Catalytic Asymmetric Synthesis of Atropisomers Bearing Multiple Chiral Elements: An Emerging Field

With the rapid development of asymmetric catalysis, the demand for the enantioselective synthesis of complex and diverse molecules with different chiral elements is increasing. Owing to the unique features of atropisomerism, the catalytic asymmetric synthesis of atropisomers has attracted a considerable interest from the chemical science community. In particular, introducing additional chiral elements, such as carbon centered chirality, heteroatomic chirality, planar chirality, and helical chirality, into atropisomers provides an opportunity to incorporate new properties into axially chiral compounds, thus expanding the potential applications of atropisomers. Thus, it is important to perform catalytic asymmetric transformations to synthesize atropisomers bearing multiple chiral elements. In spite of challenges in such transformations, in recent years, chemists have devised powerful strategies under asymmetric organocatalysis or metal catalysis, synthesizing a wide range of enantioenriched atropisomers bearing multiple chiral elements. Therefore, the catalytic asymmetric synthesis of atropisomers bearing multiple chiral elements has become an emerging field. This review summarizes the rapid progress in this field and indicates challenges, thereby promoting this field to a new horizon.

Planar Chiral Multiple Resonance Thermally Activated Delayed Fluorescence Materials for Efficient Circularly Polarized Electroluminescence

Chiral boron/nitrogen doped multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for highly efficient and color-pure circularly polarized organic light-emitting diodes (CP-OLEDs). Herein, we report two pairs of MR-TADF materials (Czp-tBuCzB, Czp-POAB) based on planar chiral paracyclophane with photoluminescence quantum yields of up to 98 %. The enantiomers showed symmetric circularly polarized photoluminescence spectra with dissymmetry factors |g_PL | of up to 1.6×10^-3 in doped films. Meanwhile, the sky-blue CP-OLEDs with (R/S)-Czp-tBuCzB showed an external quantum efficiency of 32.1 % with the narrowest full-width at half-maximum of 24 nm among the reported CP-OLEDs, while the devices with (R/S)-Czp-POAB displayed the first nearly pure green CP electroluminescence with |g_EL | factors at the 10^-3 level. These results demonstrate the incorporation of planar chirality into MR-TADF emitter is a reliable strategy for constructing of efficient CP-OLEDs.

Nitrenoid from Oxime: A Practical Synthesis of Planar Chiral Ferrocenyl Phenanthridines via Nitrene-Involved Ring-Expansion Reaction

Nitrenes and nitrenoids are highly reactive species and the proposed key intermediates in nitrogen-containing heterocyclic compound synthesis. In this work, we developed a practical method for the synthesis of phenanthridines by the reaction of oximes and Grignard reagents (with or without diethylzinc) via ring-expansion of magnesium coordinated nitrenoid complex as the key step. The method has been used to synthesize optically active planar chiral ferrocenyl phenanthridines.

Spectroscopic investigation on 1,2-substituted ferrocenes with only planar chirality: How chiroptical data are related to absolute configuration and to substituents

Single enantiomers of three 1,2-substituted ferrocene derivatives, i.e. 1-methoxymethyl-2-hydroxymethylferrocene (1), 1-formyl-2-hydroxymethylferrocene (2) and 1-iodo-2-hydroxymethylferrocene (3), sharing the common hydroxymethyl substituent and the presence of planar chirality only, were investigated for their spectroscopic (IR and UV) and chiroptical (VCD and ECD) properties. Both enantiomers of 1 were obtained for the first time in optically pure form by lipase-catalyzed kinetic resolution of the corresponding racemate (±)-1 and separately converted into formyl derivatives (+)-2 and (-)-2. The experimental spectroscopic and chiroptical data were compared with DFT calculated spectra and excellent correspondence was found for all compounds, allowing one to confirm the previously assigned absolute configurations. The common features in the VCD spectra of a doublet between 940 and 965 cm^-1 and the short-wavelength (about 200 nm) doublet and the longest wavelength band in the ECD spectra were analyzed to test whether they may be taken as markers of the absolute configuration (AC). The predominance of conformers with intramolecular hydrogen bond for the first two investigated compounds is predicted by conformational analysis and also confirmed by NMR.