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Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H2O2 Efficient Dinuclear Manganese Catalysts
journal contribution
posted on 2007-08-06, 00:00 authored by Johannes W. de Boer, Wesley R. Browne, Jelle Brinksma, Paul L. Alsters, Ronald Hage, Ben L. FeringaIn the presence of carboxylic acids the complex [MnIV2(μ-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(μ-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(μ-O)(μ-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.