Mechanical Activation of a Latent Olefin Metathesis Catalyst and Persistence of its Active Species in ROMP

Preparation, substrate scope, and activity of a previously reported mechanically activated metathesis catalyst were investigated. Scission of the catalyst under ultrasound irradiation was followed by GPC, which showed a first-order scission rate constant of 0.011 min^–1. The resulting active species was shown to have catalytic reactivity in ring closing metathesis (RCM) of various (un)­hindered substrates. Further investigations of the active species showed that it was not influenced by radicals formed during ultrasound and that the most effective method to increase its lifetime was an increase in the substrate concentration. In ring-opening metathesis polymerization (ROMP), the lifetime of the active species was shown to be several hours, in contrast to the short lifetimes found in RCM. ROMP experiments also showed that all of the latent precatalyst broken leads to active catalyst species