Open-shell organometallics: reactivity at the ligand
The purpose of this review is to show that (cooperative) ligand radical reactivity can be effectively employed in synthetic organometallic chemistry and catalysis to achieve selectivity in radical-type transformations. The ‘redox non-innocence’ of ligands, and the controlled reactivity of ‘ligand radicals’– giving rise to new, intriguing substrate transformations – allow unusual and selective radical-type substrate coupling reactions, ligand rearrangements and C-Y bond formations. In this review, several examples of fast and selective ligand-centered radical transformations in the open-shell organometallic chemistry of transition metals are described, focussing on radical-type reactions of olefin-, vinylidene-, vinyl-, alkyne-, allyl-, propargyl-, carbonyl-, and carbene ligands. Intriguing and selective substrate-substrate coupling reactions, (covalent) carbon-metal bond formations and hydrogen atom transfer reactions from and to ligand radicals are summarized. To conclude this chapter, an overview of recently disclosed new insights in the catalytic mechanism of CoII(por) catalysed olefin cyclopropanation is presented, showing that enzyme-like cooperative metal-ligand-radical reactivity is no longer reserved to real enzymes, but is a useful new concept to steer and control radical-type transformations in future bio-inspired organometallic catalysis.