Organopalladium alkene/alkyne insertion

Alkynes are more reactive than alkenes in carbopalladation. Facile insertion of internal alkynes to some Pd—C bonds (carbopalladation of alkynes) generates the alkenylpalladiums 2 and 7 by mainly selective syn addition of organopalladium species 1 and 6 to alkynes. Formally the species 2 can be generated by oxidative addition of appropriately substituted alkenyl halides 3 to Pd(0). 

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). 

The intennolecular acylpalladation corresponds to the addition of an acyl-palladium bond onto a rr-bond system of another molecule this elementary step can also be referred to as an insertion (Scheme I). This produces another organopalladium complex, which can in principle participate in subsequent propagation or termination reactions. This excludes processes that involve alkoxycarbonylation (R— = R O—) and hydrocarbonyla-tion (R— = H—). This section will focus on nonpolymeric intermolecular reactions of acylpalladium complexes with different 7r-bond systems (alkenes, imines, dienes, and alkynes). 

---