Gerhard N. Schrauzer

The structure of the C16H16·Fe3(CO)9 complex derived from iron tricarbonyl complexes of dimers of cyclo-octatetraene

The structure (IV) of the C16H16·Fe3(CO)9 complex derived from iron tricarbonyl complexes of dimers of cyclo-octatetraene has been shown to contain a vinyl radical to which one of the iron atoms in an Fe2(CO)6 group forms a π-bond while the other is σ-bonded; this result corrects an earlier structural assignment.

The Chemical Evolution of a Nitrogenase Model, XXIII. The Nature of the Active Site and the Role of Homocitric Acid in MoFe-Nitrogenase

Abstract The iron-molybdenum cofactor (FeMo-co) of bacterial nitrogenase is a heterometallic cluster of composition MoFe7S9 that is attached to the apoprotein by a coordinative Mo-N bond to the imidazole group of hisα442, and by a Fe-S bond to cysα215. The molybdenum atom of FeMo-co in the enzyme in addition is coordinated to one molecule of homocitrate (hc), which is required for maximal N2 reducing activity. The molybdenum atom in the enzyme-bound FeMo-co thus is hexacoordinated and cannot react with substrates unless free coordination sites are made available. It is proposed that the reactions of the substrates o f nitrogenase occur at a molybdenum active site consisting of a mononuclear molybdenum homocitrate complex attached to hisα442 of the apoprotein that in the functional enzyme is generated from FeMo-co by a reversible, redox-linked dissociation of the Fe7S9-cys cluster. Studies with catalytic model systems consisting of complexes of molybdenum with imidazole and hydroxocarboxylate ligands support this proposal and provide a rationale for the specific activating effect of homocitrate in nitrogenase.

Organocobalamin Reactions Relevant to the Mechanism of the α-Methyleneglutarate Mutase Enzyme [1]

Abstract The organocobalamin with methylitaconic acid attached to cobalt via its methyl group has been synthesized for the first time. This compound is believed to be an intermediate in the interconversion of methylitaconic acid and α-methyleneglutaric acid, catalysed by the coenzyme B12-dependent α-methyleneglutarate mutase enzyme. Reactions of this organocobalamin and the corresponding dimethyl ester demonstrate that cleavage of the Co-C bond leads to the rearranged α-methyleneglutarate structure under conditions where intermediate carbanions are formed.

Studies on Vitamin B12 and Related Compounds, 50 Synthesis of Substituted Alkylcobalamins from Vitamin B12r and Radicals Generated from Aldehydes, Alcohols and Ethers under "Oxidizing-Reducing" Conditions. A New Synthesis of Coenzyme B12 [1]

Organic radicals generated by the oxidation of aldehydes, alcohols and ethers under reducing conditions are trapped by vitamin B12r to yield substituted organocobalamins. From higher n-alkyl aldehydes, acylcobalamins are formed. With acetaldehyde, a mixture of acetylcobalamin and of methylcobalamin is obtained due to the spontaneous decarbonylation of the CH3CO· radical. From saturated alcohols, w-hydroxyalkylcobalamins are produced, while w-alkoxyalkylcobalamins are formed in the corresponding reactions with radicals generated from ethers. Maximum yields of the organocobalamins are obtained if reducing conditions are maintained during the oxidation of the organic substrates. This is conveniently achieved by using V(III) salts as the reductants and the slow addition of oxidants (e.g. of O2, H2O2, Fenton reagent, or of electrochemically generated oxidizing equivalents). With 5′-deoxyadenosine, 5′-deoxyadenosylcobalamin is formed.

Notizen: The Chemical Evolution of a Nitrogenase Model, XXII. Reduction of Acetylene with Catalysts Derived from Molybdate, Homocitric Acid and N-Methylimidazole and a Proposal Concerning the Active Site of Functional Azotobacter Nitrogenases

Based on model experiments it is suggested that the reduction of substrates by nitrogenase occurs at a mononuclear site containing molybdenum, homocitrate and the imidazole component of his-442 of the apoenzyme, generated from the FeMo-co by a reversible dissociation of the Fe7S8 unit from the MoFe7S8 cluster in a redox-linked process.

Amino Acids as Substrates in the Synthesis of Substituted Organocobalamins

Abstract The amino acids alanine, valine, leucine, isoleucine, norvaline, α-amino butyric acid, and β -methyl aspartic acid are utilized as substrates for organocobalamin syntheses under “oxidizing/reducing” conditions. In the presence of V+3 as the reducing agent, free radicals are generated by the reaction of oxygen radicals with the amino acids through hydrogen atom abstraction. These amino acid radicals then combine with the cobalt atom of vitamin B12r to yield the respective organocobalamins. The cobalamin obtained from the reaction of β-methyl aspartic acid is a possible intermediate in the coenzyme B12 dependent skeletal rearrangement of β-methyl aspartic acid to glutamic acid. HPLC analysis of the cobal-amins prepared from D, L-alanine indicates that the corrin ligand is relatively ineffective at promoting asymmetric syntheses under these conditions.

The Chemical Evolution of a Nitrogenase Model, 20 Alkyl Molybdates(VI) and the Mechanism of Hydrocarbon Production by Nitrogenase [1]

Abstract Colorless alkylmolybdates(VI) of composition R-MoO3-are generated in aqueous solutions by the alkaline hydrolysis of complexes R-Mo(Bpy)(0)2Br(Bpy = 2,2′-bipyridyl, R = CH3 and higher alkyl). At room temperature in alkaline aqueous solution, the new organometallic derivatives of oxomolybdate(VI) are remarkably resistant against Mo-C bond hydrolysis. Decomposition occurs more rapidly on heating, affording unrearranged alkanes according to the eq.: R-MoO3- + OH-→RH + Mo04=. In acidic solutions, the methylmolybdate(VI) species decomposes with the formation of a mixture of methane and ethane while higher alkylmolybdates carrying hydrogen in the β-position relative to molybdenum undergo Mo-C bond heterolysis by way of β-elimina-tion: R-CH2CH2-MoO3 → Mo+4 (aq) + H+ + R-CH = CH2. The Mo-C bond of alkylmolybdates is resistant to oxidants but is very sensitive to cleavage under reducing conditions. Reductive Mo-C bond cleavage occurs particularly rapidly in the presence of thiols and reduced ferredoxin model compounds. The latter reactions simulate the terminal steps of hydrocarbon producing reactions of nitrogenase with alternate substrates such as CN-, R-CN or R-NC, confirming previous mechanistic conclusions concerning the mechanism of nitrogenase action.

The Chemical Evolution of a Nitrogenase Model, XIX Simulation of the Enzymatic Reduction of Cyclopropene

Abstract The nonenzymatic reduction of cyclopropene in molybdothiol-and related model systems of nitrogenase yields cyclopropane and propylene in a pH-dependent fashion: In alkaline media, cyclopropane is formed exclusively. In acidic solutions, cyclopropane and propylene are produced. Cyclopropene is thus identified as a pH sensitive chemical probe of nitrogenase. Since substantial amounts of propylene are formed in the enzymatic reduction of cyclopropene, it follows that the active site in functioning nitrogenase is in a locally acidic environment. The protons required to sustain the acidic pH are presumably generated by ATP hydrolysis; addition of substrate amounts of ATP to nonenzymatic cyclopropene reducing systems also stimulates propylene-and cyclopropane production. The stereochemical course of cyclopropene reduction to cyclopropane is exclusively cis, both under enzymatic and nonenzymatic conditions. The formation of propylene from cyclopropene is not stereoselective and mechanistically consistent with acid catalyzed ring opening prior to reduction.