SolutionsThe Heck reaction
Exercise 1
What product is obtained in the Heck reaction of bromobenzene with styrene? And with acrylonitrile? Explain the regiochemistry and geometry.
With styrene, (E)-stilbene; with acrylonitrile, (E)-cinnamonitrile (3-phenylprop-2-enenitrile). The phenyl group adds to the less hindered terminal carbon and palladium to the carbon bearing the substituent. In the syn β-elimination, the conformation in which the two large groups are far apart is the most stable, and it gives the E alkene.
Exercise 2
What is the role of triethylamine in the Heck reaction? What would happen with an alkene lacking hydrogens on the carbon that receives palladium, such as methyl 2-methylacrylate?
The amine neutralizes the HX released from the palladium hydride, H–PdL2–X, and regenerates Pd(0); without base the cycle stops. With methyl 2-methylacrylate the carbon bearing palladium is quaternary, but it has two adjacent carbons bearing hydrogens: β-elimination can take place with an H of the CH2Ar group, giving the conjugated methyl (E)-2-methyl-3-phenylacrylate, or with an H of the methyl group, giving methyl 2-benzylacrylate. A mixture is usually obtained, whose ratio depends on the conditions.