Chiral Fluorene Research Articles

Remote C─H Bond Activation via Enantioselective Carbopalladation and 1,4-Pd Migration Cascade Process.

Carbopalladation-initiated cascade reaction involving 1,4-Pd migration is a straightforward and powerful approach to activate remote C─H bond, forging versatile fused polycyclic compounds containing fluorene fragment which are highly valuable synthetic targets. However, its asymmetric variants pose considerable challenges and have not been explored. Here the first asymmetric palladium-catalyzed tandem carbopalladation is reported, 1,4-Pd migration reaction of ortho-iodophenol-derived allyl ether under mild conditions, allowing the transformation of a wide range of substrates in good to excellent enantioselectivities, and providing a facile and straight forward access to tetracyclic dihydroindeno[1,2,3-de]chromene bearing a chiral fluorene skeleton. A good functional group tolerance, high stereoselectivity, as well as the good chiroptical properties (high fluorescence quantum yields, circular dichroism) of the products make this approach highly attractive. Moreover, density functional theory (DFT) calculations indicate that the protonation of five-membered palladacycle intermediate is more favorable rather than its direct reductive elimination process.

Catalytic Asymmetric Synthesis of Chiral Fluorenes: Recent Developments and Future Perspectives

The chiral fluorene fragment constitutes one of the most important scaffolds frequently found in numerous bioactive molecules, ligands, and advanced functional materials due to its unique physical properties. In this context, various transition metal‐ and organo‐catalyzed asymmetric reactions have been developed to prepare chiral fluorenes with high optical purity. In the present mini‐review, we summarize fifteen years of achievements in the catalytic asymmetric synthesis of chiral fluorenes and their derivatives. Particular attention is given to the strategies used and mechanistic aspects of some significant reactions. Moreover, the cutting‐edge developments in the catalytic asymmetric synthesis of silafluorenes are also highlighted.

Construction of an Axially Chiral Fluorene Nitrile-Based Framework via Benzannulation of Indene Diene with Benzoylacetonitrile

The methodology for the synthesis of fluorene-based atropisomers was developed via the strategy of aromatic ring formation. By this strategy, an efficient benzannulation of indene-based diene with benzoylacetonitrile divergently promoted by DABCO and a chiral organocatalyst was established, and various atropisomeric fluorene-based skeletons were generated in good yields, which not only provide a new strategy for the construction of atropisomeric biaryls but also offer a new member to the atropisomeric family.

Modulating the Chiroptical Response of Chiral Polymers with Extended Conjugation within the Structural Building Blocks.

Advancing the emerging area of chiral photonics requires modeling-guided concepts of chiral material design to enhance optical activity and associated optical rotatory dispersion. Herein, we introduce conformational engineering achieved by tuning polymer backbone conjugation through introduction of thiophene structural units in a chiral fluorene polymer backbone. Our theoretical calculations reveal a relationship between the structural conformation and the resultant rotational strength. We further synthesize a series of chiral fluorene-based polymers copolymerized with thiophene whose optical chirality trend is in qualitative agreement with predictions of our quantum chemical calculations. Varying the number of thiophene units in the monomer building block allows us to modulate the rotational strength by tuning the intrafibril helicity of single-stranded polymer chains, whereby the monomer conjugation is retained throughout the whole length of the polymer backbone. Our design concept delineates an underexamined approach: the concept of tuning backbone conjugation and helicity within the main chain to enhance the optical activity of chiral polymer systems.

Correlation of electronic and vibrational properties with the chiro-optical activity of polyfluorene copolymers

The rationale of this paper is to shed some light on the origin of the optical response of two similar chiral fluorene copolymers in correlation with their vibrational modes, to understand how a chiral center placed in a ramification affects the optical properties of the main chain. Various spectroscopic ellipsometric techniques, in the scope of the Stokes theory were used to characterize the optical-vibrational behavior of the polyfluorenes: ellipsometry in emission (EE), transmission (TE), and Raman (ERS). The results showed that the optical activity and the emission of the circularly polarized light depends substantially on the interaction of the chiral carbon in the ramification and the main chain through specific optically active vibrational modes, for each sample. One interesting achievement was to find the absolute dextrorotatory configuration of the studied molecules, that could induce a helicoidal structure to the entire material.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π-Conjugated Polymers by Light.

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities.Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively.Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.

Circularly Polarized Light From a Series of Chiral Fluorene Copolymers

An innovative methodology for the study of the chiro-optical properties of chiral materials based in transmission ellipsometry is described, applied to fluorene thin films-based copolymers with high optical activity. The direct chirality evaluation parameter was possible. To the best of our knowledge, this is a first report of a complete experimental procedure for that determination. Four copolymers with systematic variation in chemical structure bearing chiral centers in side chains were used.

Intramolecular Oxidative Arylations in 7-Azaindoles and Pyrroles: Revamping the Synthesis of Fused N-Heterocycle Tethered Fluorenes.

We revealed intramolecular oxidative arylations in 7-azaindoles and pyrroles that, for the first time, provided direct access to 7-azaindole- or pyrrole-fused isoindolines and tetrahydroisoquinolines. In addition, N-benzylation of 7-azaindoles or pyrroles with sterically hindered sec-benzyl alcohols by Mitsunobu reaction followed by intramolecular oxidative arylation allowed access to chiral congeners of fused isoindolines that have little precedence. A new opportunity in the design and synthesis of fluorene-based organic emitters is demonstrated in the preparation of novel fused N-heterocycle tethered fluorenes, including a chiral fluorene architecture.

Circular Selective Reflection of Light Proving Cholesteric Ordering in Thin Layers of Chiral Fluorene Polymers

Thermally annealed and aligned thin films of polyfluorene and two other chiral fluorene copolymers preferentially reflect left-circular-polarized light with wavelengths just below the onset of the lowest-allowed π–π* optical transition. This provides evidence for a left-handed cholesteric arrangement of the polymer chains in the film.

Molecular Weight Optimum in the Mesoscopic Order of Chiral Fluorene (Co)polymer Films

A chiral fluorene homopolymer (PF) and two new chiral alternating polyfluorenes, poly(fluorene-alt-benzothiadiazole) (PFBT) and poly(fluorene-alt-dithienylbenzothiadiazole) (PFDTBT), have been synthesized by palladium-catalyzed Suzuki polycondensations. In these polymers, chirality was introduced by attaching (S)-3,7-dimethyloctyl substituents to the 9-positions of the fluorene monomers. Upon thermal annealing, PF and PFBT undergo an irreversible phase transition to a liquid crystalline state that upon cooling to room temperature is vitrified and exhibits considerable circular dichroic (CD) effects. By contrast, PFDTBT forms an amorphous glass with hardly any CD activity. After fractionation by preparative size exclusion chromatography (SEC), the phase behavior of the low polydispersity polymers was studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and CD spectroscopy. Remarkably, after fractionation also for PFDTBT a CD effect was observed and for all three materials ...

Preparation of Chiral Cholestanofluorene and Its Electron-Rich Derivatives for Isolation of a Stable Cation−Radical Salt

A simple and practical synthesis of a variety of chiral fluorene derivatives is described where a cholestane moiety is attached to the carbon 9 of the fluorene ring system from readily available starting materials. An appropriately substituted fluorene derivative (i.e., R = OMe) forms a highly colored (chiral) cation-radical that can be isolated as robust hexachloroantimonate salt. Interestingly, the simplest cholestanofluorene (i.e., R = H) can also be transformed into a dibromo derivative (i.e., R = Br), a precursor to the (poly)cholestanofluorenes where the cholestane moieties will serve not only as groups that impart chirality but also allow them to be soluble in common organic solvents. The details of these works are described.