Twisted boron‐based biradicals featuring unsaturated C2R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground‐state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C2R2‐bridged systems featuring N‐heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.
Treatment of an N-silyl-B-chloro-aminoborane with substoichiometric quantities of Me3SiOTf afforded B–N coupling, whereas activation with 5 mol% of Ag[Al{OC(CF3)3}4] led to Cl/Me exchange between the boron and the silicon center....
The 3-chymotrypsin-like protease 3CLpro from SARS-CoV-2 is a potential target for antiviral drug development. In this work, three organometallic ferrocene-modified quinolinones and coumarins were compared to their benzoic acid ester analogues with regard to inhibition of 3CLpro using a HPLC-based assay with a 15mer model peptide as the substrate. In contrast to FRET-based assays, this allows direct identification of interference of buffer constituents with the inhibitors, as demonstrated by the complete abolishment of ebselen inhibitory activity in the presence of DTT as a redox protectant. The presence of the organometallic ferrocene moiety significantly increased the stability of the title compounds towards hydrolysis. Among the studied compounds, 4-ferrocenyloxy-1-methyl-quinol-2-one was identified as the most stable and potent inhibitor candidate. IC50 values determined for ebselen and this sandwich complex compound are (0.40 ± 0.07) and (2.32 ± 0.21) μM, respectively.
Boron chemistry offers many systems in which slight variations lead to drastic geometrical and electronic perturbations. This article explains the reasons for variations observed in a system consisting of two boron centers stabilized by CAAC or NHC donors and connected by C2R2 (R=Et, Me, H) bridges. For CAAC and R=Me, Et the bridge is twisted, and the molecule is a biradical. For CAAC and R=H the bridge is not twisted, leading to a closed‐shell structure. For NHC‐stabilized systems we predict closed‐shell molecules with partially twisted bridges. This is attributed to a delicate interplay of steric and electronic effects. More information can be found in the Full Paper by B. Engels et al. on page 5160.
Invited for the cover of this issue is Bernd Engels, Holger Braunschweig, Volker Engel and their coworkers at University of Würzburg. The image depicts bridged boron compounds which possess fascinating relationships between their composition and their geometrical and electronic structures, the latter ranging from closed‐shell to biradical triplet or singlet ground state. Read the full text of the article at 10.1002/chem.202004619.
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