Enantioselective Friedel–Crafts\nAlkylation\nof Pyrrole with Chalcones Catalyzed by a Dinuclear Zinc Catalyst

Abstract

A highly enantioselective Friedel–Crafts\n(F–C) alkylation\nof pyrrole with a wide range of simple nonchelating chalcone derivatives\ncatalyzed by a chiral (Zn₂EtL)_<i>n</i> (L = (<i>S,S</i>)-<b>1</b>) complex has been developed.\nThe catalyst (Zn₂EtL)_<i>n</i> complex\nwas prepared in situ by reacting the chiral ligand (<i>S,S</i>)-<b>1</b> with 2 equiv of diethylzinc. A series of β-pyrrole-substituted\ndihydrochalcones were usually formed mostly in excellent yields (up\nto 99%) and excellent enantioselectivity [up to 99% enantiomeric excess\n(ee)] by using 15 mol % catalyst loading under mild conditions. The\nabsolute stereochemistry of the products was determined to be the <i>S</i>-configuration by X-ray crystallographic analysis of <b>13g</b>. Meanwhile, a weak negative nonlinear effect was observed.\nOn the basis of the experimental results and previous reports, a possible\nmechanism was proposed to explain the origin of the asymmetric induction.