Reactivity of Pd(0)(NHC)2 (NHC: N-heterocyclic carbene) in oxidative addition with aryl halides in heck reactions

Pd(0)(NHC)(2) complexes (saturated or unsaturated NHC) may react in oxidative addition of aryl halides by two mechanisms: a dissociative mechanism involving Pd(0)(NHC) as reactive complex formed after dissociation of one carbene or an associative mechanism involving Pd(0)(NHC), as the reactive complex. The mechanism of the oxidative addition of aryl halides with Pd(0)(2)(2) (2: 1,3-di-benzyl-4,5-di-tert-butylimidazolidin-2-ylidene) is established in this work (associative mechanism) and compared to that of Pd(0)(1)2 (1: 1,3-di-tert-butylimidazolin-2-ylidene) which reacts via Pd(0)(1) in a dissociative mechanism, as reported by Cloke et al. Both complexes activate aryl chlorides at room temperature. The more reactive complex with aryl chlorides is Pd(0)(2)(2) which directly reacts in an associative mechanism. Pd(0)(2)2 is even more reactive than Pd(0)(1). Consequently, the reactivity of Pd(0)(NHC)(2) complexes in oxidative additions is not connected to the structure of the reactive species, i.e., Pd(0)(NHC)(2) vs. Pd(0)(NHC) but is more relevant to the electronic and steric properties of the carbene ligand.