Studies In the Optimization of the Suzuki-Miyaura Reaction
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Enormous efforts have been made to optimize the Pd-catalyzed Suzuki-Miyaura reaction, but there is to date no generally useful protocol and forcing conditions are often required. One reaction variable that has often been neglected is the extent to which the supposed catalysts, bisphosphinepalladium(0) complexes, are actually formed from the variety of popular precatalysts used. There is in fact little evidence that these precursors produce bis-ligated Pd(0) complexes and it is possible that the rate limiting factor may be catalyst formation. If so, then the development of an optimized method for forming these catalytic species would be a significant contribution to this field. The following work describes research efforts to determine the optimum conditions to generate PdL2 (L = PCy3, PMeBut2, PBut3) cleanly and quantitatively from Pd(3-C3H5)(5-C5H5) and Pd(3-1-Ph-C3H5)(5-C5H5). Furthermore, the conditions under which PdL3 species may exist in equilibrium with the PdL2 species are defined. NMR studies indicate that while Pd(PBut3)2 shows no inclination to increase its coordination number, Pd(PCy3)2 and Pd(PMeBut2)2 react with added phosphine to form 3:1 compounds. Equilibrium constants for dissociation of the PdL3 compounds were measured over a range of temperatures, yielding the thermodynamic parameters of dissociation and estimated Pd-P bond dissociation energies. Additionally, the generation of heteroleptic species serve to confirm the existence of 3:1 compounds. A kinetic study of the oxidative addition of PhBr to Pd(PCy3)2 was also performed. It was found that oxidative addition was first order in palladium, but that added bromide had no effect on the rate of oxidative addition. Added PCy3 inhibited oxidative addition, possibly due to the conversion of palladium(0) into the less active 3:1 compound. The formation of the catalytically less active 3:1 compounds has serious implications for many catalytic cross-coupling processes which involve catalyst formation via the slow reduction of palladium(II) in the presence of excess phosphine; for many systems, relatively little of the added palladium may actually be present as the active bisphosphinepalladium(0) compound.