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Predict the major product of the following reaction :

Methyl 6-chloronicotinate plus p-tolylmagnesium chloride with Pd(dha)2, dPPf in THF at -40 C

  • A.Methyl 6-(p-tolyl)nicotinate: a pyridine ring with a CO2Me group and a p-tolyl group
  • B.Ketone with a p-tolyl group on the carbonyl carbon attached to a 6-(p-tolyl)pyridine ring
  • C.4,4'-Dimethylbiphenyl, two p-tolyl rings joined directly
  • D.Tertiary alcohol with two p-tolyl groups and a 6-(p-tolyl)pyridin-3-yl group on a carbon bearing OH

Answer: A

The product is methyl 6-(p-tolyl)nicotinate, option A.

Pd(dha)2 with the ligand dPPf catalyses a Kumada cross-coupling: Pd(0) inserts into the activated C-Cl bond next to the ring nitrogen, the Grignard reagent transmetallates, and reductive elimination joins the two aryl groups. At -40°C the catalyst reacts with the aryl chloride much faster than the Grignard reagent attacks the ester, so the CO2Me group survives.

B and D are what an uncatalysed or warm Grignard reaction with the ester would give (ketone, then tertiary alcohol after a second addition), and they still contain the coupling product. C is the homocoupling product of the Grignard reagent, 4,4'-dimethylbiphenyl, which is only a minor side product under catalysis.

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