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

Abstract

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 α-substituent has not been achieved. Furthermore, reactions that could discriminate between the two sp2 carbon centers of such C(sp2),C(sp2)-palladacycle remain elusive. Herein, we report a regioselective three-component tandem alkylation/coupling reaction applicable for a variety of non-, α-, or β-substituted and α,β-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 σ values are observed.

Recent Advances in Alkenyl sp2 C–H and C–F Bond Functionalizations: Scope, Mechanism, and Applications

“…Subsequently, Yu and co-workers illustrated an efficient palladium-catalyzed olefinic C−H bond alkylation of 2bromostyrenes with cyclobutanols as the coupling partners enabled by a controllable aryl to vinyl 1,4-palladium migration/ ring-opening C−C cleavage cascade strategy (Scheme 29). 82 This protocol features excellent regio-and stereoselectivity, wide substrate scope, and high functional group compatibility, providing access to γ-ketone olefins in decent yields (43− 95%). Combined mechanistic investigations and DFT calculations demonstrated that the reaction occurs via oxidative addition, aryl to vinyl palladium 1,4-migration, ringopening C−C bond cleavage, and reductive elimination sequences.…”

Highly Efficient and Selective Photocatalytic Nonoxidative Coupling of Methane to Ethylene over Pd-Zn Synergistic Catalytic Sites

“…Benefiting from the synergy of two active sites, our model catalyst achieves remarkable activity and selectivity for CH 4 to C 2 H 4 conversion as well as a nearly stoichiometric H 2 evolution, benchmarked against the state-of-the-art photocatalysts. In situ spectroscopic studies reveal that the Zn sites promote the adsorption and activation of CH 4 molecules, while the Pd sites facilitate the dehydrogenation of methoxy intermediates and suppress the overoxidation [26][27][28]. Moreover, we unravel the reaction pathway for CH 4 to C 2 H 4 , in which the adsorbed CH 4 is activated and dehydrogenated to in turn generate methyl, methoxy, and methylene intermediates, and finally, the methylene radicals undergo self-coupling reactions to produce C 2 H 4 .…”

Decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy to access dibenzo[a,c]carbazoles

“…1 In the past few decades, five-membered C , C -palladacycle-based reactions have attracted widespread attention due to their versatile conversions in which two C–Pd bonds could be functionalized by trapping with coupling reagents. 2–16 Among them, of extraordinary interest to synthetic chemists are cyclizations of five-membered C , C -palladacycles with various coupling reagents such as aryl halides, 3 arynes, 4 alkynes, 5 diaziridinones, 6 diazo compounds, 7 bromophenols, 8 dibromomethane, 9 halogenated carboxylic acids, 10–14 etc ., which have emerged as a powerful tool for the construction of polycyclic compounds. Recently, the groups of Kwong, us and others achieved the transformation of five-membered C , C -palladacycles by various halogenated carboxylic acids via a carboxyl-assisted palladacycle capture strategy, leading to the construction of diverse fused polycycles (Scheme 1a).…”