Linear Triphosphines as Ligands for Metal Complexes Immobilization in Ionic Liquids: Palladium-Catalyzed Methoxylation of Alkynes

Several novel palladium triphosphine complexes have been synthesized and tested as recyclable catalysts for the methoxylation of alkynes into acetals in ionic liquids. A complete conversion of phenylacetylene was achieved with only 0.2% of [Pd(Triphos)NCMe][(PF6)2] in a methanol/[BMIM][BF4] mixture. We discovered that the addition of an ionic liquid to methanol allowed not only to increase the activity of the palladium catalyst but also to provide a recyclable catalyst which can be reused several times with a weaker drop of activity. To complete these catalytic studies, we describe the synthesis of the first poor -electron-donating/strong -electron-acceptor linear Triphosphine which, after palladium coordination, led to a better selectivity compared to its Triphos analogue. The performances of recovered ionic liquid reaction mixtures show for the first time that P-tridentate ligands efficiently immobilize palladium catalysts and lead to selective catalytic systems benign for environment.