Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Radical Activation of N-H and O-H Bonds at Bismuth(II)

The development of unconventional strategies for the activation of ammonia (NH₃) and water (H₂O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi-O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X-H bond, leading to low BDFE_X-H. As a result, radical activation of various N-H and O-H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)-N complexes engage in a unique reactivity pattern with the triad of H⁺, H^-, and H^• sources, thus providing alternative pathways for main group chemistry.

Coordination-Induced Bond Weakening

Coordination-induced bond weakening is a phenomenon wherein ligand X-H bond homolysis occurs in concert with the energetically favorable oxidation of a coordinating metal complex. The coupling of these two processes enables thermodynamically favorable proton-coupled electron transfer reductions to form weak bonds upon formal hydrogen atom transfer to substrates. Moreover, systems utilizing coordination-induced bond weakening have been shown to facilitate the dehydrogenation of feedstock molecules including water, ammonia, and primary alcohols under mild conditions. The formation of exceptionally weak substrate X-H bonds via small molecule homolysis is a powerful strategy in synthesis and has been shown to enable nitrogen fixation under mild conditions. Coordination-induced bond weakening has also been identified as an integral process in biophotosynthesis and has promising applications in renewable chemical fuel storage systems. This review presents a discussion of the advances made in the study of coordination-induced bond weakening to date. Because of the broad range of metal and ligand species implicated in coordination-induced bond weakening, each literature report is discussed individually and ordered by the identity of the low-valent metal. We then offer mechanistic insights into the basis of coordination-induced bond weakening and conclude with a discussion of opportunities for further research into the development and applications of coordination-induced bond weakening systems.

Exploring the potential of the ammine complexes M(NH3)n+ (M = Zr, Re) to activate NH3

NH3 is activated by the complex Zr(NH3)7+ through a mechanism involving radical species.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Facile proton-coupled electron transfer enabled by coordination-induced E-H bond weakening to boron

We report the facile activation of aryl E-H (ArEH; E = N, O, S; Ar = Ph or C6F5) or ammonia N-H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1-). Substantial decreases in E-H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol-1 in H2N-H BDFE upon coordination was experimentally determined.

Coordination-Induced N-H Bond Splitting of Ammonia and Primary Amine of CuI -MOFs

We report a porous three-dimensional anionic tetrazolium based Cu^I -MOF 1, which is capable of cleaving the N-H bond of ammonia and primary amine, as well as the O-H bond of H₂ O along with spontaneous H₂ evolution. In the gas-solid phase reaction of 1 with ammonia and water vapor, Cu^I -MOF 1 was gradually oxidized to NH₂ -Cu^II -MOF and OH-Cu^II -MOF, through single-crystal-to-single-crystal (SCSC) structural transformations, which was confirmed by XPS, PXRD and X-ray single-crystal diffraction. Density functional theory (DFT) demonstrated that Cu^I -MOF could lower N-H bond dissociation free energy of ammonia through coordination-induced bond weakening and promote H₂ evolution by the reduction potential of 1. To our knowledge, this is the first example of MOFs that activate ammonia and amine in gas-solid manner.

Multiple N-H and C-H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes

We report the use of the reported Fe-phthalocyanine complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe-amine complexes 1-(NH₃)₂, 1-(MeNH₂)₂, and 1-(Me₂NH)₂. Treatment of 1 or 1-(NH₃)₂ to an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical (^tBuArO^•), under NH₃ resulted in catalytic H atom abstraction (HAA) and C-N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and ^tBuArOH. Exposing 1-(NH₃)₂ to an excess of the trityl (CPh₃) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (^TrArO^•), under NH₃ did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N-H and β C-H in the alkylamine congeners, 1-(MeNH₂)₂ and 1-(Me₂NH)₂, led to multiple HAA events yielding the unsaturated cyanide complex, 1-(MeNH₂)(CN), and imine complex, 1-(MeN═CH₂)₂, respectively. Subsequent C-N bond formation was also observed in the latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential N-H and C-H HAA to generate these unsaturated products.

Photocatalytic reaction mechanisms at the gas–solid interface for environmental and energy applications

Five gas–solid photocatalytic reactions including the oxidation of NOx, VOCs and NH3, and reduction of CO2 and N2 are summarized. Besides, basic properties of gas molecules, their adsorption and activation, and various reaction pathways are analyzed.

Main-Group Metal Complexes in Selective Bond Formations Through Radical Pathways

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.

A highly site-selective radical sp3 C-H amination of azaheterocycles

This report describes the development of a novel C-H amination strategy using both a Cu(ii) Lewis acid and an organic hydrogen atom transfer catalyst to activate benzylic C-H bonds adjacent to aromatic N-heterocycles. This simple methodology demonstrates very high selectivity towards azaheterocycles without using exogenous directing groups and affords excellent site selectivity in substrates with more than one reactive position. A wide range of heterocyclic structures not compatible with previously reported catalytic systems have proven to be amenable to this approach. Mechanistic investigations strongly support a radical-mediated H-atom abstraction, which explains the observed contrast to known closed-shell Lewis acid catalyzed processes.

Thermodynamic and reactivity studies of a tin corrole-cobalt porphyrin heterobimetallic complex

A heterobimetallic complex, (TPFC)Sn-Co(TAP) (TPFC = 5,10,15-tris(pentafluorophenyl)corrole, TAP = 5,10,15,20-tetrakis(p-methoxyphenyl)porphyrin), was synthesized. The complex featured a Sn-Co bond with a bond dissociation enthalpy (BDE) of 30.2 ± 0.9 kcal mol^-1 and a bond dissociation Gibbs free energy (BDFE) of 21.0 ± 0.2 kcal mol^-1, which underwent homolysis to produce the (TPFC)Sn radical and (TAP)Co^II under either heat or visible light irradiation. The novel tin radical (TPFC)Sn, being the first four-coordinate tin radical observed at room temperature, was studied spectroscopically and computationally. (TPFC)Sn-Co(TAP) promoted the oligomerization of aryl alkynes to give the insertion products (TPFC)Sn-(CH 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C(Ar)) _n -Co(TAP) (n = 1, 2, or 3) as well as 1,3,5-triarylbenzenes. Mechanistic studies revealed a radical chain mechanism involving the (TPFC)Sn radical as the key intermediate.

Combined Iron/Hydroxytriazole Dual Catalytic System for Site Selective Oxidation Adjacent to Azaheterocycles

This report details a new method for site-selective methylene oxidation adjacent to azaheterocycles. A dual catalysis approach, utilizing both an iron Lewis acid and an organic hydroxylamine catalyst, proved highly effective. We demonstrate that this method provides complementary selectivity to other known catalytic approaches and represents an improvement over current heterocycle-selective reactions that rely on stoichiometric activation.

Evidence for a [17] π-Electronic Full-Fledged Non-innocent Gallium(III)-Corrole Radical

One-electron oxidation of a Ga^III -corrole with N(4-BrC₆ H₄ )₃ SbCl₆ resulted in an air-stable non-innocent Ga^III -corrole radical. The single-crystal X-ray crystallography of the 2,17-bis-formyl-5,10,15-tris(pentafluorophenyl)corrolato gallium(III)(chloride) radical ([3-Cl]^. ) revealed delocalization of the unpaired electron, which was further confirmed by electron spin resonance (ESR) spectroscopy and spin density distribution plot. In addition, the nucleus-independent chemical shift (NICS), anisotropy-induced current density (AICD) and harmonic oscillator model of aromaticity (HOMA) supported a [17] π-electron-conjugated (or antiaromatic) radical.

Coordination-induced weakening of ammonia, water, and hydrazine X-H bonds in a molybdenum complex

Although scores of transition metal complexes incorporating ammonia or water ligands have been characterized over the past century, little is known about how coordination influences the strength of the nitrogen-hydrogen and oxygen-hydrogen bonds. Here we report the synthesis of a molybdenum ammonia complex supported by terpyridine and phosphine ligands that lowers the nitrogen-hydrogen bond dissociation free energy from 99.5 (gas phase) to an experimentally measured value of 45.8 kilocalories per mole (agreeing closely with a value of 45.1 kilocalories per mole calculated by density functional theory). This bond weakening enables spontaneous dihydrogen evolution upon gentle heating, as well as the hydrogenation of styrene. Analogous molybdenum complexes promote dihydrogen evolution from coordinated water and hydrazine. Electrochemical and theoretical studies elucidate the contributions of metal redox potential and ammonia acidity to this effect.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole^•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(d_z2)-corrole("a_2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(d_x2-y2)-corrole(a_2u) interaction in coinage metal corroles. Less commonly encountered is the metal(d_π)-corrole("a_1u") interaction, a unique feature of formal d⁵ metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

Strategies for Corrole Functionalization

This review covers the functionalization reactions of meso-arylcorroles, both at the inner core, as well as the peripheral positions of the macrocycle. Experimental details for the synthesis of all known metallocorrole types and for the N-alkylation reactions are presented. Key peripheral functionalization reactions such as halogenation, formylation, carboxylation, nitration, sulfonation, and others are discussed in detail, particularly the nucleophilic aromatic substitution and the participation of corroles in cycloaddition reactions as 2π or 4π components (covering Diels-Alder and 1,3-dipolar cycloadditions). Other functionalizations of corroles include a large diversity of reactions, namely Wittig reactions, reactions with methylene active compounds, formation of amines, amides, and imines, and metal catalyzed reactions. At the final section, the reactions involving oxidation and ring expansion of the corrole macrocycle are described comprehensively.

Synthesis, Electronic Structure, and Reactivity Studies of a 4-Coordinate Square Planar Germanium(IV) Cation

A tetra-coordinate, square planar germanium(IV) cation [(TPFC)Ge](+) (TPFC = tris(pentafluorophenyl)corrole) was synthesized quantitatively by the reaction of (TPFC)Ge-H with [Ph3C](+)[B(C6F5)4](¯). The highly reactive [(TPFC)Ge](+) cation reacted with benzene to form phenyl complex (TPFC)Ge-C6H5 through an electrophilic pathway. The key intermediate, a σ-type germylium-benzene adduct, [(TPFC)Ge(η(1)-C6H6)](+), was isolated and characterized by single-crystal X-ray diffraction. Deprotonation of [(TPFC)Ge(η(1)-C6H6)](+) cation led to the formation of (TPFC)Ge-C6H5. [(TPFC)Ge](+) also reacted with ethylene and cyclopropane in benzene at room temperature to form (TPFC)Ge-CH2CH2C6H5 and (TPFC)Ge-CH2CH2CH2C6H5, respectively. The observed electrophilic reactivity is ascribed to the highly exposed cationic germanium center with novel frontier orbitals comprising two vacant sp-hybridized orbitals that are not conjugated to π-system. The three electron-withdrawing pentafluorophenyl groups on the corrole ligand also enhance the electrophilicity of the cationic germanium corrole.

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter, we aim to highlight the origins, development, and evolution of the PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.

NMR spectroscopy of the phenyl derivative of germanium(IV) 5,10,15-tritolylcorrole

A thoroughly structural characterization of (TTC)GePh (TTC [Formula: see text] 5,10,15-tritolylcorrole; Ph [Formula: see text] phenyl) in solution has been carried out through a combination of 2D NMR (1H-1H COSY, 1H-1H ROESY, 1H-[Formula: see text]C HSQC and 1H-[Formula: see text]C HMBC) experiments and density functional theory (DFT) calculations of the molecular and electronic structure and the shielding constants. The 1H and [Formula: see text]C chemical shifts computed at DFT-S12g and DFT-SAOP levels of theory nicely reproduce the experimental values, the agreement between theory and experiment being especially good for the DFT-S12g results. The calculations prove to be able to capture the fine details of the NMR spectra and to resolve some assignment ambiguities related to the inherent conformational flexibility of the macrocycle. The calculations also provide an explanation of the observed chemical shift trends in terms of diamagnetic and paramagnetic components of the shielding tensor.

Bond-weakening catalysis: conjugate aminations enabled by the soft homolysis of strong N-H bonds

The ability of redox-active metal centers to weaken the bonds in associated ligands is well precedented, but has rarely been utilized as a mechanism of substrate activation in catalysis. Here we describe a catalytic bond-weakening protocol for conjugate amination wherein the strong N-H bonds in N-aryl amides (N-H bond dissociation free energies ∼100 kcal/mol) are destabilized by ∼33 kcal/mol upon by coordination to a reducing titanocene complex, enabling their abstraction by the weak H-atom acceptor TEMPO through a proton-coupled electron transfer process. Significantly, this soft homolysis mechanism provides a method to generate closed-shell, metalated nucleophiles under neutral conditions in the absence of a Brønsted base.

Synthesis and reactivity studies of a tin(II) corrole complex

A series of tris(pentafluorophenyl)corrole (TPFC) tin(IV) and tin(II) complexes were prepared and studied by various characterization techniques including (1)H, (19)F, and (119)Sn NMR and UV-vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The unusual 4-coordinate, monomeric, divalent tin(II) complex [(TPFC)Sn(II)](-) (2a) showed highly efficient reactivity toward alkenes and alkyl halides via a nucleophilic addition pathway leading to the quantitative formation of alkyl stannyl corrole compounds. DFT calculations confirmed the divalent nature of the tin center in 2a, and an NBO analysis showed about 99.99% Sn lone pair character, of which 83.6% was Sn 5s and 16.35% was Sn 5p character.