Why is a dicationic digallene so reactive towards activation of strong covalent bonds? Scope and mechanistic investigations

Abstract

Herein, we investigate the reactivity of the trans-bent Ga⇄Ga double bond in the dicationic digallene [{Ga(dcpe)}₂]²⁺ (dcpe = bis(dicyclohexylphosphino)ethane) as its [pf]⁻ salt ([pf]⁻ = [Al(OR^F)₄]⁻; R^F = C(CF₃)₃), which is formed in situ within seconds. Unusually, this digallene is highly reactive towards covalent bonds and oxidatively adds even to strong E–Y σ-bonds, e.g., H–O, H–N, H–C and C–F bonds, under mild conditions, often at room temperature. Their bond activation at any cationic subvalent group 13 compound is unprecedented and the C–H bond activation is the first oxidative addition reported between any subvalent gallium compound and a neutral substrate. The scope and mechanism of the bond activation reactions were experimentally investigated by interaction with selected substrates and via isotope labelling experiments, as well as using high-level quantum chemical calculations. Mechanistically, the pronounced reactivity of the digallene can be attributed to an easily accessible asymmetric conformer with one (Lewis-acidic) planarized and one (Lewis-basic) pyramidalized reactive Ga-site, allowing for cooperative E–Y bond cleavage. In addition, the [2 + 2] cycloaddition of the Ga⇄Ga bond to CC double and triple bonds was studied: it follows a stepwise, non-concerted reaction mechanism, which allows for the catalytic isomerization of cis-olefins and may serve as the basis for follow-up functionalization reactions.