Regioselective Tandem C-H Alkylation/Coupling Reaction of ortho-Iodophenylethylenes via C,C-Pallada(II)cycles

Five-membered C,C-pallada(II)cycles are a unique class of diorganopalladium species with favorable stability and an electron-rich nature, leading to efficient sequential reactions with diverse electrophiles and nucleophiles. Specifically, the development of aryl-alkenyl-palladacycle-based transformations could provide an attractive approach with regio- and stereocontrol for the construction of multifunctionalized arylethylenes. However, currently, the C,C-pallada(II)cycle formation relies on a rigid skeleton or steric congestion in the backbone to promote cyclopalladation, and the formation of aryl-alkenyl-palladacycle without an alpha-substituent has not been achieved. Furthermore, reactions that could discriminate between the two sp(2) carbon centers of such C(sp(2)),C(sp(2))-palladacycle remain elusive. Herein, we report a regioselective three-component tandem alkylation/coupling reaction applicable for a variety of non-, alpha-, or beta-substituted and alpha,beta-disubstituted ortho-iodophenylethylenes. Electron-rich 2-pyridone ligands are employed to enable the cyclopalladation process leading to aryl-alkenyl-palladacycle intermediates, of which the two C-Pd bonds are discriminated toward alkylation by their inherent steric and electronic differences. Good linear free-energy relationships between regio-/chemoselectivities and Hammett sigma values are observed.