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 …