Syntheses, Structures, and Dimerizations of Ferrocenyl- and Fluorenylideneallenes: Push−Pull Multiple Bonds?

Protonation of 9-(ferrocenylethynyl)fluoren-9-ol (9) yields the conjugated enone derived from a Meyer−Schuster rearrangement. However, treatment of 9 with thionyl chloride at −30 °C proceeds with elimination of SO₂ to furnish 3,3-biphenylene-1-chloro-1-ferrocenylallene (14), the ¹³C NMR data of which indicate that the central carbon does not have markedly carbenic character. Upon warming, this allene readily forms the sterically highly encumbered head-to-head dimer 15, whereby the 1,2-bis(alkylidene)cyclobutane adopts a butterfly conformation with two exocyclic Cl(Fc)C moieties and a very long (∼1.65 Å) C(3)−C(4) bond linking the two spiro-bonded fluorenyl fragments. In contrast, attempts to generate the analogous ferrocenylbromoallene 16 instead yielded the hexa-1,2-dien-5-yne 17, the product of allenyl radical head-to-tail coupling. Protonation of ferroceno[2,3]inden-1-ol (25) confirmed a proposal, originally put forward by Caïs in 1965, concerning the possible diradical nature of the resulting cation; the unambiguous characterization of the ferroceno[2,3]inden-1-yl dimer (29) provides experimental verification of Caïs’s hypothesis. The X-ray crystal structures of 9-(ferrocenylethynyl)fluoren-9-ol (9) and of its isomer 1-(phenylethynyl)ferroceno[2,3]inden-1-ol (26) are reported along with, among others, the structures of 15, 17, 25, and 29. The possible role of these allenes as precursors to ferrocenyl-containing electroluminescent tetracenes is discussed.