Tetrathiaheterohelicene Phosphanes as Helical‐Shaped Chiral Ligands for Catalysis

Tetrathia[7]helicene‐based phosphanes thiaheliphos ( 2a ), n Pr‐thiaheliphos ( 2b ) and di‐ n Pr‐thiaheliphos ( 2c ) have been prepared from the 2,13‐dilithio derivatives of thiahelicenes 1a – c by reaction with an excess of Ph 2 PCl. Protection of the air‐sensitive products as BH 3 adducts, from which the phosphanes 2a – c are easily regenerated on heating with ethanol, is described. New rhodium(I) complexes 8 and 9 were obtained by reaction of 2c with [Rh(COD) 2 ] + [BF 4 ] – and [BARF] – ,respectively, which were converted by oxidation into thechelating phosphane–phosphane oxide Rh I complexes 10 and 11 . The monophosphane 5 was similarly prepared, and reaction of the dilithio species with ClP(O)(OEt) 2 gave the diphosphonate 17 , which was characterized by X‐ray crystallography. Comparison of data from DFT calculations (B3LYP/SVP) on 2c , 8 and 10 and X‐ray data for 17 reveals a strong tension within the thiaheliphos ligands that is partially relaxed by increasing the Rh–P distances and the bite angle of the chelating ligand. Compound ( P ) ‐ (+) ‐1c , obtained by chromatographic resolution, was transformed into optically pure ( P ) ‐ (+) ‐2c , which was used in the asymmetric hydrogenation of itaconic acid ester and methyl 2‐acetamidoacrylate (product ee s up to 40 %).