Evidence of a Sole Oxygen Atom Transfer Agent in Asymmetric Epoxidations with Fe-pdp Catalysts

Iron complexes with chiral tetradentate ligands based on the pdp scaffold (pdp = <i>N</i>,<i>N</i>′-bis­(2-pyridylmethyl)-2,2′-bipyrrolidine) are efficient and versatile catalysts for the highly enantioselective epoxidation of a wide range of olefins. The nature of the species responsible for oxygen atom transfer to the olefin in these reactions is under debate. In order to investigate this question, the enantioselectivity of the epoxidation reaction has been used as a mechanistic probe. The enantioselectivities obtained under different reaction conditions for two iron catalysts (<i>S,S</i>)-[Fe­(CF₃SO₃)₂(^Me2Npdp)] (<b>(</b><i><b>S,S</b></i><b>)</b>^<b>Me2N</b><b>1Fe</b>) and (<i>S,S</i>)-[Fe­(CF₃SO₃)₂(^dMMpdp)] (<b>(</b><i><b>S,S</b></i><b>)</b>^<b>dMM</b><b>1Fe</b>) have been analyzed. Reactions were performed with a series of peracids, and enantioselectivities of these reactions were compared with those obtained by combining peroxides and carboxylic acids. This analysis provides conclusive experimental evidence that the same oxidant is responsible for the asymmetric epoxidation reaction in both scenarios. The study also provides insight into the nature of the oxygen atom transfer species, as well as its mechanism of formation, offering a rational guide for defining catalytic systems with more versatile structures and improved selectivity

Asymmetric Epoxidation with H₂O₂ by Manipulating the Electronic Properties of Non-heme Iron Catalysts

A non-heme iron complex that catalyzes highly enantioselective epoxidation of olefins with H₂O₂ is described. Improvement of enantiomeric excesses is attained by the use of catalytic amounts of carboxylic acid additives. Electronic effects imposed by the ligand on the iron center are shown to synergistically cooperate with catalytic amounts of carboxylic acids in promoting efficient O–O cleavage and creating highly chemo- and enantioselective epoxidizing species which provide a broad range of epoxides in synthetically valuable yields and short reaction times

Chemoselective Aliphatic C–H Bond Oxidation Enabled by Polarity Reversal

Methods for selective oxidation of aliphatic C–H bonds are called on to revolutionize organic synthesis by providing novel and more efficient paths. Realization of this goal requires the discovery of mechanisms that can alter in a predictable manner the innate reactivity of these bonds. Ideally, these mechanisms need to make oxidation of aliphatic C–H bonds, which are recognized as relatively inert, compatible with the presence of electron rich functional groups that are highly susceptible to oxidation. Furthermore, predictable modification of the relative reactivity of different C–H bonds within a molecule would enable rapid diversification of the resulting oxidation products. Herein we show that by engaging in hydrogen bonding, fluorinated alcohols exert a polarity reversal on electron rich functional groups, directing iron and manganese catalyzed oxidation toward a priori stronger and unactivated C–H bonds. As a result, selective hydroxylation of methylenic sites in hydrocarbons and remote aliphatic C–H oxidation of otherwise sensitive alcohol, ether, amide, and amine substrates is achieved employing aqueous hydrogen peroxide as oxidant. Oxidations occur in a predictable manner, with outstanding levels of product chemoselectivity, preserving the first-formed hydroxylation product, thus representing an extremely valuable tool for synthetic planning and development

Iron Catalyzed Highly Enantioselective Epoxidation of Cyclic Aliphatic Enones with Aqueous H₂O₂

An iron complex with a <i>C</i>₁-symmetric tetradentate N-based ligand catalyzes the asymmetric epoxidation of cyclic enones and cyclohexene ketones with aqueous hydrogen peroxide, providing the corresponding epoxides in good to excellent yields and enantioselectivities (up to 99% yield, and 95% ee), under mild conditions and in short reaction times. Evidence is provided that reactions involve an electrophilic oxidant, and this element is employed in performing site selective epoxidation of enones containing two alkene sites

Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts

Fast, efficient, and highly stereoselective epoxidation with H₂O₂ is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation

Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts

Fast, efficient, and highly stereoselective epoxidation with H₂O₂ is reached by manganese coordination complexes with e-rich aminopyridine tetradentate ligands. It is shown that the electronic properties of these catalysts vary systematically with the stereoselectivity of the O-atom transfer event and exert fine control over the activation of hydrogen peroxide, reducing the amount of carboxylic acid co-catalyst necessary for efficient operation