Diverse Reactivities of Ketones toward Compounds with Low-Valent Aluminum: Reductive Coupling, Enolization, and C═O Cleavage

The reactivity of the α-diimine-ligated dialuminum(II) complex [LAl^II-Al^IIL]^2- (<b>1</b>, dialumane, L = [(2,6-<i>i</i>Pr₂C₆H₃)NC(CH₃)]₂) and the corresponding Al(I) species [LAl^I:]^- (aluminyl anion) toward ketones was systematically investigated. Reaction of complex <b>1</b> with 9-fluorenone induced pinacol coupling to afford product <b>2</b> via two-electron reduction, with one electron derived from the L^2- ligand and the other from the Al(II) center. In contrast, treatment of <b>1</b> with 1,3-dimethyl-2-imidazolidinone resulted in the formation of complex <b>3</b> through a simple Lewis acid-base addition. Furthermore, the Al(I) species [LAl:]^-, generated via reduction of dialumane <b>1</b>, reacts with benzophenone and acetophenone to yield the pinacol coupling products <b>4</b> and <b>5</b>, where both electrons are donated by the Al(I) center. Notably, sterically hindered α-hydrogen-containing ketones (1,1-diphenylacetone and 2,4-dimethyl-3-pentanone) underwent enolization to form products <b>6</b> and <b>7</b> instead of C-C coupling. Compound <b>7</b> additionally involved hydrogen migration, featuring an enolized unit (OC(R)═CR'₂) and an alkoxide moiety (OC(H)R₂). Surprisingly, reaction of [LAl:]^- with the bulkier 2,2,4,4-tetramethyl-3-pentanone led to complete C═O bond cleavage, affording product <b>8</b> via ligand-based C-C coupling and hydrogen migration from ligand L to the deoxygenated carbon. This represents the first reported instance of Group 13 elements mediating the complete cleavage of the C═O bond in ketones. The newly obtained compounds were fully characterized by EPR/NMR, IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Their electronic structures were further elucidated through DFT calculations.