Hexadehydro-Diels-Alder Reaction: Benzyne Generation via Cycloisomerization of Tethered Triynes

Cascade reactions of HDDA-benzynes with tethered cyclohexadienones: strain-driven events originating from ortho-annulated benzocyclobutenes

Intramolecular net [2 + 2] cycloadditions between benzyne intermediates and an electron-deficient alkene to give benzocyclobutene intermediates are relatively rare. Benzynes are electrophilic and generally engage nucleophiles or electron-rich π-systems. We describe here reactions in which an alkene of a tethered enone traps thermally generated benzynes in a variety of interesting ways. The number of atoms that link the benzyne to C4 of a cyclohexa-2,5-dienone induces varying amounts of strain in the intermediates and products. This leads to a variety of different reaction outcomes by way of various strain-releasing events that are mechanistically intriguing. This work demonstrates an underappreciated class of strain that originates from the adjacent fusion of two rings to both C1-C2 and C2-C3 of a benzenoid ring - i.e. 'ortho-annulation strain'. DFT computations shed considerable light on the mechanistic diversions among various reaction pathways as well as allow more fundamental evaluation of the strain in a homologous series of ortho-annulated carbocycles.

2:1 Adducts Arising from Reactions between Benzynes and 1,3,4-Oxadiazoles

2:1 adducts arise from the reaction of 2,5-diaryl-1,3,4-oxadiazoles and benzynes generated from the hexadehydro-Diels-Alder (HDDA) reaction. Density functional theory computations support a mechanistic manifold that includes a concerted SNAr process. Additionally, the benzyne trapping reaction of 2,5-dimethyl-1,3,4-oxadiazole affords an unusual acylimine-containing 2:1 adduct, which is the first case in which a dearomatized product has arisen from a HDDA reaction.

Generation of sulfones utilizing β-sulfinyl esters as masked aryl sulfinates under redox-neutral conditions

A method for generation of SVI sulfones from β-sulfinyl esters (SIV) under transition-metal-free non-oxidative mild conditions is presented. Various sulfones have been achieved with moderate to excellent yields. The advantage of using β-sulfinyl esters as masked aryl sulfinates has also been exemplified using brominated substrates. Oxygen isotope-labeling experiments indicated that the oxygen atoms incorporated into the sulfone product come from the sulfoxide of the β-sulfinyl ester. Successive β-elimination/O-addition/sulfinate esterification/β-elimination processes are proposed for the mechanism of generating SVI from SIV.

Switchable chemoselective aryne reactions between nucleophiles and pericyclic reaction partners using either 3-methoxybenzyne or 3-silylbenzyne

Arynes are known to serve as highly reactive benzene-based synthons, which have gained numerous successes in preparing functionalized arenes. Due to the superb electrophilic nature of these fleeting species, however, it is challenging to modulate the designated aryne transformation chemoselectively, when substrates possess multiple competing reaction sites. Here, we showcase our effort to manipulate chemoselective control between two major types of aryne transformations using either 3-methoxybenzyne or 3-silylbenzyne, where nucleophilic addition-triggered reactions and non-polar pericyclic reactions could be differentiated. This orthogonal chemoselective protocol is found to be applicable between various nucleophiles, i.e., imidazole, N-tosylated/N-alkyl aniline, phenol, and alcohol, and an array of pericyclic reaction partners, i.e., furan, cyclopentadiene, pyrrole, cycloheptatrienone, and cyclohexene. Beyond arylation reactions, C-N bond insertion, Truce-Smiles rearrangement, and nucleophilic annulation are appropriate reaction modes as well. Moreover, this chemoselective protocol can find potential synthetic application.

Room Temperature Diels-Alder Reactions of 4-Vinylimidazoles

In the course of studying Diels-Alder reactions of 4-vinylimidazoles with N-phenylmaleimide, it was discovered that they engage in cycloaddition at room temperature to give high yields of the initial cycloadduct as a single stereoisomer. In certain cases, the product precipitated out of the reaction mixture and could be isolated by simple filtration, thereby avoiding issues with aromatization observed during chromatographic purification. Given these results, intramolecular variants using doubly activated dienophiles were also investigated at room temperature. Amides underwent cycloaddition at room temperature in modest yields, but the initial adducts were not isolable with Nimid-benzyl-protected systems. Attempts to extend these results to the corresponding esters and hydroxamate were less successful with these substrates only undergoing cycloaddition at elevated temperatures in lower yields. Density functional theory calculations were performed to evaluate the putative transition states for both the inter- and intramolecular variants to rationalize experimental observations.

Cascade Aryne Aminoarylation for Biaryl Phenol Synthesis

We describe a transition metal-free approach to hindered 3-amino-2-aryl phenols through a cascade nucleophilic addition / Smiles-Truce rearrangement of a functionalized Kobayashi aryne precursor. Under anionic conditions, secondary alkyl amines add to the aryne intermediate to set up an aryl transfer from a neighboring sulfonate group. The use of a sulfonate, rather than the more typical sulfonamide, enables access to phenolic biaryl products that are important motifs in natural products and pharmaceuticals.

Scalable Transition-Metal-Free Synthesis of Aryl Amines from Aryl Chlorides through X@RONa-Catalyzed Benzyne Formation

Aryl amines are highly useful organic chemicals, but large-scale, transition-metal-free syntheses of aryl amines are surprisingly underdeveloped. A mild and scalable (up to 500 mmol) aryl amine synthesis from benzyne chemistry was invented using easily accessible aryl chlorides as precursors, NaH as a stoichiometric base, and a new type of sodium alkoxide cluster, X@RONa, as a catalyst. The cluster catalyst X@RONa featured an externally hydrophobic dodecameric sodium alkoxide shell housing an encapsulated center anion. The cluster made from methoxy-tert-butanol was found to be the most effective. The intramolecular version of this reaction allowed the synthesis of indolines and indoles. Experimental and computational mechanistic studies revealed that the rate-determining step was likely the transport of solid NaH into the X@RONa cluster in the organic phase.

Phosphorane-Promoted C-C Coupling during Aryne Annulations

Arynes are fleeting, high-energy intermediates that undergo myriad trapping reactions by nucleophiles. Their unusual reactivity compared to other electrophiles can spur unexpected mechanistic pathways enroute to the formation of benzenoid products. Herein we explore a particularly unique case of thermally generated arynes reacting with phosphoranes to form helical dibenzothiophenes and -selenophenes. Multiple new helical polycyclic aromatic products are reported. DP4+ and X-ray crystallographic analysis were used in tandem to confirm the structural topologies of selected products and to demonstrate the utility of DP4+ for distinguishing between isomeric polycyclic aromatic compounds. Lastly, we discuss a plausible mechanism consistent with DFT computations that accounts for the product formation; namely, ligand coupling (i.e., reductive elimination) within a hypervalent, pentacarbon-ligated σ-phosphorane furnishes the dibenzothio- or dibenzoselenophene.

Access to N-Aryl (Iso)quinolones via Aryne-Induced Three-Component Coupling Reaction

N-Aryl (iso)quinolones are of increasing interest in material and medicinal chemistry, although general routes for their provision remain underexplored, especially when compared with its N-alkyl counterparts. Herein, we report a modular and transition-metal-free, aryne-induced three-component coupling protocol that allows the facile synthesis of structurally diverse N-aryl (iso)quinolones from readily accessible halo-(iso)quinolines in the presence of water. Preliminary results highlight the applicability of our method through scale-up synthesis, downstream derivatization, and flexible synthesis involving other types of aryne precursors.

Easy Access to Phenanthridinones via Metal-Free Cascade Benzannulation and C-N Bond Formation

A concise route for the synthesis of dihydrobenzo[j]phenanthridinones has been disclosed through an aryne annulation strategy under metal-free reaction conditions. The reaction involves multiple C-C and C-N bond cleavages/formations via Diels-Alder reaction, aromatization-driven C-N bond cleavage, and amide formation.

Access to Fully Substituted Dihydroindazoles Via Hexadehydro-Diels-Alder/[3 + 2] Cycloaddition

A cascade hexadehydro-Diels-Alder (HDDA)/[3 + 2] cycloaddition reaction between tetrayne and N,N'-cyclic acylhydrazone is described. This strategy allows the efficient construction of fully substituted 2,3-dihydro-1H-indazole scaffolds which have insecticidal activity against the third instar larvae of Mythimna separata.

Selective S-Arylation of Sulfenamides with Arynes: Access to Sulfilimines

Sulfilimines, the aza analogues of sulfoxides, are of increasing interest in medicinal and agrochemical research programs. However, the development of efficient routes for their synthesis has remained relatively unexplored. In this study, we report a transition metal-free, selective S-arylation reaction between sulfenamides and arynes, enabling the facile preparation of structurally diverse sulfilimines under mild and redox-neutral conditions in good yields. The application value of our method was further demonstrated by scale-up synthesis, downstream derivatization, and robustness screen.

Synthesis of Multisubstituted Aromatics via 3-Triazenylarynes

An efficient method for generating 3-triazenylarynes from ortho-iodoaryl triflate-type precursors was developed. The generated arynes reacted with various arynophiles with high regioselectivity because of the triazenyl group. The 3-triazenylaryne precursors functioned as useful intermediates of diverse multisubstituted aromatic compounds through the transformation of the remaining triazenyl group of aryne adducts and triazenyl group-directed ortho-C-H functionalization.

Copper-Catalyzed Aryne Insertion into the Carbon-Iodine Bond of Heteroaryl Iodides

Aryne insertions into the carbon-iodine bond of heteroaryl iodides has been achieved for the first time. This novel reaction provides an efficient pathway for the synthesis of valuable building blocks 2-iodoheterobiaryls from heteroaryl iodides and o-silylaryl triflates in excellent regioselectivity. The copper(I) catalyst, which bears a N-heterocyclic carbene (NHC) ligand, is essential to accomplish the reaction. Control reactions and DFT calculations indicate that the coordination of copper, as a Lewis acid, with nitrogen atoms of heteroaryl iodides mediates the insertion of arynes into heteroaryl carbon-iodine bonds.

Sulfonium-based precise alkyl transposition reactions

S-adenosyl-L-methionine (SAM), a sulfonium-based cofactor, plays an important role in numerous biological processes as methyl donor. Inspired by the function of sulfonium motif in this nature's synthetic toolkit, we here present an aryne-activation strategy that the sulfonium intermediates in situ generated from thioethers display unique reactivity toward alkyl group transposition. Experimental and theoretical studies indicate that the reaction occurs in an intermolecular fashion where the TfO--incorporated [K(18-crown-6)] complex acts as a key promoter for this thermodynamically favored process. Next, a series of robust, easy-to-prepare sulfonium salts are designed and developed as electrophilic alkylation reagents accordingly. Both systems feature for broad scope, excellent selectivity, and simple operation. Moreover, we highlight the synthetic value through molecular editing and late-stage modification of complex scaffolds or even active pharmaceutical ingredients.

Lewis acid-catalyzed diastereoselective carbofunctionalization of bicyclobutanes employing naphthols

Traditional radical-mediated ring-opening of bicyclo[1.1.0]butanes (BCBs) for cyclobutane synthesis suffers from poor diastereoselectivity. Although few reports on BCB ring-opening via polar mechanisms are available, the Lewis acid-catalyzed diastereoselective ring-opening of BCBs using carbon nucleophiles is still underdeveloped. Herein, we report a mild and diastereoselective Bi(OTf)₃-catalyzed ring-opening of BCBs employing 2-naphthols. The anticipated carbofunctionalized trisubstituted cyclobutanes were obtained via a bicoordinated bismuth complex and the products are formed in good to excellent yields with high regio- and diastereoselectivity. The scope of the reaction was further extended using electron-rich phenols and naphthylamine. The functionalization of the synthesized trisubstituted cyclobutanes shows the synthetic utility of the present method.

The contrasting reactivity of trans- vs. cis-azobenzenes (ArN[double bond, length as m-dash]NAr) with benzynes

We report here a study that has revealed two distinct modes of reactivity of azobenzene derivatives (ArN[double bond, length as m-dash]NAr) with benzynes, depending on whether the aryne reacts with a trans- or a cis-azobenzene geometric isomer. Under thermal conditions, trans-azobenzenes engage benzyne via an initial [2 + 2] trapping event, a process analogous to known reactions of benzynes with diarylimines (ArC[double bond, length as m-dash]NAr). This is followed by an electrocyclic ring opening/closing sequence to furnish dihydrophenazine derivatives, subjects of contemporary interest in other fields (e.g., electronic and photonic materials). In contrast, when the benzyne is attacked by a cis-azobenzene, formation of aminocarbazole derivatives occurs via an alternative, net (3 + 2) pathway. We have explored these complementary orthogonal processes both experimentally and computationally.

1,3-Butadiynamides the Ethynylogous Ynamides: Synthesis, Properties and Applications in Heterocyclic Chemistry

1,3-butadiynamides-the ethynylogous variants of ynamides-receive considerable attention as precursors of complex molecular scaffolds for organic and heterocyclic chemistry. The synthetic potential of these C4-building blocks reveals itself in sophisticated transition-metal catalyzed annulation reactions and in metal-free or silver-mediated HDDA (Hexa-dehydro-Diels-Alder) cycloadditions. 1,3-Butadiynamides also gain significance as optoelectronic materials and in less explored views on their unique helical twisted frontier molecular orbitals (Hel-FMOs). The present account summarizes different methodologies for the synthesis of 1,3-butadiynamides followed by the description of their molecular structure and electronic properties. Finally, the surprisingly rich chemistry of 1,3-butadiynamides as versatile C4-building blocks in heterocyclic chemistry is reviewed by compiling their exciting reactivity, specificity and opportunities for organic synthesis. Besides chemical transformations and use in synthesis, a focus is set on the mechanistic understanding of the chemistry of 1,3-butadiynamides-suggesting that 1,3-butadiynamides are not just simple alkynes. These ethynylogous variants of ynamides have their own molecular character and chemical reactivity and reflect a new class of remarkably useful compounds.

Diastereoselective, Lewis acid-mediated Diels-Alder reactions of allenoic acid derivatives and 1,3-cyclopentadienes

Allenes with different substituents at their terminal carbon atom display axial chirality and can be obtained in enantiopure form by a photochemical deracemization protocol. It has now been studied under which conditions allenoic acid derivatives undergo a Diels-Alder reaction with 1,3-cyclopentadienes and which products result. Cyclic derivatives (lactams, lactones) underwent an exo-selective reaction catalyzed by the Lewis acid Eu(fod)₃, while acyclic derivatives yielded with high preference the endo-products (EtAlCl₂ as the preferred Lewis acid). The exocyclic double bond forms with exquisite diastereoselectivity and the chirality transfer is close to perfect. The method was applied to the synthesis of the sesquiterpenes β-santalol (1) and 10(E)-β-santalic acid (13).

A Distinct Mode of Strain-Driven Cyclic Allene Reactivity: Group Migration to the Central Allene Carbon Atom

Strained cyclic allenes are reactive species that can be trapped in a variety of complementary fashions that capitalize on their inherent high potential energy. 1,2,4-Cyclohexatrienes represent a subclass of allenes that, notably, can be conveniently generated by a net [4 + 2] cycloaddition within a 1,3-enyne bearing a tethered alkyne via a tetradehydro-Diels-Alder reaction. A limitation to the use of this type of thermally generated cyclic allene as a construct for the introduction of molecular complexity is their propensity to isomerize to benzenoids via a simple net 1,5-hydrogen atom migration. We have discovered that when the enyne component of the substrate is modified as an enol silyl ether (or an enol ester), migration of the silyl (or acyl) group can become the predominant event. Specifically, an appropriately electrophilic group can migrate from the O atom to the central allene carbon adjacent to the 1-siloxy(acyloxy) substituent. This process leads to highly substituted phenolic products (e.g., o-silyl phenols) following tautomerization of the intermediate cyclohexa-2,4-dienone. Experimental studies show that this novel mode of reactivity is general; DFT studies reveal the unimolecular nature of the group migration.

Empowering boronic acids as hydroxyl synthons for aryne induced three-component coupling reactions

Boronic acids have become one of the most prevalent classes of reagents in modern organic synthesis, displaying various reactivity profiles via C-B bond cleavage. Herein, we describe the utilization of a readily available boronic acid as an efficient surrogate of hydroxide upon activation via fluoride complexation. The hitherto unknown aryne induced ring-opening reaction of cyclic sulfides and three-component coupling of fluoro-azaarenes are developed to exemplify the application value. Different from metal hydroxides or water, this novel hydroxy source displays mild activation conditions, great functionality tolerance and structural tunability, which shall engender a new synthetic paradigm and in a broad context offer new blueprints for organoboron chemistry. Detailed computational studies also recognize the fluoride activation mode, provide in-depth insights into the unprecedented mechanistic pathway and elucidate the reactivity difference of ArB(OH) x F y complexes, which fully support the experimental data.

3-sulfonyloxyaryl(mesityl)iodonium triflates as 1,2-benzdiyne precursors with activation via ortho-deprotonative elimination strategy

Benzyne has long captivated the attention of chemists and has gained numerous synthetic achievements. Among typical benzyne generation methods, removal of two vicinal substituents from 1,2-difunctionalized benzenes, i.e., Kobayashi's protocol, are prevailing, while ortho-deprotonative elimination from mono-substituted benzene lags far behind. Despite the advantages of atom economy and ready achievability of precursors, a bottle neck for ortho-deprotonative elimination strategy resides in the weak acidity of the ortho-hydrogen, which normally demands strong bases as the activating reagents. Here, an efficient aryne generation protocol is developed, where ortho-deprotonative elimination on 3-sulfonyloxyaryl(mesityl)iodonium triflates occurs under mild conditions and the generated 3-sulfonyloxyarynes can serve as efficient 1,2-benzdiyne synthons. This array of 1,2-benzdiyne precursors can be conveniently prepared with high functional group tolerance, and densely substituted scaffolds can be accessed as well. Carbonate and fluoride salts are found to serve as efficient activating reagents, which are the weakest bases used in ortho-deprotonative elimination strategies. Particularly, this scaffold has predictable chemoselective generation of the designated aryne intermediates. The success of this ortho-deprotonative elimination protocol sets up a unique platform with a broad spectrum of synthetic applications.

Asymmetric reactions involving aryne intermediates

Although arynes are usually considered fleeting intermediates, they are highly valuable synthons because they enable the introduction of aromatic rings and the simultaneous formation of new bonds at two sites. Although catalytic reactions using transition metals are excellent method for constructing complex polycyclic aromatic molecules in a single step, the use of asymmetric catalysis for the capture of arynes remains a crucial goal for the progress of aryne chemistry. Catalytic asymmetric reactions of arenes are challenging, requiring sufficient interactions between the neutral and highly reactive short-lived aryne intermediates in a stereo-controlled fashion. In addition, spontaneous decomposition, as well as side reactions, has hindered their development and, until recently, highly enantioselective reactions using arynes had remained elusive. This Review highlights asymmetric reactions using arynes, featuring diastereoselective, enantioselective and catalytic enantioselective reactions.

Sequential Strategies to Trigger Mild Dearomative Diels-Alder Cyclizations

Dihydronaphthalenes are present in several functional molecules, but their assembly is challenging. However, a sequential strategy can induce the key annullation of an alkyne with a vinylarene under mild conditions. Products form in good yields, with ample functional tolerance via domino nucleophilic substitution and dearomative Diels-Alder and ene reactions. DFT modeling data show that alkali cations are crucial to ensure a smooth dearomative cyclization on the in situ generated intermediates.

Isolation, Reactivity, and Tunable Properties of a Strained Antiaromatic Osmacycle

Strain and antiaromaticity in compounds are recognized as two substantial destabilizing features, and consequently, realization of dual destabilizing features in a single molecule is challenging and far more difficult in a single ring. Moreover, transformation of an antiaromatic framework to different antiaromatic or aromatic species is a significant subject in antiaromatic chemistry and has attracted increasing interest. In this work, we isolated a highly strained antiaromatic metallacycle in which a cyclic metal vinylidene unit is embedded. Computational studies revealed its ring strain energies and antiaromatic character and showed that the metal incorporation and the phosphonium substituents play a crucial role in its stabilization. The mechanism of its formation has been illustrated by density functional theory (DFT) calculations and the isolation of a key intermediate. We further discovered diverse reactivities and structural reshuffling of this unusual strained antiaromatic complex according to its two destabilizing characters. We obtained two isomers of metallaindenes fused with oxiranes from the direct oxidation of the metal vinylidene or by nucleophilic addition to an isolated metallacyclocumulene formed by the reaction of metal vinylidene with hydroxide ion, achieving a reconfiguration of the antiaromatic framework. Transformations of the antiaromatic metallacycle by electrophiles to various aromatic metallaindynes have been achieved, and that a condensed Fukui function was employed to confirm the regioselectivity of the electrophilic additions, and the acid/base-induced aromaticity switch along with tunable photophysical properties were investigated. These interesting transformations not only enrich the chemistry of metal vinylidenes and antiaromatics and could also perform potentially as switchable optical materials.

Thermodynamics Evaluation of Selective Hydride Reduction for α,β-Unsaturated Carbonyl Compounds

The selective reduction of α,β-unsaturated carbonyl compounds is one of the core reactions and also a difficult task for organic synthesis. We have been attempting to study the thermodynamic data of these compounds to create a theoretical basis for organic synthesis and computational chemistry. By electrochemical measurement method and titration calorimetry, in acetonitrile at 298 K, the hydride affinity of two types of unsaturated bonds in α,β-unsaturated carbonyl compounds, their single-electron reduction potential, and the single-electron reduction potential of the corresponding radical intermediate are determined. Their hydrogen atom affinity, along with the hydrogen atom affinity and proton affinity of the corresponding radical anion, is also derived separately based on thermodynamic cycles. The above data are used to establish the corresponding "Molecule ID Card" (Molecule identity card) and analyze the reduction mechanism of unsaturated carbonyl compounds. Primarily, the mixture of any carbonyl hydride ions and Ac-tempo⁺ will stimulate hydride transfer process and create corresponding α,β-unsaturated carbonyl compounds and Ac-tempoH from a thermodynamic point of view.

Advances in the synthesis of nitrogen-containing heterocyclic compounds by in situ benzyne cycloaddition

Nitrogen-containing heterocyclic compounds are prevalent in various natural products, medicines, agrochemicals, and organic functional materials. Among strategies to prepare nitrogen-containing heterocyclic compounds, pathways involving benzyne intermediates are attractive given that they can readily assemble highly diverse heterocyclic compounds in a step-economical manner under transition-metal-free conditions. The synthesis of nitrogen-containing heterocyclic compounds from benzyne intermediates offers an alternative strategy to the conventional metal-catalyzed activation approaches. In the past years, chemists have witnessed the revival of benzyne chemistry, mainly attributed to the wide application of various novel benzyne precursors. The cycloaddition of benzynes is a powerful tool for the synthesis of nitrogen-containing heterocyclic compounds, which can be constructed by [n + 2] cyclization of benzyne intermediates in situ generated from benzyne precursors under mild reaction conditions. This review focuses on the application of cycloaddition reactions involving in situ benzynes in the construction of various nitrogen-containing heterocyclic compounds.

Construction of Functionalized 2H-Pyran-2-ones via N-Heterocyclic Carbene-Catalyzed [3 + 3] Annulation of Alkynyl Esters with Enolizable Ketones

A new synthesis of functionalized 2H-pyran-2-ones has been developed through N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynyl esters with enolizable ketones. The key to the success of this protocol relies on the use of an NHC instead of a tertiary amine as the catalyst. This protocol also features a broad substrate scope and mild metal-free conditions, offering simple and rapid access to the target molecules in a highly regioselective manner.

Synergism of Fe/Ti Enabled Regioselective Arene Difunctionalization

Regioselective difunctionalization of arenes remains a long-standing challenge in organic chemistry. We report a novel and general Fe/Ti synergistic methodology for regioselective synthesis of various polysubstituted arenes through either E/E' or Nu/E ortho difunctionalizations of arenes. Preliminary results showed that an unprecedented 1,2-Fe/Ti heterobimetallic arylene intermediate bearing two distinct C-M bonds is essential to the regioselective difunctionalization.

Fused diethynylbenzenes and phenanthrenes via arynes with alkynylsilanes

A method for the hydroalkynylation and catalytic cyclization reactions of hexadehydro-Diels-Alder-derived benzynes is described. Diethynylbenzene derivatives are generated in a one-step reaction via trimethylsilyl-alkyne groups with benzyne formed by heating the appropriate tetrayne substrate. Trimethyl(phenylethynyl)silane loses TMS and binds to the electron-deficient site on HDDA-derived benzynes, and then phenanthrene was synthesized under mild reaction conditions by transition-metal-free, base promoted intramolecular cyclization.

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

Enantiospecific Syntheses of Congested Atropisomers through Chiral Bis(aryne) Synthetic Equivalents

The synthetic equivalents of the enantiopure binaphthyl bis(aryne) atropisomers derived from BINOL (1,1'-bi-2,2'-naphtol) featuring a stereogenic axis vicinal to the two reactive triple bonds can be generated for the first time in solution in an enantiospecific manner. Using a two-step sequence based on the bidirectional [4+2] cycloaddition of furan derivatives followed by an aromatizative deoxygenation reaction, several 9,9'-bianthracenyl-based atropisomers could be prepared enantiospecifically in high enantiomeric purity. Alternatively, bidirectional reactions with anthracene, 2-bromostyrene, and perylene as the arynophiles afforded very congested bis(benzotriptycene), bis(tetraphene) and bis(anthra[1,2,3,4-ghi]perylene) nanocarbon atropisomers in equally high enantiomeric purity. In complement, cross reactions with two different arynophiles revealed possible. The unusual atropisomer prototypes described in this study open the way to enantiopure nanographene atropisomers designed for functions.

Porphyryne

Density functional theory calculations with the B3LYP*-D3 method with large STO-QZ4P basis sets unambiguously predict a singlet ground state for Zn-porphyryne. However, the calculations also predict a low singlet-triplet gap of about 0.4 eV and a high adiabatic electron affinity of 2.4 eV. Accordingly, the reactivity of porphyryne species may be dominated by electron transfer, hydrogen abstraction, and proton-coupled electron transfer processes.

Diverse Synthesis of Triarylselenonium Salts and o-(Alkoxy)aryl Aryl Selanes via Insertion of Benzyne into the Se═O Bond

An insertion of benzyne into the Se═O bond has been realized. In this reaction, two types of compounds containing selenium could be synthesized from different substrates. When diaryl selenium oxides were used, triarylselenonium salts were furnished, while o-(alkoxy)phenyl phenyl selanes could be produced from aryl alkyl selenium oxides.

Determination of the Rate Constant of the [4 + 2] Cycloaddition Between an Aryne Atropisomer and Furan in Solution

Using a specially designed prototype of a nonracemic aryne atropisomer with a low barrier to enantiomerization (ca. 36 kJ·mol^-1), it was possible to determine the kinetic constant of its cycloaddition with furan in solution by a combination of theoretical calculations and experimental measurements. It was found that the reaction half-life of this aryne atropisomer in solution with 100 equivalents of furan as the trapping reagent is <150 ns at temperatures above -20 °C.

Synthesis of Trisubstituted Oxazoles via Aryne Induced [2,3] Sigmatropic Rearrangement-Annulation Cascade

A transition-metal-free, [2,3] sigmatropic rearrangement-annulation cascade of 2-substituted thio/amino acetonitriles with arynes allowing the synthesis of 2,4,5-trisubstituted oxazoles under mild conditions has been demonstrated. The key sulfur/nitrogen ylides were generated by the initial S/N arylation followed by proton transfer, which was followed by the selective [2,3] sigmatropic rearrangement involving the -CN moiety and a subsequent annulation to afford the desired products in reasonable yields.

Quaternary Ammonium Ion-Tethered (Ambient-Temperature) HDDA Reactions

The hexadehydro-Diels-Alder (HDDA) reaction converts a 1,3-diyne bearing a tethered alkyne (the diynophile) into bicyclic benzyne intermediates upon thermal activation. With only a few exceptions, this unimolecular cycloisomerization requires, depending on the nature of the atoms connecting the diyne and diynophile, reaction temperatures of ca. 80-130 °C to achieve a convenient half-life (e.g., 1-10 h) for the reaction. In this report, we divulge a new variant of the HDDA process in which the tether contains a central, quaternized nitrogen atom. This construct significantly lowers the activation barrier for the HDDA cycloisomerization to the benzyne. Moreover, many of the ammonium ion-based, alkyne-containing substrates can be spontaneously assembled, cyclized to benzyne, and trapped in a single-vessel, ambient-temperature operation. DFT calculations provide insights into the origin of the enhanced rate of benzyne formation.

The Stannum-Ene Reactions of Benzyne and Cyclohexyne with Superb Chemoselectivity for Cyclohexyne

The stannum-ene reactions of both benzyne and cyclohexyne were realized, which is particularly suitable for cyclohexyne with a broad substrate scope and excellent chemoselectivity. Our DFT calculations via distortion/interaction analysis revealed that both stannum- and hydrogen-ene reactions with cyclohexyne have later transition states due to their higher distortion energies in the transition states than those in benzyne reactions, which lead to enhanced Pauli repulsion as the decisive factor in the interaction energy accompanied with enlarged energy gap between two types of ene reactions. Therefore, excellent chemoselectivity was disclosed in the cyclohexyne-ene reaction.

Intramolecular rhodium-catalysed [2 + 2 + 2] cycloaddition of linear chiral N-bridged triynes: straightforward access to fused tetrahydroisoquinoline core

A route for the preparation of merged symmetrical tetrahydroisoquinolines with central chirality through a rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition involving enantiopure triynes as substrates is described. The results show that linear triynes lacking a 3-atom tether can undergo efficient cyclisation. The N-tethered 1,7,13-triynes used in our approach were easily prepared from readily accessible chiral homopropargyl amides, the basic building blocks in our approach, which were efficiently obtained by diastereoselective addition of propargyl magnesium bromide to Ellman imines. Additional substitution at the benzene rings could be attained when substituted triynes at the terminal triple bonds were employed, giving access to more complex tetrahydroisoquinolines after the rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition. Among the different transition-metal catalysts, the Wilkinson complex (RhCl(PPh₃)₃) afforded higher yields in the cyclisation of linear triynes; however, triynes bearing a Br substituent at the terminal positions underwent the cyclisation more efficiently in the presence of [RhCl(CO)₂]₂.

Convenient synthesis of dipeptide structures in solution phase assisted by a thioaza functionalized magnetic nanocatalyst

In this study, a heterogeneous nanocatalyst is presented that is capable to efficiently catalyze the synthetic reactions of amide bond formation between the amino acids. This nanocatalyst which is named Fe₃O₄@SiO₂/TABHA (TABHA stands for thio-aza-bicyclo-hepten amine), was composed of several layers that increased the surface area to be functionalized with 2-aminothiazole rings via Diels-Alder approach. Firstly, various analytic methods such as Fourier-transform infrared (FTIR) and energy-dispersive X-ray (EDX) spectroscopic methods, thermogravimetric analysis (TGA), electron microscopy (EM), and UV-vis diffuse reflectance spectroscopy (UV-DRS) have been used to characterize the desired structure of the Fe₃O₄@SiO₂/TABHA catalyst. Afterward, the application of the presented catalytic system has been studied in the peptide bond formation reactions. Due to the existence of a magnetic core in the structure of the nanocatalyst, the nanoparticles (NPs) could be easily separated from the reaction medium by an external magnet. This special feature has been corroborated by the obtained results from vibrating-sample magnetometer (VSM) analysis that showed 24 emu g^-1 magnetic saturation for the catalytic system. Amazingly, a small amount of Fe₃O₄@SiO₂/TABHA particles (0.2 g) has resulted in ca. 90% efficiency in catalyzing the peptide bond formation at ambient temperature, over 4 h. Also, this nanocatalyst has demonstrated an acceptable recycling ability, where ca. 76% catalytic performance has been observed after four recycles. Due to high convenience in the preparation, application, and recyclization processes, and also because of lower cost than the traditional coupling reagents (like TBTU), the presented catalytic system is recommended for the industrial utilization.