Etienne Derat

Complexes of Borane and N-Heterocyclic Carbenes : A New Class of Radical Hydrogen Atom Donor

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

Generation and Reactions of an Unsubstituted N‐Heterocyclic Carbene Boryl Anion

Lying low: A lithiated unsubstituted N-heterocyclic carbene (NHC) boryl anion can be generated by reduction, and trapped by electrophiles (see scheme; dipp=2,6-diisopropylphenyl) to provide new substituted NHC boranes. It is yet another example of a low-valent boron compound or boron-containing reactive intermediate stabilized by an NHC, thereby extending the scope of NHC borane chemistry. Copyright

Silicates as Latent Alkyl Radical Precursors: Visible‐Light Photocatalytic Oxidation of Hypervalent Bis‐Catecholato Silicon Compounds

This works introduces hypervalent bis-catecholato silicon compounds as versatile sources of alkyl radicals upon visible-light photocatalysis. Using Ir[(dF(CF3)ppy)2(bpy)](PF6) (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, bpy = bipyridine) as catalytic photooxidant, a series of alkyl radicals, including highly reactive primary ones can be generated and engaged in various intermolecular homolytic reactions. Based on cyclic voltammetry, Stern-Volmer studies, and supported by calculations, a mechanism involving a single-electron transfer from the silicate to the photoactivated iridium complex has been proposed. This oxidative photocatalyzed process can be efficiently merged with nickel-catalyzed Csp2-Csp3 cross-coupling reactions.

Regioselective Activation of Oxo Ligands in Functionalized Dawson Polyoxotungstates

The organic side chain of tin-substituted Dawson polyoxotungstates alpha1- and alpha2-[P2W17O61{SnCH2CH2COOH}]7- can be used to direct regioselective acylations of oxo ligands in the inorganic backbone, which was examined both experimentally and computationally. Acylation of the oxo ligand gave exalted electrophilicity to the acyl moiety, and the compounds that were obtained led to direct ligation of POMs to complex organic molecules.

Expeditious synthesis of phenanthridines from benzylamines via dual palladium catalysis.

A method for the synthesis of phenanthridines from benzylamines and aryl iodides which uses a dual palladium-catalyzed process is developed. The domino sequence ends via an intramolecular amination and an oxidative dehydrogenation. No protecting group or prefunctionalization of the amine is required, and the process uses dioxygen as the terminal oxidant.

Structure and quantum chemical characterization of chloroperoxidase compound 0, a common reaction intermediate of diverse heme enzymes

We have determined the crystal structure of the chloroperoxidase (CPO) hydroperoxo reaction intermediate (CPO compound 0) at 1.75-Å resolution. The intermediate was generated through controlled photoreduction of the CPO oxygen complex during x-ray data collection, which was monitored by recording of the crystal absorption spectra. Initially, the peroxo-anion species was formed and then protonated to yield compound 0. Quantum chemical calculations indicate that the peroxo-anion species is not stable and collapses instantaneously to compound 0. Compound 0 is present in the ferric low-spin doublet ground state and is characterized by a long OO bond length of 1.5 Å and a FeO bond distance of 1.8 Å, which is also observed in the crystal structure.

C-H activation/functionalization catalyzed by simple, well-defined low-valent cobalt complexes.

A facile C-H activation and functionalization of aromatic imines is presented using low-valent cobalt catalysts. Using Co(PMe3)4 as catalyst we have developed an efficient and simple protocol for the C-H/hydroarylation of alkynes with an anti selectivity. Deuterium-labeling experiments, DFT calculations coupled with the use of a well-defined catalyst have for the first time shed light on the elusive black box of cobalt catalyzed C-H functionalization.

Quantum Mechanical/Molecular Mechanical Study on the Mechanisms of Compound I Formation in the Catalytic Cycle of Chloroperoxidase: An Overview on Heme Enzymes

The formation of Compound I (Cpd I), the active species of the enzyme chloroperoxidase (CPO), was studied using QM/MM calculation. Starting from the substrate complex with hydrogen peroxide, FeIII-HOOH, we examined two alternative mechanisms on the three lowest spin-state surfaces. The calculations showed that the preferred pathway involves heterolytic O-O cleavage that proceeds via the iron hydroperoxide species, i.e., Compound 0 (Cpd 0), on the doublet-state surface. This process is effectively concerted, with a barrier of 12.4 kcal/mol, and is catalyzed by protonation of the distal OH group of Cpd 0. By comparison, the path that involves a direct O-O cleavage from FeIII-HOOH is less favored. A proton coupled electron transfer (PCET) feature was found to play an important role in the mechanism nascent from Cpd 0. Initially, the O-O cleavage progresses in a homolytic sense, but as soon as the proton is transferred to the distal OH, it triggers an electron transfer from the heme-oxo moiety to form water and Cpd I. This study enables us to generalize the mechanisms of O-O activation, elucidated so far by QM/MM calculations, for other heme enzymes, e.g., cytochrome P450cam, horseradish peroxidase (HRP), nitric oxide synthase (NOS), and heme oxygenase (HO). Much like for CPO, in the cases of P450 and HRP, the PCET lowers the barrier below the purely homolytic cleavage alternative (in our case, the homolytic mechanism is calculated directly from FeIII-HOOH). By contrast, the absence of PCET in HO, along with the robust water cluster, prefers a homolytic cleavage mechanism.

From molecular copper complexes to composite electrocatalytic materials for selective reduction of CO2 to formic acid

The development of new energy storage technologies is central to solving the challenges facing the widespread use of renewable energies. An option is the reduction of carbon dioxide (CO2) into carbon-based products which can be achieved within an electrochemical cell. Future developments of such processes depend on the availability of cheap and selective catalysts at the electrode. Here we show that a unique well-characterized active electrode material can be simply prepared via electrodeposition from a molecular copper complex precursor. The best performances, namely activity (150 mV onset overpotential and 1 mA cm−2 current density at 540 mV overpotential), selectivity (90% faradaic yield) and stability for electrocatalytic reduction of CO2 into formic acid in DMF/H2O (97 : 3 v/v) have been obtained with the [Cu(cyclam)](ClO4)2 complex (cyclam = 1,4,8,11-tetraazacyclotetradecane) as the precursor. Remarkably the organic ligand of the Cu precursor remains part of the composite material and the electrocatalytic activity is greatly dependent on the nature of that organic component.

Radical Pd(III)/Pd(I) reductive elimination in palladium sequences

Open-shell mechanisms are often at work in catalytic sequences involving first-row transition metals while usually not considered in palladium chemistry. Herein a computational study suggests their possible relevance in catalytic methods involving paramagnetic Pd(iii) intermediates. Indeed C-C bond forming reductive elimination previously thought to occur in Pd(iv) complexes has lower barriers in neutral, radical Pd(iii) intermediates instead. These species could form upon addition on Pd(ii) of an aryl radical generated via single electron transfer from a photo-active ruthenium complex and have the perfect stereoelectronic arrangement to smoothly undergo the coupling process.