Construction of quaternary carbon centers by a base-catalyzed enantioselective aldol reaction and related reactions of trimethoxysilyl enol ethers

The aldol reactions of trimethoxysilyl enol ethers catalyzed by lithium binaphtholate were found to be powerful tools for the construction of quaternary asymmetric carbon centers. The stereoselectivities were greatly affected by the presence of water. Trimethoxysilyl enol ether derived from a cyclic ketone, such as cyclohexanone, was used as a substrate to obtain the anti-adduct preferentially under anhydrous conditions; by contrast, the syn-adduct was preferentially obtained under aqueous conditions with high stereoselectivity. The aldol-Tishchenko reaction of a trimethoxysilyl enol ether derived from acyclic ketones proceeded to give monoacyl 1,3-diol derivatives in high enantioselectivities.     

CuI-Catalyzed Coupling Reactions of 4-Iodopyrazoles and Alcohols: Application toward Withasomnine and Homologs

The direct 4-alkoxylation of 4-iodo-1H-pyrazoles with alcohols was achieved by a CuI-catalyzed coupling protocol. The optimal reaction conditions employed excess alcohol and potassium t-butoxide (2 equiv) in the presence of CuI (20 mol%) and 3,4,7,8-tetramethyl-1,10-phenanthroline (20 mol%) at 130 °C for 1 h under microwave irradiation. The present method was efficiently applied to the synthesis of withasomnine and its six- and seven-membered cyclic homologs.     

X‐ray irradiation induces local rearrangement of silica particles in swollen rubber

X‐ray photon correlation spectroscopy (XPCS) of swollen rubber containing spherical silica nanoparticles is reported. It is shown that irradiation by intense X‐rays leads to the breakdown of cross‐links, thereby inducing the local rearrangement of silica nanoparticles. This rearrangement process depends on the cross‐link density and is characterized by a compressed exponential relaxation with aging behaviour, which resembles a common feature of complex fluids observed with XPCS.     

Liquid‐Crystalline Biomacromolecular Templates for the Formation of Oriented Thin‐Film Hybrids Composed of Ordered Chitin and Alkaline‐Earth Carbonate

Macroscopically ordered inorganic thin films have been formed on unidirectionally oriented, liquid‐crystalline chitin matrices. In the presence of poly(acrylic acid) (PAA), unidirectionally oriented chitin films act as templates for the formation of oriented thin‐film crystals of alkaline‐earth carbonates such as SrCO₃ and BaCO₃. The morphology and orientation of crystals are dependent on the metal ion concentration. For SrCO₃ crystallization, unidirectional thin films and hexagonal‐shaped thin films have been deposited from 200 and 25 mm concentration strontium solutions, respectively.     

A Flow Microreactor System Enables Organolithium Reactions without Protecting Alkoxycarbonyl Groups

A flow microreactor system consisting of micromixers and microtube reactors provides an effective tool for the generation and reactions of aryllithiums bearing an alkoxycarbonyl group at para‐, meta‐, and ortho‐positions. Alkyl p‐ and m‐lithiobenzoates were generated by the I/Li exchange reaction with PhLi. The Br/Li exchange reactions with sBuLi were unsuccessful. Subsequent reactions of the resulting aryllithiums with electrophiles gave the desired products in good yields. On the other hand, alkyl o‐lithiobenzoates were successfully generated by the Br/Li exchange reaction with sBuLi. Subsequent reactions with electrophiles gave the desired products in good yields.     

Sequential O- and N-acylation protocol for high-yield preparation and modification of rotaxanes: synthesis, functionalization, structure, and intercomponent interaction of rotaxanes

A pseudorotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type rotaxane was determined by the weak CH/pi interaction between the methylene group in crown ether and the benzene ring of the axle component. The N-acylation of ammonium-type rotaxane is useful for the preparation of both functionalized rotaxanes and weak intercomponent interaction-based rotaxanes.     

Control of dominant conduction orbitals by peripheral substituents in paddle-wheel diruthenium alkynyl molecular junctions

Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C₆H₄-NCHN-C₆H₄-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF₃) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10⁻⁵ to 10⁻⁴G₀ region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO–LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO–LUMO gaps.

Dominant conduction orbitals of paddle-wheel organodiruthenium complexes can be facilely controlled by the substituents embedded in the amidinato ligands.     

Olefin cross-metathesis on proteins: investigation of allylic chalcogen effects and guiding principles in metathesis partner selection

Olefin metathesis has recently emerged as a viable reaction for chemical protein modification. The scope and limitations of olefin metathesis in bioconjugation, however, remain unclear. Herein we report an assessment of various factors that contribute to productive cross-metathesis on protein substrates. Sterics, substrate scope, and linker selection are all considered. It was discovered during this investigation that allyl chalcogenides generally enhance the rate of alkene metathesis reactions. Allyl selenides were found to be exceptionally reactive olefin metathesis substrates, enabling a broad range of protein modifications not previously possible. The principles considered in this report are important not only for expanding the repertoire of bioconjugation but also for the application of olefin metathesis in general synthetic endeavors.