Experimental study of speciation and mechanistic implications when using chelating ligands in aryl-alkynyl Stille coupling

Abstract

Neutral palladium(II) complexes [Pd(Rf)X(P–L)] (Rf = 3,5-C₆Cl₂F₃, X = Cl, I, OTf) with P–P (dppe and dppf) and P–N (PPh₂(bzN)) ligands have chelated structures in the solid-state, except for P–L = dppf and X = Cl, were chelated and dimeric bridged structures are found. The species present in solution in different solvents (CDCl₃, THF, NMP and HMPA) have been characterised by ¹⁹F and ³¹P{¹H} NMR and conductivity studies. Some [Pd(Rf)X(P–L)] complexes are involved in equilibria with [Pd(Rf)(solv)(P–L)]X, depending on the solvent and X. The ΔH° and ΔS° values of these equilibria explain the variations of ionic vs. neutral complexes in the range 183–293 K. Overall the order of coordination strength of solvents and anionic ligands is: HMPA ≫ NMP > THF and I⁻, Cl⁻ > TfO⁻. This coordination preference is determining the complexes participating in the alkynyl transmetalation from PhCCSnBu₃ to [Pd(Rf)X(P–L)] (X = OTf, I) in THF and subsequent coupling. Very different reaction rates and stability of intermediates are observed for similar complexes, revealing neglected complexities that catalytic cycles have to deal with. Rich information on the evolution of these Stille systems after transmetalation has been obtained that leads to proposal of a common behaviour for complexes with dppe and PPh₂(bzN), but a different evolution for the complexes with dppf: this difference leads the latter to produce PhCCRf and black Pd, whereas the two former yield PhCCRf and [Pd(CCPh)(SnBu₃)(dppe)] or [Pd(CCPh)(SnBu₃){PPh₂(bzN)}].