Ohlmann, D. M.; Tschauder, N.; Stockis, J.-P.; Gooßen, Dierker, M.; Gooßen, L. J. “Isomerizing Olefin Metathesis as a Strategy To Access Defined Distributions of Unsaturated Compounds from Fatty Acids” J. Am. Chem. Soc. 2012, 134, 13716-13729.
As petrochemical feedstocks are depleted, the chemical community is charged with discovering and developing new methods to access value added olefins from bio-renewable feedstocks. Natural oils and their derivatives, such as oleic acid and methyl oleate, are desirable substrates because of their ubiquity. It is well established that metathesis catalysts can transform long chain fatty acids and esters into new, useful compounds. Self and cross metathesis compounds are produced when metathesis is carried out in the presence of another olefin (typically a small hydrocarbon such as ethylene or butylene). These products subsequently can be distilled from one another. This process is an example of the “bio-refinery” concept in which renewable feedstocks are transformed into value added chemicals on scale. Ideally such processes can fulfill demand for commodity chemicals currently derived from petroleum, at least in part.
The number and identity of compounds synthesized in the process described above is limited by the position of the double bond in the fatty acid. This highlighted research paper out of Technische Universität Kaiserslautern in Germany addresses that limitation by utilizing the concept of isomerizing olefin metathesis to create well defined distributions of olefins from fatty acids, increasing the number of new compounds produced from an olefinic substrate. This development was made possible by tandem catalysis and necessitated finding isomerization and metathesis catalysts that “play nice” together – compatibility in this type of system is crucial and requires particular attention to liberated ancillary ligands on the catalysts and to available bimolecular decomposition events. A key was identifying the dimeric palladium complex [Pd(μ-Br)(tBu3P)]2 as an isomerization precatalyst. When combined with active metathesis compounds such as 1-3 (Scheme 2) it maintains its activity for isomerization and does not attenuate metathesis activity.
After separately testing the activity of several potential catalysts for isomerization and metathesis, the best catalysts were combined into one pot to demonstrate isomerizing self-metathesis of methyl oleate and oleic acid (Scheme 1). Broad product distributions (from C8-C32) were obtained at 60 °C. Interestingly, at lower temperatures isomerization selectively slowed down and product distributions were sharp and centered on self-metathesis products, allowing for a “tuning” of product identity. This is in contrast to sequential isomerization/metathesis in which the isomerization catalyst is removed prior to metathesis – product distributions are set by extent of isomerization and are typically bimodal. The cooperative, one-pot catalyst described here could be a very useful concept in a commercial setting allowing flexibility in the diversity of the product stream.
Scheme 1. Isomerizing metathesis of oleates.
Scheme 2. Examples of active metathesis and isomerization catalysts used throughout the paper
In the same vein, several more similar studies were undertaken in this research including isomerizing metathesis of simple olefins, isomerizing self-metathesis of other fatty acids, isomerizing ethenolysis of fatty acids, and isomerizing cross-metathesis of fatty acids with dicarboxylic acids. Though the details of these studies will not be discussed herein, I encourage you to read this paper in full. Some other interesting features of the chemistry stood out:
- In the cross studies, varying the ratio of cross-partner to fatty acid substrate allowed for tuning of the mean chain length
- Several metathesis catalysts were active for ethenolysis of fatty acids in the presence of isomerization catalyst
- Electron deficient olefins proved difficult cross partners and required high catalyst loadings
- Mathematical modeling using statistics software R was used to predict equilibrium distributions of product fractions and should be a useful tool for future studies
To be practiced on scale, catalyst loadings will probably need to be lowered and their availability in thousands of kilogram quantities considered. Overall however, I found this chemistry interesting and extremely promising, primarily because of its versatility in selecting desired product distributions. I look forward to watching the technology mature as new classes of substrates are considered and more active/stable catalysts are developed.