Helvetica Chimica Acta最新文献

A simple method for the electrochemical allylic oxidation of olefins to enones based on peroxide-mediated C–H activation is presented. t-Butyl hydroperoxide serves a dual role as mediator and oxygen source, avoiding the utilization of expensive transition metal catalysts, potentially explosive supporting electrolytes such as perchlorates, or additional environmentally harmful mediators. This elegant methodology allows for an operationally simple and sustainable late-stage transformation of delicate natural compounds such as terpenes or steroids into highly value-added building blocks. Such an approach provides a greener alternative to conventional oxidation methods used for industrial applications. In total, 25 examples are demonstrated with isolated yields up to 80%. The robust scalability of the method was also demonstrated, and chromatography could be avoided in scale-up simply by using recrystallization, which decreases the cost of downstream processing.

The integration of boron into π-conjugated systems is a reliable approach to enhance photophysical properties. In this context, boramidines have emerged as a promising class of fluorophores. In this work, the design, synthesis, and chiroptical properties of novel axially chiral isoquinoline-derived boramidines are reported. Thanks to metal-catalyzed cross-coupling allied with atroposelective C─H arylation strategies or chiral stationary phase HPLC resolution, enantioenriched boramidines 1 and 2 are afforded (ee 90%–99%). Comparing with more classical boramidines, investigations reveal (i) bathochromic absorption shifts in the UV–vis region (>50 nm) and (ii) emission maxima now in the green and cyan domains, along with fluorescence quantum yields reaching 70% in N₂-saturated environments. Chiroptical measurements demonstrate well-defined electronic circular dichroism (ECD) spectra and circularly polarized luminescence (CPL) signals with |g_lum| values up to 2 × 10⁻³.

Extreme ultraviolet (XUV) radiation is highly efficient for functionalization of solid materials by means of pronounced radical formation. To prevent collective effects, and study single photon effects, a low-fluence XUV source with 200 μJ/cm² was used to irradiate polymers such as poly-methyl methacrylate (PMMA) and poly-tetrafluoroethylene (PTFE). The unexposed and XUV-exposed sample domains were analyzed with time-of-flight secondary ion mass spectrometry (TOF-SIMS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The findings show that XUV photons drive photochemical modification of the surface, fragmentation, and ionization of the desorbed products. Besides a theoretical analysis, the XUV-induced surface modification of PTFE is discussed a new application for surface functionalization.

A detailed picture of the reactivity of metallacyclobutanes is key to the understanding and development of new reactions relying on these reactive intermediates, including olefin metathesis and cyclopropanation. More than 40 years ago, Miyashita and Grubbs reported experimental studies on the thermal decomposition pathways of a phosphine-supported nickelacyclobutane (NiCB) suggesting that nickel may be amenable to both cyclopropanation and olefin metathesis. Here we report a computational investigation of this system that shows that cyclopropane reductive elimination is the primary decomposition pathway for monometallic NiCBs, independently of the number of coordinated phosphine ligands. Further calculations suggest that bimetallic species may be responsible for the olefinic products of apparent [2+2] cycloreversion.

Transmetalation is a key elementary step in organometallic chemistry, for which reason there is a keen interest in better understanding the factors governing this reaction. We have previously reported the unusual transfer of a phenyl anion from boron to lithium in the gas-phase dissociation of [LiBr(^tBu)(Ph)Bpin]⁻ (Chem. Eur. J. 2024, 43, e202303653). Here, we use a combination of gas-phase fragmentation experiments and quantum chemical calculations to probe the reactivity of the related adducts [MX(R)(Ph)Bpin]⁻ (R=ⁿBu, ^tBu, Ph; X=Cl, Br, I, BF₄, BPh₄; M=Li, Na, K, Cu). We find the transfer of Ph⁻ to be much favored over that of ⁿBu⁻ and ^tBu⁻. The tendency toward transmetalation is decreased for anions X⁻ that strongly bind to the metal center M. Likewise, it is diminished for more electropositive/less electronegative metals M. According to our theoretical calculations, the coordination of a single molecule of tetrahydrofuran to the metal M also lowers the propensity for transmetalation, thereby approaching the behavior in solution. Thus, our results reveal a subtle interplay of different effects influencing the tendency toward transmetalation.

In this work, we aimed at photocaging the well-known anticancer agents dasatinib, ceritinib, gemcitabine, and combretastatin A4 with red-light activatable 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based cages using a carbonate/carbamate linking strategy. Due to the synthetic challenges discussed in this article, we only obtained two target compounds, namely two caged ceritinib compounds. The latter were characterized in-depth by nuclear magnetic resonance spectroscopy (NMR, ¹H, COSY, ¹³C), high-resolution mass spectrometry (HRMS), infrared (IR) spectroscopy, and their purity was evaluated by elemental analysis. Their photophysical characteristics were also measured including absorption and emission spectra, quantum yields, and lifetimes. Analysis of the products after irradiation of the compounds allowed us to make assumptions about the possible mechanism of the phototransformations. Moreover, we conducted biological studies to determine the phototoxicity indexes of A549 cancer cells. While the two compounds were found to be non-toxic, the BODIPY precursors themselves were found to be highly toxic upon irradiation with phototoxicity indexes up to 1400.

In senescent leaves of banana (Musa acuminata) hypermodified fluorescent chlorophyll (Chl) catabolites (hmFCCs) accumulate and give these leaves an easily observable blue fluorescence. Non-fluorescent Chl-catabolites (NCCs) are not detected, the typical colourless Chl-catabolites that accumulate in most de-greened plant leaves. Here, we report the isolation and spectroscopic structural characterization of two very lipophilic hmFCCs, named Ma-FCC-78 and Ma-FCC-79. These two novel Ma-FCCs are hypermodified with megastigmane (Megs) residues, terpenoids related to the plant hormone abscisic acid (ABA). Thus, these hypermodified Ma-FCCs from senescent banana leaves may possibly play novel physiological roles, on the one hand, in the regulation of stress response, a fundamental role of ABA, on the other hand, by providing some of the cytotoxic, antibacterial and antioxidant activities of Megs.

The use of melanin as a colorant has been significantly limited by its low solubility and the difficulty of penetrating the hair shaft or follicles. However, the proposed nanotechnological approach demonstrates that it is possible to encapsulate a melanin precursor, 5,6-dihydroxyindole (DHI), within breakable particles and trigger its oxidation into insoluble black melanin, which can be released upon the degradation of the carrier using UV light. This process can be monitored via UV spectroscopy and has also been successfully tested on hair. In fact, when applied to human white hair, the result was a brownish coloration, with melanin naturally formed and adsorbed by the hair. This mild and harmless method shows great promise for the development of nanoparticles capable of penetrating hair follicles, potentially enabling natural hair pigmentation at the roots.

The role of Mg(SPh)₂ on photocatalytic alkylation of an active methylene with a nonactivated alkene was investigated. Kinetic studies indicate that one-electron oxidation of a Mg enolate is the rate-determining step. ESR experiments suggest an interaction between Lewis acidic metals and a radical anion species of 4CzIPN. These results suggest that the higher Lewis acidity of Mg(SPh)₂ stabilizes the 4CzIPN radical anion species to enhance its relative abundance and might influence both the one-electron oxidation step and the radical addition step in the catalytic cycle, leading to enhanced reaction efficiency.