Pseudohalides Heck reaction

The Heck reaction, a palladium-catalyzed vinylic substitution, is conducted with olefins and organohalides or pseudohalides are frequently used as reactants [15, 16], One of the strengths of the method is that it enables the direct monofunctionalization of a vinylic carbon, which is difficult to achieve by other means. Numerous elegant transformations based on Heck chemistry have been developed in natural and non-natural product synthesis. Intermolecular reactions with cyclic and acyclic al-kenes, and intramolecular cyclization procedures, have led to the assembly of a variety of complex and sterically congested molecules. 

Diazonium salts are important intermediates for the preparation of halides, and azo compounds. Diazonium salts can react as pseudohalide-type electrophiles, and can therefore be used in specific protocols for the Heck Reaction or Suzuki Coupling. 

The palladium-catalyzed reaction of olefins with aryl or vinyl halides or pseudohalides in the presence of base (the Heck reaction) follows a different course from the other crosscoupling reactions after the oxidative addition step. As shown in Scheme 19.5, the olefin coordinates to the palladium after oxidative addition of the aryl or vinyl halide. Tliis coordination of olefin may occur by associative displacement of a monodentate ligand from the palladium, or it may occur by replacement of halide by the olefin to generate a cationic olefin complex. In some cases, these reactions are conducted with aryl or vinyl triflates. In this case, the olefin readily displaces the triflate to generate a cationic palladium-olefin... 

Scheme 50 Heck reaction employing a polymer-bound aryl sulfonate as a pseudohalide...

The palladium-catalyzed Heck carbonylation reaction is a powerful means of generating amides, esters, and carboxylic acids from aryl halides or pseudohalides [28]. The development of rapid, reliable, and convenient procedures for the introduction of carbonyl groups is important for the development of high throughput chemistry in general and high-speed microwave-mediated chemistry in particular. Unfortunately, the traditional method of introducing carbon monoxide into a reaction mixture via a balloon or gas tube is not practical because of the special requirements of microwave synthesis. 

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