Catalytic Synthesis of Cyclopropenium Cations with Rh-Carbynoids

Diazomethyl-λ3-iodane meets aryne: dipolar cycloaddition and C-to-N iodane shift leading to indazolyl-λ3-iodanes

Diazomethyl-λ3-iodanes have recently emerged as carbyne equivalents in organic synthesis, enabling the construction of multi-substituted carbon centers through strategic sequential activation of the diazo and iodane functional groups. Distinct from such reaction modes, we report here on the reactivity of diazomethyl-λ3-iodanes as iodane-bound 1,3-dipoles toward arynes. Equipped with bis(trifluoromethyl)benzyl alcohol-based benziodoxole (BX) moiety, diazomethyl-λ3-iodanes undergo annulation with arynes generated from ortho-silylaryl triflates and cyclic diarylhalonium salts, resulting in indazolyl-λ3-iodanes through [3 + 2] cycloaddition and carbon-to-nitrogen iodane migration. DFT calculations reveal that diazomethyl-BX prefers [3 + 2] cycloaddition with aryne over aryne insertion into the carbon-iodine(iii) bond (carboiodanation) and that the subsequent iodane migration proceeds through two consecutive 1,5-iodane shifts. The utility of these indazolyl-BXs as indazole-transfer agents has been demonstrated by α-functionalization of N,N-dimethylaniline derivatives.

Tris(aryl)cyclopropenium Ion as Organic Lewis Acid Catalyst in Carbonyl Activation Reactions

Although, in recent years, cyclopropenium salts have been explored as phase-transfer catalysts, electro-photocatalysts, H-bond donor catalysts, etc., and until now, they have not been utilized directly as Lewis acid catalysts in organic transformations. In this article, we demonstrate a "Proof of Concept" that the tris(aryl)cyclopropenium (TAC) carbocation could be utilized as an organic Lewis acid catalyst in some of the reactions involving carbonyl activation such as 1,2-addition reactions of aldehydes, 1,4-conjugate addition reactions of enones, and 1,6-vinylogous conjugate addition of dienones (p-quinone methides). The mode of activation of carbonyl group by cyclopropenium ion has been studied using NMR titrations and UV kinetics and further supported by computational calculations.

Rh-Catalyzed Enantioselective Aryl C-H Bond Cyclopropylation

Herein, we disclose the discovery and development of a site-, regio-, diastereo-, and enantioselective aryl C-H bond cyclopropylation using diazomethyl hypervalent iodine reagents, styrenes, and paddlewheel dirhodium carboxylate catalysts. A key aspect of this work was the catalytic generation of a chiral Rh(II) carbene through an electrophilic aromatic substitution with chiral Rh(II) carbynoids. The strategy allows the construction of cyclopropane rings using aryl C-H bonds from aromatic feedstocks and drug molecules and promises to reach an unexplored "cyclopropanated" chemical space highly difficult to reach by current strategies.

Diversity-Generating Skeletal Editing Transformations

ConspectusSkeletal editing, as a synthetic tool, offers the unique potential to selectively and efficiently modify the core skeleton of a target molecule at a late-stage. The main benefit of such transformations is the rapid exploration of the chemical space around lead compounds without necessitating a de novo synthesis for each new molecule. However, many skeletal editing transformations are inherently restricted to generating a single product from a single starting compound, limiting the potential for diversification, a concept central to expediting structure-activity relationship (SAR) investigations. In this Account, we describe our efforts to develop novel skeletal editing transformations in which a modification to the central motif of a molecule is performed simultaneously with the incorporation of additional functionality that can be easily varied through a judicious choice of the reagents. Specifically, we successfully developed an α-iodonium diazo-based carbynyl radical equivalent reagent that, under photoredox conditions, could facilitate the ring-expansion of indene scaffolds while enabling the insertion of over ten different functionalized carbon atoms into the corresponding naphthalene products. This concept was later extended to the design of an atomic carbon equivalent reagent that could promote mild and selective Ciamician-Dennstedt-type indole ring-expansion reactions, while simultaneously installing an oxime ester handle that could undergo further functionalization. Furthermore, we highlight recent work from our group on multiple-atom insertion reactions, namely, the development of a photocatalyzed De Mayo reaction for the ring-expansion of cyclic ketones and a photocatalyzed dearomative ring-expansion of thiophenes via small-ring insertion. In both of these cases, multiple products can be potentially accessed from a single starting material upon variation of the insertion reagent. The diversity-generating skeletal editing strategy could also be applied to single-atom transmutation, as demonstrated by the development of a nitrogen-to-functionalized carbon atom transmutation reaction to convert pyridine to benzene rings. Here, the desired transformation was achieved via a sequence of pyridine ring-opening, Horner-Wadsworth-Emmons (HWE) olefination, and ring-closure, with a judicious choice of the HWE reagent allowing the installation of a wide variety of versatile functional groups. Finally, an energy transfer-mediated quinoline ring-contraction is discussed, specifically with reference to the ways in which it does and does not fit the criteria of a skeletal editing reaction. Although formal atom deletion transformations are typically restricted to single products from each discrete substrate, this [2 + 2] cycloaddition/rearrangement cascade also involves the incorporation of an alkene into the molecule and introduces a point of variation that can be exploited for diversity generation. We hope to not only highlight the transformations reported herein but also inspire further research into this synthetic strategy to access new classes of skeletal editing transformations that, through rapid diversity generation, provide the potential to expedite SAR investigations.

Asymmetric multi-component trifunctionalization reactions with α-Halo Rh-carbenes

Multi-component multi-functionalization reactions involving active intermediates are powerful tools for rapidly generating a wide array of compounds. Metal carbynoids, with their distinct reactivity, hold great promise for developing synthetic methodologies. However, their application in catalytic transfer reactions has been hindered by the limited availability of suitable precursors. In this study, we investigate the catalytic potential of α-halo Rh-carbenes, leveraging the concept of metal carbynoids in multi-functionalization reactions. Through a chiral phosphoric acid-catalyzed asymmetric trifunctionalization, we have developed a method for synthesizing a variety of chiral α-cyclic ketal β-amino esters with high yields and excellent enantioselectivity. Our extensive experimental and computational studies reveal that α-halo Rh-carbenes exhibit carbynoid properties, which facilitate the transformation into functionalized Fischer-type Rh-carbenes through the decomposition of the C-halo bond.

Two-step continuous flow-driven synthesis of 1,1-cyclopropane aminoketones

The continuous flow telescoped synthesis of 1,1-cyclopropane aminoketones was achieved by optimizing the photocyclization of 1,2-diketones to 2-hydroxycylobutanones (HCBs) and their reaction with aryl- and alkylamines, via tandem condensation C4-C3-ring contraction reaction. With the achieved operational conditions, we were able to obtain a library of cyclopropylamines with good chemical yields, high productivity, and short residence times.

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Photocatalytic Decarboxylative Functionalization of Cyclopropenes via Cyclopropenium Cation Intermediates

A photocatalytic decarboxylative functionalization of cyclopropenes is reported. Starting from a broad range of redox-active ester-substituted cyclopropenes, cyclopropenylphthalimides can be synthesized in the absence of a nucleophile. Alternatively, different carbon and heteroatom nucleophiles can be introduced. The transformation proceeds most probably through the formation of an aromatic cyclopropenium cation, followed by trapping with the nucleophiles.

Accessing elusive σ-type cyclopropenium cation equivalents through redox gold catalysis

Cyclopropenes are the smallest unsaturated carbocycles. Removing one substituent from cyclopropenes leads to cyclopropenium cations (C3+ systems, CPCs). Stable aromatic π-type CPCs were discovered by Breslow in 1957 by removing a substituent on the aliphatic position. In contrast, σ-type CPCs-formally accessed by removing one substituent on the alkene-are unstable and relatively unexplored. Here we introduce electrophilic cyclopropenyl-gold(III) species as equivalents of σ-type CPCs, which can then react with terminal alkynes and vinylboronic acids. With catalyst loadings as low as 2 mol%, the synthesis of highly functionalized alkynyl- or alkenyl-cyclopropenes proceeded under mild conditions. A class of hypervalent iodine reagents-the cyclopropenyl benziodoxoles (CpBXs)-enabled the direct oxidation of gold(I) to gold(III) with concomitant transfer of a cyclopropenyl group. This protocol was general, tolerant to numerous functional groups and could be used for the late-stage modification of complex natural products, bioactive molecules and pharmaceuticals.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Trifluoromethyl Rhodium-Carbynoid in [2+1+2] Cycloadditions

Trifluoromethyl cationic carbyne (CF3 C+ :) possessing dual carbene-carbocation behavior emulated as trifluoromethyl metal-carbynoid (CF3 C+ =M) has not been explored yet, and its reaction characteristics are unknown. Herein, a novel α-diazotrifluoroethyl sulfonium salt was prepared and used in Rh-catalyzed three-component [2+1+2] cycloadditions for the first time with commercially available N-fused heteroarenes and nitriles, yielding a series of imidazo[1,5-a] N-heterocycles that are of interest in medicinal chemistry, in which the insertion of trifluoromethyl Rh-carbynoid (CF3 C+ =Rh) into C=N bonds of N-fused heteroarenes was involved. This strategy demonstrates synthetic applications in late-stage modification of pharmaceuticals, construction of CD3 -containing N-heterocycles, gram-scale experiments, and synthesis of phosphodiesterase 10A inhibitor analog. These highly valuable and modifiable imidazo[1,5-a] N-heterocycles exhibit good antitumor activity in vitro, thus demonstrating their potential applications in medicinal chemistry.

Ring expansion of indene by photoredox-enabled functionalized carbon-atom insertion

Skeletal editing has received unprecedented attention as an emerging technology for the late-stage manipulation of molecular scaffolds. The direct achievement of functionalized carbon-atom insertion in aromatic rings is challenging. Despite ring-expanding carbon-atom insertion reactions, such as the Ciamician-Dennstedt re-arrangement, being performed for more than 140 years, only a few relevant examples of such transformations have been reported, with these limited to the installation of halogen, ester and phenyl groups. Here we describe a photoredox-enabled functionalized carbon-atom insertion reaction into indene. We disclose the utilization of a radical carbyne precursor that facilitates the insertion of carbon atoms bearing a variety of functional groups, including trifluoromethyl, ester, phosphate ester, sulfonate ester, sulfone, nitrile, amide, aryl ketone and aliphatic ketone fragments to access a library of 2-substituted naphthalenes. The application of this methodology to the skeletal editing of molecules of pharmaceutical relevance highlights its utility.

Synthesis of Hypervalent Iodine Diazo Compounds and Their Application in Organic Synthesis

Diazomethyl-substituted iodine(III) compounds with electron-withdrawing groups (EWG) connected to diazo methyl center were a type of donor-acceptor diazo compounds with potential reaction abilities similar to ordinary diazo compounds. Although several diazomethyl-substituted iodine(III) compounds were synthesized and used in the nucleophilic substitution reactions as early as 1994, the synthesis and application of new iodine(III) diazo compounds have only been reported to a certain extent in recent years. In the presence of rhodium catalyst, photocatalyst, or nucleophiles, diazomethyl-substituted iodine(III) compounds can be converted into rhodium-carbenes, diazomethyl radicals, ester radicals or nucleophilic intermediates, which can be used as key intermediates for the formation of chemical bonds. The aim of this review is to give an overview of diazomethyl-substituted iodine(III) compounds in organic synthesis.

General Synthesis of N-Alkylindoles from N,N-Dialkylanilines via [4 + 1] Annulative Double C-H Functionalization

Strategies enabling the construction of indoles and novel polycyclic heterocycles from simple building blocks streamline syntheses in synthetic and medicinal chemistry. Herein, we report a C-H functionalization approach to N-alkylindoles proceeding via a double, site-selective C(sp3)-H/C(sp2)-H [4 + 1] annulation of readily accessed N,N-dialkylanilines. This protocol features a site-selective hydrogen atom transfer by a tuned N-tBu amidyl radical and addition of a sulfonyl diazo coupling partner, which promotes highly site-selective homolytic aromatic substitution of the (hetero)aromatic core. Mild decarboxylation of the annulation product enables the overall introduction of a carbyne equivalent into the N,N-dialkylaniline scaffold. Furthermore, the site-selectivity and mild conditions of the indolization facilitate direct access to N-alkyl indole scaffolds in late-stage functionalization (LSF) settings.

Enantioselective Multifunctionalization with Rh Carbynoids

Multifunctionalization from the interception of active intermediates is an attractive synthetic strategy for the efficient construction of complex molecular scaffolds in an atom and step economic fashion. However, the design of reactions involving metal carbynoids that exhibit carbene/carbocation behavior is currently limited, and developing catalyst-controlled highly enantioselective versions poses significant challenges. In this study, we present the first asymmetric trifunctionalization reactions with rhodium carbynoids. This reaction unveils the distinctive reactivity of the carbynoid precursor, enabling it to react with simultaneously two nucleophiles and one electrophile. This process involves the formation of two distinct carbene ylides with the alcohol/carbamate and the trapping of one ylide with the imine, resulting in the formation of three new bonds. Furthermore, this strategy allows for the divergent synthesis of a wide array of β-amino esters in high yields and exceptional enantioselectivity.

Late-Stage Aryl C-H Bond Cyclopropenylation with Cyclopropenium Cations

Herein, we disclose the first regio-, site- and chemoselective late-stage (hetero)aryl C-H bond cyclopropenylation with cyclopropenium cations (CPCs). The process is fast, operationally simple and shows an excellent functional group tolerance in densely-functionalized drug molecules, natural products, agrochemicals and fluorescent dyes. Moreover, we discovered that the installation of the cyclopropene ring in drug molecules could not only be used to shield against metabolic instability but also as a synthetic tool to reach medicinally-relevant sp3 -rich scaffolds exploiting the highly-strained nature of the cyclopropene ring with known transformations.

Electron-deficient cyclopropenium cations as Lewis acids in FLP chemistry

Cyclopropenium cations incorporating electron deficient substituents are Lewis acidic despite the presence of π-electrons. The chloride and electron affinities are examined computationally and experimentally, respectively. These cations form classic Lewis acid-base adducts with PPh3, while sterically demanding phosphines yield frustrated Lewis pairs (FLPs) which participate in FLP additions. Depending on the basicity of the phosphine used, addition to alkynes or alkyne deprotonation is observed. In either case, new C-C bonds are formed, thus extending the utility of the concept of FLP chemistry to these delocalized π-cations.

Borane-Catalyzed C-F Bond Functionalization of gem-Difluorocyclopropenes Enables the Synthesis of Orphaned Cyclopropanes

Herein, we disclose an approach to synthesize tert-alkyl cyclopropanes by leveraging C-F bond functionalization of gem-difluorocyclopropenes using tris(pentafluorophenyl)borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph2(C6D5)C3]+[(C6F5)3BF]-. We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α-tert-alkyl centers (63-93% yield). The regioselectivity of the addition to cyclopropenium ions is controlled by their steric and electronic properties and enables access to 3,3-bis(difluoromethyl)cyclopropenes in short order. The resulting cyclopropene products are readily reduced to the corresponding orphaned cyclopropanes under hydrogenation conditions. Quantum chemical calculations reveal the nature of the C-F bond cleavage steps and provide evidence for catalysis by boron and not silylated oxonium ions, though Si-F bond formation is the enthalpic driving force for the reaction.

Generating Fischer-Type Rh-Carbenes with Rh-Carbynoids

We describe the first catalytic generation of Fischer-type acyloxy Rh(II)-carbenes from carboxylic acids and Rh(II)-carbynoids. This novel class of transient donor/acceptor Rh(II)-carbenes evolved through a cyclopropanation process providing access to densely functionalized cyclopropyl-fused lactones with excellent diastereoselectivity. DFT calculations allowed the analysis of the properties of Rh(II)-carbynoids and acyloxy Rh(II)-carbenes as well as the characterization of the mechanism.