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

Protonation of 9-(ferrocenylethynyl)fluoren-9-ol (<b>9</b>) yields the conjugated enone derived from a Meyer−Schuster rearrangement. However, treatment of <b>9</b> with thionyl chloride at −30 °C proceeds with elimination of SO₂ to furnish 3,3-biphenylene-1-chloro-1-ferrocenylallene (<b>14</b>), 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 <b>15</b>, 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 <b>16</b> instead yielded the hexa-1,2-dien-5-yne <b>17</b>, the product of allenyl radical head-to-tail coupling. Protonation of ferroceno[2,3]inden-1-ol (<b>25</b>) 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 (<b>29</b>) provides experimental verification of Caïs’s hypothesis. The X-ray crystal structures of 9-(ferrocenylethynyl)fluoren-9-ol (<b>9</b>) and of its isomer 1-(phenylethynyl)ferroceno[2,3]inden-1-ol (<b>26</b>) are reported along with, among others, the structures of <b>15</b>, <b>17</b>, <b>25</b>, and <b>29</b>. The possible role of these allenes as precursors to ferrocenyl-containing electroluminescent tetracenes is discussed.