Origin of Benzophenone Ketyl in Reactions of Benzophenone with Lithium Dialkylamides. Implications for Other Possible Electron-Transfer Reactions

Article abstract of DOI:10.1021/ja00338a044

A reaction scheme is presented for the formation of benzophenone ketyl in reactions of lithium dialkylamides containing β-hydrogen atoms with benzophenone.The key steps are fast concerted β-hydride reduction of benzophenone to give lithium benzhydrolate, rate-limiting deprotonation of the lithium benzhydrolate to give dilithium benzophenone dianion, and fast electron transfer from the dianion to benzophenone to give two molecules of ketyl.Benzophenone is supplied throughout the course of the reaction by a retro-aldol reaction of the lithium salt of aldol-like adduct 4 formed early in the reaction.The r eaction scheme was confirmed by kinetic studies of the individual steps.The velocity of the retro-aldol reaction was orders of magnitude faster than that of ketyl formation.The deprotonation of lithium benzhydrolate by lithium diisopropylamide (LDA) in tetrahydrofuran at 22 deg C occurred with an apparent second-order rate constant which was approximately equal to one-half of the apparent second-order rate constant for ketyl formation when benzophenone was treated with LDA under similar conditions.The possibility that related sequences of reactions could occur when weak organic oxidants are treated with reagents that can act as hydride donors and bases is discussed.