Spectroscopic Exploration Of Catalyst Dynamics And Electrochemical Properties Of Grubbs Second Generation In Different Solvent

Abstract

The phosphine (PCy₃) ligand dissociation equilibrium of the second generation Grubbs catalyst (GII) in different solvents (dichloromethane, dichloroethane, chloroform, tetrahydrofuran, and toluene) was determined to be pseudo first order with k_obs=0.8–2.6×10⁻⁴ s⁻¹. An electrochemically reversible behavior was observed for GII dissolved in the chlorinated solvents (DCM, DCE, and CLF). The formal reduction potentials, E°′, of GII in the different chlorinated solvents increased with increasing polarity index of the solvent. In THF, however, only oxidation of GII could be detected. Upon oxidation of GII, THF (with a lone pair electron on the oxygen), which is a highly coordinative solvent, nucleophilically attaches to the electrochemically generated Ru⋅⁺, resulting in a redox inactive THF-associated compound. A correlation was established between the ease of GII oxidation in the different solvents and the rate of PCy₃ dissociation in that solvent. The % yield of GII-catalyzed cross-metathesis product in the different solvents at 40 °C increases exponentially with increasing Gutmann donor number of the solvent increased, with DCM showing the highest yield. No metathesis could be detected in THF, consistent with the electrochemical data that THF coordinates to the positively charged or active GII, rending it inactive.