Combining Nickel- and Zinc-Porphyrin Sites via Covalent Organic Frameworks for Electrochemical CO2 Reduction.

Covalent organic frameworks (COFs) are ideal platforms to spatially control the integration of multiple molecular motifs throughout a single nanoporous framework. Despite this design flexibility, COFs are typically synthesized using only two monomers. One bears the functional motif for the envisioned application, while the other is used as an inert connecting building block. Integrating more than one functional motif extends the functionality of COFs immensely, which is particularly useful for multistep reactions such as electrochemical reduction of CO2. In this systematic study, we synthesized five Ni(II)- and Zn(II)-porphyrin-based COFs, including two pure component COFs (Ni100 and Zn100) and three mixed Ni/Zn-COFs (Ni75/Zn25, Ni50/Zn50, and Ni25/Zn75). Among these, the Ni50/Zn50-COF exhibited the highest catalytic performance for the electroreduction of CO2 to CO and formate at -0.6 V vs RHE, as was observed in an H-cell. The catalytic performance of the COF catalysts was further extended to a zero-gap membrane electrode assembly (MEA) operation where, utilizing Ni50/Zn50, CH4 was detected along with CO and formate at a high current density of 150 mA cm-2. In contrast, under these conditions predominantly H2 and CO were detected at Ni100 and Zn100 respectively, indicating a clear synergistic effect between the Ni- and Zn-porphyrin units.

Kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cages.

The kinetic enantio-recognition of chiral viologen guests by planar-chiral porphyrin cage compounds, measured in terms of ΔΔG‡on, is determined by the planar-chirality of the host and influenced by the size, as measured by ion mobility-mass spectrometry, but not the chirality of its substituents.

Enantiodivergent Sulfoxidation Catalyzed by a Photoswitchable Iron Salen Phosphate Complex.

Here we describe a photoswitchable iron(III) salen phosphate catalyst, which is able to catalyze the enantiodivergent oxidation of prochiral aryl alkyl sulfides to chiral aryl alkyl sulfoxides. The stable (S)-axial isomer of the catalyst produced enantioenriched sulfoxides with the (R)-configuration in up to 75 % e.e., whereas the photoisomerized metastable (R)-axial isomer of the catalyst favored the formation of (S)-sulfoxides in up to 43 % e.e. The maximum Δe.e. value obtained in the enantiodivergent sulfoxidation was 118 %, which is identical to the maximum Δe.e. value that was measured in the enantiodivergent epoxidation of alkenes by a related recently described Mn1 catalyst. This iron-based catalyst broadens the scope of photoswitchable enantiodivergent catalysts and may be used in the future to develop a photoswitchable catalytic system that can write digital information on a polymer chain in the form chiral sulfoxide functions.

Enantiodivergent epoxidation of alkenes with a photoswitchable phosphate manganese-salen complex.

The development of enantiodivergent catalysts capable of preparing both enantiomeric products from one substrate in a controlled fashion is challenging. Introducing a switching function into the catalyst can address this challenge, allowing the chiral reaction environment to reversibly change during catalysis. Here we report a photoswitchable phosphate ligand, derived from 2,2'-biphenol, which axially coordinates as the counter ion to an achiral manganese(III) salen catalyst, providing the latter with the ability to switch stereoselectivity in the epoxidation of alkenes. The enantiomers of the chiral ligand exist as a pair of pseudo-enantiomers, which can be interconverted by irradiation with light of different wavelengths. The opposite axial chirality of these pseudo-enantiomers is efficiently transferred to the manganese(III) salen catalyst. With this switchable supramolecular catalyst, the enantioselectivity of the epoxidation of a variety of alkenes can be controlled, resulting in opposite enantiomeric excesses of the epoxide products. This transfer of chirality from a photoswitchable anionic ligand to a metal complex broadens the scope of supramolecular catalysts.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts.

Macrocyclic metal porphyrin complexes can act as shape-selective catalysts mimicking the action of enzymes. To achieve enzyme-like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20-88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes.

Polyoxygenated cyclohexene derivatives and flavonoids from the leaves of Uvaria pandensis.

Three new oxygenated cyclohexene derivatives, pandensenol D - F (1-3), two new flavanoids, pandensone A and B (4-5), and seven known compounds (6-12) were isolated from the methanol extract of the leaves of Uvaria pandensis Verdc. (Annonaceae). The structures were characterized by NMR spectroscopic and mass spectrometric analyses. The isolated metabolites were evaluated for their antibacterial activity against the Gram-positive bacteria Bacillus subtilis and Staphylococcus epidermidis, the Gram-negative bacteria Enterococcus raffinosus, Escherichia coli, Paraburkholderia caledonica, Pectobacterium carotovorum and Pseudomonas putida, and for cytotoxicity against the MCF-7 human breast cancer cell line. Out of the tested compounds, pandensenol D (1) and (6',7'-dihydro-8'α,9'ß-dihydroxy)-3-farnesylindole (12) showed weak activity, whereas (8'α,9'ß-dihydroxy)-3-farnesylindole (11) strong activity against B. subtilis. Four of the isolated compounds (1, 4, 11 and 12) showed moderate cytotoxicity against MCF-7 breast cancer cells (EC50 > 100 µM).

A new ß-hydroxydihydrochalcone from Tephrosia uniflora, and the revision of three ß-hydroxydihydrochalcones to flavanones.

The CH2Cl2/MeOH (1:1) extract of the stems of Tephrosia uniflora yielded the new ß-hydroxydihydrochalcone (S)-elatadihydrochalcone-2'-methyl ether (1) along with the three known compounds elongatin (2), (S)-elatadihydrochalcone (3), and tephrosin (4). The structures were elucidated by NMR spectroscopic and mass spectrometric data analyses. Elongatin (2) showed moderate antibacterial activity (EC50 of 25.3 µM and EC90 of 32.8 µM) against the Gram-positive bacterium Bacilus subtilis, and comparable toxicity against the MCF-7 human breast cancer cell line (EC50 of 41.3 µM). Based on the comparison of literature and predicted NMR data with that obtained experimentally, we propose the revision of the structures of three ß-hydroxydihydrochalcones to flavanones.

Oxygenated Cyclohexene Derivatives from the Stem and Root Barks of Uvaria pandensis.

Five new cyclohexene derivatives, dipandensin A and B (1 and 2) and pandensenols A-C (3-5), and 16 known secondary metabolites (6-21) were isolated from the methanol-soluble extracts of the stem and root barks of Uvaria pandensis. The structures were characterized by NMR spectroscopic and mass spectrometric analyses, and that of 6-methoxyzeylenol (6) was further confirmed by single-crystal X-ray crystallography, which also established its absolute configuration. The isolated metabolites were evaluated for antibacterial activity against the Gram-positive bacteria Bacillus subtilis and Staphylococcus epidermidis and the Gram-negative bacteria Enterococcus raffinosus, Escherichia coli, Paraburkholderia caledonica, Pectobacterium carotovorum, and Pseudomonas putida, as well as for cytotoxicity against the MCF-7 human breast cancer cell line. A mixture of uvaretin (20) and isouvaretin (21) exhibited significant antibacterial activity against B. subtilis (EC50 8.7 µM) and S. epidermidis (IC50 7.9 µM). (8'α,9'ß-Dihydroxy)-3-farnesylindole (12) showed strong inhibitory activity (EC50 9.8 µM) against B. subtilis, comparable to the clinical reference ampicillin (EC50 17.9 µM). None of the compounds showed relevant cytotoxicity against the MCF-7 human breast cancer cell line.

Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests.

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C 2-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG ‡ on = 3 kJ mol-1) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG ‡ on = 1 kJ mol-1) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C 2-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host-guest complexes, i.e. there is no thermodynamic selectivity.

Allosteric Guest Binding in Chiral Zirconium(IV) Double Decker Porphyrin Cages.

Chiral zirconium(IV) double cage sandwich complex Zr(1)2 has been synthesized in one step from porphyrin cage H21. Zr(1)2 was obtained as a racemate, which was resolved by HPLC and the enantiomers were isolated in >99.5 % ee. Their absolute configurations were assigned on the basis of X-ray crystallography and circular dichroism spectroscopy. Vibrational circular dichroism (VCD) experiments on the enantiomers of Zr(1)2 revealed that the chirality around the zirconium center is propagated throughout the whole cage structure. The axial conformational chirality of the double cage complex displayed a VCD fingerprint similar to the one observed previously for a related chiral cage compound with planar and point chirality. Zr(1)2 shows fluorescence, which is quenched when viologen guests bind in its cavities. The binding of viologen and dihydroxybenzene derivatives in the two cavities of Zr(1)2 occurs with negative allostery, the cooperativity factors α (=4 K2/K1) being as low as 0.0076 for the binding of N,N'-dimethylviologen. These allosteric effects are attributed to a pinching of the second cavity as a result of guest binding in the first cavity.

Antibacterial and cytotoxic biflavonoids from the root bark of Ochna kirkii.

The new isoflavonoid kirkinone A (1) and biflavonoid kirkinone B (2) along with six known compounds (3-8) were isolated from the methanolic extract of the root bark of Ochna kirkii. The compounds were identified by NMR spectroscopic and mass spectrometric analyses. Out of the eight isolated natural products, calodenin B (4) and lophirone A (6) showed significant antibacterial activity against the Gram-positive bacterium Bacillus subtilis with MIC values of 2.2 and 28 µM, and cytotoxicity against the MCF-7 human breast cancer cell line with EC50 values of 219.3 and 19.2 µM, respectively. The methanolic crude extract of the root bark exhibited cytotoxicity at EC50 8.4 µg/mL. The isolated secondary metabolites and the crude extract were generally inactive against the Gram-negative Escherichia coli (MIC ≥400 µg/mL). Isolation of biflavonoids and related secondary metabolites from O. kirkii demonstrates their chemotaxonomic significance to the genus Ochna and to other members of the family Ochnaceae.

Biflavanones, Chalconoids, and Flavonoid Analogues from the Stem Bark of Ochna holstii.

Two new biflavanones (1 and 2), three new bichalconoids (3-5), and 11 known flavonoid analogues (6-16) were isolated from the stem bark extract (CH3OH-CH2Cl2, 7:3, v/v) of Ochna holstii. The structures of the isolated metabolites were elucidated by NMR spectroscopic and mass spectrometric analyses. The crude extract and the isolated metabolites were evaluated for antibacterial activity against Bacillus subtilis (Gram-positive) and Escherichia coli (Gram-negative) as well as for cytotoxicity against the MCF-7 human breast cancer cell line. The crude extract and holstiinone A (1) exhibited moderate antibacterial activity against B. subtilis with MIC values of 9.1 µg/mL and 14 µM, respectively. The crude extract and lophirone F (14) showed cytotoxicity against MCF-7 with EC50 values of 11 µg/mL and 24 µM, respectively. The other isolated metabolites showed no significant antibacterial activities (MIC > 250 µM) and cytotoxicities (EC50 ≥ 350 µM).

Molecular motor-functionalized porphyrin macrocycles.

Molecular motors and switches change conformation under the influence of an external stimulus, e.g. light. They can be incorporated into functional systems, allowing the construction of adaptive materials and switchable catalysts. Here, we present two molecular motor-functionalized porphyrin macrocycles for future photo-switchable catalysis. They display helical, planar and point chirality, and are diastereomers, which differ in the relative orientation of the motor and macrocyclic components. Fluorescence, UV-vis, and 1H NMR experiments reveal that the motor-functionalized macrocycles can bind and thread different variants of viologen guests, including a one-side blocked polymeric one of 30 repeat units. The latter feature indicates that the motor systems can find the open end of a polymer chain, thread on it, and move along the chain to eventually bind at the viologen trap, opening possibilities for catalytic writing on single polymer chains via chemical routes.

Absolute configuration and host-guest binding of chiral porphyrin-cages by a combined chiroptical and theoretical approach.

Porphyrin cage-compounds are used as biomimetic models and substrate-selective catalysts in supramolecular chemistry. In this work we present the resolution of planar-chiral porphyrin cages and the determination of their absolute configuration by vibrational circular dichroism in combination with density functional theory calculations. The chiral porphyrin-cages form complexes with achiral and chiral viologen-guests and upon binding one of the axial enantiomorphs of the guest is bound selectively, as is indicated by induced-electronic-dichroism-spectra in combination with calculations. This host-guest binding also leads to unusual enhanced vibrational circular dichroism, which is the result of a combination of phenomena, such as rigidification of the host and guest structures, charge transfer, and coupling of specific vibration modes of the host and guest. The results offer insights in how the porphyrin cage-compounds may be used to construct a future molecular Turing machine that can write chiral information onto polymer chains.

Oxidation of Secondary Methyl Ethers to Ketones.

We present a mild way of converting secondary methyl ethers into ketones using calcium hypochlorite in aqueous acetonitrile with acetic acid as activator. The reaction is compatible with various oxygen- and nitrogen-containing functional groups and afforded the corresponding ketones in up to 98% yield. The use of this methodology could expand the application of the methyl group as a useful protecting group.