Activation of “Inert” Alkenyl/Aryl CO Bond and Its Application in Cross-Coupling Reactions

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Unimolecular Fragment Coupling: A New Bond-Forming Methodology via the Deletion of Atom(s)

Unimolecular fragment coupling (UFC) is defined as a reaction format, wherein atom(s) located in the middle of a molecule are extruded, and the remaining fragments are coupled. UFC is a potentially powerful strategy that is an alternative to transition-metal-catalyzed cross-coupling because the target chemical bond is formed in an intramolecular fashion, which is inherently beneficial for chemoselectivity and stereoselectivity issues. In this Perspective, we will present an overview of the recent advances in UFC reactions, which encompass those proceeding through the elimination of CO2, CO, SO2, isocyanates, N2, or single atoms primarily via transition metal catalysis.

α-Arylsulfonyloxyacrylates: attractive O-centered electrophiles for synthesis of α-substituted acrylates via Pd-catalysed Suzuki reactions

We herein report α-arylsulfonyloxyacrylates as a kind of useful and attractive O-centered electrophiles for Suzuki cross-coupling reactions. A range of α-(hetero)aryl substituted acrylates has been prepared via the palladium-catalysed C-C cross-coupling reactions between potassium (hetero)aryltrifluoroborates and α-arylsulfonyloxyacrylates. Moreover, α-arylsulfonyloxyacrylate could also react with B-alkyl-9-BBN to produce α-alkyl substituted acrylates. The synthetic application of this new method was demonstrated by the preparation of the intermediate for synthesis of retinoid X receptors-selective retinoids. These Suzuki reaction-based protocols feature broad substrate scope, generality, and mild reaction conditions.

Direct Stannylation and Silylation of Arylmethanols by Palladium Catalysis

A direct transformation of non-preactivated benzyl alcohols to benzyl stannanes and benzyl silanes was realized through Pd-catalyzed C(sp³)-O activation process. By using versatile tin and silicon sources, these reactions exhibit a broad substrate scope and a high efficiency under mild conditions, affording functionalized benzyl and allylic stannanes and benzylsilanes with high yields. The successful implementation of gram-scale stannylation/silylation as well as the one-pot Stille coupling reaction demonstrates the potential application of this method in organic synthesis. Both experimental and theoretical investigations reveal the mechanistic details of this reaction.

Diverse Alkyl-Silyl Cross-Coupling via Homolysis of Unactivated C(sp3)-O Bonds with the Cooperation of Gold Nanoparticles and Amphoteric Zirconium Oxides

Since C(sp³)-O bonds are a ubiquitous chemical motif in both natural and artificial organic molecules, the universal transformation of C(sp³)-O bonds will be a key technology for achieving carbon neutrality. We report herein that gold nanoparticles supported on amphoteric metal oxides, namely, ZrO₂, efficiently generated alkyl radicals via homolysis of unactivated C(sp³)-O bonds, which consequently promoted C(sp³)-Si bond formation to give diverse organosilicon compounds. A wide array of esters and ethers, which are either commercially available or easily synthesized from alcohols participated in the heterogeneous gold-catalyzed silylation by disilanes to give diverse alkyl-, allyl-, benzyl-, and allenyl silanes in high yields. In addition, this novel reaction technology for C(sp³)-O bond transformation could be applied to the upcycling of polyesters, i.e., the degradation of polyesters and the synthesis of organosilanes were realized concurrently by the unique catalysis of supported gold nanoparticles. Mechanistic studies corroborated the notion that the generation of alkyl radicals is involved in C(sp³)-Si coupling and the cooperation of gold and an acid-base pair on ZrO₂ is responsible for the homolysis of stable C(sp³)-O bonds. The high reusability and air tolerance of the heterogeneous gold catalysts as well as a simple, scalable, and green reaction system enabled the practical synthesis of diverse organosilicon compounds.

Deaminative Arylation and Alkenyaltion of Aliphatic Tertiary Amines with Aryl and Alkenylboronic Acids via Nitrogen Ylides

Transition-metal-catalyzed Suzuki-Miyaura coupling has significantly advanced C-C bond formation and has been well recognized in organic synthesis, pharmaceuticals, materials science and other fields. In this rapid development, cross coupling without transition metal catalyst is a big challenge in this field, and using widely existing tertiary amines as electrophiles to directly couple with boronic acids has great hurdles yet significant application prospects. Herein, we report an efficient and general deaminative arylation and alkenylation of tertiary amines (propargyl amines, allyl amines and 1H-indol-3-yl methane amines) with ary and alkenylboronic acids enabled by difluorocarbene under transition-metal-free conditions. Preliminary mechanism experiments suggest that in situ formed difluoromethyl quaternary amine salt, nitrogen ylide and tetracoordinate boron species are the key intermediates, the subsequent 1,2-metallate shift and protodeboronation complete the new coupling reaction.

Reductive Cross-Coupling of Unreactive Electrophiles

Transition-metal-catalyzed reductive coupling of electrophiles has emerged as a powerful tool for the construction of molecules. While major achievements have been made in the field of cross-couplings between organic halides and pseudohalides, an increasing number of reports demonstrates reactions involving more readily available, low-cost, and stable, but unreactive electrophiles. This account summarizes the recent results in our laboratory focusing on this topic. These findings typically include deoxygenative C-C coupling of alcohols, reductive alkylation of alkenyl acetates, reductive C-Si coupling of chlorosilanes, and reductive C-Ge coupling of chlorogermanes.The reductive deoxygenative coupling of alcohols with electrophiles is synthetically appealing, but the potential of this chemistry remains to be disclosed. Our initial study focused on the reaction of allylic alcohols and aryl bromides by the combination of nickel and Lewis acid catalysis. This method offers a selectivity that is opposite to that of the classic Tsuji-Trost reactions. Further investigation on the reaction of benzylic alcohols led to the foundation of a dynamic kinetic cross-coupling strategy with applications in the nickel-catalyzed reductive arylation of benzylic alcohols and cobalt-catalyzed enantiospecific reductive alkenylation of allylic alcohols. The titanium catalysis was later established to produce carbon radicals directly from unactivated tertiary alcohols via C-OH cleavage. The development of their coupling reactions with carbon fragments delivers new methods for the construction of all-carbon quaternary centers. These reactions have shown high selectivity for the functionalization of tertiary alcohols, leaving primary and secondary alcohols intact. Alkenyl acetates are inexpensive, stable, and environmentally friendly and are considered the most attractive alkenyl reagents. The development of reductive alkylation of alkenyl acetates with benzyl ammoniums and alkyl bromides offers mild approaches for the conversion of ketones into aliphatic alkenes.Extensive studies in this field have enabled us to extend the cross-electrophile coupling from carbon to silicon and germanium chemistry. These reactions harness the ready availability of chlorosilanes and chlorogermanes but suffer from the challenge of their low reactivity toward transition metals. Under reductive nickel catalysis, a broad range of alkenyl and aryl electrophiles couple well with vinyl- and hydrochlorosilanes. The use of alkyl halides as coupling partners led to the formation of functionalized alkylsilanes. The C-Ge coupling seems less substrate-dependent, and various common chlorogermanes couple well with aryl, alkenyl, and alkyl electrophiles. In general, functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine are tolerated. These methods provide new access to organosilicon and organogermanium compounds, some of which are challenging to obtain otherwise.

Nickel-catalyzed 1,4-aryl rearrangement of aryl N-benzylimidates via C-O and C-H bond cleavage

We report herein that nickel-catalyzed reaction of aryl imidates bearing an N-benzyl group results in 1,4-migration of an O-aryl group via the cleavage of C-O and C-H bonds. This protocol allows for the benzylic C-H bond arylation of benzylamine building blocks using phenols as an aryl source to form elaborate diarylmethylamine derivatives.

Palladium-Catalyzed Unimolecular Fragment Coupling of N-Allylamides via Elimination of Isocyanate

Transition metal-catalyzed unimolecular fragment coupling (UFC) is defined as processes that forge new chemical bonds through the extrusion of molecules, such as CO and CO₂, and the subsequent recombination of the remaining fragments. Herein, we report on a new UFC reaction that involves the palladium-catalyzed elimination of an isocyanate fragment from an amide, with the formation of carbon-carbon and carbon-heteroatom bonds. An organometallic intermediate that is relevant to the catalytic reaction was characterized by X-ray crystallography. This UFC reaction enables the late-stage transformation of an amide functionality, allowing amides to be used as a convertible directing or protecting group.

Palladium-catalyzed Hiyama cross-couplings of pyrimidin-2-yl tosylates with organosilanes

An efficient palladium-catalyzed Hiyama reaction between various pyrimidin-2-yl tosylates with organosilanes has been developed. The use of CuCl with TBAF as additive is essential for promotion of the construction of the carbon–carbon bond. This procedure shows wide functional group tolerance for electrophilic pyrimidin-2-yl tosylates and has been extended to aromatic amino-substituted pyrimidin-2-yl tosylates, affording the desired C2-aryl and alkenyl pyrimidine derivatives in good to excellent yields.

Recent Advancement on Decarbonylation Reactions Assisted by Ru-complexes: Synthetic and Mechanistic Approach

This frontier article covers the recent advancements in the ruthenium complex catalysed decarbonylation reactions of different types of carbonyl compounds and provides a direction towards the mechanistic understanding involved in...

(3 + 2) cycloaddition of 2-alkoxynaphthalenes with azaoxyallyl cations: access to benzo[e]indolones

A route to functionalized benzo[e]indolones has been reported via (3 + 2) cycloaddition of in situ generated azaoxyallyl cations with alkoxynaphthalene followed by aryl C–O bond cleavage. The intermediate adduct has been isolated and characterized.

Regio- and Stereoselective Synthesis of Enol Carboxylate, Phosphate, and Sulfonate Esters via Iodo(III)functionalization of Alkynes

β-Iodo(III)enol carboxylates, phosphates, and tosylates can be efficiently synthesized through regio- and stereoselective iodo(III)functionalization of alkynes. The combination of chlorobenziodoxole and silver salt has proven to generate a versatile cationic iodine(III) electrophile to activate alkynes and engage various carboxylic acids, triethyl phosphate, and p-toluenesulfonic acid as nucleophiles. The β-iodo(III)enol esters serve as starting materials for the synthesis of multisubstituted alkenes through sequential cross-coupling of the C-I(III) and C-O bonds.

Nickel-catalyzed cross-electrophile coupling of aryl chlorides with allylic alcohols

Nickel-catalyzed cross-electrophile coupling of aryl chlorides with allylic alcohols proceeds readily under mild conditions in the presence of zinc powder and MgCl₂ to produce allylarenes in 25-92% yields. The reaction shows high regioselectivity and E/Z-selectivity, giving linear allylation products with an E configurated double bond when 1- or 3-arylallyl alcohols were used as the substrates. Functional groups including F, CF₃, COOEt, NMe₂, OMe, SiMe₃, OH and vinyl groups as well as nitrogen-containing heterocycles were tolerated.

Facilitating room-temperature oxygen ion migration via Co-O bond activation in cobaltite films

Oxygen ion migration in strongly correlated oxides can cause dramatic changes in the crystal structure, chemical and magnetoelectric properties, which holds promising for a wide variety of applications in catalysis, energy conversion, and electronics. However, the high strength and stability of metal-oxygen (M-O) bonds cause a large thermodynamic barrier for oxygen migration. Here, we designed Co-O bond activation in cobaltite (SrCoO_x) films by Au-nanodot-decoration. Charge transfer from Au to SrCoO_x effectively weakens the Co-O bond, meanwhile Co-O-Au synergistic bonding remarkably decreases the migration barrier of oxygen ions. Fast oxygen evolution occurs at the perimeter of the Au/SrCoO_x interface, and the chemical potential gradient of O^2- drives inner ion diffusion to the surface. Consequently, bias-free topotactic phase reduction from perovskite SrCoO_3-δ to brownmillerite SrCoO_2.5 has been achieved at room temperature. Our finding explores a new dimension to accelerate oxygen ion kinetics in transition-metal oxides from the aspect of interfacial bond activation, which is significant for developing oxide/noble-metal interfaces for high-efficiency ion migration and redox catalysis at low temperature.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.

Nickel-Catalyzed Ring-Opening C-O Functionalization of peri-Xanthenoxanthenes for 8-Substituted Binaphthol Synthesis

Herein, we disclose the Ni-catalyzed ring-opening C-O functionalization of peri-xanthenoxanthenes using Grignard reagents that forms 8-monofunctionalized binaphthols. 1,2-Bis(dicyclohexylphosphino)ethane was the best ligand for alkylations and ICy for arylation. After mechanistic investigations, we assumed that the reaction proceeds via C-O reduction and subsequent C-O functionalization. To verify the mechanism, the intermediate after reduction was isolated. Moreover, the asymmetric addition, using 8-octylbinaphthol after optical resolution, was studied.

Suzuki-Miyaura Cross-Coupling of Esters by Selective O-C(O) Cleavage Mediated by Air- and Moisture-Stable [Pd(NHC)(μ-Cl)Cl]2 Precatalysts: Catalyst Evaluation and Mechanism

The cross-coupling of aryl esters has emerged as a powerful platform for the functionalization of otherwise inert acyl C-O bonds in chemical synthesis and catalysis. Herein, we report a combined experimental and computational study on the acyl Suzuki-Miyaura cross-coupling of aryl esters mediated by well-defined, air- and moisture-stable Pd(II)-NHC precatalysts [Pd(NHC)(μ-Cl)Cl]2. We present a comprehensive evaluation of [Pd(NHC)(μ-Cl)Cl]2 precatalysts and compare them with the present state-of-the-art [(Pd(NHC)allyl] precatalysts bearing allyl-type throw-away ligands. Most importantly, the study reveals [Pd(NHC)(μ-Cl)Cl]2 as the most reactive precatalysts discovered to date in this reactivity manifold. The unique synthetic utility of this unconventional O-C(O) cross-coupling is highlighted in the late-stage functionalization of pharmaceuticals and sequential chemoselective cross-coupling, providing access to valuable ketone products by a catalytic mechanism involving Pd insertion into the aryl ester bond. Furthermore, we present a comprehensive study of the catalytic cycle by DFT methods. Considering the clear advantages of [Pd(NHC)(μ-Cl)Cl]2 precatalysts on several levels, including facile one-pot synthesis, superior atom-economic profile to all other Pd(II)-NHC catalysts, and versatile reactivity, these should be considered as the 'first-choice' catalysts for all routine applications in ester O-C(O) bond activation.

Photohomolysis and Photoheterolysis in Aryl Sulfonates and Aryl Phosphates

The photochemical behaviour of selected aryl sulfonates and phosphates (ArOX) in polar and nonpolar media has been investigated by laser flash photolysis (LFP) experiments. Two main pathways have been identified, namely the photohomolysis of the ArO-X bond or the photoheterolysis of the Ar-OX bond depending on the nature of the leaving group (OX) and on the nature of the substituents on the aromatic ring. In nonpolar solvents the esters are quite photostable due to an efficient triplet deactivation. In polar solvents, the homolytic fragmentation of the ArO-S bond from the exited singlets was found in aryl sulfonates bearing moderately electron-donating groups as well as electron-withdrawing groups. In electron-rich aryl phosphates and sulfonates photoheterolysis of the Ar-OP/Ar-OS bond took place as the exclusive pathway.

Nickel-catalyzed Heck reaction of cycloalkenes using aryl sulfonates and pivalates

Nickel-catalyzed Heck reaction of cycloalkenes delivers unusual conjugated arylated isomers. Nickel(0) catalysts ligated by chelating dialkylphosphines effectively activate not only aryl triflates as electrophiles, but also less reactive aryl mesylates, tosylates and pivalates. The omission of bases allows nickel hydride species to exist long enough to perform in situ olefin isomerization of initial Heck adducts.

N-Heterocyclic Carbene-Catalyzed Truce-Smiles Rearrangement of N-Arylacrylamides via the Cleavage of Unactivated C(aryl)-N Bonds

We report on the N-heterocyclic carbene (NHC)-catalyzed Truce-Smiles rearrangement of aniline derivatives, in which an unactivated C(aryl)-N bond is cleaved, leading to the formation of a new C(aryl)-C bond. The key to the success of this reaction is the utilization of a highly nucleophilic NHC, which enables the formation of a highly nucleophilic ylide intermediate that is generated from an α,β-unsaturated amide.

Nickel-Catalyzed Cross-Coupling of Aryl 2-Pyridyl Ethers with Organozinc Reagents: Removal of the Directing Group via Cleavage of the Carbon-Oxygen Bonds

Reaction of aryl 2-pyridyl ethers with arylzinc reagents under catalysis of NiCl₂(PCy₃)₂ affords aryl-aryl cross-coupling products via selective cleavage of C_Ar-OPy bonds. The reaction features a wide substrate range and good compatibility of functional groups. β-H-free alkylzinc reagents are also applicable as the nucleophiles in the transformation, whereas β-H-containing alkylzinc reagents lead to a mixture of cross-coupling and hydrogenation products.

Electrochemical Oxidative Carbon-Atom Difunctionalization: Towards Multisubstituted Imino Sulfide Ethers

Ethers (C-O/S) are ubiquitously found in a wide array of functional molecules and natural products. Nonetheless, the synthesis of imino sulfide ethers, containing an N(sp² )=C(sp² )-O/S fragment, still remains a challenge because of its sensitivity to acid. Developed here in is an unprecedented electrochemical oxidative carbon-atom difunctionalization of isocyanides, providing a series of novel multisubstituted imino sulfide ethers. Under metal-free and external oxidant-free conditions, isocyanides react smoothly with simple and readily available mercaptans and alcohols. Importantly, the procedure exhibited high stereoselectivities, excellent functional-group tolerance, and good efficiency on large-scale synthesis, as well as further derivatization of the products.

An evaluation of palladium-based catalysts for the base-free borylation of alkenyl carboxylates

Palladium catalysis can achieve the base-free borylation of alkenyl carboxylates, enabling direct access to functionalized enones and heterocycles.

Carbon dioxide-promoted palladium-catalyzed dehydration of primary allylic alcohols: access to substituted 1,3-dienes

A carbon dioxide promoted dehydration reaction of primary allylic alcohols gives synthetically important substituted 1,3-dienes in good yields under milder conditions compared with the reaction using a heterogeneous catalyst or carbon monoxide.

Metal-free catalytic hydrocarboxylation of hexafluorobut-2-yne

An efficient method for stereoselective synthesis of trifluorinated enol esters catalyzed by base was introduced.

Nickel-catalyzed defluorinative alkylation of C(sp2)–F bonds

A nickel-catalyzed defluorinative alkylation of unactivated C(sp²)–F electrophiles using commercially available trialkylaluminum reagents, thus forming the C(sp²)–C(sp³) bonds is reported.