The role of the stabilizing/leaving group in palladium catalysed cross-coupling reactions

Despite the widespread use of well-defined PdII complexes as pre-catalysts for cross-coupling processes, the role of the throw-away ligand is still underexplored. In this work we focused on the complexes of the type [Pd(NHC)(eta 3-R-allyl)Cl] (NHC = N-heterocyclic carbene) and we investigated the influence of the R substitution on the allyl moiety. Starting from the already described [Pd(IPr)(eta 3-cinnamyl)Cl] and [Pd(IPr*)(eta 3-cinnamyl)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr* = N,N '-1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazol-2-ylidene) we prepared eight new complexes bearing new substitutions on the cinnamyl motif and we tested them in the C-N bond formation to evaluate the effect of the throw-away ligand modification in the catalytic activity. In addition, we studied the undesired formation of the less active off-cycle [PdI2(NHC)2(eta 3-R-allyl)(mu-Cl)] dimers from the corresponding PdII complexes to evaluate the role of the new throw-away ligands on the inhibition of this process. Despite the widespread use of well-defined PdII complexes as pre-catalysts for cross-coupling processes, the role of the throw-away ligand is still underexplored.