Synthesis of chiral water-soluble metalloporphyrins (Fe, Ru,): new catalysts for asymmetric carbene transfer in water

A bioinspired bifunctional catalyst: an amphiphilic organometallic catalyst for ring-closing metathesis forming liquid droplets in aqueous media

Inspired by phase-separated biopolymers with enzymatic activity, we developed an amphiphilic catalyst consisting of alternating hydrophilic oligo(ethylene glycol) and hydrophobic aromatic units bearing a Hoveyda-Grubbs catalyst center (MAHGII). MAHGII served as both a droplet-forming scaffold and a catalyst for ring-closing metathesis reactions, providing a new biomimetic system that promotes organic reactions in an aqueous environment.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

Photoredox-catalyzed intramolecular cyclopropanation of alkenes with α-bromo-β-keto esters

A mild photoredox-catalyzed intramolecular cyclopropanation of alkenes with α-bromo-β-keto esters in an aqueous medium was developed. The sequential reaction process comprising the intramolecular radical addition of α-bromo-β-keto esters to olefins under photoredox catalysis, and subsequent cyclization to form cyclopropane proceeds in one-pot under exceptionally mild conditions at room temperature in the presence of 2,6-lutidine. A broad range of substrates consisting of various alkenes and both base- and acid-sensitive functionalized esters were feasible under the reaction conditions, resulting in a wide range of functionalized bicyclic cyclopropanes.

Selective carbene transfer to amines and olefins catalyzed by ruthenium phthalocyanine complexes with donor substituents

Electron-rich ruthenium phthalocyanine complexes were evaluated in carbene transfer reactions from ethyl diazoacetate (EDA) to aromatic and aliphatic olefins as well as to a wide range of aromatic, heterocyclic and aliphatic amines for the first time. It was revealed that the ruthenium octabutoxyphthalocyanine carbonyl complex [(BuO)8Pc]Ru(CO) is the most efficient catalyst converting electron-rich and electron-poor aromatic olefins to cyclopropane derivatives with high yields (typically 80-100%) and high TON (up to 1000) under low catalyst loading and nearly equimolar substrate/EDA ratio. This catalyst shows a rare efficiency in the carbene insertion into amine N-H bonds. Using a 0.05 mol% catalyst loading, a high amine concentration (1 M) and 1.1 eq. of EDA, a number of structurally divergent amines were selectively converted to mono-substituted glycine derivatives with up to quantitative yields and turnover numbers reaching 2000. High selectivity, large substrate scope, low catalyst loading and practical reaction conditions place [(BuO)8Pc]Ru(CO) among the most efficient catalysts for the carbene insertion into amines.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Crystal structure of {4-[10,15,20-tris-(4-meth-oxy-phen-yl)porphyrin-5-yl]benzyl 2-diazo-acetato}-zinc(II)

In the title compound, [Zn(C50H36N6O5)], the ZnII cation is chelated by four pyrrole N atoms of the porphyrinate anion and coordinated by a symmetry-generated keto O atom of the diazo-ester group in a distorted square-pyramidal geometry. The mean Zn-N(pyrrole) bond length is 2.058 Å and the Zn-O(diazo-ester) bond length is 2.179 (4) Å. The zinc cation is displaced by 0.2202 (13) Å from the N4C20 mean plane of the porphyrinate anion toward the O atom; the involvement of this atom leads to a [100] polymeric chain in the crystal.

Spin modification of iron(ii) complexes via covalent (dative) and dispersion guided non-covalent bonding with N-heterocyclic carbenes: DFT, DLPNO-CCSD(T) and MCSCF studies

Spin crossover from high spin Fe(ii)-phthalocyanine to low or intermediate spin via either dative covalent or non-covalent interaction by just varying the substituent using the same core ligand.

Iron catalysts with N-ligands for carbene transfer of diazo reagents

This review provides an overview of the catalytic activity of iron complexes of nitrogen ligands in driving carbene transfers towards CC, C–H and X–H bonds. The reactivity of diazo reagents is discussed as well as the proposed reaction mechanisms.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Reusable and highly enantioselective water-soluble Ru(II)-Amm-Pheox catalyst for intramolecular cyclopropanation of diazo compounds

A reusable and highly enantioselective catalyst for the intramolecular cyclopropanation of various diazo ester and Weinreb amide derivatives was developed. The reactions catalyzed by a water-soluble Ru(II)-Amm-Pheox catalyst proceeded smoothly at room temperature, affording the corresponding bicyclic cyclopropane ring-fused lactones and lactams in high yields (up to 99%) with excellent enantioselectivities (up to 99% ee). After screening of various catalysts, the Ru(II)-Amm-Pheox complex having an ammonium group proved to be crucial for the intramolecular cyclopropanation reaction in a water/ether biphasic medium. The water-soluble catalyst could be reused at least six times with little loss in yield and enantioselectivity.

Halterman Corroles and Their Use as a Probe of the Conformational Dynamics of the Inherently Chiral Copper Corrole Chromophore

Halterman corroles have been synthesized for the first time from pyrrole and Halterman's aldehyde via Gryko's "water-methanol method". These were derivatized to the corresponding copper complexes and subsequently to the β-octabromo complexes. Electronic circular dichroism spectra were recorded for the enantiopure copper complexes, affording the first such measurements for the inherently chiral Cu corrole chromophore. Interestingly, for a given configuration of the Halterman substituents, X-ray crystallographic studies revealed both P and M conformations of the Cu-corrole core, proving that the substituents, even in conjunction with β-octabromination, are unable to lock the Cu-corrole core into a given chirality. The overall body of evidence strongly indicates a dynamic equilibrium between the P and M conformations. Such an interconversion, which presumably proceeds via saddling inversion, provides a rationale for our failure so far to resolve sterically hindered Cu corroles into their constituent enantiomers by means of chiral HPLC.

Cyclopropanations via Heme Carbenes: Basic Mechanism and Effects of Carbene Substituent, Protein Axial Ligand, and Porphyrin Substitution

Catalytic carbene transfer to olefins is a useful approach to synthesize cyclopropanes, which are key structural motifs in many drugs and biologically active natural products. While catalytic methods for olefin cyclopropanation have largely relied on rare transition-metal-based catalysts, recent studies have demonstrated the promise and synthetic value of iron-based heme-containing proteins for promoting these reactions with excellent catalytic activity and selectivity. Despite this progress, the mechanism of iron-porphyrin and hemoprotein-catalyzed olefin cyclopropanation has remained largely unknown. Using a combination of quantum chemical calculations and experimental mechanistic analyses, the present study shows for the first time that the increasingly useful C=C functionalizations mediated by heme carbenes feature an Fe^II-based, nonradical, concerted nonsynchronous mechanism, with early transition state character. This mechanism differs from the Fe^IV-based, radical, stepwise mechanism of heme-dependent monooxygenases. Furthermore, the effects of the carbene substituent, metal coordinating axial ligand, and porphyrin substituent on the reactivity of the heme carbenes was systematically investigated, providing a basis for explaining experimental reactivity results and defining strategies for future catalyst development. Our results especially suggest the potential value of electron-deficient porphyrin ligands for increasing the electrophilicity and thus the reactivity of the heme carbene. Metal-free reactions were also studied to reveal temperature and carbene substituent effects on catalytic vs noncatalytic reactions. This study sheds new light into the mechanism of iron-porphyrin and hemoprotein-catalyzed cyclopropanation reactions and it is expected to facilitate future efforts toward sustainable carbene transfer catalysis using these systems.

Unexpected formation of a μ-carbido diruthenium(iv) complex during the metalation of phthalocyanine with Ru3(CO)12 and its catalytic activity in carbene transfer reactions

Catalytic activity of novel μ-carbido Ru(iv) bisphthalocyaninate was firstly demonstrated by the olefin cyclopropanation and carbene insertion into N–H bonds.

DNA-Accelerated Catalysis of Carbene-Transfer Reactions by a DNA/Cationic Iron Porphyrin Hybrid

A novel DNA-based hybrid catalyst comprised of salmon testes DNA and an iron(III) complex of a cationic meso-tetrakis(N-alkylpyridyl)porphyrin was developed. When the N-methyl substituents were placed at the ortho position with respect to the porphyrin ring, high reactivity in catalytic carbene-transfer reactions was observed under mild conditions, as demonstrated in the catalytic enantioselective cyclopropanation of styrene derivatives with ethyl diazoacetate (EDA) as the carbene precursor. A remarkable feature of this catalytic system is the large DNA-induced rate acceleration observed in this reaction and the related dimerization of EDA. It is proposed that high effective molarity of all components of the reaction in or near the DNA is one of the key contributors to this unique reactivity. This study demonstrates that the concept of DNA-based asymmetric catalysis can be expanded into the realm of organometallic chemistry.

Computation Sheds Insight into Iron Porphyrin Carbenes' Electronic Structure, Formation, and N-H Insertion Reactivity

Iron porphyrin carbenes constitute a new frontier of species with considerable synthetic potential. Exquisitely engineered myoglobin and cytochrome P450 enzymes can generate these complexes and facilitate the transformations they mediate. The current work harnesses density functional theoretical methods to provide insight into the electronic structure, formation, and N-H insertion reactivity of an iron porphyrin carbene, [Fe(Por)(SCH3)(CHCO2Et)](-), a model of a complex believed to exist in an experimentally studied artificial metalloenzyme. The ground state electronic structure of the terminal form of this complex is an open-shell singlet, with two antiferromagnetically coupled electrons residing on the iron center and carbene ligand. As we shall reveal, the bonding properties of [Fe(Por)(SCH3)(CHCO2Et)](-) are remarkably analogous to those of ferric heme superoxide complexes. The carbene forms by dinitrogen loss from ethyl diazoacetate. This reaction occurs preferentially through an open-shell singlet transition state: iron donates electron density to weaken the C-N bond undergoing cleavage. Once formed, the iron porphyrin carbene accomplishes N-H insertion via nucleophilic attack. The resulting ylide then rearranges, using an internal carbonyl base, to form an enol that leads to the product. The findings rationalize experimentally observed reactivity trends reported in artificial metalloenzymes employing iron porphyrin carbenes. Furthermore, these results suggest a possible expansion of enzymatic substrate scope, to include aliphatic amines. Thus, this work, among the first several computational explorations of these species, contributes insights and predictions to the surging interest in iron porphyrin carbenes and their synthetic potential.

C–C bond forming reactions catalyzed by chiral metalloporphyrins

Porphyrins are abundant in nature facilitating many enzymatic reactions by being present in the active sites of many enzymes. Consequently, over the years, a number of chiral metalloporphyrins have been synthesized and have proved efficient in catalyzing C–C bond forming reactions. Herein, we review the synthesis of chiral metalloporphyrins and their catalytic activity in cyclopropanation, cyclopropenation, and C–H insertion reactions.

The ligand influence in stereoselective carbene transfer reactions promoted by chiral metal porphyrin catalysts

This Perspective illustrates the state-of-the-art of stereoselective carbene transfer reactions catalysed by chiral metal porphyrin complexes. A particular attention is focused on the active role of the porphyrin ligand to drive the carbene insertion into the target organic skeleton.

Asymmetric Kita spirolactonisation catalysed by anti-dimethanoanthracene-based iodoarenes

Rigid C₂-symmetric anti-dimethanoanthracene-based iodoarenes at 10 mol% loading catalyse the asymmetric Kita spirolactonisation of 1-naphthols with significant levels of asymmetric induction.

Iron-catalyzed transformations of diazo compounds

Although iron-promoted diazo transformations were only discovered during the 1990s, iron can undergo facile changes in its oxidation state and possesses distinct Lewis acid character, and these properties have afforded iron a privileged position as a catalyst in the transformations of diazo compounds. In this review, we have provided an overview of the iron-catalyzed diazo transformation reactions reported in the literature by the end of 2013 with the aim of stimulating further interest in this area of research.

Enantioselective iron-catalyzed intramolecular cyclopropanation reactions

An iron-catalyzed asymmetric intramolecular cyclopropanation was realized in high yields and excellent enantioselectivity (up to 97% ee) by using the iron complexes of chiral spiro-bisoxazoline ligands as catalysts. The superiority of iron catalysts exhibited in this reaction demonstrated the potential abilities of this sustainable metal in asymmetric carbenoid transformation reactions.

A water-soluble ruthenium glycosylated porphyrin catalyst for carbenoid transfer reactions in aqueous media with applications in bioconjugation reactions

Water-soluble [Ru(II)(4-Glc-TPP)(CO)] (1, 4-Glc-TPP = meso-tetrakis(4-(beta-D-glucosyl)phenyl)porphyrinato dianion) is an active catalyst for the following carbenoid transfer reactions in aqueous media with good selectivities and up to 100% conversions: intermolecular cyclopropanation of styrenes (up to 76% yield), intramolecular cyclopropanation of an allylic diazoacetate (68% yield), intramolecular ammonium/sulfonium ylide formation/[2,3]-sigmatroptic rearrangement reactions (up to 91% yield), and intermolecular carbenoid insertion into N-H bonds of primary arylamines (up to 83% yield). This ruthenium glycosylated porphyrin complex can selectively catalyze alkylation of the N-terminus of peptides (8 examples) and mediate N-terminal modification of proteins (four examples) using a fluorescent-tethered diazo compound (15). A fluorescent group was conjugated to ubiquitin via 1-catalyzed alkene cyclopropanation with 15 in aqueous solution in two steps: (1) incorporation of an alkenic group by the reaction of N-hydroxysuccinimide ester 19 with ubiquitin and (2) cyclopropanation of the alkene-tethered Lys(6) ubiquitin (23) with the fluorescent-labeled diazoacetate 15 in the presence of a catalytic amount of 1. The corresponding cyclopropanation product (24) was obtained with approximately 55% conversion based on MALDI-TOF mass spectrometry. The products 23, 24, and the N-terminal modified peptides and proteins were characterized by LC-MS/MS and/or SDS-PAGE analyses.