Carbopalladation regiochemistry

Presumably, the oxidative cyclization of 1 commences with direct palladation at the orfAo-position, forming o-arylpalladium(II) complex 3 in a fashion analogous to a typical electrophilic aromatic substitution (this notion is useful in predicting the regiochemistry of oxidative cyclizations). The mechanism of the second formal C—H bond functionalization step is not fully elucidated, but may occur either via (a) an intramolecular carbopalladation reaction (migratory insertion) followed by czHft-P-hydride elimination from 4 (Path A) (b) by o-bond metathesis (through a four-centered transition state) followed by reductive elimination (Path B) (c) by electrophilic aromatic substitution followed by C—C bond-forming reductive elimination (PathC) [9]. 

There are, in principle, two pathways for the carbopalladation reaction of allenes depending on the regiochemistry of the insertion reaction of the C—Pd bond into one of the two cumulated C=C bonds (Scheme 1). Following Path I, the reaction would afford a Csp2—Pd species 1, while a 7r-allylpalladium species 2 could be formed via Path II. 

A.i. Formation of rr-Allylpalladium Species via Stoichiometric Reactions of Carbon-Palladium Species with AUenes Regiochemistry of Carbopalladations of Allenes... 

The regiochemistry of this carbopalladation has been further confirmed by the X-ray crystal structure analysis of complex 10, which was formed by the reaction of the methyl-palladium chloride complex 9 with propadiene (Scheme... 

The regiochemistry of the carbopalladation step is considered to be controlled primarily by steric and coordination effects, whereas electronic effects appear to be less important. 

The successive coordination and iyw-addition of organopalladium(II) intermediates to olefins and alkynes generating a new C—C cr-bond are referred to as migratory insertion or carbopalladation and control the regioselective outcome in the particular reaction. The regiochemistry can be tuned by steric and electronic effects, which will be discussed in more detail for the Heck reaction in Sect. 1.7. 

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