Host–Guest Complexation Using Pillar[5]arene Crystals: Crystal-Structure Dependent Uptake,Release, and Molecular Dynamics of an Alkane Guest

Host–guest complexation has been mainly investigated in solution, and it is unclear how guest molecules access the assembled structures of host and dynamics of guest molecules in the crystal state. In this study, we studied the uptake, release, and molecular dynamics of n-hexane vapor in the crystal state of pillar[5]arenes bearing different substituents. Pillar[5]arene bearing 10 ethyl groups yielded a crystal structure of herringbone-type 1:1 complexes with n-hexane, whereas pillar[5]arene with 10 allyl groups formed 1:1 complexes featuring a one-dimensional (1D) channel structure. For pillar[5]arene bearing 10 benzyl groups, one molecule of n-hexane was located in the cavity of pillar[5]arene, and another n-hexane molecule was located outside of the cavity between two pillar[5]arenes. The substituent-dependent differences in molecular arrangement influenced the uptake, release, and molecular dynamics of the n-hexane guest. The substituent effects were not observed in host–guest chemistry in solution, and these features are unique for the crystal state host–guest chemistry of pillar[5]arenes.     

High‐Yield Diastereoselective Synthesis of Planar Chiral [2]‐ and [3]Rotaxanes Constructed from per‐Ethylated Pillar[5]arene and Pyridinium Derivatives

Planar chiral [2]‐ and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per‐ethylated pillar[5]arene ( C2 ) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)‐ C2 or (pR, pR, pR, pR, pR)‐ C2 wheels was accomplished. Furthermore, pillar[5]arene‐based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane 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 [3]rotaxane 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 pillar[5]arene‐based interlocked molecules.     

Ionic liquid pillar[5]arene: its ionic conductivity and solvent-free complexation with a guest

A room-temperature ionic liquid containing macrocyclic compound pillar[5]arene in its core was synthesized. The ionic liquid showed high thermal stability, moderate ionic conductivity and solvent-free complexation ability with the guest tetracyanoethylene.     

Nickel-catalyzed dehydrogenative [4 + 2] cycloaddition of 1,3-dienes with nitriles

Pyridines, which comprise one of the most important classes of the six-membered heterocyclic compounds, are widely distributed in nature, and the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction of two alkynes and a nitrile is one of the most powerful methods for preparing versatile, highly substituted pyridine derivatives. However, the lack of chemo- and regioselectivity is still a crucial issue associated with fully intermolecular [2 + 2 + 2] cycloaddition. The present study developed the Ni(0)-catalyzed intermolecular dehydrogenative [4 + 2] cycloaddition reaction of 1,3-butadienes with nitriles to give a variety of pyridines regioselectively.     

Pre-edge features of Ti K-edge X-ray absorption near-edge structure for the local structure of sol-gel titanium oxides.

Titanium K-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to examine the local Ti environments in the sol, gel, and xerogels of titanium oxide prepared by a sol-gel method. The xerogels were prepared by heat treatment at 200, 300, 400, 500, and 600 degrees C (denoted xero-200, xero-300, xero-400, xero-500, and xero-600), and the xerogels were doped with Fe(II), Ni(II), and Cu(II) ions (xero-Fe, xero-Ni, and xero-Cu). The local structures of the samples are compared using the pre- and post-edge features of the XANES spectra. Further analysis of the pre-edge features using Gaussian functions provides the energy and height of each peak, and the intensity ratios between peaks. Based on the resulting data, the samples can be classified into three groups: a weak Ti-Ti interaction group, an anatase-like structure group, and an anatase group. The sol, gel, xerogel, and xero-Ni are in the weak Ti-Ti interaction group, and the xero-200, xero-Fe, and xero-Cu are in the anatase-like structure group. The remaining samples are in the anatase group.     

Sequential Chiral Induction and Regulator-Assisted Chiral Memory of Pillar[5]arenes

Chirality transfer is widely observed in life processes, and many artificial chiral transfer systems have been developed. In these systems, chiral information is transferred from chiral inducers to chiral acceptors by a direct chiral induction process and a direct chiral memorization process. We have developed ternary nondirect chiral transfer systems based on pillar[5]arenes, in which a third factor was introduced as a regulator. The planar chirality of an acceptor was induced and memorized by a chiral inducer with precise control by a regulator. In the chiral induction period, the feed sequence of the chiral inducer and regulator affected the chiral induction behavior of the chiral acceptor. The chiral memory ability of the acceptor was substantially improved by the combined action of the chiral inducer and regulator.     

Nickel‐Catalyzed Synthesis of N‐Aryl‐1,2‐dihydropyridines by [2+2+2] Cycloaddition of Imines with Alkynes through T‐Shaped 14‐Electron Aza‐Nickelacycle Key Intermediates

Despite there being a straightforward approach for the synthesis of 1,2‐dihydropyridines, the transition‐metal‐catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N‐sulfonyl or ‐pyridyl group. Considering the importance of 1,2‐dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel‐catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy₃ catalyst, a reaction with N‐benzylidene‐P,P‐diphenylphosphinic amide was developed. Moreover, an application of N‐aryl imines to the reaction was also achieved by adopting N‐heterocyclic carbene ligands. The isolation of an (η²‐N‐aryl imine)nickel(0) complex containing a 14‐electron nickel(0) center and a T‐shaped 14‐electron five‐membered aza‐nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X‐ray analyses.