Heck Coupling
The Heck reaction is the cross-coupling between an unsaturated halide or triflate and an alkene in the presence of a base and a palladium catalyst.
General features:
1. The reaction conditions tolerate a wide range of functional groups on the olefin. 2. The more substituted the olefins is, the slower the reaction proceeds. 3. The reaction is not sensitive to water but the solvents need to be deoxygenated. 4. The Heck Coupling is stereospecific. Both the migratory insertion and the hydride elimination proceed with syn stereochemistry. 5. The active palladium catalyst is generated in situ from precatalysts such as Pd(OAc)2 or Pd(PPh3)4. The reaction is usually conducted in the presence of monodentate or bidentate phosphine ligands and a base. 6. Substrates with hydrogen atoms on their β-carbons undergo rapid β-hydride elimination to give olefins 7. Aryl chlorides are not always good substrates because they react very slowly, being I, Br, and OTf best suited.
Heck Coupling
Reaction Mechanism
1. Oxidative addition of an organic halide to Pd(0) to form Pd(II). 2. Pd forms a π complex with the alkene. 3. Migratory insertion: the alkene inserts in the Pd–C bond in a syn addition step. 4. β-hydride elimination leads to the π complex. 5. Decoordination of the complex followed by reductive elimination to regenerate the catalyst.
Example
Experimental Procedure
To a solution of the aryl halide (37.5 mmol, 1.0 eq) and palladium acetate (0.05 eq) in acetonitrile (380 mL) were added ethyl acrylate (1.5 eq) and triethylamine (3.0 eq) at room temperature. The reaction mixture was allowed to reflux and monitored by TLC. After complete consumption of the substrate, the reaction mixture was filtered through Celite, to remove the palladium acetate, and the residue was concentrated in vacuo. The residue was re-dissolved in DCM, and sat. aq. solution of ammonium chloride was added. The mixture was extracted with DCM. The organic layers were combined, washed with brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified by column chromatography on silica to afford the desired product (81% yield).
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