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Palladium-Catalyzed Intramolecular Reactions of (E)-2,2-Disubstituted 1-Alkenyldimethylalanes with Aryl Triflates

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posted on 2009-06-22, 00:00 authored by Eric Fillion, Vincent É. Trépanier, Jarkko J. Heikkinen, Anna A. Remorova, Rebekah J. Carson, Julie M. Goll, Adam Seed
A full account of the Pd-catalyzed intramolecular reactions of (E)-2,2-disubstituted 1-alkenyldimethylalanes with aryl triflates as an entry into polycarbocyclic structures displaying an ethyl−methyl-substituted all-carbon benzylic quaternary center is herein presented. It was found that the efficiency of the Pd-catalyzed carbon−carbon bond forming process is highly affected by the structure of the starting material, including tether length and aryl substitution pattern; substituting the position ortho to the triflate is mandatory to obtain a good yield of the carbocycle. Moreover, the formation of 1-ethyl-1-methylindanes is facile in comparison to the case for the analogous tetrahydronaphthalenes, for which the competing methyl cross-coupling reaction is equally competent. It was established through labeling studies that the carbon−carbon bond forming events are stereospecific and proceed though the intermediacy of a neopentylic sp3-gem-dimetallic palladio(II) dialkylaluminoalkane species, from which a 1,2-methyl migration from aluminum to carbon occurs. Intramolecular palladium-catalyzed reactions of 1-naphthyl triflates with (E)-2,2-disubstituted 1-alkenyldimethylalanes revealed two competing reaction pathways: arylation with sequential 1,2-alkyl migration from aluminum to carbon and intramolecular 1,2-diarylation, in which the catalytic cycle is terminated by direct arylation of the C(sp3)−Pd(II) bond. Factors such as tether length, additives, solvent polarity, and C−H···Pd interactions between the Pd(II) center and the hydrogen atom at the 8-position all influence not only the pathway taken by the (σ-aryl)palladium(II) complexes but also the subsequent reactivity of the (σ-alkyl)palladium(II) complexes.