Bismutverbindungen in der Radikalkatalyse: Übergangsmetallbismutane ermöglichen thermisch induzierte Cycloisomerisierungen

Two Faces of the Bi−O Bond: Photochemically and Thermally Induced Dehydrocoupling for Si−O Bond Formation

The diorgano(bismuth)alcoholate [Bi((C₆H₄CH₂)₂S)OPh] (1‐OPh) has been synthesized and fully characterized. Stoichiometric reactions, UV/Vis spectroscopy, and (TD‐)DFT calculations suggest its susceptibility to homolytic and heterolytic Bi−O bond cleavage under given reaction conditions. Using the dehydrocoupling of silanes with either TEMPO or phenol as model reactions, the catalytic competency of 1‐OPh has been investigated (TEMPO=(tetramethyl‐piperidin‐1‐yl)‐oxyl). Different reaction pathways can deliberately be addressed by applying photochemical or thermal reaction conditions and by choosing radical or closed‐shell substrates (TEMPO vs. phenol). Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single‐crystal X‐ray diffraction analysis, and (TD)‐DFT calculations.

Cationic Bismuth Aminotroponiminates: Charge Controls Redox Properties

The behavior of the redox‐active aminotroponiminate (ATI) ligand in the coordination sphere of bismuth has been investigated in neutral and cationic compounds, [Bi(ATI)₃] and [Bi(ATI)₂L_n][A] (L=neutral ligand; n=0, 1; A=counteranion). Their coordination chemistry in solution and in the solid state has been analyzed through (variable‐temperature) NMR spectroscopy, line‐shape analysis, and single‐crystal X‐ray diffraction analyses, and their Lewis acidity has been evaluated by using the Gutmann–Beckett method (and modifications thereof). Cyclic voltammetry, in combination with DFT calculations, indicates that switching between ligand‐ and metal‐centered redox events is possible by altering the charge of the compounds from 0 in neutral species to +1 in cationic compounds. This adds important facets to the rich redox chemistry of ATIs and to the redox chemistry of bismuth compounds, which is, so far, largely unexplored.

Bismuth Amides Mediate Facile and Highly Selective Pn-Pn Radical-Coupling Reactions (Pn=N, P, As)

The controlled release of well-defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr2 )3 ] readily release aminyl radicals [NAr2 ]. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar2 N-NAr2 , as a result of highly selective N-N coupling. The exploitation of facile homolytic Bi-Pn bond cleavage for Pn-Pn bond formation was extended to higher homologues of the pnictogens (Pn=N-As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR2 to give R2 Pn-PnR2 . Analyses by NMR and EPR spectroscopy, single-crystal X-ray diffraction, and DFT calculations reveal low Bi-N homolytic bond-dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.

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

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.