Rh-Catalyzed Aqueous Pauson-Khand-Type Cycloaddition in Microwave-Irradiated Medium

Abstract

Microwave-assisted Rh-catalyzed dual catalysis in aqueous medium is described. This tandem process transforms the enyne to cycloadduct by cascade decarbonylation of formate ester and subsequently carbonylation of enyne under microwave-irradiated conditions.

Keywords

aqueous - cycloaddition - catalysis - rhodium - microwave

References and Notes

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Representative Procedures
[Rh(COD)Cl]₂ (7.4 mg, 15.0 µmol) and dppp (13.6 mg, 33.0 µmol) were charged into the reaction vial on bench-top at room temperature. The reaction vial was then transferred to the dry box for being evacuated and backfilled with nitrogen (3 cycles). Then, 4-chlorobenzyl formate (255 mg, 1.5 mmol, 5 equiv with respected to enyne) was added under nitrogen atmosphere and the reaction mixture was stirred for 1 h. Enynes (57 mg, 0.3 mmol) and H₂O (1.0 mL) were charged into the reaction vial. The vial was air-tightened by a special designed lid and transferred to the microwave oven. The reaction mixtures were heated to 130 °C by microwave irradiation with power of 500 W for 50 min. The vials were allowed to reach r.t. Diethyl ether or EtOAc (ca. 2 mL) was added. The crude reaction mixtures were directly purified by column chromatography on silica gel using hexane-EtOAc as the eluent to afford chiral bicyclic cyclopentenones. For the asymmetric catalytic Pauson-Khand-type cyclization, the enantiomeric excesses of the products were determined by chiral HPLC analysis using Chiralcel^® AD-H columns. All characterization data matched with the literature reports (ref. 6).