ResearchPad - asymmetric-catalysis https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light‐Mediated Cycloadditions]]> https://www.researchpad.co/article/elastic_article_6757 Bind me tender: Cyclic enone‐3‐carboxylic acids (X=CH2, O) display a binding motif (in blue) for an assembly with chiral acid 1. The acid acts as photocatalyst by transferring the energy of visible photons to the substrate via its thioxanthone substituents (in yellow) enabling an enantioselective [2+2] photocycloaddition (see scheme).

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<![CDATA[Stereochemistry of the Reaction Intermediates of Prolinol Ether Catalyzed Reactions Characterized by Vibrational Circular Dichroism Spectroscopy]]> https://www.researchpad.co/article/Nf7d7f78a-ea9a-4de1-9780-ad36a2645b72

Abstract

Spectroscopic characterizations of key reaction intermediates are often considered the final confirmation of a reaction mechanism. This proof‐of‐principle study showcases the application of vibrational circular dichroism (VCD) spectroscopy for the characterization of in situ generated reaction intermediates using the key intermediates of enamine catalysis of Jørgensen–Hayashi‐type prolinol ether catalysts as model system. By comparison of experimental and computed spectra, the enamines are shown to preferentially adopt an anti‐conformation with E‐configured C=C bond. For the parent prolinol catalyst, the structure and stereochemistry of the oxazolidine side product is determined as well. This study thus demonstrates that VCD spectra can provide insights into structural preferences of organocatalysts that utilize a covalent activation mechanism. Thereby it outlines new fields of applications for VCD spectroscopy and finally adds the technique to the toolbox of physical organic chemistry for in‐depth mechanistic studies.

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<![CDATA[Bis‐Cyclometalated Indazole Chiral‐at‐Rhodium Catalyst for Asymmetric Photoredox Cyanoalkylations]]> https://www.researchpad.co/article/Nc8ed3d4d-972e-43c1-ad80-85532cf6bc34

Abstract

A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).

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