One-electron reduction of ferriporphyrins and reactions of ferric and ferrous porphyrins with a halothane-derived radical

Alkene Hydrobenzylation by a Single Catalyst That Mediates Iterative Outer-Sphere Steps

Cross-coupling catalysts typically react and unite functionally distinct partners via sequential inner-sphere elementary steps: coordination, migratory insertion, reductive elimination, etc. Here, we report a single catalyst that cross-couples styrenes and benzyl bromides via iterative outer-sphere steps: metal-ligand-carbon interactions. Each partner forms a stabilized radical intermediate, yet heterocoupled products predominate. The system is redox-neutral and, thus, avoids exogenous oxidants, resulting in simple and scalable conditions. Numerous variations of alkene hydrobenzylation are made possible, including access to the privileged heterodibenzyl (1,2-diarylethane) motif and challenging quaternary carbon variants.

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O₂ or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent Ni^III and structurally characterized mixed-valent Ni^II-Ni^IV intermediates and radical intermediates, resembling O₂ activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF₃)₂ complexes resulting in the formation of a stable Ni^III product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.

Environmental dehalogenation: chemistry and mechanism

The halogen cycle is one of the great chemical cycles on earth. Haloorganics are both synthesized and destroyed by the chemistry that controls their flux and form. The synthetic leg of the cycle is both biotic and abiotic in nature. The biotic synthesis results primarily from the biochemical activity of marine algae and kelp, although these are by no means the only sources. The abiotic process is vested in large part in volcanic eruption and emission of gases synthesized as a thermal consequence of venting the earth's core.

One-electron oxidation and reduction reactions of vitamin C derivatives: 6-bromo- and 6-chloro-6-deoxy-ascorbic acid

6-Bromo- and 6-chloro-6-deoxy derivatives of ascorbate anion are able to transfer an electron to the oxidizing radicals .OH, Br2.- and RS. with the same rate constants as the ascorbate anion itself. The resulting radicals also show the same kinetic stabilities and optical absorption spectra as the well-characterized ascorbate radical anion (lambda max = 360 nm; epsilon 360 = 330 m2 mol-1). This proves that there is no influence of the structural changes in the side chain on the antioxidant capacity of the ascorbate moiety. In contrast, measured reduction of the 6-halo-6-deoxy derivatives occurs significantly faster (up to one order of magnitude) than the reduction of unsubstituted ascorbate. For example, absolute rate constants of 4.6 x 10(9) and 2.2 x 10(7) dm3 mol-1 s-1 have been measured for the reactions of bromo-derivative with eaq- and (CH3)2COH respectively. These radical-induced reductions proceed via dissociative electron capture and, under cleavage of the C-halogen bond, yield C-6 carbon-centered radicals. In the presence of oxygen the corresponding peroxyl radical is readily formed. This radical is again able to oxidize the ascorbate moiety (rate constant 2 x 10(7) dm3 mol-1 s-1). Results are discussed in terms of biological relevance of the investigated compounds regarding their ability to act as efficient antioxidants and bioreductive antitumour agents simultaneously.

Free radical modification of prosthetic heme groups

Hemoproteins catalyze reductive and oxidative one-electron transformations. Not infrequently, the radicals produced by these one-electron reactions add to the prosthetic heme group of the enzyme and modify or terminate its catalytic function. Reactions of the radicals with the heme group include additions to the iron atom, pyrrole nitrogens, pyrrole carbons, vinyl groups, and meso carbons. The radicals involved in these reactions derive from the oxidizing agent, the substrate, or the amino acid residues of the catalytic site. The mechanism by which the radicals are generated, their steric and electronic properties, and the extent to which they have access to the heme group determine the nature and regiospecificity of the reaction. The reaction of heme prosthetic groups with radicals is relevant to the inhibition of hemoprotein enzymes, the normal and pathological degradation of heme, and our understanding of hemoprotein function.

Reversible H-atom abstraction from alcohols by thiyl radicals: determination of absolute rate constants by pulse radiolysis

Penicillamine thiyl radicals, PenS., are shown to abstract hydrogen atoms from 2-propanol and to establish an equilibrium PenS. + (CH3)2 CHOH in equilibrium PenSH + (CH3)2 COH. The rate constants for the forward and back reaction have been determined to (1.4 +/- 0.3) x 10(4) and (1.2 +/- 0.3) x 10(8) M-1 s-1, respectively, by means of pulse radiolysis. The data have been obtained from various independent methods which include direct measurements and competitive schemes involving irreversible interception of the alcohol radical by electron acceptors (e.g. CCl4, PNAP) and/or the thiyl radical by antioxidants (e.g. alpha-tocopherol). The results demonstrate that the reaction of carbon-centered radicals with thiols, in radiation biology commonly known as "repair" reaction, may be reversed and thus imply the possibility of thiyl radical induced biological damage.

The lipid peroxidation model for halogenated hydrocarbon toxicity. Kinetics of peroxyl radical processes involving fatty acids and Fe(III) porphyrins

The toxicity of halogenated alkanes originates from their metabolism by cytochrome P-450 which leads to the formation of reactive intermediates. In particular, peroxyl radicals derived from the halogenated compounds are believed to induce peroxidative chain degradation of lipids. To examine this hypothesis, radical reactions in a system involving FeIII-deuteroporphyrin as a model of cytochrome P-450, fatty acids or cholesterol, and carbon tetrachloride or the anesthetic agent halothane are studied by means of pulse radiolysis. It is shown that haloperoxyl radicals react with the fatty acids in competition with their reaction with the ferriporphyrin. Moreover, the secondary fatty acid peroxyl radicals also react efficiently with the porphyrin. A model for halogenated alkane toxicity is discussed in terms of these new findings. The importance of local oxygen concentration and structural arrangement of fatty acids around cytochrome P-450 are emphasized.