Achieving Olympicene Functionalization Three Ways

Polycyclic aromatic hydrocarbons (PAHs) and their derivatives are of interest in many fields, from materials science to supramolecular chemistry. 6H-Benzo[cd]pyrene, or olympicene, is a PAH bearing a central sp3-carbon bridge. We sought methods to modify this center position while maintaining stable tetrahedral geometry. Here, we characterize reactivity patterns of three olympicene core starting materials, with two of the three leading to center functionalization of the olympicene core, including the first use of 5H-benzo[cd]pyren-5-one as an olympicene synthon.

Helix-to-Disc Conversion of Thia[6]helicenes into Coronenes Facilitated by Sulfur Oxidation and Fluorination

Helicenes, with their unique helical structures, have long captured the interest of synthetic chemists, not only as end products, but also as versatile platforms for further chemical transformations. However, transforming [6]helicene into planar coronene typically requires harsh conditions and poses significant challenges. Herein, we demonstrate that replacing the terminal benzene ring of [6]helicene with a thiophene ring enables its photochemical transformation into coronene. Sulfur oxidation of the thiophene ring enables the corresponding thermal transformation, and the terminal tetrafluorination of the opposite benzene ring further accelerates this process, yielding 1,2-difluorocoronene, as confirmed by X-ray crystallography. The transformation begins with an intramolecular Diels-Alder reaction, whose activation energy is significantly lowered by these structural changes. Our findings underscore the utility of strategic modifications such as sulfur oxidation and fluorination in promoting this "helix-to-disc" conversion and opening new avenues for synthesizing functional polycyclic aromatics.

Transformation from triple helicene to double helicene embedding adjacent stereogenic carbon atoms and axial stereogenicity

Transformation of triple helicene (TH) to double helicene (DH) with adjacent stereogenic carbon atoms and axial stereogenicity was achieved by the unexpected conjugate addition to the central aromatic ring of TH-1. We also studied the boundary at which different reactivities to addition reactions occur in some helicenes with different π-extension.

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

The intramolecular Diels-Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F₄ -[7]helicene) undergoes a photoinduced Diels-Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F₃ -[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels-Alder reactions, but the whole domino process became slow. F₂ -[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F₂ -[7]helicene was less compressed than F₄ -[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene-fluoroarene stacking interactions and thus promoted the transformations.

Revisiting the racemization mechanism of helicenes

Herein we propose a general mechanism for the racemization of [n]helicenes up to n = 24. It is a concerted process for n = 4-7, but a multi-step mechanism is followed for n≥ 8, involving 2n- 14 intermediates. The changes in the barriers are a delicate consequence of the steric hindrance and the π-interactions.