Decomposition of a Phosphine-Free Metathesis Catalyst by Amines and Other Bronsted Bases: Metallacyclobutane Deprotonation as a Major Deactivation Pathway

Reactions are described of the second-generation Hoveyda catalyst <b>HII</b> with amines, pyridine, and DBU (1,8-diazabicyclo[5.4.0]­undec-7-ene), in the presence and absence of olefin substrates. These nitrogen bases have a profoundly negative impact on metathesis yields, but in most cases, they are innocuous toward the precatalyst. <b>HII</b> adducts were formed by primary and secondary amines (<i>n</i>-butylamine, <i>sec</i>-butylamine, benzylamine, pyrrolidine, morpholine), pyridine, and DBU at room temperature. No reaction was evident for NEt₃, even at 60 °C. On longer reaction at RT, unencumbered primary amines abstract the benzylidene ligand from <b>HII</b>. With 10 equiv of NH₂^<i>n</i>Bu, this process was complete in 12 h, affording NH^<i>n</i>Bu­(CH₂Ar) (Ar = <i>o</i>-C₆H₄–O^<i>i</i>Pr) and [RuCl­(H₂IMes)­(NH₂^<i>n</i>Bu)₄]­Cl. For benzylamine, benzylidene abstraction occurred over days at RT. No such reaction was observed for <i>sec</i>-butylamine, secondary amines, NEt₃, pyridine, or DBU. All of these bases, however, strongly inhibited metathesis of styrene by <b>HII</b>, with a general trend toward more deleterious effects with higher Bronsted basicity. Studies at 10 mol % of <b>HII</b> and 10 equiv of DBU, NEt₃, and pyrrolidine (60 °C, C₆D₆) indicated that the primary mechanism for decomposition involved base-induced deprotonation of the metallacyclobutane intermediate, rather than the Lewis base-mediated decomposition pathways previously established for the Grubbs catalysts. In the corresponding metathesis of ethylene, this decomposition process is rapid even at RT, highlighting the vulnerability of the less substituted metallacyclobutane