β-Hydride Elimination at Low-Coordinate Gold(III) Centers

This Article reports the first comprehensive study of beta-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 beta-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 beta-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 beta-hydride elimination/reinsertion into the Au-H bond. The, reaction of the cationic gold(III) methyl complex 3c with ethylene was also explored. I According to H-1 and C-13 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, beta-hydride elimination, and olefin exchange at gold(III).