Enyne metathesis

What is Enyne metathesis?

In 1985, T.J. Katz made an intriguing discovery regarding a methylene migration reaction that occurred when a biaryl 1,7-enyne was exposed to a tungsten Fischer carbene complex in the presence of 1 mol%.

Enyne metathesis produced a 1,3-diene as the end product, with a yield of 31 %. This marked the first instance of metal carbene catalyzed intramolecular redistribution of carbon-carbon multiple bonds between an alkene and an alkyne.

Enyne metathesis - general reaction scheme - ring-closing Enyne metathesis
Ring-closing Enyne metathesis
Enyne metathesis - general reaction scheme - enyne cross metathesis
Enyne cross metathesis

This process is commonly known as the intramolecular ring-closing enyne metathesis, with variants such as cross enyne metathesis between independent molecules of an alkene and an alkyne.

Subsequently, molybdenum and chromium Fischer carbene complexes were also utilized, but with a requirement of stoichiometric amounts of catalyst and generally low yields due to side reactions.

Enyne metathesis can also be catalyzed by low-valent transition metals such as Pd(II), Pt(II), Ru(II), and Ir(I) complexes. Among them, ruthenium benzylidene complexes, including Grubbs’ first and second generation catalysts, are the most widely used and efficient enyne metathesis catalysts, originally developed for olefin metathesis reactions.

The substituents of the olefin significantly impact the reaction rate, the number of different products, and their distributions, with monosubstituted alkenes reacting faster than di- or trisubstituted ones. Enynes with monosubstituted olefins form only the smallest possible ring size, and the substitution of the alkyne partner also affects the reaction rate, with terminal alkynes reacting slower than internal ones. Alkyl substituents on the alkyne generally result in high yields, whereas electron-withdrawing substituents typically lead to lower yields. The presence of ethylene gas instead of argon can substantially increase the reaction rate in certain cases, and reactions are usually conducted in dichloromethane, toluene, or benzene either at ambient temperature or at reflux.

References

Metal-catalyzed rearrangement of alkene-alkynes and the stereochemistry of metallacyclobutene ring opening.
Thomas J. Katz and Timothy M. Sivavec
Journal of the American Chemical Society 1985 107 (3), 737-738
DOI: 10.1021/ja00289a054

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