Formation of Ruthenium-Oxo Complexes: An End, and a Beginning for Stereoretentive Olefin Metathesis

Abstract

Olefin metathesis is a central methodology in modern synthetic chemistry, offering unmatched versatility in the formation of carbon-carbon bonds. Its utility is illustrated by its long-standing use in petrochemical applications, over a period now spanning decades. In synthetic contexts, ruthenium catalysts have been widely adopted owing to their ease of handling relative to the group 6 catalysts. Improvements in catalyst reliability have come with the recognition that the "robust" Ru catalysts are in fact readily degraded by Brønsted and Lewis bases. Oxygen is generally presumed to be relatively unimportant. This study challenges that assumption, setting out contexts in which oxygen sensitivity must be considered.

Much recent attention has focused on stereocontrolled metathesis. Ruthenium dithiocatecholate catalysts, in particular, have been employed in leading contexts, including stereoselective macrocyclization. Their productivity is strikingly low; however, suggesting rapid decomposition during operating conditions. This thesis work investigated the pathways involved. The dithiocatecholate catalysts Ru(S₂C₆H₂-3,6-Cl₂)(NHC)(=CHAr) (where Ar = C₆H₄-2-OiPr; NHC = H₂IPr or H₂IMes) were shown to decompose rapidly in solution in air. Quantitative formation of aldehydes ArCHO was confirmed by NMR spectroscopy, with co-formation of four-coordinate oxo complexes Ru(O)(S₂C₆H₂-3,6-Cl₂)(NHC), the structure of which was confirmed by single crystal X-ray diffraction of the H₂IPr complex. Additional experiments involving olefin metathesis in air indicated that the oxo complexes do not function as isomerization catalysts, in contrast to many of the common ruthenium decomposition products. Neither positional nor geometric isomerization of the C=C bonds was detected.

Additional work explored synthetic routes to these Ru-oxo complexes, following the discovery in parallel work that the metathesis catalyst can be regenerated by treating the H₂IPr oxo complex with the ylide Ph₃P=CHAr. With the idea of leveraging this discovery to investigate alkylidene-free entry points to olefin metathesis catalysts, routes to the oxo complexes from conventional ruthenium precursors were explored. Synthesis of the piano-stool complex Ru(S₂C₆H₂-3,6-Cl₂)(p-cymene)(PPh₃), followed by reaction with H₂IPr, yielded four-coordinate Ru(S₂C₆H₂-3,6-Cl₂)(PPh₃)(H₂IPr). Upon exposure to air, the latter afforded the target dithiocatecholate oxo complex. Attempts to expand this chemistry to the more widely used dichloride catalysts were less successful, but the oxo complex Ru(O)Cl₂(IMes)(NMe₃) was synthesized using trimethylamine N-oxide as an oxidant. These findings highlight the dual importance of oxidative degradation pathways even in Ru-catalyzed olefin metathesis, but also of Ru-oxo species as a new entry point to metathesis catalysts.