Heck reaction, palladium chemistry mechanisms

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... 

Studies regarding the nature of the catalytically active species for NHC complexes in Heck-type reactions have focused on the Mizorvki—Heck reaction and have consistently revealed a palladium(O) species as the active catalyst. The induction period is shortened upon addition of a reducing agent,and postulated intermediates of the reaction were isolated and characterized as well as employed in stoichiometric and catalytic reactions. Theoretical studies using DPT calculations showed the mechanism for NHC complexes to most likely he in agreement with phosphine chemistry. ... 

Another variant of the Heck reaction which is important in heterocyclic chemistry utilizes five membered heterocycles as olefin equivalent (2.2.)7 It is not clear whether the process, coined as heteroaryl Heck reaction follows the Heck mechanism (i. e. carbopalladation of the aromatic ring followed by //-elimination) or goes via a different route (e.g. electrophilic substitution by the palladium complex or oxidative addition into the C-H bond). Irrespective of these mechanistic uncertainties the reaction is of great synthetic value and is frequently used in the preparation of complex policyclic structures. 

The transformation that has come to be known as the Heck reaction is broadly defined as the palladium(O)-mediated coupling of an aryl or vinyl halide or triflate with an alkene. The basic mechanism for the Heck reaction of aryl halides or trifiates (as outlined in more detail in the Key Chemistry), involves initial oxidative addition of the chiral palladium(O) catalyst to afford a a-arylpalladium(II) complex. Coordination of an alkene and subsequent carbon-carbon bond formation by syn insertion provide a a-alkylpalladium(II) intermediate, which readily undergoes P-hydride elimination to release the alkene product. Finally, the hydridopalladium(II) complex has to be converted into the active palladium(O) catalyst to complete the catalytic cycle. 

Two hypothetical mechanisms have been proposed to explain the Heck reaction on the basis of Pd(II)/Pd(IV) cycles (Scheme 2.12). As discussed in Section 2.2.1, oxidative addition of aryl halides to Pd(II) precursors is both kinetically and thermodynamically difficult. The Pd(II)/Pd(IV) mechanism proposed by Shaw for the Heck reaction (Scheme 2.1) tried to elude this problem by postulating the intermediacy of anionic Pd(II) complexes with increased nucleophilicity, but it is not evident how this mechanism could be adapted to complexes containing PCP or related pincer ligands. With this problem in mind, Jensen [93] made an alternative proposal (Scheme 2.12a), which starts with the oxidative addition ofa C-H bond of the olefin to the Pd(II) pincer complex to afford a Pd(IV) vinyl-hydride intermediate. This idea was inspired by a similar reaction observed with an isostructural Ir(I) PCP complex, but such C-H bond activations are unusual in palladium chemistry. A theoretical analysis by Freeh [63] raled out such possibility, leading instead to the alternative Pd(II)/Pd(IV) cycle depicted in Scheme 2.12b. A key element... 

The palladium Heck reaction of aryl halides (and related R—X bonds) with alkenes provides a valuable route to the assembly of substituted alkenes [73]. This reaction proceeds via a mechanism similar to cross coupling chemistry, though rather than a... 

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