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SolutionsThe Suzuki coupling

Exercise 1

Propose a synthesis of 2-methylbiphenyl by Suzuki coupling. Give the two possible disconnections and how you would prepare the boronic acid for one of them.

Disconnection of the bond between the rings: (a) 2-bromotoluene + phenylboronic acid, or (b) bromobenzene + 2-methylphenylboronic acid. For (b), 2-bromotoluene is converted into the organolithium (n-BuLi, −78 °C) or the Grignard reagent, which reacts with B(OMe)3; acid hydrolysis gives the boronic acid. Coupling conditions: Pd(PPh3)4, aqueous Na2CO3, toluene or DME, 80 °C.

Exercise 2

(E)-Hex-1-enylboronic acid is coupled with (Z)-1-iodobut-1-ene. Which diene is obtained? What does this tell you about the stereochemistry of the reaction?

(3Z,5E)-Deca-3,5-diene. Oxidative addition, transmetalation and reductive elimination retain the geometry of both vinyl fragments: the Suzuki coupling is stereospecific, which is why it is widely used to prepare dienes and polyenes of defined geometry.