Mechanisms of the Mizoroki–Heck Reaction

The palladium-catalysed Mizoroki–Heck reaction is the most efficient route for the vinylation of aryl/vinyl halides or triflates. This reaction, in which a C C bond is formed, proceeds in the presence of a base (Scheme 1.1) [1, 2]. Nonconjugated alkenes are formed in reactions involving cyclic alkenes (Scheme 1.2) [1e, 2a,c,e,g] or in intramolecular reactions (Scheme 1.3) [2b,d–g] with creation of stereogenic centres. Asymmetric Mizoroki–Heck reactions may be performed in the presence of a chiral ligand [2]. The Mizoroki–Heck reaction has been intensively developed from a synthetic and mechanistic point of view, as expressed by the impressive number of reviews and book chapters [1, 2]. In the late 1960s, Heck reported that arylated alkenes were formed in the reaction of alkenes with a stoichiometric amount of [Ar–Pd Cl] or [Ar–Pd–OAc], generated in situ by reacting ArHgCl with PdCl2 or ArHgOAc with Pd(OAc)2 respectively [3]. A mechanism was proposed which involves a syn migratory insertion of the alkene into the Ar–Pd bond, followed by a syn β-hydride elimination of a hydridopalladium [HPdX] (X = Cl, OAc) (Scheme 1.4a). In the case of cyclic alkenes, in which no syn β-hydride is available, a syn β ′-hydride elimination occurs, leading to a nonconjugated alkene (Scheme 1.4b). Isomerization of the new C C bond may occur by a syn readdition of HPdX in the reverse direction, followed by a syn β ′′-hydride elimination (Scheme 1.4c) [3c].