It’s well known that olefins can build up peroxides over time, especially if stored at room temperature and exposed to light. Oxygen and peroxide impurities can accelerate the decomposition of an olefin metathesis catalyst and decrease the efficiency of the metathesis significantly. Of course higher catalyst loadings can fix that, but it’s not a perfect solution. High levels of catalyst decomposition products can lead to the olefin isomerization and other undesired products and to more difficult purification.
Many reactions would work with very low catalyst loadings (i.e. ppm levels of catalyst), but the importance of ppm levels of impurities within the substrate or the reaction solvent become magnified. Some best practices are to use dry, deoxygenated solvents and substrates. Passage of substrates through alumina prior to use or storage over alumina is beneficial. Distilling the substrates before use can help to lower the catalyst loadings significantly. Also, radical scavengers such as BHT can be added to reaction substrates. Prior to setting up a metathesis reaction, it’s best to degas the reaction mixture before adding the catalyst.
An example of how efficient cross metathesis can be using very pure substrates was published by Roy Jackson’s group (Green Chem. 2006, 8, 450). The authors were able to achieve an impressive turn over number (TON) of 470,000 in the butenolysis of methyl oleate by triple distillation of the oleate and passing 2-butene through activated alumina. Much higher catalyst loadings were required to achieve the same conversion with unpurified methyl oleate or after a single distillation.