Aryne Chemistry: Generation Methods and Reactions Incorporating Multiple Arynes

Arynes hold significance for the efficient fusion of (hetero) arenes with diverse substrates, advancing the construction of complex molecular frameworks. Employing multiple equivalents of arynes is particularly effective in the rapid formation of polycyclic cores found in optoelectronic materials and bioactive compounds. However, the inherent reactivity of arynes often leads to side reactions, yielding unanticipated products and underlining the importance of a detailed investigation into the use of multiple arynes to fine-tune their reactivity. This review centers on methodologies and syntheses in organic reactions involving multiple arynes, categorizing based on mechanisms like cycloadditions, σ-bond insertions, nucleophilic additions, and ene reactions, and discusses aryne polymerization. The categorization based on these mechanisms includes two primary approaches: the first entails multiple aryne engagement within a single step while the second approach involves using a single equivalent of aryne sequentially across multiple steps, with both requiring strict reactivity control to ensure precise aryne participation in each respective step. Additionally, the review provides an in-depth analysis of the selection of aryne precursors, organized chronologically and by activation strategy, offering a comprehensive background that supports the main theme of multiple aryne utilization. The expectation remains that this comprehensive review will be invaluable in designing advanced syntheses engaging multiple arynes.

Selective C(aryl)-O bond cleavage in biorenewable phenolics

Biorefining of lignocellulosic biomass via a lignin first approach delivers a range of products with high oxygen content. Besides pulp, a lignin oil rich in guaiacols and syringols is obtained bearing multiple C(aryl)-OH and C(aryl)-OMe groups, typically named phenolics. Similarly, technical lignin can be used but is generally more difficult to process providing lower yields of monomers. Removal of the hydroxy and methoxy groups in these oxygenated arenes is challenging due to the inherently strong C-O bonds, in addition to the steric and electronic deactivation by adjacent -OH or -OMe groups. Moreover, chemoselective removal of a specific group in the presence of other similar functionalities is non-trivial. Other side-reactions such as ring saturation and transalkylation further complicate the desired reduction process. In this overview, three different selective reduction reactions are considered. Complete hydrodeoxygenation removes both hydroxy and methoxy groups resulting in benzene and alkylated derivatives (BTX type products) which is often complicated by overreduction of the arene ring. Hydrodemethoxylation selectively removes methoxy groups in the presence of hydroxy groups leading to phenol products, while hydrodehydroxylation only removes hydroxy groups without cleavage of methoxy groups giving anisole products. Instead of defunctionalization via reduction transformation of C(aryl)-OH, albeit via an initial derivatization into C(aryl)-OX, into other functionalities is possible and also discussed. In addition to methods applying guaiacols and syringols present in lignin oil as model substrates, special attention is given to methods using mixtures of these compounds obtained from wood/technical lignin. Finally, other important aspects of C-O bond activation with respect to green chemistry are discussed.

Base-promoted tandem synthesis of 2-azaaryl indoline

A novel tandem method to synthesize 2-azaaryl indoline promoted by LiN(SiMe3)2 from 2-azaaryl methyl amine and 2-fluoro benzyl bromides was developed. Mechanistic investigation indicated that this tandem cyclization was initiated by selective benzyl C-SN2 substitution followed by an intramolecular SNAr reaction. Diverse 2-azaaryl indoles could also be obtained via simple functional transformations.

Carbon-Nitrogen Bond Formation Using Sodium Hexamethyldisilazide: Solvent-Dependent Reactivities and Mechanisms

The solvent-dependent reactivity of sodium hexamethyldisilazide (NaHMDS) toward carbon-centered electrophiles reveals reactions that are poorly represented or unrepresented in the literature, including direct aminolysis of aromatic methyl esters to give carboxamides, nitriles, or amidines, depending on the choice of solvent. SNAr substitutions of aryl halides and opening of terminal epoxides are also examined. A combination of 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic studies using [15N]NaHMDS, kinetic studies, and computational studies reveals the complex mechanistic basis of the preferences for simple aryl carboxamides in toluene and dimethylethylamine and arylnitriles or amidines in tetrahydrofuran (THF). A prevalence of dimer- and mixed dimer-based chemistry even starting from the observable NaHMDS monomer in THF solution is notable.

Regioselective transformation of 3-phosphoryl benzyne intermediates to diverse phosphorus-substituted arenes

Pre-functionalized benzyne precursors 5, 6 and 10 bearing a phosphoryl group were efficiently synthesized via a phospho-Fries rearrangement reaction on gram scales, and directly proceed various transformations to poly-substituted organophosphorus arenes in high regioselectivity.

Recent Progress in Amination Enabled by Transition-Metal-Free C(sp2)–O/C(sp2)–S Bond Cleavage Strategy

Recently, intense efforts have been dedicated to the development of novel synthetic strategies to access aromatic amines due to their importance in the pharmaceuticals, agrochemicals, materials, and natural product areas. Although numerous transition-metal-catalyzed C–N formation strategies have been described for the preparation of aromatic amines in the past few decades, complementary methods under transition-metal-free conditions are still required. We present the recent advances in the exploration of innovative amination approaches via C(sp2)–O/C(sp2)–S bond cleavage in this review.

1 Introduction

2 Stoichiometric Base-Promoted Amination

3 Base-Catalyzed Amination

4 Photoredox-Catalyzed Amination

5 Acid-Promoted Amination

6 Conclusion and Perspectives

Diverse diaryl sulfide synthesis through consecutive aryne reactions

An efficient method to synthesize diaryl sulfides with structural diversity is disclosed. Demethylative hydrothiolation of aryne intermediates generated from o-iodoaryl triflates with methylthio-substituted o-silylaryl triflates and further aryne reactions afford diverse diaryl sulfides.

Aryne Precursors for Selective Generation of 3-Haloarynes: Preparation and Application to Synthetic Reactions

The synthesis and reaction of new 3-haloaryne precursors 2a-2h were studied. The ortho-(trimethylsilyl)aryl triflate precursors 2a-2h were prepared by a simple procedure involving O-trimethylsilylation and migration of a trimethylsilyl group followed by triflation. The remarkable feature of new precursors is the selective generation of 3-haloarynes by suppressing the competitive thia-Fries rearrangement, which is the problem in the reaction using the well-known 3-haloaryne precursors. The advantage of new precursor 2a over a typical precursor 1 was confirmed by the direct comparisons in several reactions. The application of precursors 2a-2h to the syntheses of heterocycles was also reported.

One-step synthesis of benzo[b]thiophenes by aryne reaction with alkynyl sulfides

An aryne reaction with alkynyl sulfides affording benzo[b]thiophenes is disclosed. A wide range of 3-substituted benzothiophenes were synthesized from easily available o-silylaryl triflates and alkynyl sulfides in a one-step intermolecular manner. The synthesis of diverse multisubstituted benzothiophene derivatives involving a pentacyclic compound was achieved by virtue of the good functional group tolerance and versatile C2 functionalizations.

One-Pot Generation of Functionalized Benzynes from Readily Available 2-Hydroxyphenylboronic Acids

We developed a one-pot method for the generation of benzynes from a range of readily available 2-hydroxyphenylboronic acids. This method features the in situ activation of both boronic acid and hydroxyl groups of the substrate to enhance benzyne generation at 60 °C. Such mild conditions facilitate the generation of functionalized benzynes that immediately react with diverse arynophiles to produce multisubstituted fused benzenes.