Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity

The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solid-state NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl2(H2IMes)(PCy3)(═CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl2(PCy3)2(═CHR) (GI, GIm). The compiled rate constants for PCy3 dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The 31P chemical shift (CS) tensors for the PCy3 ligand exhibited the expected changes (i.e., those characteristic of an increased Ru–P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the 13C CS tensors. Whereas c...