Isolable Anion Radicals of Nitrosoarenes

Summary of main observation and conclusion The rich redox chemistry of nitrosoarenes has rendered these reactive molecules very useful in modern synthetic and material chemistry.Electrochemical studies have revealed the capability of nitrosoarenes to undergo one-electron oxidation or reduction reaction for a long time.However,the isolation and structural characterization of nitrosoarene radical compounds deviating the stabilization of transition-metal have not been achieved.Investigation on the reduction reaction of nitrosoarenes bearing steric demanding substituents has now revealed that the interaction of 2,6-dimesityl-1-nitroso-benzene(DmpNO)or 2,4,6-tri(tert-butyl)-1-nitroso-benzene(TtpNO)with KC8 and crypt-2,2,2 can produce the corresponding anion radical compound[K(crypt-2,2,2)][DmpNO](1)or[K(crypt-2,2,2)][TtpNO](2)in good isolated yield.Compounds 1 and 2 represent the first examples of isolable nitrosoarene radical compounds deviating the stabilization of transition-metal,and have been characterized by single-crystal X-ray diffraction study,electron paramagnetic resonance(EPR)spectroscopy,and elemental analysis.Theoretical study in collaboration with the characterization data revealed that the unpaired spin in[DmpNO]·-and[TtpNO]·-delocalizes on the nitroso and the central phenyl groups.     

Isolable, gold carbonyl complexes supported by N-heterocyclic carbenes

Cationic gold carbonyl complexes supported by N-heterocyclic carbene ligands, SIDipp and IDipp, have been synthesized. [(SIDipp)Au(CO)][SbF(6)] has a linear, two-coordinate gold center. [(SIDipp)Au(CO)][SbF(6)] and [(IDipp)Au(CO)][SbF(6)] display ?ν(CO) values at 2197 and 2193 cm(-1), respectively. Computational studies on [(SIMe)Au(CO)](+) indicate the presence of a strong Au(I)-CO bond.     

Isolable acetylene complexes of copper and silver

Copper and silver play important roles in acetylene transformations but isolable molecules with acetylene bonded to Cu(i) and Ag(i) ions are scarce. This report describes the stabilization of π-acetylene complexes of such metal ions supported by fluorinated and non-fluorinated, pyrazole-based chelators. These Cu(i) and Ag(i) complexes were formed readily in solutions under an atmosphere of excess acetylene and the appropriate ligand supported metal precursor, and could be isolated as crystalline solids, enabling complete characterization using multiple tools including X-ray crystallography. Molecules that display κ²-or κ³-ligand coordination modes and trigonal planar or tetrahedral metal centers have been observed. Different trends in coordination shifts of the acetylenic carbon resonance were revealed by ¹³C NMR spectroscopy for the Cu(i) and Ag(i) complexes. The reduction in acetylene _{C C} due to metal ion coordination is relatively large for copper adducts. Computational tools were also used to quantitatively understand in detail the bonding situation in these species. It is found that the interaction between the transition metal fragment and the acetylene ligand is significantly stronger in the copper complexes, which is consistent with the experimental findings. The C C distance of these copper and silver acetylene complexes resulting from routine X-ray models suffers due to incomplete deconvolution of thermal smearing and anisotropy of the electron density in acetylene, and is shorter than expected. A method to estimate the C C distance of these metal complexes based on their experimental _{C C} is also presented.

Gaseous acetylene can be trapped on copper(i) and silver(i) sites supported by pyrazole-based scorpionates to produce isolable molecules for detailed investigations and the study of metal-acetylene bonding.     

An Isolable Potassium Salt of a Borasilene–Chloride Adduct

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon–boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([ 2 ]⁻), which was obtained as an orange crystalline solid. Single‐crystal X‐ray diffraction analysis and reactivity studies on [ 2 ]⁻ confirmed the double‐bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl⁻ to the boron center. A thermal reaction of [ 2 ]⁻ afforded a bicyclic product by formal intramolecular C−H insertion across the Si=B bond of 1 , which was corroborated by a theoretical study.     

Methane Oxidation on the Surfaces of Manganese Oxides: a First Principles Study

A comprehensive density functional theory calculation was employed to investigate the possible reaction pathways and mechanisms of methane complete oxidation(CH_4 + 2O_2 → CO_2 + 2H_2O) on different manganese oxides including a-MnO_2(100) and b-MnO_2(111) surfaces. According to a coupling of the Mars-van Krevelen and Langmuir-Hinshelwood mechanism, the activation energy barrier and the reaction energy of each elementary surface reaction were determined. Our calculated results show that the detailed processes for methane oxidation on two surfaces are different due to the differences in the surface structure. The breaking of the last C–H bond of CH_4 moleculeis the rate-determining step with an activation barrier of 0.85 eV for a-MnO_2(100) surface. By contrast, the overall reaction rate on b-Mn O_2(111) surface is limited by the dissociation of the second O_2 molecule adsorbed on the vacancy site, and re-oxidation of the reduced b-MnO_2(111) surface by the gaseous oxygen requires a much higher energy barrier of 1.44 eV. As a result, the a-Mn O_2(100) exhibits superior activity and durability in the methane oxidation reaction than b-MnO_2(111) surface. The present study provides insight into understanding the structure-catalytic activity relationship of the catalysts based on manganese oxides towards the methane oxidation reaction.     

The Oxidation of Phenylhydrazine by Tyrosinase

Tyrosinase was found to catalyze the oxidation of phenylhydrazine to phenol in a reaction that did not resemble those typically performed by tyrosinase. The kinetics of this reaction was investigated by measuring the initial velocity of the formation of phenol (25 °C). The values of k _cat and K _M for the oxidation of phenylhydrazine were obtained as 11.0 s^?1 and 0.30 mM, respectively. The generation of superoxides during the oxidation of phenylhydrazine by tyrosinase was monitored by nitroblue tetrazolium (NBT) assay. In the phenylhydrazine-tyrosinase reaction, 1 mol O₂ was required for the production of 1 mol phenol and 1/6 mol superoxide. The decomposition of superoxide by superoxide dismutase enhanced the rate constant of the oxidation of phenylhydrazine. Phenol formed in the oxidation of phenylhydrazine by tyrosinase was further oxidized by tyrosinase to an o-quinone, after the oxidation of phenylhydrazine by tyrosinase was almost completed.     

An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone

The first isolable molecular silicon dicarbonate complex (bis‐NHC)Si(CO₃)₂ 2 (bis‐NHC=H₂C[{NC(H)=C(H)N(Dipp)}C:]₂, Dipp=2,6‐iPr₂C₆H₃) was synthesized by facile reaction of the bis‐N‐heterocyclic carbene stabilized silylone (bis‐NHC)Si 1 , bearing a zero‐valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis‐NHC)SiO₂ 3 supported by the bis‐NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero‐valent silicon atom in 1 by two molar equivalents of CO₂ under liberation of CO; its subsequent Lewis acid–base reaction with CO₂ leads to 2 which has been fully characterized including an single‐crystal X‐ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum‐chemically.     

The Stereoselective Oxidation of para-Substituted Benzenes by a Cytochrome P450 Biocatalyst

The serine 244 to aspartate (S244D) variant of the cytochrome P450 enzyme CYP199A4 was used to expand its substrate range beyond benzoic acids. Substrates, in which the carboxylate group of the benzoic acid moiety is replaced were oxidised with high activity by the S244D mutant (product formation rates >60 nmol.(nmol-CYP)⁻¹.min⁻¹) and with total turnover numbers of up to 20,000. Ethyl α-hydroxylation was more rapid than methyl oxidation, styrene epoxidation and S-oxidation. The S244D mutant catalysed the ethyl hydroxylation, epoxidation and sulfoxidation reactions with an excess of one stereoisomer (in some instances up to >98 %). The crystal structure of 4-methoxybenzoic acid-bound CYP199A4 S244D showed that the active site architecture and the substrate orientation were similar to that of the WT enzyme. Overall, this work demonstrates that CYP199A4 can catalyse the stereoselective hydroxylation, epoxidation or sulfoxidation of substituted benzene substrates under mild conditions resulting in more sustainable transformations using this heme monooxygenase enzyme.     

Oxidation of a water-soluble porphyrin complex by bromate

The water-soluble iron(III) complex of meso-tetra(4-sulfonatophenyl)porphine is oxidized by bromate in nearly neutral aqueous solutions in an overall six-electron three-step consecutive reaction. Kinetics and mechanism of the reaction are described.     

Dithiirane or thiosulfine intermediates in the reaction of acetyl diaroylchloromethyl disulfides with secondary amines?

Summary Since 1979,Senning's acetyl dibenzoylchloromethyl disulfide has been known as one of the first thiosulfine/dithiirane precursors. Its reactions with an excess of morpholine fulfill three advantageous conditions at once: thiosulfine or dithiirane species are intercepted by intermolecular additions, without heating, and without rearrangements. In the present work, reactions between a series of new acetyl diaroylchloromethyl disulfides ((4-X-C₆H₄-CO)₂-CCl-SSCOCH₃;X=F, Cl, Br, CH₃, and CH₃O) and an excess of morpholine are studied. As dominating products, 2-(4-morpholinyldithio)-1,3-diaryl-1,3-propanediones are obtained. The reactions are complete within several seconds. In order to obtain high yields of interception products, the mixtures have to be worked up within a few minutes. From the observation of high reaction rates at room temperature, a new reaction mechanism is deduced that involves the initial formation of dithiirane species (alone or before thiosulfine species).Dedicated to Prof.Peter K. Claus on the occasion of his 60th birthday     

The First Catalytic Asymmetric Nitro-Mannich-Type Reaction Promoted by a New Heterobimetallic Complex

The best ratio is 1:1:3 for the components Yb, K, and binaphthol in the new heterobimetallic complex, which efficiently catalyzes an asymmetric nitro-Mannich-type reaction. The desired nitro-Mannich products 2 are obtained with up to 91 % ee starting from N-phosphinoyl imines 1.