Periodic Reporting for period 1 - CarbFunction (Carbanions as Functional Groups and Building Blocks for Novel Reagents, Catalysts and Materials)
Okres sprawozdawczy: 2023-07-01 do 2025-12-31
Carbanionic compounds such as organolithium or Grignard reagents are important organometallic reagents and commonly used in organic syntheses both in academia as well as in large industrial processes. Due to the strongly polarized metal carbon bond these reagents are usually highly reactive and thus capable of performing difficult deprotonation or C-C bond formation reactions. However, this reactivity makes these reagents also sensitive towards air and moisture and thus requires their handling under strictly anhydrous conditions using special inert gas techniques. Therefore, carbanionic compounds are usually only prepared in situ and scarcely applied as isolated compounds thus limiting applications beyond their classical use as strong bases or alkylation reagents.
The carbanionic centre in polar organometallic compounds is usually engaged in metal-carbon interactions which provide additional stabilization through electrostatic attractions. Therefore, larger aggregates are formed to stabilize the anionic charge. Free carbanions without stabilizing metal-carbon inter¬action are thus usually even more reactive but can be isolated by careful molecular design. These naked carbanions are isoelectronic to simple amines. Yet, whereas amines are applied in various fields of chemistry not only as bases but also as versatile building blocks and functional groups, applications of free carbanions are very limited.
This project will change the perspective on carbanionic compounds. By careful molecular design the reactivity of carbanions will be controlled to enable their use as broadly applicable building blocks and functional groups. Experimental and computational methods will provide a fundamental understanding of the electronic structure and its influencing factors, thus allowing for a systematic use of the anionic nature and donor capacity of carbanions to reach properties and reactivities, which are not accessible via conventional strategies. Thus, we will provide a new toolbox for the design of smart anionic reagents and catalysts. The power of this concept will be demonstrated by applying carbanions in different research directions ranging from the stabilization of main group species with unusual electronic properties, to designing new bifunctional catalysts with abundant s- and p-block elements and the generation of versatile weakly coordinating anions.
The carbanionic centre in polar organometallic compounds is usually engaged in metal-carbon interactions which provide additional stabilization through electrostatic attractions. Therefore, larger aggregates are formed to stabilize the anionic charge. Free carbanions without stabilizing metal-carbon inter¬action are thus usually even more reactive but can be isolated by careful molecular design. These naked carbanions are isoelectronic to simple amines. Yet, whereas amines are applied in various fields of chemistry not only as bases but also as versatile building blocks and functional groups, applications of free carbanions are very limited.
This project will change the perspective on carbanionic compounds. By careful molecular design the reactivity of carbanions will be controlled to enable their use as broadly applicable building blocks and functional groups. Experimental and computational methods will provide a fundamental understanding of the electronic structure and its influencing factors, thus allowing for a systematic use of the anionic nature and donor capacity of carbanions to reach properties and reactivities, which are not accessible via conventional strategies. Thus, we will provide a new toolbox for the design of smart anionic reagents and catalysts. The power of this concept will be demonstrated by applying carbanions in different research directions ranging from the stabilization of main group species with unusual electronic properties, to designing new bifunctional catalysts with abundant s- and p-block elements and the generation of versatile weakly coordinating anions.
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.
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.
Breakthrough results beyond the current state of the art were achieved by employing carbanions as powerful electron-donating groups and by establishing ligand exchange reactions at carbanionic centers as a novel synthetic strategy. Owing to the superior donor strength of carbanions—which surpasses that of traditional electron-donating groups such as amines—we were able to design and synthesize highly electron-rich phosphine ligands. These ligands significantly enhanced the performance of gold and palladium catalysts, enabling catalytic transformations that were previously inaccessible using conventional ligands. As a result, this approach not only expanded the reactivity profile of well-established catalytic systems but also opened up new avenues for challenging transformations in homogeneous catalysis. The success of these ligands underlines the potential of carbanions as tunable electronic elements in catalyst design, offering promising opportunities for future applications in selective, efficient, and sustainable catalysis.
Our work on ligand exchange reactions at carbanionic centers represents a genuine breakthrough—both as a methodological innovation and as a fundamental contribution to our understanding of chemical bonding. In this context, we demonstrated that zwitterionic carbanions exhibit pronounced ambiphilic behavior, enabling them to react directly with small, often inert molecules through elimination of phosphine or even dinitrogen. Building on this reactivity, we developed a synthetically valuable methodology that allows the generation of functionalized carbanionic reagents from sustainable and readily available small molecules, such as carbon monoxide or nitrous oxide. This transformation not only highlights the unusual reactivity of these species but also points to new directions in sustainable synthesis.
Our work on ligand exchange reactions at carbanionic centers represents a genuine breakthrough—both as a methodological innovation and as a fundamental contribution to our understanding of chemical bonding. In this context, we demonstrated that zwitterionic carbanions exhibit pronounced ambiphilic behavior, enabling them to react directly with small, often inert molecules through elimination of phosphine or even dinitrogen. Building on this reactivity, we developed a synthetically valuable methodology that allows the generation of functionalized carbanionic reagents from sustainable and readily available small molecules, such as carbon monoxide or nitrous oxide. This transformation not only highlights the unusual reactivity of these species but also points to new directions in sustainable synthesis.