Marx, V. M.; Sullivan, A. H.; Melaimi, M.; Virgil; S. C.; Keitz, B. K.; Weinberger, D. S.; Bertrand, G. and Grubbs, R. H. “Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis” Angew. Chem. Int. Ed. 2015, 54, 1919-23
Ethenolysis of seed oils has significant potential as a clean and sustainable way for the production of linear α-olefins, but the high catalyst loadings required to achieve reasonable yields of terminal olefins are not cost efficient on industrial scale. Unlike the very efficient cross metathesis reactions with terminal or internal olefins, turnover Numbers (TON) of only 2,000-5,000 were achieved with the standard first and second generation Ru metathesis catalysts in the ethenolysis of Methyl Oleate (MO). First generation Grubbs catalyst shows very high selectivity for ethenolysis but decomposes very quickly in the presence of ethylene. Second generation catalysts are significantly less selective for ethenolysis due to their propensity for self-metathesis reactions.
Promising results achieved previously with CAAC catalyst 1 prompted more systematic investigation which led to the synthesis of new catalysts exhibiting remarkable TONs: the highest TON achieved was over 300,000!
In the best performing catalysts 2 and 3, increased N-aryl substitution appears to increase the selectivity for terminal olefins. The high activity for ethenolysis can be explained by the increased stabilization of the ruthenium methylidene intermediate generated in the presence of ethylene. Ruthenium methylidenes are known to decompose rapidly through the insertion of the N-aryl substituent into the methylidene carbene. The increased electron density at the ruthenium and the steric bulk at the N-aryl groups in the CAAC derived catalysts probably stabilizes this highly reactive intermediate. The increased stability of the methylidene allows the ethenolysis reaction to proceed to completion to achieve the equilibrium ratio of terminal olefins.
Last but not least, it’s noteworthy how crucial the purity of the ethylene gas was to achieve high TONs. The use of 99.95% purity almost doubled the previously reported TON for 1 achieved with 99.9% pure ethylene1. The use of even higher grade gas (99.995%) led to dramatic increase to 340,000 TON at 1 ppm catalyst loading with catalyst 2.
1TON of 35,000 was achieved for ethenolysis of MO: Schrodi, Y.; Ung, T.; Vargas, A.; Mkrtumyan, G.; Champagne, T. M.; Pederson, R. L.; Hong, S. H. Clean 2008, 36, 669-73.