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 H(2)O(2) 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.

A simple, high yielding catalytic method for the multigram scale selective epoxidation of electron-rich alkenes using near-stoichiometric H2O2 under ambient conditions is reported. The system consists of a Mn(II) salt (<0.01 mol %), pyridine-2-carboxylic acid (<0.5 mol %), and substoichiometric butanedione. High TON (up to 300 000) and TOF (up to 40 s–1) can be achieved for a wide range of substrates with good to excellent selectivity, remarkable functional group tolerance, and a wide solvent scope. It is shown that the formation of 3-hydroperoxy-3-hydroxybutan-2-one from butanedione, and H2O2 in situ, is central to the activity observed.

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 H(2)O(2) 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, H(2)O(2) 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 (1)H 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 H(2)O(2) required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.

The oxidation of alkenes, alkanes, and alcohols with H 2 O 2 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 H 2 O 2 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 H 2 O 2 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 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.

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 H(2)O(2) 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 H(2)O(2) 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.

An efficient and simple method for selective oxidation of secondary alcohols and oxidation of alkanes to ketones is reported. An in situ prepared catalyst is employed based on manganese(II) salts, pyridine-2-carboxylic acid, and butanedione, which provides good-to-excellent conversions and yields with high turnover numbers (up to 10 000) with H2 O2 as oxidant at ambient temperatures. In substrates bearing multiple alcohol groups, secondary alcohols are converted to ketones selectively and, in general, benzyl C-H oxidation proceeds in preference to aliphatic C-H oxidation.

The manganese-containing catalytic system [Mn IV,IV 2 O 3 A C H T U N G T R E N N U N G (tmtacn) 2 ] 2+ (1)/carboxylic acid (where tmtacn = N,N',N''-trimethyl-1,4,7-triazacyclononane), initially identified for the cis-dihydroxylation and epoxidation of alkenes, is applied for a wide range of oxidative transformations, including oxidation of alkanes, alcohols and aldehydes employing H 2 O 2 as oxidant. The substrate classes examined include primary and secondary aliphatic and aromatic alcohols, aldehydes, and alkenes. The emphasis is not primarily on identifying optimum conditions for each individual substrate, but understanding the various factors that affect the reactivity of the Mn-tmtacn cata-lytic system and to explore which functional groups are oxidised preferentially. This catalytic system, of which the reactivity can be tuned by variation of the carboxylato ligands of the in situ formed [Mn III,III 2

α‐Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic‐diols to α‐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 MnII catalyst and H2O2 followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to α‐hydroxy ketones can be achieved under mild (ambient) conditions.