Total Syntheses of Angular [7]-, [8]-, and [9]Phenylene by Triple Cobalt-Catalyzed Cycloisomerization: Remarkably Flexible Heliphenes

Abstract

Six of the final cyclobutadiene rings in angular [8]phenylene and [9]phenylene (1) are closed in an unprecedented cobalt-catalyzed triple cyclization of appropriate nonaynes. These strained products are the largest known phenylenes and display unusual configurational lability. Their synthesis has enabled a first estimate of the properties of the hypothetical polyheliphene.