Ortho-metalation of pyridines by cationic yttrium methyl complexes

Selective C-H bond activation of pyridines by organometallic complexes is a reaction of synthetic importance for the synthesis of functionalized pyridines. By reacting the dicationic methyl complex of yttrium YMe(thf)₆]BPh₄]₂ with substituted pyridines, both selectivity and kinetics have been studied. Electron donating properties of para-substituents of pyridines increase the rates of reaction. Hammett linear free energy relationship was found with ρ = –2.72. DFT calculations confirmed the two-step reaction consisting of ligand substitution followed by σ-bond metathesis. DFT calculations furthermore revealed for the C-H bond activation step an unusual transition state structure with a nearly linear methyl carbon-hydrogen-ortho-carbon arrangement.