalytic activity of nickel, which exhibits excellent catalytic activity for the synthesis of benzimidazoles and pyrazoles under mild conditions. Moreover, the catalyst can be easily separated and reused for seven successive cycles and maintains high catalytic activity.

Cyclotrimerization Approach to Symmetric [9]Helical Indenofluorenes: Diverting Cyclization Pathways

Catalytic cyclotrimerization routes to symmetrical [9]helical indenofluorene were explored by using different transition-metal complexes and thermal conditions. Depending on the reaction conditions, the cyclotrimerizations were accompanied by dehydro-Diels-Alder reaction giving rise to another type of aromatic compounds. Structures of both symmetrical [9]helical cyclotrimerization product as well as the dehydro-Diels-Alder product were confirmed by single-crystal X-ray diffraction analyses. Limits of enantioselective cyclotrimerization were assessed as well. DFT calculations shed light on the reaction course and the origin of diminished enantioselectivity.

Activation of alkynes by chalcogen bonding: a Se⋯π interaction catalyzed intramolecular cyclization of 1,6-diynes

The activation of the triple bond of alkynes was dominated by transition metals, while it is difficult for organocatalysts to play an effective role in this realm. Herein, we describe the activation of alkynes by chalcogen bonding, and the weak Se⋯π interaction was capable of catalyzing the intramolecular cyclization of 1,6-diynes, thus adding a new capability in the list of supramolecular catalysis.

Mechanistic insight into cobalt-mediated [2 + 2 + 2]-cycloaddition reactions with γ-alkylidenebutenolide and γ-alkylidenebuterolactam as 2π partners

The molecular complexity of recently reported cobalt(III) polycyclic complexes, resulting from an intramolecular formal (2 + 2 + 3) cycloaddition reaction on an enediyne containing a lactone moiety, has prompted us to computationally review the mechanisms of cobalt cycloaddition reactions with γ-alkylidenebutenolide or γ-alkylidenebuterolactam as 2π partners. Computed mechanisms are compared, leading to either cobalt(III)- or cobalt(I)-spiro complexes depending of both the nature of the reaction (intra- vs. intermolecular pathway) and the nature of the 2π partner (γ-alkylidenebutenolide vs. γ-alkylidenebuterolactam). These proposed mechanisms are supported by experiments, allowing us to report the synthesis and characterization of the predicted compounds.

Application of phenacyl bromide analogs as a versatile organic intermediate for the synthesis of heterocyclic compounds via multicomponent reactions

Due to the increased interest in heterocyclic compounds over the past decade, many pharmaceutical and organic chemists have explored the synthesis of various materials. Among the many organic compounds that can be synthesized in a wide range of chemical reactions, phenacyl bromide has proven to be a good, inexpensive, versatile, and efficient intermediate. This review presents an overview of the significant applications of phenacyl bromide, focusing on its role in recent synthetic advances and its utility in multicomponent reactions and literature reports for 2017 to the end of 2021.

Barrier Heights for Diels-Alder Transition States Leading to Pentacyclic Adducts: A Benchmark Study of Crowded, Strained Transition States of Large Molecules

Theoretical characterization of reactions of complex molecules depends on providing consistent accuracy for the relative energies of intermediates and transition states. Here we employ the DLPNO-CCSD(T) method with core-valence correlation, large basis sets, and extrapolation to the CBS limit to provide benchmark values for Diels-Alder transition states leading to competitive strained pentacyclic adducts. We then used those benchmarks to test a diverse set of wave function and density functional methods for the absolute and relative barrier heights of these transition states. Our results show that only a few of the tested density functionals can predict the absolute barrier heights satisfactorily, although relative barrier heights are more accurate. The most accurate functionals tested are ωB97M-V, M11plus, ωB97X-V, PBE-D3(0), M11, and MN15 with MUDs from best estimates less than 3.0 kcal. These findings can guide selection of density functionals for future studies of crowded, strained transition states of large molecules.

Overcrowded Triply Fused Carbo[7]helicene

This paper presents the synthesis and comprehensive analysis of a highly contorted and doubly negatively curved multihelicene compound, composed of three carbo[7]helicene units fused within a central six-membered ring. The synthesis of this compound involved a [2 + 2 + 2] cycloaddition reaction of 13,14-picyne, employing a Ni(0) catalyst, which exhibited superior performance compared to conventional Pd(0) catalysts. The evaluation of aromaticity in this triple carbo[7]helicene, utilizing magnetic and electronic criteria, led to noteworthy insights challenging the limitations of Clar's model of aromaticity.

Recent advances in the multicomponent synthesis of heterocycles using tetronic acid

Tetronic acid, a versatile synthon, has been extensively investigated by numerous researchers in synthetic chemistry due to its crucial role in synthesizing heterocycles which makes this compound particularly advantageous in both pharmaceutical and biological fields. Various heterocycles can be synthesized using it as a precursor via multicomponent reactions (MCRs). Dicarbonyl groups can be considered the building blocks and key structural motifs of a wide range of natural compounds, which may contain different functional groups in the synthesis of heterocyclic frameworks. This review covers the literature from 2017 to 2022, and it encompasses the different one-pot protocols for synthesizing a variety of heterocyclic molecules.

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl₂C₆H₃)₂] (1, Xyl = 2,6-Me₂C₆H₃). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC≡CH]₀/[1]₀), referred to as the low ([RC≡CH]₀/[1]₀ < 40) and high ([RC≡CH]₀/[1]₀ > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

Modular Access to meta-Substituted Benzenes via Mo-Catalyzed Intermolecular Deoxygenative Benzene Formation

The substituted benzene derivatives are essential to organic synthesis, medicinal chemistry, and material science. However, the 1,3-di- and 1,3,5-trisubstituted benzenes are far less prevalent in small-molecule drugs than other substitution patterns, likely due to the lack of robust, efficient, and convenient synthetic methods. Here, we report a Mo-catalyzed intermolecular deoxygenative benzene-forming reaction of readily available ynones and allylic amines. A wide range of unsymmetric and unfunctionalized 1,3-di- and 1,3,5-trisubstituted benzenes were obtained in up to 88% yield by using a commercially available molybdenum catalyst. The synthetic potential of the method was further illustrated by synthetic transformations, a scale-up synthesis, and derivatization of bioactive molecules. Preliminary mechanistic studies suggested that this benzene-forming process might proceed through a Mo-catalyzed aza-Michael addition/[1,5]-hydride shift/cyclization/aromatization cascade. This strategy not only provided a facile, robust, and modular approach to various meta-substituted benzene derivatives but also demonstrated the potential of molybdenum catalysis in the challenging intermolecular deoxygenative cross-coupling reactions.

Computational insights into the reactivity for the [2+5] cycloaddition reactions of norbornene-linked group 14 element/P-based and Si/group 15 element-based frustrated Lewis pairs with benzaldehyde

The element effects of Lewis acid (LA) and Lewis base (LB) on the potential energy surfaces of [2+5] cycloaddition reactions of norbornene-based G14/P-based (G14 = group 14 element) and Si/G15-based (G15 = group 14 element) frustrated Lewis pair (FLP)-type molecules with benzaldehyde were theoretically examined via density functional theory and several sophisticated methods. The theoretical findings indicated that among the above nine norbornene-linked G14/G15-based FLPs, only the Si/N-Rea, Si/P-Rea, and Si/As-Rea FLP-assisted compounds can readily undergo cycloaddition reactions with doubly bonded organic systems from kinetic and thermodynamic viewpoints. The energy decomposition analysis showed that the bonding interactions between the norbornene-based G14/G15-FLPs and benzaldehyde are better described in terms of the singlet-singlet model (donor-acceptor model) rather than the triplet-triplet model (electron-sharing model). In particular, natural orbitals for chemical valence findings revealed that the forward bonding is the lone pair (G15) → p-π*(C) interaction, which is a significantly strong FLP-to-benzaldehyde interaction. However, the back-bonding is the p-π*(G14) ← lone-pair orbital(O) interaction, which is a weak benzaldehyde-to-FLP interaction. The analyses based on the activation strain model showed that the larger the atomic radius of either the G14(LA) or the G15(LB) atom, the greater the G14⋯G15 separation distance in the norbornene-based G14/G15-FLP molecule, the smaller the orbital overlaps between G14/G15-FLP and Ph(H)CO, and the higher the activation barrier during its cycloaddition reaction with benzaldehyde.

Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

The [2 + 2 + 2] cycloaddition of 1,5-bisallenes and alkynes under the catalysis of Rh(I) with hemilabile thioether-functionalized N-heterocyclic carbene ligands is described. This protocol effectively provides an entry to different trans-5,6-fused bicyclic systems with two exocyclic double bonds in the cyclohexene ring. The process is totally chemoselective with the two internal double bonds of the 1,5-bisallenes being involved in the cycloaddition. The complete mechanism of this transformation as well as the preference for the trans-fusion over the cis-fusion has been rationalized by density functional theory calculations. The reaction follows a typical [2 + 2 + 2] cycloaddition mechanism. The oxidative addition takes place between the alkyne and one of the allenes and it is when the second allene is inserted into the rhodacyclopentene that the trans-fusion is generated. Remarkably, the hemilabile character of the sulfur atom in the N-heterocyclic carbene ligand modulates the electron density in key intermediates, facilitating the overall transformation.

Controllable carbonyl-assisted C(sp3)–C(sp3) bond reduction and reorganization

Unprecedentedly preferential reduction of unstrained C(sp3)–C(sp3) bond over ketone, hydrogenative [2+2+2]-cycloreversion of 2,4-diacylcyclohexanols, and cyclizative degradation of poly(vinylketone) have been achieved by organolanthanide catalysis.

Synthesis of 5H-chromeno[3,4-c]pyridine derivatives through ruthenium-catalyzed [2 + 2 + 2] cycloaddition

Efficient access to 5H-chromeno[3,4-c]pyridines using Ru-catalyzed [2 + 2 + 2] cycloaddition of α,ω-diynes with cyanamides was developed, providing valuable tricyclic pyridine building blocks and enabling access to a biologically relevant intermediate.

o-Quinodimethane Atropisomers: Enantioselective Synthesis and Stereospecific Transformation

o-Quinodimethanes have remarkable utility as reactive intermediates in Diels-Alder reactions, enabling significantly accelerated routes to complex polycyclic compounds. The discovery of different discrete precursors to thermally generate o-quinodimethanes thereby greatly augmented their availability and versatility. However, due to the required high temperatures and the immense reactivity of o-quinodimethanes, stereoselectivity to afford isomerically defined products still constitutes a critical challenge. Herein, we describe the accessibility of atropisomeric o-quinodimethanes, the enantioselective synthesis of their precursors, their remarkable configurational stability and the stereospecific transformation by the benzannulation of dienophiles. A catalyst-stereocontrolled [2+2+2] cycloaddition, the generation of o-quinodimethane atropisomers and ensuing stereospecific Diels-Alder reactions enabled enantioselectivities through these transient intermediates with of up to 96 : 4 e.r.

Interstellar Benzene Formation Mechanisms via Acetylene Cyclotrimerization Catalyzed by Fe+ Attached to Water Ice Clusters: Quantum Chemistry Calculation Study

Benzene is the simplest building block of polycyclic aromatic hydrocarbons and has previously been found in the interstellar medium. Several barrierless reaction mechanisms for interstellar benzene formation that may operate under low-temperature and low-pressure conditions in the gas phase have been proposed. In this work, we studied different mechanisms for interstellar benzene formation based on acetylene cyclotrimerization catalyzed by Fe⁺ bound to solid water clusters through quantum chemistry calculations. We found that benzene is formed via a single-step process with one transition state from the three acetylene molecules on the Fe⁺(H₂O)_n (n = 1, 8, 10, 12 and 18) cluster surface. Moreover, the obtained mechanisms differed from those of single-atom catalysis, in which benzene is sequentially formed via multiple steps.

Rhodium-Catalyzed [2 + 2 + 2] Cyclotrimerizations of Yndiamides with Alkynes

Yndiamides offer opportunities for the synthesis of vicinally nitrogen-disubstituted aromatics and azacycles. Here we report the Rh-catalyzed cyclotrimerization of alkynyl yndiamides with alkynes, the regiochemical outcome of which is controlled by the electronic properties of the alkyne partner, enabling the formation of 7-aminoindolines with excellent selectivity (up to >20:1 r.r.). We also report a complementary synthesis of bicyclic 1,2-dianiline derivatives by cyclotrimerization of yndiamides with terminal diynes, where slow addition of the diyne overcomes self-dimerization.

Electrochemical Rhodium-Catalyzed C-H Cyclodimerization of Alkynes to Access Diverse Functionalized Naphthalenes: Involvement of RhIV/V and RhI Dual Catalysis

The first electrochemical rhodium-catalyzed C-H cyclodimerization of alkynes for the direct construction of functionalized naphthalenes was reported. The practicality and synthetic value of this strategy were demonstrated by the readily accessible scale-up synthesis and transformation of the products. Detailed mechanistic studies evidenced that electricity played an important role during the electrochemical disproportionation (ECD) process to generate and maintain the catalytically active Rh^IV/V and Rh^I species, which conducted the direct C-H activation.

Accurate computed singlet-triplet energy differences for cobalt systems: implication for two-state reactivity

Accurate singlet-triplet energy differences for cobalt and rhodium complexes were calculated by using several wave function methods, such as MRCISD, CASPT2, CCSD(T) and BCCD(T). Relaxed energy differences were obtained by considering the singlet and triplet complexes, each at the minimum of their potential energy surfaces. Active spaces for multireference calculations were carefully checked to provide accurate results. The considered systems are built by increasing progressively the first coordination sphere around the metal. We included in our set two CpCoX complexes (Cp = cyclopentadienyl, X = alkenyl ligand), which have been suggested as intermediates in cycloaddition reactions. Indeed, cobalt systems have been used for more than a decade as active species in this kind of transformations, for which a two-state reactivity has been proposed. Most of the considered systems display a triplet ground state. However, in the case of a reaction intermediate, while a triplet ground state was predicted on the basis of Density Functional Theory results, our calculations suggest a singlet ground state. This stems from the competition between the exchange term (stabilising the triplet) and the accessibility of an intramolecular coordination (stabilising the singlet). This finding has an impact on the general mechanism of the cycloaddition reaction. Analogous rhodium systems were also studied and, as expected, they have a larger tendency to electron pairing than cobalt species.

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] (1) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: (E)-4-methyl-3-methylene-1,4-hexadiene and (3Z)-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Regio- and Chemoselective Copper-Catalyzed Formal [2+2+2] Cycloaddition of Primary Amines with Arylacetylenes to 2,4,5-Trisubstituted Pyridines

Disclosed herein is an efficient strategy for the synthesis of 2,4,5-trisubstituted pyridines via CuI/NBS-catalyzed formal intermolecular [2+2+2] cycloaddition of easily available primary amines and nonactivated terminal alkynes. Moreover, this given reaction features a new mode of cycloaddition with high regio- and chemoselectivity, good atom- and step-economy, broad substrate scope, and wide functional group compatibility. Further mechanism studies indicate that this transformation starts with oxidative alkynylation of the amine to form a propargylamine intermediate, followed by radical addition to the alkyne and intramolecular cycloaddition, delivering the pharmacologically interesting multisubstituted pyridines.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Palladium(II)-Catalyzed [2+2+1] Annulation of Alkynes and Hydroxylamines: A Rodox-Neutral Approach to Fully Substituted Pyrroles

A palladium-catalyzed [2+2+1] annulation of alkynes and hydroxylamines has been developed for the rapid construction of fully substituted pyrroles. This transformation involves sequential nucleophilic-addition of hydroxylamine to alkyne, alkyne migratory insertion, and synergistic demetallization cyclization, which provides a redox-neutral annulation approach to pyrrole derivatives. Moreover, the strategy enabled alteration of the photophysical properties of pyrrole products by varying the aryl substituents, thus leading to the development of N-functionalized tetraarylpyrroles as new fluorophores.