The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising ...way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an FeIIIFeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIIIFeIVL. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self‐adaptation of the catalyst to the substrate needs.
Self‐adaptive catalyst: An efficient diiron catalyst mediates nitrene transfer to sulfides through an FeIV active state but self‐activates to FeV when facing aliphatic substrates that are harder to oxidize.
Metal‐catalyzed nitrene transfer reactions arouse intense interest as clean and efficient procedures for amine synthesis. Efficient Rh‐ and Ru‐based catalysts exist but Fe alternatives are actively ...pursued. However, reactive iron imido species can be very short‐lived and getting evidence of their occurrence in efficient nitrene‐transfer reactions is an important challenge. We recently reported that a diiron(III,II) complex is a very efficient nitrene‐transfer catalyst to various substrates. We describe herein how, by combining desorption electrospray ionization mass spectrometry, quantitative chemical quench experiments, and DFT calculations, we obtained conclusive evidence for the occurrence of an {FeIIIFeIVNTosyl} intermediate that is very active in H‐ion and nitrene‐transfer reactions. DFT calculations revealed a strong radical character of the tosyl nitrogen atom in very low‐lying electronic configurations of the FeIV ion which are likely to confer its high reactivity.
Nitrene transfer: An FeIIIFeIV imido intermediate is identified in nitrene‐transfer reactions by desorption electrospray ionization mass spectrometry (DESI‐MS). DFT calculations show that low‐lying FeIIIFeIII‐.N‐tosyl configurations play a major role in the high reactivity of the intermediate.
Mixed-valence non-heme diiron centers are present at the active sites of a few enzymes and confer them interesting reactivities with the two ions acting in concert. Related (μ-phenoxido)diiron ...complexes have been developed as enzyme mimics. They exhibit very rich spectroscopic properties enabling independent monitoring of each individual ion, which proved useful for mechanistic studies of catalytic hydrolysis and oxidation reactions. In our studies of such complexes, we observed that these compounds give rise to a wide variety of electron transfers (intervalence charge transfer), proton transfers (tautomerism), coupled electron and proton transfers (H. abstraction and PCET). In this minireview, we present and analyze the main results illustrating the latter aspects.
Mixed-valence non-heme diiron centers are present at the active sites of a few enzymes and confer them interesting reactivities with the two ions acting in concert. Related (μ-phenoxido)diiron ...complexes have been developed as enzyme mimics. They exhibit very rich spectroscopic properties enabling independent monitoring of each individual ion, which proved useful for mechanistic studies of catalytic hydrolysis and oxidation reactions. In our studies of such complexes, we observed that these compounds give rise to a wide variety of electron transfers (intervalence charge transfer), proton transfers (tautomerism), coupled electron and proton transfers (H
abstraction and PCET). In this minireview, we present and analyze the main results illustrating the latter aspects.
Mixed‐valence non‐heme diiron centers are present at the active sites of a few enzymes and confer them interesting reactivities with the two ions acting in concert. Related (μ‐phenoxido)diiron ...complexes have been developed as enzyme mimics. They exhibit very rich spectroscopic properties enabling independent monitoring of each individual ion, which proved useful for mechanistic studies of catalytic hydrolysis and oxidation reactions. In our studies of such complexes, we observed that these compounds give rise to a wide variety of electron transfers (intervalence charge transfer), proton transfers (tautomerism), coupled electron and proton transfers (H. ion and PCET). In this minireview, we present and analyze the main results illustrating the latter aspects.
This minireview is a tribute to Prof. J.‐M. Savéant who with his group deciphered a deprotonation‐induced valence inversion in a (μ‐phenoxido)FeIIFeIII complex and revealed that it operates through a concerted proton‐electron transfer. A survey of the diverse electron and proton transfers exhibited by this family of compounds is presented and sets up the stage of the field.
Aziridination has very recently been found to be catalyzed by heme and nonheme Fe enzymes, opening the way to biotechnological developments. However, its mechanism is not fully understood owing to ...the contrasting behaviors exhibited by several Fe catalysts. Indeed, whereas a few Fe catalysts exhibit an activity dominated by inductive effects, the activity of others reveal significant and even dominant radical delocalization. Therefore, no clear and general rationale of aziridination has yet emerged. Elaborating on our previous studies, we anticipated that replacing two pyridines of a pentanitrogen ligand by two quinolines would enhance the electron affinity of the corresponding imido FeIV active species and hence its aziridination activity. This proved to be the case, and Hammett correlations indicate an electrophilic active species and dominant inductive effects. The calculated reaction profile points to a two-step mechanism with the formation of the first C–N bond being rate-determining and involving a strong charge transfer in the transition state. The aziridine ring closure in the second step is almost barrierless. A clear correlation of aziridination yields with calculated EA for Fe-catalysts indicate that the dependence of aziridination efficacy on EA of active species is a quite general feature. To generalize this analysis, we reinvestigated a catalyst exhibiting a radical delocalization dominance. Indeed, a similar two-step mechanism was found, which involves a partial charge transfer in the C–N bond formation as all other cases. The interesting point is that owing to the strong steric hindrance of the catalyst substitution, the aziridine ring closure of the intermediate benzylic radical (second step) becomes rate-determining, thus explaining the dominance of the radical delocalization effect. Eventually, a general aziridination two-step mechanism has been rationalized, and EA thus appears as the key descriptor for Fe-based catalytic aziridination that can be used in a predictable way.
High-valent oxo-metal complexes are involved in key biochemical processes of selective oxidation and removal of xenobiotics. The catalytic properties of cytochrome P-450 and soluble methane ...monooxygenase enzymes are associated with oxo species on mononuclear iron haem and diiron non-haem platforms, respectively. Bio-inspired chemical systems that can reproduce the fascinating ability of these enzymes to oxidize the strongest C-H bonds are the focus of intense scrutiny. In this context, the development of highly oxidizing diiron macrocyclic catalysts requires a structural determination of the elusive active species and elucidation of the reaction mechanism. Here we report the preparation of an Fe(IV)(µ-nitrido)Fe(IV) = O tetraphenylporphyrin cation radical species at -90 °C, characterized by ultraviolet-visible, electron paramagnetic resonance and Mössbauer spectroscopies and by electrospray ionization mass spectrometry. This species exhibits a very high activity for oxygen-atom transfer towards alkanes, including methane. These findings provide a foundation on which to develop efficient and clean oxidation processes, in particular transformations of the strongest C-H bonds.
Nitrene transfer reactions have emerged as one of the most powerful and versatile ways to insert an amine function to various kinds of hydrocarbon substrates. However, the mechanisms of nitrene ...generation have not been studied in depth albeit their formation is taken for granted in most cases without definitive evidence of their occurrence. In the present work, we compare the generation of tosylimido iron species and NTs transfer from Fe
and Fe
precursors where the metal is embedded in a tetracarbene macrocycle. Catalytic nitrene transfer to reference substrates (thioanisole, styrene, ethylbenzene and cyclohexane) revealed that the same active species was at play, irrespective of the ferrous versus ferric nature of the precursor. Through combination of spectroscopic (UV-visible, Mössbauer), ESI-MS and DFT studies, an Fe
tosylimido species was identified as the catalytically active species and was characterized spectroscopically and computationally. Whereas its formation from the Fe
precursor was expected by a two-electron oxidative addition, its formation from an Fe
precursor was unprecedented. Thanks to a combination of spectroscopic (UV-visible, EPR, Hyscore and Mössbauer), ESI-MS and DFT studies, we found that, when starting from the Fe
precursor, an Fe
tosyliodinane adduct was formed and decomposed into an Fe
tosylimido species which generated the catalytically active Fe
tosylimide through a comproportionation process with the Fe
precursor.
Multicomponent reactions are attracting strong interest because they contribute to develop more efficient synthetic chemistry. Understanding their mechanism at the molecular level is thus an ...important issue to optimize their operation. The development of integrated experimental and theoretical approaches has very recently emerged as most powerful to achieve this goal. In the wake of our recent investigation of amidine synthesis, we used this approach to explore how an Fe-catalyzed aziridination can lead to an imidazoline when run in acetonitrile. We report that the synthesis of imidazoline by combination of styrene, acetonitrile, an iron catalyst and a nitrene precursor occurs along a new kind of multicomponent reaction. The formation of imidazoline results from acetonitrile interception of a benzyl radical styrene aziridination intermediate within Fe coordination sphere, as opposed to classical nucleophilic opening of the aziridine by a Lewis acid. Comparison of this mechanism to that of amidine formation allows a rationalization of the modes of intermediates trapping by acetonitrile according to the oxidation state Fe active species. The molecular understanding of these processes may help to design other multicomponent reactions.
Integrated experimental and computational studies reveal a new mechanism for Fe-catalyzed imidazoline synthesis through combined nitrene transfer and acetonitrile attack of a styrenyl radical intermediate.
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