[workingundergrad1896]

Been trying to run an alkene metathesis on a compound that also contains an alkyne. It's a cross metathesis, which I understand generally requires 1:2 or 1:3 ratio of alkenes.

Does anyone have any references/advice/examples of an alkene metathesis being successfully run when the compound also contains an alkyne. Preferably not a macrocyclic RCM, as I've found a few examples of these and it's not really relevant to my case. Something like this:



I want a chemoselective alkene metathesis. I've been unable to find any literature examples of such a reaction. I know alkynes react in metathesis reactions, but my understanding is that alkenes generally react faster and so this could be chemoselective??

Any help much appreciated

[clarkstill]

On my phone so can’t look up scifinder etc, but I think you’ll struggle here. The initial reaction may well be fastest on the terminal alkene, however it’s then a competition between an intermolecular cross-metathesis and an intramolecular ene-yne metatheses, to form a 5-membered rings. All things being equal, I think the intramolecular reaction will win...

[Enthalpy]

Pardon my ignorance... Does something hamper metatheses among two molecules of the longest or the shortest reactant? Producing
and
Thanks!

[clarkstill]

Grubbs has addressed this very issue in great detail, see JACS 2003 11361 - well worth a read. He delineates a set of rules for which compounds will undergo selective cross-metathesis reactions, and which will be unselective or unreactive.

In short - the first molecule will form, the second will not (assuming R1 != H). However, the first homodimer can still undergo metathesis reactions since it is formed as an equilibrating mixture: the Ru can reinsert into the 1,2-disubstituted internal alkene and split the dimer apart.

Hope this makes sense!

[clarkstill]

PS for those without journal access, the paper is summarised here in a Wulff group presentation here: https://www2.chemistry.msu.edu/faculty/wulff/myweb26/Literature_pdf/2003-08-28.pdf

[workingundergrad1896]

Yes I've since found this paper and it's been pretty useful, so thanks for all your help. One my substrates would almost certainly be a type 3 alkene, and the alkene/alkyne compound is basically as i drew it in my first post, so if i were to guess this would be a type 2.

Unfortunately this was written in 2003 and since alkyne metathesis has developed a lot.

I've been using hoveydra-grubbs and so far after 1 attempt i've not had any luck, but I will have another try

[Enthalpy]

Thanks from my side too!