Diphosphorus Ligands Containing a P-Stereogenic Phosphane and a Chiral Phosphite or Phosphorodiamidite - Evaluation in Pd-Catalysed Asymmetric Allylic Substitution Reactions

Abstract

The synthesis of 14 new optically pure C 1‐symmetric phosphane-phosphinite (1 -4 ), phosphane-phosphite (5 -9 ) and phosphane-phosphorodiamidite (10 -14 ) ligands is reported. The ligands were prepared through the condensation of (2‐hydroxyphenyl)phenylphosphanes PPh(2‐PhOH)R (R = Me, t Bu and Ph) with chlorodiisopropylphosphane (1 and 2 ), chlorodiphenylphosphane (3 and 4 ), the chlorodioxaphosphepine derived from both enantiomers of 1,1′‐bi‐2‐naphthol (5 -9 ) and the chlorodiazaphosphepine derived from both enantiomers of N ,N′ ‐dimethyl‐1,1′‐binaphthyl‐2,2′‐diamine (10 -14 ) in the presence of a base. With these ligands, cationic Pd complexes of the type [Pd(η3‐C4H7)(PP′)]PF6 (Pd1 -Pd14 ) were obtained and characterised; the crystal structures of Pd1 , Pd2 and Pd13 were obtained. In solution, the complexes are present as mixtures of two diastereomers because of the lack of symmetry of the ligand and the presence of the methallyl group. The Pd complexes catalyse the allylic alkylation with dimethyl malonate and the amination with benzylamine of the model substrate rac ‐3‐acetoxy‐1,3‐diphenyl‐1‐propene (I ). For the alkylation, full conversions and good enantioselectivities (up to 96 % ee with Pd14 ) were observed.