F430 MODEL CHEMISTRY - AN INVESTIGATION OF NICKEL-COMPLEXES AS CATALYSTS FOR THE REDUCTION OF ALKYL-HALIDES AND METHYL COENZYME-M BY SODIUM-BOROHYDRIDE

The ability of nickel(II) macrocycle and coordination complexes 1-8 to mediate the reductive dehalogenation of cyclohexyl bromide and the CH 3-S bond cleavage of methyl CoM by sodium borohydride in diglyme/alcoh ol, DMF/alcohol, or acetonitrile/alcohol was investigated. Methyl CoM, or CH3SCH2CH2SO3-, is the cofactor that carries the methyl group in t he final step of methanogenesis in methanogenic bacteria. Because of t he potential for production of the heterogeneous catalyst nickel borid e during these reactions, the activities of several nickel salts that afford nickel boride when reduced with borohydride were examined for p urposes of comparison. Complexes 1-8 homogeneously catalyze the dehalo genation of cyclohexyl bromide by sodium borohydride. The facility of the reaction varies markedly with the structure of the ligands and the solvent composition. Nickel boride is a moderately active heterogeneo us catalyst for the dehalogenation of cyclohexyl bromide and produces small yields of methane from methyl CoM and borohydride. When excess n ickel boride is generated in situ, the yield of methane increases to 5 4%. The other isolated products, ethanesulfonate and a product derived from the CH3-S part of methyl CoM, show that nickel boride preferenti ally cleaves the CH2-S bond of methyl CoM, which is the opposite of th e enzymatic selectivity. Freshly prepared Raney nickel, a second heter ogeneous nickel compound, quantitatively cleaves methyl CoM to methane and ethanesulfonate. None of the complexes 1-8 produced significant a mounts of methane from methyl CoM and sodium borohydride in the mixed solvents or in aqueous solution.