Exploiting noninnocent (E,E)-dibenzylideneacetone (dba) effects in palladium(0)-mediated cross-coupling reactions: Modulation of the electronic properties of dba affects catalyst activity and stability in ligand and ligand-free reaction systems

The reactivity of palladium(0) complexes, [Pd₂ ⁰(dba-n,n′-Z)₃] (n,n′-Z = 4,4′-F; 4,4′-CF₃; 4,4′-H; 4,4′-MeO) and [Pd ⁰(dba-n,n′-Z)₂] (n,n′-Z = 4,4′-CF ₃; 4,4′-H; 3,3′,5,5′-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dban,n′-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L_nPd⁰-η²-dba interaction (in dπ-π* synergic bonding, n = 1 or 2), which ultimately increases the global concentration of catalytically active L_nPd⁰ available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n′-Z ligand stabilizes the Pd ⁰-η²-dba interaction, reducing catalytic activity. The specific type of dban,n′-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L_nPd ⁰ concentration (and the associated catalytic activity) and catalyst longevity. Chang-ing the type of dba-n,n′-Z ligand controls the concentration of L_nPd⁰ and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dban,n′-Z ligands in Heck arylation is more convoluted and dependant on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n′-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n′-Z with amine and/ or base effectively lessens its interference in the catalytic cycle.