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)Cl2Ru(CH2CH2CH2). These authors measured activation parameters for the intramolecular and intermolecular exchange processes (ΔG⧧ = 12.3 kcal mol-1 and ΔH⧧ = 13.2(5) kcal mol-1 and ΔS⧧ = −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 (ΔGCH2Cl2...