Reversible and Irreversible Higher-Order Cycloaddition Reactions of Polyolefins with a Multiple-Bonded Heavier Group 13 Alkene Analogue: Contrasting the Behavior of Systems with π-π, π-π* and π-n+ Frontier Molecular Orbital Symmetry

The heavier group 13 element alkene analogue, digallene (ArGaGaAriPr4)-Ga-iPr4 (1) [Ar-iPr4 = C6H3-2,6-(C6H3-2,6-Pr-i(2))(2)], has been shown to react readily in [n + 2] (n = 6, 4, 2 + 2) cycloaddition reactions with norbornadiene and quadricyclane, 1,3,5,7-cyclooctatetraene, 1,3-cyclopentadiene, and 1,3,5-cycloheptatriene to afford the heavier element deltacyclane species (ArGa)-Ga-iPr4(C7H8)GaAriPr4 (2), pseudoinverse sandwiches (ArGa)-Ga-iPr4(C8H8)GaAriPr4 (3, 3(iso)), and polycyclic compounds (ArGa)-Ga-iPr4(C5H6)GaAriPr4 (4) and (ArGa)-Ga-iPr4(C7H8)GaAriPr4 (5, 5(iso)), respectively, under ambient conditions. These reactions are facile and may be contrasted with other all-carbon versions, which require transition-metal catalysis or forcing conditions (temperature, pressure.), or with the reactions of the corresponding heavier group 14 species (ArEEAriPr4)-E-iPr4 (E = Ge, Sn), which give very different product structures. We discuss several mechanistic possibilities, including radical- and non-radical-mediated cyclization pathways. These mechanisms are consistent with the improved energetic accessibility of the LUMO of the heavier group 13 element multiple bond in comparison with that of a' simple alkene or alkyne. We show that the calculated frontier molecular orbitals (FMOs) of (ArGaGaAriPr4)-Ga-iPr4 are of pi-pi symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual pi-pi* FMOs of C-C pi bonds or the pi-n(+) FMOs of heavier group 14 alkyne analogues.