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Heterolytic Cleavage (heterolytic + cleavage)

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Chemistry100%

Selected Abstracts

Mechanism of Oxidation of (olefin)RhI and -IrI Complexes by H2O2

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2004
Peter H. M. Budzelaar
Abstract A DFT study of the oxidation of (tpa)MI(C2H4)+ and (dpa-R)MI(cod)+ complexes (M = Rh, Ir) by H2O2 indicates that the reaction starts with heterolytic cleavage of the peroxide O,O bond, leading to MIII(olefin)(OH)2+ species. These can then undergo cyclisation, followed by deprotonation to oxetanes. In the oxidation of COD complexes, further cyclisation to internal ethers has a modest barrier, and the observed products (ethers for Rh, oxetanes for Ir) are suggested to be the thermodynamic ones. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Nucleophilicities and Nucleofugalities of Organic Carbonates,

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2010
Nicolas Streidl
Abstract The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/Vis spectrophotometry. Substitution of the resulting second-order rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log,k = s(N + E) yielded the nucleophilicity parameters N25 = 16.03 and s25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first-order rate constants and the known electrofugality parameters Ef of benzhydrylium ions were used to determine the nucleofugality parameters Nf and sf of the ROCO2, groups by using the linear free energy relationship log,k = sf(Nf + Ef). The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO2, over MeOCO2, and iBuOCO2, to tBuOCO2, in various alcoholic and aqueous solvents. tert -Butyl carbonates (tBocO-R) are, thus, considerably more stable with respect to heterolytic cleavage of the O,R bond than other organic carbonates. [source]

Coordination chemistry of iron(III),porphyrin,antibody complexes

FEBS JOURNAL, Issue 2 2002
Influence on the peroxidase activity of the axial coordination of an imidazole on the iron atom
An artificial peroxidase-like hemoprotein has been obtained by associating a monoclonal antibody, 13G10, and its iron(III),,,,,,,,- meso -tetrakis(ortho -carboxyphenyl)porphyrin [Fe(ToCPP)] hapten. In this antibody, about two-thirds of the porphyrin moiety is inserted in the binding site, its ortho -COOH substituents being recognized by amino-acids of the protein, and a carboxylic acid side chain of the protein acts as a general acid base catalyst in the heterolytic cleavage of the O,O bond of H2O2, but no amino-acid residue is acting as an axial ligand of the iron. We here show that the iron of 13G10,Fe(ToCPP) is able to bind, like that of free Fe(ToCPP), two small ligands such as CN,, but only one imidazole ligand, in contrast to to the iron(III) of,Fe(ToCPP) that binds two. This phenomenon is general for a series of monosubstituted imidazoles, the 2- and 4-alkyl-substituted imidazoles being the best ligands, in agreement with the hydrophobic character of the antibody binding site. Complexes of antibody 13G10 with less hindered iron(III),tetraarylporphyrins bearing only one [Fe(MoCPP)] or two meso-[ortho -carboxyphenyl] substituents [Fe(DoCPP)] also bind only one imidazole. Finally, peroxidase activity studies show that imidazole inhibits the peroxidase activity of 13G10,Fe(ToCPP) whereas it increases that of 13G10,Fe(DoCPP). This could be interpreted by the binding of the imidazole ligand on the iron atom which probably occurs in the case of 13G10,Fe(ToCPP) on the less hindered face of the porphyrin, close to the catalytic COOH residue, whereas in the case of 13G10,Fe(DoCPP) it can occur on the other face of the porphyrin. The 13G10,Fe(DoCPP),imidazole complex thus constitutes a nice artificial peroxidase-like hemoprotein, with the axial imidazole ligand of the iron mimicking the proximal histidine of peroxidases and a COOH side chain of the antibody acting as a general acid-base catalyst like the distal histidine of peroxidases does. [source]

Computational study of base-induced skeletal conversion via a spirocyclic intermediate in dibenzodithiocinium derivatives by ab initio MO calculations

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2003
Keiji Okada
Abstract Reaction of 6-methyl-12-oxo-5H,7H -dibenzo[b,g][1,5]dithiocinium salt (1-SO) with methanolic KOH afforded a mixture of dibenzothiepin derivatives 2-SO. In order to clarify the intermediates of the rearrangement, ab initio MO calculations with the HF/6,31G* basis set were performed on the reaction intermediates, the transition states and related compounds. The rearrangement was explained in terms of the usual [2,3]-sigmatropic shift via a spirocyclic intermediate, followed by a 1,3-shift of the sulfonyl group. However, a different type of rearrangement was observed in 6-methyl-5H,7H -dibenzo[b,g][1,5]dithiocinium salt (1-S), giving an unexpected dibenzothiepin derivative 3-S along with a small amount of a ring-opening product 8 under the same reaction conditions. The formation of 3-S and 8 was understandable by the assumption of a cationic intermediate resulting from heterolytic cleavage at the benzyl position. Copyright © 2003 John Wiley & Sons, Ltd. [source]