The five-membered aromatic heterocycles as modulators of electronic properties of diphospholane ligands

We have demonstrated that the electronic properties at phosphorus of chiral biheteroaromatic atropisomeric diphosphines, can be efficiently modulated either by varying the five-membered aromatic heterocyclic system of the backbone or by changing the position of the phosphine group on the same heterocycle.[1] We considered extending this strategy to the very successful class of the electron-rich DuPHOSes (1), designed and realized by M. Burk. We have synthesised the first two examples of the class of C2-symmetric bis(phospholane) heterocyclic ligands (HeteroDuPHOSes) (2), characterized by a five-membered heteroaromatic system as a scaffold for the phospholane groups. [2] The synthetic approach to the thiophene- and pyrrole-based bis-phospholanes 2a and 2b, their chemical and electrochemical characterization and preliminary applicatios as ligands of Rh(l) in the asymmetric hydrogenation of functionalised carbon-carbon double bonds are discussed. Structural studies demonstrate that the geometries of UIIuPHOS, Me-DuPHOS and those of their metal complexes are very similar and hence very similar should be their facial selection ability. Substantial differences are observed instead, in the kinetics of the hydrogenation reactions of functionalised prostereogenic carbon-carbon double bonds.