Novel Chiral C3-Symmetric Monophosphite Ligands: Synthsis and Catalytic Performance in Asymmetric Hydroformylation and 1,4-Conjugate Addition

A family of the novel chiral helical C-3-symmetric monophosphite ligands has been synthesized from axially chiral (R)- or (S)-H-8-BINOL and PCl3. The performance of these ligands in asymmetric catalysis was evaluated by the Rh-catalyzed asymmetric hydroformylation of styrene, and the Cu-catalyzed asymmetric 1,4-conjugate addition of diethylzinc to acyclic enones. The catalytic hydroformylation of styrene gave rather high conversion (93%), high regioselectivity (b/n=95/5) and low enantioselectivity [28% ee (R)] using ligand (R)-2 in tetrahydrofuran at room temperature. Under the optimal 1,4-conjugate addition of ZnEt2 to chalcone 3a conditions, eleven acyclic enones 4a similar to 4k were all converted smoothly to the 1,4-adducts with high enantiomeric excess. Using Cu(OTf)(2) as catalyst precursor and Et2O as solvent at -20 degrees C, the structure of ligand (R)-3 comprising a partially hydrogenated 2,2'-(1,1'-binaphthyl)-phosphite scaffold and an adamantyl moiety was key factor in improving catalytic activity and the enantioselectivity with up to 98% yield and 72% ee (R). The sense of enantiodiscrimination of the reaction was controlled by the configuration of the diaryl phosphite moieties.