✍ Scribed by Astruc, Didier
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Alkynes have been used as precursors for aromatic compounds since Berthelot converted acetylene to benzene in 1867. The review presents new evidence on the mechanism of this first total synthesis of an aromatic compound. Presumably, its last step involves the thermocyclization of hexa-1,3-dien-5-yne to benzene. Trapping experiments as well as the study of derivatives of this highly unsaturated hydrocarbon show that the reaction takes place via isobenzene intermediates (1,2,4-cyclohexatrienes) at lower temperatures. Under high temperature pyrolysis conditions, carbenes as well as radical routes compete with the pericyclic process. In another recent application of acetylenes in the synthesis of aromatic compounds, the use of the isomeric diethynylbenzenes is discussed, which, depending on the actual substitution pattern, may be used for the construction of long extended p-systems or cyclic derivatives. When the number of triple bond substituents at the aromatic core is increased, subsystems of novel carbon allotropes become available (''graphyne''). Cyclooligomerization of acetylenes in the presence of cobalt catalysts leads to phenylenes, a class of condensed aromatic compounds hitherto available only with difficulty. Debromination of bis(propargyl bromide)-substituted aromatics provides highly reactive intermediates, which cyclodimerize to cyclophanes possessing unsaturated molecular bridges (cyclophynes). Cyclophynes are not only of interest for structural and stereochemical reasons but also as precursors of novel fullerene derivatives.
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