Triflate‐Selective Suzuki Cross‐Coupling of Chloro‐ and Bromoaryl Triflates Under Ligand‐Free Conditions

In the absence of strong ancillary ligands such as phosphines or N‐heterocyclic carbenes, palladium salts are selective for C−OTf cleavage in the room‐temperature Suzuki couplings of chloroaryl triflates in acetonitrile. Similar “ligand‐free” conditions in DMSO also promote triflate‐selective Suzuki coupling of bromoaryl triflates. This triflate selectivity complements the typical preference for reaction of bromides in prior reports of Suzuki couplings using phosphine ligands. DFT calculations and additional experimental evidence are consistent with triflate‐selective oxidative addition taking place at homogeneous, possibly mononuclear, anionic palladium supported by a solvent molecule. “Ligand‐free” room‐temperature conditions for Pd‐catalyzed Suzuki couplings of haloaryl triflates give selectivity for triflate over chloride or bromide. For chloroaryl triflates, triflate selectivity is exclusive and represents an improvement over conditions with strong ancillary ligands. The triflate selectivity with bromoaryl triflates is unprecedented for Suzuki couplings. Solvent choice is critical. Triflate‐selective oxidative addition may take place at anionic [Pd(solvent)(X)]−.