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.

A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NAr^Trip)] (3; IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene; Ar^Trip = 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide Ar^TripN₃. Spectroscopic investigations in combination with DFT calculations established a high-spin S = 2 ground spin state for 3, consistent with its long Fe–N multiple bond of 1.715(2) Å revealed by X-ray diffraction analysis. Complex 3 exhibits unusual activity of nitrene transfer and C–H bond activation in comparison to the reported iron imido complexes. Specifically, the reactions of 3 with CH₂=CHAr^CF3, an electron-deficient alkene, and CO, a strong π acid, readily afford nitrene transfer products, Ar^CF3CH=CHNHAr^Trip and Ar^TripNCO, respectively, yet no similar reaction occurs when 3 is treated with electron-rich alkenes and PMe₃. Moreover, 3 is inert toward the weak C(sp³)–H bonds in 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave the stronger C(sp)–H bond in p-trifluoromethylphenylacetylene to form an iron(II) amido alkynyl complex. Interestingly, intramolecular C(sp³)–H bond functionalization was observed by adding (p-Tol)₂CN₂ to 3. The unique reactivity of 3 is attributed to its low-coordinate nature and the high negative charge population on the imido N atom, which render its iron–imido unit nucleophilic in nature.

The two-coordinate iron(II) imido complex (IPr)Fe(NAr^Trip) (3) exhibits a high-spin ground state (S = 2) and was found to be reactive toward electron-deficient alkene, diazo compounds, terminal alkyne, et al., in which diversified reactivities of nitrene transfer, C−H bond activation, and C−N bond formation have been observed. The reactivity pattern reflects the nucleophilic nature of the imido moiety of the high-spin iron(II) complex.

Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions: Impact of Fe-C Covalency in (Me2IPr)Fe(═NAd)R2 (R = neoPe, 1-nor)

The iron(IV) imide complexes, (Me₂IPr)-R₂Fe=NAd (R = ^neoPe (3a), 1-nor (3b)) undergo migratory insertion to iron(II) amides (Me₂IPr)RFe{NR(Ad)} (R = ^neoPe (4a), 1-nor (4b)) without evidence of imidyl or free nitrene character. By increasing the field strength about iron, odd-electron reactivity was circumvented via increased covalency.

Iron-Catalyzed C-H Bond Activation

Catalytic C-H bond activation, which was an elusive subject of chemical research until the 1990s, has now become a standard synthetic method for the formation of new C-C and C-heteroatom bonds. The synthetic potential of C-H activation was first described for ruthenium catalysis and is now widely exploited by the use of various precious metals. Driven by the increasing interest in chemical utilization of ubiquitous metals that are abundant and nontoxic, iron catalysis has become a rapidly growing area of research, and iron-catalyzed C-H activation has been most actively explored in recent years. In this review, we summarize the development of stoichiometric C-H activation, which has a long history, and catalytic C-H functionalization, which emerged about 10 years ago. We focus in this review on reactions that take place via reactive organoiron intermediates, and we excluded those that use iron as a Lewis acid or radical initiator. The contents of this review are categorized by the type of C-H bond cleaved and the type of bond formed thereafter, and it covers the reactions of simple substrates and substrates possessing a directing group that anchors the catalyst to the substrate, providing an overview of iron-mediated and iron-catalyzed C-H activation reported in the literature by October 2016.

Styrene Aziridination by Iron(IV) Nitrides

Thermolysis of the iron(IV) nitride complex [PhB(tBuIm)3Fe≡N] with styrene leads to formation of the high-spin iron(II) aziridino complex [PhB(tBuIm)3Fe-N(CH2CHPh)]. Similar aziridination occurs with both electron-rich and electron-poor styrenes, while bulky styrenes hinder the reaction. The aziridino complex [PhB(tBuIm)3Fe-N(CH2CHPh)] acts as a nitride synthon, reacting with electron-poor styrenes to generate their corresponding aziridino complexes, that is, aziridine cross-metathesis. Reaction of [PhB(tBuIm)3Fe-N(CH2CHPh)] with Me3SiCl releases the N-functionalized aziridine Me3SiN(CH2CHPh) while simultaneously generating [PhB(tBuIm)3FeCl]. This closes a synthetic cycle for styrene azirdination by a nitride complex. While the less hindered iron(IV) nitride complex [PhB(MesIm)3Fe≡N] reacts with styrenes below room temperature, only bulky styrenes lead to tractable aziridino products.

A mechanistic investigation of the ruthenium porphyrin catalysed aziridination of olefins by aryl azides

A combination of DFT and kinetic studies suggests a mechanism for olefin aziridination by organic azides catalysed by ruthenium porphyrins.

Synthesis of reduction-sensitive 1,1-diarylhydrazines from 1,1-diarylamines

1-(2-Nitrophenyl)-1-phenylamine and methyl 4-((2-nitrophenyl)amino)benzoate have been transformed into their corresponding urea derivatives through the action of chlorosulfonyl isocyanate. The initial sulfimidate product from the former reaction has sufficient stability so that it can be isolated and characterized as its disodium salt, and this, as well as three other subsequent products, have been characterized by X-ray diffraction. The corresponding intermediary urea was converted into its hydrazine derivative via a Hofmann rearrangement under oxidative conditions. Density functional theory has been used to examine the nature of the intermediates and transition states for the Hofmann rearrangement. There is little theoretical indication for a cyclic aziridinonium intermediate and the transition state between the urea and the isocyanate corresponds to a reactant-like rotation of the planar singlet nitrene before migration and formation of the new N−N bond.

Comparison of the reactivity of nonheme iron(IV)-oxo versus iron(IV)-imido complexes: which is the better oxidant?

Which is better? The first detailed comparison of the reactivity of nonheme iron(IV)-imido versus nonheme iron(IV)-oxo intermediates with substrates is presented. The iron(IV)-imido variant reacts with sulfides five times faster than iron(IV)-oxo, whereas the reverse trend is observed for hydrogen atom abstraction. These observed trends are analyzed and explained.

Generation of a high-valent iron imido corrolazine complex and NR group transfer reactivity

The generation of a new high-valent iron terminal imido complex prepared with a corrolazine macrocycle is reported. The reaction of [Fe(III)(TBP8Cz)] (TBP8Cz = octakis(4-tert-butylphenyl)corrolazinato) with the commercially available chloramine-T (Na(+)TsNCl(-)) leads to oxidative N-tosyl transfer to afford [Fe(IV)(TBP8Cz(+•))(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV-vis, Mössbauer (δ = -0.05 mm s(-1), ΔE(Q) = 2.94 mm s(-1)), and EPR (X-band (15 K), g = 2.10, 2.00) spectroscopies, and together with reactivity patterns and DFT calculations has been established as an iron(IV) species antiferromagnetically coupled with a Cz-π-cation-radical (S(total) = 1/2 ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe(IV)(TBP8Cz(+•))(NTs)] is an efficient NTs transfer agent, affording the phospharane product Ph3P═NTs under both stoichiometric and catalytic conditions. Kinetic analysis of this reaction supports a bimolecular NTs transfer mechanism with rate constant of 70(15) M(-1) s(-1). These data indicate that [Fe(IV)(TBP8Cz(+•))(NTs)] reacts about 100 times faster than analogous Mn terminal arylimido corrole analogues. It was found that two products crystallize from the same reaction mixture of Fe(III)(TBP8Cz) + chloramine-T + PPh3, [Fe(IV)(TBP8Cz)(NPPh3)] and [Fe(III)(TBP8Cz)(OPPh3)], which were definitively characterized by X-ray crystallography. The sequential production of Ph3P═NTs, Ph3P═NH, and Ph3P═O was observed by (31)P NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe(III) complex was then rationally synthesized and structurally characterized from Fe(III)(TBP8Cz) and OPPh3, providing an important benchmark compound for spectroscopic studies. A combination of Mössbauer and EPR spectroscopies led to the characterization of both intermediate spin (S = 3/2 and low spin (S = 1/2) Fe(III) corrolazines, as well as a formally Fe(IV) corrolazine which may also be described by its valence tautomer Fe(III)(Cz(+•)).