Efficient novel 1,2-diphosphite ligands derived from d-mannitol in the Pd-catalyzed asymmetric allylic alkylation

A novel Pd/1,2-diphosphite catalyzed asymmetric allylic alkylation of 1,3-diarylpropenyl acetate with malonates was developed. Catalyst optimization via a variation in the protecting groups at the 1,2- and/or 5,6-positions of d-mannitol skeleton and in biaryl moieties of the ligands led to a ‘lead’ catalyst, which efficiently mediated the allylic alkylations. The activities and enantioselectivities of the reaction clearly showed that the stereogenic centers of the skeleton and the axially chiral diaryl moieties of the ligands had a synergic effect. The ligand 1,2:5,6-di-O-isopropylidene-3,4-bis[(S)-1,1′-binaphthyl-2,2′-diyl]phosphite-d-mannitol afforded excellent yields (up to 99%) and high levels of enantioselectivies (ee up to 98%) in 1,4-dioxane/CH₂Cl₂ mixture (v/v, 1:1) using [Pd(π-allyl)Cl]₂ as catalytic precursor and LiOAc as base. Dramatic changes in the sense and in the degree of the enantioselectivity depending on the configuration of the diaryl moieties of the ligands and reaction conditions were observed.