The Heck Reaction

Question 1

What is the mechanism and product for this reaction?

Answer 1

• C-Br bonds are weaker, so the palladium inserts there
• Na₂CO₃ form base in water which adds to B(OH)₂, to allow transmetallation to occur

Question 2

What is the product of this reaction?

Answer 2

Question 3

Why are alkyl halides (R(sp³)-X) rarely used in Pd cross-coupling reactions

Answer 3

Pd(II) σ-alkyl complexes rapidly decompose by beta-hydride elimination

Question 4

How does the β-hydride elimination occur with a palladium catalyst?

Answer 4

• The empty d-orbital on the palladium can accept the electron density from the C-H bond
• Resulting in a new Pd-H bond
• And Breaking of the Pd-C bond where the elctrons from this form a double bond

Question 5

What are the requirements for a β-hydride elimination to occur on palladium?

Answer 5

• A cis free coordination site on palladium is required
• Palladium is not oxidised or reduced in this process
• β-hydride elimination is syn specific, i.e. only Hₐ is eliminated in this complex

Question 6

β-hydride elimination is reversible
The reverse process is called…

Answer 6

insertion

Question 7

Carbon (R groups) can undergo insertion reactions with palladium - called ligand migration or carbometallation
What are the key features of this

Answer 7

• Pd is not oxidised or reduced
• The reaction is syn-specific (R and Pd add to the same face of the alkene
• Regioselectivity: R attached to the less substituted end of the double bond (to avoid steric clash with substituents on the alkene)
• The reverse pprocess does not occur readily

Question 8

Where have we see insertion of an alkene/alkyne before?

Answer 8

Question 9

The Heck reaction involves a β-hydride elimination and a carbopalladation
What is the product of this reaction

Answer 9

Question 10

The first step of the Heck reaction is an oxidative addition
What is the mechanism and product?

Answer 10

Question 11

The second step of the Heck reaction is alkene coordination
What dos this involve and what is the product

Answer 11

• Pd¹¹ is electrophilic due to being electron deficient
• The palladium(II) will coordinate to the π-electrons on the alkene
• Results in a coordination complex forming

Question 12

The 3rd step of the Heck reaction is Carbopalladtion (insertion)
What is the mechanism and the product of this reaction?

Answer 12

• The Pd-C and C-Ph bonds form at the same time so these groups are cis to another
• But the Pd and H must be cis for β-H elimination to occur
• This hence requires a rotation around C-C bond

Question 13

Following the Carbopalladation in the Heck reaction, a PPh₃ dissociates off Pd to give a free coordination site
This is the followed by a β-hydride elimination
What is the mechanism and product?

Answer 13

Question 14

To reform the Pd catalyst, involves a reductive elimination
What is the mechanism and final product of the Heck reaction

Answer 14

(Br on LHS should be PPh₃)