Question

Show the sequence of Heck reactions by which the following conversion takes place. Note from the molecular formula given under each structural formula that this conversion corresponds to a loss of \(\mathrm{H}\) and I from the starting material.

Short answer

Short Answer: The Heck reaction sequence for the given conversion involves a series of four steps: 1) Formation of the Pd(0) coordination complex, where the Pd(0) species coordinates with the starting material (aryl iodide and alkene); 2) Oxidative addition, where the Pd(0) catalyst reacts with the C-I bond of the aryl iodide to form a Pd(II) complex; 3) Migratory insertion, where the alkene forms a new carbon-carbon bond with the aryl group, leading to a linked Pd(II) complex; and 4) Reductive elimination, where the Pd(II) complex collapses and eliminates a hydride (H-) and iodide (I-) to return to its initial Pd(0) state, resulting in a molecule that lost one hydrogen and one iodine atom. The Pd(0) species is regenerated and can catalyze another cycle of the Heck reaction.

Step-by-step solution

Formation of the Pd(0) Coordination Complex

First, we need a Pd(0) catalyst to start the Heck reaction. A common form of Pd(0) is Pd(PPh3)4, where PPh3 is triphenylphosphine. The molecular structure of the starting material consists of an aryl iodide, which is the substrate, and an alkene, which serves as the coupling partner. The Pd(0) species can be coordinated with the alkene to form a coordination complex, which is the key step to start the Heck reaction.

Oxidative Addition

In this step, the Pd(0) catalyst undergoes oxidative addition with the C-I bond of the aryl iodide. As a result, the molecular structure goes from a Pd(0) complex to a Pd(II) complex with the iodide now bound to the metal center. This process increases the oxidation state of the palladium from +0 to +2.

Migratory Insertion

Next, the alkene forms a bond with the palladium, leading to a new carbon-carbon bond between the aryl and alkene groups. This process is called migratory insertion, as the alkene migrates into the Pd-C bond. The new structure is a Pd(II) complex with the aryl and alkene fragments covalently bonded to each other.

Reductive Elimination

Finally, reductive elimination takes place. The Pd(II) complex collapses, and the palladium center loses a hydride (H-) and iodide (I-) as \(\mathrm{H}\) and I to return to its initial Pd(0) state. This forms a new C-H bond, while the carbon-carbon bond remains the same, leading to a molecule that has lost one hydrogen and one iodine atom. The Pd(0) species can then catalyze another cycle of the Heck reaction, as it is regenerated. By following these steps, we explained the Heck reaction sequence that corresponds to the removal of \(\mathrm{H}\) and I from the starting material, which led to the formation of the target molecule.