Reactions of iron porphyrins with methyl radicals

Metal-Hydride C-C Cross-Coupling of Alkenes Through a Double Outer-Sphere Mechanism

This Synopsis covers recent reports of metal-catalyzed alkene functionalizations that likely involve iterative outer-sphere reactions in which the substrate reacts directly with a metal ligand instead of with the metal center itself. Traditional metal hydride-catalyzed alkene functionalizations involve this latter pathway whereby the alkene forms part of the metal ligand sphere (i.e. an inner-sphere reaction). In contrast, alkenes do not ligate the metal in so-called outer-sphere reactions and instead react with a metal ligand. These transformations have proved crucial for the synthesis of high fraction sp3 (Fsp3) targets, especially in hindered fragment couplings of relevance to natural product space.

Carbon quaternization of redox active esters and olefins by decarboxylative coupling

The synthesis of quaternary carbons often requires numerous steps and complex conditions or harsh reagents that act on heavily engineered substrates. This is largely a consequence of conventional polar-bond retrosynthetic disconnections that in turn require multiple functional group interconversions, redox manipulations, and protecting group chemistry. Here, we report a simple catalyst and reductant combination that converts two types of feedstock chemicals, carboxylic acids and olefins, into tetrasubstituted carbons through quaternization of radical intermediates. An iron porphyrin catalyst activates each substrate by electron transfer or hydrogen atom transfer, and then combines the fragments using a bimolecular homolytic substitution (SH2) reaction. This cross-coupling reduces the synthetic burden to procure numerous quaternary carbon---containing products from simple chemical feedstocks.

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.

Iron-Catalyzed C-C Single-Bond Cleavage of Alcohols

An iron-catalyzed deconstruction/hydrogenation reaction of alcohols through C-C bond cleavage is developed through photocatalysis, to produce ketones or aldehydes as the products. Tertiary, secondary, and primary alcohols bearing a wide range of substituents are suitable substrates. Complex natural alcohols can also perform the transformation selectively. A investigation of the mechanism reveals a procedure that involves chlorine radical improved O-H homolysis, with the assistance of 2,4,6-collidine.

Radicals in 'biologically relevant' concentrations behave differently: Uncovering new radical reactions following the reaction of hydroxyl radicals with DMSO

Methyl radicals play key roles in various chemical and biological processes. Mechanistic studies of methyl radicals with their precursor, Dimethyl Sulfoxide (DMSO), were extensively studied. Though the involved mechanisms seemed to be clarified, essentially none of the studies have been performed at conditions relevant to both biological and catalytic systems, i.e. low steady state radical concentrations. A chain-like reaction, as an inverse function of the radicals concentrations ([^•CH₃]_ss), increases the methyl radical yields. The nature of the additional products obtained differs from those commonly observed. Furthermore it is shown that methyl radicals abstract a methyl group from DMSO to yield ethane. Herein we report a novel mechanism relevant mainly at low steady state radical concentrations, which may change the understanding of certain reaction routes present in both biological systems and catalytic chemical systems. Thus the results point out that mechanistic studies have to be carried out at dose rates forming radicals at analogous concentrations to those present in the process of interest.

Free radicals induced peptide damage in the presence of transition metal ions: a plausible pathway for biological deleterious processes

When aqueous N2O-saturated solutions containing glycine-N-tert-butylamide (L) and Cr2+ (aq) or Cu+ (aq) are irradiated, transients with metal-carbon sigma-bonds are formed with rate constants of (4.4 +/- 0.5) x 10(7) and (5.2 +/- 0.3) x 10(9) M-1 s-1, respectively. In the chromium(II) system, after a fast process (k = 43 +/- 4 s-1), possibly chelation, the transient decomposes very slowly (k = 0.003 +/- 0.001 h-1) via a beta-elimination process to yield 2-methylpropene and glycinamide, i.e., a cleavage of the peptide bond takes place. However, in the copper(I) system the heterolytic cleavage of the sigma-bond and the reaction of the transient complex with L-Cu2+ compete efficiently with the beta-elimination process. The latter reaction leads to some modification of the amide. We suggest that the formation and decomposition of transients with metal-carbon sigma-bonds may describe an additional pathway for peptide damage induced by aliphatic free radical precursors (e.g., OH., H2O2) in the presence of transition metal ions.

Growth inhibition of Plasmodium falciparum involving carbon centered iron-chelate radical (L., X-)-Fe(III) based on pyridoxal-betaine. A novel type of antimalarials active against chloroquine-resistant parasites

Malaria parasites have been shown to be more susceptible to oxidative stress than their host erythrocytes. In the present work, a chloroquine resistant malaria parasite, Plasmodium falciparum (FCR-3) was found to be susceptible in vitro to a pyridoxal based iron chelator--(1-[N-ethoxycarbonylmethylpyridoxlidenium]-2-[2'-pyridyl ] hydrazine bromide--(code named L2-9). 2h exposure to 20 microM L2-9 was sufficient to irreversibly inhibit parasite growth. Desferrioxamine blocked the drug effect, indicating the requirement for iron. Oxygen however, was not essential. Spectrophotometric analysis showed that under anoxic conditions, L2-9-Fe(II) chelate undergoes an intramolecular redox reaction which presumably involves a one electron transfer and is expected to result in the formation of free radical. Spin trapping coupled to electron spin resonance (ESR) studies of L2-9-iron chelate showed that L2-9-Fe(II) produced free radicals both in the presence and absence of cells, while L2-9-Fe(III) produced free radicals only in the presence of actively metabolising cells.

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.

Enhancement of the rate of the beta-elimination of phosphate from radicals derived from glycerol-2-phosphate by Cu(I)-phenanthroline. A pulse radiolysis study

Hydroxyl radicals abstract hydrogen atoms from glycerol-2-phosphate with a specific rate constant of (7.0 +/- 1.5) x 10(8) M-1s-1 forming the beta-phospho radical as the major product. At physiological pH this radical undergoes a beta-phosphate elimination with a rate constant less than or equal to 1 x 10(3) s-1. The beta-phospho radical reacts with Cu(I)-phenanthroline to produce an unstable transient with a metal-carbon sigma-bond which has an absorbance similar to that of the cuprous phenanthroline complex in the visible region. This intermediate decomposes via a beta-elimination of phosphate with a rate constant of (1.0 +/- 1.5) x 10(4) s-1, which was independent of the acidity in the pH range 4-9.

DNA alterations induced by the carbon-centered radical derived from the oxidation of 2-phenylethylhydrazine

The possible significance of carbon-centered radicals in hydrazine-induced carcinogenesis is explored by studies of the interaction between the 2-phenylethyl radical and DNA. The radical is efficiently generated during oxidation of phenelzine (2-phenylethylhydrazine) promoted by oxyhemoglobin or ferricyanide, as demonstrated by spin-trapping experiments and analysis of the reaction products. In the ferricyanide promoted oxidation, ethylbenzene formation accounts for about 40% of the initial drug concentration, from 5 to 100 mM phenelzine. By contrast, product formation in the presence of oxyhemoglobin depends on the enzyme concentration due to the fact that the prosthetic heme is destroyed during catalytic turnover. Covalent binding of the 2-phenylethyl radical to oxyhemoglobin is demonstrated by experiments with 2-[3H]phenelzine, where tritium incorporation to the protein is inhibited by the spin-trap, alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone. The 2-phenylethyl radical is also able to alkylate DNA as suggested by electrophoretic studies with plasmid DNA, and proved by experiments with 2-[3H]-phenelzine. The carbon-centered radical has a preference for attacking guanine residues as demonstrated by the use of sequencing techniques with 32P-DNA probes. The results indicate that the 2-phenylethyl radical is an important product of phenelzine oxidation and that this species can directly damage protein and DNA.

Hemoglobin-mediated oxidation of the carcinogen 1,2-dimethylhydrazine to methyl radicals

Oxidation of 1,2-dimethylhydrazine (SDMH) catalyzed by hemoglobin is investigated by oxygen consumption studies, ESR spin-trapping experiments, and gas chromatography. Kinetic analysis and the study of the effects of superoxide dismutase, catalase, and azide on reaction rates indicate that SDMH oxidation is primarily dependent on ferric hemoglobin and autoxidatively formed H2O2. SDMH oxidation generates both methyl and hydroxyl radicals as ascertained by spin-trapping experiments with alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, 5,5-dimethyl-1-pyrroline-N-oxide, and tert-nitrosobutane. Quantitative estimates indicate that the yield of the methyl radical trapped by alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone is about 8% of the consumed oxygen. Analysis of the gaseous products by gas chromatography shows methane formation at a yield 10 times lower than that obtained for the spin-trap methyl adduct. These results are discussed within the context of the spin-trapping technique. The relative efficiencies of oxyhemoglobin in catalyzing SDMH, 2-phenylethylhydrazine, and phenylhydrazine oxidation, defined as Vmax/KM, are estimated as 1, 13, and 386, respectively. The higher efficiency obtained for the monosubstituted derivatives leads us to suggest that hemoglobin-catalyzed oxidation could be a detoxification route for these compounds. By contrast, SDMH oxidation requires a peroxidase-like activity, a fact that may be related to the efficacy and specificity of this carcinogen.

Reductive oxygenation of carbon tetrachloride: trichloromethylperoxyl radical as a possible intermediate in the conversion of carbon tetrachloride to electrophilic chlorine

Under aerobic conditions, rat liver microsomes convert carbon tetrachloride to an electrophilic form of chlorine that is trapped with 2,6-dimethylphenol to form 4-chloro-2,6-dimethylphenol. The mechanism of cytochrome P-450-catalyzed electrophilic chlorine formation from carbon tetrachloride was examined with structure-activity studies of electrophilic halogen formation and chemical and in vitro microsomal studies. 4-Chloro-2,6-dimethylphenol is not formed as a consequence of a reaction of 2,6-dimethylphenoxyl radical with carbon tetrachloride or carbon tetrachloride-induced lipid peroxyl radical formation. Only tetrahalomethanes were found to yield electrophilic halogens. The chemical oxidants hydrogen peroxide, cumene hydropheroxide, sodium periodate, and iodobenzene diacetate did not support electrophilic halogen formation from carbon tetrachloride, carbon tetrabromide, or hexachloroethane in microsomal studies. The addition of superoxide dismutase, catalase, sodium azide, or glutathione to microsomal incubations did not affect the rate of electrophilic chlorine formation, whereas Paraquat completely inhibited the reaction. The radical spin trap phenyl t-butyl nitrone (14 mM) completely inhibited electrophilic chlorine formation. The rate of electrophilic chlorine formation was highest at 2-5% atmospheric oxygen, whereas anaerobiosis completely inhibited electrophilic chlorine formation, and high oxygen tension impaired electrophilic chlorine formation. These results preclude direct oxidation of carbon tetrachloride or a reaction of superoxide anion radical with carbon tetrachloride as the initial step in electrophilic chlorine formation and suggest that the likely initial step is reductive dehalogenation of carbon tetrachloride to trichloromethyl radical which then traps oxygen to form trichloromethylperoxyl radical. Subsequent reaction of trichloromethyl peroxyl radical leads to electrophilic chlorine. These findings may have important implications concerning carbon tetrachloride-induced lipid peroxidation and carbon tetrachloride hepatotoxicity.

Reaction of monosubstituted hydrazines and diazenes with rat-liver cytochrome P450. Formation of ferrous-diazene and ferric sigma-alkyl complexes

The alkyldiazenes RN = NH (R = CH3 or C2H5) react with reduced microsomal cytochrome P450 leading to complexes exhibiting a Soret peak at 446 nm. Upon oxidation of the [cytochrome P450-Fe(II)(CH3N = NH)] complex with limited amounts of dioxygen, a new complex characterized by a Soret peak at 486 nm is formed. The latter complex was also formed upon slow reaction of methyldiazene with microsomal cytochrome P450-Fe(III) or in situ oxidation of methylhydrazine by limited amounts of O2 or ferricyanide. This complex is rapidly destroyed by O2 or ferricyanide in excess and more slowly by excess dithionite in the presence of CO. Reactions of ethyldiazene or benzyldiazene with cytochrome P450-Fe(III) afforded similar complexes characterized by Soret peaks around 480 nm. These results, when compared to those recently described on reactions of monosubstituted hydrazines RNHNH2 and diazenes RN = NH with hemoglobin and iron-porphyrins, are consistent with a [cytochrome P450-Fe(II)(RN = NH)] structure for the 446-nm-absorbing complexes and a sigma-alkyl cytochrome P450-Fe(III)-R structure for the complexes characterized by a Soret peak around 480 nm. They also suggest a sigma-cytochrome P450-Fe(III)-Ph structure for the complex derived from phenylhydrazine oxidation, recently described in the literature. Finally, they provide the first evidence that cytochrome P450-Fe(III)-R complexes are formed upon microsomal oxidation of alkyl or phenylhydrazines.

Reduction of benzyl halides by liver microsomes. Formation of 478 NM-absorbing sigma-alkyl-ferric cytochrome P-450 complexes

The benzyl halides benzyl bromide and 4-nitrobenzyl chloride are reduced anaerobically by NADPH and rat liver microsomes to yield toluene and 4-nitrotoluene, respectively. These reductions and cytochrome P-450-dependent since they are inhibited by CO and metyrapone, and are increased after pretreatment of rats by phenobarbital and 3-methylcholanthrene. During benzyl halide reduction, cytochrome P-450 complexes, which are very unstable to O2 and characterized by a Soret peak at 478 nm, are formed in steady-state concentrations. These concentrations are very dependent on pretreatment of rats and on the nature of the reducing agent (NADPH or dithionite) and the benzyl halide:4-methylbenzyl bromide and benzyl bromide lead to 478 nm absorbing complexes in the presence of NADPH whereas 4-nitrobenzyl chloride and benzyl chloride lead to such completes only in the presence of dithionite. Microsomal reductions of 4-nitrobenzyl chloride and benzyl bromide in D2O lead to partially deuterated 4-nitrotoluene and toluene. From these results, we propose a mechanism for anaerobic microsomal reduction of benzyl halides involving the intermediate formation of sigma-alkyl cytochrome P-450-Fe(III)-CH2Ar complexes which exhibit red-shifted Soret peaks around 478 nm. Toluenes, ArCH3, are formed either by protonation of the sigma-alkyl complexes or by hydrogen abstraction by the intermediate free radical ArCH2.