A Lewis base catalyst Trip-SMe (Trip = triptycenyl) for electrophilic aromatic halogenation using N-halosuccinimides (NXS) is introduced. In the presence of an appropriate activator (as a ...noncoordinating-anion source), a series of unactivated aromatic compounds were halogenated at ambient temperature using NXS. This catalytic system was applicable to transformations that are currently unachievable except for the use of Br2 or Cl2: e.g., multihalogenation of naphthalene, regioselective bromination of BINOL, etc. Controlled experiments revealed that the triptycenyl substituent exerts a crucial role for the catalytic activity, and kinetic experiments implied the occurrence of a sulfonium salt Trip-S(Me)BrSbF6 as an active species. Compared to simple dialkyl sulfides, Trip-SMe exhibited a significant charge-separated ion pair character within the halonium complex whose structural information was obtained by the single-crystal X-ray analysis. A preliminary computational study disclosed that the π system of the triptycenyl functionality is a key motif to consolidate the enhancement of electrophilicity.
•The reduction and lithiation of pillar6quinone were examined using DFT calculations.•The calculated lithiation potential trend corresponded to the voltammogram waveform.•Intermolecular lithium ...coordination of pillar6quinone was indicated.
The new redox-active hexagonal macrocycle pillar6quinone (PQ6) exhibits unique electrochemical reduction behavior. This paper describes the electrochemical redox properties of PQ6 dissolved in an aprotic solvent in the presence of Li+ ions using density functional theory calculations. These calculations show that the multi-electron reduction behavior of PQ6 is determined by the electrostatic repulsion between quinone units, in agreement with the results of voltammetric measurements. The lithiation process can be divided into two stages, comprising bridging of adjacent carbonyl groups by Li+ ions followed by the formation of coordination bonds outside the macrocycle. The trend exhibited by calculated lithiation potentials was found to correspond to the voltammogram waveform. Preliminary calculations of lithiation behavior involving two macrocycles to confirm intermolecular lithium coordination are also presented herein. The results of this work are expected to provide a better understanding of the electrochemical properties of PQ6 and facilitate the application of this compound in redox materials.
The development of a robust amide‐bond formation remains a critical aspect of N‐methylated peptide synthesis. In this study, we synthesized a variety of dipeptides in high yields, without severe ...racemization, from equivalent amounts of amino acids. Highly reactive N‐methylimidazolium cation species were generated in situ to accelerate the amidation. The key to success was the addition of a strong Brønsted acid. The developed amidation enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation. In addition, the amidation can be performed by using either a microflow reactor or a conventional flask. The first total synthesis of naturally occurring bulky N‐methylated peptides, pterulamides I–IV, was achieved. Based on experimental results and theoretical calculations, we speculated that a Brønsted acid would accelerate the rate‐limiting generation of acyl imidazolium cations from mixed carbonic anhydrides.
A variety of N‐methylated peptides were synthesized in high yield without severe racemization via the generation of acylN‐methylimidazolium cations. Brønsted acids dramatically accelerated the reaction. The developed amidation reaction enabled the synthesis of a bulky peptide in higher yield and shorter reaction time in comparison with conventional amidation reactions. The first total synthesis of pterulamides I–IV was also achieved.
All-atom molecular dynamics simulations were performed on 4-heptyl-4′-cyanobiphenyl (7CB) to study the mechanism of heat conduction in this nematic liquid crystal atomistically. To describe 7CB ...properly, the AMBER-type force field was optimized for the dihedral parameter of biphenyl and the Lennard-Jones parameters. The molecular dynamics calculation using the optimized force field well reproduced the experimental values of the isotropic-nematic phase transition temperature, density, and anisotropy of the thermal conductivity. Furthermore, the contributions of convection, intramolecular interaction, and intermolecular interaction to the thermal conductivity were determined by performing thermal conductivity decomposition analysis. According to the analysis, the contributions of convection, bond stretching, and bond bending interactions were higher in the direction parallel to the nematic director than that perpendicular to the director, which is the origin of the anisotropy in the nematic phase. This result indicates that the anisotropy is caused by well-aligned covalent bonds and high mobility parallel to the director. This quantitative description of the mechanism of heat conduction of 7CB is foreseen to provide new insights toward designing highly thermally conductive liquid-crystalline materials.
The acid dissociation constant of three benzimidazoles, namely 2,2'-bibenzodimidazole, 2,5'-bibenzodimidazole, and 5,5'-bibenzodimidazole, have been investigated by means of density functional theory ...calculations in gas phase and in aqueous solution. The theoretical approach was validated by the comparing of predicted and experimentally determined p
values in imidazole, benzimidazole, and 2-phenylbenzimidazole. From the studied compounds, 2,2'-bibenzodimidazole was found to be the most acidic, which made it a valuable candidate as a material for polymer electrolyte membrane fuel cells.
Mechanisms of the unimolecular and bimolecular isomerization of nitrile N‐oxide to isocyanate were investigated by quantum chemical calculations using the global reaction route mapping (GRRM) ...technique, which consists of the anharmonic downward distortion following (ADDF) and multi‐component artificial force induced reaction (AFIR) methods, which can exhaustively search reaction paths. In the unimolecular isomerization, isomerization paths of nitrile N‐oxide to isocyanate were exhaustively explored by the ADDF method. The energetically lowest path for 2,6‐dimethoxybenzonitrile oxide (Ar–CNO) isomerization was via oxazirine intermediate formation, which was the rate‐determining step with an activation free energy of 56.8 kcal mol–1. The high activation free energy indicates that the isomerization hardly proceeds as a thermal unimolecular reaction. Next, the bimolecular isomerization paths of Ar–CNO were explored for diradical addition, 3+2 and 3+3 cycloadditions, and self‐catalyzed isomerization through the AFIR method. The diradical addition was the energetically lowest path with an activation free energy of 55.4 kcal mol–1. Although the bimolecular isomerization was energetically favorable than the unimolecular isomerization, its activation free energy was still high for the reaction to proceed under thermal conditions. Exhaustive path searching for these unimolecular and bimolecular isomerizations suggest that these reactions are catalyzed by acid or base.
Unimolecular isomerization pathways of nitrile N‐oxides to isocyanates were exhaustively explored by the DFT calculation through the global reaction route mapping (GRRM) technique. Furthermore, bimolecular isomerization pathways were manually searched; the exhaustivity of the pathway search was confirmed by the GRRM technique.
The synthesis of alternating donor–acceptor 12 and 16cycloparaphenylenes (CPPs) has been achieved by the rhodium‐catalyzed intermolecular cross‐cyclotrimerization followed by imidation and/or ...aromatization. These alternating donor–acceptor CPPs showed positive solvatofluorochromic properties and smaller HOMO–LUMO gaps compared with nonfunctionalized CPPs, which was confirmed by the theoretical study.
Catherine wheels: The synthesis of alternating donor–acceptor 12 and 16cycloparaphenylenes (CPPs) has been achieved by the rhodium‐catalyzed intermolecular cross‐cyclotrimerization followed by imidation and/or aromatization. The alternating donor–acceptor CPPs showed positive solvatofluorochromic properties and smaller HOMO–LUMO gaps compared with nonfunctionalized CPPs, which was confirmed by the theoretical study.
It is shown that a cationic rhodium(I)/biphep complex catalyzes the cycloisomerization of 2‐(alkynylsilylethynyl)phenols, leading to alkynylmethylidene‐benzoxasiloles through concomitant silicon and ...carbon migration. This unprecedented cycloisomerization presumably proceeds via the formation of rhodium vinylidenes through 1,2‐silicon migration, followed by 1,3‐carbon (alkyne) migration via the formation of hypervalent silicon centers.
An unexpected journey: It is demonstrated that a cationic rhodium(I)/biphep complex catalyzes the cycloisomerization of 2‐(alkynylsilylethynyl)phenols, leading to alkynylmethylidene‐benzoxasiloles through concomitant silicon and carbon migration. This unprecedented cycloisomerization presumably proceeds via the formation of rhodium vinylidenes through 1,2‐silicon migration, followed by 1,3‐carbon (alkyne) migration via the formation of hypervalent silicon centers.
Planar chiral 2‐ and 3rotaxanes constructed from pillar5arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was ...diastereoselective; the obtained 2rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per‐ethylated pillar5arene (C2) wheel, and other possible types of the 2rotaxane did not form. Isolation of the enantiopure 2rotaxanes with one axle through (pS, pS, pS, pS, pS)‐C2 or (pR, pR, pR, pR, pR)‐C2 wheels was accomplished. Furthermore, pillar5arene‐based 3rotaxane was successfully synthesized by attachment of two pseudo 2rotaxanes onto a bifunctional linker. 3Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)‐C2, one (pS, pS, pS, pS, pS)‐C2 and one (pR, pR, pR, pR, pR)‐C2 (meso form), or two (pR, pR, pR, pR, pR)‐C2 wheels. The 3rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar5arene‐based interlocked molecules.
Wheels in motion: Planar chiral 2‐ and 3rotaxanes constructed from pillar5arenes as wheels and pyridinium derivatives as axles (see figure) were obtained in high yield using click reactions. The procedure developed in this study is a starting point for the creation of pillar5arene‐based interlocked molecules.
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