Syntheses, X-ray crystal structures and reactivity of fluorenylidene - and dibenzosuberenylidene -allenes: convenient precursors to dispirotetracenes, di-indenotetracenes and 2-phenyl-11bH-dibenz[cd,h]azulene

3,3-(Biphenyl-2,2'-diyl)-1-alpha,alpha,alpha-trifluoro-p-tolyl-allene, 9, sequentially forms head-to-tail, 12, cis-tail-to-tail, 13, and trans tail-to-tail, 14, 1,2-dialkylidene-cyclobutane dimers, each of which has been characterised by X-ray crystallography. Thermolysis at 180 degrees C yields the dispirotetracene, 15, and di-indenotetracene, 16; the latter compound forms an air-stable Diels-Alder adduct, 17, with N-methylmaleimide. In contrast, the dibenzo[a,d]cycloheptenylidene-allenes, (C(14)H(10))C=C=C(Br)Ph, 20a, and (C(14)H(10))C=C=C(H)Ph, 21a, do not dimerise under relatively mild conditions. However, protonation of the bromo-allene, 20a, and subsequent addition of hydride, provide a facile entry to the difficultly accessible bowl-shaped dibenz[cd,h]azulene framework, as in 30, that had not previously been structurally characterised. Among others, the X-ray crystal structures of 12, 13, 14, 17, 20a, 21a and 30 are reported.