Synthesis of Biaryls via Catalytic Decarboxylative Coupling

Palladium-Catalyzed Decarboxylative Cross-Couplings of Zinc Polyfluorobenzoates with Aryl 2-Pyridyl Esters for Accessing Polyfluorinated Aryl Ketones

An efficient decarboxylative cross-coupling of zinc polyfluorobenzoate with aryl 2-pyridyl ester under palladium catalysis, which enabled the convenient access to valuable polyfluorinated aryl ketones, was developed. The cross-coupling reactions proceeded effectively to give an array of polyfluorinated aryl ketones in moderate to excellent yields with a reasonable substrate scope and broad functional group tolerance. In addition, a one-pot reaction with the use of in situ formed zinc polyfluorobenzoate commencing from polyfluorobenzoic acid and Zn(OH)2 could be accomplished as well in a step-economical fashion. Moreover, the coupling protocol could also be successfully subjected to scale-up synthesis and readily applied in the late-stage functionalization of biologically active complex molecules.

Pd-catalyzed decarboxylative coupling of zinc polyfluorobenzoate with aryl imidazolylsulfonate for polyfluorinated biaryl synthesis

We report here an efficient decarboxylative cross-coupling of zinc polyfluorobenzoates with aryl imidazolylsulfonates under palladium catalysis, which proceeded effectively via C-O bond cleavage to afford the corresponding valuable polyfluorinated biaryls in moderate to good yields, exhibiting both reasonable substrate scope and broad functional group tolerance. In addition, late-stage functionalization of bioactive molecules and an amino acid-derived compound and scaled-up synthesis could be accomplished as well. Further exploration revealed that zinc polyfluorobenzoates are more vibrant than their potassium, sodium, and magnesium counterparts as a polyfluoroarylating reagent.

Ruthenium-Catalyzed Decarboxylative Direct 2-Pyridination of α,β-Unsaturated Carboxylic Acids

We report a ruthenium-catalyzed decarboxylative direct mono-2-pyridination of α,β-unsaturated carboxylic acids, offering a selective method for synthesizing 2-(1-arylalkenyl)pyridines. Under redox-neutral conditions, the reaction demonstrates broad substrate scope, high functional group tolerance, and significant potential for further derivatization. Mechanistic studies reveal that selective activation of the allylic C(sp3)-H bond, directed by the pyridine group, underpins the observed mono-2-pyridination. This work provides an efficient and selective strategy for the catalytic decarboxylative transformation of α,β-unsaturated carboxylic acids, expanding the toolbox for functionalizing pyridine derivatives.

Cs2CO3-N,N-Dimethylacetamide-Promoted Esterification of Arylformic Acids with a-Bromo Esters via Nucleophilic Substitution

Herein, a nucleophilic substitution reaction between arylformic acids and α-bromo esters, facilitated by a Cs2CO3-DMA system, is reported. This reaction occurs under mild conditions, yielding a diverse range of valuable lactic acid derivatives (34 examples) with good to excellent yields (up to 97%). Subtle factors, such as the ion-dipole interactions between Cs2CO3, the substrates, and the solvent, may significantly influence the dynamics of the SN2 reaction.

Aryl Acid-Alcohol Cross-Coupling: C(sp3)-C(sp2) Bond Formation from Nontraditional Precursors

Alcohols and aryl carboxylic acids are among the most commercially abundant, synthetically versatile, and operationally convenient building blocks in organic chemistry. Despite their widespread availability, the direct formation of C(sp3)-C(sp2) bonds from these functional groups remains a challenge. Recently, our group developed robust protocols to harness alcohols as alkyl radical precursors, but the activation of aryl acids remains relatively unexplored. Herein, we describe the merger of N-heterocyclic carbene (NHC)-mediated deoxygenation and nickel-mediated decarbonylation of aryl acids toward C(sp3)-C(sp2) bond formation. The utility of this method is demonstrated through the synthesis of a diverse range of aryl-alkyl cross-coupled products and the late-stage functionalization of complex molecules, including drugs, natural products, and biomolecules.

Selective Transformation of Biomass and the Derivatives for Aryl Compounds and Hydrogen via Visible-Light-Induced Radicals

ConspectusFor sustainable development, exploring renewable resources is an urgent priority. Nonfood biomass, one of the largest renewable resources on Earth, primarily comprises three key components: lignin (ca. 15-30%), cellulose (ca. 35-50%), and hemicellulose (ca. 20-30%). Theoretically, nonfood biomass can be converted into green chemicals and energy. However, most studies have focused on the generation of chemicals and carbon-based energy under harsh conditions, often resulting in lower selectivities. Therefore, further efforts to explore efficient and selective methods for producing chemicals and hydrogen (H2) are essential to promoting the practical applications of renewable biomass. In this Account, we summarize our contributions to the efficient and selective transformation of biomass and its derivatives into aryl compounds and H2. These transformations were achieved using visible-light-induced photocatalytic systems that generate active radicals to selectively cleave C-C, C-O, C-H, and O-H bonds under mild conditions, without using noble metals. First, aryl compound production was achieved by chemoselective cleavage of C-C and C-O bonds using aryl carboxyl radicals and aryl ether radical cations. Specifically, the aryl carboxyl radical in the charge-transfer complex induced the chemoselective cleavage of C-C bonds of aryl carboxylic acids, which are platform molecules derived from lignin oxidation; the aryl carboxyl radical in free form facilitated the chemoselective cleavage of C-O bonds in the model of the 4-O-5 lignin linkage. Moreover, arenols and aryl alcohols were obtained via cooperation of the aryl ether radical cation and the vanadate-induced chemoselective cleavage of the C-O bonds of the models of various lignin linkages. Second, we developed a streamlined strategy for H2 production from biomass using a one-pot, two-step route with formic acid (HCO2H) as an intermediate for H2 storage by thermocatalysis. Using this strategy by photoredox catalysis, HCO2H was initially obtained via the alkoxy radical-induced gradual cleavage of C-C bonds in cellulose, hemicellulose, glucose, and their derivatives. Subsequently, efficient H2 production from biomass-based HCO2H was realized via hydroxyl radical (·OH)-induced C-H and the following cleavage of the O-H bonds, with cooperation of the nickel catalysis. Third, the highest H2 production capability from biomass was achieved via efficient water reforming. This process utilized alkoxy radicals followed by generated carbon cations via electrocatalysis, inducing a well-organized cleavage of C-C, O-H, and C-H bonds. We anticipate that these insights will inspire the development of more efficient, stable, and cost-effective catalytic systems, accelerating the utilization of biomass as a renewable resource and driving other related significant transformations.

Synthesis of Polyfluorinated Biaryls via Palladium-Catalyzed Decarboxylative Cross-Coupling of Zinc Polyfluorobenzoates with Aryl Fluorosulfates

A palladium-catalyzed decarboxylative cross-coupling of zinc polyfluorobenzoates with aryl fluorosulfates, which proceeded efficiently via C-O bond cleavage to afford the corresponding polyfluorinated biaryls in moderate-to-good yields, was developed. The reactions exhibited both good substrate scope and broad functional group compatibility, and it could be scaled-up easily. The synthetic simplicity and practicability of the reaction was further demonstrated by one-pot manipulation by directly mixing polyfluorobenzoic acid and Zn(OH)2 in the coexistence of aryl fluorosulfate and a palladium catalyst in one flask. Further studies showed that aryl fluorosulfate is more robust than other aryl halides and pseudohalides as a arylating reagent, and zinc polyfluorobenzoate is a more effective decarboxylative polyfluoroarylating agent than their magnesium and potassium counterparts.

Ruthenium Catalyzed Ortho-Arylation Reaction of Benzoic Acids with Arylthianthrenium Salts

Arylthianthrenium salts have become key intermediates for late-stage functionalizations of drug-like molecules. Ruthenium-phosphine catalysts are now shown to enable their use as aryl sources in high-yielding ortho-C─H arylations of benzoic acids. The arylthianthrenium salts are converted chemo-selectively, leaving aryl halides and boronates untouched. The carboxylate groups are uniquely effective as directing groups, ensuring exclusive ortho-selectivity even in the presence of competing pyridine or amide groups. This makes the reaction orthogonal to cross-couplings and conventional C─H arylations. The carboxylate group can be removed via decarboxylation or serve as an anchor for downstream transformations. Mechanistic studies identify C─H ruthenation as the rate-limiting step and highlight the unique efficiency of P(Cy)₃ ligands.

Photoinduced Late-Stage Radical Decarboxylative and Deoxygenative Coupling of Complex Carboxylic Acids and Their Derivatives

The simple and efficient conversion of carboxylic acids into structurally diverse organic molecules is highly desirable in chemical synthesis. This review covers recent developments in photocatalytic methodology for late-stage transformations of complex carboxylic acids and their derivatives enabled by radical decarboxylation and deoxygenation, highlighting some representative and significant contributions in this field. These advancements are categorized based on the reactivity patterns exhibited by the carboxylic acids. Several activation modes to generate alkyl or aryl radical intermediates during decarboxylation of carboxylic acids are presented, namely, single-electron transfer (SET) oxidation, ligand-to-metal charge transfer (LMCT), SET reduction, and energy transfer (EnT) processes. On the other hand, direct activation of C-O bonds in carboxylic acids mediated by phosphoranyl radicals has been discussed and illustrates their potential synthetic application for the synthesis of complex aldehydes, ketones and amides.

Formal Transformation of Benzylic Carboxylic Acids to Phenols

Phenols play a crucial role as core structural motifs in natural products and also serve as fundamental building blocks in synthetic chemistry. Apart from the known protocols for the conversion of aryl precursors to phenols (i. e., decarboxylative oxygenation), we report here the efficient synthesis of phenols from the stable and readily available benzylic carboxylic acids under mild reaction conditions. The photocatalytic conversion of carboxylic acids to peroxides is a crucial step in this strategy, allowing the subsequent C-O bond formation via Hock rearrangement.

Site-selective decarbonylative [4 + 2] annulation of carboxylic acids with terminal alkynes by C-C/C-H activation strategy and cluster catalysis

Cycloaddition and annulation strategies are among the most powerful methods for creating molecular complexity in organic molecules. In this manuscript, we report a highly site-selective palladium-catalyzed decarbonylative [4 + 2] cyclization of carboxylic acids with terminal alkynes by a sequential C-C/C-H bond activation. Most notably, this method represents the first use of carboxylic acids as the ubiquitous and underdeveloped synthons for intramolecular cycloadditions by decarbonylative C-C bond cleavage. The method provides a solution to the long-standing challenge of the regioselective synthesis of substituted naphthalenes by cycloaddition. Mechanistic studies show that this reaction occurs through a sequential process involving the formation of key palladacycle by a sequential C-C/C-H bond activation and highly regioselective alkyne insertion enabled by cluster catalysis. Wide substrate scope for both carboxylic acids and terminal alkynes is demonstrated with high functional group tolerance. Moreover, this reaction is scalable and applicable to the synthesis of functionalized molecules featuring bioactive fragments. This reaction advances the toolbox of redox-neutral carboxylic acid interconversion to cycloaddition processes. We anticipate that this approach will find broad application in organic synthesis, drug discovery and functionalized material research fields.

Photoinduced Cu(II)-Mediated Decarboxylative Thianthrenation of Aryl and Heteroaryl Carboxylic Acids

Given that (hetero)aryl carboxylic acids are inexpensive materials available in a great variety from commercial and natural resources or synthesis, the strategies enabling their use as starting materials for preparing fine chemicals are highly sought after. Here we report a photoinduced Cu(II)-mediated protocol converting (hetero)aryl carboxylic acids into (hetero)aryl thianthrenium salts, high value-added building blocks that can undergo various subsequent transformations, creating an attractive two-step pathway for the divergent functionalization of these ubiquitous starting materials. The excellent compatibility of the method is shown by preparing a broad range of sterically and electronically varied (hetero)aryl thianthrenium salts, including derivatives of pharmaceuticals, such as ataluren, celecoxib, flavoxate, probenecid, repaglinide, and tamibarotene. The syntheses of 13 C-labeled probenecid and bioisosteres of ataluren as well as the unconventional modifications of celecoxib and flavoxate, illustrate the synthetic potential of the strategy. Mechanistic studies are in line with a reaction occurring through a photoinduced ligand-to-metal charge transfer (LMCT) of Cu(II)-arylcarboxylates, enabling radical decarboxylative carbometallation to form arylcopper(II) intermediates that in turn react with thianthrene to form the product. Noteworthy, the susceptibility of aryl thianthrenium salts to photodegradation is overcome by a Cu(I)-driven salvage loop, which continuously intercepts the transiently formed radicals and regenerates the products.

Copper-Catalyzed Intramolecular Decarboxylative C(sp2)-Heteroatom Cross-Couplings: Mechanism Insights and Synthetic Applications

Decarboxylative C(sp2)-heteroatom cross-coupling reactions hold extraordinary potential for the sustainable preparation of biologically active scaffolds. Herein, we report a copper sulfate/1,10-phenathroline catalytic system for the decarboxylative intramolecular C(sp2)-O, C(sp2)-S, and C(sp2)-N coupling reactions leading to the construction of a series of benzo[b]furans, benzo[b]thiophenes, and indole derivatives from the corresponding coumarins, thiocoumarins, or quinolones, respectively. Our mechanistic study based on benzo[b]furan formation suggests a three-step process of the transformations, which consists of (i) base-mediated hydrolytic ring opening of coumarin, (ii) copper-oxygen co-initiated radical decarboxylation, and (iii) copper-catalyzed C-heteroatom cross coupling. Application of this method in the total synthesis of egonol, a bioactive natural product, was demonstrated successfully, with an overall yield of 51.7%.

Photocatalytic decarboxylation of free carboxylic acids and their functionalization

Visible light mediated decarboxylative functionalization of carboxylic acids and their derivatives has recently emerged as a novel and powerful toolkit for small molecule activation in diverse carbon-carbon and carbon-hetero bond forming reactions. Naturally abundant highly functionalized bench-stable carboxylic acid analogs have been employed as promising alternatives to non-trivial organometallic reagents for mild and eco-benign synthetic transformation with traceless CO2 by-products. In this highlight article, we focus on the development of various photodecarboxylative functionalization strategies along with intra/inter-molecular cyclization via concerted single electron transfer (SET) or energy transfer (ET) pathways. Moreover, widely explored carboxylic acids are systematically classified here into four categories; i.e., α-keto, aliphatic, α,β-unsaturated, and aromatic analogs for a concise overview to the readership. The association of decarboxylative radical species with coupling partners to construct C-C and C-N/O/S/P/X bonds for each analogous acid has been presented in brief.

Electrochemical Decarboxylative Cross-Coupling with Nucleophiles

Decarboxylative cross-coupling reactions are powerful tools for carbon-heteroatom bonds formation, but typically require pre-activated carboxylic acids as substrates or heteroelectrophiles as functional groups. Herein, we present an electrochemical decarboxylative cross-coupling of carboxylic acids with structurally diverse fluorine, alcohol, H2O, acid, and amine as nucleophiles. This strategy takes advantage of the ready availability of these building blocks from commercial libraries, as well as the mild and oxidant-free conditions provided by electrochemical system. This reaction demonstrates good functional-group tolerance and its utility in late-stage functionalization.

Visible-Light-Induced Synthesis of Vinyl Sulfones via Decarboxylative Sulfonylation of Cinnamic Acids Using Sulfonylazides/p-Toluenesulfonylmethyl Isocyanide/β-Keto Sulfones

An efficient visible-light-induced synthesis of vinyl sulfones has been accomplished via decarboxylative sulfonylation of cinnamic acids using sulfonylazides, p-toluenesulfonylmethyl isocyanide, and β-keto sulfones as sulfonyl source, in the presence of inexpensive organic photocatalysts like rhodamine B and eosin Y. The reaction is facile, straightforward, and endowed with wide substrate scope and functional group tolerability.

Palladium-catalyzed Suzuki-Miyaura cross-coupling of carboxylic-phosphoric anhydrides via C-O bond cleavage

A robust palladium-catalyzed Suzuki-Miyaura reaction of carboxylic-phosphoric anhydrides via highly selective C(O)-O bond cleavage under inorganic base-free conditions has been reported. Carboxylic-phosphoric anhydrides, generated through activating carboxylic acids using phosphates by esterification or direct dehydrogenative reaction with phosphites, have been employed as highly reactive electrophiles for Suzuki-Miyaura cross-coupling reactions. Broad substrate scope and excellent functional group tolerance have been demonstrated to be a general and practical approach for the synthesis of highly valuable ketones.

Rh(III)-Catalyzed C-H Activation/Annulation for the Construction of Quinolizinones and Indolizines

A catalytic-condition-controlled synthesis strategy was reported to build quinolizinone and indolizine derivatives from the easily available enamide and triazole substrates with high regioselectivity and good functional group tolerance. More especially, this transformation has successfully fulfilled a C-H bond activation of terminal olefin from enamides followed by a [3 + 3] and a [2 + 3] cyclization cascade under different catalytic conditions, respectively, to provide two kinds of potentially biologically active heterocyclic scaffolds with a ring-junction nitrogen atom. Mechanistically, the methoxyamine formyl group serves as either a traceless directing group (DG) or an oxidizing DG via the C-N and C-C cleavage in this protocol.

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

Palladium-Catalyzed Decarbonylative Sonogashira Alkynylation of Carboxylic-Phosphoric Anhydrides

We report the first palladium-catalyzed decarbonylative alkynylation of carboxylic-phosphoric anhydrides via highly selective C(O)-O bond cleavage. Carboxylic-phosphoric anhydrides are highly active carboxylic acid derivatives, which are generated through activating carboxylic acids using phosphates by esterification or direct dehydrogenative coupling with phosphites. Highly valuable internal alkynes have been generated by the present method, and the efficiency of this approach has been demonstrated through a wide substrate scope and excellent functional group tolerance.

Merging Photocatalytic Doubly-Decarboxylative Csp 2 -Csp 2 Cross-Coupling for Stereo-Selective (E)-α,β-Unsaturated Ketones Synthesis

A photocatalytic domain of doubly decarboxylative Csp 2 -Csp 2 cross coupling reaction is disclosed. Merging iridium and palladium photocatalysis manifested carbon-carbon bonds in a tandem dual-radical pathway. Present catalytic platform efficiently cross-coupled α, β-unsaturated acids and α-keto acids to afford a variety of α, β-unsaturated ketones with excellent (E)-selectivity and functional group tolerance. Mechanistically, photocatalyst implicated through reductive quenching cycle whereas cross coupling proceeded over one electron oxidative pallado-cycle.

Catalytic π-π Interactions Triggered Photoinduced Synthesis of Biaryls

A highly regioselective photocatalytic method to access a variety of biaryl motifs under metal-free conditions has been developed. The organophotocatalyst is involved in π-π stacking interactions with the alkyne species, which promotes this photocatalytic process with thiophene. Mechanistic studies have shed light on these interactions and the overall process. Along with a broad functional-group tolerance and excellent regioselectivity, this protocol has been utilized in the late-stage functionalization of pharmaceuticals and other natural products.

Mild Pd-Catalyzed Decarboxylative Cross-Coupling of Zinc(II) Polyfluorobenzoates with Aryl Bromides and Nonaflates: Access to Polyfluorinated Biaryls

We developed a Pd-catalyzed decarboxylative cross-coupling of zinc polyfluorobenzoates, which were used as precursors for producing zinc reagents in situ, with aryl bromides and nonaflates, providing a mild and efficient pathway for the synthesis of polyfluorinated biaryls. This protocol exhibits a broad substrate scope and excellent functional tolerance. Moreover, the versatility of this approach was demonstrated by the straightforward late-stage modification of drugs, biologically active molecules, and pesticides, indicating its potential significance in drug discovery.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Palladium-Catalyzed Decarbonylative Annulation of 2-Arylbenzoic Acids with Internal Alkynes toward Phenanthrenes

A palladium-catalyzed decarbonylative annulation of 2-arylbenzoic acids with internal alkynes via C(sp2)-H activation has been developed. A series of phenanthrenes were produced in moderate to good yield with good functional group tolerance. The mechanism study indicated that the C(sp2)-H activation should be the rate-determining step during the reaction.

Skeletal Editing of Dibenzolactones to Fluorenes via Ni- or Pd-Catalyzed Decarboxylation

The skeletal editing of dibenzolactones to fluorenes by Ni- or Pd-catalyzed decarboxylation is reported. In contrast to previously reported intramolecular decarboxylative couplings, inductively electron-withdrawing ortho substituents on the aryl carboxylate moiety and metal additives are not required. The decarboxylation reaction proceeds cleanly and can be applied to the skeletal editing of a natural product analogue. Mechanistic observations are consistent with stabilization of the carboxylate-ligated Ni complex over the Ni-carboxylate ion pair, which is the key factor in promoting the challenging decarboxylation step in the catalytic cycle.

Copper-Catalyzed Asymmetric Cyanation of Propargylic Radicals via Direct Decarboxylation of Propargylic Carboxylic Acids

Chiral propargylic cyanides are often used as small-molecule feedstocks for the introduction of chiral centers into various valuable products and complex molecules. Here, we have developed a highly atom-economical strategy for the chiral copper complex-catalyzed synthesis of chiral propargylic cyanides. Propargylic radicals can be smoothly obtained by direct decarboxylation of the propargylic carboxylic acids without preactivation. The reactions show excellent selectivity and functional group compatibility. Gram-scale reaction and several conversion reactions from chiral propargylic cyanide have demonstrated the synthetic value of this strategy.

Rh(II)/Pd(0) Dual Catalysis: Interception of Ammonium Ylide with Allyl Palladium to Construct 2,2-Disubstituted Tetrahydroquinoline Derivatives

New synthetic methods to construct 2,2-disubstituted tetrahydroquinoline derivatives are of significant value in pharmaceutical chemistry. Herein, a Rh(II)/Pd(0) dual-catalyzed diazo α-aminoallylation reaction has been developed between allylpalladium(II) and ammonium ylides derived from the Rh₂(OAc)₄-mediated intramolecular N-H bond insertion reaction of diazo compounds, affording various 2,2-disubstituted tetrahydroquinoline derivatives in good yields up to 93% with high chemoselectivities under mild reaction conditions. A substrate scope investigation reveals broad ester substituent tolerance, and control experiments provide the basis for a proposed reaction mechanism.

Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Herein, we report a mild, operationally simple, multicatalytic method for the synthesis of β,γ-unsaturated ketones via allylic acylation of alkenes. Specifically, the method combines N‑heterocyclic carbene catalysis, hydrogen atom transfer catalysis, and photoredox catalysis for cross-coupling reactions between a wide range of feedstock carboxylic acids and readily available olefins to afford structurally diverse β,γ-unsaturated ketones without olefin transposition. The method could be used to install acyl groups on highly functionalized natural-product-derived compounds with no need for substrate pre-activation, and C-H functionalization proceed with excellent site selectivity. To demonstrate the potential applications of the method, we convert a representative coupling product into various useful olefin synthons.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp2)-C(sp3) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp2)-C(sp3) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.

Multimetallic-Catalyzed C-C Bond-Forming Reactions: From Serendipity to Strategy

The use of two or more metal catalysts in a reaction is a powerful synthetic strategy to access complex targets efficiently and selectively from simple starting materials. While capable of uniting distinct reactivities, the principles governing multimetallic catalysis are not always intuitive, making the discovery and optimization of new reactions challenging. Here, we outline our perspective on the design elements of multimetallic catalysis using precedent from well-documented C-C bond-forming reactions. These strategies provide insight into the synergy of metal catalysts and compatibility of the individual components of a reaction. Advantages and limitations are discussed to promote further development of the field.

Titania Nanorod-Supported Mercaptoundecanoic Acid-Grafted Palladium Nanoparticles as a Highly Reusable Heterogeneous Catalyst for Substrate-Dependent Ullmann Coupling and Debromination of Aryl Bromides

Herein, by implanting palladium nanoparticles (Pd NPs) onto titanium dioxide (TiO₂) nanorods (NRs) through 11-mercaptoundecanoic acid (MUA), we devised a robust heterogeneous catalyst. The formation of Pd-MUA-TiO₂ nanocomposites (NCs) was authenticated using Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, atomic absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. Pd NPs were synthesized directly onto TiO₂ nanorods without the MUA support for comparative studies. As a means of evaluating the endurance and competency of Pd-MUA-TiO₂ NCs compared to their counterpart (Pd-TiO₂ NCs), both were used as the heterogeneous catalyst for Ullmann coupling of a wide variety of aryl bromides. When Pd-MUA-TiO₂ NCs were used, the reaction produced high yields of homocoupled products (54-88%), whereas the yield was only 76% when Pd-TiO₂ NCs were used. Moreover, Pd-MUA-TiO₂ NCs impressed with their outstanding reusability property, allowing over 14 reaction cycles without losing efficiency. On the flip side, just after seven reaction cycles, the productivity of Pd-TiO₂ NCs dropped around 50%. Presumably, the strong affinity of Pd for the thiol groups of MUA allowed for the substantial control of leaching out of Pd NPs during the reaction. Nonetheless, another crucial feature of the catalyst is that the di-debromination reaction took place with an excellent yield of 68-84% from di-aryl bromides with long alkyl chains instead of macrocyclic or dimerized products. It is worth mentioning that AAS data confirmed that only 0.30 mol % catalyst loading was sufficient to activate a broad substrate scope with large functional group tolerance.

Metal -free PhI(OAc)2-oxidized decarboxylation of propiolic acids towards synthesis of α-acetoxy ketones and insights into general decarboxylation with DFT calculations

A metal-free oxidative decarboxylation reaction of propiolic acids mediated by hypervalent iodine(III) reagents is described. This decarboxylative C-O bond-forming reaction used a combination of (diacetoxyiodo)benzene and aromatic, heteroaromatic or aliphatic propiolic acids to give the corresponding α-acetoxy ketones. Preliminary mechanistic studies based on both DFT calculations and high-resolution mass spectroscopy (HRMS) suggested that the reaction proceeded through decarboxylation to form a propargyl iodide intermediate. This reaction provides an attractive alternative to existing methods for the exclusive synthesis of α-acyloxy ketones.

Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling

Complex [(DIPeP BDI)Ca]2 (C6 H6 ), with a C6 H6 2- dianion bridging two Ca2+ ions, reacts with benzene to yield [(DIPeP BDI)Ca]2 (biphenyl) with a bridging biphenyl2- dianion (DIPeP BDI=HC[C(Me)N-DIPeP]2 ; DIPeP=2,6-CH(Et)2 -phenyl). The biphenyl complex was also prepared by reacting [(DIPeP BDI)Ca]2 (C6 H6 ) with biphenyl or by reduction of [(DIPeP BDI)CaI]2 with KC8 in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [(DIPP BDI)CaI(THF)]2 with K/KI was extracted with benzene (DIPP=2,6-CH(Me)2 -phenyl) giving crystalline [(DIPP BDI)Ca(THF)]2 (biphenyl) (52 % yield). Reduction of [(DIPeP BDI)SrI]2 with KC8 gave highly labile [(DIPeP BDI)Sr]2 (C6 H6 ) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(DIPeP BDI)Sr]2 (biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C6 H6 2- dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.

Photoredox-Mediated Deoxygenative Radical Additions of Aromatic Acids to Vinyl Boronic Esters and gem-Diborylalkenes

A new method to access β-keto-gem-diborylalkanes, by direct deoxygenative radical addition of aromatic carboxylic acids to gem-dibortlalkenes, is described. The reaction proceeds under mild photoredox catalysis and involves the photochemical C-O bond activation of aromatic carboxylic acids in the presence of PPh3 . It generates an acyl radical, which further undergoes an additional reaction with gem-diborylalkenes to form an α-gem-diboryl alkyl radical intermediate, which then reduces to the corresponding anion, which after protonation, affords the β-keto-gem-diborylalkane product. Moreover, the same scenario has been extended to the vinyl boronic esters, for example, gem-(Ar, Bpin)-alkenes, and gem-(Alkyl, Bpin)-alkenes. Importantly, this protocol provides a general platform for the late-stage functionalization of bio-active and drug molecules containing a carboxylic acid group.

Tandem Palladium/Copper-Catalyzed Decarboxylative Approach to Benzoimidazo- and Imidazophenanthridine Skeletons

A protocol for a tandem Pd/Cu-catalyzed intermolecular cross-coupling cascade between o-bromobenzoic acids and 2-(2-bromoaryl)-1H-benzo[d]imidazoles or the corresponding imidazoles is presented. The protocol provides conceptually novel and controlled access to synthetically useful N-fused (benzo)imidazophenanthridine scaffolds with high efficiency, a broad substrate scope, and excellent functional group compatibility.

Carboxylic Acids as Adaptive Functional Groups in Metallaphotoredox Catalysis

The development of palladium-catalyzed cross-coupling methods for the activation of C(sp2)-Br bonds facilitated access to arene-rich molecules, enabling a concomitant increase in the prevalence of this structural motif in drug molecules in recent decades. Today, there is a growing appreciation of the value of incorporating saturated C(sp3)-rich scaffolds into pharmaceutically active molecules as a means to achieve improved solubility and physiological stability, providing the impetus to develop new coupling strategies to access these challenging motifs in the most straightforward way possible. As an alternative to classical two-electron chemistry, redox chemistry can enable access to elusive transformations, most recently, by interfacing abundant first-row transition-metal catalysis with photoredox catalysis. As such, the functionalization of ubiquitous and versatile functional handles such as (aliphatic) carboxylic acids via metallaphotoredox catalysis has emerged as a valuable field of research over the past eight years.In this Account, we will outline recent progress in the development of methodologies that employ aliphatic and (hetero)aromatic carboxylic acids as adaptive functional groups. Whereas recent decarboxylative functionalization methodologies often necessitate preactivated aliphatic carboxylic acids in the form of redox-active esters or as ligands for hypervalent iodine reagents, methods that enable the direct use of the native carboxylic acid functionality are highly desired and have been accomplished through metallaphotoredox protocols. As such, we found that bench-stable aliphatic carboxylic acids can undergo diverse transformations, such as alkylation, arylation, amination, and trifluoromethylation, by leveraging metallaphotoredox catalysis with prevalent first-row transition metals such as nickel and copper. Likewise, abundant aryl carboxylic acids are now able to undergo halogenation and borylation, enabling new entry points for traditional, primarily palladium- or copper-catalyzed cross-coupling strategies. Given the breadth of the functional group tolerance of the employed reaction conditions, the late-stage functionalization of abundant carboxylic acids toward desired targets has become a standard tool in reaction design, enabling the synthesis of various diversified drug molecules. The rapid rise of this field has positively inspired pharmaceutical discovery and will be further accelerated by novel reaction development. The achievement of generality through reaction optimization campaigns allows for future breakthroughs that can render protocols more reliable and applicable for industry. This article is intended to highlight, in particular, (i) the employment of aliphatic and (hetero)aryl carboxylic acids as powerful late-stage adaptive functional handles in drug discovery and (ii) the need for the further development of still-elusive and selective transformations.We strongly believe that access to native functionalities such as carboxylic acids as adaptive handles will further inspire researchers across the world to investigate new methodologies for complex molecular targets.

Visible-Light-Induced Acylative Coupling of Benzoic Acid Derivatives with Alkenes to Dihydrochalcones

A strategy was developed for the visible-light-induced photocatalytic synthesis of dihydrochalcone via the deoxygenation and coupling of benzoic acid derivatives with alkenes using diphenyl sulfide as the O-transfer reagent. Under mild photoredox conditions, a series of dihydrochalcone derivatives were produced in moderate to good yields. A mechanism for the visible-light-induced free-radical coupling was proposed on the basis of the control experiments. The protocol provides a new strategy the generation of acyl radicals from carboxylic acids and the synthesis of dihydrochalcones.

Palladium-Catalyzed Direct Decarbonylative Cyanation of Aryl Carboxylic Acids

The direct transformation of aryl carboxylic acids to aryl nitrile compounds is an interesting topic because carboxylic acids are not only abundant in nature but are also inexpensive and stable. Here, the synthesis of a series of aryl nitriles by palladium-catalyzed decarbonylative cyanation of carboxylic acids without base has been achieved. The successful decarbonylative cyanation of drug molecules and Gram-scale reaction to verify the practicality and operability of this method are analyzed.

Metal-Free Catalysis in C-C Single-Bond Cleavage: Achievements and Prospects

This review article emphasizes the C-C bond cleavage in organic synthesis via metal-free approach. Conventional organic synthesis mainly deals with the reactive π bonds and polar σ bonds. In contrast, the ubiquitous C-C single bonds are inherently stable and are less reactive, which poses a challenge to synthetic chemists. Although inert, such C-C single-bond cleavage reactions have gained attention amongst synthetic chemists, as they provide unique and more straightforward routes, with significantly fewer steps. Several review articles have been reported regarding the activation and cleavage of C-C bonds using different transition metals. However, given the high cost and toxicity of many of these metals, the development of strategies under metal-free conditions is of utmost importance. Though many research articles have been published in this area, no review article has been reported so far. Herein, we discuss the reactions in a more concise way from the year 2012 to today, with emphasis on important reactions. Mechanisms of all the reactions are also well addressed. We believe that this review will be beneficial for the readers who work in this field.

Preparation of glycosyl carboxylic acids via stereoselective synthesis and oxidative cleavage of C-vinyl glycosides

We have developed an improved cyanide-free strategy for the synthesis of glycosyl carboxylic acids, employing stereoselective C-vinyl glycosylation and oxidative cleavage of C-vinyl glycosides as key steps. Compared to our previous work, the amount of NaIO₄ required for the oxidative cleavage step is reduced significantly from 18 equivalents to 4.5 equivalents. This modification not only is advantageous in terms of operation and costs but also avoids the over-oxidation problem, thus greatly expanding the substrate scope, which is evidenced by the fact that 10 out of 21 glycosyl carboxylic acids synthesized are undocumented. With differently O5-protected furanosyl acids in hand, we demonstrate that an electron-rich protecting group is beneficial for the decarboxylative arylation of furanosyl carboxylic acids. This represents a rare example of protecting groups affecting the reaction efficiency in radical C-glycosylation. As C-vinyl glycosides can be prepared stereoselectively and the oxidative step is stereoretentive, the approach provides an effective means to access 1,2-trans or 1,2-cis glycosyl acids, which would be a valuable alternative to the cyanide-based synthesis of glycosyl carboxylic acids.

Visible light-induced deoxygenation/cyclization of salicylic acid derivatives and aryl acetylene for the synthesis of flavonoids

A visible-light-induced photocatalytic strategy for the synthesis of flavonoids has been developed through the deoxygenative/cyclization reaction of salicylic acid derivatives with aryl acetylene using diphenyl sulfide as an O-transfer reagent. Based on the controlled experiments, the mechanism of visible-light-induced free radical coupling cyclization was proposed. The protocol obtained 51 flavonoids in good yields and has been successfully applied to the synthesis of some natural flavones.

Ru-Catalyzed C-H Arylation of Acrylic Acids with Aryl Bromides

In the presence of a [Ru(p-cymene)Cl₂]₂/triethylphosphine/lithium carbonate catalyst system, aryl bromides undergo (Z)-selective couplings with unprotected 2-arylacrylic acids to form (Z)-diarylacrylic acids. This vinylic C-H functionalization proceeds in high yields of up to 94% and (Z/E)-ratios of up to 99:1, tolerating a wide range of functional groups. Mechanistic studies indicate that the vinylic C-H activation proceeds via base-assisted cyclometalation rather than via a Heck-type mechanism, which explains its orthogonal stereoselectivity.

A Unified Approach to Decarboxylative Halogenation of (Hetero)aryl Carboxylic Acids

Aryl halides are a fundamental motif in synthetic chemistry, playing a critical role in metal-mediated cross-coupling reactions and serving as important scaffolds in drug discovery. Although thermal decarboxylative functionalization of aryl carboxylic acids has been extensively explored, the scope of existing halodecarboxylation methods remains limited, and there currently exists no unified strategy that provides access to any type of aryl halide from an aryl carboxylic acid precursor. Herein, we report a general catalytic method for direct decarboxylative halogenation of (hetero)aryl carboxylic acids via ligand-to-metal charge transfer. This strategy accommodates an exceptionally broad scope of substrates. We leverage an aryl radical intermediate toward divergent functionalization pathways: (1) atom transfer to access bromo- or iodo(hetero)arenes or (2) radical capture by copper and subsequent reductive elimination to generate chloro- or fluoro(hetero)arenes. The proposed ligand-to-metal charge transfer mechanism is supported through an array of spectroscopic studies.

Decarboxylative Borylation and Cross-Coupling of (Hetero)aryl Acids Enabled by Copper Charge Transfer Catalysis

We report a copper-catalyzed strategy for arylboronic ester synthesis that exploits photoinduced ligand-to-metal charge transfer (LMCT) to convert (hetero)aryl acids into aryl radicals amenable to ambient-temperature borylation. This near-UV process occurs under mild conditions, requires no prefunctionalization of the native acid, and operates broadly across diverse aryl, heteroaryl, and pharmaceutical substrates. We also report a one-pot procedure for decarboxylative cross-coupling that merges catalytic LMCT borylation and palladium-catalyzed Suzuki-Miyaura arylation, vinylation, or alkylation with organobromides to access a range of value-added products. The utility of these protocols is highlighted through the development of a heteroselective double-decarboxylative C(sp²)-C(sp²) coupling sequence, pairing copper-catalyzed LMCT borylation and halogenation processes of two distinct acids (including pharmaceutical substrates) with subsequent Suzuki-Miyaura cross-coupling.