Mechanistic Insights into a Palladium-Catalyzed Quaternary Carbon-Editing Strategy: A DFT Study

Direct editing of quaternary carbon remains highly challenging. In this study, we computationally investigated a palladium-catalyzed quaternary carbon-editing strategy using density functional theory (DFT) to elucidate its principal characteristics and address key mechanistic issues. A quaternary carbon-editing mechanism driven by sequential Pd migration was established. The results indicate that the total free energy barrier for the transformation is 29.5 kcal mol-1, which is reasonable under the studied reaction conditions, with 1,3-PdIV migration identified as the rate-determining step. Distortion-interaction (D/I) analysis revealed that smaller distortion energy is responsible for the selective palladation. These calculations confirm that 1,3-PdIV migration is kinetically more favorable than 1,3-PdII migration. Selectfluor can effectively lower the barrier to 1,3-PdIV migration, thereby facilitating the conversion. Furthermore, the calculations indicate that the amide bond in the starting reactant (1) plays a critical role in this strategy, particularly in selective palladation and 1,3-Pd migration. Notably, we discovered a novel mechanism involving 1,2-methyl/PdIV dyotropic rearrangement and β-hydride elimination. This process exhibits a significantly lower free energy barrier, with methyl migration and HF elimination occurring simultaneously to form a C═C double bond. Thus, these findings enhance the understanding of quaternary carbon-editing strategies and can potentially provide theoretical support for future research.

Skeletal Rearrangements of Amides via Breaking Inert Bonds

Skeletal rearrangements of amides provide rapid access to complex nitrogen-containing scaffolds from simple readily available starting materials. While classical reactions such as the Hofmann and Curtius rearrangements have been widely utilized in organic synthesis, recent advances in amide activation strategies have brought new types of transformations and offered many new applications. This review focuses on the development of amide skeletal rearrangement reactions over the past two decades. The content is organized based on the initial bond cleavage pathways: C─N bond cleavage, C─C bond cleavage, and C═O bond activation.

Visible Light-Induced C─C Bond Cleavage for the Construction of Ketone Using Molecular Oxygen

The selective cleavage of carbon-carbon bonds by photocatalysis with the participation of oxygen for ketone synthesis and drug derivatization is reported. In this oxidation-based ketone formation reaction, molecular oxygen complexes with the substrate, which is subsequently photoexcited to generate a charge transfer (CT) state. This CT state facilitates energy transfer with molecular oxygen, producing singlet oxygen, which facilitates hydrogen atom process and promotes the subsequent cleavage of the carbon-carbon bond, ultimately leading to the formation of aryl ketone compounds. 48 aryl ketone compounds and pharmaceutical derivatives were synthesized using this method. A notable feature of this reaction is its catalyst-free, additive-free, and transition metals-free nature combined with green, mild, and environmentally friendly reaction conditions enabling high yields.

Hydrogen-Bond-Assisted Ru(III)-Catalyzed C-C Bond Activation in 1,3-Dicarbonyls: A Direct Route to Multi-Substituted Pyrroles

Unprecedented CO-Cα bond cleavage of 1,3-dicarbonyls and enaminone, catalyzed by a well-defined Ru(III)-complex (1) featuring a redox-active triamine ligand (L1) with a free -NH2 arm, opening a new route to accessing substituted pyrroles with broad substrate scope and functional group tolerance in good isolated yields via multicomponent coupling of 1,3-dicarbonyls, amines, and diol, is reported. The hydrogen bonding interaction offered by 1 facilitates the formation of critical reaction intermediates, favoring the formation of pyrroles.

Nickel-Catalyzed Electroreductive Heck Reaction: A Divergent Synthesis of C7- or C2-Arylindolines

Herein, we present a novel electrochemically driven nickel-catalyzed Heck reaction of readily available N-arylacrylindoles for the construction of both 7-arylindolines and 2-arylindolines. The reaction proceeds in an undivided electrochemical cell and employs a sacrificial zinc anode as the reductant. This catalytic protocol features a consecutive electroreductive process under exceedingly mild reaction conditions, with moderate yields. Detailed mechanistic studies indicate that the reaction involves a domino sequence of intramolecular electroreductive Heck arylation and selective C-C or C-O bond cleavage, which could be accomplished by aryl migration or carbethoxy removal.

Pd-Catalyzed Aerobic C-H Carbonylative Esterification of Imidazo[1,2-a]pyridines with Alcohols as the Carbonyl Source

A simple and practical method has been developed for the carbonylative esterification of imidazo[1,2-a]pyridines via C(sp2)-H bond functionalization using alkyl alcohols under mild reaction conditions. The carbonyl fragment is sourced from radical-mediated C-C cleavage of the alcohols, providing a green, safe, and economic alternative to traditional carbonyl sources like carbon monoxide. Through this strategy, a number of imidazo[1,2-a]pyridine-3-carboxylates were obtained from simple substrates by a single step.

Palladium-Catalyzed C(O)-C Bond Cleavage of Unstrained Ketones Assisted with Aryl Handles: An Approach to Diaryl Ketones

C-C bond cleavage reactions have achieved remarkable progress in molecular deconstruction and skeleton editing. In this study, we describe a palladium-catalyzed synthesis of diaryl ketones through a sequence of α-arylation and aerobic oxidative C-C bond cleavage. This transformation features good functional group compatibility, especially for highly reactive groups, including -OH, NH2, and -CHO. Furthermore, this approach allows for gram-scale synthesis in a low catalyst loading manner. It also streamlines the synthesis of a variety of drugs or their intermediates. Mechanism studies reveal the essential role of utilizing a bulky ligand and the existence of air as the reaction atmosphere.

Skeletal Modification via Activation of Relatively Unstrained C-C Bonds

ConspectusMethods that can directly modify the skeletons of complex molecules have become increasingly attractive for preparing novel analogues without the need for de novo synthesis in drug discovery processes. Among the various skeletal modification approaches, those targeting unstrained C-C bonds are particularly challenging to realize, owing to the relative inertness of these bonds toward common reagents. Compared to C-H or C-X (X: heteroatom) bonds, the activation of unstrained C-C bonds is often not thermodynamically and/or kinetically favorable. As a result, strategies relying on highly strained substrates or oxidative conditions are generally employed, which inevitably limit the scope and applications of C-C bond activation reactions. Hence, the development of redox-neutral catalytic C-C activation methods remains highly sought after for late-stage skeletal modification of complex bioactive compounds.In this Account, we summarize our recent progress in skeletal modifications through the catalytic activation of relatively unstrained C-C bonds. Enabled by transient or removable directing groups (DGs), the scope of C-C bond activation can be greatly expanded, encompassing a wide range of substrates, including ketones, amides, lactams, and biaryls. Consequently, different types of skeletal modification transformations have been developed. The major topics covered include the following: (1) Skeletal rearrangement and "cut-and-sew" transformations of cyclic ketones: we developed an aminopyridine/Rh-N-heterocyclic carbene (NHC) cooperative catalysis system that specifically targets the α-C-C bond of cyclic ketones. For substrates bearing a β-aryl substitution, the rhodacycle formed after the C-C bond activation can undergo an intramolecular C-H activation, resulting in the skeletal rearrangement from cyclopentanones/cyclohexanones to 1-tetralones/1-indanones. Additionally, the "cut-and-sew" transformations between indanones and ethylene or alkynes have been realized to offer a two-carbon ring expansion. (2) Chain homologation of linear amides and downsizing of lactams: the Rh-NHC activation system can be extended to the linear amides and lactams through preinstalling removable DGs. This approach has provided some new tools for precise amide modifications, including tunable homologation of tertiary amides via a "hook-and-slide" strategy and the downsizing transformation of lactams. (3) "Cut-and-sew" transformations of biphenols: using the preinstalled phosphinite DGs, unstrained 2,2'-biphenols can undergo split cross-coupling with various aryl iodides. When diiodide coupling partners are used, an interesting phenylene insertion into the aryl-aryl bond of biphenols can be achieved, which represents another type of "cut-and-sew" transformation.Collectively, these methods provide a reliable means to manipulate inert molecular scaffolds and offer new bond-disconnecting strategies to access useful structural motifs. The applications of these methods in the synthesis of bioactive natural products and complex analogues underscore their practical significance. Mechanistic insights gained from these studies are also discussed, which are expected to inspire future endeavors in this field.

Pd-Catalyzed Skeletal Rearrangement via C(sp3)-C(sp3) Activation to Access α,β-Unsaturated δ/γ-Lactone

The activation of non-polar aliphatic C-C bonds represents a significant challenge that remains to be addressed in the field of Pd(II) catalysis. In this study, we present a dual ligand approach as a means of addressing this issue. The process entails lactonization of cyclobutane and cyclopropane carboxylic acid via the activation of non-polar C(sp3)-C(sp3) bond despite the high feasibility of both οrtho-C(sp2)-H and β or γ -C(sp3)-H activation. Density Functional Theory (DFT) calculations were performed to reveal the mechanistic insights and elucidate the role of dual ligands in facilitating this challenging transformation.

Sc(OTf)3-Catalyzed C-C Bond Cleavage of Unactivated Acylazaarenes with 1,2-Diamines to Afford Azaarenes and Imidazoles

We report a method for cleaving the C(O)-sp2C bonds of unactivated acylazaarenes via benzimidazoline intermediates using a mild redox neutral catalytic system involving scandium triflate. This method avoids the need for preactivation or transition metal catalysts, enabling efficient C-C bond cleavage in a broad range of substrates, including 2-acylimidazoles, 2-acylpyridines, 2-acylpyrole, and even nonchelating 3-acylindole, in which direct C-C bond cleavage has not been previously achieved.

Palladium-catalyzed carbon-carbon bond cleavage of primary alcohols: decarbonylative coupling of acetylenic aldehydes with haloarenes

In the current work, a palladium-catalyzed C-C bond cleavage reaction of primary alcohols has been developed. This transformation was characterized by a broad substrate scope, superior functional group tolerance, and high efficiency for selective C-C bond cleavage and was then followed by alkynyl-aryl cross coupling. Mechanism studies indicated that the propargyl alcohols underwent β-H elimination to form aldehydes rather than having undergone β-C elimination. The corresponding aldehyde intermediates then proceeded through a decarbonylation and coupling reaction with haloarenes to yield diarylacetylenes.

Conversion of aromatic methyl ketones to esters and carboxylic acids using o-phthalaldehyde as an oxidant

Herein we describe a two-step conversion of aromatic methyl ketones to esters and carboxylic acids employing o-phthalaldehyde as an oxidant. In the first step, o-phthalaldehyde oxidizes the methyl group to 1-indanone, which acts as a leaving group in a subsequent regioselective retro-Claisen condensation to form esters and carboxylic acids. The mild oxidation conditions ensure the method is applicable to a broad range of substrates. Additionally, the two-step method is operationally simple and scalable, and can also be performed in a single pot.

Aza-[4 + 2]-cycloaddition of benzocyclobutenones into isoquinolinone derivatives enabled by photoinduced regio-specific C-C bond cleavage

The activation of C-C bond of benzocyclobutenones under mild reaction conditions remains a challenge. We herein report a photoinduced catalyst-free regio-specific C1-C8 bond cleavage of benzocyclobutenones, enabling the generation of versatile ortho-quinoid ketene methides for aza-[4 + 2]-cycloaddition with imines, which offers a facile route to isoquinolinone derivatives, including seven family members of protoberberine alkaloids, gusanlung A, B, D, 8-oxotetrahydroplamatine, tetrahydrothalifendine, tetrahydropalmatine, and xylopinine. Furthermore, the catalytic enantioselective version of this strategy is also realized by merging synergistic photocatalysis and chiral Lewis acid catalysis. Mechanistic studies provide compelling evidence to rationalize the photoisomerization/cycloaddition cascade process.

Aerobic Reconstruction of Amines to Amides: A C-N/C-C Bond Cleavage Approach

Herein, an aerobic reconstruction of amines to amides via C(sp3)-N bond and C(sp2)-C(sp3) bond cleavage is described. This method features a metal-free reaction, insensitivity to oxygen or moisture, and ambient air as the terminal oxidant. Preliminary mechanistic studies suggest that the reaction pathway of amine oxidation, followed by imine exchange and Beckmann rearrangement, is involved.

Efficient C(sp3)-P(V) bond cleavage and reconstruction of free α-aminophosphonates via palladium catalysis

Transition metal-catalyzed cleavage and reconstruction of the C-P bond provides a highly efficient and rapid method for the transformation of organophosphine compounds. In this study, a novel and general protocol for the palladium-catalyzed C(sp3)-P(V) bond cleavage of free α-aminophosphonates and subsequent functionalization via C-P bond recombination has been developed. The reaction exhibits high reactivity between the C(sp3)-P bond and halides, accommodating a wide range of substrates and enabling the rapid synthesis of aryl, alkenyl, and alkyl organophosphine molecules. Additionally, the synthetic utility is validated by gram-scale synthesis, and the reaction process is corroborated by mechanistic experiments.

Recent Advances in Electrochemical Carboxylation with CO2

ConspectusCarbon dioxide (CO2) is recognized as a greenhouse gas and a common waste product. Simultaneously, it serves as an advantageous and commercially available C1 building block to generate valuable chemicals. Particularly, carboxylation with CO2 is considered a significant method for the direct and sustainable production of important carboxylic acids. However, the utilization of CO2 is challenging owing to its thermodynamic stability and kinetic inertness. Recently, organic electrosynthesis has emerged as a promising approach that utilizes electrons or holes as environmentally friendly redox reagents to produce reactive intermediates in a controlled and selective manner. This technique holds great potential for the CO2 utilization.Since 2015, our group has been dedicated to exploring the utilization of CO2 in organic synthesis with a particular focus on electrochemical carboxylation. Despite the significant advancements made in this area, there are still many challenges, including the activation of inert substrates, regulation of selectivity, diversity in electrolysis modes, and activation strategies. Over the past 7 years, our team, with many great experts, has presented findings on electrochemical carboxylation with CO2 under mild conditions. In this context, we primarily highlight our contributions to selective electrocarboxylations, encompassing new reaction systems, selectivity control methods, and activation approaches.We commenced our research by establishing a Ni-catalyzed electrochemical carboxylation of unactivated aryl halides and alkyl bromides in conjunction with a useful paired anodic reaction. This approach eliminates the need for sacrificial anodes, rendering the carboxylation process sustainable. To further utilize the widely existing yet cost-effective alkyl chlorides, we have developed a deep electroreductive system to achieve carboxylation of unactivated alkyl chlorides and poly(vinyl chloride), allowing the direct modification and upgrading of waste polymers.Through precise adjustment of the electroreductive conditions, we successfully demonstrated the dicarboxylation of both strained carbocycles and acyclic polyarylethanes with CO2 via C-C bond cleavage. Furthermore, we have realized the dicarboxylative cyclization of unactivated skipped dienes to produce the valuable ring-tethered adipic acids through single-electron reduction of CO2 to the CO2 radical anion (CO2•-). In terms of the asymmetric carboxylation, Guo's and our groups have recently achieved the nickel-catalyzed enantioselective electroreductive carboxylation reaction using racemic propargylic carbonates and CO2, paving the way for the synthesis of enantioenriched propargylic carboxylic acids.In addition to the aforementioned advancements, Lin's and our groups have also developed new electrolysis modes to achieve regiodivergent C-H carboxylation of N-heteroarenes dictated by electrochemical reactors. The choice of reactors plays a crucial role in determining whether the hydrogen atom transfer (HAT) reagents are formed anodically, consequently influencing the carboxylation pathways of N-heteroarene radical anions in the distinct electrolyzed environments.

A DFT Comparison of C-C Reductive Coupling from Terminal Cyanido and Cyaphido Complexes of Nickel

The density functional theory study of the thermal C-C reductive coupling from terminal cyanido and hypothetical cyaphido complexes of [Ni(dmpe)] (dmpe = 1,2-bis(dimethylphosphino)ethane) revealed the key reaction intermediate in the reductive C-CP coupling being a σ-CC complex unlike an η2-aryl complex in the Ni C-CN system, as already observed in our previous studies. The reaction in THF is endothermic by 4.9 kcal/mol for cyanido with a 32.0 kcal/mol activation barrier and exothermic by 28.5 kcal/mol for cyaphido with an 11.3 kcal/mol activation barrier. To compare our results with the existing experimental data, we chose mesityl as the aryl group and also studied the CP reaction with [Pt(dmpe)] and [Pt(dmpm)] (dmpe = 1,2-bis(dimethylphosphino)methane) fragments. Our findings are consistent with the thermodynamically uphill photolytic C-CP bond activation in phosphaalkynes with Pt and a faster thermal back-reaction with [Pt(dmpe)] compared to that of [Pt(dmpm)]. Based on the natural population analysis, when the polarity of the C-C bond is inverted, the sign of ΔG° is also inverted.

Reversible C-C bond formation in group 4 metal complexes: nitrile extrusion via β-aryl elimination

Pyridylamides of zirconium and hafnium with [C,N,N]-ligands reversibly insert nitriles into M-CAr bonds leading to an observable equilibrium between the starting [C,N,N]-complexes and newly formed [N,N,N]-complexes with a ketimide moiety in a 7-membered metallacycle. The discovered reversible insertion of nitriles into M-CAr bonds represents an unprecedented example of β-aryl elimination from a ketimide ligand in early transition metal complexes. Experimental and computational studies suggest thermodynamic and electronic reasons for this reactivity. Weak orbital overlap between the ketimide nitrogen and the metal, and an unfavorable 7-membered metallacycle destabilize the product of insertion into the M-CAr bond, while the pyridylamide moiety acts as a directing group making the reverse process viable. The influence of non-chelate spectator ligands on the metal center and substituents in nitrile on the thermodynamic stability of the [N,N,N]-complexes was also studied. Exploiting β-carbon elimination in complexes of early transition metals may extend the range of catalysts that are accessible for C-C activation processes in the future.

Transforming cyclopropanes to enamides via σ-C-C bond eliminative borylation

Recent strides in C-H borylation have significantly expanded our toolkit for the preparation of organoboronates. Nevertheless, avenues alternative to obtain these compounds via σ-C-C cleavage, thereby facilitating molecular scaffold editing, remain scarce. Several methodologies have been proposed for hydroboration of cyclopropanes by activating C-C bonds, conventionally relying on noble and hazardous metal catalysts to control reaction outcomes. Here, we present a strategy for crafting stereochemically precise γ-borylenamides through ring-opening of cyclopropanes avoiding any metallic entities. Boryl species, generated through a ternary reaction with BCl3, cyclopropanes, and a tertiary amine, selectively undergo C-C bond eliminative borylation under the directing of N-acyl group, thereby ensuring enhanced selectivity and efficiency along the reaction pathway. Such inherently stereoconvergent approach accommodates precursors of diverse geometries, including cis/trans isomeric blends.

Quaternary Carbon Editing Enabled by Sequential Palladium Migration

Peripheral functionalization of a quaternary carbon via C(sp3)-H bond activation has made significant progress in recent years. However, direct editing of a quaternary carbon through Csp3-Csp3 bond cleavage and refunctionalization of nonstrained acyclic molecules remain underexploited. Herein we report a reaction in which a methyl group attached to a quaternary carbon is shifted to its neighboring secondary carbon with concurrent oxidation of the quaternary C-C single bond to the C═C double bond. Specifically, morpholinyl amide of 2,2-dimethyl alkanoic acids is converted to 2-methylene-3-methyl alkanoic acid derivatives in the presence of a catalytic amount of palladium acetate, Selectfluor and sodium carbonate. Control experiments suggest that the reaction proceeds via a sequence of selective C(sp3)-H activation of the methyl group, oxidation of the resulting C(sp3)-PdII to PdIV intermediate followed by unprecedented 1,3-PdIV migration, 1,2-methyl/PdIV dyotropic rearrangement and finally, β-Hydride elimination. In this domino process, palladium migrates successively from the primary to the secondary and finally to the quaternary carbon, leading to the concurrent functionalization of a primary, a secondary, and a quaternary carbon.

Post-Synthetic Modification of Calix[4]arene Framework by Iridium-Mediated Alkyne Insertion into an Inert C-C Bond: A Novel Strategy for Unsymmetrical Macrocycles

As a novel synthetic method for unsymmetrical macrocycles, we herein developed a post-synthetic modification of calix[4]arenes by insertion of a terminal alkyne into an inert C(methylene)-C(aryl) bond of the macrocyclic framework. In this transformation, C-iridated calix[4]arenes, readily synthesized through C-H bond activation of the parent calix[4]arene, undergoes C(methylene)-C(aryl) bond cleavage followed by insertion of an alkyne to provide a ring-expanded calix[4]arene complex. Removal of the iridium metal from the resulting complex was readily accomplished by a simple treatment with an acid. The developed sequential method affords novel unsymmetrical, monofunctionalized macrocyclic compounds via 3 steps from the parent calix[4]arene in good yield. The unique unsymmetrical structures of the alkyne-inserted macrocycles were evaluated by X-ray single crystal analyses. On the basis of theoretical calculations, we propose a reaction mechanism involving an uncommon C-C bond cleavage step through δ-carbon elimination for the ring enlargement process.

Selective nitrogen insertion into aryl alkanes

Molecular structure-editing through nitrogen insertion offers more efficient and ingenious pathways for the synthesis of nitrogen-containing compounds, which could benefit the development of synthetic chemistry, pharmaceutical research, and materials science. Substituted amines, especially nitrogen-containing alkyl heterocyclic compounds, are widely found in nature products and drugs. Generally, accessing these compounds requires multiple steps, which could result in low efficiency. In this work, a molecular editing strategy is used to realize the synthesis of nitrogen-containing compounds using aryl alkanes as starting materials. Using derivatives of O-tosylhydroxylamine as the nitrogen source, this method enables precise nitrogen insertion into the Csp2-Csp3 bond of aryl alkanes. Notably, further synthetic applications demonstrate that this method could be used to prepare bioactive molecules with good efficiency and modify the molecular skeleton of drugs. Furthermore, a plausible reaction mechanism involving the transformation of carbocation and imine intermediates has been proposed based on the results of control experiments.

Palladium/Norbornene Cooperatively Catalyzed Modular Trifunctionalization of 2-Bromoaryl Ketone via a Decarbonylation Process

Palladium/norbornene cooperatively catalyzed Catellani-type reactions were normally limited to aryl iodides as substrates. The employment of aryl bromides has remained challenging. Herein a Pd/NBE cooperatively catalyzed Catellani-type reaction of 2-bromoaryl ketone is described. The 2-bromoaryl ketone was employed as both substrates and arylation reagents with a Heck acceptor. A decarbonylation process of the ketones also occurred in the reaction, finishing the modular ispo-Heck/ortho,ortho-diarylation in one pot. It provided the functionalized m-triphenyl derivatives with three new C-C bonds in moderate to excellent yields which exhibited good regioselectivities and functional group tolerance.

Aromatization-driven deconstructive functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis

Aromatization-driven deconstruction and functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis is developed. The aromatization effect was introduced to synergistically drive unstrained cyclic C-C bond cleavage, with the aim of overcoming the ring-size limitation of nitrogen-centered radical induced deconstruction of carbocycles. Herein, we demonstrate the synergistic photoredox/nickel catalyzed deconstructive cross-coupling of spiro dihydroquinazolinones with organic halides. Remarkably, structurally diverse organic halides including aryl, alkenyl, alkynyl, and alkyl bromides were compatible for the coupling. In addition, this protocol is also characterized by its mild and redox-neutral conditions, excellent functional group compatibility, high atom economy, and easy scalability. A telescoped procedure involving condensation and ring-opening/coupling was found to be accessible. This work provides a complementary strategy to the existing radical-mediated C-C bond cleavage of unstrained carbocycles.

A Metallomicellar Catalyst for Controlled Oxidation of Alcohols and Lignin Mimics in Water using Open Air as Oxidant

Alcohol groups and β-O-4 (C-C) linkages are widespread in biomass feedstock that are abundant renewable resource for value-added chemicals. The development of sustainable protocols for direct oxidation or oxidative cleavage of feedstock materials in a controlled fashion, using open air as an oxidant is an intellectually stimulating task to produce industrially important value-added carbonyls. Further, the oxidative depolymerization of lignin into fine chemicals has evoked interest in recent times. Herein, we report the first example of a catalyst system that could activate molecular oxygen from atmospheric air for controlled oxidation and oxidative cleavage/depolymerization of feedstock materials such as alcohols, β-O-4 (C-C) linkages and real lignin in water under open air conditions. The selectivity of carbonyl products is controlled by altering the pH between ~7.0 and ~12.0. The current strategy highlights the non-involvement of any external co-catalyst, oxidant, radical additives, and/or destructive organic solvents. The catalyst shows a wide substrate scope and eminent functional group tolerance. The upscaled multigram synthesis using an inexpensive catalyst and easily available oxidant evidences the practical utility of the developed protocol. A plausible mechanism has been proposed with the help of a few controlled experiments, and kinetic and computational studies.

Copper/O2-Mediated Oxidative C-C Activation of Nitriles for Selective Acylation-Bromination of Anilines

This study reports selective dual amino acylation and C-H bromination of aniline compounds enabled by Cu/O2 catalyst systems. This method involves crucial oxidation-induced C-CN bond cleavage of α-methylene nitriles to generate an acylcyanide intermediate that is facilely intercepted by anilines. After amino acylation, the Cu(II) precatalyst in combination with NBS generates Cu(III)-Br in situ that engages in selective electrophilic para- or ortho-C-H bromination. The substrate scope, mechanistic aspects, and late-stage functionalization of biologically active anilines are studied. This study shows the synthetic potential of oxidative C-CN bond activation of nitriles for the development of valuable reactions.

Palladium Catalysis Enabled Sequential C(sp3)-H/C-C Activation: Access to Vinyl γ-Lactams

A Pd(II)-catalyzed tandem reaction of aliphatic amides with vinylcyclopropanes (VCPs) was accomplished by merging C(sp3)-H and C-C activation. The reaction of VCP revealed alkenylation/cyclization, followed by ring opening via C-C cleavage, delivering vinyl γ-lactams with (E)-selectivity. The role of ligands, site-selectivity, functional group diversity, mechanistic insight, and synthetic utilities are important practical features.

Radical Addition-Enabled C-C σ-Bond Cleavage/Reconstruction to Access Functional Indanones: Total Synthesis of Carexane L

C-C σ-bond cleavage and reconstruction is a significant tool for structural modification in synthetic chemistry but it remains a formidable challenge to perform on unstrained skeletons. Herein, we describe a radical addition-enabled C-C σ-bond cleavage/reconstruction reaction of unstrained allyl ketones to access various functional indanones bearing a benzylic quaternary center. The synthetic utility of this method has been showcased by the first total synthesis of carexane L, an indanone-based natural product.

Sodium-Mediated Reductive C-C Bond Cleavage Assisted by Boryl Groups

In contrast to the well-established oxidative C=C double bond cleavage to give the corresponding carbonyl compounds, little is known about reductive C=C double bond cleavage. Here we report that C-C single bond cleavage in 1,2-diaryl-1,2-diborylethanes proceeds by reduction with sodium metal to yield α-boryl benzylsodium species. In combination with our previous reductive diboration of stilbenes, the overall transformation represents reductive cleavage of the C=C double bonds of stilbene to yield α-boryl-α-sodiated toluenes. This reductive two-step C=C double bond cleavage is applicable to ring-opening or ring-expansion reactions of polycyclic aromatic hydrocarbons.

Split cross-coupling via Rh-catalysed activation of unstrained aryl-aryl bonds

Constructive functionalization of unstrained aryl-aryl bonds has been a fundamental challenge in organic synthesis due to the inertness of these bonds. Here we report a split cross-coupling strategy that allows two-fold arylation with diverse aryl iodides through cleaving unstrained aryl-aryl bonds of common 2,2'-biphenols. The reaction is catalyzed by a rhodium complex and promoted by a removable phosphinite directing group and an organic reductant. The combined experimental and computational mechanistic studies reveal a turnover-limiting reductive elimination step that can be accelerated by a Lewis acid co-catalyst. The utility of this coupling method has been illustrated in the modular and simplified syntheses of unsymmetrical 2,6-diarylated phenols and skeletal insertion of phenyl units.

Palladium-Catalyzed Directed Carbon-Carbon Bond Activation of Aryl Nitriles for Cyano Transfer

Herein, we report the C-H cyanation of indoles via a palladium-catalyzed directed C-CN activation reaction using aryl nitrile as a cyano source. The employment of the phenoxy-oriented group is the key to the cleavage of the C-CN bond. This protocol features a broad substrate scope, good efficiency, and high regioselectivity. Furthermore, the practical application of this protocol was showcased in the late-stage functionalization and synthesis of indole derivatives, which were derived from drugs and natural products, through the process of cyanation.

Diastereo- and Enantioselective Synthesis of Tetracyclic Cycloheptanols through (4+3) Annulation via C-C/C-H Activation Cascade

Transition metal-catalyzed annulations of four-membered rings via C-C activation are powerful tools to construct complex fused and bridged ring systems. Despite significant progress in (4+1), (4+2) and (4+4) annulations, the (4+3) annulation remains unexplored. Herein, we develop an asymmetric Rh-catalyzed intramolecular (4+3) annulation of α-arylalkene-tethered benzocyclobutenols for the synthesis of dihydrofuran-annulated dibenzocycloheptanols with two discontinuous chiral carbon centers via a C-C and C-H activation cascade. The reaction features excellent diastereo- and enantioselectivities and 100 % atom economy, and is applicable to late-stage modification of complex molecules.

Palladium-Catalyzed Deuteration of Arylketone Oxime Ethers

We report herein the development of palladium-catalyzed deacylative deuteration of arylketone oxime ethers. This protocol features excellent functional group tolerance, heterocyclic compatibility, and high deuterium incorporation levels. Regioselective deuteration of some biologically important drugs and natural products are showcased via Friedel-Crafts acylation and subsequent deacylative deuteration. Vicinal meta-C-H bond functionalization (including fluorination, arylation, and alkylation) and para-C-H bond deuteration of electro-rich arenes are realized by using the ketone as both directing group and leaving group, which is distinct from aryl halide in conventional dehalogenative deuteration.

Rhodium(III)-Catalyzed Oxidative 1,3-Aryl Migration of α-Aryl Allylic Alcohols

A Rh(III)-catalyzed oxidative 1,3-aryl migration of α-arylallylic alcohols via Csp2-Csp3 σ bond activation has been developed. This method provides an efficient strategy to allow for allylic alcohol-based skeleton rearrangement, in which various secondary and tertiary α-arylallylic alcohols are rapidly converted to β-aryl-α, β-unsaturated ketones and aldehydes.

Lewis Acid-Promoted Oxidative Cleavage of Carbon-Carbon Bonds: Synthesis of N-Arylated Lactam-Type Iminosugars

In this paper, a mild strategy for the oxidative cleavage of carbon-carbon bonds catalyzed by Lewis acid was developed in air condition at room temperature. Under such conditions, the bis-carbonyl compounds 3 were directly afforded from the reaction of D-ribose tosylate 1 and aniline in excellent yields through the oxidative cleavage of the key intermediate iminium-ion A and its tautomer enamine B. A series of N-arylated lactam-type iminosugars 5 were then successfully obtained by removing the isopropylidene group from 3 with the aid of the condensation agent DCC. Additionally, reduction of A and the removal of the isopropylidene group could provide N-arylated iminosugars 4. This strategy enables the oxidative cleavage of carbon-carbon bonds under mild conditions and facilitates the synthesis of the novel iminosugars with potent biological activity.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

Pd-Catalyzed Formal [2 + 2]-Retrocyclization of Cyclobutanols via 2-Fold Csp3-Csp3 Bond Cleavage

In this work, we describe the unexpected 2-fold Csp3-Csp3 bond cleavage suffered by cyclobutanols in the presence of a catalytic amount of Pd(OAc)2 and promoted by the bulky biaryl JohnPhos ligand. Overall, the sequential cleavage of a strained and an unstrained Csp3-Csp3 bond leads to the formal [2 + 2]-retrocyclization products, namely, styrene and acetophenone derivatives. This procedure might enable the use of cyclobutanols as masked acetyl groups, resisting harsh conditions in organic synthesis.

Mechanistic Investigation of the Rhodium-Catalyzed Transfer Hydroarylation Reaction Involving Reversible C-C Bond Activation

Carbon-carbon (C-C) bonds are ubiquitous but are among the least reactive bonds in organic chemistry. Recently, catalytic approaches to activate C-C bonds by transition metals have demonstrated the synthetic potential of directly reorganizing the skeleton of small molecules. However, these approaches are usually restricted to strained molecules or rely on directing groups, limiting their broader impact. We report a detailed mechanistic study of a rare example of catalytic C-C bond cleavage of unstrained alcohols that enables reversible ketone transfer hydroarylation under Rh-catalysis. Combined insight from kinetic analysis, in situ nuclear magnetic resonance (NMR) monitoring, and density functional theory (DFT) calculations supports a symmetric catalytic cycle, including a key reversible β-carbon elimination event. In addition, we provide evidence regarding the turnover-limiting step, the catalyst resting state, and the role of the sterically encumbered NHC ligand. The study further led to an improved catalytic system with the discovery of two air-stable precatalysts that showed higher activity for the transformation in comparison to the original conditions.

ortho-Fluoro or ortho Effect? Oxidative Addition of Zerovalent Nickel into the C-CN Bond of Substituted Benzonitriles

A recent study of the oxidative addition of zerovalent Ni to the C-CN bond of F-substituted benzonitriles showed significantly increased stabilization of the C-CN oxidative addition products with o-F groups (-6.6 kcal/mol per o-F) compared to m-F groups (-1.8 kcal/mol per m-F). To answer the question of whether this is an o-F effect or an ortho effect, in this study the effect of CF3 and CH3 groups on the oxidative addition of the [Ni(dmpe)] fragment [dmpe = 1,2-bis(dimethylphosphino)ethane] to the C-CN bond of benzonitriles has been studied. A density functional theory study of the reaction pathway between η2-CN complexes and the C-CN oxidative addition products shows stabilization of the C-CN oxidative addition product with the electron-withdrawing CF3 group and destabilization with the electron-donating CH3 group in both tetrahydrofuran and toluene. There is a slightly larger ortho effect with CF3 (-7.4 kcal/mol) than with F. However, due to steric crowding, two o-CF3 groups did not show considerably more stabilization than one o-CF3 group. There is a linear relationship between ΔG° and the number of meta groups (2.0 kcal/mol stabilization per m-CF3 and 0.8 kcal/mol destabilization per m-CH3). On the basis of natural population analysis, as the C-CN bond becomes more polarized, the stability of the C-CN oxidative addition products with respect to the η2-CN complexes increases.

Rhodium catalyzed tunable amide homologation through a hook-and-slide strategy

Preparation of diverse homologs from lead compounds has been a common and important practice in medicinal chemistry. However, homologation of carboxylic acid derivatives, particularly amides, remains challenging. Here we report a hook-and-slide strategy for homologation of tertiary amides with tunable lengths of the inserted carbon chain. Alkylation at the α-position of the amide (hook) is followed by highly selective branched-to-linear isomerization (slide) to effect amide migration to the end of the newly introduced alkyl chain; thus, the choice of alkylation reagent sets the homologation length. The key step involves a carbon-carbon bond activation process by a carbene-coordinated rhodium complex with assistance from a removable directing group. The approach is demonstrated for introduction of chains as long as 16 carbons and is applicable to derivatized carboxylic acids in complex bioactive molecules.

Transition-Metal-Catalyzed C-C Bond Formation from C-C Activation

ConspectusC-C single bonds are ubiquitous in organic compounds. The activation and subsequent functionalization of C-C single bonds provide a unique opportunity to synthesize conventionally inaccessible molecules through the rearrangement of carbon skeletons, often with a favorable atom and step economy. However, the C-C bonds are thermodynamically and kinetically inert. Consequently, the activation of C-C bonds is particularly attractive yet challenging in the field of organic chemistry. In the past decade, we sought to develop efficient strategies to carry out transition-metal-catalyzed diverse C-C cleavage/C-C forming reactions and to obtain some insights into the intrinsic reactivities of different C-C bonds. With our efforts, readily available alcohols, carboxylic acids, and ketones served as suitable substrates for the catalytic C-C coupling reactions, which are reviewed in this Account. In 2009, we observed a Ni-catalyzed cross coupling of aryl nitriles with arylboronic esters through C-CN cleavage. Encouraged by these results, we are interested in transition-metal-catalyzed C-C bond activation. Due to their broad availability, we then turned our attention to C-C cleavage of carboxylic acids. Rhodium-catalyzed decarbonylative coupling of carboxylic acids with (hetero)arenes was then achieved through oxidative addition of in situ formed, more reactive mixed anhydrides to Rh(I) without the need for oxidants that are commonly required for the decarboxylative coupling of carboxylic acids. Subsequently, the decarbonylation of more challenging unstrained aryl ketones was realized under Rh catalysis assisted by N-containing directing groups. Following this work, a group exchange of aryl ketones with carboxylic acids was achieved through 2-fold C-C bond cleavage. By employing the chelation strategy, Rh-catalyzed C-C bond activation of secondary benzyl alcohols was also accomplished through β-carbon elimination of the rhodium alcoholate intermediates. The competing oxidation of secondary alcohols to ketones via β-hydrogen elimination of the same intermediates was suppressed as thermodynamically favorable five-membered rhodacycles are formed after β-carbon elimination. Different types of transformations of alcohols, including the Heck-type reaction with alkenes, cross coupling with arylsilanes, and Grignard-type addition with aldehydes or imines, have been achieved, showing the great potential of secondary alcohols in the formation of C-C bonds. These C-C bond-forming reactions are complementary to traditional cross couplings of aryl halides with organometallic reagents. However, these transformations produce small molecules as byproducts. To improve the atom economy, we then investigated C-C bond transformations of strained-ring cyclic compounds. Ni-catalyzed intermolecular cyclization of benzocyclobutenones with alkynes was recently achieved via the uncommon cleavage of the C1-C8 bond by employing a removable blocking strategy. Rh-catalyzed intramolecular annulation of benzocyclobutenols with alkynes was also achieved. In summary, our developments demonstrate the great potential of transition-metal-catalyzed C-C bond activation for the formation of new C-C bonds. To further expand the synthetic utility of C-C bond activation, more efforts are required to expand the substrate scope and to achieve earth-abundant metal-catalyzed transformations.

Pd-Catalyzed Activation of Carbon-Carbon Bonds in Hydroxymethylfurfural Derivatives

Palladium-catalyzed activation of C-C bonds in organic molecules is a powerful tool for the synthesis of value-added compounds. 5-Hydroxymethylfurfural (HMF) derivatives are a promising class of biomass-derived chemicals that have received considerable attention due to their potential applications in the synthesis of biologically active molecules and materials. However, the selective activation of unstrained C-C bonds is a challenging task, mainly due to their relatively high bond dissociation energies. Herein, we report a palladium-catalyzed method for the efficient C-C bond activation of HMF derivatives, enabling their arylation with iodobenzenes. Mechanistic studies, including reaction-profile analysis, competition experiments and head-space IR spectroscopy suggest a decarboxylative mechanism.

Aerobic oxidative C-C bond cleavage and functionalization for the synthesis of value-added chemicals

The aerobic oxidative cleavage of C-C bonds is an attractive and sustainable route for constructing valuable molecules such as esters, nitriles, and amides. Traditionally homogeneous catalytic systems for C-C bond cleavage required harsh conditions, stoichiometric oxidants, and noble metal catalysts to overcome the thermodynamic and kinetic barriers of C-C bonds, imposing environmental concerns of the transformation. Therefore, developing efficient, low-cost, and environmentally benign methods for C-C bond cleavage is of great importance and a cutting-edge area in modern chemistry. This feature article summarizes the sustainable aerobic oxidative C-C bond cleavage method developed by our group in the past 5 years. Fundamental principles in catalyst design, substrate scope, and mechanism for C-C bond cleavage are also discussed.

Mechanistic Investigation of the Nickel-Catalyzed Transfer Hydrocyanation of Alkynes

The implementation of HCN-free transfer hydrocyanation reactions on laboratory scales has recently been achieved by using HCN donor reagents under nickel- and Lewis acid co-catalysis. More recently, malononitrile-based HCN donor reagents were shown to undergo the C(sp3)-CN bond activation by the nickel catalyst in the absence of Lewis acids. However, there is a lack of detailed mechanistic understanding of the challenging C(sp3)-CN bond cleavage step. In this work, in-depth kinetic and computational studies using alkynes as substrates were used to elucidate the overall reaction mechanism of this transfer hydrocyanation, with a particular focus on the activation of the C(sp3)-CN bond to generate the active H-Ni-CN transfer hydrocyanation catalyst. Comparisons of experimentally and computationally derived 13C kinetic isotope effect data support a direct oxidative addition mechanism of the nickel catalyst into the C(sp3)-CN bond facilitated by the coordination of the second nitrile group to the nickel catalyst.

Ruthenium-Catalyzed Activation of Nonpolar C-C Bonds via π-Coordination-Enabled Aromatization

Activation of C-C bonds allows editing of molecular skeletons, but methods for selective activation of nonpolar C-C bonds in the absence of a chelation effect or a driving force derived from opening of a strained ring are scarce. Herein, we report a method for ruthenium-catalyzed activation of nonpolar C-C bonds of pro-aromatic compounds by means of π-coordination-enabled aromatization. This method was effective for cleavage of C-C(alkyl) and C-C(aryl) bonds and for ring-opening of spirocyclic compounds, providing an array of benzene-ring-containing products. The isolation of a methyl ruthenium complex intermediate supports a mechanism involving ruthenium-mediated C-C bond cleavage.

Rhodium-Catalyzed Intramolecular Nitrogen Atom Insertion into Arene Rings

In this study, we describe the direct insertion of an intramolecular nitrogen atom into an aromatic C-C bond. In this transformation, carbamoyl azides are activated by a Rh catalyst and subsequently directly inserted into the C-C bond of an arene ring to access fused azepine products. This transformation is challenging, owing to the existence of a competitive C-H amination pathway. The use of a paddlewheel dirhodium complex Rh2(esp)2 effectively inhibited the undesired C-H insertion. Density functional theory calculations were performed to reveal the reaction mechanism and origin of the chemoselectivity of the Rh-catalyzed reactions. The novel fused azepine products are highly robust and allow for downstream diversification.

Iron-Catalyzed Alkoxyl Radical-Induced C-C Bond Cleavage/gem-Difluoroalkylation Cascade

An inexpensive iron-catalyzed alkoxyl radical-induced C-C bond cleavage/gem-difluoroalkylation cascade is presented. Regulated by the structure of alkoxyl radical precursors, fluorinated distal diketones were synthesized through a ring-opening strategy and difluoroalkylated medium-sized lactones and macrolactones were constructed via a ring-expansion strategy. Both protocols proceeded under mild and redox neutral conditions with a broad substrate scope and good functional group compatibility.

Cleavage of an Aromatic C-C Bond in Ferrocene by Insertion of an Iridium Nitrido Nitrogen Atom

The intermolecular cleavage of C-C bonds is a rare event. Herein, we report on a late transition-metal terminal nitrido complex, which upon oxidation undergoes insertion of the nitrido nitrogen atom into the aromatic C-C bond of ferrocene. This reaction path was confirmed through ¹⁵N and deuterium isotope labeling experiments of the nitrido complex and ferrocenium, respectively. Cyclic voltammetry and UV/vis spectroscopy monitoring of the reaction revealed that oxidation is the initial step, yielding the tentative radical cationic nitrido complex, which is experimentally supported by extended X and Q-band electron paramagnetic resonance (EPR) and ENDOR, UV/vis, vT ¹H NMR, and vibrational spectroscopic data. Density functional theory (DFT) and multireference calculations of this highly reactive intermediate revealed an S = 1/2 ground state. The high reactivity can be traced to the increased electrophilicity in the oxidized complex. Based on high-level PNO-UCCSD(T) calculations and UV/vis kinetic measurements, it is proposed that the reaction proceeds by initial electrophilic exo attack of the nitrido nitrogen atom at the cyclopentadienyl ring and consecutive ring expansion to a pyridine ring.

Ligand-Controlled Nickel-Catalyzed Reactions of Benzocyclobutenones with Alkynyltrifluoroborates: Diverse Construction of Polysubstituted Naphthols

Ligand-controlled nickel-catalyzed selective cleavage of the C1-C2 or C1-C8 bond of benzocyclobutenones (BCBs) is reported. The delicate selection of dpppe or PMe₃ as the ligand led to predictably divergent synthesis of a wide range of 1-naphthols and 2-naphthols without C2 and C3 substituents, respectively, from BCBs and potassium alkynyltrifluoroborate, and the increase in the amount of PMe₃ resulted in tandem reaction of 2 equiv of BCB with the borate to afford 3,4,5-trisubstituted 2-naphthols. The fabulous ligand effect resulted in the facile and unique construction of multisubstituted naphthols with well-controlled regioselectivity and a high degree of structural diversity.

Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols

Sigmatropic rearrangement provides a versatile strategy to site-selectively reorganize carbon-skeleton with high atom- and step-economy. Herein, we disclose a Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols via C-C σ bond activation. A variety of α-aryl-allylic alcohols and α-aryl-propargyl alcohols could undergo in-situ 1,2- or 1,3- sigmatropic rearrangements to allow for converting to complex structural arylethyl- and arylvinyl- carbonyl compounds under a simple catalytic system. More importantly, this catalysis model can be further applied to assemble macrocyclic ketones through bimolecular [2n + 4] coupling-cyclization and monomolecular [n + 1] ring-extension. The presented skeleton rearrangement would be a useful tool complementary to the traditional molecular rearrangement.