Asymmetric electrosynthesis: Recent advances in catalytic transformations

Asymmetric synthesis is one of the most important and valuable research fields in modern organic chemistry. Since the use of electricity as a traceless oxidant or reductant under electrochemical conditions, highly reactive intermediates can selectively generate under mild reaction conditions through more environmentally benign conditions compared with the reactions using common chemical oxidants or reductants. Thus, the merging electrochemistry with asymmetric catalysis would provide a unique and powerful approach for the synthesis of optically active compounds under oxidative or reductive conditions. Selected recent (2018–2020) examples of enantioselective electro-organic synthesis using transition metal catalysts or organocatalysts are summarized in this short review. These examples are divided to oxidative and reductive transformations and described together with brief reaction mechanisms.     

Synthesis of nonplanar bipyridyls bridged by disilane and disiloxane and their phosphorescent copper complexes

Disilane- and disiloxane-bridged bipyridyls ( DSBPy and DSOBPy ) were prepared and their optical properties were investigated in comparison with those of previously reported monosilane- and monogermane-bridged counterparts. The UV–visible absorption and photoluminescence bands of DSBPy and DSOBPy were blue-shifted as a result of elongation of the bridging units from monosilane and monogermane to disilane and disiloxane, likely due to the enhanced twisting of the bipyridyl units. Phosphorescent complexes DSBPy–Cu and DSOBPy–Cu were prepared by the interaction of DSBPy and DSOBPy with Cu₂I₂(PPh₃)₂. X-ray diffraction studies of their single-crystal structures revealed polymeric structures composed of repeat units of DSBPy or DSOBPy and [Cu^II(PPh₃)]₂. Organic light-emitting diodes with the ITO/PEDOT:PSS/ DSBPy–Cu or DSOBPy–Cu :PCTSQ/TAZ/Al structure were fabricated to examine the applications of the complexes as electroluminescent materials. The devices emitted yellow light with emission maxima at approximately 600 nm, and maximal luminance reached 120 and 190 cd m⁻² for devices based on DSBPy–Cu and DSOBPy–Cu , respectively. The performance of the DSOBPy–Cu -based device was improved by using TAZ as the dopant of the emissive layer, and luminance was increased to 390 cd m⁻².     

Preparation of polymethyl methacrylate with well-controlled stereoregularity by anionic polymerization in an ionic liquid solvent

This study investigates the effect of ionic liquids (ILs) on the anionic polymerization of methyl methacrylate (MMA). Polymethyl methacrylate (PMMA), an isotactic polymer, is prepared by anionic polymerization at a high reaction temperature with an IL that acts as both solvent and additive. The most plausible reaction mechanism is determined using ¹H NMR and Fourier-transform infrared spectroscopy. The electrostatic interaction between MMA and the IL increases the apparent steric hindrance in MMA, resulting in the isotactic PMMA.     

A Compact Representation for Modular Semilattices and Its Applications

Order - A modular semilattice is a semilattice generalization of a modular lattice. We establish a Birkhoff-type representation theorem for modular semilattices, which says that every modular...     

Synthesis of (Z)-β-(Carbonylamino)alkenylindium through Regioselective anti-Carboindation of Ynamides and Its Transformation to Multisubstituted Enamides

The regioselective anti-carboindation of ynamides by using InBr₃ and silylated nucleophiles was developed to synthesize (Z)-β-(carbonylamino)alkenylindiums. The X-ray crystallographic analysis of an alkenylindium suggested that the reaction proceeded in an anti-addition fashion. In contrast to reported syn-carbometalations of ynamides by using organometallics, a cooperation of InBr₃ and silylated nucleophiles to ynamides achieved an anti-addition, which was supported by DFT calculations. The scope of substrates included various ynamides and silylated nucleophiles, such as silyl ketene acetals and silyl ketene imines. The transformation of synthesized alkenylindiums by iodination, radical coupling, and Pd-catalyzed cross-coupling successfully afforded trisubstituted enamines with high regio- and stereoselectivities.     

A Light-Harvesting/Charge-Separation Model with Energy Gradient Made of Assemblies of meta-Pyridyl Zinc Porphyrins

Self-assembly of porphyrins is a fascinating topic, not only for mimicking chlorophyll assemblies in photosynthetic organisms, but also for the potential of creating molecular-level devices. Herein, zinc porphyrin derivatives bearing a meta-pyridyl group at the meso position were prepared and their assemblies studied in chloroform. Among the porphyrins studied, one with a carbamoylpyridyl moiety gave a distinct ¹H NMR spectrum in CDCl₃, which allowed the supramolecular structure in solution to be probed in detail. Ring-current-induced chemical-shift changes in the ¹H NMR spectrum, together with vapor-pressure osmometry and diffusion-ordered NMR spectroscopy, among other evidence, suggested that the porphyrin molecules form a trimer with a triangular cone structure. Incorporation of a directly linked porphyrin–ferrocene dyad with the same assembling properties in the assemblies led to a rare example of a light-harvesting/charge-separation system in which an energy gradient is incorporated and reductive quenching occurs.     

Syntheses of Key Monomers for Advanced Polymer Materials Using Cyclodextrin as Catalyst

Terephthalic acid was synthesized by the carboxylation of benzoic acid with carbon tetrachloride in aqueous sodium hydroxide solution in the presence of cyclodextrin (CyD) and copper powder as catalyst. By the use of β-CyD at the initial molar ratio to benzoic acid of 0.5, the carboxylation at 60 °C for 7 hours produced terephthalic acid in 75 mol% yield with 87% selectivity. The selective synthesis of 4,4'-biphenyldicarboxylic acid in 70 mol% yield was achieved by the carboxylation of 4-biphenylcarboxylic acid with carbon tetrachloride in the presence of β-Cyd under similar conditions. The carboxylation of 2-naphthalene carboxylic acid with carbon tetrachloride using β-Cyd under similar conditions produced 2,6-naphthalenedicarboxylic acid in 67 mol% yield with 84% selectivity. When α-CyD and γ-CyD were used in place of β-Cyd, both the yields and the selectivities of the dicarboxylic acids were markedly small. In the absence of CyD, carboxylation did not proceed. Inclusion complex formations between β-Cyd and aromatic monocarboxylic acids were indicated by the ¹H chemical shifts of the β-Cyd. The reaction mechanism was discussed on the basis of inclusion complex formation.     

Macromolecule-Metal Complexes for Gas Separation

Macromolecule-copper(I) chloride complexes have been prepared for the separation of carbon monoxide and ethylene from gas mixtures with hydrogen, nitrogen, carbon dioxide, methane, and water. A toluene solution of a polystyrene-aluminum copper(I) chloride complex can separate carbon monoxide selectively and retains this function even on contact with gas containing water. Strong charge-transfer absorption bands have been found in the 380-500 nm region for the toluene solution of polystyrene-aluminum copper(I) chloride complex. A solution of 1, 3-diphenylpropane-aluminum copper(I) chloride complex also exhibits similar absorption bands. A continuous variation plot using the chemical shift change in ¹³C-NMR shows a 1:1 interaction between 1,3-diphenylpropane and aluminum copper(I) chloride. These results suggest a two-way interaction between the adjacent phenyl groups of polystyrene and aluminum copper(I) chloride. A resin bead of crosslinked polystyrene-aluminum copper(I) chloride complex has been prepared as a solid adsorbent. The water resistance of the solid macromolecular complex depends on the nature of the solvent used in the preparation of the solid adsorbent. Carbon disulfide is a suitable solvent. A selective adsorbent of ethylene has been prepared from a macroreticular polystyrene resin with primary and secondary amino groups and copper(I) chloride. The selectivity of ethylene against ethane and that of carbon monoxide against carbon dioxide increase with an increasing amount of supported copper(I) chloride.