Rate and Mechanism of the Reaction of Alkenes with Aryl Palladium Complexes Ligated by a Bidentate P,P Ligand in Heck Reactions



The regioselectivity of the Heck reaction is supposed to be highly affected by the electronic properties of the alkene and the ionic or neutral character of the aryl palladium(II) complexes involved in the reaction with alkenes. In Heck reactions performed in dmf, [Pd(dppp){dppp(O)}Ph]+ (dppp=1,2-bis(diphenylphosphino)propane) is generated in the oxidative addition of PhI with [Pd0(dppp)(OAc)] formed in situ from Pd(OAc)2 associated to two equivalents of dppp. [Pd(dppp){dppp(O)}Ph]+ is not very reactive with alkenes (styrene or methyl acrylate); however, it reacts with iodide ions (released in the catalytic reactions) to give [Pd(dppp)IPh] and with acetate ions (used as base) to give [Pd(dppp)(OAc)Ph]. [Pd(dppp)(OAc)Ph] reacts with styrene and methyl acrylate exclusively by an ionic mechanism, that is, via the cationic complex [Pd(dppp)(dmf)Ph]+ formed by dissociation of the acetate ion. The reaction of [Pd(dppp)IPh] is more complex and substrate dependent. It reacts with styrene exclusively by the ionic mechanism via [Pd(dppp)(dmf)Ph]+. [Pd(dppp)IPh] (neutral mechanism) and [Pd(dppp)(dmf)Ph]+ (ionic mechanism) react in parallel with methyl acrylate. [Pd(dppp)(dmf)Ph]+ is more reactive than [Pd(dppp)IPh] but is always generated at lower concentration.