Terminal Alkyne Activation by an Al(I)-Centered Anion: Impact on the Mechanism of Alkali Metal Identity

The group 1 alumanyls, [{SiN ^Dipp}AlM] ₂ (M = K, Rb, Cs; SiN ^Dipp = {CH ₂SiMe ₂NDipp} ₂), display a variable kinetic facility (K < Rb < Cs) toward oxidative addition of the acidic C-H bond of terminal alkynes to provide the corresponding alkali metal hydrido(alkynyl)aluminate derivatives. Theoretical analysis of the formation of these compounds through density functional theory (DFT) calculations implies that the experimentally observed changes in reaction rate are a consequence of the variable stability of the [{SiN ^Dipp}AlM] ₂ dimers, the integrity of which reflects the ability of M ⁺ to maintain the polyhapto group 1-arene interactions necessary for dimer propagation. These observations highlight that such “on-dimer” reactivity takes place sequentially and also that the ability of each constituent Al(I) center to effect the activation of the organic substrate is kinetically differentiated.