Synthesis of Allenes via Palladium-Catalyzed Hydrogen-Transfer Reactions:  Propargylic Amines as an Allenyl Anion Equivalent

Synthesis of allenes has been achieved by using palladium-catalyzed hydrogen-transfer reactions. Various propargylic amines, which were readily prepapred from iodobenzenes and propargylic amines by Sonogashira coupling reaction, underwent the hydrogen-transfer reaction in the presence of Pd₂dba₃·CHCl₃/(C₆F₅)₃P catalyst at 100 °C in dioxane for 24 h, giving the corresponding allenes in 43−99% yields. Various propargylic alcohols containing a propargylic aminomethyl group, synthesized by the addition of lithium acetylides of N,N-diisopropylprop-2-ynylamine to aldehydes and a ketone, also underwent the hydrogen-transfer reaction in the presence of Pd₂dba₃·CHCl₃ catalyst and (C₆F₅)₃P at 80 °C in dioxane, giving the corresponding allenes in 56−92% yields. In the current transformation, propargylic amines can be handled as an allenyl anion equivalent and introduced into various electrophiles to be transformed into allenes under palladium-catalyzed conditions.