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. Squeezing cross‐coupling products: Complex cis‐[Pd(C6F5)2(THF)2] was used as a meter to quantify directly the efficiency of ligands to promote reductive elimination in carbon–carbon bond formation. The ligand could be ranked and compared to others. The complex is a good precatalyst for palladium(0) catalytic species PdLn.