Iron-Catalyzed Ring-Closing C−O/C−O Metathesis of Aliphatic Ethers

Carbonylation Reactions at Carbon-Centered Radicals with an Adjacent Heteroatom

Heteroatoms are essential to living organisms and present in almost all molecules with medicinal usage. The catalytic functionalization at the carbon-centered radical with an adjacent heteroatom provides an effective way to value added moiety while retaining the unique physicochemical and pharmacological properties of heteroatoms, which can promote the development of pharmaceutical and fine chemical production. Carbonylative transformation was discovered nearly a century ago which is an efficient method for the synthesis of carbonyl-containing molecules with potent applications in both industry and academia. Despite numerous advances in new reaction development, carbonylative transformation involving adjacent heteroatom carbon radical remain a subject that deserves to be discussed. In this minireview, we systematically summarized and discussed the recent advances in carbonylative transformations involving carbon-centered radicals with an adjacent heteroatom, including oxygen (O), nitrogen (N), phosphorus (P), silicon (Si), sulfur (S), boron (B), fluorine (F), and chlorine (Cl). The related reaction mechanism was also discussed.

Hyperconjugation-Driven Isodesmic Reaction of Indoles and Anilines: Reaction Discovery, Mechanism Study, and Antitumor Application

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional-group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five-membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology.

Intermolecular C-C/C-N σ-bond metathesis enabled by visible light

Transition-metal-catalyzed double/triple bond metathesis reactions have been well-established due to the ability of transition-metal catalysts to readily interact with π bonds, facilitating the progression of the entire reaction. However, activating σ-bonds to induce σ-bond metathesis is more challenging due to the absence of π bonds and the high bond energy of σ bonds. In this study, we present a novel photo-induced approach that does not rely on transition metals or photosensitizers to drive C-C and C-N σ-bond metathesis reactions. This method enables the cross-coupling of tertiary amines with α-diketones via C-C and C-N single bonds cleavage and recombination. Notably, our protocol exhibits good compatibility with various functional groups in the absence of transition metals and external photosensitizers, resulting in the formation of aryl alkyl ketones and aromatic amides in good to high yields. To gain insights into the mechanism of this pathway, we conducted controlled experiments, intermediate trapping experiments, and DFT (Density Functional Theory) calculations. This comprehensive approach allowed us to elucidate the detailed mechanism underlying this transformative reaction.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

DFT investigation of the DDQ-catalytic mechanism for constructing C-O bonds

In this study, we investigated the photo-catalytic mechanisms for the construction of C-O bonds from arenes (benzene, 2',6'-dimethyl-[1,1'-biphenyl]-2-carboxylic acid, or 2,4-dichloro-1-fluorobenzene), catalyzed by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). All the structures for the Gibbs free surfaces were calculated at the M06-2X-D3/ma-def2-SVP level in the SMD solvent model. Also, TDDFT calculations of DDQ were performed at the PBE1PBE-D3/ma-def2-SVP level in the SMD solvent model. The computational results indicated that DDQ, serving as a photo-catalyst, would be excited under visible light of 450 nm, aligning well with experimental observations as reflected in the UV-vis spectrum. Gibbs free energy surface analyses of the three reactions suggested that the path involving 3DDQ* activating the reactant (-COOH, H2O, or CH3OH) is favorable. Additionally, the role of O2 was investigated, revealing that it could facilitate the recycling of DDQ by lowering the energy barrier for the conversion of the DDQH˙ radical (not DDQH2) into DDQ. The use of ρhole and ρele can reveal the photo-catalytic reaction and charge transfer processes, while localized orbital locator isosurfaces and electron spin density isosurface graphs were employed to analyze structures and elucidate the single electron distribution. These computational results offer valuable insights into the studied interactions and related processes, shedding light on the mechanisms governing C-O bond formation from arenes catalyzed by DDQ.

Ring Expansion toward Disila-carbocycles via Highly Selective C-Si/C-Si Bond Cross-Exchange

Herein, we successfully inhibited the preferential homodimerization and C-Si/Si-H bond cross-exchange of benzosilacyclobutenes and monohydro-silacyclobutanes and achieved the first highly selective C-Si/C-Si bond cross-exchange reaction by deliberately tuning the Ni-catalytic system, which constitutes a powerful and atom-economical ring expansion method for preparing medium-sized cyclic compounds bearing two silicon atoms at the ring junction, which are otherwise inaccessible. The DFT calculation explicitly elucidated the pivotal role of Si-H bond at silacyclobutanes and the high ring strain of two substrates in realizing the two C-Si bonds cleavage and reformation in the catalytic cycle.

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.

Synthesis of fungicidal morpholines and isochromenopyridinones via acid-catalyzed intramolecular reactions of isoindolinones

Acid-catalyzed intramolecular cyclization or rearrangement of isoindolinone derivatives is described. 3-Hydroxy/ethoxy-3,4-dihydro-6H-[1,4]-oxazino-[3,4-a]-isoindol-6-ones are obtained in moderate to good yields. Further acid-catalyzed intramolecular rearrangement reactions give 6H-isochromeno-[4,3-b]-pyridin-6-ones. The mild reaction conditions with convenient starting materials show broad substrate scope and provide the target compounds as novel pesticide leads with good fungicidal or systemical acquired resistance activities.

Synthesis and properties of photoluminescent phosphorus-doped triptycenes

A new class of phosphorus-doped triptycenes was designed and synthesized via a Diels-Alder reaction between alkynylphosphonates and anthracene, followed by oxidative cyclization. The packing interaction and molecular alignment in the single crystals revealed that the weak C-H⋯π (2.825 Å) interaction guides the self-assembly of phosphindole oxide iptycenes. The photophysical and electrochemical properties of these photoluminescent phosphorus-doped iptycenes were characterized to gain a deeper understanding of their fluorescence tunability. The presence of functional groups on the phenyl ring of the P-doped fin and the chemical environment of the P atom both had an effect on the fluorescence emission.

Generality-oriented optimization of enantioselective aminoxyl radical catalysis

Catalytic enantioselective methods that are generally applicable to a broad range of substrates are rare. We report a strategy for the oxidative desymmetrization of meso-diols predicated on a nontraditional catalyst optimization protocol by using a panel of screening substrates rather than a singular model substrate. Critical to this approach was rational modulation of a peptide sequence in the catalyst incorporating a distinct aminoxyl-based active residue. A general catalyst emerged, providing high selectivity in the delivery of enantioenriched lactones across a broad range of diols, while also achieving up to ~100,000 turnovers.

Ring-closing C-O/C-O metathesis of ethers with primary aliphatic alcohols

In canonical organic chemistry textbooks, the widely adopted mechanism for the classic transetherifications between ethers and alcohols starts with the activation of the ether in order to weaken the C-O bond, followed by the nucleophilic attack by the alcohol hydroxy group, resulting in a net C-O/O-H σ-bond metathesis. In this manuscript, our experimental and computational investigation of a Re₂O₇ mediated ring-closing transetherification challenges the fundamental tenets of the traditional transetherification mechanism. Instead of ether activation, the alternative activation of the hydroxy group followed by nucleophilic attack of ether is realized by commercially available Re₂O₇ through the formation of perrhenate ester intermediate in hexafluoroisopropanol (HFIP), which results in an unusual C-O/C-O σ-bond metathesis. Due to the preference for the activation of alcohol rather than ether, this intramolecular transetherification reaction is therefore suitable for substrates bearing multiple ether moieties, unparalleled by any previous methods.

A Green Route to Benzyl Phenyl Sulfide from Thioanisole and Benzyl Alcohol over Dual Functional Ionic Liquids

Benzyl phenyl sulfide is a kind of important chemicals with wide usage, which is mainly prepared through a nucleophilic reaction of thiophenol with benzyl chlorides or benzyl alcohols, suffering from inherent drawbacks, such as low efficiency, requirements for equivalent acid or base catalysts and formation of harmful byproducts and waste. Herein, we report a green route to access various benzyl phenyl sulfide derivatives in good to excellent yields under mild conditions via the reaction of thioanisoles with benzyl alcohols over ionic liquid 1-propylsulfonate-3-methylimidazolium trifluoromethanesulfonate ([SO₃ HPrMIm][OTf]). Mechanism investigation indicates that the synergic effect of cation and anion of [SO₃ HPrMIm][OTf] activates thioanisoles and benzyl alcohols via hydrogen bonding, thus catalyzes the dehydration of benzyl alcohol to dibenzyl ether and the subsequent metathesis reaction between dibenzyl ether and benzyl phenyl sulfide, finally generating benzyl phenyl sulfide derivatives. This is a simple, highly efficient, and green approach to produce benzyl phenyl sulfide derivatives, which has promising application potentials.

Silicon Catalyzed C-O Bond Ring Closing Metathesis of Polyethers

The Lewis superacid bis(perchlorocatecholato)silane catalyzes C-O bond metathesis of alkyl ethers with an efficiency outperforming all earlier reported systems. Chemoselective ring contractions of macrocyclic crown ethers enable substrate-specific transformations, and an unprecedented ring-closing metathesis of polyethylene glycols allows polymer-selective degradation. Quantum chemical computations scrutinize a high Lewis acidity paired with a simultaneous low propensity for polydentate substrate binding as critical for successful catalysis. Based on these mechanistic insights, a second-generation class of silicon Lewis superacid with enhanced efficacy is identified and demonstrated.

Ionic-Liquid-Catalyzed Approaches under Metal-Free Conditions

ConspectusMetal-free catalysis is a promising protocol to access chemicals without metal contamination. Ionic liquids (ILs) that are entirely composed of organic cations and inorganic/organic anions have emerged as promising alternatives to molecular solvents and metal catalysts due to their unique properties such as structural tunability, the coexistence of multiple interactions among ions (e.g., electrostatic interaction, hydrogen bonding, van de Waals forces, acid/base interactions, hydrophilic/hydrophobic interactions, etc.), unique affinity for a wide range of chemicals, good chemical and thermal stability, and quite low volatility. ILs have shown potential applications in various chemical processes.In this Account, we systematically described our most recent work on IL-catalyzed approaches under metal-free conditions. The first section presents the IL-catalyzed strategies toward the transformation of CO2 to value-added chemicals, focusing on the CO2-reactive IL-catalyzed CO2 transformation to various heterocycles and the IL-catalyzed reductive transformation of CO2 to chemicals. In these approaches, we designed task-specific ILs that are able to chemically capture and activate CO2 via forming anion-based carbonate/carbamate or cation-based carboxylate/carbamate intermediates, thus further accomplishing its transformation to a series of heterocycles including quinazoline-2,4(1H,3H)-diones, cyclic carbonates, 2-oxazolidinones, oxazolones, and benzimidazolones under metal-free conditions. For the IL-catalyzed approaches to reducing CO2 with hydrosilanes to chemicals, we employed ILs capable of activating the Si-H bond in hydrosilanes and the N-H bond in amine substrates via H-bonding, thus achieving the reductive transformation of CO2 to formamides, benzimidazoles, and benzothiazoles via cooperative catalysis. The second section describes our finding on the IL-catalyzed hydration of the C≡C bond in propargylic alcohols. Azolate anion-based ILs that can chemically capture CO2 via the formation of carbamates could serve as robust nucleophiles to attack the C≡C bond in propargylic alcohols and then efficiently catalyze the hydration of propargylic alcohols to produce α-hydroxy ketones with the assistance of atmospheric CO2 gas under metal-free conditions. The third section unveils the cooperative catalysis strategy of hydrogen bond donors and acceptors of ILs for chemical reactions. In the hydrogen-bonding catalysis protocols, cations of the ILs act as H-bond donors and anions, as acceptors, forming H-bonds with the reactant molecules, respectively, in opposite ways, which can cooperatively catalyze the ring-closing C-O/C-O bond metathesis reactions of aliphatic diethers to O-heterocycles, the dehydrative etherification of alcohols to ethers, and direct oxidative esterification of alcohols to esters. We believe that these IL-catalyzed metal-free processes and strategies display promising practical applications, and their commercialization would bring great benefits to the production of the as-afforded value-added chemicals.

Hydrogen-bond donor and acceptor cooperative catalysis strategy for cyclic dehydration of diols to access O-heterocycles

Dehydrative cyclization of diols to O-heterocycles is attractive, but acid and/or metal-based catalysts are generally required. Here, we present a hydrogen-bond donor and acceptor cooperative catalysis strategy for the synthesis of O-heterocycles from diols in ionic liquids [ILs; e.g., 1-hydroxyethyl-3-methyl imidazolium trifluoromethanesulfonate ([HO-EtMIm][OTf])] under metal-free, acid-free, and mild conditions. [HO-EtMIm][OTf] is tolerant to a wide diol scope, shows performance even better than H₂SO₄, and affords a series of O-heterocycles including tetrahydrofurans, tetrahydropyrans, morpholines, dioxanes, and thioxane in high yields. Mechanism investigation indicates that the IL cation and anion serve as hydrogen-bond donor and acceptor, respectively, to activate the C─O and O─H bonds of alcohol via hydrogen bonds, which synergistically catalyze dehydrative cyclization of diols to O-heterocycles. Notably, the products could be spontaneously separated after reaction because of their immiscibility with the IL, and the IL could be recycled. This green strategy has great potential for application in industry.

Iron-Catalyzed Carbonyl–Alkyne and Carbonyl–Olefin Metathesis Reactions

Construction of carbon–carbon bonds is one of the most important tools for the synthesis of complex organic molecules. Among multiple possibilities are the carbonyl–alkyne and carbonyl–olefin metathesis reactions, which are used to form new carbon–carbon bonds between carbonyl derivatives and unsaturated organic compounds. As many different approaches have already been established and offer reliable access to C=C bond formation via carbonyl–alkyne and carbonyl–olefin metathesis, focus is now shifting towards cost efficiency, sustainability and environmentally friendly metal catalysts. Iron, which is earth-abundant and considered as an eco-friendly and inexpensive option in comparison to traditional metal catalysts, fulfils these requirements. Hence, the focus of this review is on recent advances in the iron-catalyzed carbonyl–alkyne, carbonyl–olefin and related C–O/C–O metathesis reactions. The still large research potential for ecologically and economically attractive and sustainable iron-based catalysts is demonstrated.

Understanding the mechanism and reactivity of Pd-catalyzed C-P bond metathesis of aryl phosphines: a computational study

Transition metal-catalyzed single bond metathesis has recently emerged as a useful strategy for functional group transfer. In this work, we explored the mechanism and reactivity profile of Pd/PhI-cocatalyzed C-P bond metathesis between aryl phosphines using density functional theory (DFT) calculations. The overall single bond metathesis involves two Pd(ii)-catalyzed C-P reductive eliminations and two Pd(0)-catalyzed C-P oxidative additions, which allows the reversible C-P bond cleavage and formation of the phosphonium cation. Distortion/interaction analysis indicates that the facile C-P bond cleavage and formation of the phosphonium cation are due to the involvement of coordinating aryl phosphine in the process. In addition, the substituent effects on the reaction kinetics and thermodynamics of metathesis were computed, which provides helpful mechanistic information for the design of related single bond metathesis reactions.

Overcoming Selectivity Issues in Reversible Catalysis: A Transfer Hydrocyanation Exhibiting High Kinetic Control

Reversible catalytic reactions operate under thermodynamic control, and thus, establishing a selective catalytic system poses a considerable challenge. Herein, we report a reversible transfer hydrocyanation protocol that exhibits high selectivity for the thermodynamically less favorable branched isomer. Selectivity is achieved by exploiting the lower barrier for C-CN oxidative addition and reductive elimination at benzylic positions in the absence of a cocatalytic Lewis acid. Through the design of a novel type of HCN donor, a practical, branched-selective, HCN-free transfer hydrocyanation was realized. The synthetically useful resolution of a mixture of branched and linear nitrile isomers was also demonstrated to underline the value of reversible and selective transfer reactions. In a broader context, this work demonstrates that high kinetic selectivity can be achieved in reversible transfer reactions, thus opening new horizons for their synthetic applications.

Morpholine As a Scaffold in Medicinal Chemistry: An Update on Synthetic Strategies

Morpholine is a frequently used heterocycle in medicinal chemistry and a privileged structural component of bioactive molecules. This is mainly due to its contribution to a plethora of biological activities as well as to an improved pharmacokinetic profile of such bioactive molecules. The synthesis of morpholines is a subject of much study due to their biological and pharmacological importance, with the last such review being published in 2013. Here, an overview of the main approaches toward morpholine synthesis or functionalization is presented, emphasizing on novel work which has not been reviewed so far. This review is an update on synthetic strategies leading to easily accessible libraries of bioactives which are of interest for drug discovery projects.

Diastereoselective Intramolecular Hydride Transfer Triggered by Electrophilic Halogenation of Aryl Alkenes

Diastereoselective hydride transfer could be triggered by electrophilic halogenation (bromination or fluorination) of homoallylic alcohol O-Bn ethers. The resulting diastereomerically enriched haloalkyl alcohols underwent subsequent intramolecular nucleophilic substitution to afford the corresponding tetrahydrofurans.

Two-Photon Absorption and Cell Imaging of Fluorene-Functionalized Epicocconone Analogues

Epicocconone 1 is a natural chromophore isolated from the fungus Epicoccum nigrum that has shown applications in proteomics and fluorescent microscopy thanks to its unique pro-fluorescence properties. The modification of the skeleton of the natural product by replacing the triene side chain by a fluorenyl scaffold can noticeably increase the fluorophore's absorption coefficient. The synthesis of the analogues of the natural product has been made possible by the use of a palladium-catalyzed carbonylation reaction, allowing the construction of the β-keto-dioxinone key intermediate. Two-photon absorption cross-section measurements of the fluorenyl epicocconone analogues show a structure dependency with values ranging from 60 to 280 GM and live cell imaging show intense staining of intracellular vesicle-like structures around the nucleus.