Donor-Induced Decomposition of the Grubbs Catalysts: An Intercepted Intermediate

The σ-alkyl species RuCl2(CH2PCy3)­(py)3 (3a) is intercepted on adding pyridine to the first-generation Grubbs catalyst 1a during RCM or to the isolated resting-state species RuCl2(PCy3)2(CH2) (2a). Complex 3a is formed by pyridine-induced displacement of PCy3 and nucleophilic attack of the liberated PCy3 on the methylidene carbon. The rapid, near-quantitative conversion of 2a into 3a indicates that nucleophilic attack by PCy3 is the primary deactivating event. Once formed, 3a decomposes more slowly via several competing pathways. One such pathway involves elimination of the σ-alkyl ligand as [CH3PCy3]Cl (A), following proton and chloride abstraction. Observation of nearly 80% 3a during RCM by 1a in the presence of pyridine confirms the relevance of this behavior to metathesis and implicates the resting-state methylidene 2a as the vulnerable species, rather than the metallacyclobutane intermediate. Any donor capable of displacing PCy3 and stabilizing a five-coordinate methylidene adduct is predicted to trigger the same deactivation sequence, steric factors permitting.