Bond cyclometallation reaction across

Electrophihc reagents, inclnding Br2, CI2, NCS-SCN, and I2, will react rapidly and in high yield to add across the Pd-C bond. Mineral acids will cleave the bond in cyclometalated complexes as well. Since cyclometalation with palladinm halide complexes generates HCl, frequently a weak base, such as sodium acetate, is added to prevent reversal of the cyclometalation reaction. 

In the case of alkyl leaving groups, mechanistic studies show that a-bond metathesis pathways are active, proceeding via four-centre, four-electron transition states. In the case of unsaturated functional groups, the reaction involves addition of the CH bond across the r-component, so that the substrate hydrogen atom remains (labelling studies) within the product molecule. The cyclometallation of 2,6-dimethylphenoxide by a tantalum alkylidene ligand has also been demonstrated. ... 

Cyclometalation has been applied to synthesis in the Murai reaction. The substrate has to contain a group, a ketone in Eq. 14.42, that binds the metal complex. Cyclometalation gives an intermediate aryl that undergoes insertion with a vinyl silane followed by reductive elimination to give the product the result is a C-H addition across a C=C bond. ... 

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