Catalytic Synthesis of N-Heterocycles via Direct C(sp3)-H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

[CuII{(LISQ)˙-}2] (H2L: thioether-appended o-aminophenol ligand) monocation triggers change in donor site from N2O2 to N2O(2)S and valence-tautomerism

Using a potentially tridentate o-aminophenol-based redox-active ligand H₂L¹ (2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) in its deprotonated form, [Cu(L¹)₂] has been synthesized and crystallized as [Cu^II(L¹)₂]·CH₂Cl₂ (1·CH₂Cl₂). A cyclic voltammetry experiment (in CH₂Cl₂; V vs. SCE (saturated calomel electrode)) on 1·CH₂Cl₂ exhibits two oxidative (E = 0.20 V (peak-to-peak separation, ΔE_p = 100 mV) and E = 0.90 V (ΔE_p = 140 mV)) and two reductive (E = -0.52 V (ΔE_p = 110 mV) and E = -0.92 V (ΔE_p = 120 mV)) responses. Upon oxidation using a stoichiometric amount of [Fe^III(η⁵-C₅H₅)₂](PF₆), 1·CH₂Cl₂ yielded [Cu(L¹)₂](PF₆) (2). Structural analysis (100 K) reveals that 1·CH₂Cl₂ is a four-coordinate bis(iminosemiquinonato)copper(ii) complex (CuN₂O₂ coordination), and that the thioethers remain uncoordinated. The twisted geometry of 1 (distorted tetrahedral) results in considerable changes in the electronic structure, compared to well-known square-planar analogues. Crystallographic analysis of 2 both at 100 K and at 293 K reveals that it is effectively a four-coordinate complex with a CuN₂OS coordination; however, a substantial interaction with the other phenolate O is observed. The metal-ligand bond distances and metric parameters associated with the o-aminophenolate rings indicate a valence-tautomeric (VT) equilibrium involving monocationic (iminosemiquinonato)(iminoquinone)copper(ii) and bis(iminoquinone)copper(i). Complex 1·CH₂Cl₂ is a three-spin system and a magnetic study (4-300 K) established that it has a S = 1/2 ground-state, owing to the strong antiferromagnetic coupling between the unpaired spin of the copper(ii) and the iminosemiquinonate(1-) π-radical anion. Electron paramagnetic resonance (EPR) spectral studies corroborate this result. Complex 2 is diamagnetic and the existence of VT in 2 was probed using variable-temperature (248-328 K) ¹H NMR and EPR (100-298 K) spectral measurements and X-ray photoelectron spectroscopic studies at 298 K. Remarkably, modification of the well-studied 2-anilino-4,6-di-tert-butylphenol by incorporation of a benzylthioether arm leads to the occurrence of VT in 2. The electronic structure of 1·CH₂Cl₂ and 2 has been assigned using density functional theory (DFT) calculations at the B3LYP-D3 level of theory. Time-dependent (TD)-DFT calculations have been performed to elucidate the origin of the observed UV-VIS-NIR absorptions.

Enantioselective intramolecular C-H amination of aliphatic azides by dual ruthenium and phosphine catalysis

By combining a chiral-at-metal ruthenium catalyst with catalytic amounts of tris(p-fluorophenyl)phosphine (both 1 mol%), the challenging catalytic enantioselective ring-closing C(sp³)-H amination of unactivated aliphatic azides has been achieved with high enantioselectivities.

The catalytic enantioselective intramolecular C(sp³)-H amination of aliphatic azides represents an efficient method for constructing chiral saturated cyclic amines which constitute a prominent structural motif in bioactive compounds. We report a dual catalytic system involving a chiral-at-metal bis(pyridyl-NHC) ruthenium complex and tris(4-fluorophenyl)phosphine (both 1 mol%), which facilitates the cyclization of aliphatic azides to chiral α-aryl pyrrolidines with enantioselectivities of up to 99% ee, including a pyrrolidine which can be converted to the anti-tumor alkaloid (R)-(+)-crispine. Mechanistically, the phosphine activates the organic azide to form an intermediate iminophosphorane and transfers the nitrene unit to the ruthenium providing an imido ruthenium intermediate which engages in the highly stereocontrolled C–H amination. This dual catalysis combines ruthenium catalysis with the Staudinger reaction and provides a novel strategy for catalyzing enantioselective C–H aminations of unactivated aliphatic azides.

Transition Metal-Free Intermolecular C(sp2)-H Direct Amination of Furanones via a Redox Pathway

A direct C(sp²)-H amination of 2-furanones under metal-free conditions was realized. This unprecedented intermolecular C-H to C-N conversion provides rapid access to 4-amino-furanone derivatives and novel aza-heterocycle fused furanone skeletons. A redox mechanism based on a double-Michael-addition intermediate INT2 is proposed and detected by spectrometry.

Copper-catalyzed nitrene transfer/cyclization cascade to synthesize 3a-nitrogenous furoindolines and pyrroloindolines

Copper-catalyzed nitrene transfer for amination/cyclization of tryptophols and tryptamines to generate the corresponding indole alkaloids in good to excellent yields was successfully developed.

Ligand-free iron-catalyzed benzylic C (sp3)–H amination of methylarenes with N-fluorobenzenesulfonimide

The first biocompatible iron-catalyzed benzylic C (sp³)–H amination of methylarenes with NFSI under ligand and additional oxidant free conditions is described.

Recent advances in Fe-catalyzed C–H aminations using azides as nitrene precursors

The intramolecular Fe-catalyzed amination of C–H bonds using azides as nitrene precursors represents an elegant approach toward N-heterocycles. This review summarizes the most recent achievements while focussing on fundamental mechanistic aspects.

Unveiling the mechanism and regioselectivity of iron-dipyrrinato-catalyzed intramolecular C(sp3)–H amination of alkyl azides

The mechanism of iron-catalyzed C(sp³)–H amination was established, in which regioselectivity arose from both radical stability and ring strain.

Stabilization of different redox levels of a tridentate benzoxazole amidophenoxide ligand when bound to Co(iii) or V(v)

The electronic structure of Co and V complexes of a tridentate benzoxazole-containing aminophenol ligand NNOH2 were characterized by both experimental and theoretical methods.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Recent Progress on Cyclic Nitrenoid Precursors in Transition-Metal-Catalyzed Nitrene-Transfer Reactions

Nitrene-transfer reactions are powerful synthetic tools for the direct incorporation of nitrogen atoms into organic molecules. The discovery of novel nitrene-transfer reactions has been dominantly supported not only by improvements in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since pioneering work involving the use of organic azides and iminoiodinanes as practical synthetic tools for nitrogen-containing compounds was reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles is comprehensively described, showing the recent remarkable progress in this chemistry.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above -2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of -C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of "metric oxidation state" (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal-ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.

A non-innocent pincer H3LONS ligand and its corresponding octahedral low-spin Fe(iii) complex formation via ligand-centric homolytic S-S bond scission

In the presence of FeCl3 and Et3N, a ligand H4Ldtda(AP) underwent S-S bond cleavage and generated a pincer non-innocent H3LONS ligand, which formed a homoleptic, six-coordinate, low-spin Fe(iii) complex (1). The complex comprised two 2-iminobenzosemiquinone (1-) π-radicals and one thiyl π-radical.

Rh(III)-Catalyzed C-H Activation of Boronic Acid with Aryl Azide

A Rh(III)-catalyzed C-H activation of boronic acid with aryl azide to obtain unsymmetric carbazoles, 1 H-indoles, or indolines has been developed. The reaction constructs dual distinct C-N bonds via sp²/sp³ C-H activation and rhodium nitrene insertion. Synthetically, this approach represents an access to widely used carbazole derivatives. The practical application to CBP and unsymmetric TCTA derivatives has also been performed. Mechanistic experiments and DFT calculations demonstrate that a five-membered rhodacycle species is the key intermediate.

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes

Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe₂ complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe₂ complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

Ferrate(ii) complexes with redox-active formazanate ligands

The synthesis of mono(formazanate) iron complexes is described. In the presence of tetrabutylammonium halides, salt metathesis reactions afford the ferrate(ii) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products are characterized in detail. The high-spin ferrate(ii) complexes show cyclic voltammograms that are consistent with reversible, ligand-based one-electron reduction. The halides in these ferrate(ii) compounds are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv.) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) iron(ii) complexes is established.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

An iridium(iii/iv/v) redox series featuring a terminal imido complex with triplet ground state

The iridium(iii/iv/v) imido redox series [Ir(NtBu){N(CHCHPtBu2)2}]0/+/2+ was synthesized and examined spectroscopically, magnetically, crystallographically and computationally. The monocationic iridium(iv) imide exhibits an electronic doublet ground state with considerable 'imidyl' character as a result of covalent Ir-NtBu bonding. Reduction gives the neutral imide [Ir(NtBu){N(CHCHPtBu2)2}] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO2) and nucleophiles (PMe3), respectively, is reported.

Metal-organic layers stabilize earth-abundant metal-terpyridine diradical complexes for catalytic C-H activation

We report the synthesis of a terpyridine-based metal-organic layer (TPY-MOL) and its metalation with CoCl2 and FeBr2 to afford CoCl2·TPY-MOL and FeBr2·TPY-MOL, respectively. Upon activation with NaEt3BH, CoCl2·TPY-MOL catalyzed benzylic C-H borylation of methylarenes whereas FeBr2·TPY-MOL catalyzed intramolecular Csp3 -H amination of alkyl azides to afford pyrrolidines and piperidines. X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy, UV-Vis-NIR spectroscopy, and electron paramagnetic spectroscopy (EPR) measurements as well as density functional theory (DFT) calculations identified M(THF)2·TPY-MOL (M = Co or Fe) as the active catalyst with a MII-(TPY˙˙)2- electronic structure featuring divalent metals and TPY diradical dianions. We believe that site isolation stabilizes novel MII-(TPY˙˙)2- (M = Co or Fe) species in the MOLs to endow them with unique and enhanced catalytic activities for Csp3 -H borylation and intramolecular amination over their homogeneous counterparts. The MOL catalysts are also superior to their metal-organic framework analogs owing to the removal of diffusion barriers. Our work highlights the potential of MOLs as a novel 2D molecular material platform for designing single-site solid catalysts without diffusional constraints.

Intramolecular metal–ligand electron transfer triggered by co-ligand substitution

Labile co-ligands are attached to a dinuclear copper(i) complex with a redox-active bridging guanidine ligand. Their substitution triggers electron-transfer from the copper atoms to the guanidine.

Regio- and Stereoselective Iron-Catalyzed Oxyazidation of Enamides Using a Hypervalent Iodine Reagent

A novel regio- and diastereoselective iron-catalyzed intermolecular oxyazidation of enamides using various azidobenziodoxolone (ABX) derivatives is presented. A variety of α-N₃ amino derivatives and of α-N₃ piperidines were synthesized in good yields and under mild reaction conditions. The reaction involves a radical process using cheap FeCl₂ as the initiator.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin-Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1^•-)Rh^III(1^•-)]⁺ ([2]⁺), starting from the low-valent electron reservoir [Rh^I]. The less stable monoradical [Rh^III(1^•-)Cl₂(PPh₃)₃] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]⁺, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t₂⁶) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]⁺ lies near ∼2.3 μ_B over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1^•-)Rh^III(1^•-)]⁺ have been found experimentally (J ≈ -5 cm^-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.

Direct Comparison of C-H Bond Amination Efficacy through Manipulation of Nitrogen-Valence Centered Redox: Imido versus Iminyl

Reduction of previously reported iminyl radical (ArL)FeCl(•N(C6H4-p-tBu)) (2) with potassium graphite furnished the corresponding high-spin (S = 5/2) imido (ArL)Fe(N(C6H4-p-tBu)) (3) (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin). Oxidation of the three-coordinate imido (ArL)Fe(NAd) (5) with chlorotriphenylmethane afforded (ArL)FeCl(•NAd) (6) with concomitant expulsion of Ph3C(C6H5)CPh2. The respective aryl/alkyl imido/iminyl pairs (3, 2; 5, 6) have been characterized by EPR, zero-field 57Fe Mössbauer, magnetometry, single crystal X-ray diffraction, XAS, and EXAFS for 6. The high-spin (S = 5/2) imidos exhibit characteristically short Fe-N bonds (3: 1.708(4) Å; 5: 1.674(11) Å), whereas the corresponding iminyls exhibit elongated Fe-N bonds (2: 1.768(2) Å; 6: 1.761(6) Å). Comparison of the pre-edge absorption feature (1s → 3d) in the X-ray absorption spectra reveals that the four imido/iminyl complexes share a common iron oxidation level consistent with a ferric formulation (3: 7111.5 eV, 2: 7111.5 eV; 5: 7112.2 eV, 6: 7112.4 eV) as compared with a ferrous amine adduct (ArL)FeCl(NH2Ad) (7: 7110.3 eV). N K-edge X-ray absorption spectra reveal a common low-energy absorption present only for the iminyl species 2 (394.5 eV) and 6 (394.8 eV) that was assigned as a N 1s promotion into a N-localized, singly occupied iminyl orbital. Kinetic analysis of the reaction between the respective iron imido and iminyl complexes with toluene yielded the following activation parameters: Ea (kcal/mol) 3: 12.1, 2: 9.2; 5: 11.5, 6: 7.1. The attenuation of the Fe-N bond interaction on oxidation from an imido to an iminyl complex leads to a reduced enthalpic barrier [Δ(ΔH‡) ≈ 5 kcal/mol]; the alkyl iminyl 6 has a reduced enthalpic barrier (1.84 kcal/mol) as compared with the aryl iminyl 2 (3.84 kcal/mol), consistent with iminyl radical delocalization into the aryl substituent in 2 as compared with 6.