Theoretical Investigation of C−H/Olefin Coupling Catalyzed by Zirconium(IV) Complexes

Abstract

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 α-picoline to propene to produce 2-Me-6-iPr-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 η2-pyridyl complex Cp2Zr(η2-2-Me-6-pyridyl)+ (1) to yield the azametallacycle Cp2Zr{η2-C,N-CH2CHMe-(2-Me-6-pyridyl)}+ (2), (ii) hydrogenolysis of 2 to produce Cp2Zr(H)(2-Me-6-iPr-pyridine)+ (3), (iii) ligand substitution of 3 by α-picoline to release 2-Me-6-iPr-pyridine and form Cp2Zr(H)(2-Me-pyridine)+ (4), and (iv) C−H activation of 4 to release H2 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 H2, the origin of the regioselectivity of the C−H activation and insertion steps, and the preference for α-picoline/propene coupling over propene hydrogenation.