Helvetica Chimica Acta最新文献

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.

Optical control of molecular beams is intriguing as it promises to become a new tool for mass spectrometry and quantum interferometry, where a single or two photons deterministically remove a tailored tag from a larger molecular structure, e. g., a polypeptide. This cleavage process can change the charge state of the macromolecule, provide reporting signals for both fragments by mass spectrometry and it can selectively remove the fragments from a molecular beam by the momentum recoil generated in the dissociation process. Here, we explore a series of porphyrin derivatives as candidates for photocleavage in the gas phase. They share a large, conjugated core which promises a high absorption cross section for visible light. We present the individualization and beam formation of candidate molecules and study their photo-dissociation under tunable, visible radiation. We observe a significant wavelength shift and broadening in the photocleavage cross section for molecules in the gas phase compared to those in solution and we find that a single photon can suffice to trigger the cleavage process.

The carbonylation of methyl nitrite (MN) to synthesize dimethyl carbonate (DMC) is a promising route, but the lack of efficient and stable catalysts remains a major challenge for large-scale applications. In this study, by adjusting the alkalinity of the initial gel, NaY zeolites with tunable acidity were synthesized, and the amount of Lewis acid sites was optimized. Appropriate amounts of Lewis acid sites facilitated the interaction between NaY zeolites and Pd species, which was beneficial for stabilizing the oxidation state of Pd and promoting CO adsorption and activation, thereby contributing to the formation of the key intermediate *COOCH₃. The catalyst PdCu/NaY-1 (alkalinity in the initial gel Na₂O=9) exhibited significant improvement in catalytic performance for the synthesis of DMC, with the WTY_DMC of 1604 g kg_cat⁻¹⋅h⁻¹, the conversion of CO to DMC (C_CO) of 88.7 %, and the DMC selectivity based on CO (S_DMC/CO) up to 100 %.

Protonation and tautomerization significantly impact the conformational landscape and non-covalent interactions in bis(oxazoline) (BOX) ligands, which are crucial for their applications in catalysis. These interactions influence properties such as binding affinity and catalytic efficiency. While tautomerization has been reported for neutral BOX ligands, its role in protonated forms remains less understood. Here we report the structural and spectroscopic characterization of (S,S−Ph-BOX)H⁺ and its tautomerization-resistant derivative (S,S−Ph-diMeBOX)H⁺. We show through IRMPD spectroscopy and TIMS experiments, combined with DFT calculations that (S,S−Ph-BOX)H⁺ is present in its tautomeric form, while a broader conformational landscape is observed for (S,S−Ph-diMeBOX)H⁺, in which an N−H⋅⋅⋅N proton-shared conformer is identified. These findings provide a deeper understanding of the relationship between tautomerization and non-covalent interactions in protonated BOX ligands, offering insights for the design of catalytic systems.

The rich chemistry of East Indian Sandalwood oil has fascinated chemists for over a century. Whilst the synthesis on industrial scale of the principal odour vector (−)-(Z)-β-Santalol 2, has resisted all attempts to date, many synthetic fragrant sandalwood ingredients have been discovered and manufactured on industrial scale. This precious wood is still listed as endangered and appears on the CITES red list, despite an increased number of plantations being developed. This review will focus on the interesting chemistry of Santalum album (Linn.) essential oil and the subsequent efforts by researchers to find a viable substitute for this highly appreciated oil both by investigating novel structures and using biotech approaches. The emphasis will be on the approaches to these highly appreciated odorants made from renewable resources that are currently manufactured on multi-MT scale. Lesser emphasis will be placed on close structural analogues that have not been commercially successful.