Platinum(0)-mediated C–O bond activation of ethers via an SN2 mechanism

A computational study of the C(methyl)–O bond activation of fluorinated aryl methyl ethers by a platinum(0) complex Pt(PCyp3)2 (Cyp = cyclopentyl) (N. A. Jasim, R. N. Perutz, B. Procacci and A. C. Whitwood, Chem. Commun., 2014, 50, 3914) demonstrates that the reaction proceeds via an SN2 mechanism. Nucleophilic attack of Pt(0) generates an ion pair consisting of a T-shaped platinum cation with an agostic interaction with a cyclopentyl group and a fluoroaryloxy anion. This ion-pair is converted to a 4-coordinate Pt(II) product trans-[PtMe(OArF)(PCyp3)2]. Structure-reactivity correlations are fully consistent with this mechanism. The Gibbs energy of activation is calculated to be substantially higher for aryl methyl ethers without fluorine substituents and higher still for alkyl methyl ethers. These conclusions are in accord with the experimental results. Further support was obtained in an experimental study of the reaction of Pt(PCy3)2 with 2,3,5,6-tetrafluoro-4-allyloxypyridine yielding the salt of the Pt(η3-allyl) cation and the tetrafluoropyridinolate anion [Pt(PCy3)2(η3-allyl)][OC5NF4]. The calculated activation energy for this reaction is significantly lower than that for fluorinated aryl methyl ethers

Design of an enantioselective artificial metallo-hydratase enzyme containing an unnatural metal-binding amino acid

The design of artificial metalloenzymes is a challenging, yet ultimately highly rewarding objective because of the potential for accessing new-to-nature reactions. One of the main challenges is identifying catalytically active substrate-metal cofactor-host geometries. The advent of expanded genetic code methods for the in vivo incorporation of non-canonical metal-binding amino acids into proteins allow to address an important aspect of this challenge: the creation of a stable, well-defined metal-binding site. Here, we report a designed artificial metallohydratase, based on the transcriptional repressor lactococcal multidrug resistance regulator (LmrR), in which the non-canonical amino acid (2,2'-bipyridin-5yl) alanine is used to bind the catalytic Cu(II) ion. Starting from a set of empirical pre-conditions, a combination of cluster model calculations (QM), protein-ligand docking and molecular dynamics simulations was used to propose metallohydratase variants, that were experimentally verified. The agreement observed between the computationally predicted and experimentally observed catalysis results demonstrates the power of the artificial metalloenzyme design approach presented here

IrI(η4-diene) precatalyst activation by strong bases: formation of an anionic IrIII tetrahydride

The reaction between [IrCl(COD)]2 and dppe in a 1:2 ratio was investigated in detail under three different conditions. [IrCl(COD)(dppe)], 1, is formed at room temperature in the absence of base. In the presence of a strong base at room temperarture, hydride complexes that retain the carbocyclic ligand in the coordination sphere are generated. In isopropanol, 1 is converted into [IrH(1,2,5,6-η2:η2-COD)(dppe)] (2) on addition of KOtBu, with k12 = (1.11±0.02)·10-4 s-1, followed by reversible isomerisation to [IrH(1-κ-4,5,6-η3-C8H12)(dppe)] (3) with k23 = (3.4±0.2)·10-4 s-1 and k32 = (1.1±0.3)·10-5 s-1 to yield an equilibrium 5:95 mixture of 2 and 3. However, when no hydride source is present in the strong base (KOtBu in benzene or toluene), the COD ligand in 1 is deprotonated, followed by β-H elimination of an IrI-C8H11 intermediate, which leads to complex [IrH(1-κ-4,5,6-η3-C8H10)(dppe)] (4) selectively. This is followed by its reversible isomerisation to 5, which features a different relative orientation of the same ligands (k45 = (3.92±0.11)·10-4 s-1; k54 = (1.39±0.12)·10-4 s-1 in C6D6), to yield an equilibrated 32:68 mixture of 4 and 5. DFT calculations assisted in the full rationalization of the selectivity and mechanism of the reactions, yielding thermodynamic (equilibrium) and kinetic (isomerization barriers) parameters in excellent agreement with the experimental values. Finally, in the presence of KOtBu and isopropanol at 80 °C, 1 is tranformed selectively to K[IrH4(dppe)] (6), a salt of an anionic tetrahydride complex of IrIII. This product is also selectively generated from 2, 3, 4 and 5 and H2 at room temperature, but only when a strong base is present. These results provide an insight into the catalytic action of [IrCl(LL)(COD)] complexes in the hydrogenation of polar substrates in the presence of a base

Interaction of molecular H

The interaction of molecular hydrogen with some d6-ML5 and d8-ML4 metal complexes is studied by means of Extended Huckel calculations. In a dissymétric ligand field (σ-donor and π-acceptor ligands), the structure of the complex is governed, for the d6-ML5 metal fragment, by the amplitude of electron transfer toward [math], which is maximum for H2 lying in the plane of σ-donors. On the other hand, the structure of the complex with the d8-ML4 metal fragment is governed by the four-electron repulsion between z2 and σH2 orbitals : the addition of H2 leading to the dihydride adduct is easier in the plane of the π-acceptor ligands which decrease this repulsion at the transition state

Cobalt-Catalyzed Vinylation of Aromatic Halides Using β‑Halostyrene: Experimental and DFT Studies

A new protocol for the direct cobalt-catalyzed vinylation of aryl halides using β-halostyrene has been developed in order to form functionalized stilbenes. A variety of aromatic halides featuring different reactive group were employed. This method proceeded smoothly with a total retention of the double bond configuration in the presence of triphenylphosphine as ligand. Preliminary DFT calculations rationnalize these results and proposed a reaction pathway in agreement with the experimental conditions. This procedure offers a new route to the stereoselective synthesis of stilbenes

Nature of Cp*MoO 2 + in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations

International audienceA stopped-flow study of the Cp*MoO3- protonation at low pH (down to zero) in a mixed H2O−MeOH (80:20) solvent at 25 °C allows the simultaneous determination of the first acid dissociation constant of the oxo−dihydroxo complex, [Cp*MoO(OH)2]+ (pKa1 = −0.56), and the rate constant of its isomerization to the more stable dioxo−aqua complex, [Cp*MoO2(H2O)]+ (k-2 = 28 s-1). Variable-temperature (5−25 °C) and variable-pressure (10−130 MPa) kinetics studies have yielded the activation parameters for the combined protonation/isomerization process (k-2/Ka1) from Cp*MoO2(OH) to [Cp*MoO2(H2O)]+, viz., ΔH⧧ = 5.1 ± 0.1 kcal mol-1, ΔS⧧ = −37 ± 1 cal mol-1 K-1, and ΔV⧧ = −9.1 ± 0.2 cm3 mol-1. Computational analysis of the two isomers, as well as the [Cp*MoO2]+ complex resulting from the dissociation of water, reveals a crucial solvent effect on both the isomerization and the water dissociation energetics. Introducing a solvent model by the conductor-like polarizable continuum model and especially by explicitly inclusion of up to three water molecules in the calculations led to the stabilization of the dioxo−aqua species relative to the oxo−dihydroxo isomer and to the substantial decrease of the energy cost for the water dissociation process. The presence of a water dissociation equilibrium is invoked to account for the unusually low effective acidity (pKa1‘ = 4.19) of the [Cp*MoO2(H2O)]+ ion. In addition, the computational study reveals the positive role of external water molecules as simultaneous proton donors and acceptors, having the effect of dramatically lowering the isomerization energy barrier