2-Bromo[6]helicene as a Key Intermediate for [6]Helicene Functionalization

Keto-enol equilibrium: stable tautomers of ortho-, meta-, and para-hydroquinones in large aromatics

The synthesis of [6]helicene para-quinone starting from the 1,4-dimethoxy-[6]helicene derivative is presented. The demethylation reaction with boron tribromide led to unexpected results. Instead of the expected para-hydroquinone, the diketone tautomeric form was isolated. In contrast to the 1,4-hydroquinone and 1,4-dihydroxynaphthalene, the stable tautomers for the [4] and [6]helicenes were the aromatic diketones. These experimental results were corroborated by calculations. Additional calculations showed that these tautomeric species were indeed the stable forms of 1,4 and 1,3-hydroquinones when present in larger aromatics, in drastic contrast with 1,2-dihydroxy-aromatics (known as catechol).

Helitwistacenes-Combining Lateral and Longitudinal Helicity Results in Solvent-Induced Inversion of Circularly Polarized Light

Helicity is expressed differently in ortho- and para-fused acenes-helicenes and twistacenes, respectively. While the extent of helicity is constant in helicenes, it can be tuned in twistacenes, and the handedness of flexible twistacenes is often determined by more rigid helicenes. Here, we combine helicenes with rigid twistacenes consisting of a tunable degree of twisting, forming helitwistacenes. While the X-ray structures reveal that the connection does not affect the helicity of each moiety, their electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra are strongly affected by the helicity of the twistacene unit, resulting in solvent-induced sign inversion. ROESY NMR and TD-DFT calculations support this observation, which is explained by differences in the relative orientation of the helicene and twistacene moieties.

Porphyrin- and Bodipy-helicene conjugates: syntheses, separation of enantiomers and chiroptical properties

The synthesis of several new compounds containing a chromophore and a helicenic moiety is reported. The preparation, characterisation and some physico-chemical studies are detailed. In particular, the two enantiomers of several chiral molecules of this type were separated by chiral HPLC (both analytically and in a preparative way) and their racemisation rates were determined for short-lived species. Electronic circular dichroism (ECD) and circular polarised luminescence (CPL) measurements were performed for the compounds with a very long racemisation half-life. Chiral porphyrins and Bodipys both gave ECD and CPL responses over a large area of the visible spectrum.

Helicity-modulated remote C-H functionalization

Remote C-H functionalization is highly important for the conversion and utilization of arenes, but the conventional routes are comprehensively developed with the assistance of transition metal catalysts or templates. We report a facile metal/template-free electrochemical strategy for remote C-H functionalization in a helical system, where aromatic or aliphatic hydrogen act as a directing group to promote the alkoxylation at the opposite site of the helical skeleton by generating a unique helical "back-biting" environment. Such helicity-modulated C-H functionalization is prevalent for carbo[n]helicenes (n = 6 to 9, primitive or substituted) and hetero[6]helicenes and also occurs when the aryl hydrogen on the first position is replaced by a methyl group or a phenyl group. Thus, the relatively inert helicene skeleton can be precisely furnished with a rich array of alkoxy pendants with tunable functional moieties. Notably, the selective decoration of a methoxy group on N-methylated aza[6]helicene close or distant to the nitrogen atom leads to distinct luminescence variation upon changing the solvents.

Effect of Terminal Fluorination on Chiroptical Properties of Carbo[5-8]helicenes: A Systematic Computational Study at the RI-ADC(2) Level

In this article, effects of di-, tetra-, and octafluorination on the structural and chiroptical properties of carbo[5-8]helicenes are reported. Three fluorinated derivatives are designed from each parent carbohelicene by substituting either one, two, or four hydrogens at each terminal ring with fluorine atoms. Excited states properties such as UV-vis and CD spectra of all the six fluorinated carbohelicenes are computed at the ADC(2)/def2-TZVP level, and the results are compared against the results of their respective parent carbohelicene. In addition, CPL properties are also computed at the same level of theory. In the case of carbo[5]helicene (5H), g_CPL decreases with an increase in the degree of fluorination. A similar observation is made in carbo[6]helicene (6H) too, although the value for tetrafluorinated 6H is slightly larger than for difluorinated 6H. Di- and tetrafluorination in carbo[7]helicene (7H), and all types of considered fluorination in carbo[8]helicene (8H) produce improved g_CPL results. Results for fluorescence rate constants are also shown. Results are analyzed in terms of the transition dipole moment vectors and the angles between those.

Helicenic N-heterocyclic carbene copper(I) complex displaying circularly polarized blue fluorescence

Enantiopure copper(I) chloride complexes bearing a monodentate N-(carbo[6]helicenyl)-NHC ligand have been prepared and characterized experimentally and computationally. Their high stability enables the stereochemistry to be probed by X-ray crystallography and NMR spectroscopy. The resolved enantiomeric complexes emit circularly polarized blue fluorescence with g_lum ∼1.3 × 10^-3 in solution. The photophysical and chiroptical properties of these systems, with their helicene-centred origin, are similar to those of the organic helicene-benzimidazole precursor proligand, although the reverse axial chirality configuration is preferentially observed for the complex compared to the ligand.

Synthesis of 2-Phospha[7]helicene, a Helicene with a Terminal Phosphinine Ring

A synthetic strategy toward phosphahelicenes containing a terminal phosphinine ring has been explored. The 4-phenyl-6-methyl-2-phospha[7]helicene was prepared from starting 2-bromobenzo[c]phenanthrene in 12% overall yield in 12 steps. The synthetic approach involves introduction of the phosphorus function prior to photocyclization forming the final helicene skeleton, followed by the formation of a phosphorus hexacycle. The structure of the first phosphahelicene with a terminal phosphinine ring was confirmed by X-ray crystallography.

Synthesis of a helicene-fused porphyrin leading to a π-extended chiral chromophore

The preparation of several covalently linked [6]-helicene-porphyrins is reported. A fused [6]-helicene-porphyrin π-extended aromatic system was isolated, the enantiomers separated and the chiroptical properties determined. The oxidative cyclodehydrogenation proved to be very effective for six-membered fused helical systems, but not suited for the formation of five-membered fused systems.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Enantioselective Synthesis of Dithia[5]helicenes and their Postsynthetic Functionalization to Access Dithia[9]helicenes

A highly enantioselective synthesis of 5,13-disubstituted dibenzo[d,d']benzo[1,2-b:4,3-b']dithiophenes is reported. Key for the successful assembly of these helical architectures is the last two successive Au-catalyzed intramolecular alkyne hydroarylation events. Specifically, the second cyclization is the enantiodetermining step of the whole process and provides the desired helicenes with excellent ee values when a TADDOL-derived 1,2,3-(triazolium)phosphonite moiety (TADDOL: α,α,α',α'-tetraaryl-1,3-dioxolane-4,5-dimethanol) is employed as an ancillary ligand. The absolute stereochemistry of the newly prepared structures has been determined by X-ray crystallography to be P; the optical properties of these heterohelicenes are also reported. A three-step procedure was subsequently developed that allows the transformation of the initially obtained dithia[5]helicenes into dithia[9]helicenes without erosion of the enantiopurity.

Flavin-Helicene Amphiphilic Hybrids: Synthesis, Characterization, and Preparation of Surface-Supported Films

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin-[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin-2,4-dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2-nitroso[6]helicene and 6-methylamino-3-methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1-monoolein) matrix was developed.

Triskelion-shaped iridium-helicene NHC complex

Triskelion-shaped cycloiridiated complexes with three N-[6]helicenyl-NHC ligands were prepared with uncommon diastereoselectivities and their configurations were assigned using NMR analyses. They show strong ECD and OR, and yellow CP phosphorescence.

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.

Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs

The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The ¹H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.

Perylene Diimide-Embedded Double [8]Helicenes

The rational design and modification of the helix is of significance for fully promoting properties of configurationally stable materials for various applications in chiral science. Herein, a straightforward, sterically less demanding synthetic approach involving hybridization between two [6]helicene subunits and a perylene diimide (PDI) scaffold are presented, affording perylene diimide-embedded double [8]helicenes (PD8Hs) which represent the highest double carbohelicenes reported thus far. Due to the structural features of PDI and [6]helicene, the PD8Hs have six stereoisomers including two pairs of enantiomers and two mesomers. Such structural diversity is unprecedented in the realm of double helicenes. The absolute configuration of these PD8Hs was unambiguously confirmed by single-crystal X-ray diffraction analyses, revealing that the subtle configurational differences lead to great variation in the superhelical structure and molecular packing arrangement. Due to the embedding of the PDI chromophore, the PD8Hs possess outstanding fluorescence quantum yields of approximately 30%. Two pairs of enantiomers were resolved by chiral HPLC, and the chiroptical properties were evaluated using circular dichroism and circularly polarized luminescence spectroscopy, of which PD8H-6R exhibited excellent chiroptical performances in both the absorption and emission ranges with dissymmetry factors |g_abs| of 0.012 and |g_lum| of 0.002.

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.

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.