Catalytic cycle for cross-coupling

Scheme 2-12 A general catalytic cycle for cross-coupling.

Scheme 12.18 outlines the general catalytic cycle for cross-coupling, the key steps of which are (a) oxidative addition of R -X to Pd(0), (b) cis-trans isomerization of a square planar Pd complex (if necessary), (c) transmetalation of R-M to give R -Pd-R, (d) trans-cis isomerization (if necessary), and (e) reductive elimination of R -R and regeneration of Pd(0). 

In summary, these results demonstrate that air-stable POPd, POPdl and POPd2 complexes can be directly employed to mediate the rate-limiting oxidative addition of unactivated aryl chlorides in the presence of bases, and that such processes can be incorporated into efficient catalytic cycles for a variety of cross-coupling reactions. Noteworthy are the efficiency for unactivated aryl chlorides simplicity of use, low cost, air- and moisture-stability, and ready accessibility of these complexes. Additional applications of these air-stable palladium complexes for catalysis are currently under investigation. 

The reductive elimination/oxidative addition is of practical importance in catalytic cycles, for example the rhodium/methyl iodide catalysed carbonylation of methanol. In organic synthesis the palladium or nickel catalysed cross-coupling presents a very common example involving oxidative addition and reductive elimination. A simplified scheme is shown in Figure 2.19 [17],... 

Palladium-catalyzed cross-coupling reaction of organostannanes with organic halides, triflates, etc. For the catalytic cycle, see Kumada coupling on page 345. 

Figure 3.44 Catalytic cycle for the Pd-catalyzed Sonogashira cross-coupling the cuprous acetylide intermediate is highlighted in gray.

Fig. 52 Catalytic cycles for cobalt-catalyzed cross-coupling reactions...

Scheme 7. Suggested catalytic cycle for copper-catalyzed cross coupling reaction ofthiophenol and aryl halide based on detected species by ESI-MS 54. ...

A general catalytic cycle for the cross-coupling reaction of organometallics involves an oxidative addition-transmetallation-reductive elimination sequence, as depicted in Scheme... 

The catalytic cycle for the Suzuki cross-coupling reaction involves an oxidative addition (to form RPd(II)X)-transmetalation-reductive elimination sequence. The transmeta-lation between the RPd(lI)X intermediate and the organoboron reagent does not occur readily until a base, such as sodium or potassium carbonate, hydroxide or alkoxide, is present in the reaction mixture. The role of the base can be rationalized by its coordination with the boron to form the corresponding ate-complex A, thereby enhancing the nucleophilicity of the organic group, which facilitates its transfer to palladium. Also, the base R O may activate the palladium by formation of R-Pd-OR from R-Pd-X. 

The General Catalytic Cycle for Pd-Catalyzed Cross-Coupling Reactions... 

The reaction shown can be considered a combination of a Pd-catalyzed ene reaction and a Suzuki cross-coupling.224 Propose a catalytic cycle for the reaction. One of the key intermediates is structure J. 

The generally accepted catalytic cycle for the Buchwald-Hartwig amination mirrors that of other palladium catalyzed cross-coupling reactions.10 11 irThere is an oxidative addition (A to B), followed by an exchange on palladium (B to C), and finally a reductive elimination (C to D and A). The main difference involves the exchange step. In a Suzuki, or Stille, reaction this step proceeds through a discrete transmetallation event, whereas... 

Many transmetalation reactions take place as part of a catalytic cycle. For example, aryl (as well as heterocyclic and vinyl) halides, tosylates, and triflates form biaryls with numerous aryl-metal partners. The cross-coupling reactions (which are increasingly being used to prepare other products besides biaryls) are usually named after... 

The general reaction mechanism has been shown to involve typical steps for cross-coupling [98, 113]. Oxidative addition of an aryl halide generates a Pd(II) species that undergoes transmetalation to form a Pd(II)-thiolate. C-S reductive elimination provides the aryl sulfide and regenerates the Pd(0) catalyst. More recently, Hartwig reported a detailed mechanistic analysis of the Pd/Josiphos system derived from different Pd precursors. The dominant Pd species were found to be off the catalytic cycle, which accounted for differences in rates between stoichiometric and catalytic reactions [114]. Thioketones are also effective thiolate nucleophiles for C-S bond formation. The reaction involves tandem Pd-catalyzed thioenolate alkylation, followed by 5-arylation (8) [102]. Presumably, the arylation process proceeds by a similar mechanism to related Pd-catalyzed transformations. 

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