Synthesis and Transformations of NH-Sulfoximines

Recent years have seen a marked increase in the occurrence of sulfoximines in the chemical sciences, often presented as valuable motifs for medicinal chemistry. This has been prompted by both pioneering works taking sulfoximine containing compounds into clinical trials and the concurrent development of powerful synthetic methods. This review covers recent developments in the synthesis of sulfoximines concentrating on developments since 2015. This includes extensive developments in both S−N and S−C bond formations. Flow chemistry processes for sulfoximine synthesis are also covered. Finally, subsequent transformations of sulfoximines, particularly in N-functionalization are reviewed, including N−S, N−P, N−C bond forming processes and cyclization reactions.     

Incorporation of β-Alanine in Cu(II) ATCUN Peptide Complexes Increases ROS Levels,DNA Cleavage and Antiproliferative Activity**

Redox-active Cu(II) complexes are able to form reactive oxygen species (ROS) in the presence of oxygen and reducing agents. Recently, Faller et al. reported that ROS generation by Cu(II) ATCUN complexes is not as high as assumed for decades. High complex stability results in silencing of the Cu(II)/Cu(I) redox cycle and therefore leads to low ROS generation. In this work, we demonstrate that an exchange of the α-amino acid Gly with the β-amino acid β-Ala at position 2 (Gly2→β-Ala2) of the ATCUN motif reinstates ROS production (^•OH and H₂O₂). Potentiometry, cyclic voltammetry, EPR spectroscopy and DFT simulations were utilized to explain the increased ROS generation of these β-Ala2-containing ATCUN complexes. We also observed enhanced oxidative cleavage activity towards plasmid DNA for β-Ala2 compared to the Gly2 complexes. Modifications with positively charged Lys residues increased the DNA affinity through electrostatic interactions as determined by UV/VIS, fluorescence, and CD spectroscopy, and consequently led to a further increase in nuclease activity. A similar trend was observed regarding the cytotoxic activity of the complexes against several human cancer cell lines where β-Ala2 peptide complexes had lower IC₅₀ values compared to Gly2. The higher cytotoxicity could be attributed to an increased cellular uptake as determined by ICP-MS measurements.     

Rb[Li5Si2O7] – A Latecomer in the Family of Alkali Lithosilicates Hiding a Green-Emitting Lithosilicate

In order to expand the field of alkali lithosilicates, a new representative of the substance class with a previously unknown structure type was found based on solid-state synthesis. The novel compound with the sum formula Rb[Li₅Si₂O₇] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) Å by means of single-crystal X-ray diffraction. The structure consists of a highly condensed lithosilicate framework, built up of corner- and edge-linked [LiO₄]-tetrahedra and [Si₂O₇]-units, and the rubidium ions aligned in channels. Suitable crystals of the material were obtained using sealed tantalum ampoules as reaction tube at a temperature of 750 °C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu²⁺-doped Rb[Li₅Si₂O₇] reveals an intense green luminescence. In-depth crystal analysis shows that a core-shell formation is present even for apparently high quality single-crystals. As a minority phase, the known green phosphor RbLi[Li₃SiO₄]₂:Eu²⁺ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li₅Si₂O₇] dominating the single-crystal diffraction pattern.     

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co₃O₄, MgCo₂O₄, Co₂FeO₄, Co₂AlO₄ and CoFe₂O₄. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H₂O₂ decomposition. Studying the effect of dominant surface termination, isotropic Co₃O₄ and CoFe₂O₄ catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co³⁺ plays the major role for high activity. In H₂O₂ decomposition, Co²⁺ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction.     

Platinum Indolylphosphine Fluorido and Polyfluorido Complexes: An Interplay between Cyclometallation,Fluoride Migration,and Hydrogen Bonding

The reaction of [PtCl₂(COD)] (COD=1,5-cyclooctadiene) with diisopropyl-2-(3-methyl)indolylphosphine (iPr₂P(C₉H₈N)) led to the formation of the platinum(ii ) chlorido complexes, cis-[PtCl₂{iPr₂P(C₉H₈N)}₂] ( 1 ) and trans-[PtCl₂{iPr₂P(C₉H₈N)}₂] ( 2 ). The cis-complex 1 reacted with NEt₃ yielding the complex cis-[PtCl{κ²-(P,N)-iPr₂P(C₉H₇N)}{iPr₂P(C₉H₈N)}] ( 3 ) bearing a cyclometalated κ²-(P,N)-phosphine ligand, while the isomer 2 with a trans-configuration did not show any reactivity towards NEt₃. Treatment of 1 or 3 with (CH₃)₄NF (TMAF) resulted in the formation of the twofold cyclometalated complex cis-[Pt{κ²-(P,N)-iPr₂P(C₉H₇N)}₂] ( 4 ). The molecular structures of the complexes 1–4 were determined by single-crystal X-ray diffraction. The fluorido complex cis-[PtF{κ²-(P,N)-iPr₂P(C₉H₇N)}{iPr₂P(C₉H₈N)}] ⋅ (HF)₄ ( 5 ⋅ (HF)₄) was formed when complex 4 was treated with different hydrogen fluoride sources. The Pt(ii ) fluorido complex 5 ⋅ (HF)₄ exhibits intramolecular hydrogen bonding in its outer coordination sphere between the fluorido ligand and the NH group of the 3-methylindolyl moiety. In contrast to its chlorido analogue 3 , complex 5 ⋅ (HF)₄ reacted with CO or the ynamide 1-(2-phenylethynyl)-2-pyrrolidinone to yield the complexes trans-[Pt(CO){κ²-(P,C)-iPr₂P(C₉H₇NCO)}{iPr₂P(C₉H₈N)}][F(HF)₄] ( 7 ) and a complex, which we suggest to be cis-[Pt{C=C(Ph)OCN(C₃H₆)}{κ²-(P,N)-iPr₂P(C₉H₇N)}{iPr₂P(C₉H₈N)}][F(HF)₄] ( 9 ), respectively. The structure of 9 was assigned on the basis of DFT calculations as well as NMR and IR data. Hydrogen bonding of HF and NH to fluoride was proven to be crucial for the existence of 7 and 9 .     

The Photophysics and Photochemistry of Melanin- Like Nanomaterials Depend on Morphology and Structure

Melanin-like nanomaterials have found application in a large variety of high economic and social impact fields as medicine, energy conversion and storage, photothermal catalysis and environmental remediation. These materials have been used mostly for their optical and electronic properties, but also for their high biocompatibility and simplicity and versatility of preparation. Beside this, their chemistry is complex and it yields structures with different molecular weight and composition ranging from oligomers, to polymers as well as nanoparticles (NP). The comprehension of the correlation of the different compositions and morphologies to the optical properties of melanin is still incomplete and challenging, even if it is fundamental also from a technological point of view. In this minireview we focus on scientific papers, mostly recent ones, that indeed examine the link between composition and structural feature and photophysical and photochemical properties proposing this approach as a general one for future research.     

On-Surface Formation of Cyano-Vinylene Linked Chains by Knoevenagel Condensation

The rapid development of on-surface synthesis provides a unique approach toward the formation of carbon-based nanostructures with designed properties. Herein, we present the on-surface formation of CN-substituted phenylene vinylene chains on the Au(111) surface, thermally induced by annealing the substrate stepwise at temperatures between 220 °C and 240 °C. The reaction is investigated by scanning tunneling microscopy and density functional theory. Supported by the calculated reaction pathway, we assign the observed chain formation to a Knoevenagel condensation between an aldehyde and a methylene nitrile substituent.     

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ-enamide aldehydes bearing a fully substituted, benzylic stereogenic center – a structural motif common in many natural products and therapeutically active compounds – with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4- or 1,5- functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.     

Azimuthal Dipolar Rotor Arrays on Surfaces

A set of dipolar molecular rotor compounds was designed, synthesized and adsorbed as self-assembled 2D arrays on Ag(111) surfaces. The title molecules are constructed from three building blocks: (a) 4,8,12-trioxatriangulene (TOTA) platforms that are known to physisorb on metal surfaces such as Au(111) and Ag(111), (b) phenyl groups attached to the central carbon atom that function as pivot joints to reduce the barrier to rotation, (c) pyridine and pyridazine units as small dipolar units on top. Theoretical calculations and scanning tunneling microscopy (STM) investigations hint at the fact that the dipoles of neighboring rotors interact through space through pairs of energetically favorable head-to-tail arrangements.     

Negishi Cross-Coupling Provides Alkylated Tryptophans and Tryptophan Regioisomers

Mild transition-metal catalysed cross-couplings enable direct functionalisation of biocatalytically halogenated tryptophans with alkyl iodides, representing a new alternative for late-stage derivatisations of halogenated aromatic amino acids. Moreover, this strategy enables preparation of (homo)tryptophan regioisomers in a simple two-step synthesis using a Pd-catalysed Negishi cross coupling. This method provides access to non-canonical constitutional surrogates of tryptophan, ready for use in peptide synthesis.