Ethenolysis Without Ethene: A Practical and Safe Lab Method to Introduce Terminal Alkene Groups via Metathesis Exemplified for Chiral Natural Product Chemistry

Abstract The introduction of terminal alkene groups through ethenolysis is a well‐established metathesis process. However, while being widely applied in industry, this reaction is not performed without risk in organic synthesis laboratories due to the hazardous properties of ethylene. In this contribution, we present an ethylene‐free ethenolysis, exemplified for a terminal alkene formation at the side chain of 12‐oxophytodienoic acid (12‐OPDA), a chiral naturally occurring plant‐hormone with internal alkene groups. This novel type of ethenolysis was achieved through ruthenium‐catalyzed metathesis with a triisopropylsilyl (TIPS)‐group carrying terminal alkene, acting as ethene surrogate. The newly formed terminal alkene was isolated in high purity and the structure could be unequivocally identified via NMR‐spectroscopical methods as well as mass spectrometry. We propose a mechanism which follows a pathway similar to the one of a degenerate metathesis that is driven by steric hindrance between the two alkenes during the catalytic cycle. This hypothesis is supported by our further experimental findings. The successful transfer of this concept to other alkene substrates and studies with a second catalyst indicate that this method for ethylene‐free introduction of terminal alkene groups through the use of bulky reagents avoiding formation of “classic” metathesis products could be applied to a wide range of other substrates, in particular complex natural products, as well.