Redox process Heck reaction

The Heck reaction was discovered in the early 1970s and is extremely useful for rapidly assembling carbon skeletons. This reaction is unique to palladium A great deal of information is known about the reaction. For example, the success of the reaction depends on each of the three steps involved. Electron-donating groups decrease the reactivity of alkenyl halides and triflates toward Pd(0), whereas electron-withdrawing group increase the rate of oxidative addition. In cases where Pd(II) salts are used, it is assumed that they are converted to Pd(0) by some redox process. 

The Heck reaction has now been reviewed448,449. Evidence for the formation of zerova-lent palladium from (AcO Pd and Ph3P via a redox process has been provided450. This explains the origin of Pd(0) required for certain palladium-catalysed reactions in cases where Pd(II) is added to the reaction as the primary form of the Pd-catalyst. Thallium has been found to accelerate the Heck-type cyclization-carbonylation451. 

Of special significance are the catalytic properties of small metal clusters. At their surface such clusters have a large number of atoms with a low coordination number to which substrates bind. Catalytic reactions are being studied in hydrogenation, hydrosilylation, hydration, and the Heck reaction. Metal clusters are also of importance with regard to redox and electron transfer processes such as the photochemical decomposition of water (fuel cells) and photocatalytic hydrogenation. 

Myers oxidative decarboxylative Heck reaction became the prototype for a whole series of regiospecific oxidative couplings in which carboxylic acids adopt the reactivity of aryl electrophiles in the corresponding redox-neutral processes [67-72]. Crabtree et al. developed a process in which arenes react with aromatic carboxylates under C-H activation in the presence of a palladium catalyst and excess silver carbonate to yield biaryls. This reaction is useful especially for intramolecular couplings (Scheme 20) [73, 74]. Recently, a palladium-free, silver-catalyzed radical variant has been disclosed [78]. 

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