Beyond olefins: new metathesis directions for synthesis

Multicomponent Synthesis of 3(2H)-Furanones Initiated by Copper(II)-Catalyzed Alkyne-Carbonyl Cross Metathesis

The cooperative Lewis and Brønsted acid catalysis makes convergent synthesis of 3(2H)-furanones through a three-component coupling of 1,3-diynes, alkyl glyoxylates and water. Control experiments support that Lewis acid-catalyzed highly chemo-, regio- and stereoselective alkyne-carbonyl metathesis of 1,3-diynes and alkyl glyoxylates might be the initial step of this multicomponent annulation. Further chemo- and regioselective hydration of the alkyne-carbonyl metathesis product and subsequent oxa-Michael addition promoted by Brønsted acid results in the formation of two C-O bonds of the five-membered oxygen heterocycle.

Ru- and Co-Catalyzed Intermolecular Carbonyl-Alkyne Metathesis Reactions of 1H-Indene-1,2,3-triones with Internal Alkynes

The catalyst-dependent intermolecular carbonyl-alkyne metathesis (CAM) reaction of 1H-indene-1,2,3-triones with internal alkynes was realized using Ru and Co catalysts. 2-(2-Oxo-1,2-diphenylethylidene)-1H-indene-1,3(2H)-dione derivatives were obtained using a Ru catalyst, whereas S-alkyl 2-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)-2-phenylethanethioates were prepared using a Co catalyst. These transformations led to the synthesis of α,β-unsaturated carbonyl compounds with a broad substrate scope, excellent regiocontrol, and gram-scale amenability. This catalytic strategy using a Co or Ru catalyst has rarely been described for other established CAM catalysts.

A metal-catalysed functional group metathesis approach to the carbon isotope labelling of carboxylic acids

The distribution, metabolism and ultimate fate of molecules within the body is central to the activity of pharmaceuticals. However, the introduction of radioisotopes into the metabolically stable carbon sites on drugs to probe these features typically requires toxic, radioactive gases such as [14C]CO and [14C]CO2. Here we describe an approach to directly carbon-label carboxylic-acid-containing pharmaceuticals via a metal-catalysed functional group exchange reaction, forming 14C-labelled carboxylic-acid-containing drugs without radioactive gases, in one pot, using an easily available and handled carboxylic acid 14C source. To enable this process, a functional group metathesis of carbon-carbon covalent bonds in acid chloride functionalities is developed, exploiting the ability of nickel catalysts to both reversibly activate carbon-chloride bonds and exchange functionalities between organic molecules. The drug development applicability is illustrated by the direct incorporation of the 14C label or 13C label into an array of complex aryl, alkyl, vinyl and heterocyclic carboxylic acid drugs or drug candidates without gases or a special apparatus, at ambient conditions and without loss of the radiolabel.

Platform for Multiple Isotope Labeling via Carbon-Sulfur Bond Exchange

In this work, we explore a nickel-catalyzed reversible carbon-sulfur (C-S) bond activation strategy to achieve selective sulfur isotope exchange. Isotopes are at the foundation of applications in life science, such as nuclear imaging, and are essential tools for the determination of pharmacokinetic and dynamic profiles of new pharmaceuticals. However, the insertion of an isotope into an organic molecule remains challenging, and current technologies are element-specific. Despite the ubiquitous presence of sulfur in many biologically active molecules, sulfur isotope labeling is an underexplored field, and sulfur isotope exchange has been overlooked. This approach enables us to move beyond standardized element-specific procedures and was applied to multiple isotopes, including deuterium, carbon-13, sulfur-34, and radioactive carbon-14. These results provide a unique platform for multiple isotope labeling and are compatible with a wide range of substrates, including pharmaceuticals. In addition, this technology proved its potential as an isotopic encryption device for organic molecules.

Enantioselective Ring-Closing Aminomethylamination of Allylic Aminodienes with Aminals Triggered by C-N Bond Metathesis

A conceptually novel strategy utilizing a cyclopalladated complex as an electrophile to activate the C-N bond for the C-N bond metathesis between allylamines and aminals is developed, which enables an efficient ring-closing aminomethylamination of allylic aminodienes and aminals. The reaction proceeds under mild reaction conditions and displays a remarkable scope. Utilizing a modified Trost-type diphosphine as the ligand, this method enables the efficient synthesis of 5-10-membered aminoallylated chiral N-heterocycles in good yields with high enantiomeric excess values.

Catalytic Intermolecular Deoxygenative Coupling of Carbonyl Compounds with Alkynes by a Cp*Mo(II)-Catalyst

Carbonyl is highly accessible and acts as an essential functional group in chemical synthesis. However, the direct catalytic deoxygenative functionalization of carbonyl compounds via a putative metal carbene intermediate is a formidable challenge due to the requirement of a high activation energy for the cleavage of strong C═O double bonds. Here, we report a class of bench stable and readily available Cp*Mo(II)-complexes as efficient deoxygenation catalysts that could catalyze the direct intermolecular deoxygenative coupling of carbonyl compounds with alkynes. Enabled by this powerful Cp*Mo(II)-catalyst, various valuable heteroarenes (10 different classes) were obtained in generally good yields and remarkable chemo- and regioselectivities. Mechanistic studies suggested that this reaction might proceed via a sequence of C═O double bonds cleavage, carbene-alkyne metathesis, cyclization, and aromatization processes. This strategy not only provided a general catalytic platform for the rapid preparation of heteroarenes but also opened a new window for the applications of Cp*Mo(II)-catalysts in organic synthesis.

Synthesis and Properties of 4- and 10-Benzoyl-1-azapyrenes

A series of 4- and 10-benzoyl-1-azapyrenes were prepared by a combination of Pd-catalyzed cross-coupling reactions and Brønsted-acid-mediated alkyne-carbonyl-metathesis (ACM). The photophysical and electrochemical properties of the products were studied and compared to theoretical results.

Carbon-Atom Exchange between [MC2]+ (M = Os and Ir) and Methane: on the Thermodynamic and Dynamic Aspects

Gas-phase reactions of [OsC2]+ and [IrC2]+ with methane at ambient temperature have been studied using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. Both [OsC2]+ and [IrC2]+ undergo carbon-atom exchange reactions with methane. The associated mechanisms for the two systems are found to be similar. The differences in the rates of carbon isotope exchange reactions of methane with [MC2]+ (M = Os and Ir) are explained by several factors like the energy barrier for the initial H3C-H bond breaking processes, the molecular dynamics, orbital interactions, and the H-binding energies of the pivotal steps. Besides, the number of participating valence orbitals might be one of the keys to regulate the rate in the key step. The present findings may provide useful ideas and inspiration for designing similar processes.

Indium-catalyzed inter- and intramolecular dithianyl-alkyne metathesis reactions

Herein, we reported an efficient indium catalyzed dithianyl-alkyne metathesis (DAM) reaction. This strategy allows for the formation of a new C-C double bond and valuable C-S bonds during the metathesis event, and was successfully applied to the synthesis of diverse vinyl dithianyl substituted organic molecules.

Quantum Neural Network Inspired Hardware Adaptable Ansatz for Efficient Quantum Simulation of Chemical Systems

The variational quantum eigensolver is a promising way to solve the Schrödinger equation on a noisy intermediate-scale quantum (NISQ) computer, while its success relies on a well-designed wave function ansatz. Inspired by the quantum neural network, we propose a new hardware heuristic ansatz where its expressibility can be improved by increasing either the depth or the width of the circuit. Such a character makes this ansatz adaptable to different hardware environments. More importantly, it provides a general framework to improve the efficiency of the quantum resource utilization. For example, on a superconducting quantum computer where circuit depth is usually the bottleneck and the qubits thus cannot be fully used, circuit depth can be significantly reduced by introducing ancilla qubits. Ancilla qubits also make the circuit less sensitive to noises in practical application. These results open a new avenue to develop practical applications of quantum computation in the NISQ era.

Metal-Free Tunable 1,2-Difunctionalization of Terminal Alkynes: Synthesis of β-Substituted α,β-Unsaturated Ketones

A metal-free tunable 1,2-difunctionalization of the terminal alkynes showcasing a tandem installation of C-C and C-S bonds has been developed. The key enabling factor for the approach is the use of acetic acid as an acyl source to synthesize β-substituted α,β-unsaturated ketones. The reaction at room temperature leads to the regioselective acylation at the terminal carbon of alkynes, whereas at -78 °C, the acylation occurs at the more substituted carbon.

Gold(I)-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis for the Synthesis of Butenolides

Alkyne-carbonyl metathesis is a type of carbon-carbon forming reaction involving the construction a carbon-carbon double bond and a carbonyl group in one transformation. Herein, a Au(I)-catalyzed ring-closing alkyne-carbonyl metathesis protocol has been developed to make densely substituted γ-butenolides from propargyl α-ketoesters. It features 100 % atom economy, excellent substrate flexibility and benign functional group tolerance. Mechanistic studies demonstrate that the coordinative interaction between the gold catalyst and the alkyne might initiate the transfer of an oxygen atom and the formation of the carbon-carbon double bond. By using this gold-catalyzed ring-closing alkyne-carbonyl metathesis as a key step reaction, four naturally occurring butenolide-type compounds including decumbic acid (45 % yield for 3 steps), deoxyisosporothric acid (32 % yield for 5 steps), lichesterinic acid (34 % yield for 5 steps) and isomuronic acid (6 % yield for 8 steps) have been synthesized starting from commercially available starting materials.

Ruthenium-Catalyzed Chemo-Selective Carbene Insertion into C-H Bond of Styrene over Cyclopropanation: C-C Bond Formation

The C-C bond formation reactions are important in organic synthesis. Heck reaction is known to arylate the terminal carbon of olefins; however, direct alkylation of the terminal carbon of olefin is limited. Herein, we report a novel ruthenium-catalyzed selective cross-coupling reaction of styrene and α-diazoesters to form a new C-C bond over cyclopropanation via the C-H insertion process for the first time. Using this novel methodology, a wide variety of substrates have been utilized and a variety of α-vinylated benzylic esters and densely functionalized olefins have been synthesized with good stereoselectivity under mild reaction conditions. The overall reaction process proceeds through the carbene insertion into styrene to form the desired products in good to excellent yields with proper stereoselectivity. The selective C-H inserted product, wide substrate scope, and excellent functional group tolerance are the best features of this work.

Iron mediated reductive cyclization/oxidation for the generation of chemically diverse scaffolds: An approach in drug discovery

Chemically diverse scaffolds represent a main source of biologically important starting points in drug discovery. Herein, we report the development of such diverse scaffolds from nitroarene/ nitro(hetero)arenes using a key synthetic strategy. In a pilot-scale study, the synthesis of 10 diverse scaffolds was achieved. The 1,7-phenanthroline, thiazolo[5,4-f]quinoline, 2,3-dihydro-1H-pyrrolo[2,3-g]quinoline, pyrrolo[3,2-f]quinoline, 1H-[1,4]oxazino[3,2-g]quinolin-2(3H)-one, [1,2,5]oxadiazolo[3,4-h]quinoline, 7H-pyrido[2,3-c]carbazole, 3H-pyrazolo[4,3-f]quinoline, pyrido[3,2-f]quinoxaline were obtained from nitro hetero arenes in ethanol using iron-acetic acid treatment followed by reaction under oxygen atmosphere. This diverse library is compliant with the rule of five for drug-likeness. The mapping of chemical space represented by these scaffolds revealed a significant contribution to the underrepresented chemical diversity. Crucial to the development of this approach was the mapping of biological space covered by these scaffolds which revealed neurotropic and prophylactic anti-inflammatory activities. In vitro, neuro-biological assays revealed that compounds 14a and 15a showed excellent neurotropic potential and neurite growth compared to controls. Further, anti-inflammatory assays (in vitro and in vivo models) exhibited that Compound 16 showed significant anti-inflammatory activity by attenuating the LPS-induced TNF-α and CD68 levels by modulating the NFkB pathway. In addition, treatment with compound 16 significantly ameliorated the LPS-induced sepsis conditions, and pathological abnormalities (in lung and liver tissues) and improved the survival of the rats compared to LPS control. Owing to their chemical diversity along with bioactivities, it is envisaged that new quality pre-clinical candidates will be generated in the above therapeutic areas using identified leads.

Enantioselective synthesis of α,α-diarylketones by sequential visible light photoactivation and phosphoric acid catalysis

Chiral ketones and their derivatives are useful synthetic intermediates for the synthesis of biologically active natural products and medicinally relevant molecules. Nevertheless, general and broadly applicable methods for enantioenriched acyclic α,α-disubstituted ketones, especially α,α-diarylketones, remain largely underdeveloped, owing to the easy racemization. Here, we report a visible light photoactivation and phosphoric acid-catalyzed alkyne-carbonyl metathesis/transfer hydrogenation one-pot reaction using arylalkyne, benzoquinone, and Hantzsch ester for the expeditious synthesis of α,α-diarylketones with excellent yields and enantioselectivities. In the reaction, three chemical bonds, including C═O, C─C, and C─H, are formed, providing a de novo synthesis reaction for chiral α,α-diarylketones. Moreover, this protocol provides a convenient and practical method to synthesize or modify complex bioactive molecules, including efficient routes to florylpicoxamid and BRL-15572 analogs. Computational mechanistic studies revealed that C-H/π interactions, π-π interaction, and the substituents of Hantzsch ester all play crucial roles in the stereocontrol of the reaction.

Construction of 4-Acyl-2-quinolones through Sc(OTf)3-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis

An atom-economical protocol to construct densely substituted 4-acyl-2-quinolones from N-(2-alkynylphenyl)-α-ketoamides has been developed through Sc(OTf)₃-catalyzed ring-closing alkyne-carbonyl metathesis. Mechanistic experimental studies support that coordinative interaction between Sc(OTf))₃ and the substrate, the formation of an oxetene intermediate, and an electrocyclic ring-opening of the oxetene might be involved.

Rhodium-catalyzed intramolecular reductive aldol-type cyclization: Application for the synthesis of a chiral necic acid lactone

A rhodium-catalyzed intramolecular reductive aldol-type cyclization is described to give β-hydroxylactones with high diastereoselectivities. The stereoselectivity of this cyclization is highly solvent dependent and can give syn- or anti-β-hydroxylactones with high diastereoselectivity. This methodology was also applied to the synthesis of a chiral necic acid lactone which is a structural component of the pyrrolizidine alkaloid monocrotaline.

Iron-Catalyzed Olefin Metathesis: Recent Theoretical and Experimental Advances

The "metathesis reaction" is a straightforward and often metal-catalyzed chemical reaction that transforms two hydrocarbon molecules to two new hydrocarbons by exchange of molecular fragments. Alkane, alkene and alkyne metathesis have become an important tool in synthetic chemistry and have provided access to complex organic structures. Since the discovery of industrial olefin metathesis in the 1960s, many modifications have been reported; thus, increasing scope and improving reaction selectivity. Olefin metathesis catalysts based on high-valent group six elements or Ru(IV) have been developed and improved through ligand modifications. In addition, significant effort was invested to realize olefin metathesis with a non-toxic, bio-compatible and one of the most abundant elements in the earth's crust; namely, iron. First evidences suggest that low-valent Fe(II) complexes are active in olefin metathesis. Although the latter has not been unambiguously established, this review summarizes the key advances in the field and aims to guide through the challenges.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe₂CH₂C₆H₄)(mesityl)Sb]⁺ (A) and [(NMe₂C₆H₄)(mesityl)Sb]⁺ (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.

Ring contraction in synthesis of functionalized carbocycles

Carbocycles are a key and widely present structural motif in organic compounds. The construction of structurally intriguing carbocycles, such as highly-strained fused rings, spirocycles or highly-functionalized carbocycles with congested stereocenters, remains challenging in organic chemistry. Cyclopropanes, cyclobutanes and cyclopentanes within such carbocycles can be synthesized through ring contraction. These ring contractions involve re-arrangement of and/or small molecule extrusion from a parental ring, which is either a carbocycle or a heterocycle of larger size. This review provides an overview of synthetic methods for ring contractions to form cyclopropanes, cyclobutanes and cyclopentanes en route to structurally intriguing carbocycles.

Toward Homogeneous Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Metathesis Reactions

The carbonyl-olefin metathesis (COM) reaction is an attractive approach for the formation of a new carbon-carbon double bond from a carbonyl precursor. In principle, this reaction can be promoted by the activation of the carbonyl group with a Brønsted acid catalyst; however, it is often complicated as a result of unwanted side reactions under acidic conditions. Thus, there have been only a very few examples of Brønsted-acid-catalyzed COM reactions, all of which required specially designed setups. Herein, we report a new practical homogeneous Brønsted-acid-catalyzed protocol using nitromethane, a readily available solvent, to promote intramolecular ring-closing COM reactions.

Explaining Regiodivergent Vinylations with Vinylbenziodoxolones**

Vinylbenziodoxolones have recently been identified as efficient hypervalent iodine(III) reagents for electrophilic vinylations under transition metal‐free conditions. Their unique reactivity allows synthesis of either internal or terminal alkenes, depending on the nucleophile class. This paper constitutes the first mechanistic investigation of VBX vinylations, and makes use of NMR studies, deuterium labelling and computations to rationalize the observed regio‐ and stereochemical outcome. Internal alkene formation in S‐vinylation was found to proceed through the ligand coupling mechanism typical of diaryliodonium salts, whereas terminal alkene formation in P‐vinylations took place via a phosphinous acid‐coordinated VBX complex, which underwent concerted deprotonation and Michael‐type addition. Subsequent base‐assisted protonation and E2 elimination delivered the terminal alkene. The findings can be used to predict the regioselectivity in vinylations of other nucleophile classes.

Beyond classical sulfone chemistry: metal- and photocatalytic approaches for C-S bond functionalization of sulfones

The exceptional versatility of sulfones has been extensively exploited in organic synthesis across several decades. Since the first demonstration in 2005 that sulfones can participate in Pd-catalysed Suzuki-Miyaura type reactions, tremendous advances in catalytic desulfitative functionalizations have opened a new area of research with burgeoning activity in recent years. This emerging field is displaying sulfone derivatives as a new class of substrates enabling catalytic C-C and C-X bond construction. In this review, we will discuss new facets of sulfone reactivity toward further expanding the flexibility of C-S bonds, with an emphasis on key mechanistic features. The inherent challenges confronting the development of these strategies will be presented, along with the potential application of this chemistry for the synthesis of natural products. Taken together, this knowledge should stimulate impactful improvements on the use of sulfones in catalytic desulfitative C-C and C-X bond formation. A main goal of this article is to bring this technology to the mainstream catalysis practice and to serve as inspiration for new perspectives in catalytic transformations.

Rhodium-Catalyzed Three-Component Reaction of Alkynes, Arylzinc Chlorides, and Iodomethanes Producing Trisubstituted/Tetrasubstituted Alkenes with/without 1,4-Migration

A three-component reaction of alkynes, arylzinc chlorides, and iodomethanes was found to proceed in the presence of a rhodium catalyst to give high yields of trisubstituted/tetrasubstituted alkenes. The usual arylzinc chlorides only gave trisubstituted alkenes, generated through a migratory carbozincation-cross-coupling sequence, where 1,4-Rh migration from an alkenyl carbon to an aryl carbon occurred. In contrast, 5-membered heteroarylzinc chlorides only gave the tetrasubstituted alkenes via a carborhodation-cross-coupling pathway without 1,4-migration.

Hydrogen Bonding Networks Enable Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis

Synthetic chemists have learned to mimic nature in using hydrogen bonds and other weak interactions to dictate the spatial arrangement of reaction substrates and to stabilize transition states to enable highly efficient and selective reactions. The activation of a catalyst molecule itself by hydrogen‐bonding networks, in order to enhance its catalytic activity to achieve a desired reaction outcome, is less explored in organic synthesis, despite being a commonly found phenomenon in nature. Herein, we show our investigation into this underexplored area by studying the promotion of carbonyl‐olefin metathesis reactions by hydrogen‐bonding‐assisted Brønsted acid catalysis, using hexafluoroisopropanol (HFIP) solvent in combination with para‐toluenesulfonic acid (pTSA). Our experimental and computational mechanistic studies reveal not only an interesting role of HFIP solvent in assisting pTSA Brønsted acid catalyst, but also insightful knowledge about the current limitations of the carbonyl‐olefin metathesis reaction.

Regioselective Synthesis of Naphthothiophenes by Pd Catalyzed Cross-Coupling Reactions and Alkyne-Carbonyl Metathesis

Naphthothiophenes were prepared from commercially available 2,3-dibromothiophenes in two steps by one-pot Suzuki/Sonogashira or Sonogashira/Suzuki coupling reactions, followed by intramolecular alkyne-carbonyl-metathesis reactions. The final cyclization reaction proceeds in the presence of p-toluenesulfonic acid and provides a rapid access to two series of isomeric naphthothiophenes.

Olefin Metathesis in Confined Spaces: The Encapsulation of Hoveyda-Grubbs Catalyst in Peanut, Square, and Capsule Shaped Hollow Silica Gels

In this study, Hoveyda-Grubbs 2nd generation (HG2) catalyst was encapsulated in hollow mesoporous silica gels with various morphologies (peanut, square, and capsule) by reducing the pore size of the mesoporous...

Expanding the Protecting Group Scope for the Carbonyl Olefin Metathesis Approach to 2,5-Dihydropyrroles

Chiral pyrrolidine derivatives are important building blocks for natural product synthesis. Carbonyl olefin metathesis has recently emerged as a powerful tool for the construction of such building blocks from chiral amino acid derivatives. Here, we demonstrate that the supramolecular resorcinarene catalyst enables access to chiral 2,5-dihydropyrroles under Brønsted acid catalysis. Moreover, this catalytic system even tolerated Lewis-basic-protecting groups like mesylates that are not compatible with alternative catalysts. As expected for conversion inside a closed cavity, the product yield and selectivity depended on the size of the substrates.

Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis: Synthesis of Phenanthrenes via Phosphomolybdic Acid as a Catalyst

Compared with the impressive achievements of catalytic carbonyl-olefin metathesis (CCOM) mediated by Lewis acid catalysts, exploration of the CCOM through Brønsted acid-catalyzed approaches remains quite challenging. Herein, we disclose a synthetic protocol for the construction of a valuable polycycle scaffold through the CCOM with the inexpensive, nontoxic phosphomolybdic acid as a catalyst. The current annulations could realize carbonyl-olefin, carbonyl-alcohol, and acetal-alcohol in situ CCOM reactions and feature mild reaction conditions, simple manipulation, and scalability, making this strategy a promising alternative to the Lewis acid-catalyzed COM reaction.

Synthesis of Dibenzotropones by Alkyne-Carbonyl Metathesis

Dibenzocycloheptanones (dibenzotropones) were prepared by Brønsted acid mediated intramolecular alkyne-carbonyl metathesis (ACM) reactions. The cyclization precursors are readily available by Sonogashira reaction of 2-bromobenzoyl chloride with terminal alkynes, followed by Suzuki reactions with benzaldehydes. The ACM reactions are highly modular and atom economic and allow for the construction of two regioisomeric series of dibenzotropones.

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂ , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂ IMe, Me₂ Iⁱ Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n ; n=3, 4, 5, 6] and Ph₂ C=CPh₂ . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂ CPh₂ and cyclo-(ArP)₄ CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

Thiophosphonium-Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl-Alkyne Metathesis

While the metathesis reaction between alkynes and thiocarbonyl compounds has been thoroughly studied, the reactivity of alkynes with isoelectronic main group R₂E=S compounds is rarely reported and unknown for [R₂P=S]⁺ analogues. We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C–C=S heterodiene structure and thus undergo hetero-Diels–Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C−C=S heterodiene structure and thus undergo hetero-Diels−Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

Heterogeneously Ni-Pd nanoparticle-catalyzed base-free formal C-S bond metathesis of thiols

This study rationally designed a heterogeneously catalyzed system (i.e., using Ni-Pd alloy nanoparticles supported on hydroxyapatite (Ni-Pd/HAP) under an H₂ atmosphere) achieving an efficient base-free formal C-S bond metathesis of various thiols via suppression of the Ni catalysis deactivation.

Synthesis of α-(aminoethyl)-α,β-enones via alkyne aza-Prins cyclization and their synthetic application to pyrrolidines

We developed a synthetic method for α-(aminoethyl)-α,β-enones from aryl-substituted homopropargyl sulfonamides and aldehydes, representing the first synthesis of conjugated enones via alkyne aza-Prins cyclization. These products could be converted into pyrrolidines by a formal 5-endo-trig cyclization.

Ni-catalyzed regio- and stereo-defined intermolecular cross-electrophile dialkylation of alkynes without directing group

The development of straightforward synthesis of regio- and stereodefined alkenes with multiple aliphatic substituents under mild conditions is an unmet challenge owing to competitive β-hydride elimination and selectivity issues. Herein, we report the nickel-catalyzed intermolecular cross-dialkylation of alkynes devoid of directing or activating groups to afford multiple aliphatic substituted alkenes in a syn-selective fashion at room temperature. The combination of two-electron oxidative cyclometallation and single-electron cross-electrophile coupling of nickel enables the syn-cross-dialkylation of alkynes at room temperature. This reductive protocol enables the sequential installation of two different alkyl substituents onto alkynes in a regio- and stereo-selective manner, circumventing the tedious preformation of sensitive organometallic reagents. The synthetic utility of this protocol is demonstrated by efficient synthesis of multi-substituted unfunctionalized alkenes and diverse transformations of the product.

Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction

Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Brønsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Brønsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive. TfOH's conjugate base is also found to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction. The mechanism explains available experimental information, including the skipped diene species that appear transiently before product formation. The present study clarifies the mechanism for activation of intramolecular carbonyl-olefin substrates by Brønsted acids and provides important insights that will help develop this exciting class of catalysts.

A unique high-diversity natural product collection as a reservoir of new therapeutic leads

We review the successful application of computer-aided methods to screen a unique and high-diversity in house collection library composed of around 1000 individual natural products.