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 Introduction
2 Polycationic Phosphines: Stronger Acceptors than Phosphites
3 Inconveniences Derived from the Use of (Poly)cationic phosphines
4 A Second Generation of Cationic Ligands: α-Pyridiniophosphines
5 Chiral α-Cationic Phosphines
6 α-Radical Phosphines and (Poly)cationic Phosphine Oxides
7 Conclusions and Outlook
Key words
α-cationic phosphines - ligand design - π-acid catalysis - hydroarylation - asymmetric
catalysis - helicenes
