A New Class of Bis-Phospholane Ligands for Asymmetric Catalysis

The class of the bis-phospholane ligands, invented by Mark Burk, were introduced in rhodium-catalyzed asymmetric hydrogenation reactions of prostereogenic carbon-carbon double bonds in 1990.1 The most successful ligands of this series are the dialkyl-DuPHOSes2 (1). Their most outstanding peculiarities are the dialkyl substitution at the phosphorus atoms which improves their electronic availability and the rigidity of the structure surrounding the metal core which improves the efficiency of chirality transfer during the reaction. A breakdown of these very powerful mediators is, however, the difficulty in tuning their electronic properties. We have recently given the preliminary examples of a new class of C2-bis(phospholane) ligands having the general formula 2,3 characterized by a five-membered heteroaromatic system as scaffold for the phospholane groups. The strategy for this design is the same we had successfully applied in the class of the biheteroaromatic diphosphine ligands 4,4 where the heterocyclic five-membered system allows a fine tuning of the electronic availability at phosphorus5 either by changing the supporting heterocyclic system, or the position of the phosphorus atoms on it. The synthetic approach to the bis-phospholane 3, its chemical and electrochemical characterization and preliminary applications as ligand of Rh(I) in the asymmetric hydrogenation of ?-(N-acylamino) acrylate derivatives are discussed.