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Example 3: Ethylene Exchange in Grubb’s Catalysts

exp data for ethylene exchange exp data for ethylene exchange 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 Cα and Cβ in 1 and 2) intermolecular exchange, the degenerate exchange of free ethylene with 1.

proposed mechanism for ethylene exchange 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.

proposed mechanism for ethylene exchange 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.

computational results used to propose a mechanism for ethylene exchange

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).

computational results used to propose a mechanism for ethylene exchange

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.

computational results used to discuss associative versus dissociative mechanism for ethylene exchange 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.

  • Webster, J. Am. Chem. Soc. 2007, 129, 7490-7491.
  • Grubbs, Ed. Handbook of Metathesis; Wiley-VCH: New York, 2003.
  • Trnka, Grubbs, Acc. Chem. Res. 2001, 34, 18-29.
  • Schrock, Hoveyda, Angew. Chem. Int. Ed. 2003, 42, 4592-4633.
  • Schrock, Angew. Chem., Int. Ed. 2006, 45, 3748-3759.
  • Grubbs, Angew. Chem., Int. Ed. 2006, 45, 3760-3765.
  • Romero, Piers, J. Am. Chem. Soc. 2007, 129, 1698-1704.
  • Anderson, Hickstein, O'Leary, Grubbs, J. Am. Chem. Soc. 2006, 128, 8386-8387.
  • Romero, Piers, J. Am. Chem. Soc. 2005, 127, 5032-5033.

Recent News

Aug 2015
$6 million NSF RII Track-2 FEC: "Feeding and Powering the World" funded!

July 2014
Webster Group moved to Mississippi State University

June 2014
Dr. Webster co-organized and presented at "Making and Breaking Bonds with Light" at Telluride Science Research Center

April 2014
Katie Leigh accepted tenure-track faculty position! Congratulations Katie!