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
[LAlII–AlIIL]2– (1, dialumane, L = [(2,6-iPr2C6H3)NC(CH3)]2) and the corresponding
Al(I) species [LAlI:]− (aluminyl anion)
toward ketones was systematically investigated. Reaction of complex 1 with 9-fluorenone induced pinacol coupling to afford product 2 via two-electron reduction, with one electron derived from
the L2– ligand and the other from the Al(II) center.
In contrast, treatment of 1 with 1,3-dimethyl-2-imidazolidinone
resulted in the formation of complex 3 through a simple
Lewis acid–base addition. Furthermore, the Al(I) species [LAl:]−, generated via reduction of dialumane 1, reacts with benzophenone and acetophenone to yield the pinacol
coupling products 4 and 5, 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 6 and 7 instead
of C–C coupling. Compound 7 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 8 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.