Methane Monooxygenase Mimic Asymmetric Oxidation: Self-Assembling μ-Hydroxo, Carboxylate-Bridged Diiron(III)-Catalyzed Enantioselective Dehydrogenation

Mimicking naturally occurring metalloenzymes to enrich the diversity of catalytic asymmetric oxidation reactions is a long-standing goal for modern chemistry. Toward this end, a range of methane monooxygenase (MMO) mimic chiral carboxylate-bridged (μ-hydroxo) diiron(III) dimer complexes using salan as basal ligand and sodium aryl carboxylate as additive have been designed and synthesized. The chiral diiron complexes exhibit efficient catalytic reactivity in dehydrogenative kinetic resolution of indolines using environmentally benign hydrogen peroxide as oxidant. In particular, complex C9 bearing sterically encumbered salan ligands and a 2-naphthoate bridge is identified as the optimal catalyst in terms of chiral recognition. Further investigation reveals that this MMO mimic chiral catalyst can be readily generated by self-assembly under the dehydrogenation conditions. The self-assembling catalytic system is applicable to a series of indolines with multiple stereocenters and diverse substituent patterns in high efficiency with a high level of chiral recognition (selectivity factor up to 153). Late-stage dehydrogenative kinetic resolution of bioactive molecules is further examined.

Biomimetic Non-Heme Iron-Catalyzed Epoxidation of Challenging Terminal Alkenes Using Aqueous H2O2 as an Environmentally Friendly Oxidant

Catalysis mediated by iron complexes is emerging as an eco-friendly and inexpensive option in comparison to traditional metal catalysis. The epoxidation of alkenes constitutes an attractive application of iron(III) catalysis, in which terminal olefins are challenging substrates. Herein, we describe our study on the design of biomimetic non-heme ligands for the in situ generation of iron(III) complexes and their evaluation as potential catalysts in epoxidation of terminal olefins. Since it is well-known that active sites of oxidases might involve imidazole fragment of histidine, various simple imidazole derivatives (seven compounds) were initially evaluated in order to find the best reaction conditions and to develop, subsequently, more elaborated amino acid-derived peptide-like chiral ligands (10 derivatives) for enantioselective epoxidations.

A Bottom Up Approach Towards Artificial Oxygenases by Combining Iron Coordination Complexes and Peptides

The combination of peptides and a chiral iron coordination complex catalyzes high yield highly asymmetric epoxidation with aqueous hydrogen peroxide.

Supramolecular systems resulting from the combination of peptides and a chiral iron coordination complex catalyze asymmetric epoxidation with aqueous hydrogen peroxide, providing good to excellent yields and high enantioselectivities in short reaction times. The peptide is shown to play a dual role; the terminal carboxylic acid assists the iron center in the efficient H₂O₂ activation step, while its β-turn structure is crucial to induce high enantioselectivity in the oxygen delivering step. The high level of stereoselection (84–92% ee) obtained by these supramolecular catalysts in the epoxidation of 1,1′-alkyl ortho-substituted styrenes, a notoriously challenging class of substrates for asymmetric catalysis, is not attainable with any other epoxidation methodology described so far. The current work, combining an iron center ligated to N and O based ligands, and a peptide scaffold that shapes the second coordination sphere, may be seen as a bottom up approach towards the design of artificial oxygenases.

Biologically inspired non-heme iron-catalysts for asymmetric epoxidation; design principles and perspectives

Iron coordination complexes with nitrogen and oxygen donor ligands have long since been known to react with peroxides producing powerful oxidizing species. These compounds can be regarded as simple structural and functional models of the active sites of non-heme iron dependent oxygenases. Research efforts during the last decade have uncovered basic principles and structural coordination chemistry motifs that permit us to control the chemistry that evolves when these iron complexes react with peroxides, in order to provide powerful metal-based, but at the same time selective, oxidising agents. Oxidation methodologies with synthetic value are currently emerging from this approach. The current review focuses on asymmetric epoxidation, a reaction which has large value in synthesis, and where iron/H2O2 based methodologies may represent not only a sustainable choice, but may also expand the scope of state-of-the-art oxidation methods. Basic principles that underlay catalyst design as well as H2O2 activation are discussed, whilst limitations and future perspectives are also reviewed.

A porphyrin-inspired iron catalyst for asymmetric epoxidation of electron-deficient olefins

An in situ formed porphyrin-inspired iron complex that catalyzes asymmetric epoxidation of di- and trisubstituted enones is described. The reaction provides highly enantioenriched α,β-epoxyketones (up to 99% ee). The practical utility of the new catalyst system is demonstrated by the gram-scale synthesis of optically pure epoxide. Hammett analysis suggests that the transition state of the reaction is electron-demanding and the active oxidant is electrophilic.

Chiral octahedral complexes of Co(iii) as catalysts for asymmetric epoxidation of chalcones under phase transfer conditions

Stereochemically inert and positively charged chiral complexes of Co(iii) were shown to catalyze the asymmetric epoxidation of chalcones with H₂O₂ under phase transfer conditions.

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.

Asymmetric epoxidation of alkenes catalyzed by a porphyrin-inspired manganese complex

A novel strategy for catalytic asymmetric epoxidation of a wide variety of olefins by a porphyrin-inspired chiral manganese complex using H2O2 as a terminal oxidant in excellent yield with up to greater than 99% ee has been successfully developed.

An iron catalyst for oxidation of alkyl C-H bonds showing enhanced selectivity for methylenic sites

Many are called but few are chosen: A nonheme iron complex catalyzes the oxidation of alkyl C-H bonds by using H(2)O(2) as the oxidant, showing an enhanced selectivity for secondary over tertiary C-H bonds (see scheme).

Synthesis and optical resolution of a Cu(I) double-stranded helicate with ketimine-bridged tris(bipyridine) ligands

A tetranuclear Cu(I) double-stranded helicate was synthesized from ketimine-bridged tris(bipyridine) ligands and Cu(I) ions, and the racemate was successfully resolved by diastereomeric salt formation using an optically pure phosphate anion followed by anion exchange with NaPF(6) without racemization.

Iron-catalyzed asymmetric epoxidation of β,β-disubstituted enones

The combination of Fe(OTf)(2) and novel phenanthroline ligands enables the catalytic asymmetric epoxidation of acyclic β,β-disubstituted enones, which have been a heretofore inaccessible substrate class. The reaction provides highly enantioenriched α,β-epoxyketones (up to 92% ee) that can be further converted to functionalized β-ketoaldehydes with an all-carbon quaternary center.