Development of Catalytic Cross-Coupling Reactions Based on Transition Metal→Z-type Ligand Interactions

Abstract

Polyfunctional ligands containing Lewis acid (LA) moieties have attracted great interest over the last decade on account of their interesting properties. One of the most notable features is their ability to function as σ-electron acceptor (Z-type) ligands for transition metals (TMs). The presence of LA moiety around a TM strongly influences the reactivity of the TM complex and allows unconventional metal-ligand cooperation, leading to versatile applications in catalysis. Z-type ligands are typically used to generate electrophilic catalysts by withdrawing electrons from the TM via TM→Z-type ligand interactions. The cooperative activation of σ-bonds in TM→LA interactions has been observed in a number of catalytic systems. In addition, the interconversion between X- and Z-type ligands has been found to promote efficient cross-coupling reactions. This article describes our recent results on Z-type ligand chemistry. First, a new type of Pd-borane cooperation is described, in which Pd cross-coupling reactions operate via an unconventional mechanism involving anionic Pd(0) species. This cooperation enables the hydro- / deutero-dehalogenation of a variety of (hetero)aryl chlorides. Next, the first cross-coupling reactions of the fluoro-silanes and -germanes as coupling partners are introduced. The coordination of the fluoro-silanes and -germanes with the TM as Z-type ligands presents a new approach for bond activation. In this case, the TM acts as a Lewis base and efficiently weakens the Si-F and Ge-F bonds via TM→σ^*(E-F) interactions (E＝Si and Ge). The synergistic action of an external LA allows for the cleavage of strong Si-F and Ge-F bonds and converts the Z-type silane and germane ligands into X-type silyl and germyl moieties, respectively. This could be employed in the Si-C and Ge-C cross-coupling reactions of fluoro-silanes and -germanes.