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

Abstract

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 degrees 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 degrees 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.

Cu-catalyzed cross-dehydrogenative coupling: A versatile strategy for C–C bond formations via the oxidative activation of sp 3 C–H bonds

“…Propargylic amines are of great pharmaceutical interest and are synthetically useful intermediates for various nitrogen compounds as well as carbohydrate derivatives (58). Two main methods have been used to construct propargylic amines (Scheme 5): Path A represents stoichiometric nucleophilic reactions (59,60), and Path B represents transition metal-catalyzed reactions of alkynes with imines generated from aldehydes and amines.…”

<b>SYNTHESIS OF ALLENIC NAPHTHALENE DERIVATIVES</b>

“…Treatment of acetoxyl 4a with SmI 2 -THF solution, 2-propanol as a proton source and Pd (PPh 3 ) 4 yielded allene 5a and acetylene 6a [9] isolated by column chromatography in 24 % and 57 % yields, respectively. The acetylene 6a is the appropriate starting material for 1-allenylnaphthalene 7 [10]. The reaction of acetylene 6a with sodium ethoxide at rt in THF gave rise to 1-allenylnaphthalene 7 in 74 % yield (Scheme 1).…”

Synthesis and Reactivity of Allenylporphyrins

“…Work by Nakamura et al utilizing propargyl amines followed by Pd-catalyzed hydrogen transfer is one of the more recent advances in this area which seemed to offer the best potential for success. [22] The first step of this synthesis involved a Sonogashira reaction analogous to that described in Scheme 5, using N,N-diisopropylprop-2-yn-1-amine (15) as the alkyne partner. The resultant propargylporphyrin 16 was then dissolved in a minimum amount of chloroform and heated to reflux in the presence of Pd 2 (dba) 3 as the precatalyst and P(C 6 F 5 ) 3 as the ligand source.…”