Chiral Manganese Aminopyridine Complexes: the Versatile Catalysts of Chemo‐ and Stereoselective Oxidations with H₂O₂

Abstract

In the last decade, manganese(II) complexes with N-donor tetradentate aminopyridine ligands emerged as efficient catalysts of enantioselective epoxidation of olefins and direct selective oxidation of CÀH groups in complex organic molecules, with environmentally benign oxidant hydrogen peroxide. In this personal account, we summarize the progress of these catalysts with regard to ligands design, structure-reactivity correlations, evaluation of the substrate scope, as well as mechanistic studies, shedding light on the nature of active sites and the mechanisms of selective oxygenations. Several practically promising catalytic syntheses with the aid of Mn aminopyridine catalysts are exemplified.

Aromatic C−H Hydroxylation Reactions with Hydrogen Peroxide Catalyzed by Bulky Manganese Complexes

“…Generally, activation of H 2 O 2 by manganese or iron complexes supported by aminopyridine ligands leads to electrophilic oxidants [1h,2e,g,10a,b] . It is proposed that the starting Mn(II) complex is first oxidized to a Mn(III) hydroperoxo species, which then converts to a Mn(V) oxo complex through O−O bond heterolysis, [1m,10a,b] this last step being assisted by the carboxylic acid additive (Scheme 4). We propose that the aromatic hydroxylation reaction occurs via a similar oxidizing species, without the involvement of oxygen‐centered radicals.…”

Manganese Catalyzed Enantioselective Epoxidation of α,β‐Unsaturated Amides with H2O2

“…Shibasaki and co‐workers developed a series of catalyst systems, based on lanthanide metal complexes, employing TBHP as oxidant [10–12] for the asymmetric epoxidation of trans ‐enamides. In recent years, manganese complexes with chiral bis ‐amino‐ bis ‐pyridine and structurally related ligands have emerged as challenging catalysts for enantioselective epoxidations of various olefins (unfunctionalized alkenes, unsaturated ketones and esters) with environmentally benign oxidant hydrogen peroxide [13–16] …”

Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

“…We rationalize that 4-pyrrolidinopyridine is a stronger Nheteroaromatic electron donor moiety compared to DMAP and pyridine (pK a = 18.33, 17.95 and 12.53 for 4-pyrrolidinopyridine, DMAP and pyridine, respectively), 77,78 and accordingly we believe that complex (S,S)-1 and 2, containing the tetradentate aminopyridine ligands with the pyrrolidine substituents, will better stabilize the active oxidant that is being formed upon reaction of the complex with H 2 O 2 , that is the high-valent manganese-oxo species. 29,37,38,63,[85][86][87] Thus, we have tested manganese complexes (S,S)-1 and 2 (1 mol%) in catalytic benzylic oxidation reactions in the presence of acetic acid as additive and 2,2,2-trifluoroethanol (TFE) as solvent, using propylbenzene (6, 0.2 mmol) as model substrate (Table 2). For comparison purposes, manganese complexes 3 and 4 were also tested for the same oxidation reaction.…”

“…37 It has been well established that such complexes activate H 2 O 2 , in most cases with the help of a carboxylic acid as an additive, to generate powerful and selective electrophilic metal-oxo species. 22,23,38 On the one hand, the use of both nonheme iron and manganese complexes with aminopyridine ligands in asymmetric epoxidation reactions has been described extensively with high yields and enantioselectivities. On the other hand, the enantioselective oxidation of aliphatic C(sp 3 )-H bonds remains amongst the most challenging reactions, in which aminopyridine-based manganese complexes have particularly appeared as effective catalysts in the last years.…”