Promoting Difficult Carbon-Carbon Couplings: Which Ligand Does Best?

A Pd complex, cis-[Pd(C F ) (THF) ] (1), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C-C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd L. Complex 1 ranks the coupling ability of some popular ligands in the order P Bu >o-TolPEWO-F≈tBuXPhos>P(C F ) ≈PhPEWO-F>P(o-Tol) ≈THF≈tBuBrettPhos≫Xantphos≈PhPEWO-H≫PPh according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈P Bu ≈o-TolPEWO-F>PhPEWO-F>P(C F ) ≫tBuBrettPhos>THF≈P(o-Tol) >Xantphos>PhPEWO-H≫PPh . This "meter" also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.