Revisiting Self-Metathesis of Fatty Acids and Esters: Contribution to Renewable Chemicals Under the Carbon Rainbow Strategy.

The practicality of self-metathesis of oleic substrates was compared using methyl esters (FAME) and technical oleic acid. Both reactions proved more challenging than those reported for high-purity substrates (≥95% oleic acid or ester) in the literature. Efficient self-metathesis was achieved for both feedstocks with the second-generation Grubbs catalyst (Ru2). For rapeseed-derived FAME, conversion reached ≥70% (based on oleic, linoleic, and linolenic esters). However, the yield of the valuable C18:1 diester (2) did not exceed 40%. In a larger-scale test, the isolated yield was only 36%. The product was contaminated with other metathesis products (e.g., C21:2 diester), which were difficult to separate. In contrast, self-metathesis of 85% technical oleic acid (containing 9.5% linoleic acid) in concentrated n-heptane solution gave better results. An isolated yield of 48% of pure (E)-C18:1 diacid (16) was obtained at a low catalyst loading of 45 ppm Ru2. The improvement resulted from the low solubility of the (E)-diacid, which precipitated and shifted the equilibrium toward product formation. The obtained (E)-C18:1 diacid and its saturated analogue (C18:0 diacid) are promising 100% bio-based building blocks for renewable fine chemicals and biodegradable polymers.