Organopalladium reaction mechanisms

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 ... 

The results summarized under the headings 1 and 2 rather conclusively indicate that, under the conditions used, the reaction must proceed via hydropaUadation. For the other steps, a combination of oxidative addition and reductive elimination, as in the Chalk-Harrod mechanism, is plausible, but other possibilities involving metathetical processes may not be ruled out. It should be noted that the two reactions employed by Brookhart and co-workers " and Hayashi and co-workers are substantially different. It is therefore very likely that the mechanistic conclusions made in their studies are both fundamentally correct and that, as in many other reactions of organopalladium compounds, more than one mechanism operates for a given type or class of organopalladium reactions. 

The reaction mechanism usually begins with oxidative addition of the organic halide to the catalyst (Scheme 13.16). Subsequently, the reaction of the organopalladium species 65 with a base gives intermediate 66, which undergoes trans-metalation with the boronate to form the organopalladium... 

In considering the mechanisms involved in organopalladium chemistry, several general points should be kept in mind. Frequently, reactions involving organopalladium... 

Palladium-catalyzed coupling reactions of the Heck type have in many instances involved indole and pyrrole derivatives. Although the mechanisms are complex, organopalladium species are implicated (84H(22)1493). Vinylation of A-substituted-3 -iodoindoles with amidoacrylate groups provides a useful functionalization of indoles (Scheme 81) (90JOM(39l)C23). Yields are improved in intramolecular reactions, e.g. (406 — 407) and (408 — 409) <92H(34)219,91CPB2830). 

Organopalladium(II) intermediates formed by oxidative addition of sp2 and sp3 organic halides and related compounds to Pd(0) species undergo a variety of synthetically useful reactions (e.g., Heck reaction) (191). For example, Pd complexes catalyze substitutive C—C bond formation between olefins and organic halides by the mechanism shown in Scheme 80 (192). The initially formed organo-Pd(II) intermediate adds across the C—C bond, and subsequent /3-elimination of Pd(II) hydride affords the final product. Other organic compounds that have electronegative... 

Several examples of the cyclization of indole derivatives with alkenic side chains in the 3-position have been reported.6 In these examples, palladium chloride in combination with silver tetrafluoroborate is the cyclizing agent. The palladium tetrafluoroborate, presumably formed, should be a very reactive palladating species and probably is the reason why these reactions proceed at room temperature, although the mechanism is not yet completely clear. These reactions were worked up reductively (by addition of sodium borohydride) in order to reduce the expected alkenic product or any relatively stable organopalladium complexes that may have been formed (equation 4).6... 

In keeping with a mechanistic emphasis, the book was reorganized. The chapter on mechanism is now Chapter 5 instead of Chapter 10. Thus the first six chapters focus on the mechanistic and structural underpinnings of organic chemistry. Synthetic aspects of organic chemistry are then discussed from a mechanistic and structural point of view. Several new sections have been added and others expanded. An expanded discussion of resonance and aromaticity is found in Chapter 1. A section on organopalladium chemistry and olefin metathesis has been added to Chapter 8 as they relate to current methods of carbon-carbon bond formation. Chapter 9 on free-radical reactions for carbon-carbon bond formation has been revised. The discussion of Diels-Alder chemistry has been moved to Chapter 10 and expanded. A number of new problems have been added which serve to further illustrate the principles developed in each chapter. Finally, thanks to input from many people who have read dris text and taught from it, the discussion has been further honed and errors corrected. 

Terminal alkynes can be coupled directly to aryl and to vinyl halides in the presence of a palladium catalyst and a base. The mechanism of this reaction appears to involve oxidative addition of the sp halide to palladium(O), followed by alkynylation of the intermediate organopalladium halide and reductive elimination of the disubstituted alkyne (equations 17 and... 

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. 

Arylidene butyrolactones were also accessible by addition of nucleophiles onto a Pd(II)-activated alkyne. Thus, the oxidative addition of Pd(0) to 617 in the presence of a base gave 618 in 70% yield (Scheme 102) (96T11463). The proposed mechanism of this reaction was suggested to involve an oxidative insertion of Pd(0) into the Ar-I bond to give the organopalladium species 619. A subsequent addition of the pendant carboxylate to the activated alkyne provides metallocycle 620 that undergoes reductive elimination to furnish 618. Similarly, subjecting 621 to the same reaction conditions produced 622 in 77% yield. 

Palladium-catalysed processes typically utilise only 1-5 mol% of the catalyst and proceed through small concentrations of transient palladium species there is a sequence of steps, each with an organopalladium intermediate, and it is important to become familiar with these basic organopalladium processes in order to rationalise the overall conversion. Concerted, rather than ionic, mechanisms are the rule, so it is misleading to compare them too closely with apparently similar classical organic mechanisms, however curly arrows can be used as a memory aid (in the same way as one may use them for cycloaddition reactions), and this is the way in which palladium-catalysed reactions are explained in the following discussion. (For convenience, an organometallic component can be referred to as the nucleophilic partner and the halide as the electrophilic partner, but this should not necessarily be taken to imply reactivity as defined in classical chemistry. Also, references to the halide should be understood to include all related substrates, such as triflates.)... 

Evidence for this mechanism is the observation that the reaction of an enol triflate with Pd(PPh3)4 in the presence of LiCl forms a truns-organopalladium(II) chloride complex which is able to catalyze the coupling of the enol triflate with tributyl(vinyl)tin to afford the expected product75. For p-chlorophenyl triflate, both trans-organopalladium(II) triflate and chloride complexes were isolated131,133 (equations 124 and 125). 

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