Chiral phosphite ligands derived from l-(+)-tartaric acid: synthesis and application in the Cu-catalyzed 1,4-conjugate addition of organozincs to cyclic enones

Abstract

A new series of chiral phosphite ligands derived from readily available l-(+)-tartaric acid and binaphthol derivatives were designed and prepared in convenient synthetic routes. These ligands were successfully employed in the Cu-catalyzed asymmetric conjugate addition of organozincs to cyclic enones. The ligands bearing (R)- and (S)-binaphthol and/or 3,3′-bis(trimethylsilyl)-2,2′-dihydroxy-1,1′-binaphthol afforded both enantiomers of the product in high yields and good enantioselectivities (up to 95% ee). The chiral organocopper reagent did exhibit high enantioselectivities for cyclic enones bearing different ring sizes. The stereochemically matched combination of l-(+)-tartaric acid backbone and (R)-binaphthyl in ligand 1-N-benzylpyrrolidine-3,4-bis[(R)-1,1′-binaphthyl-2,2′-diyl]phosphite-l-tartaric acid was essential to afford 95% (R) ee for 3-ethylcyclopentanone, 95% ee (S) for 3-ethyl-cyclohexanone, and 63% ee (R) for 3-ethylcycloheptanone in ether as solvent, using (CuOTf)2·C6H6 as a catalytic precursor. Moreover, the sense of the enantio discrimination of the products was mainly determined by the configuration of the binaphthyl phosphite moieties.