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.

σ-烷基物种RuCl₂(CH₂PCy₃)(py)₃（3a）可在闭环复分解反应（Ring-Closing Metathesis，RCM）过程中，通过向第一代格拉布催化剂（Grubbs catalyst）1a内加入吡啶，或是向分离得到的休眠态物种RuCl₂(PCy₃)₂(=CH₂)（2a）中加入吡啶而被捕获得到。配合物3a的生成路径为：吡啶诱导三环己基膦（PCy₃）发生配体取代解离，解离出的PCy₃对亚甲基碳发生亲核进攻。2a能够快速且近乎定量地转化为3a，这表明PCy₃的亲核进攻是该体系的主要失活过程。3a生成后，会通过多条竞争路径发生缓慢分解，其中一条路径涉及在攫取质子与氯离子后，将σ-烷基配体以[CH₃PCy₃]Cl（A）的形式消除。在吡啶存在的条件下，由1a催化的RCM反应中可观测到近80%的3a，这证实了该失活行为与复分解反应密切相关，同时表明易发生失活的物种是休眠态亚甲基配合物2a，而非金属环丁烷中间体（metallacyclobutane intermediate）。若空间位阻条件允许，任何能够取代PCy₃并稳定五配位亚甲基加合物的给体配体，都将触发相同的失活序列。