Nickel‐Catalyzed Carboxylation of Benzylic C−N Bonds with CO2

A user‐friendly Ni‐catalyzed reductive carboxylation of benzylic C?N bonds with CO₂ is described. This procedure outperforms state‐of‐the‐art techniques for the carboxylation of benzyl electrophiles by avoiding commonly observed parasitic pathways, such as homodimerization or β‐hydride elimination, thus leading to new knowledge in cross‐electrophile reactions.     

Nickel‐Catalyzed Activation of Acyl C−O Bonds of Methyl Esters

We report the first catalytic method for activating the acyl C?O bonds of methyl esters through an oxidative‐addition process. The oxidative‐addition adducts, formed using nickel catalysis, undergo in situ trapping to provide anilide products. DFT calculations are used to support the proposed reaction mechanism, to understand why decarbonylation does not occur competitively, and to elucidate the beneficial role of the substrate structure and the Al(OtBu)₃ additive on the kinetics and thermodynamics of the reaction.     

Rh‐Catalyzed Direct Carboxylation of Alkenyl C−H Bonds of Alkenylpyrazoles

The Rh‐catalyzed direct carboxylation of alkenyl C?H bonds was achieved by using pyrazole as a removable directing group. In the presence of 5 mol% RhCl₃ ? 3H₂O, 6 mol% P(Mes)₃, and 2 equiv. of AlMe₂(OMe), the alkenyl C?H bond of various alkenylpyrazoles was directly carboxylated in good yields under CO₂ atmosphere. Furthermore, several useful transformations of the pyrazole moiety of the product were achieved to afford synthetically useful carboxylic acid derivatives in good yields.     

Cobalt‐Catalyzed Oxidase C−H/N−H Alkyne Annulation: Mechanistic Insights and Access to Anticancer Agents

Cp*‐free cobalt‐catalyzed alkyne annulations by C?H/N?H functionalizations were accomplished with molecular O₂ as the sole oxidant. The user‐friendly oxidase strategy proved viable with various internal and terminal alkynes through kinetically relevant C?H cobaltation, providing among others step‐economical access to the anticancer topoisomerase‐I inhibitor 21,22‐dimethoxyrosettacin. DFT calculations suggest that electronic effects control the regioselectivity of the alkyne insertion step.     

Activation of Aromatic C−C Bonds of 2,2’‐Bipyridine Ligands

4,4’‐Disubstituted‐2,2′‐bipyridine ligands coordinated to Mo^II and Re^I cationic fragments become dearomatized by an intramolecular nucleophilic attack from a deprotonated N‐alkylimidazole ligand in cis disposition. The subsequent protonation of these neutral complexes takes place on a pyridine carbon atom rather than at nitrogen, weakening an aromatic C?C bond and affording a dihydropyridyl moiety. Computational calculations allowed for the rationalization of the formation of the experimentally obtained products over other plausible alternatives.     

The Critical Role of Reductive Steps in the Nickel‐Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C−O Bonds

The hydrogenolysis of the aromatic C?O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (k_H/k_D=5.7) for the reactions of diphenyl ether under H₂ and D₂ atmosphere and a positive dependence of the rate on H₂ chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate‐limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C?O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C?O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).     

Transition‐Metal‐Mediated Cleavage of C−C Bonds in Aromatic Rings

Metal‐mediated cleavage of aromatic C?C bonds has a range of potential synthetic applications: from direct coal liquefaction to synthesis of natural products. However, in contrast to the activation of aromatic C?H bonds, which has already been widely studied and exploited in diverse set of functionalization reactions, cleavage of aromatic C?C bonds remains Terra incognita. This Minireview summarizes the recent progress in this field and outlines key challenges to be overcome to develop synthetic methods based on this fundamental organometallic transformation.     

Mechanistic Insight into the Copper‐Catalyzed Regiodivergent Silacarboxylation of Allenes with CO2

DFT calculations were performed to investigate the detailed reaction mechanisms in the copper‐catalyzed regiodivergent silacarboxylation of allenes. According to our calculations, the catalysis would bifurcate at the allene silylcupration step, followed by CO₂ insertion, eventually leading to the carboxylated vinylsilane or allylsilane products. The gaps between the two silylcupration barriers were predicted to be ?2.3, ?0.4, and 2.2 kcal mol^?1 when using (rac)‐Me‐DuPhos, dcpe, and PCy₃ (+H₂O) as the ligands, which nicely accounted for the experimental vinylsilane/allylsilane ratios of 93:7, 50:50, and 15:85, respectively. By means of transition‐state‐energy decomposition, we found that the energy penalty of catalyst deformation into its transition‐state geometry was the key factor in determining the direction of the reaction. The switchable regioselectivity by using different P ligands could be ascribed to structural changes of the Cu?Si and Cu?P bonds during the silylcupration process.     

CO2 Conversion into Esters by Fluoride‐Mediated Carboxylation of Organosilanes and Halide Derivatives

A one‐step conversion of CO₂ into heteroaromatic esters is presented under metal‐free conditions. Using fluoride anions as promoters for the C?Si bond activation, pyridyl, furanyl, and thienyl organosilanes are successfully carboxylated with CO₂ in the presence of an electrophile. The mechanism of this unprecedented reaction has been elucidated based on experimental and computational results, which show a unique catalytic influence of CO₂ in the C?Si bond activation of pyridylsilanes. The methodology is applied to 18 different esters, and it has enabled the incorporation of CO₂ into a polyester material for the first time.     

Mechanistic Insight into the Intramolecular Benzylic C−H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers

The intramolecular benzylic C?H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal–metal bonding characters were investigated and the structures featuring either a small HOMO–LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one‐electron oxidation of the bimetallic center. The hydrogen‐abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru–Ru and Pd–Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh–Rh and Pd–Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd–Ni and Pd–Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh–Rh‐catalyzed reactions whereas the Pd–Co‐catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru–Ru‐ and Pd–Mn‐catalyzed reactions and in the triplet nitrene involved Pd–Ni and Pd–Zn reactions. The calculations suggest the feasibility of the Pd–Mn, Pd–Co, and Pd–Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C?H amination catalysis.     

A Chiral Nitrogen Ligand for Enantioselective,Iridium‐Catalyzed Silylation of Aromatic C−H Bonds

Iridium catalysts containing dative nitrogen ligands are highly active for the borylation and silylation of C−H bonds, but chiral analogs of these catalysts for enantioselective silylation reactions have not been developed. We report a new chiral pyridinyloxazoline ligand for enantioselective, intramolecular silylation of symmetrical diarylmethoxy diethylsilanes. Regioselective and enantioselective silylation of unsymmetrical substrates was also achieved in the presence of this newly developed system. Preliminary mechanistic studies imply that C−H bond cleavage is irreversible, but not the rate‐determining step.     

Mechanistic Insights into the Rhenium‐Catalyzed Alcohol‐To‐Olefin Dehydration Reaction

Rhenium‐based complexes are powerful catalysts for the dehydration of various alcohols to the corresponding olefins. Here, we report on both experimental and theoretical (DFT) studies into the mechanism of the rhenium‐catalyzed dehydration of alcohols to olefins in general, and the methyltrioxorhenium‐catalyzed dehydration of 1‐phenylethanol to styrene in particular. The experimental and theoretical studies are in good agreement, both showing the involvement of several proton transfers, and of a carbenium ion intermediate in the catalytic cycle.     

Iron‐Catalyzed C−H Activation with Propargyl Acetates: Mechanistic Insights into Iron(II) by Experiment,Kinetics, Mössbauer Spectroscopy,and Computation

An iron‐catalyzed C?H/N?H alkyne annulation was realized by using a customizable clickable triazole amide under exceedingly mild reaction conditions. A unifying mechanistic approach combining experiment, spectroscopy, kinetics, and computation provided strong support for facile C?H activation by a ligand‐to‐ligand hydrogen transfer (LLHT) mechanism. Combined Mössbauer spectroscopic analysis and DFT calculations were indicative of high‐spin iron(II) species as the key intermediates in the C?H activation manifold.     

Mechanistic Insights into Palladium‐Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study

The catalytic cycles of palladium‐catalyzed silylation of aryl iodides, which are initiated by oxidative addition of hydrosilane or aryl iodide through three different mechanisms characterized by intermediates R₃Si−Pd^II−H (Cycle A), Ar−Pd^II−I (Cycle B), and Pd^IV (Cycle C), have been explored in detail by hybrid DFT. Calculations suggest that the chemical selectivity and reactivity of the reaction depend on the ligation state of the catalyst and specific reaction conditions, including feeding order of substrates and the presence of base. For less bulky biligated catalyst, Cycle C is energetically favored over Cycle A, through which the silylation process is slightly favored over the reduction process. Interestingly, for bulky monoligated catalyst, Cycle B is energetically more favored over generally accepted Cycle A, in which the silylation channel is slightly disfavored in comparison to that of the reduction channel. Moreover, the inclusion of base in this channel allows the silylated product become dominant. These findings offer a good explanation for the complex experimental observations. Designing a reaction process that allows the oxidative addition of palladium(0) complex to aryl iodide to occur prior to that with hydrosilane is thus suggested to improve the reactivity and chemoselectivity for the silylated product by encouraging the catalytic cycle to proceed through Cycles B (monoligated Pd⁰ catalyst) or C (biligated Pd⁰ catalyst), instead of Cycle A.     

Arene‐Free Ruthenium(II/IV)‐Catalyzed Bifurcated Arylation for Oxidative C−H/C−H Functionalizations

Experimental and computational studies provide detailed insight into the selectivity‐ and reactivity‐controlling factors in bifurcated ruthenium‐catalyzed direct C?H arylations and dehydrogenative C?H/C?H functionalizations. Thorough investigations revealed the importance of arene‐ligand‐free complexes for the formation of biscyclometalated intermediates within a ruthenium(II/IV/II) mechanistic manifold.     

Facile P−C/C−H Bond‐Cleavage Reactivity of Nickel Bis(diphosphine) Complexes

Unusual cleavage of P?C and C?H bonds of the P₂N₂ ligand, in heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes under mild conditions, results in the formation of an iminium formyl nickelate featuring a C,P,P‐tridentate coordination mode. The structures of both the heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes and the resulting iminium formyl nickelate have been characterized by NMR spectroscopy and single‐crystal X‐ray diffraction analysis. Density functional theory (DFT) calculations were employed to investigate the mechanism of the P?C/C?H bond cleavage, which involves C?H bond cleavage, hydride rotation, Ni?C/P?H bond formation, and P?C bond cleavage.     

Palladium‐Catalyzed Intramolecular Carbene Insertion into C(sp3)−H Bonds

A palladium‐catalyzed carbene insertion into C(sp³)?H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium‐catalyzed C(sp³)?C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)?H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.     

Mechanistic Insights into Copper‐Catalyzed Sonogashira–Hagihara‐Type Cross‐Coupling Reactions: Sub‐Mol % Catalyst Loadings and Ligand Effects

An efficient catalytic system for Sonogashira–Hagihara‐type reactions displaying ligand acceleration in the copper‐catalyzed formation of C(sp²)? C(sp) bonds is described. The structure of the ligand plays a key role for the coupling efficiency. Various copper sources show excellent catalytic activity, even in sub‐mol % quantities. A wide variety of substituents is tolerated in the substrates. Mechanistic details have been revealed by kinetic measurements and DFT calculations.     

A Light/Ketone/Copper System for Carboxylation of Allylic C−H Bonds of Alkenes with CO2

A photo‐induced carboxylation reaction of allylic C?H bonds of simple alkenes with CO₂ is prompted by means of a ketone and a copper complex. The unique carboxylation reaction proceeds through a sequence of an endergonic photoreaction of ketones with alkenes forming homoallyl alcohol intermediates and a thermal copper‐catalyzed allyl transfer reaction from the homoallyl alcohols to CO₂ through C?C bond cleavage.     

Dry Reforming of Methane on a Highly‐Active Ni‐CeO2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking

Ni‐CeO₂ is a highly efficient, stable and non‐expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO₂ at temperatures as low as 300 K, generating CH_x and CO_x species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density‐functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO_2?x(111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CH_x or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.