Alkali‐Metal Aminotroponiminates: Selectivities and Equilibria in Reversible Radical Coupling of Delocalized π‐Electron Systems

Abstract

Aminotroponiminates (ATIs) have recently been shown to belong to the growing class of redox‐active ligands. The choice of the metal center allowed to switch between reversible electron transfer (M=Rh) and reductively induced dimerization (M=Na). Here, we investigate if the reductively induced dimerization of ATIs is a more general phenomenon for their alkali‐metal complexes. Lithium ATI complexes are shown to undergo reductively induced dimerizations, which are equilibrium reactions and chemically reversible. The choice of the metal center (Li vs. Na), the substitution pattern at the nitrogen atoms of the ATI ligands, and the solvent critically influence the regioselectivity and diastereoselectivity of the radical‐dimerization reactions. Potassium ATIs are shown to be susceptible to side reactions, more specifically a reduction accompanied by hydrogen‐atom transfer. Products and intermediates of the reductively induced dimerizations were characterized by techniques including NMR and EPR spectroscopy, cyclic voltammetry, DFT calculations, single‐crystal X‐ray diffraction, and mass spectrometry.

Main‐Group Metal Complexes in Selective Bond Formations Through Radical Pathways

“…Furthermore, the same strategy also provides control over the diastereoselectivity of radical C−C coupling reactions. This was exemplified, for instance, by the dimerization of 21 , which was generated in situ and detected by EPR spectroscopy (Scheme b) . Initially a 1:1 mixture of the meso ‐ and the rac ‐product ( meso ‐22 and rac ‐22 ) was obtained.…”

“…In the lithium compound 8 , the spin density is predominantly localized at the C‐4 position of the ATI ligand backbone, which is simultaneously the only reactive site for subsequent C−C coupling (Scheme a; also see section 3.1). Formal exchange of Li for Na gives radical 9 , which shows a significantly altered spin‐density distribution with equal spin density in positions C‐4 (decreased from 0.36 to 0.29) and C‐7 (increased from 0.22 to 0.29; Scheme b) . Subsequent dimerization of 9 occurs exclusively at the C‐7 position, suggesting that phenomena such as aggregation/chelation control are also important when it comes to understanding selectivities of reactions with 9 (see sections 2.6, 3.1).…”

ADUC‐Preise 2020

“…Sein Lieblingselement ist das Bismut . Ein Forschungsbericht seiner Gruppe über Alkalimetall‐Aminotroponiminate zierte das Titelbild von Chemistry—A European Journal ; in dieser Zeitschrift veröffentlichte er vor kurzem auch eine Übersicht zu Hauptgruppenmetallkomplexen in selektiven radikalischen Bindungsknüpfungsprozessen…”

ADUC Prizes 2020

“…As a Research Group Leader at the Institute of Inorganic Chemistry of Julius Maximilians University Würzburg, he studies complexes of main‐group metals for applications in synthesis and catalysis, bismuth being his favorite element . Original research by his group on alkali‐metal aminotroponiminates was selected for a cover feature in Chemistry—A European Journal , and his Review on main‐group metal complexes in selective bond formations via radical pathways has just appeared in the same journal…”