Heck reaction Shaw mechanism

Not all Heck reactions depend on Pd° catalysts. There are plenty of cases in which Pd salts are efficient. Furthermore, the catalytic species of the system Pd(OAc)2/N(C2H5)3 is undefined until now. Suggestions that the amine reduces the palladium, to be converted into an immonium salt, seem not to have a reliable experimental basis [48]. A mechanism which is based on the oxidation states +II and +IV of Pd has been proposed by Shaw to account for the possibility of using palladium(II) catalysts without previous reduction to palladium(O) [81]. 

Scheme 2.1 Shaw s hypothetical Pd(ll)/Pd(IV) mechanism for the Heck reaction catalyzed by paUadacycles.

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... 

Shaw, B.L., Perera, S.D. and Staley, E.A. (1998) Highly active, stable, catalysts for the Heck reaction further suggestions on the mechanism. Chem. Commun., 1361-2. 

The above mechanism received two blows from different directions in the mid-1990s. Herrmann and co-workers observed that a Pd(II) metallacycle. A, in the presence of n-Bu4NBr is an excellent catalyst for the Heck reaction, but the reaction only starts after addition of the olefin. This led Shaw to propose a different mechanism, at least for these... 

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