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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species
Published on: August 16, 2018
Diverse Reactivities of Ketones toward Compounds with Low-Valent Aluminum: Reductive Coupling, Enolization, and C═O
Yihu Zhang1, Fangfeng Chen1, Zhenzhou Sun2
1Key Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710127, China.
Abstract:
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.
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