Mechanism of Cis-Dihydroxylation and Epoxidation of Alkenes by Highly H₂O₂ 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.

A mechanistic study on the disproportionation and oxidative degradation of phenothiazine derivatives by manganese(III) complexes in phosphate acidic media

“…Recently, a bridged manganese(V) porphyrin dimer was shown to oxidize water into dioxygen, [20] and reversible O-O bond cleavage was observed for a manganese corrole system [21,22], and the design of novel, long-lived manganese(III) complexes, which can be a model of the precursor of the endogenous high-valent peroxo and oxo intermediates, has been published recently [23]. A special emphasis has been focused on manganese species as effective catalysts in the degradation reactions of many organic species [24,25]. In this study, a detailed account of the mechanistic features as well as the kinetic parameters for the oxidative transformation of phenothiazines by manganese(III) is provided.…”

Redox‐State Dependent Ligand Exchange in Manganese‐Based Oxidation Catalysis

“…-0.2 V in the presence of carboxylic acids in water. de Boer et al [92] observed a similar effect of carboxylic acid in acetonitrile. The species formed initially upon reduction is the colourless Mn II Mn II complex 9c, which can undergo subsequent oxidation to form initially the Mn III 2 complexes 9d followed by a loss of H 2 O to form 9a ( Figure 7 and Figure 8).…”

“…In the case of carboxylato bridged complexes based on the tmtacn ligand (1,4,7- [92] have shown by ESI-MS that the rate of exchange of the μ-O bridge is dependent on the substituent on the carboxylato ligands. For example, whereas for 7 in CH 3 (Figure 2).…”

“…[40,172] More recently, attention has focused on understanding the mechanism by which 18 catalyses reactions and, in particular, the role played by the various additives that were reported to enhance catalytic efficiency. [5,92,117,119] de Boer et al showed that carboxylic acids served multiple roles in facilitating the oxidation of alkenes by 18; that is, to protonate 18 and thereby shift its reduction potential positively to allow reduction, to suppress disproportionation of H 2 O 2 , to act as a ligand in, for example, 9a and to stabilise the complex under the reaction conditions. Notably, a substantial lag period prior to the onset of substrate conversion was observed in reactions with 18 in which H 2 O 2 was added continuously.…”

Oxidation of phenothiazine dyes by manganese(III) in sulfuric acid solution

“…Thus, manganese(III) is employed in water splitting during the light reactions of green plant photosynthesis [1] and in enzymatic and nonenzymatic redox catalysis in mammalian systems [2][3][4]. It is also a powerful one-electron oxidant and a versatile, selective redox catalyst frequently used in laboratory and industrial organic synthesis involving oxygen transfer reactions to alkanes, alkenes and compounds containing nitrogen and sulfur atoms as well as carbon-carbon bond generation [5][6][7][8][9][10][11][12][13][14][15][16].…”