title: Mechanism of cis-dihydroxylation and epoxidation of alkenes by highly H(2)O(2) efficient dinuclear manganese catalysts.
abstract: In the presence of carboxylic acids the complex [Mn(IV)2(micro-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) is shown to be highly efficient in catalyzing the oxidation of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidation of alkenes is shown to be dependent on the carboxylic acid employed. High turnover numbers (t.o.n. > 2000) can be achieved especially w.r.t. cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. the terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(micro-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidation state, H2O, and carboxylic acid concentration and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation analysis and isotope labeling studies. The results confirm that the complexes of the type [Mn2(micro-O)(micro-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chemistry, are the resting state of the catalytic system and that they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidation catalysis and in understanding related biological systems such as dinuclear catalase and arginase enzymes.
authors.affiliation: Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen, The Netherlands.
authors.forename: Johannes W
authors.initials: JW
authors.lastname: de  Boer
authors.forename: Wesley R
authors.initials: WR
authors.lastname: Browne
authors.forename: Jelle
authors.initials: J
authors.lastname: Brinksma
authors.forename: Paul L
authors.initials: PL
authors.lastname: Alsters
authors.forename: Ronald
authors.initials: R
authors.lastname: Hage
authors.forename: Ben L
authors.initials: BL
authors.lastname: Feringa
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry; D002384:Catalysis; D002621:Chemistry / Q000379:methods*; D004563:Electrochemistry / Q000379:methods; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007554:Isotopes; D009682:Magnetic Resonance Spectroscopy; D008345:Manganese / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000475:Alkenes; D002264:Carboxylic Acids; D007554:Isotopes; D012997:Solvents; D008345:Manganese; D006861:Hydrogen Peroxide; D010100:Oxygen