Computational Insights into Degenerate Ethylene Exchange with a Grubbs-Type Catalyst

Romero and Piers recently reported a mechanistic study in which they detailed the intramolecular exchange of adjacent methylene groups of a ruthenacyclobutane and the intermolecular degenerate exchange of free ethylene with a second-generation Grubbs-type catalyst, a 14-electron ruthenacyclobutane, (NHC)Cl 2 Ru(CH 2 CH 2 CH 2 ). These authors measured activation parameters for the intramolecular and intermolecular exchange processes (Δ G ⧧ = 12.3 kcal mol -1 and Δ H ⧧ = 13.2(5) kcal mol -1 and Δ S ⧧ = −15(2) eu, respectively). The current study applies density functional theory calculations to address their proposed exchange mechanisms. For intramolecular exchange, the formation of a structure with coordinated ethylene proceeds from the cyclobutane species via a direct rotational bond-breaking transition state, which produces an ethylene which is essentially perpendicular to the remaining methylene unit; and the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (Δ G CH 2 Cl 2 ⧧,-50°C = 14.4 kcal mol -1 ) is close to the observed free energy of activation. For intermolecular exchange, ethylene binds to the metal of the ruthenacyclobutane complex, a rotational bond-breaking transition state produces a bis-ethylene, methylene intermediate, from which the ethylene, formally of the metallocyclobutane, dissociates. For intermolecular exchange, the computed free energy of activation including solvation for the pathway with trans-disposed chlorides (Δ G CH 2 Cl 2 ⧧,-50°C = 18.6 kcal mol -1 ) is close to the observed free energy of activation (Δ G CD 2 Cl 2 ⧧,-50°C = 16.9 kcal mol -1 ). A dissociative mechanism could also be operative at elevated temperatures.