Remote Amino Acid Recognition Enables Effective Hydrogen Peroxide Activation at a Manganese Oxidation Catalyst

Abstract

Precise delivery of a proton plays a key role in O 2 activation at iron oxygenases, enabling the crucial OÀ O cleavage step that generates the oxidizing high-valent metaloxo species. Such a proton is delivered by acidic residues that may either directly bind the iron center or lie in its second coordination sphere. Herein, a supramolecular strategy for enzyme-like H 2 O 2 activation at a biologically inspired manganese catalyst, with a nearly stoichiometric amount (1-1.5 equiv) of a carboxylic acid is disclosed. Key for this strategy is the incorporation of an α,ω-amino acid in the second coordination sphere of a chiral catalyst via remote ammonium-crown ether recognition. The properly positioned carboxylic acid function enables effective activation of hydrogen peroxide, leading to catalytic asymmetric epoxidation. Modulation of both amino acid and catalyst structure can tune the efficiency and the enantioselectivity of the reaction, and a study on the oxidative degradation pathway of the system is presented.

Reversible Deactivation of Manganese Catalysts in Alkene Oxidation and H2O2 Disproportionation

“…Elaboration of the catalyst and carboxylic acid cocatalyst and optimization of reaction conditions, e.g., catalyst and carboxylic acid concentration, temperature, etc., have led to high yields and enantioselectivities. In particular, sterically encumbered carboxylic acids provide the highest enantioselectivities in epoxidation, hinting at a role as ligands in the active catalytic species, and supramolecular interactions can increase the effective (local) concentration of acid at the catalyst requiring only a few equivalents with respect to the catalyst . These latter observations also indicate that carboxylic acids act as coligands to the catalyst.…”

“…In particular, sterically encumbered carboxylic acids provide the highest enantioselectivities in epoxidation, hinting at a role as ligands in the active catalytic species, 23 and supramolecular interactions can increase the effective (local) concentration of acid at the catalyst requiring only a few equivalents with respect to the catalyst. 25 These latter observations also indicate that carboxylic acids act as coligands to the catalyst. The synthetic accessibility of the pyridyl amine class of ligands has facilitated variation in the structure of the ligands over the last decade, most notably with the aim to improve enantioselectivity and substrate scope.…”

Bioinspired complexes confined in well-defined capsules: getting closer to metalloenzyme functionalities

“…[76][77][78] Furthermore, highly precise and predictable C-H oxidation reactions, enabled by substrate positioning at crownether tailored Mn or Fe bioinspired catalysts, were reported by Costas, Olivo and Di Stephano. [79][80][81] However, despite both progresses in the development of (i) caged bioinspired complexes and (ii) open models displaying the functional second coordination sphere, the preparation of bioinspired complexes combining a hydrophobic cavity with weak binding units has rarely been explored. 82 For instance, the impact of the functionalized second coordination sphere on metal ion lability and host-guest binding was very lately evidenced using confined Zn(II) complexes displaying a calix [6]arene-based cavity decorated with phenol or quinone units.…”

Trading Symmetry for Stereoinduction in Tetradentate, non‐C2‐Symmetric Fe(II)‐Complexes for Asymmetric Catalysis

“…The vast majority of these complexes evinces a C 2 ‐ or pseudo‐ C 2 ‐symmetric topology with examples of non‐ C 2 ‐symmetric complexes being an exemption (Figure 1a and 1b). [18–21] Commonly, the synthesis of ligands that are C 2 ‐ or pseudo‐ C 2 ‐symmetric is more convenient and the related catalysts exhibit better performance in regard to asymmetric induction and activity [17] …”