Counterintuitive Mechanisms of the Addition of Hydrogen and Simple Olefins to Heavy Group 13 Alkene Analogues

The mechanism of the reaction of olefins and hydrogen with dimetallenes ArMMAr (Ar = aromatic group; M = Al or Ga) was studied by density functional theory calculations and experimental methods. The digallenes, for which the most experimental data are available, are extensively dissociated to gallanediyl monomers, :GaAr, in hydrocarbon solution, but the calculations and experimental data showed also that they react with simple olefins, such as ethylene, as intact ArGaGaAr dimers via stepwise [2 + 2 + 2] cycloadditions due to their considerably lower activation barriers vis-a-vis the gallanediyl monomers, :GaAr. This pathway was preferred over the [2 + 2] cycloaddition of olefin to monomeric :GaAr to form a gallacyclopropane ring with subsequent dimerization to yield the 1,2-digallacyclobutane intermediate and, subsequently, the 1,4-digallacyclohexane product. The calculations showed also that the addition of H-2 to digallene proceeds by a different mechanism involving the initial addition of one equivalent of H-2 to form a 1,2-dihydride intermediate. This reacts with a second equivalent of H-2 to give two ArGaH2 fragments which recombine to give the observed product with terminal and bridging H-atoms, Ar(H)Ga(mu-H)(2)Ga(H)Ar. The computations agree with the experimental observation that the :GaAriPra (Ar-iPrg = C6H-2,6-(C6H3-2,4,6-Pr-i(3))(2)-3,5-Pr-i(2)), which does not associate even in the solid state, does not react with ethylene or hydrogen. Calculations on the reaction of propene with ArAlAlAr show that, in contrast to the digallenes, addition involves an open-shell transition state consistent with the higher singlet diradical character of dialuminenes.