Organic Chemistry 2 · Organometallic carbon–carbon bonds
4-Acetylbiphenyl by Suzuki coupling of 4-bromoacetophenone
Conditions: Pd(PPh3)4 (cat.), K2CO3, toluene/H2O, heat
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The mechanism, step by step
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Oxidative addition
Pd(0) inserts into the C–Br bond of the 4-bromoacetophenone in one concerted step, giving a palladium(II) complex with the aryl group and bromide on the metal. The halide-bearing ring carbon cannot do SN2, so the metal is what activates the C–Br bond. The electron-withdrawing acetyl group speeds this step.
The curved arrows
- A palladium(0) lone pair forms the new Pd–C bond to the carbon bearing the halogen.
- The breaking C–Br bond pair forms the Pd–Br bond, so palladium goes from 0 to +2.
Carbonate activates boron
A carbonate oxygen donates a lone pair into the empty p orbital of boron, forming an anionic tetracoordinate boronate. The negative boron makes the B–C bond electron-rich enough to hand its phenyl group to palladium.
The curved arrows
- A carbonate lone pair fills the empty p orbital of boron, forming the O–B bond.
Transmetalation
The phenyl group moves from the boronate to palladium as bromide leaves the metal. The ketone carbonyl is not touched: the weakly nucleophilic boronate and the carbonate base do not add to it, which is why Suzuki couplings tolerate ketones.
The curved arrows
- The B–C bond pair forms the new Pd–C(phenyl) bond.
- The Pd–Br bond pair leaves with bromide.
Reductive elimination
The two aryl groups on palladium couple and the metal returns to Pd(0). The new biaryl bond joins the carbon that carried bromine to the carbon that carried boron.
The curved arrows
- The Pd–C bond pair to the acetylphenyl ring forms the new C–C bond.
- The Pd–C(phenyl) bond pair returns to palladium, regenerating Pd(0).
4-Acetylbiphenyl
The phenyl ring is now attached para to the acetyl group, giving 4-acetylbiphenyl (1-(biphenyl-4-yl)ethan-1-one) with its ketone intact. Pd(0) re-enters the cycle; bromide and a boric acid–carbonate adduct are the by-products.
What you'll practice
Draw one Suzuki cycle that couples 4-bromoacetophenone with phenylboronic acid, including carbonate activation of boron, and explain why the ketone survives.
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