The 1,3,7,9 tetramethyl 5 phenyl 5,6 dihydro 4H thieno[3',4':5,6] phosphepino[3,4 c]thiophene: the First Member of the Class of Chiral Heterophosphepines: Synthesis, Electronic and Steric Properties, Metal Complexes and Catalytic Activity

Over the last few years the metal complexes of a wide range of monodentate P-donor ligands has been proved to equal or even to outperform their analogous bidentate counterparts in some asymmetric reactions.1 A crucial role is played by the electronic properties of the phosphorous centre which controls both kinetics and the stereoselection ability of the corresponding metal complex used as catalyst. We are currently involved in the synthesis of some members of a new modular class of electronically tunable chiral phosphepines where the nature of the heteroaromatic scaffold controls both the electronic properties of the phosphorus atom and the geometry of the seven-membered ring. The synthesis of the first member of this class of ligands, the bithienophosphepine 1, is reported. The synthetic path involves the corresponding oxide: semipreparative HPLC of the latter on a chiral stationary phase followed by reduction with HSiCl3 gave 1 in both the enantiomerically pure forms. The complete structural characterization of 1 was achieved by single-crystal X-ray analysis, which showed that the dihedral angle is quite lower than those shown by analogues carbocyclic systems, like binepine.2 The electronic availability was evaluated electrochemically in comparison to other phosphepines: the lower the electrochemical peak potential, the electron richer the phosphane. The phosphorous centre of 1 was surprisingly found one of the electron-poorest in the series. The bithienophosphepine 1 has been preliminarily employed as ligand of Rh in the enantioselective hydrogenation of some functionalized carbon-carbon double bonds, and as ligand of Pt for the asymmetric alkoxyciclyzation of 1,6-enynes.3