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

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).