Olefin metathesis reactions, which exchange the substituents about the double bonds of the alkenes, are important for a variety of chemical syntheses. A metallocyclobutane intermediate has been implicated in the mechanism of the Grubbs-type catalyzed olefin metathesis. Romero and Piers reported an experimental mechanistic study of ethylene exchange with a 14-electron ruthenacyclobutane, (NHC)Cl₂Ru(CH₂CH₂CH₂), where NHC is an N-heterocyclic carbene (1,3-dimesitylimidazolidin-2-ylidene), derived from a second generation Grubbs catalyst, see Scheme. They described 1) intramolecular exchange of C_α and C_β in 1 and 2) intermolecular exchange, the degenerate exchange of free ethylene with 1.

For intramolecular carbon exchange, one could envision a three-step mechanism that would in the first step cleave the metallocyclobutane C-C bond (to form a methylene, ethylene complex), in the second step rotate the coordinated ethylene, and in the third step form the C-C bond (to form the metallocyclobutane, with exchanged α and β carbons). This three-step proposed mechanism is quite reasonable.

For intermolecular ethylene exchange, one could envision a variety of mechanisms. Ethylene could bind to 1 or 2 and a postulated metallocylohexane transition state or intermediate could be involved in the exchange. These structures could have either trans-disposed or cis-disposed chloride analogues (our computational results indicate a multistep mechanism with structures that have trans-disposed, not cis-disposed chlorides).

We applied PBE density functional theory calculations to test various mechanisms and consider dissociative versus associative mechanisms at elevated temperatures at which these olefin metathesis catalysts are also used.

For intramolecular ethylene exchange, our computational results indicate a single-step mechanism (to form a methylene, ethylene complex) starting from the ruthenacyclobutane complex (1) directly producing the ethylene ruthenium carbene complex (2) by a rotational bond-breaking transition state (TS-1-2, see Figure directly above).

For intermolecular ethylene exchange, our computational results indicate a multi-step mechanism starting from 3 (ethylene associated with 1), ethylene binds to the metal (through TS-3-5), which produces the η²-ethylene ruthenacyclobutane complex (5). A rotational bond-breaking transition state (TS-5-6) produces 6 (a bis ethylene methylene complex), from which the ethylene trans to NHC can dissociate (through TS-4-6). A second ruthenacyclobutane complex with associated ethylene (9) is formed through TS-4-9 (similar to TS-1-2). While the proposed pathway is not an energetically symmetric pathway, it does not violate the principle of microscopic reversibility.

An alternative mechanism which involves a ruthenacyclohexane intermediate proceeds from 5, η²-ethylene rotates (through TS-5-7) to form 7. A high-energy ruthenacyclohexane intermediate (8) is formed through TS-7-8. This pathway with a ruthenacyclohexane intermediate is higher in energy than the first described intermolecular ethylene exchange pathway.

Romero and Piers suggested that a competing mechanism involving dissociation of ethylene from 1 at elevated temperatures could become competitive with an associative mechanism. To test that suggestion, the relative free energy of ethylene loss from 1 was computed. A rise of the temperature to 100 °C (from -50 °C) increases the stability of separated ethylene and the methylene fragment by ~11.8 kcal mol^-1 when compared to 3. Therefore, ethylene dissociation from 1 could become the operative pathway at elevated temperatures.

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