Ultra-Fast Initiating Ruthenium Catalysts for Low-Temperature Metathesis

By: William Sommer, Aldrich ChemFiles 2009,9.6, 4.

Aldrich ChemFiles 2009,9.6, 4.

Traditional Grubbs catalyst systems are five-coordinate ruthenium complexes containing a neutral ligand, which is typically a phosphine, or in the case of the Hoveyda-Grubbs catalysts, a styrenyl ether. Ligand dissociation is required to provide the active catalyst, but is slow at low temperatures. Therefore, traditional Grubbs catalysts are not reactive at low temperatures. The Piers group developed preformed 4-coordinate cationic complexes that do not require ligand dissociation prior to reaction. The reaction progress was examined for the cyclization of diethyldiallylmalonate to provide the corresponding cyclopentene derivative using both the Piers catalyst and the Grubbs catalyst at 0 °C (Scheme 3).1 At this temperature, the Grubbs catalysts (2nd Generation) was found to be a weak initiator and after 4 h, the reaction had progressed to 25% completion, while the cationic complex had progressed to > 90% completion after 2 h. The initiation rate of the cationic catalyst at 0 °C was found to be comparable to the initiation rate of the Grubbs catalyst (2nd Generation) at 35 °C. 

Scheme 3

In a subsequent study, Piers and coworkers examined the intermediates in an olefin metathesis reaction by NMR at –50 °C (Scheme 4). This was the first direct observation of a ruthenacyclobutane intermediate and provided evidence for a symmetrical intermediate.7 It also illustrated the stabilizing effect of the N-heterocyclic carbene ligand on the Ru(IV) species.

Scheme 4

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  1. Romero, P. E. et al. Angew. Chem. Int. Ed. 2004, 43,6161.

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