Nickel-catalyzed catalyst transfer polycondensation (CTP) of thiophenes is an efficient strategy for the controlled synthesis of polythiophenes. However, a detailed view of its reaction mechanism has remained elusive with unresolved questions regarding the geometry and bonding of critical Ni(0) thiophene intermediates. Herein, we provide experimental and computational evidence of structurally characterized square planar η2-Ni(0)–thiophene species and their relevance to the mechanism of CTP. These results confirm the viability of C,C-η2 bound intermediates in CTP of thiophenes, providing an electronic rationale for the stability of such species, and thus that such processes can proceed as living polymerizations. We further show that C,S-κ2 species may also be relevant in nickel-catalyzed CTP of thiophenes, providing new avenues for exploitation and optimization.
The electronic nature of Ni π‐complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π‐complexes, Ni(dtbpe)(X) (dtbpe=1,2‐bis(di‐tert‐butyl)phosphinoethane; X=alkene or carbonyl containing π‐ligands), is probed using a combination of 31P NMR, Ni K‐edge XAS, Ni Kβ XES, and DFT calculations. These complexes are best described as square planar d10 complexes with π‐backbonding acting as the dominant contributor to M−L bonding to the π‐ligand. The degree of backbonding correlates with 2JPP from NMR and the energy of the Ni 1s→4pz pre‐edge in the Ni K‐edge XAS data, and is determined by the energy of the π*ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π‐acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d10 electronic configuration. This ligand‐induced backbonding can be formally described as a 3‐centre‐4‐electron (3c‐4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ‐donors to the π*ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.
Three new polyketide compounds (1-3), a new quinolone alkaloid (4), and seven known polyketide derivatives were identified from the cultures of Penicillium sp. I09F 484, a strain isolated from the rhizosphere soil of the plant Picea asperata from Kanas Lake, Xinjiang, China. Their structures were elucidated by extensive spectroscopic data analysis. The absolute configurations of 1 and 4 were established by quantum chemical time-dependent density functional theory electronic circular dichroism calculation and Marfey's method, respectively. Compounds 1 and 2 displayed inhibitory activity against New Delhi metallo-β-lactamase 1 with IC₅₀ values of 94.9 and 87.9 μM, respectively.
The details of ligand-induced backbonding in nickel diphosphine π complexes are explored using nickel L-edge (3d←2p) X-ray absorption spectroscopy as a means of quantifying the degree of backbonding derived from direct Ni 3d donation into the π ligand.
A series of novel N-phenylbenzamide derivatives were synthesized and their anti-EV 71 activities were assayed in vitro. Among the compounds tested, 3-amino-N-(4-bromophenyl)-4-methoxybenzamide (1e) was active against the EV 71 strains tested at low micromolar concentrations, with IC 50 values ranging from 5.7 ± 0.8-12 ± 1.2 μM, and its cytotoxicity to Vero cells (TC 50 = 620 ± 0.0 μM) was far lower than that of pirodavir (TC 50 = 31 ± 2.2 μM). Based on these results, compound 1e is a promising lead compound for the development of anti-EV 71 drugs.
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