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An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

ABSTRACT: A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B₂ Br₄ (SMe₂ )₂ . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy₃ )] afforded two diborene-Au^I π complexes, while reaction with DurBH₂ , P₄ and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to Ag^I via its B=C double bond.

SUBMITTER: Lu W

PROVIDER: S-EPMC9299934 | biostudies-literature | 2022 Jan

REPOSITORIES: biostudies-literature

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An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

Lu Wei W Jayaraman Arumugam A Fantuzzi Felipe F Dewhurst Rian D RD Härterich Marcel M Dietz Maximilian M Hagspiel Stephan S Krummenacher Ivo I Hammond Kai K Cui Jingjing J Braunschweig Holger H

Angewandte Chemie (International ed. in English) 20211203 3

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with ...[more]

PMID: 34750945

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Project description:Ruthenium catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands can attain very high productivities in olefin metathesis, owing to their resistance to unimolecular decomposition. Because the propagating methylidene species RuCl2(CAAC)(=CH2) is extremely susceptible to bimolecular decomposition, however, turnover numbers in the metathesis of terminal olefins are highly sensitive to catalyst concentration, and hence loadings. Understanding how, why, and how rapidly the CAAC complexes partition between the precatalyst and the active species is thus critical. Examined in a dual experimental-computational study are the rates and basis of initiation for phosphine-free catalysts containing the leading CAAC ligand C1 Ph , in which a CMePh group α to the carbene carbon helps retard degradation. The Hoveyda-class complex HC1 Ph (RuCl2(L)(=CHAr), where L = C1 Ph , Ar = C6H3-2-O i Pr-5-R; R = H) is compared with its nitro-Grela analogue (nG-C1 Ph ; R = NO2) and the classic Hoveyda catalyst HII (L = H2IMes; R = H). t-Butyl vinyl ether (tBuVE) was employed as substrate, to probe the reactivity of these catalysts toward olefins of realistic bulk. Initiation is ca. 100× slower for HC1 Ph than HII in C6D6, or 44× slower in CDCl3. The rate-limiting step for the CAAC catalyst is cycloaddition; for HII, it is tBuVE binding. Initiation is 10-13× faster for nG-C1 Ph than HC1 Ph in either solvent. DFT analysis reveals that this rate acceleration originates in an overlooked role of the nitro group. Rather than weakening the Ru-ether bond, as widely presumed, the NO2 group accelerates the ensuing, rate-limiting cycloaddition step. Faster reaction is caused by long-range mesomeric effects that modulate key bond orders and Ru-ligand distances, and thereby reduce the trans effect between the carbene and the trans-bound alkene in the transition state for cycloaddition. Mesomeric acceleration may plausibly be introduced via any of the ligands present, and hence offers a powerful, tunable control element for catalyst design.

Project description:Misfolding of the amyloid-β peptide (Aβ) and its subsequent aggregation into toxic oligomers is one of the leading causes of Alzheimer's disease (AD). As a therapeutic approach, cyclic peptides have been modified in many ways and developed as a potential class of amyloid aggregation inhibitors. Head-to-tail cyclic peptides with alternating d, l amino acids inhibit amyloid aggregation significantly. On the other hand, excess deposition of copper, iron, and zinc enhances amyloid aggregation. Dysregulation of these metal ions in the brain triggers aggregation by binding to the Aβ peptide. Therefore, specific metal chelators have been developed for disrupting the Aβ-metal complex, thereby reducing toxicity and restoring metal ion homeostasis. Herein, we report the development of a head-to-tail cyclic peptidomimetic with a copper chelating ligand attached. The designed peptidomimetic inhibits amyloid aggregation significantly in a two-fold molar ratio to the Aβ peptide, as confirmed by the thioflavin T (ThT) fluorescence assay, dynamic light scattering (DLS), transmission electron microscopy (TEM), and Congo-red stained birefringence studies. The chelating ligand attached to the cyclic peptide binds efficiently to Cu2+ but weakly to Zn2+ and Fe2+, thereby exhibiting profound selectivity, probed using UV-visible spectroscopy, thioflavin T (ThT) fluorescence assay, tyrosine (TYR10) fluorescence assay, isothermal titration calorimetry (ITC) and transmission electron microscopy (TEM). The non-toxicity of the designed peptidomimetics and their ability to reduce aggregating Aβ-fragment induced cytotoxicity was confirmed by the MTT assay on the mouse neuronal cell line. Further, the molecular interaction between the peptidomimetics and the Aβ-fragment was confirmed by Förster resonance energy transfer (FRET) studies using fluorescently labeled analogs. Cytotoxicity and cell internalization were also confirmed. A preliminary mechanistic investigation indicates that the peptidomimetic works by a synergistic effect of conformational restriction and metal sequestration. Such peptidomimetics can shed light on the mechanism of aggregation and a novel therapeutic approach.

Dataset Information

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

Publications

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

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