Nitrene Radical Research Articles

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 CoIII iminyl (ArL)CoBr(•N(C6H4-p-tBu)) or CoIV 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 CoII 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.

Pyrrole Functionalization by Copper‐Catalyzed Nitrene Transfer Reactions

AbstractThe catalytic functionalization of pyrroles by incorporation of a nitrene group is reported. The Cα‐H bond of 1H‐pyrrole is amidated upon the formal insertion of the NTs (Ts=p‐toluenesulfonyl) group catalyzed by TpBr3Cu(NCMe) (TpBr3=hydrotris(3,4,5‐tribromo‐pyrazolyl)borate). N‐substituted pyrroles also verify the same transformation. The mechanism proposal is similar to that previously described for benzene amidation with the same catalyst and PhI=NTs, which takes place through aziridine formation, ring opening and 1,2‐hydrogen shift. A cascade reaction involving the coupling of 2,5‐dimethylfuran, 1,2,3‐trimethyl‐pyrrole and a nitrene NTs group is also described, leading to a 1,2‐dihydropyridine‐imine compound.

Site-Selective Functionalization of Polydopamine Films via Aryl Azide-Based Photochemical Reaction

Although the post functionalization of versatile polydopamine (pD) films enables the expression of functional groups of interest on target substrates, their immobilization generally lacks site-selectivity within a surface platform. In this paper, we report a site-selective conjugation method to functionalize pD films based on the combination of simple photolithographic techniques. The essence of the chemical functionalization strategy was the use of the aryl azide group, whose photolysis induces the generation of reactive nitrene groups that functionalize hydrocarbon networks of pD films. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses indicated that the pD film was functionalized primarily with aryl azide groups under UV irradiation and the introduction of a photomask enabled the pattern generation of the initiator that was tethered with the aryl azide group. A subsequent surface-initiated polymerization on their substrates produced a patterned polymer brush, expressing their own functionality on the surface platform.

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 [CoIII(TAMLred)]– is truly redox noninnocent and that one-electron oxidation affords [CoIII(TAMLsq)]. Multireference (CASSCF) calculations show that the electronic structure of [CoIII(TAMLsq)] 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 (TAMLred or TAMLsq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [CoIII(TAMLsq)] or [CoIII(TAMLred)]– with PhINNs results in the formation of [CoIII(TAMLq)(N•Ns)] and [CoIII(TAMLq)(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.

How Does Azo Bond Cleave in the Gas Phase? Computational and Experimental Study on the Fragmentation Mechanism of Protonated Sudan I

AbstractAs a cheap but harmful additive in food industry, 1‐phenylazo‐2‐naphthol (Sudan I) has attracted lots of attentions, which mainly focused on the determinations using the HPLC‐ESI‐MS/MS techniques. However, the fragmentation mechanisms are generally scarce. In this paper, the fragmentation pathways of protonated Sudan I in gas‐phase were investigated by a combination of electrospray ionization‐tandem mass spectrometry and density functional theory (DFT) calculations. Initially, the cleavage of azo bond is triggered by ionizing proton migration from the oxygen atom of O‐protonated Sudan I to adjacent N atom through a six‐member ring transition state, leading to a single ion‐neutral complex (INC‐s). Subsequently, the direct decomposition of INC‐s gives the naphtol nitrene ion at m/z 156. Meanwhile, INC‐s may occur an intermolecular single electron transfer (SET) reaction via the crossing points (INC‐mecp), affording the triple ion‐neutral complex (INC‐t) consisting of naphtol nitrene radical and aniline radical ion. Finally, followed by the decomposition of INC‐t, the aniline radical ion at m/z 156 is formed. Further studies on Electrostatic potential map, Natural population analysis (NPA) and frontier molecular orbitals (FMOs) during the proposed mechanism are also found to be in good agreement with experimental observations. Taken together, our results presented here not only provide mechanistic information for better understanding the Sudan I, but also give insights into other related harmful additives in food industry.

Radical-type Reactions Controlled by Cobalt: From Carbene Radical Reactivity to the Catalytic Intermediacy of Reactive o-Quinodimethanes

In this account, we summarize our recent efforts in the fields of ‘open-shell organometallic chemistry’ and ‘metalloradical catalysis’. We focus in particular on the use of so-called ‘carbene radicals’ for the synthesis of a variety of useful synthons for organic chemistry. We further show that unexpected reactivity arises from catalytic synthesis of unusual o-quinone methide and o-quinodimethane intermediates that undergo subsequent rearrangements to uncommon products.1 Introduction2 General (Fischer-Type) Carbene and Nitrene Reactivity and Their Relation to Carbene and Nitrene Radical Reactivity3 Carbene and Nitrene (Radical) Precursors4 Formation and Intrinsic Radical-Type Reactivity of Carbene and Nitrene Radicals5 Types of Cobalt Catalysts Used in Reactions Involving Carbene and Nitrene Radicals6 Applications of Cobalt-Catalyzed Ring-Closure Reactions via ­Carbene Radicals7 Intermediacy of o-Quinone Methide and o-Quidodimethanes in Carbene Ring-Closing Reactions8 Conclusion

Porphyrin Co(III)-Nitrene Radical Mediated Pathway for Synthesis of o-Aminoazobenzenes.

Azobenzenes are versatile compounds with a range of applications, including dyes and pigments, food additives, indicators, radical reaction initiators, molecular switches, etc. In this context, we report a general method for synthesizing o-aminoazobenzenes using the commercially available cobalt(II) tetraphenyl porphyrin [CoII(TPP)]. The net reaction is a formal dimerization of two phenyl azides with concomitant loss of two molecules of dinitrogen. The most commonly used methodology to synthesize azobenzenes is based on the initial diazotization of an aromatic primary amine at low temperatures, which then reacts with an electron rich aromatic nucleophile. As such, this limits the synthesis of azobenzenes with an amine functionality. In contrast, the method we report here relies heavily on the o-amine moiety and retains it in the product. The reaction is metal catalyzed and proceeds through a porphyrin Co(III)-nitrene radical intermediate, which is known to form on activation of organic azides at the cobalt center. The synthesized o-aminoazobenzenes are bathochromatically shifted, as compared to azobenzenes without amine substituents. Based on the crystal structure of one of the products, strong H-bonding between the N-atom of the azo functionality and the H of the NH2 substituent is shown to stabilize the trans isomeric form of the product. The NH2 substituents offers possibilities for further functionalization of the synthesized azo compounds.

Application of [Co(Corrole)]– Complexes in Ring‐Closing C–H Amination of Aliphatic Azides via Nitrene Radical Intermediates

The synthesis, characterisation and application of anionic [CoII(Cor)]– (Cor = corrole) metalloradicals in the ring‐closing C–H amination of an aliphatic azide in the presence of Boc2O (Boc = tert‐butyloxycarbonyl) to give the corresponding Boc‐protected N‐heterocyclic product tert‐butyl 2‐phenylpyrrolidine‐1‐carboxylate are reported. This is the first example of the use of metalloradical cobalt(II) corrole complexes in nitrene‐transfer reactions. On the basis of DFT calculations, the reaction is proposed to proceed via discrete open‐shell nitrene radical intermediates bearing most of their spin density at the nitrene nitrogen atom. The [CoII(Cor)]– complexes are substantially faster catalysts than the corresponding neutral [CoII(porphyrin)] complexes when applied in the same ring‐closing C–H amination reaction under identical reaction conditions. Increasing the electron density at cobalt(II) therefore has a positive influence on the reaction rate.

Frontispiece: Nitrene Radical Intermediates in Catalytic Synthesis

Frontispiece: Nitrene Radical Intermediates in Catalytic Synthesis

Nitrene Radical Intermediates in Catalytic Synthesis.

Nitrene radical complexes are reactive intermediates with discrete spin density at the nitrogen‐atom of the nitrene moiety. These species have become important intermediates for organic synthesis, being invoked in a broad range of C−H functionalization and aziridination reactions. Nitrene radical complexes have intriguing electronic structures, and are best described as one‐electron reduced Fischer type nitrenes. They can be generated by intramolecular single electron transfer to the “redox non‐innocent” nitrene moiety at the metal. Nitrene radicals generated at open‐shell cobalt(II) have thus far received most attention in terms of spectroscopic characterization, reactivity screening, catalytic nitrene‐transfer reactions and (computational and experimental) mechanistic studies, but some interesting iron and precious metal catalysts have also been employed in related reactions involving nitrene radicals. In some cases, redox‐active ligands are used to facilitate intramolecular single electron transfer from the complex to the nitrene moiety. Organic azides are among the most attractive nitrene precursors in this field, typically requiring pre‐activated organic azides (e.g. RSO2N3, (RO)2P(=O)N3, ROC(=O)N3 and alike) to achieve efficient and selective catalysis. Challenging, non‐activated aliphatic organic azides were recently added to the palette of reagents useful in synthetically relevant reactions proceeding via nitrene radical intermediates. This concept article describes the electronic structure of nitrene radical complexes, emphasizes on their usefulness in the catalytic synthesis of various organic products, and highlights the important developments in the field.

Elusive Terminal Copper Arylnitrene Intermediates

AbstractWe report herein three new modes of reactivity between arylazides N3Ar with a bulky copper(I) β‐diketiminate. Addition of N3ArX3 (ArX3=2,4,6‐X3C6H2; X=Cl or Me) to [iPr2NN]Cu(NCMe) results in triazenido complexes from azide attack on the β‐diketiminato backbone. Reaction of [iPr2NN]Cu(NCMe) with bulkier azides N3Ar leads to terminal nitrenes [iPr2NN]Cu]=NAr that dimerize via formation of a C−C bond at the arylnitrene p‐position to give the dicopper(II) diketimide 4 (Ar=2,6‐iPr2C6H3) or undergo nitrile insertion to give diazametallocyclobutene 8 (Ar=4‐Ph‐2,6‐iPr2C6H2). Importantly, reactivity studies reveal both 4 and 8 to be “masked” forms of the terminal nitrenes [iPr2NN]Cu=NAr that undergo nitrene group transfer to PMe3, tBuNC, and even into a benzylic sp3 C−H bond of ethylbenzene.

Cobalt(II)-Catalyzed Synthesis of Sulfonyl Guanidines via Nitrene Radical Coupling with Isonitriles: A Combined Experimental and Computational Study

A Co(II)-catalyzed synthesis of sulfonyl guanidines by using amines, isonitriles, and organic azides as nitrene sources has been developed. This protocol provides an environmentally friendly and simple strategy for the synthesis of sulfonyl guanidine derivatives by employing a range of substrates and will find potential applications in organic synthesis. The computational and EPR studies suggested the formation of guanidine derivatives via a cobalt–nitrene radical intermediate.

Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C-H Amination of Aliphatic Azides: A Nitrene-Radical Approach to Saturated Heterocycles.

Cobalt‐porphyrin‐catalysed intramolecular ring‐closing C−H bond amination enables direct synthesis of various N‐heterocycles from aliphatic azides. Pyrrolidines, oxazolidines, imidazolidines, isoindolines and tetrahydroisoquinoline can be obtained in good to excellent yields in a single reaction step with an air‐ and moisture‐stable catalyst. Kinetic studies of the reaction in combination with DFT calculations reveal a metallo‐radical‐type mechanism involving rate‐limiting azide activation to form the key cobalt(III)‐nitrene radical intermediate. A subsequent low barrier intramolecular hydrogen‐atom transfer from a benzylic C−H bond to the nitrene‐radical intermediate followed by a radical rebound step leads to formation of the desired N‐heterocyclic ring products. Kinetic isotope competition experiments are in agreement with a radical‐type C−H bond‐activation step (intramolecular KIE=7), which occurs after the rate‐limiting azide activation step. The use of di‐tert‐butyldicarbonate (Boc2O) significantly enhances the reaction rate by preventing competitive binding of the formed amine product. Under these conditions, the reaction shows clean first‐order kinetics in both the [catalyst] and the [azide substrate], and is zero‐order in [Boc2O]. Modest enantioselectivities (29–46 % ee in the temperature range of 100–80 °C) could be achieved in the ring closure of (4‐azidobutyl)benzene using a new chiral cobalt‐porphyrin catalyst equipped with four (1S)‐(−)‐camphanic‐ester groups.

Cobalt(ii)-catalyzed bis-isocyanide insertion reactions with sulfonyl azides via nitrene radicals: chemoselective synthesis of sulfonylamidyl amide and 3-imine indole derivatives.

A chemoselective Co(ii)-catalyzed effective synthesis of sulfonylamidyl amide and 3-imine indole derivatives by using isocyanides and sulfonyl azides has been developed. This protocol provides a new, environmentally friendly and simple strategy for the efficient synthesis of the sulfonylamidyl amide and 3-imine indole derivatives with a wide range of substrates in the absence of any oxidants and additives.

Catalytic C-H amination driven by intramolecular ligand-to-nitrene one-electron transfer through a rhodium(iii) centre.

Werner type six-coordinate rhodium(iii) complexes coordinated by a planar trianionic ligand and two axial aniline ligands are synthesised. The trianionic ligand behaves as a redox-active ligand to form a ligand radical species upon one-electron oxidation of the complex. The rhodium(iii) complexes catalyse C-H amination of external substrates such as xanthene with tosylazide as the nitrene source. DFT-calculation and kinetic deuterium isotope effects indicate that a di-radical rhodium(iii) complex formed by one-electron transfer from the redox-active ligand to the nitrene group works as a reactive intermediate to induce aliphatic C-H activation.

DFT analysis of the electronic structure of Fe(IV) species active in nitrene transfer catalysis: influence of the coordination sphere.

Nitrene transfer reactions to various hydrocarbon molecules can be efficiently catalyzed by Fe complexes through a mechanism reminiscent of the oxygen transfer function of oxygenase enzymes. Such enzymes exhibit a high-valent iron oxo Fe(IV) = O as the active species, and it has also been proposed that an analogous species, i.e., Fe(IV) = NR (NR being the nitrene group) is responsible for the nitrene transfer activity. We describe here the influence of the Fe(IV) coordination sphere on some key parameters for nitrene transfer efficacy, such as the spin state of the Fe(IV) cation, the electronic affinity, and the bond dissociation energy of the NHR moiety. We explore here the electronic properties of Fe(IV) = NTs (NTs = tolylsulfonylimido group) mononuclear complexes with ligands involving phenolate and nitrogen donor groups, as catalytic properties with such ligands have been found to be quite promising. Six tetradentate ligands were studied, which derive from three different scaffolds: 2-methylenepyridine-N,N-bis(2-methylene-4,6-dichlorophenol) and 2-methylenepyridine-N,N-bis(2-methylene-4,6-dimethylphenol), N,N-dimethyl-N',N'-bis(2-methylene-4,6-dichlorophenol) ethylenediamine, and N,N-dimethyl-N',N'- bis(2-methylene-4,6-dimethylphenol) ethylenediamine, N,N'-bis(2-methylene-4,6-dichlorophenol)-N,N'-dimethyl-1,2-diaminoethane and N,N'-bis(2-methylene-4,6-dimethylphenol)-N,N'-dimethyl-1,2-diaminoethane. Thanks to thorough DFT computations, we present some rationalization of the electronic properties of the resulting Fe(IV) = NTs complexes in relation to their coordination sphere and compare them to other Fe(IV) nitrene active species. We show in particular the important role of the anionic character and strong π-donation of the phenolate groups.

Characterization and Reactivity Studies of a Terminal Copper–Nitrene Species

AbstractHigh‐valent terminal copper–nitrene species have been postulated as key intermediates in copper‐catalyzed aziridination and amination reactions. The high reactivity of these intermediates has prevented their characterization for decades, thereby making the mechanisms ambiguous. Very recently, the Lewis acid adduct of a copper–nitrene intermediate was trapped at −90 °C and shown to be active in various oxidation reactions. Herein, we describe for the first time the synthesis and spectroscopic characterization of a terminal copper(II)–nitrene radical species that is stable at room temperature in the absence of any Lewis acid. The azide derivative of a triazamacrocyclic ligand that had previously been utilized in the stabilization of aryl–CuIII intermediates was employed as an ancillary ligand in the study. The terminal copper(II)–nitrene radical species is able to transfer a nitrene moiety to phosphines and abstract a hydrogen atom from weak C−H bonds, leading to the formation of oxidized products in modest yields.

Characterization and Reactivity Studies of a Terminal Copper-Nitrene Species.

High-valent terminal copper-nitrene species have been postulated as key intermediates in copper-catalyzed aziridination and amination reactions. The high reactivity of these intermediates has prevented their characterization for decades, thereby making the mechanisms ambiguous. Very recently, the Lewis acid adduct of a copper-nitrene intermediate was trapped at -90 °C and shown to be active in various oxidation reactions. Herein, we describe for the first time the synthesis and spectroscopic characterization of a terminal copper(II)-nitrene radical species that is stable at room temperature in the absence of any Lewis acid. The azide derivative of a triazamacrocyclic ligand that had previously been utilized in the stabilization of aryl-CuIII intermediates was employed as an ancillary ligand in the study. The terminal copper(II)-nitrene radical species is able to transfer a nitrene moiety to phosphines and abstract a hydrogen atom from weak C-H bonds, leading to the formation of oxidized products in modest yields.

Hydrogen adsorption and storage on Palladium – functionalized graphene with NH-dopant: A first principles calculation

We conducted a detailed theoretical investigation of the structural and electronic properties of single and double sided Pd-functionalized graphene and NH-doped Pd-functionalized graphene, which are shown to be efficient materials for hydrogen storage. Nitrene radical dopant was an effective addition required for enhancing the Pd binding on the graphene sheet as well as the storage of hydrogen. We found that up to eight H2 molecules could be adsorbed by double-sided Pd-functionalized graphene at 0K with an average binding energy in the range 1.315–0.567eVA gravimetric hydrogen density of 3.622wt% was reached in the Pd-functionalized graphene on both sides. The binding mechanism of H2 molecules came not only the polarization mechanism between Pd and H atoms but also from the binding of the Pd atoms on the graphene sheet and the orbital hybridization. The most crucial part of our work is measuring the effect of nitrene radical on the H2 adsorption on Pd-functionalized graphene. Our calculations predicted that the addition of NH radicals on Pd-functionalized graphene enhance the binding of H2 molecules, which helps also to avoid the desorption of Pd(H2)n (n=1–5) complexes from graphene sheet. Our results also predict Pd-functionalized NH-doped graphene is a potential hydrogen storage medium for on-board applications.

ChemInform Abstract: Substrate‐Controlled Rh(II)‐Catalyzed Single‐Electron‐Transfer (SET): Divergent Synthesis of Fused Indoles.

AbstractStyryl azides (I), (III), (V), (VII) and (IX) undergo Rh‐catalyzed intramolecular ring expansion producing a series of indole‐fused azetidines (II) and 1H‐carbazoles or related derivatives (IV), (VI), (VIII) and (X), resp., in moderate to good yields via a Rh2(III,II) nitrene radical.