Tropanes as Scaffolds for Phosphorus–Olefin Ligands and Their Application in Asymmetric Catalysis

Tropane-derived phosphorus–olefin hybrid ligands bearing various combinations of P-units (chiral BINOL-derived units/achiral PPh2-units) with a tropene skeleton (chiral/achiral) have been synthesized and used in Cu-catalysed conjugate 1,4-additions of diethylzinc to cyclic enones and in Rh-catalysed 1,4-additions of phenylboronic acid (Hayashi–Miyaura reaction) with cyclic and acyclic enones. Whereas in Cu-catalysed reactions only moderate yields and ee values (up to 49 % ee) were obtained, up to 97 % ee was observed in the Hayashi–Miyaura reaction with various substrates. The most promising results in both reactions were achieved with ligands 11 and 15, which contain a chiral alkene/achiral P-unit or an achiral alkene/chiral P moiety, respectively, and where the metal is complexed on the exo side of the tropene skeleton. In contrast, ligands with an endo coordination mode showed only low (13) or no activity (12) in all of the 1,4-additions. NMR and ESI-MS experiments revealed the presence of two cationic [RhI(15)2]+ species in solution.