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Chase A. Salazar Joseph J. Gair Kaylin N. Flesch Ilia A. Guzei Jared C. Lewis Shannon S. Stahl 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(27):10965-10969
Mono-N-protected amino acids (MPAAs) are increasingly common ligands in Pd-catalyzed C−H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C−H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand-accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand-to-metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C−H activation and catalytic C−H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50–100-fold increase in rate when only 0.05 equivalents of MPAA are present relative to PdII. These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to PdII enables a single MPAA to support C−H activation at multiple PdII centers. 相似文献
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Dr. Marzia Dell'Aera Dr. Filippo Maria Perna Dr. Paola Vitale Dr. Angela Altomare Prof. Alessandro Palmieri Dr. Lewis C. H. Maddock Leonie J. Bole Dr. Alan R. Kennedy Prof. Eva Hevia Prof. Vito Capriati 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(40):8742-8748
We report the first transition metal catalyst- and ligand-free conjugate addition of lithium tetraorganozincates (R4ZnLi2) to nitroolefins. Displaying enhanced nucleophilicity combined with unique chemoselectivity and functional group tolerance, homoleptic aliphatic and aromatic R4ZnLi2 provide access to valuable nitroalkanes in up to 98 % yield under mild conditions (0 °C) and short reaction time (30 min). This is particularly remarkable when employing β-nitroacrylates and β-nitroenones, where despite the presence of other electrophilic groups, selective 1,4 addition to the C=C is preferred. Structural and spectroscopic studies confirmed the formation of tetraorganozincate species in solution, the nature of which has been a long debated issue, and allowed to unveil the key role played by donor additives on the aggregation and structure of these reagents. Thus, while chelating N,N,N’,N’-tetramethylethylenediamine (TMEDA) and (R,R)-N,N,N’,N’-tetramethyl-1,2-diaminocyclohexane (TMCDA) favour the formation of contacted-ion pair zincates, macrocyclic Lewis donor 12-crown-4 triggers an immediate disproportionation process of Et4ZnLi2 into equimolar amounts of solvent-separated Et3ZnLi and EtLi. 相似文献
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Transport in Porous Media - We develop an analytical model describing the flow of NaCl– $$\hbox {H}_2\hbox {O}$$ in a saturated porous medium adjacent to a hot vertical wall and apply the... 相似文献
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Dongjie Zhang Miguel A. Soto Lev Lewis Wadood Y. Hamad Mark J. MacLachlan 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(12):4735-4740
Cellulose nanocrystals (CNCs) spontaneously assemble into gels when mixed with a polyionic organic or inorganic salt. Here, we have used this ion-induced gelation strategy to create functional CNC gels with a rigid tetracationic macrocycle, cyclobis(paraquat-p-phenylene) ( CBPQT 4+). Addition of [ CBPQT ]Cl4 to CNCs causes gelation and embeds an active host inside the material. The fabricated CNC gels can reversibly absorb guest molecules from solution then undergo molecular recognition processes that create colorful host–guest complexes. These materials have been implemented in gel chromatography (for guest exchange and separation), and as elements to encode 2- and 3-dimensional patterns. We anticipate that this concept might be extended to design a set of responsive and selective gel-like materials functioning as, for instance, water-pollutant scavengers, substrates for chiral separations, or molecular flasks. 相似文献
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Journal of Heuristics - This paper proposes two heuristic algorithms for finding fixed-length circuits and cycles in undirected edge-weighted graphs. It focusses particularly on a largely... 相似文献
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High‐Energy Long‐Lived Mixed Frenkel–Charge‐Transfer Excitons: From Double Stranded (AT)n to Natural DNA 下载免费PDF全文
Dr. Ignacio Vayá Dr. Johanna Brazard Dr. Miquel Huix‐Rotllant Dr. Arun K. Thazhathveetil Prof. Frederick D. Lewis Dr. Thomas Gustavsson Prof. Dr. Irene Burghardt Dr. Roberto Improta Dr. Dimitra Markovitsi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(14):4904-4914
The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV‐induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine–thymine double‐stranded structures (AT)n. Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time‐dependent (TD)‐DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine‐to‐thymine charge‐transfer states. Emission from such high‐energy long‐lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for π–π* states (≥0.1). An increase in the size of the system quenches π–π* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between π–π* and charge‐transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n: high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive π–π* states, giving rise to delayed fluorescence. 相似文献