A step by step mechanistic study of nickel catalyzed Kumada aryl-aryl coupling reactions: the crucial role of a pincer PPP ligand

In the bimonthly Debye lunch lectures, young scientists from the Debye Institute for Nanomaterials Science present their research. 

This Debye Lunch Lecture, A step by step mechanistic study of nickel catalyzed Kumada aryl-aryl coupling reactions: the crucial role of a pincer PPP ligand, will be given by Pablo Pérez-García from the Organic Chemistry and Catalysis research group.

Abstract

Cross-coupling reactions leading to the formation of carbon-carbon bonds are among the most important synthetic tools that chemists use to produce new molecules in academic and industrial research. These reactions are mostly catalyzed by transition metal complexes, for which the metal center is chosen considering efficient reactivity and sustainability aspects. High availability and diversity are essential characteristics for useful coupling substrates; this is the case of aryl Grignard reagents making Kumada cross-coupling reactions very versatile. Three basic steps can describe its classical mechanism: oxidative addition, transmetallation, and reductive elimination. A detailed mechanistic understanding of each of these steps is crucial for the development of new efficient, and sustainable catalytic methodologies. Nickel is more abundant than palladium but also offers an enhanced reactivity for the activation of challenging substrates as alkyl halides by radical pathways. Nevertheless, the easy access of nickel to radical oxidation states as Ni^I increases the probability of generating off-cycle species and complicates the understanding of the mechanism through detailed studies. For this reason, a rational design and a judicious choice of the surrounding ligand are pivotal for taming and understanding nickel reactivity. Herein, we describe how a pincer PPP ligand allowed the detailed mechanistic study of the coupling of aryl iodides with aryl Grignard reagents. The coordination framework of ligand PPP generated unique five coordinated intermediates easily characterized by NMR spectroscopy. Additionally, we were able to identify off-cycle Ni^I species and well as study their reactivity. Finally, all our experimental data were supported by DFT calculations carried out by the group of Prof. Jeremy Harvey at KU Leuven.

Pérez-García, P. M.; Darù, A.; Scheerder, A. R.; Lutz, M.; Harvey, J. N.; Moret, M.-E. Organometallics 2020, 8, 1139-1144.