1,4-Diynes from alkynyl–propargyl coupling reactions

This review deals with state-of-the-art propargylation of metal-alkynyls. After outlining the importance of the CCCsp3CC sequence in organic and organometallic molecules, processes for obtaining these ‘skipped diynes’ from metal-alkynyls (or metal acetylides CC[M]) and propargyl electrophiles (XCR1R2CC or XCRCC) are classified according to the nature of the metal (M=Li, Na, K, Mg, Cu, Zn, Al, Si, CMe2OAl(OR)2, …) and to the hybridization state of the electrophilic carbon (sp3, sp2). The allenic/propargylic regioselectivity being crucial, emphasis is placed on methods resorting to pre-complexation of the propargylic moiety with transition-metals: η2/η3 coordination with cobalt, and η1 coordination with ruthenium are specifically discussed. Whenever such processes have been used for definite synthetic purposes, the ultimate target is mentioned. For comparison, alternative routes to skipped diynes are illustrated by a few recent examples.