Reactivity of Vinyl Epoxides/Oxetanes/Cyclopropanes toward Arynes: Access to Functionalized Phenanthrenes

The reactivity of vinyl epoxides/oxetanes/cyclopropanes toward arynes has been demonstrated under mild conditions to give the corresponding phenanthrenes in moderate to good yields. This transition-metal-free cascade process involves a series of Diels-Alder reaction, ring-opening aromatization, and ene reaction. Various functionalized phenanthrenes could be synthesized utilizing the versatile hydroxy group. Interestingly, vinyl epoxides/oxiranes experience preferentially the Diels-Alder reaction toward arynes over nucleophilic attack of epoxides/oxiranes.

Three-Component, Diastereoselective [6 + 3] Annulation of Tropone, Imino Esters, and Arynes

A transition-metal-free, three-component, and diastereoselective [6 + 3] annulation reaction employing tropone, imino esters, and arynes allowing the synthesis of bridged azabicyclo[4.3.1]decadienes is demonstrated. The key nitrogen ylides for the [6 + 3] annulation were generated by the addition of imino esters to the arynes followed by a proton transfer. The nitrogen ylides undergo a regioselective addition to tropone to furnish the desired products in moderate to good yields with good functional group tolerance under mild conditions.

Transition-Metal-Free C2-Functionalization of Pyridines through Aryne Three-Component Coupling

The direct C2-functionalization of pyridines through a transition-metal-free protocol by using aryne multicomponent coupling is demonstrated. The reaction allowed a broad-scope synthesis of C2-substituted pyridine derivatives bearing the -CF₃ group in good yields with α,α,α-trifluoroacetophenones as the third component. Activated keto esters could also be employed as the third component in this formal 1,2-di(hetero)arylation of ketones. Performing the reaction under dilute conditions inhibited the competing pyridine-aryne polymerization pathway. Nucleophilic attack by the initially generated pyridylidene intermediate on the carbonyl followed by an S_N Ar process resembling the Smiles rearrangement affords the desired products.

Synthesis of α-Aminophosphonates via Phosphonylation of an Aryne-Imine Adduct

Multicomponent phosphonylation is accomplished upon the reaction of an imine with an aryne generated in situ in the presence of a dialkyl phosphite. This transition-metal-free protocol shows a broad substrate scope, providing a variety of α-aminophosphonates in moderate to good yields. A plausible mechanism for the reaction is proposed based on a deuterium exchange experiment.

Aziridinium Ylides: Underutilized Intermediates for Complex Amine Synthesis

Harnessing the chemistry of onium ylide intermediates generated from transition metal catalysis is a powerful strategy to convert simple precursors into complex scaffolds. While the chemistry of onium ylides has been studied for over three decades, transformations of aziridinium ylides have just recently emerged as a versatile way to exploit the strain of these reactive intermediates to furnish densely functionalized N-heterocycles in a highly stereocontrolled manner. Herein, we provide a short overview of the key concepts and recent developments in this area, with a focus on how mechanistic studies to delineate the factors controlling the reactivity of aziridinium ylides can stimulate fruitful future investigations.

Rapid Synthesis of Zwitterionic Phosphonium Benzoates by a Three-Component Coupling Involving Phosphines, Arynes and CO2

A mild and easy to perform multicomponent coupling involving phosphines, arynes generated from 2-(trimethylsilyl)aryl triflates, and CO₂ allowing the transition-metal-free synthesis of zwitterionic phosphonium benzoates has been developed. The reaction proceeds via the generation of 1 : 1 zwitterionic intermediates from phosphines and arynes followed by the interception with CO₂ to deliver the carboxylates in moderate to good yields instead of the anticipated benzooxaphosphol-3(1H)-ones.

Reaction of Vinyl Aziridines with Arynes: Synthesis of Benzazepines and Branched Allyl Fluorides

We report the cycloaddition between vinyl aziridines and arynes. Depending on the reaction conditions and the choice of the aryne precursor, the aziridinium intermediate can be trapped through two distinct mechanistic pathways. The first one proceeds through a formal [5+2] cycloaddition to furnish valuable multi-substituted benzazepines. In the second pathway, the aziridinium is intercepted by a fluoride ion to afford allylic fluorides in good yields. Both reactions proceed stereospecifically and furnish enantiopure benzazepines and allylic fluorides.

1,2-Aminohalogenation of arynes with amines and organohalides

An unprecedented use of inexpensive organohalides as halogen electrophiles to trap the zwitterion intermediates generated from amines and arynes has been developed to access structurally diverse tertiary 2-haloanilines. Effective organohalides include carbon tetrachloride, hexachloroethane, N-chlorosuccinimide, carbon tetrabromide, fluorotribromomethane, N-bromosuccinimide, carbon tetraiodide, and N-iodosuccinimide.

Stereoselective Multicomponent Reactions in the Synthesis or Transformations of Epoxides and Aziridines

Small ring heterocycles, such as epoxides and aziridines, are present in several natural products and are also highly versatile building blocks, frequently involved in the synthesis of numerous bioactive products and pharmaceuticals. Because of the potential for increased efficiency and selectivity, along with the advantages of environmentally benign synthetic procedures, multicomponent reactions (MCRs) have been explored in the synthesis and ring opening of these heterocyclic units. In this review, the recent advances in MCRs involving the synthesis and applications of epoxides and aziridines to the preparation of other heterocycles are discussed emphasizing the stereoselectivity of the reactions.

The aryne Sommelet–Hauser rearrangement

An aryne induced transition-metal-free and mild Sommelet–Hauser rearrangement of tertiary benzylamines for the synthesis of α-aryl amino acid derivatives in moderate to good yields and with broad substrate scope is presented.

Intra- versus Intermolecular Carbon-to-Carbon Proton Transfers in the Reactions of Arynes with Nitrogen Nucleophiles: A DFT Study

Carbon-to-carbon proton transfer (PT) is an elementary reaction in organic chemistry. Herein we report a systematic DFT study on the carbon-to-carbon proton transfers in the reactions of arynes with nitrogen nucleophiles, including tertiary amines, aziridines, imines, and N-heteroarenes. DFT calculations indicated that the Gibbs energies of activation for the 1,4- and 1,5-proton transfers involved are lower than 20 kcal/mol in most cases. We also found that the Bell-Evans-Polanyi principle applies to the 1,4-proton transfers, indicating that the more exergonic an intramolecular proton transfer is (the more stable a carbanion is generated), the faster it will be. Previous experimental studies have shown that, in the presence of a carbon nucleophile (NuH; e.g., CHCl₃ and MeCN) as the third component (also as the solvent), intermolecular proton abstractions may compete with the intramolecular proton transfers and lead to different products. Our theoretical rationalization of these competitions is that the introduction of an electron-withdrawing group at the acidic site can accelerate the intramolecular proton transfer dramatically (owing to the generation of a more stable carbanion), making it compete with the intermolecular processes. In addition, we have also discussed the competition between intra- and intermolecular proton transfers when NuH is used as the reactant rather than the solvent.

A highly enantioselective asymmetric Darzens reaction catalysed by proline based efficient organocatalysts for the synthesis of di- and tri-substituted epoxides

A new class of easily available and readily tunable proline based chiral organocatalysts was found to efficiently catalyse an unprecedented highly enantioselective asymmetric Darzens reaction of α-chloroketones and substituted α-chloroketones with various aldehydes, which directly produces optically active di- and tri-substituted chiral epoxides with higher product yields (up to 97%) and excellent ee's (up to 99%) under mild reaction conditions.

Multiheterocyclic Motifs via Three-Component Reactions of Benzynes, Cyclic Amines, and Protic Nucleophiles

A broadly general, three-component reaction strategy for the construction of compounds containing multiple heterocycles is described. Thermal benzyne formation (by the hexadehydro-Diels-Alder (HDDA) reaction) in the presence of tertiary cyclic amines and a protic nucleophile (HNu) gives, via ring-opening of intermediate ammonium ion/Nu- ion pairs, heterocyclic products. Many reactions are efficient even when the stoichiometric loading of the three reactants approaches unity. Use of HOSO2CF3 as the HNu gives ammonium triflate intermediates, which can then be ring opened by an even wider variety of nucleophiles.

Tandem nucleophilic addition–cycloaddition of arynes with α-iminoesters: two concurrent pathways to imidazolidines

The tandem nucleophilic addition-cycloaddition reaction of arynes with α-iminoesters gave functionalized imidazolidine derivatives with high yields, complete regioselectivities, and excellent diastereo- and enantioselectivities.

Diverse ring opening of thietanes and other cyclic sulfides: an electrophilic aryne activation approach

An electrophilic aryne-activated ring opening reaction of saturated sulfur heterocycles is achieved with a wide range of nucleophiles.

Recent advances in molecular rearrangements involving aryne intermediates

A concise account on the recent developments in molecular rearrangements proceeding via the aryne intermediate is presented thereby shedding light on the synthetic potential of this versatile intermediate.

Mechanistic Duality in Tertiary Amine Additions to Thermally Generated Hexadehydro-Diels-Alder Benzynes

Reported here are studies directed at understanding the mechanism of tertiary amine addition to hexadehydro-Diels-Alder (HDDA)-generated benzynes. Tertiary amines are presumed to engage benzynes by generation of a zwitterionic intermediate. Simple trialkylamines undergo intermolecular protonation by a protic nucleophile to give an aryl ammonium intermediate that is then dealkylated. Amines containing acidified β-protons undergo an intramolecular elimination to give the aniline and an alkene. Finally, amino alcohols react at either of their N- or O atoms, depending upon the extent of internal hydrogen bonding.

An aryne-based three-component access to α-aroylamino amides

A novel multicomponent reaction among arynes, tertiary α-monosubstituted α-isocyanoacetamides and water was discovered to access densely functionalized α-aroylamino amides. The stereoconservative course of the MCR was investigated.

Employing Arynes in Diels-Alder Reactions and Transition-Metal-Free Multicomponent Coupling and Arylation Reactions

Arynes are highly reactive intermediates having several applications in organic synthesis for the construction of various ortho-disubstituted arenes. Traditionally, arynes are generated in solution from haloarenes under strongly basic conditions. However, the scopes of many of the aryne reactions are limited because of the harsh conditions used for their generation. The renaissance of interest in aryne chemistry is mainly due to the mild conditions for their generation by the fluoride-induced 1,2-elimination of 2-(trimethylsilyl)aryl triflates. This Account is focused on the Diels-Alder reaction of arynes and their transition-metal-free application in multicomponent couplings as well as arylation reactions. The Diels-Alder reaction of arynes is a powerful tool for constructing benzo-fused carbocycles and heterocycles. In 2012, we developed an efficient, broad-scope, and scalable Diels-Alder reaction of pentafulvenes with arynes affording benzonorbornadiene derivatives. Subsequently, we accomplished the Diels-Alder reaction of arynes with dienes such as 1,2-benzoquinones and tropones. Moreover, we uncovered a transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes by the reaction of arynes with styrenes that proceeds via a Diels-Alder/ene-reaction cascade. In addition, we demonstrated the reaction of arynes with indene/benzofurans, which proceeds via a tandem [4 + 2]/[2 + 2] sequence. Multicomponent coupling (MCC) involving arynes mainly comprises the initial addition of a nucleophile to the aryne followed by interception of the aryl anion intermediate with an electrophile (provided the nucleophilic and electrophilic moieties do not belong to the same molecule). We have disclosed aryne MCCs initiated by N-heterocycles such as (iso)quinoline, pyridine, and aziridines. When (iso)quinoline is used as the nucleophilic trigger and N-substituted isatin as the third component, the reaction affords spirooxazino(iso)quinolines via 1,4-dipolar intermediates. Unexpectedly, using pyridine affords indolin-2-ones, where the reaction proceeds via the pyridylidene intermediate. Additionally, we developed the phosphine-triggered aryne MCCs for the synthesis of functionalized benzooxaphospholes. In another phase of our work, we studied the synthetic utility of CO2 as a one-carbon synthon in aryne MCCs for the synthesis of phthalimides. Engaging arynes as an aryl source is one of the transition-metal-free methods for arylation reactions. We have demonstrated the N-arylation of aromatic tertiary amines and O-arylation of aliphatic alcohols using arynes. It is anticipated that the chemistry of arynes will continue to prosper and will lead to surprising developments for the synthesis of various 1,2-disubstituted arenes of molecular complexity and structural diversity. Future challenges in this area include the utility of arynes in enantioselective transformations and the synthesis and reactions of exotic heterocyclic arynes.