Suzuki Coupling
The Suzuki (or Suzuki-Miyaura) reaction is a Pd catalyzed cross-coupling between organoboronic acids or esters and organic halides or triflates under basic conditions to create a C–C sigma bond. It provides the stereoselective synthesis of (E)-alkenes by the reaction of alkenylboranes with aryl halides in the presence of a palladium catalyst.
General features:
1. It offers mild reaction conditions. 2. Wide scope of commercially available reagents. 3. Easy work-up. 4. Boronic acids are environmentally safer than organostannanes, Stille coupling. 5. It needs the presence of a base and side reactions may occur: depending on the substrates, racemization of chiral products, or aldol condensations.
Suzuki Coupling
Reaction Mechanism
1. Oxidative addition of an organic halide to Pd(0) to form the Pd(II) species. 2. Anion exchange or metathesis between the anion of the base and the one attached to the catalyst. 3. Transmetalation between Pd(II) and the alkylborate. 4. Reductive elimination to form the C–C sigma bond and regeneration of the catalyst.
Experimental Procedure
A round-bottom flask was charged with the organic halide (0.22 mmol, 1.0 eq), the boronic acid (1.1 eq), Pd2(dba)3 (0.05 eq), 2-(ditert-butylphosphino)-biphenyl (JohnPhos, 0.2 eq), and cesium carbonate (3.0 eq). THF (2.5 mL) and water (0.5 mL) were added sequentially to the flask, and the resulting reaction mixture was heated to 40 °C under argon atmosphere for 2.5 h. The reaction mixture was allowed to cool to room temperature, quenched with saturated aqueous NH4Cl, and extracted with EtOAc. The combined organic extracts were washed with brine, dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by flash column chromatography to afford the pure product (90% yield).
Additional Examples
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