[AB01]

Hi,

Oxidation state of palladium in [(allyl)PdCl]2 is II, so in palladium catalyzed coupling reactions how it undergoes oxidative addition by reacting with P and O donor ligands.( after bonding with P and O donor ligands it becomes Pd (0) so I am looking for the mechanism)
Can anyone explain me please.

Thanks,

Abby

[movies]

This depends on the other reaction conditions.  What exact reaction are you interested in?

[AB01]

This depends on the other reaction conditions.  What exact reaction are you interested in?

In the following article Cho,E.J.;Senecal, T. D. ;Kinzel,T.;Zhang,Y.;Watson,D.A.;Buchwald,S.L., Science ,2010, 328, 679.

They are doing trifluoromethylation of aryl halide so the starting material is aryl halide which  in the presence of [(allyl)PdCl]2, brettphos or ruphos ligands (which are O and P donors) , TESCF3( source of trifluoromethyl anion ), KF and dioxan as solvent gives trifluoromethylated aromatic compound. In this reaction Pd goes from Pd(0) to Pd(II) but Pd in [(allyl)PdCl]2 is already in Pd(II) state.

[movies]

I doubt they have actually studied the process under those reaction conditions, but to my eye there are three reasonable possibilities. 

The first is quite simple: reductive elimination of allyl chloride.  In many cases, addition of Pd(0) to an allyl–X is reversible, and the trifluoromethylation reaction is at relatively high temp, so there is likely enough energy to get the reductive elimination and then allyl chloride could boil out of the solution.

The second would be addition of F^– to the allyl group in a standard pi-allyl Pd reaction.  There have been a couple papers on similar reactions recently (although these were focused on generating allylic fluoride products).  The addition of F^– would form allyl fluoride and KCl along with Pd(0).

The third possibility would be essentially analogous to the aryl trifluoromethylation reaction: transmetallation of –CF₃ onto the allyl–Pd complex and reductive elimination of allyl–CF₃.

Can you think of an experiment to tell which of these processes is active?