Fragments of fullerenes: novel syntheses, structures and reactions

Abstract. The utility of high-temperature gas-phase cyclization reactions for constructing bowl-shaped polycyclic aromatic hydrocarbons related to C6o was first demonstrated by our simple synthesis of corannulene, 1. This strategy has subsequently been extended to the preparation of larger fullerene fragments and promises to play a central role in future syntheses of even more spectacular molecular bowls, baskets and tubes. Isomerization reactions of benzenoid hydrocarbons at high temperatures in the gas phase have also been studied and are now known to occur by reversible benzene ring contractions to fulvene intermediates. The relationship of these small molecule rearrangements to the mechanisms of fullerene production and isomerization (“annealing”) is discussed. Finally, the conversion of simple benzenoid hydrocarbons to fragments of fullerenes containing five-membered rings with two hydrogens fewer is shown to be a common high-temperature reaction of bay region polycyclic aromatic compounds, and a preliminary mechanistic proposal is offered along with supporting experimental evidence. The landmark synthesis of corannulene, 1, by Barth and Lawton was first announced in 1966 (l), more than two decades prior to the earliest preparative synthesis of C a and the higher fullerenes (2). This marvelous bowl-shape