Semimasked 1,1′-diethynylferrocenes: synthetic concepts, preparations, and reactions

Due to the inherent instability of 1,1′-diethynylferrocene, the respective coupling chemistry for the access of oligonuclear systems requires stepwise preparative sequences involving consecutive ethyne deprotection, or the conversion of latent ethyne precursor functionalities, respectively. The new derivatives 1-acetyl-1′-ethynylferrocene, 3, and, preferably, 1-ethynyl-1′-formylferrocene, 9, turned out to be the most favorable starting compounds. The subsequent synthetic chemistry, as well as the X-ray structures of selected starting and target derivatives are presented. A unique intramolecular coupling product, 12, represents the first ladder-type tricyclic metallocenophane system exhibiting high ring strain. Supplementary novelties concerning monoacetylenic parent systems are also presented.