β-Hydride Elimination at Low-Coordinate Gold(III) Centers
Résumé
This Article reports the first comprehensive study of β-hydride elimination at gold(III). The stability/fate of gold(III) alkyl species have been investigated experimentally and computationally. A series of well-defined cationic cyclometalated gold(III) alkyl complexes [(P,C)gold(III)(R)][NTf2] [(P,C) = 8-diisopropylphosphino-naphthyl; R = Me, nPr, nBu] have been synthesized and spectroscopically characterized. While the cationic gold(III) methyl derivative 3c is stable for days at room temperature, the gold(III) n-propyl and n-butyl complexes 3a,b readily undergo β-hydride elimination at low temperature to generate propylene and 2-butenes, respectively. The formation of internal olefins from the gold(III) n-butyl complex 3b shows that olefin isomerization takes place after β-hydride elimination. Computational studies indicate that this isomerization proceeds through a chain-walking mechanism involving a highly reactive gold(III) hydride intermediate and a sequence of β-hydride elimination/reinsertion into the Au-H bond. The reaction of the cationic gold(III) methyl complex 3c with ethylene was also explored. According to 1H and 13C NMR spectroscopy, a mixture of propylene, 1-butene, and 2-butenes is formed. DFT calculations provide detailed mechanistic insights and support the occurrence of migratory insertion of ethylene, β-hydride elimination, and olefin exchange at gold(III). © 2016 American Chemical Society.
Mots clés
13C NMR spectroscopy
Computational studies
Ethylene
gold
Gold compounds
Hydride elimination
Hydrides
Isomerization
Isomers
Methyl complexes
Methyl derivatives
Migratory insertion
Nuclear magnetic resonance spectroscopy
Olefin isomerization
Olefins
Propylene
Synthesis (chemical)
Temperature
Walking mechanism