Cationic (η1-allyl)-palladium complexes as feasible intermediates in catalyzed reactions

A computational study on the structure and reactivity of (eta(1)-allyl)-Pd complexes has been performed on model systems containing different neutral and cyclometalating ligands. A comparison of the reactivity of these compounds with the usually proposed (eta(3)-allyl)-Pd derivatives has been performed to investigate the feasibility of the participation of the former as intermediates in Pd-catalyzed reactions. These complexes are more stable than (eta(3)-allyl)-Pd derivatives with a cyclometalated PCP tridentate ligand. Intermediates, transition states, and activation and reaction energies have been calculated for several reactions of cationic and neutral complexes: (a) nucleophilic attack of NH3 on the allyl system, (b) nucleophilic attack of the gamma-allyl carbon on formaldehyde, (c) pallada-ene reactions of sigma-allyl complexes with ethylene, and (d) pallada-ene type reactions of sigma-allyl complexes with acetylene. Our results show that (eta(1)-allyl)-Pd complexes are probably not involved as intermediates in the Tsuji-Trost reactions with NH3; that even cationic complexes would be able to react as nucleophiles with aldehydes in two different ways, and that pallada-ene type reactions of these complexes with alkenes and alkynes show more stable transition states than those corresponding to the usual mechanisms involving prior coordination of the unsaturated species followed by 1,2 migratory insertion in either sigma- or d-allyl bonds.