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

Abstract

Mechanistic proposals for the C–H activation reaction enabled by mono-N-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 (NBu₄)[Pd(κ²-N,O–AcN–CHR–COO)(C₆F₅)py] (Ac = MeC(O); R = H, Me) and [Pd(κ²-N,O–MeNH–CH₂–COO)(C₆F₅)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 C₆F₅–C₆H₄Me 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.