Enantioselective [2+2+2] cycloisomerisation of alkynes in the synthesis of helicenes: The search for effective chiral ligands

Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems

Advanced molecular nanocarbons are now in the spotlight reflecting the basic discoveries of fullerenes, carbon nanotubes, and graphene. This research area includes also the chemistry, physics, and nanoscience of nonplanar polycyclic hydrocarbons, many of which exhibit helical chirality, such as iconic helicenes and their congeners. The combination of unique π-electron systems with the chirality phenomenon makes them highly attractive in various fields of science. Helicenes are polyaromatic compounds that are composed of all-angularly annulated benzene units, but other (hetero)cycles can also be embedded into their backbone. Even though they do not contain any stereogenic center, they are inherently chiral owing to the helical shape they adopt. Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and, therefore, can be obtained in an enantiopure form through a racemate resolution or asymmetric synthesis. An amazing array of synthetic methods for their preparation has been developed, but only a few of them have passed the tough scrutiny to be general, robust and practical methods such as traditional photocyclodehydrogenation of diaryl olefins and recently developed transition-metal-catalyzed [2 + 2 + 2] cycloisomerization of π-electron systems, which is discussed in this Account. Alkyne [2 + 2 + 2] cycloisomerization is a highly exergonic process and is therefore suitable for forming the strained helicene backbone, three (or more) cycles of which are closed in a single operation. The typical starting materials are aromatic triynes (optionally cyanodiynes or ynedinitriles) or tetraynes with diynes that undergo intramolecular or intermolecular cyclization, respectively, catalyzed by various complexes mainly of Ni⁰, Co^I, or Rh^I. Utilizing this synthetic methodology, various [5]-, [6]-, [7]-, [9]-, [11]-, [13]-, [16]-, [17]-, and [19]helicenes or their congeners, including functionalized derivatives, can be effectively prepared. Moreover, asymmetric synthesis (both catalytic and stoichiometric) of nonracemic helicenes has already been demonstrated. It relies on [2 + 2 + 2] cycloisomerization of centrally chiral triynes followed by an asymmetric transformation of the first order (controlled by the 1,3-allylic-type strain) or on enantioselective [2 + 2 + 2] cycloisomerization of alkynes catalyzed by chiral complexes mainly of Ni⁰ or Rh^I. Intriguingly, advanced helical architectures were formed such as the longest helicenes (up to oxa[19]helicene by closing 12 rings in a single synthetic operation) or laterally extended helicenes (e.g., pyreno[7]helicenes). Utilizing the aforementioned synthetic methodology, the tailor-made helical molecular nanocarbons are now better accessible to be applied in enantioselective catalysis, chirality sensing, spintronics (based on chirality induced spin selectivity), chiroptics (to produce circularly polarized light emission), organic/molecular electronics, or chiral single molecule devices.

Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings

The first racemization-stable helicene derivatives fluorinated at terminal rings, 1,2,3,4-tetrafluoro[6]helicene (6) and 1,2,3,4,13,14,15,16-octafluoro[6]helicene (15), were synthesized via the Wittig reaction followed by oxidative photocyclization in an overall yield of 41% of 6 and 76% of 15. The changed electronic structure in fluorinated helicenes was reflected in a slight shift of UV-vis absorption, fluorescence excitation, and emission spectra maxima when compared to unsubstituted [6]helicene. Cyclic voltammetry revealed a moderate decrease in the HOMO-LUMO gap with increasing fluorination. The specific rotation of tetrafluoro[6]helicene 6 enantiomers was found to be approximately 25% lower than that of unsubstituted [6]helicene. The theoretical study of the racemization barrier suggested a reasonable shift toward higher energy with increasing fluorination. The increasing fluorination also significantly affected the intermolecular interactions in the crystal lattice. The observed CH···F interactions led to the formation of 1D-molecular chains in the crystal structures of both fluorinated helicenes.