Reductive Coupling of Ketones: Contrasting the Reactivity of an Aluminyl and a Gallyl Anion

The synthesis of a new potassium gallyl system, K[Ga(NON)] ([NON]^2– = [O(SiMe₂NDipp)₂]^2–, Dipp = 2,6-iPr₂C₆H₃), has been reported, and its reactivity toward a variety of ketones has been studied. The potassium gallyl is initially isolated as the contacted dimeric pair (CDP) [K{Ga(NON)}]₂ but can be converted to the monomeric ion pair (MIP) (NON)Ga–K(TMEDA)₂ or the separated ion pair (SIP) [K(222-crypt)][Ga(NON)] upon addition of coordinating polydentate ligands. The reaction of the CDP with two equivalents of benzophenone or 9-fluorenone per gallium center generates homocoupled pinacolate products. However, the reaction with acetophenone initially forms a gallium 1,3-phenylethene-1-olate, which C–C couples to a second equivalent of acetophenone to form an unprecedented gallium 1,3-diphenylbutane-1,3-diolate. This reactivity is contrasted with the analogous potassium aluminyl CDP [K{Al(NON)}]₂, where preferential formation of the pinacolate product is observed under the same reaction conditions.