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 …