Amine‐Mediated Degradation in Olefin Metathesis Reactions that Employ the Second‐Generation Grubbs Catalyst

Abstract

Abstract Amine‐mediated decomposition during olefin metathesis reactions that employ the second‐generation Grubbs catalyst is studied. For most amines, the dominant deactivation pathway involves ejection of the PCy 3 (Cy=cyclohexyl) ligand by the amine, followed by abstraction of the methylidene moiety from the resting‐state species RuCl 2 (H 2 IMes)(PCy 3 )(=CH 2 ) (H 2 IMes=1,3‐dimesityl‐4,5‐dihydroimidazol‐2‐ylidene) as [MePCy 3 ]Cl. An exception is highly basic 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), which is slow to degrade the resting‐state methylidene complex, and for which the phosphonium byproduct is not observed. However, DBU is shown to rapidly attack a species generated during catalysis, most probably the metallacyclobutane intermediate.