Enantioselective Friedel–Crafts Alkylation of Pyrrole with Chalcones Catalyzed by a Dinuclear Zinc Catalyst

A highly enantioselective Friedel–Crafts (F–C) alkylation of pyrrole with a wide range of simple nonchelating chalcone derivatives catalyzed by a chiral (Zn₂EtL)_<i>n</i> (L = (<i>S,S</i>)-<b>1</b>) complex has been developed. The catalyst (Zn₂EtL)_<i>n</i> complex was 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 dihydrochalcones were usually formed mostly in excellent yields (up to 99%) and excellent enantioselectivity [up to 99% enantiomeric excess (ee)] by using 15 mol % catalyst loading under mild conditions. The absolute 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. On the basis of the experimental results and previous reports, a possible mechanism was proposed to explain the origin of the asymmetric induction