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 AriPr4GaGaAriPr4 (1) [AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)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 AriPr4Ga­(C7H8)­GaAriPr4 (2), pseudoinverse sandwiches AriPr4Ga­(C8H8)­GaAriPr4 (3, 3iso), and polycyclic compounds AriPr4Ga­(C5H6)­GaAriPr4 (4) and AriPr4Ga­(C7H8)­GaAriPr4 (5, 5iso), 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 AriPr4EEAriPr4 (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 AriPr4GaGaAriPr4 are of π–π symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual π–π* FMOs of C–C π bonds or the π–n+ FMOs of heavier group 14 alkyne analogues.