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Pure Appl. Chem., 2009, Vol. 81, No. 1, pp. 1-17

From allenes to tetracenes: Syntheses, structures, and reactivity of the intermediates*

Emilie V. Banide1, Pascal Oulié2 and Michael J. McGlinchey3

1 Novartis Ringaskiddy Ltd., Ringaskiddy, Cork, Ireland
2 Laboratory of Basic and Applied Heterochemistry, Paul Sabatier University, Toulouse, France
3 School of Chemistry and Chemical Biology, University College Dublin, Belfield Dublin 4, Ireland

Abstract: Each step of the conversion of a series of 9-alkynyl-9H-fluorenes into the corresponding fluorenylidene-allenes that dimerize and proceed sequentially via head-to-tail and tail-to-tail dialkylidene-cyclobutanes, en route to electroluminescent tetracenes, has been characterized X-ray crystallographically. Allenes possessing substituents of very different electronic and steric character, such as aryl, halogeno, silyl, phosphino, and ferrocenyl, exhibit novel and unexpected reactivity patterns. The silyl-allenes dimerize to yield 1,2-bis(fluorenylidene)cyclobutanes of intrinsic C2 symmetry as a result of the overlapping fluorenylidenes with their large wingspans. Thermal rearrangement of a bis(fluorenyl)-bis(trimethylsilyl)-diallene generates the tetrabenzo-quatercyclopentadiene, C60H36, which represents 60 % of the C60 framework. An attempt to isolate a "push-pull" allene, whose central carbon possesses carbene character, was made by incorporating a cation-stabilizing substituent (ferrocenyl) and an aromatic anionic moiety (fluorenide) at the termini. However, the allene underwent facile dimerization to the very heavily congested 3,4-di(spirofluorenyl)-1,2-bis(ferrocenyl-chloromethylene)cyclobutane that exhibits a very long (1.65 Å) C(3)-C(4) bond. Extension of this chemistry to dibenzosuberenylidene-allenes led to a straightforward route to the hitherto difficultly available dibenz[c,d,h]azulene system. Moreover, the reaction of 5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol with dicobalt octacarbonyl yielded, surprisingly, the first isolated example of a (μ-alkyne)Co2(CO)52-alkene)complex, the long-sought first intermediate in the proposed mechanism of the Pauson-Khand reaction (PKR).
*Pure Appl. Chem. 81, 1-84 (2009). A collection of invited, peer-reviewed articles by the winners of the 2008 IUPAC Prize for Young Chemists.