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

• Published in: Journal of the American Chemical Society Vol. 129; no. 24; pp. 7490 - 7491
• Main Author: Webster, Charles Edwin
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20.06.2007
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja071588d

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 (ΔG 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 (ΔG CH 2 Cl 2 ⧧,-50°C = 18.6 kcal mol-1) is close to the observed free energy of activation (ΔG CD 2 Cl 2 ⧧,-50°C = 16.9 kcal mol-1). A dissociative mechanism could also be operative at elevated temperatures.