Question

As has been demonstrated in the text, when the starting alkene has \(\mathrm{CH}_{2}\) as its terminal group, the Heck reaction is highly stereoselective for formation of the \(E\) isomer. Here, the benzene ring is abbreviated \(\mathrm{C}_{6} \mathrm{H}_{5}\)-. Show how the mechanism proposed in the text allows you to account for this stereoselectivity.

Short answer

Answer: The Heck reaction with an alkene containing a terminal methyl group shows stereoselectivity towards the E-isomer due to steric interactions between the large groups during the migratory insertion step. The syn-insertion transition state has more steric interaction between the alkene and the aryl group, while the anti-insertion transition state minimizes steric interactions due to the arrangement of these groups. Consequently, the anti-insertion transition state, which leads to the E-isomer, is favored as it has a lower energy.

Step-by-step solution

Recall the general mechanism of the Heck reaction

The Heck reaction involves the coupling of an alkene and an aryl halide to form a substituted alkene. In the initial step, the palladium(0) catalyst forms a complex with the aryl halide, undergoing oxidative addition to create a palladium(II) complex. Next, the alkene coordinates to the palladium(II) complex, and migratory insertion takes place to form the carbon-carbon bond. Finally, the product is released, and the palladium(0) is regenerated through a reductive elimination process.

Mechanism with Terminal Methyl Group

Consider an alkene with a terminal methyl group (\(\mathrm{CH}_{2}\)) reacting with an aryl halide (\(\mathrm{C}_{6} \mathrm{H}_{5}\)-X) through the Heck reaction. The alkene coordinates to the activated aryl-palladium(II) complex; once the migratory insertion occurs, two possible transition states can be considered: syn-insertion and anti-insertion. The syn-insertion leads to the Z-isomer, while the anti-insertion leads to the E-isomer.

Evaluate Steric Interactions

Compare the steric interactions between the syn and anti transition states. Remember that the transition state with less steric interaction is favored as it has lower energy. In the case of syn-insertion, there is more steric interaction between the alkene and the aryl group (large groups are close to each other), whereas for anti-insertion, there is minimal steric interaction between the alkene and the aryl group. This difference in steric interactions leads to a preference for the formation of the E-isomer, thereby explaining the observed stereoselectivity. In conclusion, the mechanism proposed in the text adequately accounts for the observed stereoselectivity towards the E-isomer in the Heck reaction with an alkene containing a terminal methyl group. The key factor is the steric interaction between the large groups during the migratory insertion step, which is minimized in the case of anti-insertion.