Carbo[n]helicenes Restricted to Enantiomerize: An Insight into the Design Process of Configurationally Stable Functional Chiral PAHs

The most important stereodynamic feature of carbo[n]helicenes is the interconversion of their enantiomers. The Gibbs activation energy (ΔG^≠(T)) of this process, which determines the rate of enantiomerization, dictates the configurational stability of [n]helicenes. High values of ΔG^≠(T) are required for applications of functional chiral molecules incorporating [n]helicenes or helicene substructures. This minireview provides an overview of the mechanism, recent developments, and factors affecting the enantiomerization of [n]helicenes, which will accelerate the design process of configurationally stable functional chiral molecules based on helicene substructures. Additionally, this minireview addresses the misconception and irregularities in the recent literature on how the terms “racemization” and “enantiomerization” are used as well as how the activation parameters are calculated for [n]helicenes and related compounds.

Twisted arenes

The nonbonded steric interactions of substituents at the crowded C4 and C5 positions of phenanthrene cause the aromatic system to twist out of planarity. Similarly, the presence of substituents at the C1 and C12 positions of benzo[c]phenanthrene and at the C1 and C14 positions of dibenzo[c,g]phenanthrene are responsible for the helical twists of the aromatic frameworks. Highly substituted acenes, such as octamethylnaphthalene and decaphenylanthracene, also exhibit substantial end-to-end twists. The X-ray structures of these compounds allow direct measurements of the extent of the structural distortions. The configurational stabilities of many twisted arenes have also been determined.

Novel synthesis of 4,5-diarylphenanthrenes via C2-C6 cyclization of benzannulated enyne-allenes

A new synthetic pathway to the 4,5-diarylphenanthrenes 8 having a helical twist in their structures was developed. The synthetic sequence involves condensation of the diketone 5 with 2 equiv of the lithium acetylides derived from the diacetylenes 4 followed by protonation to produce the propargylic alcohols 6. Reduction of 6 with triethylsilane in the presence of trifluoroacetic acid furnished the tetraacetylenic hydrocarbons 7 in nearly quantitative yields. Treatment of 7 with potassium tert-butoxide under refluxing toluene at 110 degrees C for up to 10 h then furnished the 4,5-diarylphenanthrenes 8. Apparently, the transformation from 7 to 8 involves initial prototropic isomerizations to form the benzannulated enyne-allenes 9. Two subsequent formal intramolecular Diels-Alder reactions via the biradicals 10 and 12 derived from the C(2)-C(6) cyclizations then led to 13, which in turn underwent tautomerizations to give 8. The structure of 8a was established by the X-ray structure analysis, showing that the two phenyl substituents are bent away from each other and the central aromatic system is severely distorted with a helical twist. The existence of a helical twist in 8 imposed by the aryl groups at the 4- and 5-positions was also revealed with a set of AB (1)H NMR signals for the diastereotopic methylene hydrogens on the five-membered rings.

Faurine and O-methylfaurine, two novel benzyl-aporphine dimers from Thalictrum fauriei

Two new benzyl-aporphine dimers, faurine (1) and its O-methyl ether (2), were obtained from Thalictrum fauriei and their structures determined by spectral and chemical methods. Faurine (1), the first dimer with the diphenyl ether connection at C-1 of the aporphine, did not undergo a Hofmann degradation, yet formed a chiral phenanthrene product (6) with cyanogen bromide. Two nondimeric chiral aporphine alkaloids, O-acetylisocorydine and O-methylisocorydine, gave the corresponding phenanthrene products 7 and 8, respectively, under the same conditions, but only compound 7 was optically active.