Historical Perspective of the Heck Reaction

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Reaction mechanism The catalytic cycle for the Heck reaction involves a series of transformations around the palladium catalyst. The palladium(0) compound required in this cycle is generally prepared in situ from a palladium(II) precursor [3]. For instance, palladium(II) acetate is reduced by triphenylphosphine to di(triphenylphosphine)palladium(0) and triphenylphosphine is oxidized to triphenylphosphine oxide in step 1. Step 2 is an oxidative addition in which palladium inserts itself in the aryl to bromide bond. In step 3, palladium forms a π complex with the alkene and in step 4 the alkene inserts itself in the palladium - carbon bond in a syn addition step. Step 5 is a torsional strain relieving rotation and step 6 is a Beta-hydride elimination step with the formation of a new palladium - alkene π complex. This complex is destroyed in step 7. T he palladium(0) compound is regenerated by reductive elimination of the palladium(II) compound by potassium carbonate in the final step 8. In the course of the reaction the carbonate is stoichiometrically consumed and palladium is truly a catalyst and used in catalytic amounts. 6
Oxidative addition is often the rate-determining step in a catalytic cycle. The relative reactivity decreases in the order of I > OTf > Br >> C1. Aryl and 1-alkenyl halides activated by the proximity of electron-withdrawing groups are more reactive to the oxidative addition than those with donating groups, thus allowing the use of chlorides such as 3-chloroenone for the cross-coupling reaction Reductive elimination of organic partners reproduces the palladium(0) complex. The reaction takes place directly from cis-complex, and the transcomplex reacts after its isomerization to the corresponding cis-complex (eqs 1 and 2). The order of reactivity is diaryl- > (alky1)aryl- > dipropyl- > diethyl- > dimethylpalladium(II), suggesting participation by the n-orbital of aryl group during the bond formation (eq 1) [3]. • A general catalytic cycle for cross-coupling Heck reaction 7
Page 1 and 2: Dimitra Kovala-Demertzi Section of
Page 3 and 4: Aryl and vinyl halides- Aryl and vi
Page 5: Solvents-Heck reactions are conduct
Page 9 and 10: Amino-Heck reaction In the amino-He
Page 11 and 12: All of the palladacycles shown in t
Page 13 and 14: The monoanionic HSal4Et ligand is c
Page 15 and 16: 3.3 Palladium nanoparticles as cata
Page 17: Figure 2. Possible pathways involve

Historical Perspective of the Heck Reaction

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