Measuring the Relative Reactivity of the Carbon-Hydrogen Bonds of Alkanes as Nucleophiles

Cross-Coupling of Gaseous Alkanes with (Hetero)Aryl Bromides via Dual Nickel/Photoredox Catalysis

Gaseous alkanes represent the most abundant carbon-based chemical feedstocks in our planet. However, the intrinsic inertness of their C-H bonds has rendered the use of these alkanes very difficult for purposes beyond aerobic combustion and energy intensive processes. Thus, clean and energy-efficient transformations for their use in synthetic organic chemistry are still rare. Here we report a catalytic methodology for the direct cross-coupling of gaseous alkanes with (hetero)aryl bromides through the combination of metallaphotoredox-mediated hydrogen atom transfer and nickel catalysis. This protocol provides an efficient platform for the addition of short alkyl groups into diverse (hetero) aromatic rings, providing a wide range of high-value alkyl(hetero)arenes, and bypassing the longstanding need of using preactivated alkylating agents in C(sp2)-C(sp3) cross-couplings. The method features high chemoselectivity, regioselectivity and a remarkable functional group tolerance, operates under mild conditions, and exhibits operational simplicity.

Alkanes C1-C6 C-H Bond Activation via a Barrierless Potential Energy Path: Trifluoromethyl Carbenes Enhance Primary C-H Bond Functionalization

In this mixed computational and experimental study, we report a catalytic system for alkane C1-C6 functionalization in which the responsible step for C-H bond activation shows no barrier in the potential energy path. DFT modeling of three silver-based catalysts and four diazo compounds led to the conclusion that the TpFAg═C(H)CF3 (TpF = fluorinated trispyrazolylborate ligand) carbene intermediates interact with methane without a barrier in the potential energy surface, a prediction validated by experimentation using N2═C(H)CF3 as the carbene source. The array of alkanes from propane to n-hexane led to the preferential functionalization of the primary sites with unprecedented values of selectivity for an acceptor diazo compound. The lack of those barriers implies that selectivity can no longer be controlled by differences in the energy barriers. Molecular dynamics calculations (with propane as the model alkane) are consistent with the preferential functionalization of the primary sites due to a higher concentration of such C-H bonds in the vicinity of the carbenic carbon atom.

Molecular Dynamic Simulations of Aqueous Micellar Organometallic Catalysis: Methane Functionalization as a Case Study

Molecular Dynamics (MD) simulations constitute a powerful tool that provides a 3D perspective of the dynamical behavior of chemical systems. Herein the first MD study of the dynamics of a catalytic organometallic system, in micellar media, is presented. The challenging methane catalytic functionalization into ethyl propionate through a silver-catalyzed process has been targeted as the case study. The intimate nature of the micelles formed with the surfactants sodium dodecylsulfate (SDS) and potassium perfluorooctane sulfonate (PFOS) has been ascertained, as well as the relative distribution of the main actors in this transformation, namely methane, the diazo reagent and the silver catalyst, the latter in two different forms: the initial compound and a silver-carbene intermediate. Catalyst deactivation occurs with halide containing surfactants dodecyltrimethylammonium chloride (DTAC) and Triton X-100. Computed simulations allow explaining the experimental results, indicating that micelles behave differently regarding the degree of accumulation and the local distribution of the reactants and their effect in the molecular collisions leading to net reaction.

Effect of α-Substitution on the Reactivity of C(sp3)-H Bonds in Pd0-Catalyzed C-H Arylation

We report mechanistic studies on the reactivity of different α-substituted C(sp3)-H bonds, -CHnR (R = H, Me, CO2Me, CONMe2, OMe, and Ph, as well as the cyclopropyl and isopropyl derivatives -CH(CH2)2 and -CHMe2) in the context of Pd0-catalyzed C(sp3)-H arylation. Primary kinetic isotope effects, kH/kD, were determined experimentally for R = H (3.2) and Me (3.5), and these, along with the determination of reaction orders and computational studies, indicate rate-limiting C-H activation for all substituents except when R = CO2Me. This last result was confirmed experimentally (kH/kD ∼ 1). A reactivity scale for C(sp3)-H activation was then determined: CH2CO2Me > CH(CH2)2 ≥ CH2CONMe2 > CH3 ≫ CH2Ph > CH2Me > CH2OMe ≫ CHMe2. C-H activation involves AMLA/CMD transition states featuring intramolecular O → H-C H-bonding assisted by C-H → Pd agostic bonding. The "AMLA coefficient", χ, is introduced to quantify the energies associated with these interactions via natural bond orbital 2nd order perturbation theory analysis. Higher barriers correlate with lower χ values, which in turn signal a greater agostic interaction in the transition state. We believe that this reactivity scale and the underlying factors that determine this will be of use for future studies in transition-metal-catalyzed C(sp3)-H activation proceeding via the AMLA/CMD mechanism.

"Sandwich" Diimine-Copper Catalyzed Trifluoroethylation and Pentafluoropropylation of Unactivated C(sp3 )-H Bonds by Carbene Insertion

We report here "sandwich"-diimine copper complex-catalyzed trifluoroethylation and pentafluoropropylation of unactivated C(sp3 )-H bonds in alkyl esters, halides, and protected amines by employing CF3 CHN2 and CF3 CF2 CHN2 reagents. Reactions proceed in dichloromethane solvent at room temperature. Identical C-H functionalization conditions and stoichiometries are employed for generality and convenience. Selectivities for C-H insertions are higher for compounds possessing stronger electron-withdrawing substituents. Preliminary mechanistic studies point to a mechanism involving a pre-equilibrium forming a "sandwich"-diimine copper-CF3 CHN2 complex followed by rate-determining loss of nitrogen affording the reactive copper carbene. It reacts with trifluoromethyldiazomethane about 6.5 times faster than with 1-fluoroadamantane explaining the need for slow addition of the diazo compound.

Intramolecular Interception of the Remote Position of Vinylcarbene Silver Complex Intermediates by C(sp3 )-H Bond Insertion

The trapping of the elusive vinylogous position of a vinyl carbene with an aliphatic C(sp³ )-H bond has been achieved for the first time during a silver-catalyzed carbene/alkyne metathesis (CAM) process. A Tp^x -containing silver complex first promotes the generation of a donor-acceptor silver carbene which triggers CAM, generating a subsequent donor-donor vinyl silver carbene species, which then undergoes a selective vinylogous C(sp³ )-H bond insertion, leading to the synthesis of a new family of benzoazepines. Density functional theory (DFT) calculations unveil the reaction mechanism, which allows proposing that the C-H bond insertion reaction takes place in a stepwise manner, with the hydrogen shift being the rate determining step.

Site-Selective C-H Allylation of Alkanes: Facile Access to Allylic Quaternary sp3 -Carbon Centers

The construction of allylic quaternary sp³ -carbon centers has long been a formidable challenge in transition-metal-catalyzed alkyl-allyl coupling reactions due to the severe steric hindrance. Herein, we report an effective carbene strategy that employs well-defined vinyl-N-triftosylhydrazones as a versatile allylating reagent to enable direct assembly of these medicinally desirable structural elements from low-cost alkane feedstocks. The reaction exhibited excellent site selectivity for tertiary C-H bonds, broad scope (>60 examples and >20 : 1:0 r. r.) and good efficiency, even on a gram-scale, making it a convenient alternative to the well-known Trost-Tsuji allylation reaction for the formation of alkyl-allyl bonds. Combined experimental and computational studies were employed to unravel the mechanism and origin of site- and chemoselectivity of the reaction.

Carbene-Controlled Regioselective Functionalization of Linear Alkanes under Silver Catalysis

Control of the regioselectivity in the functionalization of C-H bonds of linear alkanes C₂H_2n+2 via carbene transfer from diazo compounds is restricted to the use of rhodium-based catalysts, which govern the reaction outcome employing donor-acceptor diazo reagents. At variance with that catalyst-controlled strategy, we present an alternative approach in which employing the appropriate silver complexes containing trispyrazolylborate ligands as catalysts with large differences in their steric and electronic properties, the regioselection is mainly governed by the diazo reagent, which leads to the functionalization of primary or secondary sites of linear alkanes (lacking any activating or directing groups). Donor-acceptor aryl diazoacetates exclusively provide the functionalization of the secondary sites of hexane or pentane, whereas acceptor ethyl diazoacetate leads to an unprecedented level of primary functionalization.

Highly electrophilic silver carbenes

Catalytic carbene transfer reactions are fundamental transformations in modern organic synthesis, which enable direct access to diverse structurally complex molecules. Despite diazo precursors playing a crucial role in catalytic carbene transfer reactions, most reported methodologies take into account only diazoacetates or related compounds. This is primarily because diazoalkanes, unless they contain a resonance stabilizing group, are more susceptible to violent exothermic decomposition. In this feature article, we present an alternative approach to carbene-transfer reactions based on the formation of highly electrophilic silver carbenes from N-sulfonylhydrazones, where the high electrophilicity of silver carbenes stems from the weak interaction between silver and the carbenic carbon. These precursors are readily accessible, stable, and environmentally sustainable. Using the strategy that employs highly electrophilic silver carbenes, it is possible to develop novel intermolecular transformations involving non-stabilized carbenes, including C(sp³)-H insertion, C(sp³)-C(O) insertion, cycloaddition, and defluorinative functionalization. The silver-catalyzed carbene transfer reactions described here have high efficiency, unusual reactivity, exceptional selectivity, and a reaction pathway that differs from typical transition metal-catalyzed reactions. Our research provided fundamental insight into silver carbene chemistry, and we hope to apply this mode of catalysis to other more general transformations, including asymmetric transformations.

Chiral Bismuth-Rhodium Paddlewheel Complexes Empowered by London Dispersion: The C-H Functionalization Nexus

Heterobimetallic [BiRh] tetracarboxylate catalysts endowed with 1,3-disilylated phenylglycine paddlewheels benefit from interligand London dispersion. They were originally designed for asymmetric cyclopropanation but are now shown to perform very well in asymmetric C-H functionalization reactions too. Because of the confined ligand sphere about the derived donor/acceptor carbenes, insertions into unhindered methyl groups are kinetically favored, although methylene units also react with excellent levels of asymmetric induction; even gaseous ethane is a suitable substrate. Moreover, many functional groups in both partners are tolerated. The resulting products are synthetically equivalent to the outcome of traditional asymmetric ester alkylation, allylation, benzylation, propargylation and aldol reactions and therefore constitute a valuable nexus to more conventional chemical logic.

Selective Functionalization of Arene C(sp2)–H Bonds by Gold Catalysis: The Role of Carbene Substituents

The complete regioselective incorporation of carbene units to nonactivated arene rings has been achieved employing gold(I) catalysts bearing alkoxydiaminophosphine ligands, with readily available, nonelaborated ethyl 2-phenyldiazoacetate as the carbene source. These results are in contrast with the scarce precedents which required highly elaborated diazo substrates. Density functional theory (DFT) calculations have revealed the important role of the R group in the C(R)CO₂Et fragment, which dramatically affects the energy profile of this transformation.

Silver-catalyzed site-selective C(sp3)-H benzylation of ethers with N-triftosylhydrazones

The insertion of carbenes into the α-C-H bonds of ethers represents one of the most powerful approaches to access polysubstituted α-branched ethers. However, intermolecular carbene insertions remain challenging, since current approaches are generally limited to the use of toxic and potentially explosive α-diazocarbonyl compounds. We now report a silver-catalyzed α-C-H benzylation of ethers using bench-stable N-triftosylhydrazones as safe and convenient carbene precursors. This approach is well suited for both inter- and intramolecular insertions to deliver medicinally relevant homobenzylic ethers and 5-8-membered oxacycles in good yields. The synthetic utility of this strategy is demonstrated by its easy scalability, broad scope with valuable functional groups, high regioselectivity, and late-stage functionalization of complex oxygen-containing molecules. The relative reactivities of different types of silver carbenes and C-H bonds were also investigated by experments and DFT calculations.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Copper-Catalyzed Dehydrogenative Amidation of Light Alkanes

The functionalization of C-H bonds in light alkanes, particularly to form C-N bonds, remains a challenge. We report the dehydrogenative coupling of amides with C1-C4 hydrocarbons to form N-alkyl amide products with tBuOOtBu as oxidant, and a copper complex of a phenanthroline-type ligand as catalyst. The reactions occurred in good yields in benzene or supercritical carbon dioxide as solvents. This strategy allowed for the determination of the relative reactivity of these alkane C-H bonds toward this amination process and showed, in contrast to prior work with larger alkanes, that the reactivity correlated with bond dissociation energies.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Asymmetric Intramolecular Dearomatization of Nonactivated Arenes with Ynamides for Rapid Assembly of Fused Ring System under Silver Catalysis

Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

Iodine-Catalyzed Selective Functionalization of Ethane in Oleum: Toward a Direct Process for the Production of Ethylene Glycol from Shale Gas

Direct valorization of ethane, a substantial component of shale gas deposits, at mild conditions remains a significant challenge, both from an industrial and an academic point of view. Herein, we report iodine as an efficient and selective catalyst for the functionalization of ethane in oleum at low temperatures and pressures. A thorough study of relevant reaction parameters revealed iodine to be remarkably more active than the previously reported "Periana/Catalytica" catalyst under optimized conditions. As a result of a fundamentally different catalytic cycle, iodine yields the bis-bisulfate ester of ethylene glycol (HO₃SO-CH₂-CH₂-OSO₃H, EBS), whereas for state-of-the-art platinum-based catalysts ethionic acid (HO₃S-CH₂-CH₂-OSO₃H, ETA) is obtained as the main product. Our findings open up an attractive route for the direct conversion of ethane toward ethylene glycol.

Metal-free tandem carbene N-H insertions and C-C bond cleavages

A metal-free C-H [5 + 1] annulation reaction of 2-arylanilines with diazo compounds has been achieved, giving rise to two types of prevalent phenanthridines via highly selective C-C cleavage. Compared to the simple N-H insertion manipulation of diazo, this method elegantly accomplishes a tandem N-H insertion/SEAr/C-C cleavage/aromatization reaction, and the synthetic utility of this new transformation is exemplified by the succinct syntheses of trisphaeridine and bicolorine alkaloids.

A Quantitative Model for Alkane Nucleophilicity Based on C-H Bond Structural/Topological Descriptors

A first quantitative model for calculating the nucleophilicity of alkanes is described. A statistical treatment was applied to the analysis of the reactivity of 29 different alkane C-H bonds towards in situ generated metal carbene electrophiles. The correlation of the recently reported experimental reactivity with two different sets of descriptors comprising a total of 86 parameters was studied, resulting in the quantitative descriptor-based alkane nucleophilicity (QDEAN) model. This model consists of an equation with only six structural/topological descriptors, and reproduces the relative reactivity of the alkane C-H bonds. This reactivity can be calculated from parameters emerging from the schematic drawing of the alkane and a simple set of sums.

Methane functionalization in water with micellar catalysis

The functionalization of methane in water as the reaction medium (where it is nearly insoluble) at room temperature using micellar catalysis is described. Aggregates are formed from surfactant molecules and act as methane concentrators, also trapping the catalyst (a silver-based complex) and the diazo reagent (ethyl diazoacetate, EDA), providing yields of ethyl propionate up to 14% (referred to as EDA). This is the first example of methane being functionalized in water at room temperature.

Improving Catalyst Activity in Hydrocarbon Functionalization by Remote Pyrene-Graphene Stacking

A copper complex bearing an N-heterocyclic carbene ligand with a pyrene "tail" attached to the backbone has been prepared and supported on reduced graphene oxide (rGO). The free and supported copper materials have been employed as homogeneous and heterogeneous catalysts in the functionalization of hydrocarbons such as n-hexane, cyclohexane, and benzene through incorporation of the CHCO₂ Et unit from ethyl diazoacetate. The graphene-anchored complex displays higher reaction rates and induces higher yields than its soluble counterpart, features that can be rationalized in terms of a decrease in electron density at the metal center due to a remote net electronic flux from the supported copper complex to the graphene surface.