Cyclic tris-[5]helicenes with single and triple twisted Möbius topologies and Möbius aromaticity

• Published in: Chemical science (Cambridge) 2018-12, Vol.9 (48), p.893-8936
• Main Authors: Naulet, Guillaume, Sturm, Ludmilla, Robert, Antoine, Dechambenoit, Pierre, Röhricht, Fynn, Herges, Rainer, Bock, Harald, Durola, Fabien
• Format: Article
• Language: English
• Subjects: Aromaticity; Benzene; Cartesian coordinates; Chemical Sciences; Chemical synthesis; Chemistry; Conjugation; Crystallography; Electrons; Fragments; Material chemistry; NMR; Nuclear magnetic resonance; Ring currents; Single crystals; Topology; X-ray diffraction
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c8sc02877j

A number of singly (180°) twisted, largely single-stranded and thus conformationally rather fragile, Möbius molecules have been synthesized within the last 15 years, which are aromatic with 4 n electrons, thus violating the Hückel rule. Annulenes with significantly higher twist ( e.g. 540°) that retain a full cyclic conjugation path have been elusive, mainly because of the high strain and loss of orbital overlap. Recently, a topological strategy was devised to project the "twist" into "writhe", thus reducing the strain. However, orbital overlap was still severely reduced within the flexible building blocks. We now present a single and a triple twisted annulene with fully conjugated peripheries. They are unique in their pronounced band shape and conformational robustness as they are made up of three fully kata-condensed [5]helicene fragments. The triple twisted molecule exhibits a strong diatropic ring current in the outer periphery, even though the π system includes 4 n electrons. The diatropic current is counterbalanced by a paratropic current in the σ system, resulting in no net manifestation of macrocyclic aromaticity. The key step of the synthesis of both Möbius compounds is a Perkin condensation of complementary bifunctional bismaleates leading to a flexible macrocycle containing alternating benzene and biphenyl fragments. Subsequent photocyclization yields a separable mixture of rigid diastereomeric tris-helicene macrocycles of the above topologies. Synthesis and theoretical study of rigid conjugated macrocycles having persistent geometries of single and triple twisted Möbius strips.