First regeneration of a ruthenium-based olefin matathesis catalyst and the use of di-grignard reagents to form metallacyclobutane complexes

Includes bibliographical references (pages 145-150)A method for the regeneration of a decomposed ruthenium-based olefin metathesis catalyst to an in situ ruthenium-indenylidene complex was developed. The regeneration method allows for the treatment of the isolated decomposed inorganic product from the first-generation Hoveyda-Grubbs catalyst with a previously prepared derivative of propargyl alcohol. The structure of the regenerated catalyst was characterized by 31P {1H} NMR spectroscopy and High-Resolution Mass Spectrometry. The structure of the regenerated complex was confirmed by comparing to a previously prepared ruthenium-indenylidene from [Ru(p-cymene)Cl2(PCy3)] starting material. The regenerated catalyst is air stable and possesses catalytic activity similar to that of the first-generation Hoveyda-Grubbs catalyst and the previously prepared ruthenium-indenylidene in RCM. This research has provided a means to potentially recycle expensive ruthenium metal from commercial catalysts that decomposed due to metathesis conditions. This study has provided much needed insight into the reactivity of the putative decomposition product of first-generation Hoveyda-Grubbs catalyst. The synthesis of a gem-dimethyl di-Grignard reagent was successful. This di-Grignard was prepared in synthetic yields that were comparable to previously published quantities. The methyl substituents on the propane backbone served to effectively protect the structure from undergoing elimination decomposition pathways, which served to reduce synthetic side-reactions and resulting by-products amongst the product profile. This gem-dimethyl di-Grignard reagent was successfully characterized by NMR spectroscopy. The synthesis of a previously reported metallacyclobutane of molybdenum was successful by reaction of the gem-dimethyl di-Grignard reagent with a molybdenum di-chloride complex. This molybdenacyclobutane was characterized by NMR spectroscopy. A ligand that has been reported to be active in the Ziegler-Natta polymerization of ethylene and the ROMP of olefins was successfully synthesized. This ligand was used to attempt chelation to a molybdenum solvent-adduct to afford a metal-ligand tri-chloride complex. This synthesis proved difficult in its effective characterization by spectroscopic techniques. It is anticipated that upon isolation of this product, treatment with a standard reducing agent will afford the corresponding di-chloride metal-ligand complex. This di-chloride metal complex will be amenable to treatment with the previously prepared gem-dimethyl di-Grignard reagent to afford a novel metallacyclobutane of molybdenum complex. Given the precedence of the ligand used in this chemistry, it is hoped that this novel metallacyclobutane will be active in catalyzing olefin metathesis reactions