From Allenes to Tetracenes: A Synthetic and Structural Study of Silyl‐ and Halo‐Allenes and Their Dimers

9‐Alkynylfluoren‐9‐ols of type C 13 H 8 (OH)–C≡C–R, where R is phenyl ( 1a ), p ‐tolyl ( 1b ), p ‐methoxyphenyl ( 1c ), or trimethylsilyl ( 1d ), react with HBr to yield the corresponding bromoallenes C 12 H 8 C=C=C(Br)R ( 13a – d ). Lithiation and hydrolysis of 13d yields 3,3‐(biphenyl‐2,2′‐diyl)‐1‐(trimethylsilyl)allene ( 20 ), which forms successively the head‐to‐tail dimer 1‐(9‐fluorenylidene)‐4‐(trimethylsilyl)‐2‐[(trimethylsilyl)methylene]spiro[cyclobutane‐3,9′‐[9 H ]fluorene] ( 21 ) and the tail‐to‐tail dimer trans ‐3,4‐bis(trimethylsilyl)‐1,2‐di(fluorenylidene)cyclobutane ( 23 ) both of which exhibit long (ca. 1.6 Å) C(3)–C(4) bonds. Treatment of 21 and 23 with TBAF brings about desilylation to 22 and 24 , respectively. In the latter case, removal of the bulky trimethylsilyl groups reduces the C(3)–C(4) bond length in 1,2‐di(fluorenylidene)cyclobutane ( 24 ) to a more normal value of 1.547 Å; however, the large wingspan of the severely overlapping fluorenylidene moieties retains the C 2 symmetry of the system. Prolonged exposure to sunlight of the disilylated head‐to‐tail dimer 21 results in homolysis of the long C(3)–C(4) bond, generation of a peroxide, and ultimately formation of the lactol 25 . X‐ray crystallographic data are reported for, among others, 1c , 13a , 21 , 22 , 23 , 24 , 25 , and also for 3,3‐(biphenyl‐2,2′‐diyl)‐1‐chloroallene ( 12 ).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)