Synthesis of Carbo[6]helicene Derivatives Grafted with Amino or Aminoester Substituents from Enantiopure [6]Helicenyl Boronates

(BO)2 -Doped Tetrathia[7]helicene: A Configurationally Stable Blue Emitter

Helicenes combine two central themes in chemistry: extended π-conjugation and chirality. Hetero-atom doping preserves both characteristics and allows modulation of the electronic structure of a helicene. Herein, we report the (BO)₂ -doped tetrathia[7]helicene 1, which was prepared from 2-methoxy-3,3'-bithiophene in four steps. 1 is formally derived by substituting two (Mes)B-O moieties in place of (H)C=C(H) fragments in two benzene rings of the parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)° between the two terminal thiophene rings. The (P)-/(M)-1 enantiomers were separated by chiral HPLC and are configurationally stable at room temperature. The experimentally determined enantiomerization barrier of 27.4±0.1 kcal mol^-1 is lower than that of tetrathia[7]helicene (39.4±0.1 kcal mol^-1 ). The circular dichroism spectra of (P)- and (M)-1 show a perfect mirror-image relationship. 1 is a blue emitter (λ_em =411 nm) with a photoluminescence quantum efficiency of Φ_PL =6 % (cf. tetrathia[7]helicene: λ_em ≈405 nm, Φ_PL =5 %).

Circularly Polarized Fluorescent Helicene-Boranils: Synthesis, Photophysical and Chiroptical Properties

Mono- and di-boranil-substituted helicenes were prepared by BF₂ -borylation of the corresponding anils, readily synthesized by condensation of 2-amino- and 2,15-diamino-helicenes with 4-(diethylamino)salicylaldehyde. After enantiomeric resolution using HPLC, their chiroptical properties including circularly polarized fluorescence in solution and in PMMA films were investigated and rationalized with the help of NMR, X-ray and quantum-chemical calculations.

Recent Advances in Functionalizations of Helicene Backbone

Helicenes, chiral members of the family of polyaromatic hydrocarbons, have been increasingly used in a variety of applications in recent years. Despite their intriguing properties, wider utilization is hindered by difficult functionalization of its skeleton, which would provide access to finely tuned derivatives of desired properties. Herein, the recent advancements in the field of helicene functionalization are discussed with an emphasis on different types of transformations, their versatility, and regioselectivity.

Photocyclization of diarylethylenes with a boronate moiety: a useful synthetic tool to soluble PAH building blocksf

The synthesis of ten ortho-fused PAHs bearing boronic pinacol ester groups (BPin) is reported. The products are obtained via modification of Mallory photocyclization in 45-99% yields. Among them are examples of highly strained molecules such as [4]helicene derivatives with BPin substituents in the cavity. The method allows double C-C coupling and tolerates more than one BPin functionality.

Photochemical Functionalization of Helicenes

Herein, a visible-light photochemical approach for practical helicene functionalization at very mild reaction conditions is described. The photochemical reactions allow for the regiospecific and innate late-stage functionalization of helicenes and are easily executed either through the activation of C(sp2 )-Br bonds in helicenes using K2 CO3 as inorganic base or direct C(sp2 )-H helicene bond functionalization under oxidative photoredox reaction conditions. Overall, using these transformations six different functional groups are introduced to the helicene scaffold through C-C and four different C-heteroatom bond-forming reactions.

Reactivity and Synthetic Applications of Multicomponent Petasis Reactions

The Petasis boron-Mannich reaction, simply referred to as the Petasis reaction, is a powerful multicomponent coupling reaction of a boronic acid, an amine, and a carbonyl derivative. Highly functionalized amines with multiple stereogenic centers can be efficiently accessed via the Petasis reaction with high levels of both diastereoselectivity and enantioselectivity. By drawing attention to examples reported in the past 8 years, this Review demonstrates the breadth of the reactivity and synthetic applications of Petasis reactions in several frontiers: the expansion of the substrate scope in the classic three-component process; nonclassic Petasis reactions with additional components; Petasis-type reactions with noncanonical substrates, mechanism, and products; new asymmetric versions assisted by chiral catalysts; combinations with a secondary or tertiary transformation in a cascade- or sequence-specific manner to access structurally complex, natural-product-like heterocycles; and the synthesis of polyhydroxy alkaloids and biologically interesting molecules.