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

A Pd complex, cis-[Pd(C6F5)2(THF)2] (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 Pd0L. Complex 1 ranks the coupling ability of some popular ligands in the order PtBu3>o-TolPEWO-F≈tBuXPhos>P(C6F5)3≈PhPEWO-F>P(o-Tol)3≈THF≈tBuBrettPhos≫Xantphos≈PhPEWO-H≫PPh3 according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈PtBu3≈o-TolPEWO-F>PhPEWO-F>P(C6F5)3≫tBuBrettPhos>THF≈P(o-Tol)3>Xantphos>PhPEWO-H≫PPh3. This “meter” also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.