Three-component reaction of small-ring cyclic amines with arynes and acetonitrile

Fluoride-Mediated Aryne 1,2-Difunctionalization Involving C═S Bond Heterolysis

We have successfully synthesized a series of bidentate ligands by utilizing 2-(trimethylsilyl)phenyl trifluorosulfonate as a precursor for the benzyl group. This method proceeded by inserting a polythiourea into the C═S π-bond, intramolecular ring proton migration, and ring opening. Salient features of this strategy are mild reaction conditions, a novel product structure, excellent stereochemistry, and a good functional group tolerance. Furthermore, a series of density functional theory calculations were performed to gain insights into the transfer mechanism.

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.

Synthesis of 15N-labeled heterocycles via the cleavage of C-N bonds of anilines and glycine-15N

A nitrogen replacement process that directly incorporates the 15N atom of glycine-15N into anilines was reported. The process involves a Csp2-N bond cleavage of anilines driven by dearomatization and a Csp3-N bond cleavage of glycine-15N driven by aromatization. A variety of 15N-labeled aromatic heterocycles can be prepared via this process.

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.

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.

Regioselective and Stereodivergent Synthesis of Enantiomerically Pure Vic-Diamines from Chiral β-Amino Alcohols with 2-Pyridyl and 6-(2,2'-Bipyridyl) Moieties

In this report, we describe the synthetic elaboration of the easily available enantiomerically pure β-amino alcohols. Attempted direct substitution of the hydroxyl group by azido-functionality in the Mitsunobu reaction with hydrazoic acid was inefficient or led to a diastereomeric mixture. These outcomes resulted from the participation of aziridines. Intentionally performed internal Mitsunobu reaction of β-amino alcohols gave eight chiral aziridines in 45-82% yield. The structural and configuration identity of products was confirmed by NMR data compared to the DFT calculated GIAO values. For 1,2,3-trisubstituted aziridines slow configurational inversion at the endocyclic nitrogen atom was observed by NMR at room temperature. Moreover, when aziridine was titrated with Zn(OAc)₂ under NMR control, only one of two N-epimers directly participated in complexation. The aziridines underwent ring opening with HN₃ to form the corresponding azido amines as single regio- and diastereomers in 90-97% yield. Different results were obtained for 1,2-disubstituted and 1,2,3-trisubstituted aziridines. For the later aziridines ring closure and ring opening occurred at different carbon stereocenters, thus yielding products with two inverted configurations, compared to the starting amino alcohol. The 1,2-disubstituted aziridines produced azido amines of the same configuration as the starting β-amino alcohols. To obtain a complete series of diastereomeric vic-diamines, we converted the amino alcohols into cyclic sulfamidates, which reacted with sodium azide in S_N2 reaction (25-58% overall yield). The azides obtained either way underwent the Staudinger reduction, giving a series of six new chiral vic-diamines of defined stereochemistries.

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.

Synthesis of Oxazolidinones by using Carbon Dioxide as a C1 Building Block and an Aluminium-Based Catalyst

Oxazolidinone synthesis through the coupling of carbon dioxide and aziridines was catalysed by an aluminium(salphen) complex at 50-100 °C and 1-10 bar pressure under solvent-free conditions. The process was applicable to a variety of substituted aziridines, giving products with high regioselectivity. It involved the use of a sustainable and reusable aluminium-based catalyst, used carbon dioxide as a C₁ source and provided access to pharmaceutically important oxazolidinones as illustrated by a total synthesis of toloxatone. This protocol was scalable, and the catalyst could be recovered and reused. A catalytic cycle was proposed based on stereochemical, kinetic and Hammett studies.

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.

Synthesis of Arenesulfonyl Fluorides via Sulfuryl Fluoride Incorporation from Arynes

Transition-metal-free multicomponent reactions involving aryne precursors, secondary amines, and sulfuryl fluoride are reported herein. Zwitterionic intermediates formed from the reaction of arynes with amine nucleophiles can capture SO₂F₂ under mild conditions, offering a novel and practical protocol for the synthesis of 2-dialkyl-, 2-alkylaryl-, or 2-diarylamino-substituted arenesulfonyl fluoride derivatives in good to excellent yields.

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.

Intramolecular hydride transfer onto arynes: redox-neutral and transition metal-free C(sp3)-H functionalization of amines

Transition metal-free intramolecular hydride transfer onto arynes is reported for the first time. This unique transformation is utilized in redox-neutral intermolecular α-functionalization reactions of different tertiary amines, generating C(sp3)-C(sp3/sp2/sp) bonds in a single synthetic operation. Deuterium labeling studies support initial cleavage of the α-C-H bond via intramolecular 1,5-hydride transfer onto the aryne, which leads to activation of a range of integrated pronucleophiles and ultimately affords a new approach to cross-dehydrogenative coupling reactions which utilize aryne intermediates.

A multifunctionalized strategy of indoles to C2-quaternary indolin-3-ones via a TEMPO/Pd-catalyzed cascade process

A method for combinative oxidative homo dimerization and cyanomethylation of free indole derivatives catalysed by TEMPO and Pd(OAc)₂ was demonstrated for the first time. This new methodology is both atom and step efficient and is applicable to a broad scope of substrates, allowing the synthesis of a range of synthetically valuable 2-(2-(1H-indol-3-yl)-3-oxoindolin-2-yl)acetonitriles in moderate to excellent yields.

Aryne triggered [2,3]-sigmatropic rearrangement of allyl and propargyl thioethers

An efficient protocol for [2,3]-sigmatropic rearrangement of allyl and propargyl thioethers is reported. The key sulfonium ylide intermediate is in situ formed via S-arylation of arynes. This transition metal-free method allows for ready access to a wide array of functionalized thioether derivatives in good to excellent yields.

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.

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.

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.

Three-component carboarylation of unactivated imines with arynes and carbon nucleophiles

With 2-(trimethylsilyl)aryl triflates as aryne precursors, the title reaction proceeds smoothly to afford functionalized tertiary amines under transition metal-free conditions.

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.

Aryne Compatible Solvents are not Always Innocent

Arynes are important and versatile intermediates in a variety of transformations. Commonly used solvents for aryne chemistry include acetonitrile and dichloromethane. Although rarely reported, the reactive nature of aryne intermediates makes them prone to side reactions, which sometimes involve solvent participation. Acetonitrile and dichloromethane are not always innocent solvents and can participate in aryne-based reactions. These results are presented in the context of ongoing mechanistic investigations of the triple aryne-tetrazine reaction.

Employing carboxylic acids in aryne multicomponent coupling triggered by aziridines/azetidines

The transition-metal-free aryne multicomponent coupling (MCC) involving carboxylic acids initiated by aziridines/azetidines leading to the synthesis of N-aryl β/γ-amino alcohol derivatives has been developed.

Synthesis of functionalized amino epoxides by a three-component coupling involving aziridines, arynes and aldehydes

A three-component coupling of arynes, aziridines and aldehydes/isatins resulting in the synthesis of N-aryl α-amino epoxides has been demonstrated.

Oxidative C(sp3)–H functionalization of acetonitrile and alkanes with allylic alcohols under metal-free conditions

Direct oxidative C(sp³)–H functionalization of acetonitrile and alkanes with α,α-diaryl allylic alcohols has been developed.