Aromatic Fluorine as a Versatile Control Element for the Construction of Molecules with Helical Chirality

Abstract

The p–p stacking and edge–face contacts between aromatic groups occupy prominent positions among noncovalent interactions, with helicity being one the most significant kinds of supramolecular organization driven by these interactions. Apart from stabilizing the structure of large molecules such as DNA, helicity has also been explored in the realm of synthetic small molecules. The so-called helicenes were originally developed merely as aesthetically pleasing molecules, however their unusual optical and electronic properties have attracted a great deal of attention of late. The chiral backbones of these molecules have given them roles in a variety of applications ranging from asymmetric catalysis to molecular motors and remote chirality sensors. Furthermore, the original helicene synthesis involving stilbene photocyclizations has evolved into more modern approaches, including various ring-closing methods such as domino Diels–Alder reactions, palladium-catalyzed arylations, cobalt-catalyzed cycloisomerizations of aromatic triynes, and ruthenium-catalyzed olefin metathesis. Despite these important advances, the synthesis of chiral helicenes is still tedious and requires considerable synthetic prowess. Due to their ability to undergo directional aggregation, helical molecules that are easy to prepare and modify are of considerable significance. Our group has been interested in versatile precursors to helically chiral molecules that would be amenable to regioand stereoselective structural alterations at a late stage of synthesis; molecules meeting these criteria are practically unknown. Herein we describe a methodology that allows us to generate several families of helically chiral compounds by straightforward intramolecular fluorine substitution. We have long been interested in molecules that contain aromatic fluorine. To the best of our knowledge, there are no reports on fluorine-containing helically chiral compounds apart from a few intriguing supramolecular perfluoroalkane helicates. Fluorine substitution is known to modulate aromatic/aromatic interactions by affecting the HOMO– LUMO gap, which leads to a strong propensity for the molecules to aggregate. Extended wave function delocalization in the resulting materials leads to high electron mobility, as observed upon going from pentacene to perfluoropentacene. The versatile 1,1’-binaphthalene2,2’-diol (binol, 1) skeleton was taken as a starting point for our purposes. The synthesis and applications of several fluorine-substituted binol analogues with various fluorination patterns have been developed by us and others previously. The act of fluorination causes significant perturbation in the electronic character of binol (Figure 1) with no substantial steric consequences.