[Markovnikov]

I've always thought that the metal needed to be coordinated to the C=O double bond and bound to an alkyl/alkene  to be able to perform a CO insertion.

But how does this following reaction take place? What I'm wondering is how the hepta-1-en-6-yne does the oxidative addition? What I suggested was a sigma-bond methathesis pathway, but I'm not sure. Any suggestions?

The reaction is similiar to the Paulson-Khand reaction, but it utilitizes a Rh-catalyst instead of Co.

[movies]

Have you investigate the mechanism of the Pauson–Khand reaction?


A couple of things that may help you:

1) The oxidative step in the mechanism is a bit different than what you would see in a cross-coupling reaction, so look for something other than a direct insertion of the metal into a sigma-bond.

2) Think about what you are used to seeing in the course of a CO insertion reaction.  What sort of oxidative transformation of the substrate + metal would get you to one of those typical intermediates.  It may be useful to work backwards from the product.

3) When CO is a ligand on a metal it typically binds through the lone pair on C, not through the pi-bond.

[Markovnikov]

Thanks for the reply again!

Well, it looks like the metal coordinates to the triple bond and the double bond and then the triple bond inserts itself into the double bond. But does the M still bind to both the alkene (now a single bond) and the alkyne (now a double bond)?

[movies]

Exactly.  That sort of transformation is called a cyclometallation and is oxidative with respect to the metal because you transfer a lone pair from the metal to make one of the C–M sigma-bonds.

So what happens next?

[Markovnikov]

Never heard of them before! I need to study more on these... any good references that you could recommend? I only seem to find a lot of article when i google cyclometallation...

I'm guessing that the next step is coordination to CO and then insertion and then reductive elimination to form the last ring.

Will the metal require a "free space" to perform a cyclometallation?

[movies]

Yep, sounds like you have it well in hand after that first step.  I think cyclometallations usually do require free coordination sites, but I suppose how many and in what arrangement will be dependent on what metal is doing the chemistry.

I don't have a great reference that talks about cyclometallation, but there is a section on page 106 of Transition Metals in the Synthesis of Complex Organic Molecules by Hegedus.

I should say that the mechanism is probably a little bit different when other metals are used.  For example, if Co₂(CO)₁₂ is the metal then the metals will form a complex with the alkyne initially and then undergo more typical migratory insertion steps.

[Markovnikov]

If Co₂(CO)₁₂ was the metal, wouldn't it just be an intermolecular Paulson-Khand reaction? Or is Paulson-Khand a kind of cyclometallation? :X

I tried looking up on the book up on Google Books, too bad 106 wasn't available, and my library doesn't have that one in! :/ Oh well.

Thanks for the help movies! Much appreciated! =)

[movies]

Well, the mechanism with Co₂(CO)₁₂ is somewhat different - that is what I was getting at.  That cobalt dimer readily forms complexes with alkynes that are quite stable and isolable.  The mechanism to generate the ring (the rest of the P–K reaction) is probably not a concerted cyclometallation step but a series of migratory insertion steps of alkyl–cobalt bonds onto the alkene.  Do you see the difference?

Too bad you couldn't find the book.  It is probably covered in most organometallics texts, that is just the one that I have handy at my desk.

[Markovnikov]

Hmm, it was mentioned in my coursebook, "The Organometallic Chemistry of Transition Metals" - Crabtree, but wasn't described at all. Oh well.

[movies]

Ah, I looked in that book too, but didn't mention it since it didn't really talk about it.