Within this research project, we aimed to establish "non-traditional" directions in carbanion chemistry, moving beyond their classical role as strongly nucleophilic and basic reagents.
In one sub-project, we investigated the use of carbanions as functional groups, leveraging their enhanced electron-donating properties for applications in homogeneous catalysis. We developed zwitterionic phosphines, which enabled the first C–C cross-coupling protocol involving highly reactive organopotassium compounds. Due to the high catalytic activity of our ligands, we achieved efficient bond formation across a broad range of aryl chlorides. In addition, we introduced a new class of electron-rich phosphines by positioning a carbanionic center adjacent to the phosphorus atom. This was accomplished through geometric twisting of the double bond in phosphines functionalized with N-heterocyclic olefins. These ligands demonstrated excellent performance in gold catalysis, exhibiting superior activity compared to structurally analogous ligands lacking a carbanionic motif.
In a further subproject, we aimed at developing carbanionic ligands for the formation of main group element complexes to enable the activation of strong bonds and small molecules. During the course of these studies, we observed that ylidic ligands undergo phosphine elimination when exposed to reagents such as carbon monoxide or nitrous oxide. Recognizing that this decomposition pathway reflected a broader reactivity trend, we transformed what was initially an undesired side reaction into a deliberate and synthetically valuable transformation. We were able to compellingly demonstrate the synthetic potential of these anionic species, which readily reacted with small molecules to form highly functionalized products. Notably, reactions with sustainable building blocks such as carbon dioxide enabled the efficient construction of value-added compounds. In addition, these reagents facilitated access to structurally diverse heterocycles. As such, this methodology represents a significant advancement not only in synthetic methodology but also in sustainable chemistry, particularly with regard to the utilization of abundant, non-fossil-based feedstocks.
In a third line of research, we set out to demonstrate that carbanions can be stabilized to such a degree that they become virtually inert and may function as weakly coordinating anions (WCAs). Indeed, we successfully synthesized a highly stabilized carbanion that exhibited exceptional stability, even in the presence of air and water, along with very weak coordinating ability. This enabled the use of the carbanion as a chemically inert counterion for stabilizing highly reactive main-group cations.
Taken together, our studies underscore the versatility of carbanions beyond their traditional roles as nucleophiles and bases. We demonstrated that carbanions can be tailored to function as weakly coordinating anions, strong electron-donating functional groups, and even ambiphilic reagents, thereby enabling the synthesis of highly functionalized compounds from sustainable small molecules.
