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)n (L = (S,S)-1) complex has been developed.
The catalyst (Zn2EtL)n complex
was prepared in situ by reacting the chiral ligand (S,S)-1 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 S-configuration by X-ray crystallographic analysis of 13g. 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.