PhenTAA: A Redox-Active N-Macrocyclic Ligand Featuring Donor and Acceptor Moieties.

Here, we present the development and characterization of the novel PhenTAA macrocycle as well as a series of [Ni(RPhenTAA)] complexes featuring two sites for ligand-centered redox-activity. These differ in the substituent R (R = H, Me, or Ph) and overall charge of the complex n (n = -2, -1, 0, +1, or +2). Electrochemical and spectroscopic techniques (CV, UV/vis-SEC, X-band EPR) reveal that all redox events of the [Ni(RPhenTAA)] complexes are ligand-based, with accessible ligand charges of -2, -1, 0, +1, and +2.

Carbene Radicals in Transition-Metal-Catalyzed Reactions.

Discovered as organometallic curiosities in the 1970s, carbene radicals have become a staple in modern-day homogeneous catalysis. Carbene radicals exhibit nucleophilic radical-type reactivity orthogonal to classical electrophilic diamagnetic Fischer carbenes. Their successful catalytic application has led to the synthesis of a myriad of carbo- and heterocycles, ranging from simple cyclopropanes to more challenging eight-membered rings.

Understanding Off-Cycle and Deactivation Pathways in Radical-Type Carbene Transfer Catalysis.

Transition metal radical-type carbene transfer catalysis is a sustainable and atom-efficient method to generate C-C bonds, especially to produce fine chemicals and pharmaceuticals. A significant amount of research has therefore been devoted to applying this methodology, which resulted in innovative routes toward otherwise synthetically challenging products and a detailed mechanistic understanding of the catalytic systems. Furthermore, combined experimental and theoretical efforts elucidated the reactivity of carbene radical complexes and their off-cycle pathways.

Cobalt(II)-tetraphenylporphyrin-catalysed carbene transfer from acceptor-acceptor iodonium ylides via N-enolate-carbene radicals.

Square-planar cobalt(II) systems have emerged as powerful carbene transfer catalysts for the synthesis of numerous (hetero)cyclic compounds via cobalt(III)-carbene radical intermediates. Spectroscopic detection and characterization of reactive carbene radical intermediates is limited to a few scattered experiments, centered around monosubstituted carbenes. Here, we reveal the formation of disubstituted cobalt(III)-carbene radicals derived from a cobalt(II)-tetraphenylporphyrin complex and acceptor-acceptor λ-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application.

Substrate-Controlled Product Divergence: Conversion of CO2 into Heterocyclic Products.

Substituted epoxy alcohols and amines allow substrate-controlled conversion of CO2 into a wide range of heterocyclic structures through different mechanistic manifolds. This new approach results in an unusual scope of CO2-derived products by initial activation of CO2 through either the amine or alcohol unit, thus providing nucleophiles for intramolecular epoxy ring opening under mild reaction conditions. Control experiments support the crucial role of the amine/alcohol fragment in this process with the nucleophile-assisted ring-opening step following an SN i pathway, and a 5-exo-tet cyclization, thus leading to heterocyclic scaffolds.