Super Brønsted acid catalysis

Br?nsted acid catalysis has emerged as a new class of catalysis in modern organic synthesis. However, in order to make the utility of the Br?nsted acid catalysis as broad as the well-developed Lewis acid catalysis, it is desirable to develop Br?nsted acids demonstrating both high reactivities and selectivities. In this feature article, we will present our achievement in the design and development of strong Br?nsted acids and their application to organic reactions. Furthermore, we will describe the Tf(2)NH-catalyzed Mukaiyama aldol reaction of super silyl enol ethers. We also will highlight the differences in reactivity and chemo- and stereo-selectivity between Br?nsted and Lewis acid catalysis.     

Radical Copolymerization of Methyl Methacrylate with Styrene in Several Solvents

Radical copolymerization of styrene (M₁) with methyl methacrylate has been carried out in several solvents. The monomer reactivity ratio (r) was affected by the solvent. The values of log 1/r, Q₂, and e₂ were found to increase with the decreases of the V _C[dbnd]O and v _n[dbnd]c stretcning frequencies determined in the corresponding solvents.     

Kinetics of the Photopolymerization of Vinyl Monomers by Bis(Isopropylxanthogen) Disulfide. Design of Block Copolymers

Bis(isopropylxanthogen) disulfide (BX) has been used as a photoinitiator with various vinyl monomers at 30°C. The kinetics of polymerization of styrene (St) and methyl methacrylate (MMA) at 30°C were studied for various concentrations of monomer and initiator. The observed deviations in polymerization rate from simple kinetic theory could be explained in terms of primary radical termination. The fraction of primary radical terminating chains was obtained as a function of various concentrations. The ratio of the rate constants for chain initiation and chain termination by a primary radical was determined to be 3.34 ± 10⁷ for St and 2.60 ± 10⁷ for MMA. The number-average degree of polymerization (DP _n) of polymers obtained by photopolym-erization with BX was found to increase linearly with conversion. However, the DP _n extrapolated to zero conversion was in good agreement with that calculated on the basis of the kinetic scheme. It was found that BX had interesting properties for the design of block copolymers, i.e., BX acts as a terminator and a chain transfer agent as well as an initiator in these polymerizations. The polymers obtained with BX contained two reactive isopropyl xanthate groups bonded at their chain ends, which could also act as macrophotoinitiators.     

Dinuclear nickel complexes of phenolbased dinucleating macrocycles: Synthesis,structure, and properties

Abstract

Dinuclear Ni₂(II,II) complexes with the formula [Ni₂(R^m,n)](ClO₄)₂ ((m,n)= (2,2) (1), (2,3) (2), (2,4) (3)) have been obtained where (R^m,n)^2- denotes the macrocycles containing two 2,6-bis(iminomethyl)-4-methylphenolate entities combined through two lateral chains, -(CH₂)_m- and -(CH₂)_n-, at the imino nitrogens. [Ni₂(R^2,2)](ClO₄)₂ (1) crystallizes in the triclinic crystal system, space group P 1, with Z=1, a=8.396(2) Å, b= 10.021(2) Å, c=8.104(2) Å, α=109.56(2)°, β=99.40(2)°, γ=79.89(2)°, V=628.5(3) ų and Z=1. The refinement converges with R=0.0384 and R_w=0.0415 for 2075 reflections with | Fo | > 3[sgrave](| Fo |). In the centrosymmetric [Ni₂(R^2,2)]²⁺, a pair of Ni(II) ions are bridged by two phenolic oxygens with the Ni···Ni separation of 2.801(1) Å. Each Ni assumes a planar configuration with Ni-O bond distances of 1.842(3) and 1.838(3) Å and Ni-N bond distances of 1.814(3) and 1.823(3) Å. In the solid state, 1 is diamagnetic (S₁=S₂=0) whereas [Ni₂(R^2,3)]-(ClO₄)₂ (2) and [Ni₂(R^2,4)](ClO₄)₂ (3) are of a mixed-spin (S₁=0, S₂=1). In DMSO and pyridine all the complexes assume high-spin (S₁=S₂=1). The Ni₂(II,II) complexes are electrochemically reduced in DMSO or pyridine to Ni₂(I,II) and Ni₂(I,I) complexes. The conproportionation constants of the Ni₂(I,II) complexes are determined to be 3.4X10⁴-1.2X10⁵ in DMSO and 1.6X10³-2.6X10⁵ in pyridine. The Ni₂(I,II) and Ni₂(I,I) complexes of 1–3 have been prepared by electrolysis in DMSO. The mixed-valent complexes of 1 and 2 are characterized by an intervalence (IV) transition band at 790 and ～ 700 nm, respectively, and belong to Class II using the classification of Robin and Day. The Ni₂(I,II) complex of 3 shows no IT band (Class I). The Ni₂(I,II) complexes of 1-3 show well-resolved ESR spectra due to the spin-coupled S_T = 1/2 ground-state. The Ni₂(I,I) complexes of 1-3 are all ESR- innocent probably due to the strong antiferromagnetic interaction.     

A Brønsted acid catalyst for the enantioselective protonation reaction

A highly reactive and robust chiral Br?nsted acid catalyst, chiral N-triflyl thiophosphoramide, was developed. The first metal-free Br?nsted acid catalyzed enantioselective protonation reaction of silyl enol ethers was demonstrated using this chiral Br?nsted acid catalyst. The catalyst loading could be reduced to 0.05 mol % without any deleterious effect on the enantioselectivity.     

Absorption-based highly sensitive and reproducible biochemical oxygen demand measurement method for seawater using salt-tolerant yeast Saccharomyces cerevisiae ARIF KD-003

Salt-tolerant yeast Saccharomyces cerevisiae ARIF KD-003 was applied to highly sensitive and reproducible absorbance-based biochemical oxygen demand (BOD_AB-ScII) measurement for seawater. In the previous work, we have studied the BOD_AB-ScI method using normal Baker's yeast S. cerevisiae, and the excellent feature of the Baker's yeast as uniformly sustainable in solution could successfully be utilized. However, the BOD_AB-ScI responses were disappeared by the existence of chloride ion as well as seawater. In the present method, uniformity in solution was also observed with S. cerevisiae ARIF KD-003, and salt-tolerance of the yeast was observed even in saturate concentration of sodium chloride. Next, characterizations of the influences of pH and incubation temperature were investigated. After optimum conditions were obtained, two calibration curves were made between 0.33 and 22 mg O₂ L⁻¹ BOD using standard solution of glucose glutamic acid (GGA) or mixture of GGA and artificial seawater. Then, excellent reproducibility as the averages of relative standard deviation (R.S.D._av) in two calibration curves (nine points each) was successfully obtained at 1.10% at pure water or 1.03% at artificial seawater standard, respectively. In addition, the 3σ lower detection limit was calculated to be 0.07 mg O₂ L⁻¹ BOD, and 0.11 mg O₂ L⁻¹ BOD was experimentally detected by increase of the sample volume at 1.5-folds. The storage stability of the S. cerevisiae ARIF KD-003 was obtained at least 4 weeks.     

Bioactive constituents from Chinese natural medicines. XXVIII. Chemical structures of acyclic alcohol glycosides from the roots of Rhodiola crenulata

Five new glycosides, creosides I, II, III, IV, and V, were isolated from the methanolic extract of the roots of Rhodiola crenulata, together with 21 known compounds. The chemical structures of new constituents were elucidated on the basis of chemical and physicochemical evidence.     

Medicinal flowers. XXI. Structures of perennisaponins A, B, C, D, E, and F, acylated oleanane-type triterpene oligoglycosides, from the flowers of Bellis perennis

Six new acylated oleanane-type triterpene oligoglycosides, perennisaponins A (1), B (2), C (3), D (4), E (5), and F (6), were isolated from the flowers of Bellis perennis (Daisy flower) together with 14 saponins, nine flavonoids, and two glycosides. The structures of 1-6 were elucidated on the basis of chemical and physicochemical evidence.     

Lutidyl Radical Photoelectron Spectra Reveal Additive Substituent Effects on Benzyl Derivatives’ Ionization Energy

Understanding how isomerism influences photoelectron spectra helps in the assignment and analysis of reactive mixtures, especially for heterocycles with numerous isomers. Threshold photoelectron spectra of lutidyl radical isomers, i. e., benzyl derivatives with a nitrogen heteroatom and a methyl substituent, are recorded using vacuum ultraviolet synchrotron radiation. The radicals are produced by flash pyrolysis from aminomethyl methylpyridine precursors. Experimental ionization energies are determined to be 7.54, 7.50, and 7.45 eV for 2,4-, 2,6- and 3,5-lutidyl, respectively, in excellent agreement with composite method calculations. Franck–Condon simulations aid the TPES assignment but are also shown to exhibit artifacts if large-amplitude motions, notably the methyl internal rotation are assumed to be active in the double harmonic approximation. Based on calculated adiabatic ionization energies (AIE) of benzyl, picolyl, and xylyl radicals, the N and CH₃ substituent effects are found to be additive, position-dependent and decrease in the para>ortho≳meta order in magnitude with the nitrogen heteroatom increasing and the methyl substituent decreasing the AIE. These effects are discussed in light of the charge distribution upon ionization. The additivity of the substituent effects also helps predict the influence of substituents on the binding energy of the unpaired electron in analogous radicals.