Theoretical investigation of C-H/olefin coupling catalyzed by zirconium(IV) complexes

Density functional theory calculations at the B3LYP level have been performed to investigate the mechanism of the zirconocene-catalyzed addition of the ortho C-H bond of alpha-picoline to propene to produce 2-Me-6-Pr-i-pyridine. The computational results support the proposed mechanism, which involves (i) 2-Me-pyridine dissociation from [Cp2Zr(2-Me-6-pyridyl)(2-Me-pyridine)](+) followed by the insertion of propene into the Zr-C bond of the eta(2)-pyridyl complex Cp2Zr(eta(2)-2-Me-6-pyridyl)(+) (1) to yield the azametallacycle Cp2Zr{eta(2)-C,N-CH2CHMe-(2-Me-6-pyridy)}(+) (2), (ii) hydrogenolysis of 2 to produce Cp2Zr(H)(2-Me-6-Pr-i-pyridine)(+) (3), (iii) ligand substitution of 3 by alpha-picoline to release 2-Me-6-Pr-i-pyridine and form Cp2Zr(H)(2-Me-pyridine)(+) (4), and (iv) C-H activation of 4 to release H-2 and regenerate 1. Consistent with the experimental results, the 2-Me-pyridine dissociation from [Cp2Zr(2-Me-6-pyridyl)(2-Me-pyridine)](+) followed by the propene insertion of 1 and the hydrogenolysis of 2 are calculated to be the rate-determining steps. The calculations provide new insights into the role of the cocatalyst H-2, the origin of the regioselectivity of the C-H activation and insertion steps, and the preference for alpha-picoline/propene coupling over propene hydrogenation.