Computational Study of a New Heck Reaction Mechanism Catalyzed by Palladium(II/IV) Species

Abstract

In this theoretical study on the Heck reaction we explore the feasibility of an alternative pathway that involves a PdII/PdIV redox system. Usually, the catalytic cycle is formulated based on a Pd0/PdII mechanism. We performed quantum chemical calculations using density functional theory on a model system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. Accordingly, the PdII/PdIV mechanism is most likely to occur in the equatorial plane of an octahedral PdIV complex. The energy profiles of both reaction pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C-I bond to the palladium centre. This is a rate-determining step of the PdII/PdIV mechanism, while it is facile for a Pd0 catalyst. The calculations show that intermediate ligand detachment and reattachment is necessary in the course of the oxidative addition to PdII. Therefore, we expect the PdII/PdIV mechanism to be only feasible if a weakly coordinating ligand is present.

A novel SiO2 supported Pd metal catalyst for the Heck reaction

“…Instead, it appears likely without further evidence that this low catalytic activity results from the catalysis of the (Pd(OAc) 2 + BQ) system via Pd 2+ /Pd 4+ . The Heck reaction mechanisms with palladacycles and other Pd 2+ compounds involving a Pd 2+ /Pd 4+ catalytic cycle have been advanced [26][27][28][29][30][31][32][33] and the formation of a transient palladacycle by the reaction of Pd(OAc) 2 with an olefin has been proposed [26]. In the present Heck reaction system with Pd/ SiO 2 (O), most of Pd 2+ /SiO 2 is practically converted to Pd 0 /SiO 2 that coexists with the remaining Pd 2+ /SiO 2 .…”

Mechanisms of the Mizoroki–Heck Reaction

“…DFT calculations have, however, established that the oxidative addition of iodobenzene with a Pd(II) complex is energetically not favoured at all [58]. Some Mizoroki-Heck reactions proceed with an induction period when the base is a tertiary amine [55b], but no induction period is observed when an acetate salt is used as a base [27,55].…”

Oxidative addition of aryl chlorides to palladium N-heterocyclic carbene complexes and their role in catalytic arylamination

“…Previous computational studies employing bis-ligated palladium have found similar g 2 -arene complexes and oxidative addition transition states to those reported here [23,[25][26][27]. The most directly comparable study is that of Cundari and Deng [28] who model the catalytic cycle for Pd(PH 3 ) and Pd(PH 3 ) 2 .…”

“…The majority of these involve bis-phosphine palladium catalysts [23][24][25][26][27]. Cundari [28] has recently reported a density functional study based on BuchwaldÕs work [29][30][31] with very bulky phosphines that compares mono-and bis-phosphine palladium moieties as catalysts [28].…”