[Rutherford]

What is the mechanism of the transformation below? KOH and ethylene glycol hydrolyze the cyano group to a carboxylic group, but what happens in the following step?
How can H₂/Pt remove the iodine afterwards?

[discodermolide]

The next step, with base and iodine is an iodolactonisation.
There is no H₂/Pt in your scheme.

[Rutherford]

It is after this transformation. So how does H₂ remove the iodine?

[discodermolide]

It is after this transformation. So how does H₂ remove the iodine?

Normal hydrogenation causes de-iodination, presumably on the metal surface, pick your mechanism.

[Rutherford]

So it is hydrogenolysis? Can't think of the mechanism for it. I need help.

[orgopete]

So it is hydrogenolysis? Can't think of the mechanism for it. I need help.

Yes.

I am a lumper. I prefer to write mechanism that have as large a degree of consistency with other chemistry as I can. In my book, I suggest palladium reacts similarly in hydrogenation reactions as it does in the Heck reaction. I had believed the Heck reaction mechanism may be one of the better elucidated mechanisms. If so, the zero valence palladium inserts into the C-I bond, H2 coordinates with Pd, exchange reaction as in Heck (pdt forms), and reductive elimination returns zero valence palladium. If this is so, then you may expect the donation of electrons in the insertion to have a similar reactivity as the halides (I>Br>Cl>>>F) in Grignard formation.

[Rutherford]

So it is hydrogenolysis? Can't think of the mechanism for it. I need help.

H2 coordinates with Pd, exchange reaction as in Heck (pdt forms), and reductive elimination returns zero valence palladium.

Hm... In Heck reaction, this step seems as an electrophilic addition to a double bond, but here, as a sigmatropic shift, as there are only sigma bond. This seems somewhat strange to me. I though more of a free-radical mechanism.

[Rutherford]

I found an image where the hydrogenolysis mechanism is repsresented so that the hydrogen molecule is incorporated between the atoms of the bond going to be broken. How reasonable this seems? I can't make a judge.

[orgopete]

So it is hydrogenolysis? Can't think of the mechanism for it. I need help.

H2 coordinates with Pd, exchange reaction as in Heck (pdt forms), and reductive elimination returns zero valence palladium.

Hm... In Heck reaction, this step seems as an electrophilic addition to a double bond, but here, as a sigmatropic shift, as there are only sigma bond. This seems somewhat strange to me. I though more of a free-radical mechanism.

That is how I see it. In a Heck reaction, a double bond adds or coordinates with the Pd. In reduction, H2 adds to Pd.

Go to the Wikipedia example (showing bromobenzene). Don't pay a lot of attention to the groups reacting, per se. Consider them as sources of electrons. If H2 replaces the olefin in the mechanism, then the equivalent to intermediate 2 can react with it as shown for step B. The product of that reaction will be benzene and HPdBr. Reductive elimination of HBr gives zero valence Pd. The zero valence Pd can insert again to continue the cycle.

In a catalytic cycle, the zero valence Pd inserts into H2. You may think of H2 as though it were the smallest halogen. If you again compare this with the Heck catalytic cycle, now the alkene coordinates as shown in Wikipedia. Addition of H2 can give the same reaction you are interested in. In the absence of sufficient H2, the addition can reverse and explain how catalytic reduction can convert a cis olefin to trans.

[Rutherford]

Wouldn't a hydride ion need to leave Pd at the end (like the bromide ion is at the end of Heck reaction)? That, again seems strange.

[orgopete]

Wouldn't a hydride ion need to leave Pd at the end (like the bromide ion is at the end of Heck reaction)? That, again seems strange.

Like a Heck reaction, yes, hydride no. The Heck in Wikipedia loses HBr in the end to regenerate zero valence Pd. In your deiodination reaction, Pd loses HI. It loses a proton and a halide, not a hydride and halogen(+).

If you are comparing the hydrogenation reaction, a Pd•H2 complex doesn't need to lose anything to return to zero valence, it is already able to react with an alkene. How and why these coordination/valence changes occur is something for an inorganic chemist to explain.

[Rutherford]

Ah, right, it doesn't need to. I always thought that hydrogenation happens through free-radical mechanism (adsorption on metal surface), why isn't that the case?

[Rutherford]

In the Heck mechanism the palladium is not in metal form but it's a complex compound with ligands coordinated to palladium. Is the mechanism the same for the metal and complex form of palladium?

[orgopete]

I assume an alkene coordinates with Pd the same way a ligand does. If a ligand binds too tightly, it will stop the reaction. The details of the Heck reaction tell us about palladium chemistry. I assume it applies to hydrogenation and hydrogenolysis.

[Rutherford]

Interesting. Thank you for the explanations.