Ruthenium-Catalyzed Metathesis with Directly Functionalized Olefins.

Abstract

Olefin metathesis (OM) has become a widely used tool in organic and material syntheses. As catalyst development has advanced, functional group tolerance has increased. Unfortunately, vinyl halides were incompatible with OM catalysts and attempts at cross-metathesis (CM) with vinyl halides failed. Given the usefulness of alkenyl halides in metal-catalyzed cross-coupling reactions, improvement of CM systems employing vinyl halides would be beneficial. Research goals included determining why vinyl halides were not tolerated by ruthenium-based OM catalysts and developing systems in which vinyl halides participate in CM. Ruthenium monohalomethylidene complexes, [Ru(=CHX)(H2IMes)LCl2] (X = F, Cl; L = PCy3, 2 py), which are intermediates in CM with vinyl halides, were found to undergo decomposition through loss of HX, deactivating the catalyst. Decomposition of the monochloromethylidene complexes can be hindered by removing one of the neutral ligands (L) from the system, making CM with chlorinated olefins successful. The monofluoromethylidene complexes synthesized were less susceptible to decomposition. However, the monofluoromethylidene intermediates act as a thermodynamic well, shutting down CM. If metathesis product formation is energetically favored, then the thermodynamic stability of the monofluoromethylidene intermediate relative to Ru-alkylidene intermediates can be overcome by this added driving force. Therefore, vinyl fluoride is an effective substrate for ring-opening cross-metathesis with cyclooctene and enyne metathesis with a number of alkynes. We also pioneered a new subfield of OM referred to as Fischer carbene cross-metathesis (FCM), in which the thermodynamic stability of Fischer carbene complexes was circumvented by removing the need to form a ruthenium alkylidene complex during the catalytic cycle. FCM involves CM of electron-rich olefins such as ethyl vinyl ether with functionalized 1,2-disubstituted alkenes such as styryl acetate to form β-ethoxystyrene. FCM has allowed for productive CM with a number of electron-rich olefins including vinyl fluoride which have previously been detrimental to Ru-based catalysts. Finally, the decomposition process of the monohalomethylidene complexes led to the discovery of a facile synthesis of new Ru-benzylidyne species. The synthesis and reactivity of a number of previously unknown Ru-benzylidyne complexes was studied and has important implications for the development of Ru-based alkyne metathesis catalysts, which have yet to be realized.