Selective alkane transformations via radicals and radical cations: insights into the activation step from experiment and theory

Photocatalytic, Oxidative Cleavage of C-C Bond in Lignin Models and Native Lignin

It is challenging to realize the selective C-C bond cleavage of lignin β-O-4 linkages for production of high-value aromatic chemicals due to its intrinsic inertness and complex structure. Here we report a light-driven, chlorine-radical-based protocol to realize the oxidative C-C bond cleavage in various lignin model compounds catalyzed by commercially available TPT and CaCl2, achieving high conversion and good to high product yields at room temperature. Mechanistic studies reveal that the preferential activation of Cβ-H bond facilitates the oxidation and C-C bond cleavage of lignin β-O-4 model via chlorine radical. Furthermore, this method is also applicable to the depolymerization of natural lignin extracts, furnishing the aromatic oxygenates from the cleavage of Cα-Cβ bonds. This study provides experimental foundations to the depolymerization and valorization of lignin into high value-added aromatic compounds.

Universal inter-molecular radical transfer reactions on metal surfaces

On-surface synthesis provides tools to prepare low-dimensional supramolecular structures. Traditionally, reactive radicals are a class of single-electron species, serving as exceptional electron-withdrawing groups. On metal surfaces, however, such species are affected by conduction band screening effects that may even quench their unpaired electron characteristics. As a result, radicals are expected to be less active, and reactions catalyzed by surface-stabilized radicals are rarely reported. Herein, we describe a class of inter-molecular radical transfer reactions on metal surfaces. With the assistance of aryl halide precursors, the coupling of terminal alkynes is steered from non-dehydrogenated to dehydrogenated products, resulting in alkynyl-Ag-alkynyl bonds. Dehalogenated molecules are fully passivated by detached hydrogen atoms. The reaction mechanism is unraveled by various surface-sensitive technologies and density functional theory calculations. Moreover, we reveal the universality of this mechanism on metal surfaces. Our studies enrich the on-surface synthesis toolbox and develop a pathway for producing low-dimensional organic materials.

Photoinduced cerium-catalyzed C-H acylation of unactivated alkanes

Ketones are ubiquitous motifs in the realm of pharmaceuticals and natural products. Traditional approaches to accessing these species involve the addition of metal reagents to carboxyl compounds under harsh conditions. Herein, we report a cerium-catalyzed acylation of unactivated C(sp3)-H bonds using bench-stable acyl azolium reagents under mild and operationally-friendly conditions. This reaction exhibits excellent generality, accommodating a wide range of feedstock chemicals such as cycloalkanes and acyclic compounds as well as facilitating the late-stage functionalization of pharmaceuticals. We demonstrate further applications of our strategy with a three-component radical relay reaction and an enantioselective N-heterocyclic carbene (NHC) and cerium dual-catalyzed reaction.

One-Pot Synthesis of Sulfonamides from Unactivated Acids and Amines via Aromatic Decarboxylative Halosulfonylation

The coupling of carboxylic acids and amines to form amide linkages is the most commonly performed reaction in the pharmaceutical industry. Herein, we report a new strategy that merges these traditional amide coupling partners to generate sulfonamides, important amide bioisosteres. This method leverages copper ligand-to-metal charge transfer (LMCT) to convert aromatic acids to sulfonyl chlorides, followed by one-pot amination to form the corresponding sulfonamide. This process requires no prefunctionalization of the native acid or amine and extends to a diverse set of aryl, heteroaryl, and s-rich aliphatic substrates. Further, we extend this strategy to the synthesis of (hetero)aryl sulfonyl fluorides, which have found utility as "click" handles in chemical probes and programmable bifunctional reagents. Finally, we demonstrate the utility of these protocols in pharmaceutical analogue synthesis.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

Oils are hydrophobic, but their degradation is frequently found to be accelerated in the presence of water microdroplets. The direct chemical consequences of water-oil contact have long been overlooked. We show that aqueous microdroplets in emulsified water-hexadecane (C16H34) mixtures can spontaneously produce CO2, •H, H2, and short-chain hydrocarbons (mainly C1 and C2) as detected by gas chromatography, electron paramagnetic resonance spectroscopy, and mass spectrometry. This reaction results from contact electrification at the water-oil microdroplet interface, in which reactive oxygen species are produced, such as hydrated hydroxyl radicals and hydrogen peroxide. We also find that the H2 originates from the water microdroplet and not the hydrocarbon it contacts. These observations highlight the potential of interfacial contact electrification to produce new chemistry.

Infrared spectrum of the 1-cyanoadamantane cation: evidence of hydrogen transfer and cage-opening upon ionization

The radical cations of diamondoids are important intermediates in their functionalization reactions and are also candidates as carriers for astronomical absorption and emission features. Although neutral diamondoids have been studied extensively, information regarding their radical cations is largely lacking, particularly for functionalized diamondoid derivatives. Herein, we characterize the structure of the 1-cyanoadamantane radical cation (C10H15CN+, AdCN+) using infrared photodissociation (IRPD) spectroscopy of mass selected AdCN+N2 clusters in the XH stretch range (2400-3500 cm-1) and dispersion-corrected density functional theory calculations (B3LYP-D3BJ/cc-pVTZ). A group of three distinct CH stretch bands are observed in the 2800-3000 cm-1 range, in addition to a highly redshifted absorption at 2580 cm-1 attributed to the acidic CH proton predicted by calculations. An unexpected broad absorption peaking at 3320 cm-1 is also detected and assigned to an NH stretch mode based on its width and frequency. Calculations indicate that hydrogen atom transfer (HAT) from the adamantyl cage (C10H15, Ady) to the N atom of the CN group yields lower energy structures, with an open-cage isomer exhibiting such hydrogen transfer being the global minimum on the potential energy surface. The energy barriers involved in the formation of this open-cage isomer are also lower than those calculated for generation of the analogous open-cage 1-amantadine cation isomer which has previously been identified by IRPD. The combined consideration of IRPD spectra and calculations indicates a major population of the nascent canonical closed-cage isomer and a smaller population of the global minimum isomer featuring both cage-opening and hydrogen transfer.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Microhydration of the adamantane cation: intracluster proton transfer to solvent in [Ad(H2O)n=1-5]+ for n ≥ 3

Radical cations of diamondoids are important intermediates in their functionalization reactions in polar solvents. To explore the role of the solvent at the molecular level, we characterize herein microhydrated radical cation clusters of the parent molecule of the diamondoid family, adamantane (C₁₀H₁₆, Ad), by infrared photodissociation (IRPD) spectroscopy of mass-selected [Ad(H₂O)_n=1-5]⁺ clusters. IRPD spectra of the cation ground electronic state recorded in the CH/OH stretch and fingerprint ranges reveal the first steps of this fundamental H-substitution reaction at the molecular level. Analysis of size-dependent frequency shifts with dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the proton of Ad⁺ as a function of the degree of hydration, the structure of the hydration shell, and the strengths of the CH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. For n = 1, H₂O strongly activates the acidic C-H bond of Ad⁺ by acting as a proton acceptor in a strong CH⋯O ionic H-bond with cation-dipole configuration. For n = 2, the proton is almost equally shared between the adamantyl radical (C₁₀H₁₅, Ady) and the (H₂O)₂ dimer in a strong C⋯H⋯O ionic H-bond. For n ≥ 3, the proton is completely transferred to the H-bonded hydration network. The threshold for this size-dependent intracluster proton transfer to solvent is consistent with the proton affinities of Ady and (H₂O)_n and confirmed by collision-induced dissociation experiments. Comparison with other related microhydrated cations reveals that the acidity of the CH proton of Ad⁺ is in the range of strongly acidic phenol⁺ but lower than for cationic linear alkanes such as pentane⁺. Significantly, the presented IRPD spectra of microhydrated Ad⁺ provide the first spectroscopic molecular-level insight of the chemical reactivity and reaction mechanism of the important class of transient diamondoid radical cations in aqueous solution.

The Role of the Redox Non-Innocent Hydroxyl Ligand in the Activation of O2 Performed by [Ni(H)(OH)]. Chemistry 2023;29:e202203128. [PMID: 36447369 DOI: 10.1002/chem.202203128] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The cationic complex [Ni(H)(OH)]⁺ was previously found to activate dioxygen and methane in gas phase under single collision conditions. These remarkable reactivities were thought to originate from a non-classical electronic structure, where the Ni-center adopts a Ni(II), instead of the classically expected Ni(III) oxidation state by formally accepting an electron from the hydroxo ligand, which formally becomes a hydroxyl radical in the process. Such radicaloid oxygen moieties are envisioned to easily react with otherwise inert substrates, mimicking familiar reactivities of free radicals. In this study, the reductive activation of dioxygen by [Ni(H)(OH)]⁺ to afford the hydroperoxo species was investigated using coupled cluster, multireference ab initio and density functional theory calculations. Orbital and wave function analyses indicate that O₂ binding tranforms the aforementioned non-classical electronic structure to a classical Ni(III)-hydroxyl system, before O₂ reduction takes place. Remarkably, we found no evidence for a direct involvement of the radicaloid hydroxyl in the reaction with O₂ , as is often assumed. The function of the redox non-innocent character of the activator complex is to protect the reactive electronic structure until the complex engages O₂ , upon which a dramatic electronic reorganization releases internal energy and drives the chemical reaction to completion.

Microhydrated clusters of a pharmaceutical drug: infrared spectra and structures of amantadineH+(H2O)n

Solvation of pharmaceutical drugs has an important effect on their structure and function. Analysis of infrared photodissociation spectra of amantadineH⁺(H₂O)_n=1-4 clusters in the sensitive OH, NH, and CH stretch range by quantum chemical calculations (B3LYP-D3/cc-pVTZ) provides a first impression of the interaction of this pharmaceutically active cation with water at the molecular level. The size-dependent frequency shifts reveal detailed information about the acidity of the protons of the NH₃⁺ group of N-protonated amantadineH⁺ (AmaH⁺) and the strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred cluster growth begins with sequential hydration of the NH₃⁺ group by NH⋯O ionic H-bonds (n = 1-3), followed by the extension of the solvent network through OH⋯O H-bonds. However, smaller populations of cluster isomers with an H-bonded solvent network and free N-H bonds are already observed for n ≥ 2, indicating the subtle competition between noncooperative ion hydration and cooperative H-bonding. Interestingly, cyclic water ring structures are identified for n ≥ 3, each with two NH⋯O and two OH⋯O H-bonds. Despite the increasing destabilization of the N-H proton donor bonds upon gradual hydration, no proton transfer to the (H₂O)_n solvent cluster is observed up to n = 4. In addition to ammonium cluster ions, a small population of microhydrated iminium isomers is also detected, which is substantially lower for the hydrophilic H₂O than for the hydrophobic Ar environment.

Minisci reaction of heteroarenes and unactivated C(sp3)-H alkanes via a photogenerated chlorine radical

Here, an efficient Minisci reaction of heteroarenes and unactivated C(sp³)-H alkanes was achieved using an inexpensive FeCl₃ as a photocatalyst. The photogenerated chlorine radical contributed to the HAT of C-H and subsequently initiated this reaction. Surprisingly, salt water and even seawater can act as a chlorine radical source, which provided an enlightening idea for future organic synthesis methods.

Identification of Alkoxy Radicals as Hydrogen Atom Transfer Agents in Ce-Catalyzed C-H Functionalization

The intermediacy of alkoxy radicals in cerium-catalyzed C-H functionalization via H-atom abstraction has been unambiguously confirmed. Catalytically relevant Ce(IV)-alkoxide complexes have been synthesized and characterized by X-ray diffraction. Operando electron paramagnetic resonance and transient absorption spectroscopy experiments on isolated pentachloro Ce(IV) alkoxides identified alkoxy radicals as the sole heteroatom-centered radical species generated via ligand-to-metal charge transfer (LMCT) excitation. Alkoxy-radical-mediated hydrogen atom transfer (HAT) has been verified via kinetic analysis, density functional theory (DFT) calculations, and reactions under strictly chloride-free conditions. These experimental findings unambiguously establish the critical role of alkoxy radicals in Ce-LMCT catalysis and definitively preclude the involvement of chlorine radical. This study has also reinforced the necessity of a high relative ratio of alcohol vs Ce for the selective alkoxy-radical-mediated HAT, as seemingly trivial changes in the relative ratio of alcohol vs Ce can lead to drastically different mechanistic pathways. Importantly, the previously proposed chlorine radical-alcohol complex, postulated to explain alkoxy-radical-enabled selectivities in this system, has been examined under scrutiny and ruled out by regioselectivity studies, transient absorption experiments, and high-level calculations. Moreover, the peculiar selectivity of alkoxy radical generation in the LMCT homolysis of Ce(IV) heteroleptic complexes has been analyzed and back-electron transfer (BET) may have regulated the efficiency and selectivity for the formation of ligand-centered radicals.

Electronic spectroscopy of cationic adamantane clusters and dehydrogenated adamantane in helium droplets

We report the first helium-tagged electronic spectra of cationic adamantane clusters, along with its singly, doubly, and triply dehydrogenated analogues embedded in helium droplets. Absorption spectra were measured by recording the evaporation of helium atoms as a function of laser wavelength in the range of 300-2150 nm. Experimental spectra are coupled with simulated spectra obtained from quantum chemical calculations. The spectrum of cationic adamantane agrees with the electronic photodissociation spectrum measured previously, with an additional low-energy absorption at around 1000 nm. The spectra of the dehydrogenated molecules present broad absorptions exclusively in the high-energy region (300-600 nm). For the higher order adamantane dimer and trimer ions, strong absorptions are observed in the low-energy region (900-2150 nm), rationalised by transitions delocalised over two adamantane units.

Gauging Radical Stabilization with Carbenes

Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.

C(sp3)-H Ritter amination by excitation of in situ generated iodine(III)-BF3 complexes

Visible light excitation of iodine(III)-BF₃ complex enables the formation of carbocations from C(sp³)-H bonds. The complexes are generated catalytically from iodoarene, carboxylate ligand, the oxidizing agent Selectfluor, and the Lewis acid BF₃. This modular catalytic system allows the formation of synthetically valuable amine derivatives without a metal- or photocatalyst.

Photoinduced C(sp3)-H Chalcogenation of Amide Derivatives and Ethers via Ligand-to-Metal Charge-Transfer

A photoinduced, iron(III) chloride-catalyzed C-H activation of N-methyl amides and ethers leads to the formation of C-S and C-Se bonds via a ligand-to-metal charge transfer (LMCT) process. This methodology converts secondary and tertiary amides, sulfonamides, and carbamates into the corresponding amido-N,S-acetal derivatives in good yields. Mechanistic work revealed that this transformation proceeds through a hydrogen atom transfer (HAT) involving chlorine radical intermediates.

Infrared spectra and structures of protonated amantadine isomers: detection of ammonium and open-cage iminium ions

The protonated form of amantadine (1-C₁₀H₁₅NH₂, Ama), the amino derivative of adamantane (C₁₀H₁₆, Ada), is a wide-spread antiviral and anti-Parkinsonian drug and plays a key role in many pharmaceutical processes. Recent studies reveal that the adamantyl cage (C₁₀H₁₅) of Ama can open upon ionization leading to distonic bicyclic iminium isomers, in addition to the canonical nascent Ama⁺ isomer. Herein, we study protonation of Ama using infrared photodissociation spectroscopy (IRPD) of Ar-tagged ions and density functional theory calculations to characterize cage and open-cage isomers of AmaH⁺ and the influence of the electron-donating NH₂ group on the cage-opening reaction potential. In addition to the canonical ammonium isomer (AmaH⁺(I)) with an intact adamantyl cage, we identify at least one slightly less stable protonated bicyclic iminium ion (AmaH⁺(II)). While the ammonium ion is generated by protonation of the basic NH₂ group, AmaH⁺(II) is formally formed by H addition to a distonic bicyclic iminium ion produced upon ionization of Ama and subsequent cage opening.

Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N2 , and H2 O

Radical cations of diamondoids, a fundamental class of very stable cyclic hydrocarbon molecules, play an important role in their functionalization reactions and the chemistry of the interstellar medium. Herein, we characterize the structure, energy, and intermolecular interaction of clusters of the amantadine radical cation (Ama⁺, 1‐aminoadamantane) with solvent molecules of different interaction strength by infrared photodissociation (IRPD) spectroscopy of mass‐selected Ama⁺L_n clusters, with L=Ar (n≤3) and L=N₂ and H₂O (n=1), and dispersion‐corrected density functional theory calculations (B3LYP−D3/cc‐pVTZ). Three isomers of Ama⁺ generated by electron ionization are identified by the vibrational properties of their rather different NH₂ groups. The ligands bind preferentially to the acidic NH₂ protons, and the strength of the NH…L ionic H‐bonds are probed by the solvation‐induced red‐shifts in the NH stretch modes. The three Ama⁺ isomers include the most abundant canonical cage isomer (I) produced by vertical ionization, which is separated by appreciable barriers from two bicyclic distonic iminium ions obtained from cage‐opening (primary radical II) and subsequent 1,2 H‐shift (tertiary radical III), the latter of which is the global minimum on the Ama⁺ potential energy surface. The effect of solvation on the energetics of the potential energy profile revealed by the calculations is consistent with the observed relative abundance of the three isomers. Comparison to the adamantane cation indicates that substitution of H by the electron‐donating NH₂ group substantially lowers the barriers for the isomerization reaction.

Efficient Oxidation of Cyclohexane over Bulk Nickel Oxide under Mild Conditions

Nickel oxide powder was prepared by simple calcination of nickel nitrate hexahydrate at 500 °C for 5 h and used as a catalyst for the oxidation of cyclohexane to produce the cyclohexanone and cyclohexanol—KA oil. Molecular oxygen (O₂), hydrogen peroxide (H₂O₂), t-butyl hydrogen peroxide (TBHP) and meta-chloroperoxybenzoic acid (m-CPBA) were evaluated as oxidizing agents under different conditions. m-CPBA exhibited higher catalytic activity compared to other oxidants. Using 1.5 equivalent of m-CPBA as an oxygen donor agent for 24 h at 70 °C, in acetonitrile as a solvent, NiO powder showed exceptional catalytic activity for the oxidation of cyclohexane to produce KA oil. Compared to different catalytic systems reported in the literature, for the first time, about 85% of cyclohexane was converted to products, with 99% KA oil selectivity, including around 87% and 13% selectivity toward cyclohexanone and cyclohexanol, respectively. The reusability of NiO catalyst was also investigated. During four successive cycles, the conversion of cyclohexane and the selectivity toward cyclohexanone were decreased progressively to 63% and 60%, respectively, while the selectivity toward cyclohexanol was increased gradually to 40%.

Formation and Evolution of H2C3O+• Radical Cations: A Computational and Matrix Isolation Study

The family of isomeric H₂C₃O^+• radical cations is of great interest for physical organic chemistry and chemistry occurring in extraterrestrial media. In this work, we have experimentally examined a unique synthetic route to the generation of H₂C₃O^+• from the C₂H₂···CO intermolecular complex and also considered the relative stability and monomolecular transformations of the H₂C₃O^+• isomers through high-level ab initio calculations. The structures, energetics, harmonic frequencies, hyperfine coupling constants, and isomerization pathways for several of the most important H₂C₃O^+• isomers were calculated at the UCCSD(T) level of theory. The complementary FTIR and EPR studies in argon matrices at 5 K have demonstrated that the ionized C₂H₂···CO complex transforms into the E-HCCHCO^+• isomer, and this latter species is supposed to be the key intermediate in further chemical transformations, providing a remarkable piece of evidence for kinetic control in low-temperature chemistry. Photolysis of this species at λ = 410-465 nm results in its transformation to the thermodynamically most stable H₂CCCO^+• isomer. Possible implications of the results and potentiality of the proposed synthetic strategy to the preparation of highly reactive organic radical cations are discussed.

Theoretical study of water-assisted reaction between methane and nitrosonium cation

The water-assisted reaction between CH4 and NO+ has been studied employing post Hartree–Fock and density functional theory methods. Two reaction pathways were considered: (i) a frontside NO+ attack on the C–H bond of methane with retention of configuration and (ii) a backside attack with inversion of configuration. The first pathway leads to a thermodynamically more favorable hydrate of N-protonated nitrosomethane, and the second one leads to its O-protonated isomer. The catalytic effect of a single water molecule is expressed in decreasing the activation energy for the elementary step by about a factor of two for the frontside mechanism and by a factor of four to five for the backside one when compared with the calculated literature data on water-free methane nitrosation. The combination of the activation strain model of reactivity and the energy decomposition analysis reveals that the activation barriers are largely determined by the relative stability of the termolecular reactant complexes. The crucial factor that stabilizes these complexes comes from the electrostatic attraction. The catalytic effect of the key water molecule is decreased with introduction of additional one or two explicit water molecules, which form a coordinate O→N bonding with NO+. On the contrary, an additional water molecule hydrogen bonded to the key catalytic water molecule enhances the catalytic effect.

Ozone Treatment Increases the Release of VOC from Barley, Which Modifies Seed Germination

Ozone is widely used to control pests in grain and has an impact on seed germination. The germination process involves multiple secondary metabolites, such as volatile organic compounds (VOCs), which are altered under ozone treatment. Here, an optimized solid-phase microextraction coupled with gas chromatography-mass spectrometry was implemented to explore changes in VOCs from barley seeds under ozone treatment. The data demonstrated that barley released both a greater variety and quantity of VOCs under oxidative stress. The number of alcohols and hydrocarbons gradually decreased, whereas aldehydes and organic acids markedly increased with increasing ozone treatment time. Acetic acid was identified as a potential ozone stress-specific marker. Furthermore, the dosage-dependent function of acetic acid on the germination of barley was verified, namely, a low dosage of acetic acid increased the germination and vice versa. This study provided new insights into how barley responds to ozone treatment and highlighted the role of acetic acid in seed germination.

Titanium(IV) Chloride-Catalyzed Photoalkylation via C(sp3)-H Bond Activation of Alkanes

Despite the sophistication of C-H functionalization as one of the most powerful tools in organic synthesis, methodology for performing hydrogen-atom transfer of unactivated alkanes remains rather scarce. Herein, we describe chlorine radical-catalyzed C(sp³)-H photoalkylation using titanium(IV) chloride via a ligand-to-metal charge transfer process. Enabled by the unique properties of this abundant metal salt, the reaction not only effected the coupling of various alkanes with radical acceptors but also was shown to be applicable to direct photoalkylation of aromatic ketones.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Infrared Spectrum of the Amantadine Cation: Opening of the Diamondoid Cage upon Ionization

Radical cations of diamondoids, a fundamental class of highly stable cycloalkanes, are intermediates in functionalization reactions and possibly present in the interstellar medium. Herein, we characterize the structure of the radical cation of 1-amantadine (1-C₁₀H₁₅NH₂⁺, Ama⁺), the amino derivative of the parent adamantane (C₁₀H₁₆⁺, Ada⁺), by infrared spectroscopy and density functional theory calculations. The structural isomers of Ama⁺ produced by electron ionization are probed by infrared photodissociation of cold Ar-tagged ions. In addition to the canonical nascent Ama⁺ isomer with an intact C₁₀H₁₅ cage, we identify two distonic bicyclic iminium isomers in which the adamantyl cage opens upon ionization, one of which is lower in energy than the cage isomer. The reaction profile with barriers and intermediates for this cage-opening reaction are determined. Comparison with Ada⁺ suggests that this type of ionization-induced cage-opening may be a common feature for diamondoids and important for their reactivity.

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

Direct radical functionalization methods to access substituted adamantanes and diamondoids

Adamantane derivatives have diverse applications in the fields of medicinal chemistry, catalyst development and nanomaterials, owing to their unique structural, biological and stimulus-responsive properties, among others. The synthesis of substituted adamantanes and substituted higher diamondoids is frequently achieved via carbocation or radical intermediates that have unique stability and reactivity when compared to simple hydrocarbon derivatives. In this review, we discuss the wide range of radical-based functionalization reactions that directly convert diamondoid C-H bonds to C-C bonds, providing a variety of products incorporating diverse functional groups (alkenes, alkynes, arenes, carbonyl groups, etc.). Recent advances in the area of selective C-H functionalization are highlighted with an emphasis on the H-atom abstracting species and their ability to activate the particularly strong C-H bonds that are characteristic of these caged hydrocarbons, providing insights that can be applied to the C-H functionalization of other substrate classes.

The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3. Molecules 2021;26:molecules26196028. [PMID: 34641573 PMCID: PMC8512126 DOI: 10.3390/molecules26196028]

The activation reactions of methane mediated by metal carbide ions MC₃⁺ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC₃⁺ (M = Ir and Pt) ions reacted with CH₄ at room temperature forming MC₂H₂⁺/C₂H₂ and MC₄H₂⁺/H₂ as the major products for both systems. Besides that, PtC₃⁺ could abstract a hydrogen atom from CH₄ to generate PtC₃H⁺/CH₃, while IrC₃⁺ could not. Quantum chemical calculations showed that the MC₃⁺ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C-H activation took place on the Ir atom for IrC₃⁺. The terminal carbon atom was the reactive site for the first C-H bond activation of PtC₃⁺, which was beneficial to generate PtC₃H⁺/CH₃. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.

Homogeneous catalytic C(sp3)-H functionalization of gaseous alkanes

The conversion of light alkanes into bulk chemicals is becoming an important challenge as it effectively avoids the use of prefunctionalized alkylating reagents. The implementation of such processes is, however, hampered by their gaseous nature and low solubility, as well as the low reactivity of the C-H bonds. Efforts have been made to enable both polar and radical processes to activate these inert compounds. In addition, these methodologies also benefit significantly from the development of a suitable reactor technology that intensifies gas-liquid mass transfer. In this review, we critically highlight these developments, both from a conceptual and a practical point of view. The recent expansion of these mechanistically-different methods have enabled the use of various gaseous alkanes for the development of different bond-forming reactions, including C-C, C-B, C-N, C-Si and C-S bonds.

Catalytic Oxidations with Meta-Chloroperoxybenzoic Acid (m-CPBA) and Mono- and Polynuclear Complexes of Nickel: A Mechanistic Outlook

Selective catalytic functionalization of organic substrates using peroxides as terminal oxidants remains a challenge in modern chemistry. The high complexity of interactions between metal catalysts and organic peroxide compounds complicates the targeted construction of efficient catalytic systems. Among the members of the peroxide family, m-chloroperoxybenzoic acid (m-CPBA) exhibits quite complex behavior, where numerous reactive species could be formed upon reaction with a metal complex catalyst. Although m-CPBA finds plenty of applications in fine organic synthesis and catalysis, the factors that discriminate its decomposition routes under catalytic conditions are still poorly understood. The present review covers the advances in catalytic C–H oxidation and olefine epoxidation with m-CPBA catalyzed by mono- and polynuclear complexes of nickel, a cheap and abundant first-row transition metal. The reaction mechanisms are critically discussed, with special attention to the O–O bond splitting route. Selectivity parameters using recognized model hydrocarbon substrates are summarized and important factors that could improve further catalytic studies are outlined.

Iron-Catalyzed C(sp3)–H Alkylation through Ligand-to-Metal Charge Transfer

We report the FeCl3-catalyzed alkylation of nonactivated C(sp3)–H bonds. Photoinduced ligand-to-metal charge transfer at the iron center generates chlorine radicals that then preferentially abstract hydrogen atoms from electron-rich C(sp3)–H bonds distal to electron-withdrawing functional groups. The resultant alkyl radicals are trapped by electron-deficient olefins, and the catalytic cycle is closed by Fe(II) recombination and protodemetalation.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Cobalt-Catalyzed Intermolecular C-H Amidation of Unactivated Alkanes

Alkanes are an abundant and inexpensive source of hydrocarbons; thus, development of new methods to convert the hydrocarbon feedstocks to value-added chemicals is of high interest. However, it is challenging to achieve such transformation in a direct and selective manner mainly due to the intrinsic inertness of their C-H bonds. We herein report a tailored Cp*Co(III)(LX)-catalyzed efficient and site-selective intermolecular amidation of unactivated hydrocarbons including light alkanes. Electronic modulation of the cobalt complexes led to the enhanced amidation efficiency, and these effects were theoretically rationalized by the FMO analysis of presupposed cobalt nitrenoid species. Under the current cobalt protocol, a secondary C-H bond selectivity was observed in various nonactivated alkanes to reverse the intrinsic tertiary preference, which is attributed to the steric demands of the cobalt system that imposes difficulties in accessing tertiary C-H bonds. Experimental and computational studies suggested that the putative triplet Co nitrenoids are transferred to the C-H bonds of alkanes via a radical-like hydrogen abstraction pathway.

Copper Catalyzed C(sp3)-H Bond Alkylation via Photoinduced Ligand-to-Metal Charge Transfer

Utilizing catalytic CuCl₂ we report the functionalization of numerous feedstock chemicals via the coupling of unactivated C(sp³)-H bonds with electron-deficient olefins. The active cuprate catalyst undergoes Ligand-to-Metal Charge Transfer (LMCT) to enable the generation of a chlorine radical which acts as a powerful hydrogen atom transfer reagent capable of abstracting strong electron-rich C(sp³)-H bonds. Of note is that the chlorocuprate catalyst is an exceedingly mild oxidant (0.5 V vs SCE) and that a proposed protodemetalation mechanism offers a broad scope of electron-deficient olefins, offering high diastereoselectivity in the case of endocyclic alkenes. The coupling of chlorine radical generation with Cu reduction through LMCT enables the generation of a highly active HAT reagent in an operationally simple and atom economical protocol.

Microhydration of substituted diamondoid radical cations of biological relevance: infrared spectra of amantadine+-(H2O)n = 1-3 clusters

Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure, reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the widespread pharmaceutical drug amantadine (C16H15NH2, Ama) by infrared photodissociation (IRPD) spectroscopy of mass-selected Ama+Wn = 1-3 clusters (W = H2O) recorded in the NH, CH, and OH stretch range of the cation ground electronic state. Analysis of the size-dependent frequency shifts by dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the protons of the NH2 group of Ama+ and the structure and strength of the NHO and OHO hydrogen bonds (H-bonds) of the hydration network. The preferred sequential cluster growth begins with hydration of the two acidic NH protons of the NH2 group (n = 1-2) and continues with an extension of the H-bonded hydration network by forming an OHO H-bond of the third W to one ligand in the first hydration subshell (n = 3), like in the W2 dimer. For n = 2, a minor population corresponds to Ama+W2 structures with a W2 unit attached to Ama+via a NHW2 H-bond. Although the N-H proton donor bonds are progressively destabilized by gradual microhydration, no proton transfer to the Wn solvent cluster is observed in the investigated size range (n ≤ 3). Besides the microhydration structure, we also obtain a first impression of the structure and IR spectrum of bare Ama+, as well as the effects of both ionization and hydration on the structure of the adamantyl cage. Comparison of Ama+ with aliphatic and aromatic primary amine radical cations reveals differences in the acidity of the NH2 group and the resulting interaction with W caused by substitution of the cycloalkyl cage.

Using the Thiyl Radical for Aliphatic Hydrogen-Atom Transfer: Thiolation of Unactivated C-H Bonds

A metal- and catalyst-free thiyl-radical-mediated activation of alkanes is described. Tetrafluoropyridinyl disulfide is used to perform thiolation of the C-H bonds under irradiation with 400 nm light-emitting diodes. The key C-H activation step is believed to proceed via hydrogen-atom abstraction effected by the fluorinated thiyl radical. Secondary, tertiary, and heteroatom-substituted C-H bonds can be involved in the thiolation reaction. The resulting sulfides have wide potential as photoredox-active radical precursors in reactions with alkenes and heteroarenes.

Two-Step Synthesis of Heptacyclo[6.6.0.02,6 .03,13 .04,11 .05,9 .010,14 ] tetradecane from Norbornadiene: Mechanism of the Cage Assembly and Post-synthetic Functionalization

A selective and scalable two-step approach to the dimerization of norbornadiene (NBD) into its thermodynamically most stable dimer, heptacyclo[6.6.0.02,6 .03,13 .04,11 .05,9 .010,14 ] tetradecane, (HCTD) is reported. Calculations indicate that the reaction starts with the Rh-catalyzed stepwise homo Diels-Alder cyclisation of NBD into its exo-cis-endo dimer. Treatment of this compound with acid promotes its evolution to HCTD via a [1,2]-sigmatropic rearrangement. The assemblies of 7,12-disubstituted cages from 7-(alkyl/aryl) NBDs, as well as the selective post-synthetic C-H functionalization of the core HCTD scaffold at position C1, or positions C1 and C4 are described.

The effects of metals and ligands on the oxidation of n-octane using iridium and rhodium “PNP” aminodiphosphine complexes

Ir and Rh “PNP” complexes with different ligands are utilized for the oxidation of n-octane. Based on the obtained conversion, selectivity, and the characterized recovered catalysts, it is found that the combination of Ir and the studied ligands does not promote the redox mechanism that is known to result in selective formation of oxo and peroxo compounds [desired species for C(1) activation]. Instead, they support a deeper oxidation mechanism, and thus higher selectivity for ketones and acids is obtained. In contrast, these ligands seem to tune the electron density around the Rh (in the Rh-PNP complexes), and thus result in a higher n-octane conversion and improved selectivity for the C(1) activated products, with minimized deeper oxidation, in comparison to Ir-PNP catalysts.

Synthesis of amino-diamondoid pharmacophores via photocatalytic C-H aminoalkylation

We report a direct C-H aminoalkylation reaction using two light-activated H-atom transfer catalyst systems that enable the introduction of protected amines to native adamantane scaffolds with C-C bond formation. The scope of adamantane and imine reaction partners is broad and deprotection provides versatile amine and amino acid building blocks. Using readily available chiral imines, the enantioselective synthesis of the saxagliptin core and rimantadine derivatives is also described.