Palladium mono--protected amino acid complexes: experimental validation of the ligand cooperation model in C-H activation.

Mechanistic proposals for the C-H activation reaction enabled by mono--protected amino acid ligands (MPAAs) have been supported by DFT calculations. The direct experimental observation of the ligand-assisted C-H activation has not yet been reported due to the lack of well-defined isolated palladium complexes with MPAAs that can serve as models. In this work, palladium complexes bearing chelating MPAAs (NBu)[Pd(κ-,-AcN-CHR-COO)(CF)py] (Ac = MeC(O); R = H, Me) and [Pd(κ-,-MeNH-CH-COO)(CF)py] have been isolated and characterized. Their evolution in a solution containing toluene leads to the C-H activation of the arene and the formation of the CF-CHMe coupling products. This process takes place only for the ligands with an acyl protecting group, showing the cooperating role of this group in a complex with a chelating MPAA, therefore experimentally validating this working model. The carboxylate group is inefficient in this C-H activation.