Multibridged Chiral Naphthalene Oligomers with Continuous Extreme-Cisoid Conformation

2,2'-Tethered Binaphthyl-Embedded One-Handed Helical Ladder Polymers: Impact of the Tether Length on Helical Geometry and Chiroptical Property

Synthetic breakthroughs diversify the molecules and polymers available to chemists. We now report the first successful synthesis of a series of optically-pure 2,2'-tethered binaphthyl-embedded helical ladder polymers based on quantitative and chemoselective ladderization by the modified alkyne benzannulations using the 4-alkoxy-2,6-dimethylphenylethynyl group as the alkyne source, inaccessible by the conventional approach lacking the 2,6-dimethyl substituents. Due to the defect-free helix formation, the circular dichroism signal increased by more than 6 times the previously reported value. The resulting helical secondary structure can be fine-tuned by controlling the binaphthyl dihedral angle in the repeating unit with variations in the 2,2'-alkylenedioxy tethering groups by one carbon atom at a time. The optimization of the helical ladder structures led to a strong circularly polarized luminescence with a high fluorescence quantum yield (28 %) and luminescence dissymmetry factor (2.6×10-3 ).

Chiral Binaphthalene Building Blocks for Self-Assembled Nanoscale CPL Emitters

The introduction of biuret hydrogen-bonding sites onto chiral binaphthalene-based chromophores was investigated as a route to sub-micron-sized, vesicle-like aggregates endowed with chiroptical properties. The synthesis was conducted from the corresponding chiral 4,4'-dibromo-1,1'-bis(2-naphthol) via Suzuki-Miyaura coupling to afford luminescent chromophores whose emission spectrum could be tuned from blue to yellow-green through extension of the conjugation. For all compounds, the spontaneous formation of hollow spheres with a diameter of ca. 200-800 nm was evidenced by scanning electron microscopy, along with strong asymmetry in the circularly polarized absorption spectra. For some compounds, the emission also displayed circular polarization with values of g_lum = ca. 10^-3 which could be increased upon aggregation.

Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10^-4 to -1.4 × 10^-3). Computational calculations were used to elucidate the changes in photophysical properties.

Solid-State and Theoretical Investigations of Some Banister-Type Macrocycles with 2,2'-Aldoxime-1,1'-Biphenyl Units

In the context of helical chirality, bridging of biphenyl units leads to banister-type compounds and the stability of the resulted atropisomers may increase dramatically if suitable changes are performed in the linker unit that coils around the biphenyl moiety. A rigorous density functional theory (DFT) study was conducted for macrocycles containing rigid oxime ether segments connected to the biphenyl backbone in order to determine how the rotation barriers are influenced by the presence of either a flexible oligoethyleneoxide or a more rigid m–xylylene component in the macrocycle. The calculated values for the racemization barrier were in good agreement with those obtained experimentally and confirm the benefit of introducing a more rigid unit in the macrocycle on the stability of atropisomers. Solid-state data were obtained and computed data were used to assess the contribution brought by supramolecular associations observed in the lattice to the stabilization of the crystal structure. Beside introducing rigidity in the linker, complexation of flexible macrocycles with alkali metal ions is also contributing to the stability of atropisomers, leading to values for the racemization barrier matching that of the rigid macrocycle. Using diethylammonium cation as guest for the macrocycle, a spectacular increase in the barrier to rotation was observed for the resulted pseudo[2]rotaxane.

Catalytic Dynamic Kinetic Resolutions in Tandem to Construct Two-Axis Terphenyl Atropisomers

The defined structure of molecules bearing multiple stereogenic axes is of increasing relevance to materials science, pharmaceuticals, and catalysis. However, catalytic enantioselective approaches to control multiple stereogenic axes remain synthetically challenging. We report the catalytic synthesis of two-axis terphenyl atropisomers, with complementary strategies to both chlorinated and brominated variants, formed with high diastereo- and enantioselectivity. The chemistry proceeds through a sequence of two distinct dynamic kinetic resolutions: first, an atroposelective ring opening of Bringmann-type lactones produces a product with one established axis of chirality, and second, a stereoselective arene halogenation delivers the product with the second axis of chirality established. In order to achieve these results, a class of Brønsted basic guanidinylated peptides, which catalyze an efficient atroposelective chlorination, is reported for the first time. In addition, a complementary bromination is reported, which also establishes the second stereogenic axis. These bromo-terphenyls are accessible following the discovery that chiral anion phase transfer catalysis by C₂-symmetric phosphoric acids allows catalyst control in the second stereochemistry-determining event. Accordingly, we established the fully catalyst-controlled stereodivergent synthesis of all possible chlorinated stereoisomers while also demonstrating diastereodivergence in the brominated variants, with significant levels of enantioselectivity in all cases.

Bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C-C stereogenic axes

We report the bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C–C stereogenic axes. These are controlled by a two-fold central-to-axial chirality conversion upon oxidative aromatization.

We report the bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C–C stereogenic axes obtained by a two-fold central-to-axial chirality conversion upon oxidative aromatization. The key enantioenriched centrally chiral bis-dihydrobenzofuran precursors were synthesized via a bidirectional diastereo- and enantio-selective organocatalyzed domino reaction between simple achiral and easily accessible dihydroxylated aromatics and chloronitroalkenes. Moreover, the stereodivergent nature of the methodology was established by synthesizing both diastereomers of a non-symmetrically functionalized bis-axially chiral oligoarene.

Synthesis and Photochemical Properties of Axially Chiral Bis(dinaphthofuran)

Both enantiomers of axially chiral bis(dinaphthofuran) were prepared in only two steps from 1'-hydroxy-4'-methoxy-2,2'-binaphthalenyl-1,4-dione, followed by optical resolution via high-performance liquid chromatography (HPLC) using a chiral stationary phase (CSP). The absolute configurations were determined by comparison of experimental and calculated vibrational circular dichroism (VCD) spectra. Synthetic bis(dinaphthofuran) exhibited a broad and unstructured emission derived from an intramolecular excimer in both solution and solid state. The methylene bridge brings both chromophores close to each other and induces significant changes in the photophysical behavior. Chiral bis(dinaphthofuran) displayed a bathochromic shift in emission, as compared to the racemic mixture in the solid state. Furthermore, the compounds showed high circularly polarized luminescence (CPL) with a dissymmetry factor ( glum) of 10-3 at 405 nm upon excitation at 265 nm in a methanol solution. This compound serves as a model for the design and synthesis of organic materials having CPL activity.

Catalyst-Controlled Stereodivergent Synthesis of Atropisomeric Multiaxis Systems

Molecular scaffolds with multiple rotationally restricted bonds allow a precise spatial positioning of functional groups. However, their synthesis requires methods addressing the configuration of each stereogenic axis. We report here a catalyst-stereocontrolled synthesis of atropisomeric multiaxis systems enabling divergence from the prevailing stereochemical reaction path. By using ion-pairing catalysts in arene-forming aldol condensations, a strong substrate-induced stereopreference can be overcome to provide structurally well-defined helical oligo-1,2-naphthylenes. The configuration of up to four stereogenic axes was individually catalyst-controlled, affording quinquenaphthalenes with a unique topology.

Intense excimer CPL of pyrenes linked to a quaternaphthyl

(R,R,R)-Quaternaphthyl possessing multiple pyrenes emits CPL with high Φ_FL and g_lum values both in solution and in the solid state.

Conformational and Optical Characteristics of Unidirectionally Twisted Binaphthyl-Bipyridyl Cyclic Dyads

An axially chiral binaphthyl-bipyridyl cyclic dyad in which the two units are connected by short -CH2O- linkers was synthesized. Experimental and theoretical analyses indicate that the (R)-binaphthyl unit in the dyad induces (R)-chirality in the bipyridyl unit, both in the solid state and in solution. It is shown that vibrational circular dichroism (VCD) is useful to determine the twisting pattern of 2,2'-bipyridyl compounds. The dyad shows crystallization-induced emission enhancement (CIEE).

Inducing axial chirality in a "Geländer" oligomer by length mismatch of the oligomer strands

Helical molecules are not only esthetically appealing due to their structural beauty, they also display unique physical properties as a result of their chirality. We describe herein a new approach to "Geländer" oligomers by interlinking two oligomer strands of different length. To compensate for the dimensional mismatch, the longer oligo(benzyl ether) oligomer wraps around the oligophenyl backbone. The new "Geländer" oligomer 1 was assembled in a sequence of functional-group transformations and cross-coupling steps followed by final cyclizations based on nucleophilic substitution reactions, and was fully characterized, including X-ray diffraction analysis. The isolation of pure enantiomers enabled the racemization process to be studied by circular dichroism spectroscopy.

Oligonaphthofurans: fan-shaped and three-dimensional π-compounds

Using a bottom-up method, we prepared a series of oligonaphthofurans composed of alternating naphthalene rings and furan rings. The largest compound (compound 25) contained 8 naphthalene units and 7 furan units. DFT calculations revealed that these compounds were fan-shaped molecules and each naphthalene ring was oriented in an alternate mountain-valley fold conformation because of steric repulsion by the hydrogens at the peri-positions. We investigated the optical properties that derived from their fan-shaped and mountain-valley sequences. As the number of aromatic rings of the oligonaphthofurans increased, the peaks of the longest wavelength absorptions in the UV-vis spectra (HOMO-LUMO energy gap) of these compounds steadily red-shifted, although the shapes of spectra were not sustained because of the decreasing molar absorption coefficients (ε's) of their λ(max). We compared our results with those reported for other types of oligoaromatic compounds such as acenes 1, ethene-bridged p-phenylenes 2, rylenes 3, oligofurans 4, and oligonaphthalenes 5. The slopes of the plots between the transition energies (HOMO-LUMO energy gap) of the oligoaromatic compounds and the reciprocal of the number of aromatic rings indicated that the efficiency of π conjugation of the oligonaphthofurans was comparable with that of linear and rigid acenes and rylenes. The higher-order compounds 22 and 25 aggregated even under high dilution conditions (~10(-6) M).