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In my project I have a coupling that was planned to be done using Pd (Suzuki or Stille possibly).

Initially, I used an aryl chloride (the SM had already the chloride so I went with it), but the oxidative addition was slow. Changing to iodide helped but still slow (can't use triflate as would be destroyed in an earlier step if carry through).

In my lectures, changing the halide is always given as a way of gaining selectivity, this is also what is taught in Clayden if you look for the information on organometallics. But a postdoc suggested changing the catalyst, as apparently changing the electronics in palladium also helps. Is this the case? Or is OA just controlled by the strength of the C–X bond?

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  • $\begingroup$ You should try small-scale reactions with half a dozen ligands as in my experience the choice of ligand is key to the success of a coupling reaction. $\endgroup$ – Waylander Jul 30 '17 at 18:33
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To quote Hartwig:

Oxidative addition to more electron-rich metal centres tends to be more favourable than oxidative addition at metal poor centres [1]

This is generally a thermodynamic issue, and can be rationalised quite easily if one considers that the change in electron density at the metal centre during the oxidative addition process (note, theres a difference between formal oxidation state and the 'actual' electronics about the metal centre).

As you point out, most catalysts systems used are either Pd0 to begin with, or generate Pd0 via a Pd2 pre catalyst (the palladium needs to be in its 0 oxidation state for oxidative addition to occur, usually, though 2 -> 4 is also possible in certain circumstances).

Your confusion seems to have came from the actual nature of the active catalyst, although you'll always end up with Pd0 , the metal centre has ligands attached, and these ligands are able to tune various properties at the metal centre.

The issue with trying to optimise a reaction ( as you seem to be doing) is that there is an almost infinite number of combinations of catalyst + ligand that you could consider, and indeed chemical companies such as Sigma-Aldrich even sell screening kits which provide many of the most common.

My honest advice in your situation would be to have a thorough search of the literature and try a few combinations of ligand + catalyst that you have available in your lab. Doing the optimisation rationally can be extremely interesting (doing it properly requires changing catalysts, ligands, solvents, temperatures, additives etc and is all time consuming, though often done in industry during optimisation before scale-up), but is of little use synthetically if you simply need to drag material through.


[1]: Hartwig, J. Organotransition Metal Chemistry- From Bonding to Catalysis; University Science Books:Herndon , 2010

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