Macrocyclic lactones, ketones, and ethers can be obtained in the High-Concentration Ring-Closing Metathesis (HC-RCM) reaction in high yield and selectivity at concentrations 40 to 380 times higher than those typically used by organic chemists for similar macrocyclizations. The new method consists of using tailored ruthenium catalysts together with applying vacuum to distill off the macrocyclic product as it is formed by the metathetical backbiting of oligomers. Unlike classical RCM, no large quantities of organic solvents are used, but rather inexpensive nonvolatile diluents, such as natural or synthetic paraffin oils. Moreover, use of a protecting atmosphere or a glovebox is not needed, as the new catalysts are perfectly moisture and air stable. In addition, some other cyclic compounds previously reported as unobtainable by RCM in neat conditions, or in high dilutions even, can be formed with the help of the HC-RCM method.
Macrocyclic musk belongs to a well-known and valued class of the fragrance family. Originally, natural musks were obtained from rectal musk glands which often led to the death of the animals. Recently, a lot of effort was invested to obtain such macrocycles in a synthetic way. This research presents a study on the preparation of macrocyclic lactones with the musk scent by ring-closing metathesis (RCM) using biomass-derived starting materials: oleic and 9-decenoic acid. An experimental rule correlating the C-C double bond substitution pattern in the starting diene and the yield for the RCM macrocyclization was proposed.
A wide
set of 65 diverse Ru metathesis catalysts was investigated
in the ethenolysis reaction of biosourced ethyl oleate to allow the
comparison between the catalyst structure and its activity and selectivity.
Handling of the oleic substrate, weighing of the catalysts, and charging
the reactor were done in air, with exclusion of a glovebox or Schlenk
techniques. A catalyst bearing the unsymmetrical N-heterocyclic ligand
featuring a thiophene fragment (
Ru-63
) was selected to
offer the best combination between high selectivity and sufficient
activity under conditions mimicking oil industry practice. A proof-of-concept
large-scale ethenolysis experiment was also done with the selected
catalyst to prove its high selectivity at the 1 L scale reaction with
a 90% pure non-distilled substrate.
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