Synthesis of New C_2-SymmetricaI Bissulfonamide Ligands and Application in the Enantioselective Addition of Alkynylzinc to Aldehydes and Ketones

摘要

Novel C_2-symmetrical bissulfonamide li-gands were easily prepared in three simple steps and applied in the enantioselective addition of alky-nylzinc reagents to aldehydes.Compound 7a was found to be an outstanding ligand for this reaction.When the catalyst loading,4 mol % of 7a,was used,high enantioselectivity with an ee value up to 97% was achieved under very mild conditions.When the amount of ligand 7a was lowered to 2 mol %,excellent levels of enantiocontrol up to 95% ee were still achieved.So far the most economic catalyst system was developed for the addition of terminal alkynes to aromatic aldehydes in terms of ligand loading and enantioselectivity.Moreover,ligand 7a in combination with Ti(O-i-Pr)_4 was found to be effective in promoting the addition reaction of an alkynylzinc reagent to unactivated simple ketones under very mild conditions.The corresponding chiral tertiary propar-gylic alcohols were obtained with enantiomeric excesses of up to 81%.