Phosphine allene Ligands: Coordination and Catalytic Applications – PALCOCA

Access to new phosphines as chiral ligands by modulating their electronic and geometrical properties is a major challenge in the field of asymmetric catalysis to obtain optically pure compounds of interest. Diphosphines with axial chirality meet these requirements. However, the axial chirality of allenes has been little exploited in asymmetric catalysis. We were the first to study the coordination of 1,3-bis(diphenylphosphino)-1,3-diphenylallene. Monometallic complexes of Pd(II) and Pt(II) were obtained where the allene is bent. The bimetallic Au(I) complex shows exclusive gold coordination to the phosphines and none to the allene. After their resolution by preparative chiral HPLC, promising ee were obtained in cycloisomerisation reactions of 1,6-enynes. The ANR PALCOCA aims to study the potential of these ligands in enantioselective catalytic processes. To begin with, a family of allene diphosphines will be synthesized by modulating the nature of all substituents. Each enantiomer will be obtained by chiral preparative HPLC separation of the corresponding borane diphosphine followed by a non-racemizing deprotection step. The synthesis of allene diphosphine monoxides and allene diphosphine monoborane will be considered, very interesting hemilabile ligands which are capable of coordinating a wide variety of transition metals. The new organometallic complexes obtained will be studied and tested in asymmetric catalysis. The reactivity of diphosphine-boranes in borenium form will also be studied.
Our first results show the strong potential of this ligand, which we want to explore on different facets in the PALCOCA project. The stability of the species involved and the reaction mechanisms will be studied using various spectroscopic techniques including NMR, mass and DFT modelling.