Mechanistic Studies on 14-Electron Ruthenacyclobutanes:  Degenerate Exchange with Free Ethylene

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at −50 °C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C 2 v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Cα−Cβ agostic interactions with the Ru center. Intramolecular exchange of Cα and Cβ is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Cα and Cβ with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2CCH2] (4.8(3) × 10-4 M-1 s-1). Activation parameters for this process are ΔH ⧧ = 13.2(5) kcal mol-1 and ΔS ⧧ = −15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.