α‑Dicationic Chelating Phosphines: Synthesis and Application to the Hydroarylation of Dienes

A series
of new P^P-chelating ligands constituted by a dicationic
−[P­(H2Im)2]+2 unit (H2Im = 1,3-dimethyl-4,5-dihydroimidazol-2-ylidene) and a −PPh2 group connected through structurally different backbones
have been synthesized. Evaluation of their reactivity toward different
metal centers provides evidence that the dicationic fragment, otherwise
reluctant to coordinate metals, readily participates in the formation
of chelates when embedded into such a scaffold. Moreover, it significantly
enhances the Lewis acidity of the metals to which it coordinates.
This property has been used to develop a Rh catalyst that efficiently
triggers the hydroarylation of dienes with electron-rich aromatic
molecules. Kinetic studies and deuterium-labeling experiments, as
well as density functional theory calculations, were performed in
order to rationalize these findings.