Mechanism catalytic organopalladium

The reaction mechanism differs from that of other catalytic hydrogenations that also are carried out in the presence of palladium as catalyst, e.g. that of olefins. Presumably an organopalladium species is formed as an intermediate, which then reacts with the hydrogen ... 

This chapter describes the fundamentals of palladium catalysis in the context of heterocyclic chemistry, including the basic mechanisms of many useful transformations along with a number of new synthetic and mechanistic developments. The majority of the Pd-catalyzed reactions described in this book proceed via catalytic cycles that are comprised of eight fundamental organopalladium transformations shown below [1]. Most of these transformations can occur via more than one mechanistic pathway, and in some instances the precise mechanisms have not been fully elucidated. 

In the Heck reaction (Fig. 3, pp. 76 and 343), an organopalladium (II) complex inserts an olefin and eliminates "Pd-H", resulting in an overall vinylic alkylation. The thermal catalytic mixture is identical to the one used for sonochemical activation. The mechanism is not completely clarified electrochemistry has, however, recently brought new information. The sonochemical version of the Heck reaction allows obtaining the product in 6 h at 50°C instead of 24 h at 80 C for the purely thermal one.i The process could be rationalized either at substitution step 1 by interfacial effects or in step 2, where an anionic transition metal complex attacks the substrate. 

The mechanism is supported by fundamental studies of the behavior of model organopalladium complexes.f The processes are composed of two catalytic cycles that lead to a-keto ester and ester, respectively. The catalytic cycle for giving a-keto ester (Cycle I) is analogous to the one proposed for the formation of a-keto amide as shown in... 

The elementary steps implicated in Mechanism I were extensively studied by Norton and co-workers, who were able to independently synthesize, isolate, and characterize all the organopalladium intermediates involvedJ This elegant and thorough work unequivocally showed that intramolecular insertion of the (alkoxycarbonyl)palladium species on the unsaturated bond in 3-en-l-ols, 3-yn-l-ols, and 4-yn-l-ols occurs exclusively in a syn-exo fashion (path a). It is important to point out, however, that in a catalytic reaction Pd(0)-initiated mechanisms like II-IV cannot be ruled out Clearly, in the model system used by Norton there was no alternative to Mechanism I, since an alkoxycarbonylpalladium complex was synthesized first and the proton was added at the end of the sequence. 

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