α-Cationic Phosphines: from Curiosities to Powerful Ancillary Ligands

Abstract

The distinguishing feature of α-cationic phosphines is the presence of at least one substituent, normally (hetero)cyclic and positively charged, which is directly attached to the phosphorus atom. As result from this unique substitution pattern, the thus designed ligands depict significantly diminished donor properties if compared with their neutral counterparts. Thus, if in a hypothetical catalytic cycle, the step that determines the rate is facilitated by an increase of the electrophilicity at the metal center; then, the use of α-cationic ancillary phosphines can be highly beneficial. This fact, combined with their easy syntheses and stability, which allows an easy handling, make α-cationic phosphines a useful tool for the synthetic practitioner. Our research on the topic demonstrates that generally a remarkable ligand acceleration effect is observed when α-cationic phosphines are employed in Au(I)- and Pt(II)-promoted cycloisomerizations; moreover, in some cases even otherwise not operative transformations can be promoted. This Account describes how we entered into the topic, our efforts, and those of others to understand the coordination behavior of α-cationic phosphines and further develop their range of applications in catalysis; but it also identifies the drawbacks associated with their use, which limit their range of application.1 Introduction2 Polycationic Phosphines: Stronger Acceptors than Phosphites3 Inconveniences Derived from the Use of (Poly)cationic phosphines4 A Second Generation of Cationic Ligands: α-Pyridiniophosphines5 Chiral α-Cationic Phosphines6 α-Radical Phosphines and (Poly)cationic Phosphine Oxides7 Conclusions and Outlook

Manganese‐Mediated Synthesis of NHC‐Phosphine Ligand Precursors

“…In compounds A – B , the positioning of the PR 2 fragment excludes the C,P ‐chelating coordination mode but gives access to cationic phosphine ligands where the close proximity of the positive charge has an impact on the electronic properties [25–27] and can favor catalytic processes requiring electron‐poor metal complexes [28–30] . For systems C‐1 , different coordination modes have been evidenced, namely the monodentate coordination of the NHC, [31–33] the chelating coordination of NHC and phosphine donors, [34,35] and the bridging coordination mode where the two L‐type donors can be linked to different metal centers [31,36,37] .…”

Synthesis of an Indazole/Indazolium Phosphine Ligand Scaffold and Its Application in Gold(I) Catalysis

“…These ligands’ properties are easily tuned by chemical modification, allowing for simple yet impactful changes to catalytic activity . One method for modification that has seen increasing use is the insertion of a positive charge in the ligand backbone . This positive charge has differing impacts on the coordinated metal depending on its position.…”

“… 1 One method for modification that has seen increasing use is the insertion of a positive charge in the ligand backbone. 2 This positive charge has differing impacts on the coordinated metal depending on its position. Over the past few years, the Gabbaï group has synthesized a group of acridinium- ( A ) and xanthylium-based ( B ) γ-cationic phosphines that position the cationic charge at a remote position to withdraw electron density orthogonally from gold while minimizing the reduction in σ-donation from the phosphine ( Figure 1 ).…”