Remarkable Ability of the Benzylidene Ligand To Control Initiation of Hoveyda–Grubbs Metathesis Catalysts. Issue 21 (22nd June 2016) - Basak, Tymoteusz
Abstract : The structure of the chelating benzylidene ligand offers the unique ability to control the initiation of Hoveyda–Grubbs metathesis catalysts. Apart from steric and electronic effects acting on the step involving opening of the chelate ring, changes related to the following ligand‐exchange process may also play a critical role. Our mechanistic model reveals that ligands substituted at the 6‐position of the benzylidene ring enter the metathesis cycle in a nonoptimal chelating conformation, and thus the coordination number of the ruthenium center transiently increases to six (associative mechanism). In effect, the synthesis and initiation of the catalysts becomes difficult, and the energy barrier of the ligand‐exchange process is controlled by the structure of the coordinating OR group. Moreover, we explain how isomeric naphthalene ligands affect the catalytic performance by an indivisible combination of steric and π‐electron delocalization effects. Abstract : Initiation of Hoveyda–Grubbs metathesis catalysts requires opening of the chelate ring, followed by ligand exchange in the initial turnover of the metathesis cycle. Our studies reveal that substitution at the 6‐position of the benzylidene ring influences the latter process and shifts the complexes to the latent systems (Mes = 2, 4, 6‐trimethylphenyl).