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

Iron-Catalyzed Radical Activation Mechanism for Denitrogenative Rearrangement Over C(sp3 )-H Amination

An iron-catalyzed denitrogenative rearrangement of 1,2,3,4-tetrazole is developed over the competitive C(sp³ )-H amination. This catalytic rearrangement reaction follows an unprecedented metalloradical activation mechanism. Employing the developed method, a wide number of complex-N-heterocyclic product classes have been accessed. The synthetic utility of this radical activation method is showcased with the short synthesis of a bioactive molecule. Collectively, this discovery underlines the progress of radical activation strategy that should find wide application in the perspective of medicinal chemistry, drug discovery and natural product synthesis research.

Efficient Amination of Activated and Non-Activated C(sp3 )-H Bonds with a Simple Iron-Phenanthroline Catalyst

A readily available catalyst consisting of iron dichloride in combination with 1,10-phenanthroline catalyzes the ring-closing C-H amination of N-benzoyloxyurea to form imidazolidin-2-ones in high yields. The C-H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non-activated aliphatic C(sp3 )-H bonds, and it also works for C(sp2 )-H bonds. The surprisingly simple method can be performed under open flask conditions.

Mechanistic and chemoselective insights on sp3- and sp2-C–H bond aminations: Fe- vs. Ir-based catalysis

The mechanisms of Fe- and Ir-catalyzed sp³- and sp²-C–H bond aminations of a styryl substrate have been studied using the BPW91 method, with an emphasis on the origin of sp³-to-sp²-C–H amination chemoselectivity.

Direct intramolecular C(sp3)–H bond sulfonamidation to synthesize benzosultam derivatives under metal-free conditions

An efficient intramolecular C(sp3)–H bond amination under metal-free conditions has been developed to provide a straightforward method for the synthesis of 5-membered cyclic sulfonamides (benzosultams).

O-Benzoylhydroxylamines as Alkyl Nitrene Precursors: Synthesis of Saturated N-Heterocycles from Primary Amines

We introduce O-benzoylhydroxylamines as competent alkyl nitrene precursors. The combination of readily available, stable substrates and a proficient rhodium catalyst provides a straightforward means for the construction of various pyrrolidine rings from the corresponding primary amines. Preliminary mechanistic investigation suggests that the structure of the nitrene precursor plays a role in determining the nature of the resulting reactive intermediate.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Iron-Catalyzed Amination of Strong Aliphatic C(sp3)-H Bonds

A concept for intramolecular denitrogenative C(sp³)-H amination of 1,2,3,4-tetrazoles bearing unactivated primary, secondary, and tertiary C-H bonds is discovered. This catalytic amination follows an unprecedented metalloradical activation mechanism. The utility of the method is showcased with the short synthesis of a bioactive molecule. Moreover, an initial effort has been embarked on for the enantioselective C(sp³)-H amination through the catalyst design. Collectively, this study underlines the development of C(sp³)-H bond functionalization chemistry that should find wide application in the context of drug discovery and natural product synthesis.

Cobalt complexes of redox noninnocent azo-aromatic pincers. Isolation, characterization, and application as catalysts for the synthesis of quinazolin-4(3H)-ones

Herein we report the synthesis, characterization and catalytic application of three new cobalt(ii)-complexes of redox noninnocent arylazo ligands, 2-(phenylazo)-1,10-phenanthroline (L^1a), 2-(4-chlorophenylazo)-1,10-phenanthroline (L^1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L²) respectively. The reaction of L^1a with Co^IICl₂·6H₂O produced a μ-dichloro bridged binuclear cobalt(ii)-complex [Co(L^1a)₂Cl₂] (1a) while the same reaction when carried out with 2-(4-chlorophenyl)azo-1,10-phenanthroline (L^1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L²) ligands produced two new mononuclear five-coordinate cobalt(ii)-complexes 1b and 2 respectively. In complex 1a and 1b, the ligands L^1a and L^1b are coordinated to the cobalt(ii)-center in a tridentate mode utilizing all of its nitrogen donor sites while in complex 2 one of the azo-donor sites of the ligand L² remain pendant. All these complexes were characterized using available spectroscopic techniques and DFT studies. We further explored the potential of these complexes as catalysts for the synthesis of pharmaceutically important organic compounds via the functionalization of alcohols. A variety of substituted quinazolin-4(3H)-ones were synthesized under aerobic conditions via the coupling of alcohols and 2-aminobenzamide using 1b as the catalyst. Mechanistic investigations revealed that both cobalt and the arylazo scaffold act synergistically during catalysis.

C-H Amination Mediated by Cobalt Organoazide Adducts and the Corresponding Cobalt Nitrenoid Intermediates

Treatment of (^ArL)CoBr (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin) with a stoichiometric amount of 1-azido-4-(tert-butyl)benzene N₃(C₆H₄-p-^tBu) furnished the corresponding four-coordinate organoazide-bound complex (^ArL)CoBr(N₃(C₆H₄-p-^tBu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N₂) was observed at room temperature to afford a ligand functionalized product via a [3 + 2] annulation, which can be mediated by a high-valent nitrene intermediate such as a Co^III iminyl (^ArL)CoBr(^•N(C₆H₄-p-^tBu)) or Co^IV imido (^ArL)CoBr(N(C₆H₄-p-^tBu)) complex. The presence of the proposed intermediate and its viability as a nitrene group transfer reagent are supported by intermolecular C-H amination and aziridination reactivities. Unlike (^ArL)CoBr(N₃(C₆H₄-p-^tBu)), a series of alkyl azide-bound Co^II analogues expel N₂ only above 60 °C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via α-H-atom abstraction. The corresponding N₂-released structures were observed via single-crystal-to-crystal transformation, suggesting formation of a Co-nitrenoid intermediate in solid-state. Alternatively, the alkyl azide-bound congeners supported by a more sterically accessible dipyrrinato scaffold ^tBuL (^tBuL = 5-mesityl-(1,9-di-tert-butyl)dipyrrin) facilitate intramolecular 1,3-dipolar cycloaddition as well as C-H amination to furnish 1,2,3-dihydrotriazole and substituted pyrrolidine products, respectively. For the C-H amination, we observe that the temperature required for azide activation varies depending on the presence of weak C-H bonds, suggesting that the alkyl azide adducts serve as viable species for C-H amination when the C-H bonds are (1) proximal to the azide moiety and (2) sufficiently weak to be activated.

The first crystallographically characterised ruthenium(vi) alkylimido porphyrin competent for aerobic epoxidation and hydrogen atom abstraction

The syntheses of [Ru^VI(Por)(NAd)(O)] and [Ru^VI(2,6-F₂-TPP)(NAd)₂] have been described. [Ru^VI(2,6-F₂-TPP)(NAd)(O)] capable of catalysing aerobic epoxidation of alkenes has been characterised by X-ray crystallography with Ru[double bond, length as m-dash]NAd and Ru[double bond, length as m-dash]O bond distances being 1.778(5) Å and 1.760(4) Å (∠O-Ru-NAd: 174.37(19)°), respectively. Its first reduction potential is 740 mV cathodically shifted from that of [Ru^VI(2,6-F₂-TPP)(O)₂].

Intramolecular Csp3-H/C-C bond amination of alkyl azides for the selective synthesis of cyclic imines and tertiary amines

The intramolecular Csp³–H and/or C–C bond amination is very important in modern organic synthesis due to its efficiency in the construction of diversified N-heterocycles. Herein, we report a novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines through selective Csp³–H and/or C–C bond cleavage. Two C–N single bonds or a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N double bond are efficiently constructed in these transformations. The carbocation mechanism differs from the reported metal nitrene intermediates and therefore enables metal-free and new transformation.

A novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines has been developed. The aliphatic C–H or C–C bond was selectively cleaved with the efficient formation of two C–N single bonds or a CN double bond.

Mechanism and Chemoselectivity of Mn-Catalyzed Intramolecular Nitrene Transfer Reaction: C–H Amination vs. C=C Aziridination

The reactivity, mechanism and chemoselectivity of the Mn-catalyzed intramolecular C–H amination versus C=C aziridination of allylic substrate cis-4-hexenylsulfamate are investigated by BP86 density functional theory computations. Emphasis is placed on the origins of high reactivity and high chemoselectivity of Mn catalysis. The N p orbital character of frontier orbitals, a strong electron-withdrawing porphyrazine ligand and a poor π backbonding of high-valent MnIII metal to N atom lead to high electrophilic reactivity of Mn-nitrene. The calculated energy barrier of C–H amination is 9.9 kcal/mol lower than that of C=C aziridination, which indicates that Mn-based catalysis has an excellent level of chemoselectivity towards C–H amination, well consistent with the experimental the product ratio of amintion-to-aziridination I:A (i.e., (Insertion):(Aziridination)) >20:1. This extraordinary chemoselectivity towards C–H amination originates from the structural features of porphyrazine: a rigid ligand with the big π-conjugated bond. Electron-donating substituents can further increase Mn-catalyzed C–H amination reactivity. The controlling factors found in this work may be considered as design elements for an economical and environmentally friendly C–H amination system with high reactivity and high chemoselectivity.

DFT Mechanistic Account for the Site Selectivity of Electron-Rich C(sp3)-H Bond in the Manganese-Catalyzed Aminations

DFT study suggests that the C-H cleavage involved in the C(sp³)-H amination catalyzed by manganese perchlorophthalocyanine complex [Mn^III(ClPc)]⁺ proceeds via hydride transfer (HYT), instead of hydrogen atom transfer (HAT), thus elucidating why the reaction favors aminating the electron-rich C-H bond, rather than that with smaller bond dissociation energy preferred by HAT. Detailed analyses indicate the HYT actually occurs via concerted electron and hydrogen atom transfers and the redox-active ClPc ligand enables the HYT.

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]^Ph (H₂[OO]^Ph = [1,1′:4′,1′’-terphenyl]-2,2′’-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]^Ph(THF)₂ (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr₂([OO]^Ph)₂ (in CH₂Cl₂/THF). The electronic structure and bonding in Cr[OO]^Ph(THF)₂ were probed by density functional theory calculations. Isolated Cr₂([OO]^Ph)₂ undergoes facile reaction with 4-MeC₆H₄N₃, 4-MeOC₆H₄N₃, or 3,5-Me₂C₆H₃N₃ to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]^Ph(N(4-MeC₆H₄))₂ was confirmed by X-ray crystallography. The reaction of Cr₂([OO]^Ph)₂ with bulkier azides N₃R (MesN₃, AdN₃) forms paramagnetic products, formulated as Cr[OO]^Ph(NR). The attempted formation of a Cr–alkylidene complex (using N₂CPh₂) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]^Ph(NNCPh₂)₂. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr₂([OO]^Ph)₂ or H₂[OO]^Ph with NO⁺ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Iron-catalysed 1,2-aryl migration of tertiary azides

1,2-Carbon to nitrogen aryl migration of α,α-diaryl tertiary azides was realized by using FeCl₂ and N-heterocyclic carbene SIPr·HCl as a catalyst.

Ligand Redox Noninnocence in [CoIII(TAML)]0/- Complexes Affects Nitrene Formation

The redox noninnocence of the TAML scaffold in cobalt-TAML (tetra-amido macrocyclic ligand) complexes has been under debate since 2006. In this work, we demonstrate with a variety of spectroscopic measurements that the TAML backbone in the anionic complex [Co^III(TAML^red)]^– is truly redox noninnocent and that one-electron oxidation affords [Co^III(TAML^sq)]. Multireference (CASSCF) calculations show that the electronic structure of [Co^III(TAML^sq)] is best described as an intermediate spin (S = 1) cobalt(III) center that is antiferromagnetically coupled to a ligand-centered radical, affording an overall doublet (S = ¹/₂) ground-state. Reaction of the cobalt(III)-TAML complexes with PhINNs as a nitrene precursor leads to TAML-centered oxidation and produces nitrene radical complexes without oxidation of the metal ion. The ligand redox state (TAML^red or TAML^sq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [Co^III(TAML^sq)] or [Co^III(TAML^red)]^– with PhINNs results in the formation of [Co^III(TAML^q)(N^•Ns)] and [Co^III(TAML^q)(N^•Ns)₂]^–, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron-reduced Fischer-type nitrene radicals (N^•Ns^–) that are intermediates in catalytic nitrene transfer to styrene. These nitrene radical species were characterized by EPR, XANES, and UV–vis spectroscopy, high-resolution mass spectrometry, magnetic moment measurements, and supporting CASSCF calculations.

Synthesis, characterization and C-H amination reactivity of nickel iminyl complexes

Metalation of the deprotonated dipyrrin (^AdFL)Li with NiCl₂(py)₂ afforded the divalent Ni product (^AdFL)NiCl(py)₂ (1) (^AdFL: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce 1, affording the monovalent Ni synthon (^AdFL)Ni(py) (2) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni complexes (^AdFL)Ni(NMes) (3) and (^AdFL)Ni(NAd) (4), respectively. Both 3 and 4 were characterized by multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s → 3d transitions (3: 8333.2 eV; 4: 8333.4 eV) than Ni^I or unambiguous Ni^II analogues. N K-edge X-ray absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only for 3 and 4 (395.4 eV) that was assigned via TDDFT as an N 1s promotion into a predominantly N-localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the continuum of imido (i.e., NR²⁻) to iminyl (i.e., ²NR⁻) formulations, the complexes are best described as Ni^II-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (S = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom abstraction to afford the amide (^AdFL)Ni(NHAd) (5), whereas no reaction was observed upon treatment of the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at room temperature, exclusively affording the N-1-adamantyl-benzylidene (6). Slow addition of the organoazide substrate (4-azidobutyl)benzene (7) with 2 exclusively forms 4-phenylbutanenitrile (8) as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H₂-elimination from the intermediate Ni hydride ketimide intermediate.

Nickel-supported nitrenoids exhibit iminyl character, as determined by multi-edge XAS and TDDFT analysis, demonstrate efficacy for C–H activation and nitrene transfer chemistry.

Nickel complexes of ligands derived from (o-hydroxyphenyl) dichalcogenide: delocalised redox states of nickel and o-chalcogenophenolate ligands

Two monoanionic nickel complexes Bu4N[Ni(LSeO)2] (1) and Bu4N[Ni(LSO)2] (2) (H2LSeO = 3,5-di-tert-butyl-2-hydroxyselenophenol and H2LSO = 3,5-di-tert-butyl-2-hydroxythiophenol) were synthesised by reductive cleavage of the respective 2,2'-dichalcogenobis(4,6-di-tert-butylphenol) (H2LX-X; X = Se, S) with nickel(ii) salts. The crystal structures of 1 and 2 confirm the reductive X-X bond cleavage with the concomitant formation of the corresponding monoanionic square planar complex, where quinoidal distortions of the aromatic rings are observed. The monoanionic complexes (1 and 2) are paramagnetic (S = 1/2), exhibiting rhombic EPR signals, and the g anisotropies are well correlated with the spin-orbit coupling of chalcogenides. The spectral data indicate that the ligands H2LXO in 1 and 2 are redox non-innocent and stabilise the square planar S = 1/2 nickel complexes with a highly delocalised unpaired electron. DFT calculations further support the delocalised electronic structures of the nickel complexes.

Synthesis and redox chemistry of Pd(ii) complexes of a pincer verdazyl ligand

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH₃CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH₃CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH₃CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

Heterocyclization involving benzylic C(sp3)-H functionalization enabled by visible light photoredox catalysis

A general and efficient method for heterocyclization involving benzylic C(sp³)-H functionalization enabled by visible light photoredox catalysis to access a wide range of structurally diverse oxygen as well as nitrogen heterocycles up to a gram scale is reported. The potential application of this new methodology is demonstrated by the total synthesis of (-)-codonopsinine and (+)-centrolobine. Herein it is proposed that selectfluor, unlike a fluorinating reagent, acts as an oxidative quencher and a hydrogen radical acceptor.

Synthesis of Sultams and Cyclic N-Sulfonyl Ketimines via Iron-Catalyzed Intramolecular Aliphatic C-H Amidation

Cyclic sulfonamides (sultams) play a unique role in drug discovery and synthetic chemistry. A direct synthesis of sultams by an intramolecular C(sp³)-H amidation reaction using an iron complex in situ derived from Fe(ClO₄)₂ and aminopyridine ligand is reported. This strategy features a readily available catalyst and tolerates a broad variety of substrates as demonstrated by 22 examples (up to 89% yield). A one-pot iron-catalyzed amidation/oxidation procedure for the synthesis of cyclic N-sulfonyl ketimines is also realized with up to 92% yield (eight examples). The synthetic utility of the method is validated by a gram-scale reaction and derivatization of the products to ring-fused sultams.

Uncatalyzed Oxidative C-H Amination of 9,10-Dihydro-9-Heteroanthracenes: A Mechanistic Study

A new method for the one‐step C−H amination of xanthene and thioxanthene with sulfonamides is reported, without the need for any metal catalyst. A benzoquinone was employed as a hydride (or two‐electron and one‐proton) acceptor. Moreover, a previously unknown and uncatalyzed reaction between iminoiodanes and xanthene, thioxanthene and dihydroacridines (9,10‐dihydro‐9‐heteroanthracenes or dihydroheteroanthracenes) is disclosed. The reactions proceed through hydride transfer from the heteroarene substrate to the iminoiodane or benzoquinone, followed by conjugate addition of the sulfonamide to the oxidized heteroaromatic compounds. These findings may have important mechanistic implications for metal‐catalyzed C−H amination processes involving nitrene transfer from iminoiodanes to dihydroheteroanthracenes. Due to the weak C−H bond, xanthene is an often‐employed substrate in mechanistic studies of C−H amination reactions, which are generally proposed to proceed via metal‐catalyzed nitrene insertion, especially for reactions involving nitrene or imido complexes that are less reactive (i.e., less strongly oxidizing). However, these substrates clearly undergo non‐catalyzed (proton‐coupled) redox coupling with amines, thus providing alternative pathways to the widely assumed metal‐catalyzed pathways.

Iron-Catalyzed Intramolecular Amination of Aliphatic C-H Bonds of Sulfamate Esters with High Reactivity and Chemoselectivity

It is challenging to develop simple and low cost catalytic systems while maintaining high reactivity and selectivity. An iron-catalyzed intramolecular C-H amination of sulfamate esters using simple and cheap ligands is reported with general substrate scope (31 examples, up to 95% yield). The addition of second ligand, bipyridine, is able to accelerate the reaction and increase the yield. The ready availability of these iron catalysts provides a promising approach to selective introduction of nitrogen into hydrocarbon feedstock.