Abstract
Because of the thermodynamic stability of the aromatic sextet, the tendency of many unsaturated compounds, such as alkenes and alkynes, to form aromatic systems is readily comprehensible. Thus, the Reppe cyclotrimerization of alkynes to benzenes by agency of (Ph/sub 3/P)/sub 2/(CO)/sub 2/Ni is no cause for wonder. But the similar cyclotetramerization of certain alkynes to the non-aromatic cyclooctatetraenes by nickel(0) or nickel (II) cyanide must, in contrast, be astonishing. Yet a closer study of the behavior of both nickel(0) complexes and of alkali metals reveals that both metals are able to disrupt aromatic systems almost as readily as they can induce their formation. Basic both to the formation and the disruption of aromatics by low-valent metal reagents M/sup 0/ appears to be the possibility of electron transfer from the metal to a sufficiently low-lying antibonding molecular orbital of the reagent hydrocarbon. This paper analyzes the formation and chemical behavior of intermediates with examples drawn from reactions of lithium and of nickel. Emphasis is on the formation of new C-C sigma-bonds in aromatic systems with the preliminary disruption of C-C pi-bonds. Such processes will be shown to hold great promise for the rational construction of polycyclic aromatics.
Published Version
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