1,3‐Diethynylallenes: Stable Monomers, Length‐Defined Oligomers, Asymmetric Synthesis, and Optical Resolution

Abstract A series of differently substituted 1,3‐diethynylallenes (DEAs) have been synthesized, confirming that the previously introduced construction protocols tolerate a variety of functional groups. The new DEAs bear at least one polar group to facilitate enantiomer separations on chiral stationary phases and to allow further functionalization. They are thermally and environmentally stable compounds since bulky substituents next to the cumulene moiety suppress the tendency to undergo [2+2] cyclodimerization. A series of length‐defined oligomers were obtained as mixtures of stereoisomers by oxidative coupling of a monomeric DEA under Glaser–Hay conditions. The electronic absorption data indicate a lack of extended π‐electron conjugation across the oligomeric backbone due to the orthogonality of the allenic π‐systems. Remarkably, even complex mixtures of stereoisomers only yield one single set of NMR signals, which underlines the low stereodifferentiation in acyclic allenoacetylenic structures. Optical resolution of DEAs represents an amazing challenge, and preliminary results on the analytical level are reported. Asymmetric synthesis by Pd‐mediated S N 2′‐type cross‐coupling of an alkyne to an optically pure bispropargylic precursor opens another promising route to optically active allenes with stereoselectivities currently reaching up to 78 % ee . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)