A bimetallic cooperative catalysis model has been reported for the asymmetric Friedel-Crafts (F-C) alkylation of indoles with trifluoromethyl pyruvates using Trost's intramolecular dinuclear zinc complex as the catalyst. This dinuclear zinc catalyst was prepared in situ by reacting the chiral ligand (S,S)-L2b with 2 equiv of ZnEt. A series of trifluoromethyl alcohol and indole-containing biological compounds were formed in moderate to good yields (up to 95%) with good enantioselectivity (up to 88% enantiomeric excess (ee)) in the presence of 10 mol % catalyst under mild conditions. A synergistic transition state model was proposed to explain the origin of the asymmetric induction.
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.
A series
of new nonsymmetric semi-azacrown ether ligands were developed
and applied to the asymmetric Michael/cyclic keto-imine formation/Friedel–Crafts
alkylation reactions of 3-amino oxindole hydrochlorides and β,γ-unsaturated
α-keto amides. A diversity of 2,5-pyrrolidinyl dispirooxindoles
containing two nonadjacent spiro-quaternary stereocenters were obtained
in excellent diastereoselectivities and moderate to excellent enantioselectivities
(up to 95% ee). A possible catalytic cycle was proposed to explain
the origin of the asymmetric induction.
A new ligand can be easily prepared, and its intramolecular dinuclear zinc complexes act as a high performance catalyst for the asymmetric alternating copolymerization of cyclohexene oxide and CO2 under very mild conditions (1 atm CO2 , room temperature), affording completely alternating polycarbonates with up to 93.8 % enantiomeric excess (ee) and 98 % yield. A high Mn value of 28 600 and a relatively narrow polydispersity (Mw /Mn ratio) of 1.43 were also achieved.
A direct asymmetric Friedel-Crafts (F-C) alkylation reaction between a wide range of indoles and ethyl 2-(4-methoxyphenylimino)acetate catalyzed by Trost's dinuclear complex is reported. A series of 3-indolylglycine derivatives were synthesized in enantioselectivity of up to >99% enantiomeric excess (ee) using 10 mol% catalyst loading under mild conditions. This atom economic reaction could be run on a gram scale without impacting its enantioselectivity. The absolute stereochemistry of catalytic products was determined by correlation with a known configuration compound. A possible mechanism was proposed for the asymmetric induction.
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