Ligand-Controlled Regiodivergence in the Copper-Catalyzed [2,3]- and [1,2]-Rearrangements of Iodonium Ylides

The "cesium effect" magnified: exceptional chemoselectivity in cesium ion mediated nucleophilic reactions

Chemodivergent construction of structurally distinct heterocycles from the same precursors by adjusting specific reaction parameters is an emergent area of organic synthesis; yet, understanding of the processes that underpin the reaction divergence is lacking, preventing the development of new synthetic methods by systematically harnessing key mechanistic effects. We describe herein cesium carbonate-promoted oxadiaza excision cross-coupling reactions of β-ketoesters with 1,2,3-triazine 1-oxides that form pyridones in good to high yields, instead of the sole formation of pyridines when the same reaction is performed in the presence of other alkali metal carbonates or organic bases. The reaction can be further extended to the construction of synthetically challenging pyridylpyridones. A computational study comparing the effect of cesium and sodium ions in the oxadiaza excision cross-coupling reactions reveals that the cesium-coordinated species changes the reaction preference from attack at the ketone carbonyl to attack at the ester carbon due to metal ion-specific transition state conformational accommodation, revealing a previously unexplored role of cesium ions that may facilitate the development of chemodivergent approaches to other heterocyclic systems.

A highly diastereoselective strain-release Doyle-Kirmse reaction: access to functionalized difluoro(methylene)cyclopropanes

Difluoro(methylene)cyclopropanes (F2MCPs) show better anti-cancer properties and chemical reactivities compared to their nonfluorinated analogues. However, catalytic stereoselective methods to access these privileged motifs still remain a challenging goal. The Doyle-Kirmse reaction is a powerful strategy for the concomitant formation of carbon-carbon and carbon-sulfur bonds. Although the enantioselective variants of this reaction have been achieved with high levels of selectivity, the methods that control the diastereoselectivity have been only moderately successful. Herein, we report a catalytic, highly diastereoselective strain-release Doyle-Kirmse reaction for synthesizing functionalized F2MCPs using an inexpensive copper catalyst. The transformation proceeds under mild conditions and displays excellent functional group compatibility on both diazo compounds and difluorocyclopropenyl methyl sulfane/selane derivatives. Furthermore, the obtained products were efficiently transformed into valuable building blocks, such as functionalized spiroheterocycles, difluorocyclopropanes, and skipped dienes.

Rhodium(II)-Catalyzed Hinsberg Dearomatization Using Trimethylsilyldiazomethane

Rhodium(II) catalyzes carbene transfer from trimethylsilyldiazomethane to arylmethyl thioethers, generating sulfonium ylides that undergo [2,3]-sigmatropic rearrangement, punching quaternary centers into aromatic rings. The reaction works well with naphthalene, indole, and benzofuran ring systems, but the reaction is unsuccessful with the monocyclic benzene homologue. For aryl thioethers, Rh2(OAc)4 gives good results. For alkyl thioethers, the yields improve with Rh2(cap)4. Surprisingly, thioesters and thiocarbamates are also competent substrates for the reaction.

Hypervalent Iodine Mediated Synthesis of Sulfinamidines from Sulfenamides

In this study, we present a novel, efficient method for the oxidative amination of sulfenamides using diacetoxyiodobenzene (PhI(OAc)2) and amines under basic conditions. This innovative technique streamlines the synthesis of sulfinamidines under mild, metal-free conditions, achieving outstanding yields of up to 99%. Furthermore, we propose possible pathways that elucidate the observed molecular sequence of events in this reaction. This cutting-edge approach not only advances the synthesis of valuable sulfinamidine compounds but also expands the synthetic toolbox available to chemists, paving the way for future discoveries in organic synthesis and potential applications in medicinal chemistry.

A highly efficient method to access unprotected C-3 bifunctional quaternary 3-allyl-3-(amino)oxindoles

A highly efficient Rh(II) catalyzed non-radical protocol to access NH-free C-3 bifunctional oxindoles, which possess 3-allyl and 3-amino simultaneously, was first achieved by employing an intermolecular [2,3]-sigmatropic rearrangement reaction between diazooxindoles and tertiary allylic amines. Utilizing readily available allylamines as the nitrogen and allyl source concurrently, a wide range of bio-active 3-allyl-3-(amino)oxindoles were obtained in excellent yields under very mild reaction conditions; meanwhile, the TON can be up to 90 000. Our study addresses a gap in the literature by investigating intermolecular rearrangements of ammonium ylides with diazoamides, which have been relatively understudied.

Hypervalent Iodine-Mediated Synthesis of Sulfinimidate Esters from Sulfenamides

In this study, we present a novel and efficient approach for the oxidative esterification of sulfenamides using phenyliodonium diacetate, enabling the synthesis of sulfinimidate esters and sulfilimines under mild and metal-free conditions, with yields reaching up to 99%. The protocol is readily scalable and compatible with a diverse range of substrates and functional groups, and we demonstrate its potential for late-stage functionalization of pharmacologically relevant molecules. Furthermore, we propose a plausible reaction mechanism to account for the observed sequence of events.

A rhodium-catalyzed ylide formation/Smiles rearrangement reaction of chalcogenide ether and triazoles

A Rh(ii)-catalyzed highly stereoselective chalcogenide ylide formation/Smiles rearrangement reaction of diaryl thioether/selenoethers and triazoles was successfully developed.

Recent advances in metal-catalysed asymmetric sigmatropic rearrangements

Asymmetric sigmatropic rearrangement is a powerful organic transformation via substrate-reorganization to efficiently increase molecular complexity from readily accessible starting materials. In particular, a high level of diastereo- and enantioselectivity can be readily accessed through well-defined and predictable transition states in [3,3], [2,3]-sigmatropic rearrangements, which have been widely applied in the synthesis of various chiral building blocks, natural products, and pharmaceuticals. In recent years, catalytic asymmetric sigmatropic rearrangements involving chiral metal complexes to induce stereocontrol have been intensively studied. This review presents an overview of metal-catalysed enantioselective versions of sigmatropic rearrangements in the past two decades, mainly focusing on [3,3], [2,3], and [1,3]-rearrangements, to show the development of substrate design, new catalyst exploitation, and novel cascade processes. In addition, their application in the asymmetric synthesis of complex natural products is also exemplified.

Catalytic rearrangements of onium ylides in aromatic systems

Onium ylides are reactive intermediates that undergo versatile chemical transformations to give structurally interesting compounds. Rearrangement reactions of onium ylides are of great importance to synthetic organic chemists, as they provide efficient methods for C-C bond formations as well as installation of new stereogenic centers in molecules. Traditionally, onium ylides have been shown to undergo two types of rearrangements, namely, [2,3]- and [1,2]-rearrangements. In recent years, there have been tremendous developments in the field of metal-catalyzed onium ylide rearrangements through catalytic generation of ylide intermediates from diazocompounds. Several examples of selective catalytic onium ylide rearrangements involving sulfonium, oxonium, ammonium, as well as iodonium ylides have been developed over the years especially in allylic and propargylic systems. However, when the π-system that takes part in the rearrangement is part of an aromatic ring, the selectivity for rearrangements of reactive onium ylides is more challenging. In this review, we discuss recent advances in catalyst control of onium ylide rearrangements of aromatic systems.

Enantioselective [1,2]-Stevens Rearrangement of Thiosulfonates to Construct Dithio-Substituted Quaternary Carbon Centers

An enantioselective [1,2] Stevens rearrangement was realized by using chiral guanidine and copper(i) complexes. Bis-sulfuration of α-diazocarbonyl compounds was developed through using thiosulfonates as the sulfenylating agent. It was undoubtedly an atom-economic process providing an efficient route to access novel chiral dithioketal derivatives, affording the corresponding products in good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er). A novel catalytic cycle was proposed to rationalize the reaction process and enantiocontrol.

An asymmetric [1,2] Stevens rearrangement was realized via chiral guanidine and copper(i) complexes. A series of novel chiral dithioketal derivatives were obtained with good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er).

Modular and stereoselective synthesis of tetrasubstituted vinyl sulfides leading to a library of AIEgens

Tetraarylethylenes exhibit intriguing photophysical properties and sulfur atom frequently play a vital role in organic photoelectric materials and biologically active compounds. Tetrasubstituted vinyl sulfides, which include both sulfur atom and tetrasubstituted alkenes motifs, might be a suitable skeleton for the discovery of the new material molecules and drug with unique functions and properties. However, how to modular synthesis these kinds of compounds is still challenging. Herein, a chemo- and stereo-selective Rh(II)-catalyzed [1,4]-acyl rearrangements of α-diazo carbonyl compounds and thioesters has been developed, providing a modular strategy to a library of 63 tetrasubstituted vinyl sulfides. In this transformation, the yield is up to 95% and the turnover number is up to 3650. The mechanism of this reaction is investigated by combining experiments and density functional theory calculation. Moreover, the "aggregation-induced emission" effect of tetrasubstituted vinyl sulfides were also investigated, which might useful in functional material, biological imaging and chemicalnsing via structural modification.

Copper-Catalyzed Asymmetric Diyne Cyclization via [1,2]-Stevens-Type Rearrangement for the Synthesis of Chiral Chromeno[3,4-c]pyrroles

Here, we report a copper-catalyzed asymmetric cascade cyclization/[1,2]-Stevens-type rearrangement via a non-diazo approach, leading to the practical and atom-economic assembly of various valuable chiral chromeno[3,4-c]pyrroles bearing a quaternary carbon stereocenter in generally moderate to good yields with wide substrate scope and excellent enantioselectivities (up to 99 % ee). Importantly, this protocol not only represents the first example of catalytic asymmetric [1,2]-Stevens-type rearrangement based on alkynes but also constitutes the first asymmetric formal carbene insertion into the Si-O bond.

Advances in [1,2]-Sigmatropic Rearrangements of Onium Ylides via Carbene Transfer Reactions

This review article summarizes progress made on [1,2]-sigmatropic rearrangements using carbenes in the ylide formation step. While other rearrangements, such as the [2,3]-sigmatropic, Doyle–Kirmse, or Sommelet–Hauser rearrangements, have been studied in detail over the past decades, investigations on [1,2]-sigmatropic rearrangements are still limited. Based on the application of diazoalkanes as carbene precursors, research on diazoalkanes in ylide formation reactions started flourishing in the 1990s. This Short Review covers milestones from the advent of [1,2]-sigmatropic rearrangements using carbenes to generate ammonium, oxonium and other ylide species, and should serve as an overview to further promote research in this area.

1 Introduction

2 Ammonium Ylides

3 Oxonium Ylides

4 Sulfonium and Selenium Ylides

5 Halonium Ylides

6 Conclusion and Outlook

Ni-Catalyzed Ligand-Controlled Regiodivergent Reductive Dicarbofunctionalization of Alkenes

Transition-metal-catalyzed dicarbofunctionalization of alkenes involving intramolecular Heck cyclization followed by intermolecular cross-coupling has emerged as a powerful engine for building heterocycles with sterically congested quaternary carbon centers. However, only exo-cyclization/cross-coupling products can be obtained; endo-selective cyclization/cross-coupling has not been reported yet and still poses a formidable challenge. We herein report the first example of catalyst-controlled dicarbofunctionalization of alkenes for the regiodivergent synthesis of five- and six-membered benzo-fused lactams bearing all-carbon quaternary centers. Using a chiral Pyrox- or Phox-type bidentate ligand, 5-exo cyclization/cross-couplings proceed favorably to produce indole-2-ones in good yields with excellent regioselectivity and enantioselectivities (up to 98% ee). When C6-carboxylic acid-modified 2,2'-bipyridine was used as the ligand, 3,4-dihydroquinolin-2-ones were obtained in good yields through 6-endo-selective cyclization/cross-coupling processes. This transformation is modular and tolerant of a variety of functional groups. The ligand rather than the substrate structures precisely dictates the regioselectivity pattern. Moreover, the synthetic value of this regiodivergent protocol was demonstrated by the preparation of biologically relevant molecules and structural scaffolds.

Catalyst-Controlled Regiodivergence in Rearrangements of Indole-Based Onium Ylides

We have developed catalyst-controlled regiodivergent rearrangements of onium-ylides derived from indole substrates. Oxonium ylides formed in situ from substituted indoles selectively undergo [2,3]- and [1,2]-rearrangements in the presence of a rhodium and a copper catalyst, respectively. The combined experimental and density functional theory (DFT) computational studies indicate divergent mechanistic pathways involving a metal-free ylide in the rhodium catalyzed reaction favoring [2,3]-rearrangement, and a metal-coordinated ion-pair in the copper catalyzed [1,2]-rearrangement that recombines in the solvent-cage. The application of our methodology was demonstrated in the first total synthesis of the indole alkaloid (±)-sorazolon B, which enabled the stereochemical reassignment of the natural product. Further functional group transformations of the rearrangement products to generate valuable synthetic intermediates were also demonstrated.

Recent Perspectives on Rearrangement Reactions of Ylides via Carbene Transfer Reactions

Among the available methods to increase the molecular complexity, sigmatropic rearrangements occupy a distinct position in organic synthesis. Despite being known for over a century sigmatropic rearrangement reactions of ylides via carbene transfer reaction have only recently come of age. Most of the ylide mediated rearrangement processes involve rupture of a σ-bond and formation of a new bond between π-bond and negatively charged atom followed by simultaneous redistribution of π-electrons. This minireview describes the advances in this research area made in recent years, which now opens up metal-catalyzed enantioselective sigmatropic rearrangement reactions, metal-free photochemical rearrangement reactions and novel reaction pathways that can be accessed via ylide intermediates.

Access to Vinyl Ethers and Ketones with Hypervalent Iodine Reagents as Oxy-Allyl Cation Synthetic Equivalents

We report an Umpolung strategy of enol ethers to generate oxy-allyl cation equivalents based on the use of hypervalent iodine reagents. Under mild basic conditions, the addition of nucleophiles to aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily available in two steps from silyl alkynes, resulted in the stereoselective formation of substituted aryl enol ethers. The reaction was most efficient with phenols as nucleophiles, but preliminary results were also achieved for C- and N- nucleophiles. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent was transferred. The obtained aryl enol ethers could then be transformed into α-difunctionalized ketones by oxidation. The described "allyl cation"-like reactivity contrast with the well-established "vinyl-cation" behavior of alkenyl iodonium salts.

Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Organic azides are useful synthetic intermediates, which demonstrate broad reactivity. Unlike most organic azides, allylic azides can spontaneously rearrange to form a mixture of isomers. This rearrangement has been named the Winstein rearrangement. Using allylic azides can result in low yields and azide racemization in some synthetic contexts due to the Winstein rearrangement. Effort has been made to understand the mechanism of the Winstein rearrangement and to take advantage of this process. Several guiding principles can be used to identify which azides will produce a mixture of isomers and which will resist rearrangement. Selective reaction conditions can be used to differentiate the azide isomers in a dynamic manner. This review covers all aspects of allylic azides including their synthesis, their reactivity, the mechanism of the Winstein rearrangement, and reactions that can selectively elaborate an azide isomer. This review covers the literature from Winstein's initial report to early 2019.

A removable functional group strategy for regiodivergent Wittig rearrangement products

[1,2] and [2,3] Wittig rearrangements are competing reaction pathways, often leading to uncontrollable product distribution. We employ a single removable functional group to fulfill the dual role of attaining a reversible [2,3] and stabilizing radical intermediate for the [1,2] path to obtain both the Wittig products selectively for a broad range of substrates.

Reagent-controlled regiodivergence in the [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides

A regiodivergent [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides based on the N–O bond cleavage is achieved by changing reaction conditions.

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.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.

Copper-catalyzed [1,2]-rearrangements of allylic iodides and aryl α-diazoacetates

The [1,2]- and [2,3]-rearrangements of iodonium ylides are synthetically useful reactions for the generation of functionalized α-iodoesters. Allylic iodides are coupled with α-diazoesters in the presence of a copper catalyst and a ligand to generate iodonium ylides, which undergo metal-mediated rearrangements. By fine-tuning the structure of the ligand, we have reversed the regioselectivity of copper-catalyzed reactions of iodonium ylides from [2,3]- to [1,2]-rearrangements with the use of alternate bipyridine ligands. The preference for [1,2]-rearrangements was further improved by using bulky aryl α-diazoester substrates. Several α-iodoesters with a diverse range of functional groups were generated in good yields (up to 88% yield) and high regioselectivities (up to >95:5 regioisomeric ratio). A deuterium-labeled substrate was utilized to gain insight into the mechanism of the reaction.

Copper-Catalyzed Enantio-, Diastereo-, and Regioselective [2,3]-Rearrangements of Iodonium Ylides

The first highly enantioselective, diastereoselective, and regioselective [2,3]-rearrangement of iodonium ylides has been developed as a general solution to catalytic onium ylide rearrangements. In the presence of a chiral copper catalyst, substituted allylic iodides couple with α-diazoesters to generate metal-coordinated iodonium ylides, which undergo [2,3]-rearrangements with high selectivities (up to >95:5 r.r., up to >95:5 d.r., and up to 97 % ee). The enantioenriched iodoester products can be converted stereospecifically into a variety of onium ylide rearrangement products, as well as compounds that are not accessible by classical onium ylide rearrangements.

NHC Ligands Tailored for Simultaneous Regio- and Enantiocontrol in Nickel-Catalyzed Reductive Couplings

An exceptionally hindered class of enantiopure NHC ligands has been developed. While racemic forms had previously been utilized, a scalable and practical route to the enantiopure form of this ligand class is described utilizing a Buchwald-Hartwig N,N-diarylation in a highly sterically demanding environment. Using this newly accessible ligand class, nickel-catalyzed enantioselective reductive coupling reactions of aldehydes and alkynes have been developed. These studies illustrate that the newly available NHC ligands are well suited for simultaneous control of regio- and enantioselectivity, even in cases with internal alkynes possessing only very subtle steric differences between two aliphatic substituents. The steric demand of the new ligand class enables a complementary regiochemical outcome compared with previously described enantioselective processes. Using this method, a number of allylic alcohol derivatives were efficiently obtained with high regioselectivity (up to >95:5) and high enantioselectivity (up to 94% ee). The reaction conditions can also be extended to the reaction of aldehydes and allenes, providing silyl-protected allylic alcohol derivatives possessing a terminal methylene substituent. Computational studies have explained the origin of the exceptional steric demand of this ligand class, the basis for enantioselectivity, and the cooperative relationship of the aldehyde, alkyne, and ligand in influencing enantioselectivity.

Regioselective switching approach for the synthesis of α and δ carboline derivatives

A metal-free protocol for accessing both α and δ-carboline derivatives, starting from a common indolylchalcone oxime ester precursor is reported.

Hemin-catalyzed sulfonium ylide formation and subsequently reactant-controlled chemoselective rearrangements

The Stevens and Sommelet–Hauser rearrangements can be controlled by the electronic property of the substituents on the benzyl group together with solvent effects.