Catalytic Asymmetric Intramolecular Hydroamination of Alkynes in the Presence of a Catalyst System Consisting of Pd(0)-Methyl Norphos (or Tolyl Renorphos)-Benzoic Acid

Enantiomerically pure methyl Norphos (A), tolyl Norphos (B), CF3 Norphos (C), methyl Renorphos (D), and tolyl Renorphos (E) were synthesized and used as chiral bisphosphine ligands for the catalyst system, Pd2(dba)3·CHCl3/PhCOOH, in an intramolecular hydroamination of aminoalkynes 15. Among the Norphos series, methyl Norphos (A) was the best ligand for the hydroamination, and the corresponding five- and six-membered nitrogen heterocycles 16 were obtained in high yields with high enantioselectivities. Among the Renorphos series, tolyl Renorphos (E) gave the best result; both methyl Norphos (A) and tolyl Renorphos (E) afforded high yields and high enantioselectivities. NMR investigation using Me-Norphos revealed that this ligand was oxidized gradually in the presence of Pd2(dba)3·CHCl3 in C6D6 even under the conditions using Ar atmosphere to give Me-Norphos oxide, which prevented the intramolecular hydroamination. On the other hand, Me-Norphos was rather stable in C6D6 in the absence of the palladium catalyst under Ar atmosphere and was not converted to its oxide even after 3 days. The gradual oxidation of ligands (A and E) in the presence of the Pd catalyst is perhaps a reason why 20 mol % of A or E was needed to obtain high yields and high eeʼs of 16.