Reducing the Environmental Impact of Olefin Metathesis Reactions

As with most catalytic processes, olefin metathesis was discovered by accident as a result of the study of Ziegler polymerizations with alternate metal systems.[1] The recent development of ruthenium olefin metathesis catalysts with high activity and functional group tolerance has expanded the scope of this reaction to synthesize organic compounds and to form new C-C bonds.[2,3] Many variants of this very useful and versatile reaction have been developed in the meantime including self-metathesis (SM), cross-metathesis (CM), ring-closing metathesis (RCM), ring-opening metathesis (ROM), ROM polymerization (ROMP) as well as acyclic diene metathesis polymerization (ADMET).[2,3]
Within this project three reactions were investigated, which were suggested by the Nobel Prize laureate R. H. Grubbs as a useful, general and easily applicable platform for catalyst evaluation: CM of allyl benzene with excess cis-1,4-diacetoxy-2-butene, CM of methyl acrylate with 5 hexenyl acetate and RCM of diethyldiallyl malonate.[4] As also typically described for other olefin metathesis reactions, these reactions were performed in the organic solvent dichloromethane, which is toxic and environmentally unfriendly. Since the aim of this project is to optimize the reaction conditions for these reactions and to avoid the use of toxic solvents, we studied these reactions in detail at different concentrations in two solvents as well as in bulk. Dichloromethane and an environmentally friendly fatty acid derived solvent were used for comparison. Our studies clearly showed that the mentioned reactions can be performed in bulk, thus completely avoiding organic solvents and thus highly reducing the environmental impact of such reactions. As a very positive side effect, the reactions in bulk usually required less catalyst and often provided better conversions than their counterparts in organic solvent, most likely due to the higher concentration of the reactants. Moreover, our studies also revealed that methyl esters of capric and lauric acid are suitable non-toxic and thus environmentally friendly solvents for the investigated olefin metathesis reactions.

References:
[1] R. H. Grubbs, Tetrahedron, 2004, 60, 7117.
[2] S. H. Hong, R. H. Grubbs, Org Lett., 2007, 9 (10), 1955.
[3] R. H. Grubbs, Angew. Chem. Int. Ed. 2006, 45, 3760.
[4] R. H. Grubbs et al, Organometallics, 2006, 25, 5740.