Synthesis of 3‑Indolylglycine Derivatives via Dinuclear Zinc Catalytic Asymmetric Friedel–Crafts Alkylation Reaction

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

Asymmetric Friedel–Crafts Alkylation of Indoles with Trifluoromethyl Pyruvate Catalyzed by a Dinuclear Zinc Catalyst

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 (<i>S,S</i>)-<b>L2b</b> with 2 equiv of ZnEt₂. 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

Diastereomeric Aziridine Carbinol Catalyzed Enantioselective Arylation Reaction: Toward the Asymmetric Synthesis of Both Enantiomers of Chiral 3‑Aryl Phthalide

The diastereomeric aziridine carbinols are applied, respectively, as efficient chiral ligand in the catalysis of asymmetric arylation and sequential arylation-lactonization cascade. The two diastereomers, which are facilely synthesized from the same chiral source, function as pseudo enantiomers in arylation of aromatic aldehydes providing the different enantiomers of the diarylmethanols with almost the same excellent enantioselectivities. The arylation method is also carried out in tandem with lactonization process to afford a concise synthetic approach to both enantiomers of optically active 3-aryl phthalide

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

Asymmetric Copolymerization of Cyclopentene Oxide and CO₂ Using a Dinuclear Zinc–AzePhenol Catalyst: Enlightened by DFT Calculations

Optically active polycarbonates (PCs) are considered as candidates for new and valuable materials because of their well-defined chemical structures and special physical properties. Previous studies on asymmetric alternating copolymerization of cyclopentene oxide (CPO) and CO₂ regarding chiral zinc catalysts provided poly­(cyclopentene carbonate) (PCPC) with moderate enantioselectivity, and thus, the development of highly efficient catalysts for this enantioselective polymerization is highly desirable. This research work is enlightened by the DFT calculations. In this paper, we clearly describe the use of intramolecular dinuclear zinc–AzePhenol complex as a high performance catalyst for the asymmetric copolymerization of CPO and CO₂, affording completely alternating PCPC under very mild conditions (1 atm CO₂, 30 °C) in 98% yield with >99% enantioselectivity for (<i>S,S</i>)-configuration. The dinuclear catalyst is prepared in situ from the reaction of multidentate semiazecrown ether ligand and ZnEt₂, followed by treatment with an alcohol additive. In addition, our previous studies indicated that this catalyst also showed excellent enantioselectivity in the asymmetric copolymerization of cyclohexene oxide (CHO) and CO₂. In order to obtain more information on the mechanism of the catalytic copolymerization, the chemical structures of PCPC are characterized by ¹H NMR and ¹³C NMR spectroscopy, and the nonlinear effect is also investigated in this copolymerization. A plausible catalytic cycle for the present reaction system is outlined. The reaction of chiral ligand with ZnEt₂, followed by the ethyl group exchange with EtOH, affords the ethoxy-bridged dinuclear zinc complex. The copolymerization reaction is initiated by the insertion of CO₂ into the Zn–OEt bond to give a carbonate–ester-bridged complex. The two zinc centers are situated sufficiently close to each other to allow a synergistic effect in the copolymerization, meaning that one zinc atom acts as Lewis acid to activate the epoxide, the other is responsible for carbonate propagation through the nucleophilic attack of carbonate ester at the back side of the <i>cis</i>-epoxide by a six-membered transition state. Furthermore, the dinuclear zinc structure of the catalyst remains intact throughout the catalytic copolymerization. The proposed mechanism implies that the intramolecular dinuclear zinc catalyst is very important for future research into the copolymerization of other epoxides with CO₂