A straightforward,
user-friendly, efficient protocol for the one
pot, ZnI
2
-catalyzed allenylation of terminal alkynes with
pyrrolidine and ketones, toward trisubstituted allenes, is described.
Trisubstituted allenes can be obtained under either conventional heating
or microwave irradiation conditions, which significantly reduces the
reaction time. A sustainable, widely available, and low-cost metal
salt catalyst is employed, and the reactions are carried out under
solvent-free conditions. Among others, synthetically valuable allenes
bearing functionalities such as amide, hydroxyl, or phthalimide can
be efficiently prepared. Mechanistic experiments, including kinetic
isotope effect measurements and density functional theory (DFT) calculations,
suggest a rate-determining [1,5]-hydride transfer during the transformation
of the intermediate propargylamine to the final allene.
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