The reaction of cyclohexene with iodo-benzene under Heck conditions forms E, a coupling product with the new phenyl group on the allylic carbon, but none of the “expected” coupling product F with the phenyl group bonded directly to the carbon–carbon double bond.

a. Draw a stepwise mechanism that illustrates how E is formed.

b. Step [2] in Mechanism 26.2 proceeds with syn addition of Pd and R' to the double bond.What does the formation of E suggest about the stereochemistry of the elimination reaction depicted in Step [3] of Mechanism 26.2?

The Heck reaction is a type of C-C coupling reaction that involves a palladium catalyst. The reactants for a Heck reaction is an alkyl or aryl halide and an alkene.The general reaction is given below:

Heck reaction

Step 1: Oxidative addition

Step 1

Step 2: Syn addition

Step 2

Step 3: Syn elimination

Step 3

The product that is not formed is:

The product that is not formed

The product is not formed because the trans hydrogen has to undergo elimination, violating the pathway of syn elimination.

Step 4: Reductive elimination

Step 4

The mechanism of Heck Reaction, as given in segment 26.2, is shown below:

mechanism of Heck reaction

1. The first step is the oxidative addition to R’-X to form the organopalladium reagent.
2. The second step is the addition of R’ and Pd to the alkene group.
3. The third step is eliminating hydrogen and palladium to form a double bond and adding the hydrogen to the palladium center,whichsuggests that for stereochemistry in the formation of E, the syn elimination of H and Pd is necessary.
4. Reductive elimination is carried out to get the catalyst back, and HI is formed as a by-product.