Manganese catalysed oxidations with hydrogen peroxide:applications and mechanistic insights

Het doel van het onderzoek dat in dit proefschrift wordt beschreven was het ontwikkelen van nieuwe milieuvriendelijke en economisch duurzame methoden voor de selectieve oxidatie van organische verbindingen, in het bijzonder cis-dihydroxylering en epoxidatie van alkenen, oxidatie van alcoholen en aldehyden en selectieve C-H activatie. De voornaamste uitdaging was om deze reacties uit te voeren met een eenvoudige katalysator gebaseerd op mangaan en met waterstofperoxide. Tijdens het onderzoeksproject werd al snel een belangrijke doorbraak gemaakt: de ontdekking van een compleet nieuwe katalysator concept dat meteen geschikt was voor toepassingen op zowel het lab als in de industrie en het werd vervolgens gepatenteerd in samenwerking met industriële partners in het project. De voornaamste focus van het promotieonderzoek was om te ontrafelen hoe dit katalysator systeem werkt en om te komen tot verbeterd inzicht zodat het katalysator systeem nog bredere toepassing kan vinden in de toekomst

Oxidation of Vicinal Diols to -Hydroxy Ketones with H2O2 and a Simple Manganese Catalyst

-Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic-diols to -hydroxy ketones with H2O2 can be achieved with an in situ prepared catalyst based on manganese salts and pyridine-2-carboxylic acid. Furthermore the same catalyst is effective in alkene epoxidation, and it is shown that alkene oxidation with the Mn-II catalyst and H2O2 followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to -hydroxy ketones can be achieved under mild (ambient) conditions

Mechanisms in manganese catalysed oxidation of alkenes with H2O2

<p>The development of new catalytic systems for cis-dihydroxylation and epoxidation of alkenes, based on atom economic and environmentally friendly concepts, is a major contemporary challenge. In recent years, several systems based on manganese catalysts using H2O2 as the terminal oxidant have been developed. In this review, selected homogeneous manganese catalytic systems, including 'ligand free' and pyridyl amine ligand based systems, that have been applied to alkene oxidation will be discussed with a strong focus on the mechanistic studies that have been carried out.</p>

Mechanism of Alkene, Alkane, and Alcohol Oxidation with H2O2 by an in Situ Prepared Mn-II/Pyridine-2-carboxylic Acid Catalyst

The oxidation of alkenes, alkanes, and alcohols with H2O2 is catalyzed efficiently using an in situ prepared catalyst comprised of a MnII salt and pyridine-2-carboxylic acid (PCA) together with a ketone in a wide range of solvents. The mechanism by which these reactions proceed is elucidated, with a particular focus on the role played by each reaction component: i.e., ketone, PCA, MnII salt, solvent, etc. It is shown that the equilibrium between the ketone cocatalysts, in particular butanedione, and H2O2 is central to the catalytic activity observed and that a gem-hydroxyl-hydroperoxy species is responsible for generating the active form of the manganese catalyst. Furthermore, the oxidation of the ketone to a carboxylic acid is shown to antecede the onset of substrate conversion. Indeed, addition of acetic acid either prior to or after addition of H2O2 eliminates a lag period observed at low catalyst loading. Carboxylic acids are shown to affect both the activity of the catalyst and the formation of the gem-hydroxyl-hydroperoxy species. The molecular nature of the catalyst itself is explored through the effect of variation of MnII and PCA concentration, with the data indicating that a MnII:PCA ratio of 1:2 is necessary for activity. A remarkable feature of the catalytic system is that the apparent order in substrate is 0, indicating that the formation of highly reactive manganese species is rate limiting

Manganese catalyzed cis-dihydroxylation of electron deficient alkenes with H2O2

A practical method for the multigram scale selective cis-dihydroxylation of electron deficient alkenes such as diethyl fumarate and N-alkyl and N-aryl-maleimides using H2O2 is described. High turnovers (>1000) can be achieved with this efficient manganese based catalyst system, prepared in situ from a manganese salt, pyridine-2-carboxylic acid, a ketone and a base, under ambient conditions. Under optimized conditions, for diethyl fumarate at least 1000 turnovers could be achieved with only 1.5 equiv. of H2O2 with d/l-diethyl tartrate (cis-diol product) as the sole product. For electron rich alkenes, such as cis-cyclooctene, this catalyst provides for efficient epoxidation.

A Common Active Intermediate in the Oxidation of Alkenes, Alcohols and Alkanes with H2O2 and a Mn(II)/Pyridin-2-Carboxylato Catalyst

The mechanism and the reactive species involved in the oxidation of alkenes, and alcohols with H2O2, catalysed by an in situ prepared mixture of a MnII salt, pyridine-2-carboxylic acid and a ketone is elucidated using substrate competition experiments, kinetic isotope effect (KIE) measurements, and atom tracking with 18O labelling. The data indicate that a single reactive species engages in the oxidation of both alkenes and alcohols. The primary KIE in the oxidation of benzyl alcohols is ca. 3.5 and shows the reactive species to be selective despite a zero order dependence on substrate concentration, and the high turnover frequencies (up to 30 s−1) observed. Selective 18O labelling identifies the origin of the oxygen atoms transferred to the substrate during oxidation, and is consistent with a highly reactive, e. g., [MnV(O)(OH)] or [MnV(O)2], species rather than an alkylperoxy or hydroperoxy species

Mechanism of Alkene, Alkane, and Alcohol Oxidation with H₂O₂ by an in Situ Prepared Mn^II/Pyridine-2-carboxylic Acid Catalyst

The oxidation of alkenes, alkanes, and alcohols with H₂O₂ is catalyzed efficiently using an in situ prepared catalyst comprised of a Mn^II salt and pyridine-2-carboxylic acid (PCA) together with a ketone in a wide range of solvents. The mechanism by which these reactions proceed is elucidated, with a particular focus on the role played by each reaction component: i.e., ketone, PCA, Mn^II salt, solvent, etc. It is shown that the equilibrium between the ketone cocatalysts, in particular butanedione, and H₂O₂ is central to the catalytic activity observed and that a <i>gem</i>-hydroxyl-hydroperoxy species is responsible for generating the active form of the manganese catalyst. Furthermore, the oxidation of the ketone to a carboxylic acid is shown to antecede the onset of substrate conversion. Indeed, addition of acetic acid either prior to or after addition of H₂O₂ eliminates a lag period observed at low catalyst loading. Carboxylic acids are shown to affect both the activity of the catalyst and the formation of the <i>gem</i>-hydroxyl-hydroperoxy species. The molecular nature of the catalyst itself is explored through the effect of variation of Mn^II and PCA concentration, with the data indicating that a Mn^II:PCA ratio of 1:2 is necessary for activity. A remarkable feature of the catalytic system is that the apparent order in substrate is 0, indicating that the formation of highly reactive manganese species is rate limiting

The unexpected role of pyridine-2-carboxylic acid in manganese based oxidation catalysis with pyridin-2-yl based ligands

A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H2O2 as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H2O2 and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by 1H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H2O2 required decreases from 6–8 equiv. with the pyridin-2-yl based ligands to 1–1.5 equiv. with pyridine-2-carboxylic acid.