Tautomerism and Atropisomerism in Free-Base (meso)-Strapped Porphyrins: Static and Dynamic Aspects

A synthetic tactic to substitute axial ligands in sterically demanding Ru(II)porphyrinates

We report a synthetic strategy that allows for the preparation of sterically encumbered heteroleptic Ru(II)porphyrinates with the desired configuration of stable/inert and weak/labile axial ligands to direct reactions between substrates to exclusively occur at the sterically encumbered face. To demonstrate the method, we describe the synthesis of a strapped-Ru(II)porphyrinate bearing a stable/inert triphenylphosphine (PPh₃) bulky axial ligand coordinated exo to the central cavity and a weak/labile methanol molecule coordinated at the internal axial position. With this axial ligand configuration, the reported Ru(II)porphyrinate exclusively promotes carbene transfer reactions to olefins through the central cavity, which has been verified by the selective formation of cycloprane-linked [2]rotaxanes.

Modular Synthesis of trans-A2 B2 -Porphyrins with Terminal Esters: Systematically Extending the Scope of Linear Linkers for Porphyrin-Based MOFs

Differently functionalized porphyrin linkers represent the key compounds for the syntheses of new porphyrin-based metal-organic frameworks (MOFs), which have gathered great interest within the last two decades. Herein we report the synthesis of a large range of 5,15-bis(4-ethoxycarbonylphenyl)porphyrin derivatives, through Suzuki and Sonogashira cross-coupling reactions of an easily accessible corresponding meso-dibrominated trans-A2 B2 -porphyrin with commercially available boronic acids or terminal alkynes. The resulting porphyrins were fully characterized through NMR, MS, and IR spectroscopy and systematically investigated through UV/Vis absorption. Finally, selected structures were saponified to the corresponding carboxylic acids and subsequently proven to be suitable for the synthesis of surface-anchored MOF thin films.

Ru(II)Porphyrinate-based molecular nanoreactor for carbene insertion reactions and quantitative formation of rotaxanes by active-metal-template syntheses

Selectivity in N–H and S–H carbene insertion reactions promoted by Ru(II)porphyrinates currently requires slow addition of the diazo precursor and large excess of the primary amine and thiol substrates in the reaction medium. Such conditions are necessary to avoid the undesirable carbene coupling and/or multiple carbene insertions. Here, the authors demonstrate that the synergy between the steric shielding provided by a Ru(II)porphyrinate-based macrocycle with a relatively small central cavity and the kinetic stabilization of otherwise labile coordinative bonds, warranted by formation of the mechanical bond, enables single carbene insertions to occur with quantitative efficiency and perfect selectivity even in the presence of a large excess of the diazo precursor and stoichiometric amounts of the primary amine and thiol substrates. As the Ru(II)porphyrinate-based macrocycle bears a confining nanospace and alters the product distribution of the carbene insertion reactions when compared to that of its acyclic version, the former therefore functions as a nanoreactor.

Selectivity in carbene insertion reactions promoted by Ru(II)porphyrinates is achieved only upon careful control of substrate stoichiometry. Here, the authors demonstrate that endotopic catalysis and formation of mechanical bonds enables carbene insertions to occur selectively and in quantitative yield regardless of substrate stoichiometry.

Conformational Re-engineering of Porphyrins as Receptors with Switchable N-H⋅⋅⋅X-Type Binding Modes

The selectivity and functional variability of porphyrin cofactors are typically based on substrate binding of metalloporphyrins wherein the pyrrole nitrogen units only serve to chelate the metal ions. Yet, using the porphyrin inner core system for other functions is possible through conformational engineering. As a first step towards porphyrin “enzyme‐like” active centers, a structural and spectroscopic study of substrate binding to the inner core porphyrin system shows that a highly saddle‐distorted porphyrin with peripheral amino receptor groups (1, 2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetrakis(2‐aminophenyl)porphyrin) coordinates analytes in a switchable manner dependent on the acidity of the solution. The supramolecular ensemble exhibits exceptionally high affinity to and selectivity for the pyrophosphate anion (2.26±0.021)×10⁹  m⁻¹. ¹H NMR spectroscopic studies provided insight into the likely mode of binding and the characterization of atropisomers, all four of which were also studied by X‐ray crystallography.

Chiral Bifunctional Metalloporphyrin Catalysts for Kinetic Resolution of Epoxides with Carbon Dioxide

Chiral binaphthyl-strapped Zn(II) porphyrins with triazolium halide units were synthesized as bifunctional catalysts for kinetic resolution of epoxides with CO₂. Several catalysts were screened by changing the linker length and nucleophilic counteranions, and the optimized catalyst accelerated the enantioselective reaction at ambient temperature to produce optically active cyclic carbonates and epoxides.

Chiroptical and catalytic properties of doubly binaphthyl-strapped chiral porphyrins

Doubly (R)-binaphthyl-strapped porphyrins with methylene chains were synthesized.

NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused Structure Delays the Proton Transfer

We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H₂QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with ¹H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two nonfused pyrroles, and the NH tautomerism of 3 was evidenced by variable-temperature (VT) ¹H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (ΔS^‡ = 89 J mol^-1 K^-1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state, as evidenced by the crystal structure and NMR measurements. On the basis of the kinetic studies and density functional theory calculations, the stability of intermediates in the NH tautomerism of 3 and the transition states have been discussed in detail.

Unexpected Interactions between Alkyl Straps and Pyridine Ligands in Sulfur-Strapped Porphyrin Nanorings

Strapped or “basket-handle” porphyrins have been investigated previously as hemoglobin mimics and catalysts. The facial selectivity of their interactions with axial ligands is a sensitive test for noncovalent bonding. Here the binding of pyridyl ligands to zinc porphyrins with thioester-linked alkyl straps is investigated in solution by NMR spectroscopy and UV–vis titration, and in the solid state by X-ray crystallography. We expected that coordination of the axial ligand would occur on the less hindered face of the porphyrin, away from the strap. Surprisingly, attractive interactions between the strap and the ligand direct axial coordination to the strapped face of the porphyrin, except when the strap is short and tight. The strapped porphyrins were incorporated into π-conjugated cyclic porphyrin hexamers using template-directed synthesis. The strap and the sulfur substituents are located either inside or outside the porphyrin nanoring, depending on the length of the strap. Six-porphyrin nanorings with outwardly pointing sulfur anchors were prepared for exploring quantum interference effects in single-molecule charge transport.

Carboxylic-group-functionalized single-walled carbon nanohorns as peroxidase mimetics and their application to glucose detection

Carboxylic-group-functionalized single-walled carbon nanohorns (SWCNHs-COOH) have been found to possess peroxidase-like activity for the first time. Similar to natural peroxidase, SWCNHs-COOH can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine by H2O2 to produce a blue color solution. Compared with horseradish peroxidase, SWCNHs-COOH exhibit higher activity and stability under harsh reaction conditions. The catalytic activity of SWCNHs-COOH depends on the concentration of H2O2. A colorimetric method for glucose detection was developed by combining the SWCNH-COOH catalytic reaction and the generation of H2O2 by the enzymatic oxidation of glucose with glucose oxidase. Taking into account the advantages of good stability, high biocompatibility in aqueous solutions, being metal-catalyst free, and high purity, SWCNHs-COOH are expected to have potential applications in biotechnology and clinical diagnostics as enzymatic mimics.