Alkynes palladium-catalyzed reaction with alkenyl halides

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Like hydroalumination and hydrozirconation, hydroboration of alkynes also provides a convenient and Stereospecific route to alkenyl metal reagents. However, initial attempts to achieve palladium-catalyzed cross-coupling of alkenylboranes with alkenyl halides were unsuccessful, due to the poor carbanionic character of these reagents. Later, Suzuki discovered that the desired transformation could be effected in the presence of an alkoxide or hydroxide base weaker bases, such as sodium acetate or triethylamine, were not generally effective. The reaction is suitable for the preparation of ( , )-, ( ,Z)- and (Z,Z)-dienes. Since reactions of alkenylboronates are higher yielding than those of alkenylboranes, the recent availability of (Z)-l-alkenylboronates " substantially improves the Suzuki method for the preparation of (Z)-alkenes. An extension of the methodology to the synthesis of trisubstituted alkenes has also been reported. " ... 

Palladium-catalyzed coupling reactions of alkenyl/aryl halides or triflates with alkenes, alkynes, or organometaUic reagents are readily used during the synthesis of pharmaceuticals because of the high yield and great selectivity that can be achieved under mild reaction conditions in these processes [108]. At the same time, there are only a few examples for the same reaction carried out in carbon monoxide atmosphere. 

Carbopalladation is the reaction of a cr-bonded organopalladium complex I with an unsaturated molecule (such as an alkene 2) to yield the migratory insertion product 3 (Scheme 1). The reaction is tremendously flexible, allowing for a wide variety of structural types for both reactants 1 and 2. The precursors of palladium complexes 1 are commonly alkenyl or aryl halides or triflates (8 and 9, respectively), the reaction of which is more commonly termed the Heck reaction. Allylic systems 10, which react to provide -Tr-allylpalladium complexes, can participate in the reaction as can benzylic precursors 11. Acylpalladium complexes 12 also react and are commonly generated in the same reaction vessel by Pd-catalyzed carbonylation. Their unsaturated reaction partners include alkenes 2, alkynes 4, dienes 6, allenes, and arenes, all of which can be electron rich or poor. Carbopalladation occurs in a syn fashion allowing the installation of stereocenters (2- 3) or control of alkene geometry (4- 5). 

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