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

Abstract A Pd complex, cis ‐[Pd(C 6 F 5 ) 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 Pd 0 L. Complex  1 ranks the coupling ability of some popular ligands in the order P t Bu 3 > o ‐TolPEWO‐F≈ t BuXPhos>P(C 6 F 5 ) 3 ≈PhPEWO‐F>P( o ‐Tol) 3 ≈THF≈ t BuBrettPhos≫Xantphos≈PhPEWO‐H≫PPh 3 according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked t BuXPhos≈P t Bu 3 ≈ o ‐TolPEWO‐F>PhPEWO‐F>P(C 6 F 5 ) 3 ≫ t BuBrettPhos>THF≈P( o ‐Tol) 3 >Xantphos>PhPEWO‐H≫PPh 3 . This “meter” also detects some other possible virtues or complications of ligands such as t BuXPhos or t BuBrettPhos.