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

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.

Effect of Terminal Fluorination on Chiroptical Properties of Carbo[5-8]helicenes: A Systematic Computational Study at the RI-ADC(2) Level

In this article, effects of di-, tetra-, and octafluorination on the structural and chiroptical properties of carbo[5-8]helicenes are reported. Three fluorinated derivatives are designed from each parent carbohelicene by substituting either one, two, or four hydrogens at each terminal ring with fluorine atoms. Excited states properties such as UV-vis and CD spectra of all the six fluorinated carbohelicenes are computed at the ADC(2)/def2-TZVP level, and the results are compared against the results of their respective parent carbohelicene. In addition, CPL properties are also computed at the same level of theory. In the case of carbo[5]helicene (5H), g_CPL decreases with an increase in the degree of fluorination. A similar observation is made in carbo[6]helicene (6H) too, although the value for tetrafluorinated 6H is slightly larger than for difluorinated 6H. Di- and tetrafluorination in carbo[7]helicene (7H), and all types of considered fluorination in carbo[8]helicene (8H) produce improved g_CPL results. Results for fluorescence rate constants are also shown. Results are analyzed in terms of the transition dipole moment vectors and the angles between those.

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.