Mononuclear or Dinuclear? Mechanistic Study of the Zinc-Catalyzed Oxidative Coupling of Aldehydes and Acetylenes.

Although zinc catalysis is widely used in organic synthesis, very few studies on the dinuclear zinc mechanism have been reported. Here, a dinuclear zinc pathway is proposed for the Zn(OTf)2 -catalyzed oxidative coupling of aldehydes with terminal alkynes. DFT calculations revealed that the deprotonation of the terminal alkyne would preferentially lead to the formation of a dinuclear zinc intermediate. The nucleophilic addition of this intermediate to an aldehyde, followed by an Oppenauer-type oxidation was investigated theoretically according to the mono- and dinuclear pathways. The formation of a dinuclear zinc intermediate from a mononuclear alkynyl zinc complex was exergonic, favoring the dinuclear zinc pathway. The subsequent protonation and regeneration of the active dinuclear catalyst were also evaluated by DFT calculations. The oxidizabilities of various aldehydes and ketones were evaluated to determine the best oxidant for this step. Trifluoroacetaldehyde was predicted to be a better oxidant for this reaction because its calculated energy barrier for the Oppenauer-type oxidation step was much lower than that of the other carbonyl complexes.