Creation of negatively curved polyaromatics enabled by annulative coupling that forms an eight-membered ring

Abstract

The discoveries of new forms of carbon have always opened doors to new science and technology. In 1991, three-dimensional (3D) periodic carbon crystals with negative Gaussian curvatures that consist of six- and eight-membered rings were proposed. To realize these 3D periodic carbon crystals (known as Mackay crystals), methods for creating polyaromatic structures embedding eight-membered rings must be developed. Here we report two annulative coupling reactions that form an eight-membered ring through catalytic C–H functionalization. We have discovered that bay-chlorinated polyaromatics undergo either annulative dimerization or cross-coupling with biphenylene in the presence of a palladium catalyst to form various hitherto inaccessible polyaromatics embedding an eight-membered ring. The threefold annulative cross-coupling of 1,5,9-trichlorotriphenylene allowed construction of a highly curved nanocarbon. The present work not only demonstrates the potential of annulative coupling for constructing octagonal nanocarbons but also provides a conceptual pathway for the synthetic realization of 3D periodic carbon crystals. The Mackay crystal is a proposed—but synthetically unachieved—nanocarbon molecule that is anticipated to have many desirable properties. Now, a strategy based on annulative coupling of chlorophenanthrene derivatives is reported, allowing streamlined access to an important substructure.