Bond Activation by Metal-Ligand Cooperation

Bond Activation by Metal-Ligand Cooperation Design of Green Catalytic Reactions Based on Aromatization-Dearomatization of Pincer Complexes... 

Recently, we have discovered a new mode of bond activation by metal—Ugand cooperation based on aromatization-dearomatization of pyridine- and acridine-based heteroaromatic pincer complexes [50-60]. Deprotonation of a pyridinyl-methylenic proton of a pyridine-based pincer complex can lead to dearomatization. The dearomatized complex can then activate a chemical bond (H-Y, Y = H, OR, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Fig. 1). The overall process does not involve a change in the metal oxidation state. In this chapter, we describe the novel, environmentally benign catalytic transformations that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

Like in the case of 2, the bipyridine-based Ru(II)-PNN pincer complex 6 plausibly displays a novel type of metal-ligand cooperative activity through aromatization-dearomatization processes. Recently, a DFT calculation on classical C-O cleavage vs our newly reported C-N bond breaking in the amide hydrogenation reaction was reported by Cantillo [69]. 

Gunanathan C, Milstein D (2011) Metal-ligand cooperation by aromatization-dearomatization a new paradigm in bond activation and great catalysis. Ace Chem Res 44 588... 

In this chapter, we have successfully developed bifunctional chiral rhodium complexes bearing chiral phebox ligands that can be used in catalytic asymmetric reactions. The N,C,N meridional geometry with the rhodium-carbon covalent bond is the key character in the phebox complexes. The metal-phebox cooperative bifunctionality significantly contributes reactivity and selectivity in the catalytic asymmetric reactions. Furthermore, the prototype of the bifunctional catalyst can be explained to a wide range of asymmetric catalytic reactions promoted by the Lewis acids, hydrides, enolates, and bory active species. Their diversity further broadens the range of opportunities for asymmetric catalysis. 

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