The Hexadehydro-Diels-Alder Cycloisomerization Reaction Proceeds by a Stepwise Mechanism

Rapid (≤25 °C) cycloisomerization of anhydride-tethered triynes to benzynes - origin of a remarkable anhydride linker-induced rate enhancement

The hexadehydro-Diels-Alder (HDDA) reaction is a cycloisomerization between a conjugated diyne and a tethered diynophile that generates ortho-benzyne derivatives. Considerable fundamental understanding of aryne reactivity has resulted from this body of research. The multi-yne cycloisomerization substrate is typically pre-formed and the (rate-limiting) closure of this diyne/diynophile pair to produce the isomeric benzyne generally requires thermal input, often requiring reaction temperatures of >100 °C and times of 16-48 h to achieve near-full conversion. We report here that diynoic acids can be dimerized and that the resulting substrate, having a 3-atom anhydride linker (i.e., O[double bond, length as m-dash]COC[double bond, length as m-dash]O), then undergoes HDDA cyclization within minutes at or below room temperature. This allows for the novel in situ assembly and cyclization of HDDA benzyne precursors in an operationally simple protocol. Experimental kinetic data along with DFT computations are used to identify the source of this surprisingly huge rate acceleration afforded by the anhydride linker: >107 faster than the analogous multi-yne having, instead, a CH2OCH2 ether linker.

Alder-ene reactions of arynes to form medium-sized and macrocyclic frameworks of sizes up to a 46-membered ring

Macrocyclization via the intramolecular Alder-ene reaction of arynes to construct carbo- and hetero-macrocycles fused with an indoline or isoindoline moiety is described. By installing ether, ester, alkene, and cyclic tethers at an appropriate location between the aryne and the ene donor, macrocycles up to a 46-membered ring could be constructed.

Single-Pot Reaction of Novel Fused Indolizine Derivatives

An unprecedented fused hexacyclic indolizine derivative generated by a single-pot reaction has been developed. This model overcomes the use of catalysts, inefficient atom economy, low yields, and limitations of lengthy steps. Density functional theory calculations reveal the reaction mechanism in which the oxygen molecule plays a crucial role in trapping the penultimate zwitterionic or biradical intermediate to generate the observed products.

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.

Light-Induced Generation and Cycloaddition Reactions of Benzyne: Synthesis of Naphthoxindoles E and Annulated Indolizines

By virtue of their high electrophilic nature, benzynes serve as reactive dienophiles in numerous cycloaddition reactions. However, in situ generation of benzyne involves either base-mediated thermal reactions, low-temperature conditions, or metal-catalyzed reactions of substituted arenes. This limits the applicability of benzynes as suitable dipolarophiles in cycloaddition reactions. Herein, we have reported a UVA (365 nM)-induced in situ generation of benzynes (from triazenyl benzoic acid) and subsequently their [4 + 2] Diels-Alder and [3 + 2] cycloaddition reactions with appropriate reaction partners such as N-protected alkylidene oxindole carboxylates and pyridinium ylides to afford naphthoxindoles E and pyrido[2,1-a]isoindole, respectively, in moderate to excellent yield. The reactions occurred at room temperature and under reagent-free reaction conditions. Each of these building blocks is pharmaceutically relevant; hence, this highlights an interesting strategy to access these classes of compounds.

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.

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.

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.

Development of an Allenyne-Alkyne [4+2] Cycloaddition and its Application to Total Synthesis of Selaginpulvilin A

A new [4+2] cycloaddition of allenyne-alkyne is developed. The reaction is believed to proceed with forming an α,3-dehydrotoluene intermediate. This species behaves as a σπ-diradical to react with a hydrogen atom donor, whereas it displays a zwitterionic reactivity toward weak nucleophiles. The efficiency of trapping α,3-dehydrotoluene depends not only on its substituents but also the trapping agents. Notable features of the reaction are the activating role of the extra alkyne of the 1,3-diyne that reacts with the allenyne moiety and the opposite mode of trapping with oxygen and nitrogen nucleophiles. Oxygen nucleophiles result in the oxygen-end incorporation at the benzylic position of the α,3-dehydrotoluene, whereas with amine nucleophiles the nitrogen-end is incorporated into the aromatic core. Relying on the allenyne-alkyne cycloaddition as an enabling strategy, a concise total synthesis of phosphodiesterase-4 inhibitory selaginpulvilin A is realized.

Expedient Access to Polyaromatic Biaryls by Unconventional Ag-Catalyzed Cycloaromatization of Alkynylthiophenes and Au-Catalyzed Double C-H Activation

An unconventional approach for the regioselective synthesis of polyaromatic biaryls via site-selective Ag-catalyzed twofold electrophilic cycloisomerization followed by Au-catalyzed double C-H activation is described. The developed process allows the synthesis of highly decorated biaryls with excellent regioselectivity. As revealed by DFT computations, the reaction represents a rare example of C1-C5 endo-exo and C1-C6 endo-endo cycloaromatization. The formation of the 6-membered ring is predicted to be the fruit of an uncommon SEAr on a vinyl carbocation.

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.

Carbene catalyzed C(sp3)–Cl activation of chlorinated solvents for benzyne chlorination

A new mode of N-heterocyclic carbene (NHC) organocatalysis was discovered, in which the normally inert chlorinated solvents were activated by carbene to realize the chlorination of hexa-dehydro-Diels-Alder derived benzynes.

Silicon as a powerful control element in HDDA chemistry: redirection of innate cyclization preferences, functionalizable tethers, and formal bimolecular HDDA reactions

The 1,3-diyne and diynophile in hexadehydro-Diels–Alder (HDDA) reaction substrates are typically tethered by linker units that consist of C, O, N, and/or S atoms. We describe here a new class of polyynes based on silicon-containing tethers that can be disposed of and/or functionalized subsequent to the HDDA reaction. The cyclizations are efficient, and the resulting benzoxasiloles are amenable to protodesilylation, halogenation, oxygenation, and arylation reactions. The presence of the silicon atom can also override the innate mode of cyclization in some cases, an outcome attributable to a β-silyl effect on the structure of intermediate diradicals. Overall, this strategy equates formally to an otherwise unknown, bimolecular HDDA reaction and expands the versatility of this body of aryne chemistry.

A designer silicon-containing linker enables HDDA chemistry that complements known modes of reactivity. Subsequent removal of the Si liberates a benzenoid product that is formally the result of an intermolecular HDDA reaction.

Fully Substituted Conjugate Benzofuran Core: Multiyne Cascade Coupling and Oxidation of Cyclopropenone

An unprecedented C═C double bond cleavage of cyclopropenone and dioxygen activation by multiyne cascade coupling has been developed. This chemistry provides a novel, simple, and efficient approach to synthesize fully substituted conjugate benzofuran derivatives from simple substrates under mild conditions. The density functional theory (DFT) calculations reveal that the unique homolytic cleavages of cyclopropenone and molecular oxygen are crucial to the success of this reaction.

Copper-catalyzed asymmetric cyclization of alkenyl diynes: method development and new mechanistic insights

Metal carbenes have proven to be one of the most important and useful intermediates in organic synthesis, but catalytic asymmetric reactions involving metal carbenes are still scarce and remain a challenge. Particularly, the mechanistic pathway and chiral induction model in these asymmetric transformations are far from clear. Described herein is a copper-catalyzed asymmetric cyclization of alkenyl diynes involving a vinylic C(sp²)–H functionalization, which constitutes the first asymmetric vinylic C(sp²)–H functionalization through cyclopentannulation. Significantly, based on extensive mechanistic studies including control experiments and theoretical calculations, a revised mechanism involving a novel type of endocyclic copper carbene via remote-stereocontrol is proposed, thus providing new mechanistic insight into the copper-catalyzed asymmetric diyne cyclization and representing a new chiral control pattern in asymmetric catalysis based on remote-stereocontrol and vinyl cations. This method enables the practical and atom-economical construction of an array of valuable chiral polycyclic-pyrroles in high yields and enantioselectivities.

A copper-catalyzed asymmetric cyclization of alkenyl diynes involving a vinylic C(sp²)–H functionalization is reported, enabling the construction of various valuable chiral polycyclic-pyrroles in high yields and enantioselectivities.

Mechanistic and Kinetic Factors of ortho-Benzyne Formation in Hexadehydro-Diels-Alder (HDDA) Reactions

With the rapid development of the hexadehydro-Diels-Alder reaction (HDDA) from its first discovery in 1997, the question of whether a concerted or stepwise mechanism better describes the thermally activated formation of ortho-benzyne from a diyne and a diynophile has been debated. Mechanistic and kinetic investigations were able to show that this is not a black or white situation, as minor changes can tip the balance. For that reason, especially, linked yne-diynes were studied to examine steric, electronic, and radical-stabilizing effects of their terminal substituents on the reaction mechanism and kinetics. Furthermore, the influence of the nature of the linker on the HDDA reaction was explored. The more recently discovered photochemical HDDA reaction also gives ortho-arynes, which display the same reactivity as the thermally generated ones, but their formation might not proceed by the same mechanism. This minireview summarizes the current state of mechanistic understanding of the HDDA reaction.

"Kobayashi Benzynes" as Hexadehydro-Diels-Alder Diynophiles

We report here elaboration of a cascade strategy for naphthyne formation using Kobayashi benzynes as diynophiles in a subsequent in situ hexadehydro-Diels-Alder reaction. Density functional theory computations suggest that the strained benzynes act as "super-diynophiles" in this transformation. The reaction requires only mildly basic and ambient temperature conditions and allows for rapid construction of various naphthalenic products. The trapping efficiencies of several arynophiles were determined using the benzyne to naphthyne cyclization as an internal clock reaction.

Gold-Catalyzed Formal Hexadehydro-Diels-Alder/Carboalkoxylation Reaction Cascades

A dual gold-catalyzed hexadehydro-Diels-Alder/carboalkoxylation cascade reaction is reported. In this transformation, the gold catalyst participated in the hexadehydro-Diels-Alder step, switching the mechanism from a radical type to a cationic one, and then the catalyst activated the resulting aryne to form an ortho-Au phenyl cation species, which underwent a carboalkoxylation rearrangement rather than the expected aryne-ene reaction.

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

The hexadehydro-Diels-Alder (HDDA) reaction is the thermal cyclization of an alkyne and a 1,3-diyne to generate a benzyne intermediate. This is then rapidly trapped, in situ, by a variety of species to yield highly functionalized benzenoid products. In contrast to nearly all other methods of aryne generation, no other reagents are required to produce an HDDA benzyne. The versatile and customizable nature of the process has attracted much attention due not only to its synthetic potential but also because of the fundamental mechanistic insights the studies often afford. The authors have attempted to provide here a comprehensive compilation of publications appearing by mid-2020 that describe experimental results of HDDA reactions.

Facile preparation and conversion of 4,4,4-trifluorobut-2-yn-1-ones to aromatic and heteroaromatic compounds

The concise preparation of 4,4,4-trifluorobut-2-yn-1-ones by the oxidation of the readily accessible corresponding propargylic alcohols as well as their utilization as Michael acceptors for the construction of aromatic and heteroaromatic compounds are reported.

Selectivity between an Alder–ene reaction and a [2 + 2] cycloaddition in the intramolecular reactions of allene-tethered arynes

Substituent-dependent reactivity and selectivity in the intramolecular reactions of arynes tethered with an allene are described.

Direct observation of o-benzyne formation in photochemical hexadehydro-Diels-Alder (hν-HDDA) reactions

Reactive ortho-benzyne derivatives are believed to be the initial products of liquid-phase [4 + 2]-cycloadditions between a 1,3-diyne and an alkyne via what is known as a hexadehydro-Diels–Alder (HDDA) reaction. The UV/VIS spectroscopic observation of o-benzyne derivatives and their photochemical dynamics in solution, however, have not been reported previously. Herein, we report direct UV/VIS spectroscopic evidence for the existence of an o-benzyne in solution, and establish the dynamics of its formation in a photoinduced reaction. For this purpose, we investigated a bis-diyne compound using femtosecond transient absorption spectroscopy in the ultraviolet/visible region. In the first step, we observe excited-state isomerization on a sub-10 ps time scale. For identification of the o-benzyne species formed within 50–70 ps, and the corresponding photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions, we employed two intermolecular trapping strategies. In the first case, the o-benzyne was trapped by a second bis-diyne, i.e., self-trapping. The self-trapping products were then identified in the transient absorption experiments by comparing their spectral features to those of the isolated products. In the second case, we used perylene for trapping and reconstructed the spectrum of the trapping product by removing the contribution of irrelevant species from the experimentally observed spectra. Taken together, the UV/VIS spectroscopic data provide a consistent picture for o-benzyne derivatives in solution as the products of photo-initiated HDDA reactions, and we deduce the time scales for their formation.

We report the transient ultraviolet/visible absorption spectrum of an o-benzyne species in solution for the first time.

Copper-Catalyzed Asymmetric Reaction of Alkenyl Diynes with Styrenes by Formal [3 + 2] Cycloaddition via Cu-Containing All-Carbon 1,3-Dipoles: Access to Chiral Pyrrole-Fused Bridged [2.2.1] Skeletons

The generation of metal-containing 1,3-dipoles from metal carbenes represents a significant advance in 1,3-dipolar cycloaddition reactions. However, these transformations have so far been limited to reactions based on diazo compounds or triazoles as precursors. Herein, we disclose a copper-catalyzed enantioselective reaction of alkenyl N-propargyl ynamides with styrene derivatives by formal [3 + 2] cycloaddition via Cu-containing all-carbon 1,3-dipoles, which constitutes a novel way for the generation of metal-containing 1,3-dipoles via metal carbenes. This protocol allows the practical and atom-economical synthesis of valuable chiral pyrrole-fused bridged [2.2.1] skeletons in moderate to good yields (up to 90% yield) with excellent diastereoselectivities (dr > 50/1) and generally excellent enantioselectivities (up to >99% ee).

Access to Benzoxazepines and Fully Substituted Indoles via HDDA Coupling

The HDDA-derived benzyne intermediate was captured by oxazolines based on the addition reaction of benzyne to the C═N double bond. Benzoxazepine derivatives, fused benzoxazepine derivatives, and fully substituted indoles are synthesized in one step. The reaction does not require any catalyst or additives. Possible reaction mechanisms are presented.

Consecutive HDDA and TDDA reactions of silicon-tethered tetraynes for the synthesis of dibenzosilole-fused polycyclic compounds and their unique reactivity

Silicon-tethered tetraynes possessing a 1,3-diyne moiety underwent consecutive hexadehydro- and tetradehydro-Diels-Alder reactions to give a series of fused polycyclic aromatic compounds containing a dibenzosilole skeleton. The benzene ring in the product acted as a 1,3-diene and reacted with the active alkyne as well as oxygen to provide [4 + 2] cycloadducts.

Construction of Benzopolycycles via Pd-Catalyzed Intermolecular Cyclization of 2,7-Alkadiynylic Carbonates with Terminal Propargyl Tertiary Alcohols

A palladium-catalyzed highly regioselective and chemoselective intermolecular cyclization of 2,7-alkadiynylic carbonates with terminal propargyl tertiary alcohols to construct benzopolycycles containing furan and pyrrole moieties has been developed with a very broad scope. Polycycles containing spirane structures or dispirane structures could be also smoothly synthesized in moderate to good yields. The reaction enjoys excellent regioselectivity.

Mechanism and Regioselectivity of an Unsymmetrical Hexadehydro-Diels-Alder (HDDA) Reaction

Hoye reported intramolecular hexadehydro-Diels-Alder (HDDA) reactions to generate arynes that functionalize natural product phenols and amines. In their studies, Hoye found that unsymmetrical tetraynes selectively form a single aryne. We report density functional theory (DFT) calculations that reveal the factors controlling the regioselectivity.

Isomerizations of Propargyl 3-Acylpropiolates via Reactive Allenes

Thermal isomerizations of various propargyl 3-acylpropiolates are described. Many result in the formation of 3-acylbutenolides. These reactions appear to proceed through intermediate 2,3-dehydropyrans (strained cyclic allenes), which then isomerize in a previously unobserved fashion. Competitive processes that provide additional mechanistic insights are also described.

Construction of Condensed Polycyclic Aromatic Frameworks through Intramolecular Cycloaddition Reactions Involving Arynes Bearing an Internal Alkyne Moiety

Facile synthetic methods for condensed polycyclic aromatic compounds via aryne intermediates are reported. The generation of arynes bearing a (3-arylpropargyl)oxy group from the corresponding o-iodoaryl triflate-type precursors efficiently afforded arene-fused oxaacenaphthene derivatives, which were formed through intramolecular [2+4] cycloaddition. Extending the method to the generation of arynes bearing a 1,3-diyne moiety led to a continuous generation of naphthalyne intermediate through the hexadehydro Diels-Alder reaction involving the aryne triple bond. This novel type of aryne-relay chemistry enabled the synthesis of a unique aminoarylated oxaacenaphthene derivative and highly ring-fused anthracene derivatives.

Photochemical Hexadehydro-Diels-Alder Reaction

We demonstrate that the hexadehydro-Diels-Alder (HDDA) cycloisomerization reaction to produce reactive benzyne derivatives can be initiated photochemically. As with the thermal variant of the HDDA process, the reactive intermediates are formed in the absence of reagents or the resulting byproducts required for the generation of benzynes by traditional methods. This photo-HDDA (or hν-HDDA) reaction occurs at much lower temperatures (including even at -70 °C) than the thermal HDDA, but the benzynes produced behave in the same fashion with respect to their trapping reactions, suggesting they are of the same electronic state.

Intramolecular Inverse Electron-Demand [4 + 2] Cycloadditions of Ynamides with Pyrimidines: Scope and Density Functional Theory Insights

4-Aminopyridines are valuable scaffolds for the chemical industry in general, from life sciences to catalysis. We report herein a collection of structurally diverse polycyclic fused and spiro-4-aminopyridines that are prepared in only three steps from commercially available pyrimidines. The key step of this short sequence is a [4 + 2]/retro-[4 + 2] cycloaddition between a pyrimidine and an ynamide, which constitutes the first examples of ynamides behaving as electron-rich dienophiles in [4 + 2] cycloaddition reactions. In addition, running the ihDA/rDA reaction in continuous mode in superheated toluene, to overcome the limited scalability of MW reactions, results in a notable production increase compared to batch mode. Finally, density functional theory investigations shed light on the energetic and geometric requirements of the different steps of the ihDA/rDA sequence.

Methodology and applications of the hexadehydro-Diels–Alder (HDDA) reaction

Hexadehydro-Diels–Alder (HDDA) reactions between alkynes and 1,3-diynes readily generate highly reactive and synthetically useful arynes.