Frédéric Avenier

A Diiron(III,IV) Imido Species Very Active in Nitrene‐Transfer Reactions

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 {Fe(III) Fe(IV) NTosyl} intermediate that is very active in H-abstraction and nitrene-transfer reactions. DFT calculations revealed a strong radical character of the tosyl nitrogen atom in very low-lying electronic configurations of the Fe(IV) ion which are likely to confer its high reactivity.

Intramolecular aromatic hydroxylation mediated by a dinuclear iron complex: an oxo-FeIV FeIV active intermediate is suggested

In the presence of oxygen atom donors [XO, e.g., m-chloroperbenzoic acid (m-CPBA), o-tert-butylsulfone iodosyl benzene (ArIO)] the benzyl group of the ligand in a mixed-valent FeIIFeIII complex is almost quantitatively ortho-hydroxylated to a phenolate terminally bound to one iron in the derived FeIIIFeIII complex. All available experimental evidence concurs to suggest that this reaction involves an oxo-FeIVFeIV intermediate.

Combining Medium Effects and Cofactor Catalysis: Metal-Coordinated Synzymes Accelerate Phosphate Transfer by 108

The systematic exploration of the modification of polyethylene imine with guanidinium and octyl groups has led to the identification of a catalyst, CD6, which accelerates the phosphate transfer reaction of HPNP (2-hydroxypropyl-4-nitrophenyl phosphate) in the presence of divalent metals such as Zn(2+), Co(2+), Mg(2+) or Ni(2+). CD6 exhibits saturation kinetics that are described by Michaelis-Menten parameters K(m) ranging from 2.5-8 mM and k(cat) ranging from 0.0014-0.09 s(-1). For Zn(II)-CD6 this corresponds to an overall acceleration k(cat)/k(uncat) of 3.8x10(5) and a catalytic proficiency (k(cat)/K(m))/k(uncat) of 1.5x10(8). Catalysis by Zn(II)-CD6 is specifically inhibited by inorganic phosphate, allowing turnover regulation by product inhibition. This effect stands in contrast to Zn(II)-catalysed transesterification of HPNP in water or by the synzymes Co(II)-CD6 and Ni(II)-CD6, with which no such interference by product is observed. These characteristics render synzyme Zn(II)-CD6 an efficient enzyme model that reflects enzyme-like properties in a wide range of features.

Redox Self‐Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

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 FeIII FeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIII FeIV L. 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.

Bio-inspired electron-delivering system for reductive activation of dioxygen at metal centres towards artificial flavoenzymes

Development of artificial systems, capable of delivering electrons to metal-based catalysts for the reductive activation of dioxygen, has been proven very difficult for decades, constituting a major scientific lock for the elaboration of environmentally friendly oxidation processes. Here we demonstrate that the incorporation of a flavin mononucleotide (FMN) in a water-soluble polymer, bearing a locally hydrophobic microenvironment, allows the efficient reduction of the FMN by NADH. This supramolecular entity is then capable of catalysing a very fast single-electron reduction of manganese(III) porphyrin by splitting the electron pair issued from NADH. This is fully reminiscent of the activity of natural reductases such as the cytochrome P450 reductases with kinetic parameters, which are three orders of magnitude faster compared with other artificial systems. Finally, we show as a proof of concept that the reduced manganese porphyrin activates dioxygen and catalyses the oxidation of organic substrates in water.

A diiron complex mediates an intramolecular aliphatic hydroxylation by various oxygen donors

In the presence of hydrogen peroxide, m-chloroperbenzoic acid or an iodosyl arene, the tert-butyl group of the ligand H(L-t-Bu) in the complex [Fe2(L-t-Bu)(mpdp)]2+ is quantitatively hydroxylated to a butanolate terminally bound to one iron in [Fe2(L-t-Bu - H + O)(mpdp)]2+, and mass spectrometry experiments indicate that the reaction proceeds according to different mechanisms.

Reduction of a tris(picolyl)amine copper(II) complex by a polymeric flavo-reductase model in water

Dioxygen activation at copper(i) centres is of primary importance for the development of sustainable oxidation catalysis, but regeneration of copper(i) centres after each catalytic cycle remains a major problem for multi-turn-over catalysis. This work demonstrates that an artificial reductase, made of flavin cofactors incorporated into a water soluble polymer, efficiently reduces a Cu(ii)TPA complex in the presence of NADH in water.