Abstract:The role of the carboxylic acid as co-catalyst in metalloporphyrin catalytic hydrogen peroxide oxidations is discussed, taking into account its dependence on the ratio relatively to the catalyst. The catalytic efficiency and stability of the catalyst in the presence of an excess of carboxylic acid suggests that a metallo-acylperoxo complex can be the effective oxidation intermediate.
“…Transition metal acylperoxo complexes have been previously invoked as direct oxidizing species in the oxidations of various substrates such as triphenylphosphine, alkanes, and olefins. ,− The detailed mechanism of oxygen transfer in most cases remained unclear, in particular whether the reaction can proceed in a concerted manner or by a stepwise mechanism (via O–O bond cleavage of the Mn acylperoxo species, followed by reaction with the substrate). , In principle, our experimental data (i.e., (1) quantitative epoxide yields (in most cases) in catalyst systems Mn complex/peracids (see Tables and ) without traces of products of radical type oxidations and (2) high cis / trans ratios (10.1···17.9) in Z -stilbene epoxidations with peracids, much higher than that for the oxidations by alkyl hydroperoxides (1.4···2.0, Table , entries 4 and 5 and 7 and 8) yet somewhat lower than that for the uncatalyzed concerted Prilezhaev epoxidation (Table , entry 18) tentatively support the prevalent concerted mechanism (Scheme ). In Scheme , S may be a molecule of a chiral additive, which most likely accounts for the nonzero (still not so high) enantioselectivities of chalcone epoxidation by peracids in the presence of achiral catalyst 6 and chiral additive boc-( l )-proline (Table , entries 5–7).…”
It
has been demonstrated that chiral manganese aminopyridine complexes
[LMnII(OTf)2] are able to epoxidize olefins
with excellent enantioselectivities (up to 99% ee, several examples) with various terminal oxidants (H2O2, alkyl hydroperoxides, peroxyacids, and iodosylarenes).
The mechanisms of enantioselective olefin epoxidations on aminopyridine
manganese(II) complexes [LMnII(OTf)2] with different
oxidants are considered. The analysis of reaction products and epoxidation
enantioselelctivity, Hammett correlations, and isotopic (18O) labeling studies bear evidence in favor of the formation of different
oxidizing species in the presence of different terminal oxidants.
The addition of cocatalytic additives (carboxylic acid or H2O) dramatically changes the catalytic behavior of catalyst systems
Mn complex/hydrogen peroxide and Mn complex/alkyl hydroperoxide; in
the presence of additives, the catalyst systems with H2O2 and alkyl hydroperoxides demonstrate very similar behaviors.
Remarkably, in the presence of chiral additive Boc-protected (l)-proline, achiral Mn aminopyridine
complexes catalyze the epoxidation of chalcone in an enantioselective
fashion (with up to 60% ee), representing a rare
example of chiral environment amplification.
“…Transition metal acylperoxo complexes have been previously invoked as direct oxidizing species in the oxidations of various substrates such as triphenylphosphine, alkanes, and olefins. ,− The detailed mechanism of oxygen transfer in most cases remained unclear, in particular whether the reaction can proceed in a concerted manner or by a stepwise mechanism (via O–O bond cleavage of the Mn acylperoxo species, followed by reaction with the substrate). , In principle, our experimental data (i.e., (1) quantitative epoxide yields (in most cases) in catalyst systems Mn complex/peracids (see Tables and ) without traces of products of radical type oxidations and (2) high cis / trans ratios (10.1···17.9) in Z -stilbene epoxidations with peracids, much higher than that for the oxidations by alkyl hydroperoxides (1.4···2.0, Table , entries 4 and 5 and 7 and 8) yet somewhat lower than that for the uncatalyzed concerted Prilezhaev epoxidation (Table , entry 18) tentatively support the prevalent concerted mechanism (Scheme ). In Scheme , S may be a molecule of a chiral additive, which most likely accounts for the nonzero (still not so high) enantioselectivities of chalcone epoxidation by peracids in the presence of achiral catalyst 6 and chiral additive boc-( l )-proline (Table , entries 5–7).…”
It
has been demonstrated that chiral manganese aminopyridine complexes
[LMnII(OTf)2] are able to epoxidize olefins
with excellent enantioselectivities (up to 99% ee, several examples) with various terminal oxidants (H2O2, alkyl hydroperoxides, peroxyacids, and iodosylarenes).
The mechanisms of enantioselective olefin epoxidations on aminopyridine
manganese(II) complexes [LMnII(OTf)2] with different
oxidants are considered. The analysis of reaction products and epoxidation
enantioselelctivity, Hammett correlations, and isotopic (18O) labeling studies bear evidence in favor of the formation of different
oxidizing species in the presence of different terminal oxidants.
The addition of cocatalytic additives (carboxylic acid or H2O) dramatically changes the catalytic behavior of catalyst systems
Mn complex/hydrogen peroxide and Mn complex/alkyl hydroperoxide; in
the presence of additives, the catalyst systems with H2O2 and alkyl hydroperoxides demonstrate very similar behaviors.
Remarkably, in the presence of chiral additive Boc-protected (l)-proline, achiral Mn aminopyridine
complexes catalyze the epoxidation of chalcone in an enantioselective
fashion (with up to 60% ee), representing a rare
example of chiral environment amplification.
“…Imidazole remains coordinated to the Mn throughout the reaction, whereas carboxylic acid helps cleave the peroxide O−O bond leading to a reactive Mn−oxo intermediate 14 . Oxomanganese species are well-established intermediates in Mn−porphyrin-mediated epoxidations with H 2 O 2 . − The oxo−Mn(V) intermediate has been isolated and its formation further studied using spectroscopic techniques − 5 Possible Catalytic Cycle Showing the Importance of Imidazole and Carboxylates in the Quici Modified Manganese−Porphyrin Epoxidation System …”
“…Among these, Mn(β-NO 2 TDCPP)Cl provided the highest conversion and selectivity due to the presence of a nitro group. The cocatalysts for hydrogen peroxide activation included the buffering substances, i.e., ammonium acetate [111], imidazole [112], and pyridine plus benzoic acid [113]. However, the evidence of pyridine oxidation was also observed [114].…”
Catalysts are well-known to convert alkylbenzenes at high thermal condition to a number of useful products. However, the current schemes of transformation are not suitable for the hazard-free industrial applications because their reactive intermediates are transformed to a variety of side products that often retard the optimum yield and cause environmental pollutions. It is also observed that the formation of products depends on a wide range of parameters which are extremely difficult to control and often incur extra cost. Recently, heterogeneous catalysts have received huge commercial interests for the oxidation of alkylbenzene into carbonyl compounds which are platform chemicals in various synthetics and fine chemicals. This review is an up-to-date documentary on various catalysts used for the oxidation of alkyl-substituted benzenes along with their reaction condition and selectivity profiles. This work updates our knowledge for the selection and/or design of novel catalysts for the chemists and engineers in the industrial and academic settings.
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