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

Abstract

Reactions are described of the second-generation Hoveyda catalyst HII 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 are innocuous toward the pre-catalyst. HII adducts were formed by primary and secondary amines (n-butylamine, sec-butylamine, benzylamine, pyrrolidine, morpholine), pyridine, and DBU at room temperature. No reaction was evident for NEt 3 , even at 60 °C. On longer reaction at RT, unencumbered primary amines abstract the benzylidene ligand from HII. With 10 equiv NH 2 n Bu, this process was complete in 12 h, affording NH n Bu(CH 2 Ar) (Ar = o-C 6 H 4 -O i Pr) and [RuCl(H 2 IMes)(NH 2 n Bu) 4 ]Cl. For benzylamine, benzylidene abstraction occurred over days at RT. No such reaction was observed for sec-butylamine, secondary amines, NEt 3 , pyridine, or DBU. All of these bases, however, strongly inhibited metathesis of styrene by HII, with a general trend toward more deleterious effects with higher Bronsted basicity. Studies at 10 mol% HII and 10 equiv DBU, NEt 3 , and pyrrolidine (60 °C, C 6 D 6 ) 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 monosubstituted metallacyclobutane.

In a Quest for Selectivity Paired with Activity: A Ruthenium Olefin Metathesis Catalyst Bearing an Unsymmetrical Phenanthrene‐Based N‐Heterocyclic Carbene

“…Diminished isomerisation during a metathesis process is usually connected with higher stability of a given Ru alkylidene propagating species . One of the factors affecting the rate of decomposition is the presence of ethylene, as it is known that ruthenium methylidenes are the least stable active species …”

Olefin Metathesis at the Dawn of Implementation in Pharmaceutical and Specialty‐Chemicals Manufacturing

“…[44] Acid treatment is thus important in quenching the basicity of the nitrogen functionalities present (as well as the basicity of free PCy 3 ,i nc ases where ap hosphinecontaining catalyst, such as GII,i su sed). Lewis and Brønsted bases are now known to trigger decomposition of the key methylidene and metallacyclobutane intermediates.…”

The Future of Ethenolysis in Biobased Chemistry

“…Recent work on the degradation pathway show that catalysts containing a phosphine group undergo displacement of the phosphine by an amine, followed by an attack of the ruthenium alkylidene species by the phosphine . For catalysts without phosphine, the Bronsted–Lowry base deprotonates the metallacyclobutane that is formed in the catalytic cycle . It should be noted that both these degradation pathways are enhanced in metathesis reactions with ethylene present.…”