Synthesis of Diaryl Ethers: A Long-Standing Problem Has Been Solved

A breakthrough in the synthesis of diaryl ethers has been achieved as shown in Equation (1). The coupling of phenols with aryl boronic acids in the presence of copper(II ) acetate and a base proceeds under very mild conditions (room temperature) as described simultaneously by Evans et al. and Chan et al. Examples: R¹=2-Cl, 2-I, 2-OMe, 4-tBu, 4-CH₂CH(NHCOOtBu)CO₂Me, 3,5-tBu₂; R²=2-Me, 3-OMe, 3-NO₂, 4-H, 4-F, 4-OMe.     

Nonconventional Stereochemical Issues in the Design of the Synthesis of the Vancomycin Antibiotics: Challenges Imposed by Axial and Nonplanar Chiral Elements in the Heptapeptide Aglycons

Controlling the elements of planar and axial chirality are the principal challenges in the synthesis of the aglycon of vancomycin. Vancomycin is the prototypical member of the glycopeptide family of antibiotics which are effective for the treatment of infections by methicillin-resistant Staphylococcus aureus. The first total syntheses of the vancomycin and eremomycin aglycons provide insight into the influence of structure on kinetic and thermodynamic control of atropselective macrocyclizations.     

Sulfonate N-Heterocyclic Carbene–Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C−C,C−B,C−H,and C−Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.     

Polyanionic Lattice Modifications Leading to High-Entropy Sodium Ion Conductors: Mathematical Solution of Accessible Compositions

Sodium zirconium double phosphate NaZr₂(PO₄)₃ can be used as a starting point for investigations of high-entropy materials. Apart from the frequently used approach of partial substitution with four or more different transition metal cations, this class of materials also allows multiple substitutions of the phosphate groups. Herein modifications of the polyanionic lattice are considered and high-entropy compositions are numerically determined with up to eight elements on the central tetrahedral lattice site of the so-called NaSICON structure. For this study, the chemical formula was fixed as Na₃Zr₂(EO₄)₃ with E=B, Al, Si, P, As, Sb, S, Se and Te. The number of compositions increases exponentially with the increasing number of elements involved and with decreasing equal step size for each element. The maximum number of 237258 compositions is found for Na₃Zr₂([B,Al,Si,P,As,Sb,S,Se]O₄)₃ with a step size of 0.1 mol/formula unit. Of this compositional landscape, 143744 compositions fulfil the definitions of high-entropy materials. The highest entropy factor of ΔS_config/R=-2.0405 is attributed to the compositions Na₃Zr₂(B_0.5Al_0.6Si_0.4P_0.3As_0.3Sb_0.3S_0.3Se_0.3)O₁₂ and Na₃Zr₂(B_0.6Al_0.5Si_0.4P_0.3As_0.3Sb_0.3S_0.3Se_0.3)O₁₂.     

Amine-containing tertiary phosphine-substituted diiron ethanedithioate (edt) complexes Fe2(μ-edt)(CO)6-nLn (n= 1, 2): Synthesis,protonation, and electrochemical properties

As diiron subsite models of [FeFe]-hydrogenases for catalytic proton reduction to hydrogen (H₂), a new series of the phosphine-substituted diiron ethanedithiolate complexes Fe₂(μ-edt)(CO)_6-nL_n (n = 1, 2) were prepared from the variable substitutions of all-CO precursor Fe₂(μ-edt)(CO)₆ ( A ) and tertiary phosphines (L1-L4) under different reaction conditions. While the Me₃NO-assisted substitutions of A and one equiv. ligands L1-L4 [L = Ph₂P(CH₂NHBu^t), Ph₂P(CH₂CH₂NH₂), Ph₂P(NHBu^t), and Ph₂P(C₆H₄Me-p)] produced the monosubstituted complexes Fe₂(μ-edt)(CO)₅L ( 1 – 4 ) in good yields, the refluxing xylene solution of A and two equiv. ligand L1 prepared complex Fe₂(μ-edt)(CO)₅{κ¹-Ph₂P(CH₂NHBu^t)} ( 1 ) in low yield. Meanwhile, the UV-irradiated toluene solution of A and two equiv. ligand L3 resulted in the rare formation of the disubstituted complex Fe₂(μ-edt)(CO)₄{κ¹, κ¹-(Ph₂PNHBu^t)₂} ( 5 ) in low yield, whereas the Me₃NO-assisted substitution of A and two equiv. ligand L4 afforded the disubstituted complex Fe₂(μ-edt)(CO)₄{κ¹, κ¹-(Ph₂PC₆H₄Me-p)₂} ( 6 ) in good yield. All the model complexes 1 – 6 have been characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for 1 , 3 , 5 by X-ray crystallography. Further, the protonations of complexes 1 – 4 are studied and compared with excess acetic acid (HOAc) and trifluoroacetic acid (TFA) by using FT-IR and NMR techniques. Additionally, the electrochemical and electrocatalytic properties of model complexes 1 – 6 are investigated and compared by cyclic voltammetry (CV), suggesting that they are electrocatalytically active for proton reduction to H₂ in the presence of HOAc.     

Generalized gradients,bid–ask spreads,and market equilibrium

Abstract

This paper considers economies in which each agent valuates various goods by own generalized gradients. Taken together and appropriately scaled, the latter determine bid–ask spreads. When all such spreads are nil, market equilibrium prevails. Crucial for the arguments is a money commodity which denominates agents' rates of exchange or substitution. Equilibrium obtains when rates coincide across agents. The results may facilitate detailed modelling of market micro-structure, direct deals, and agent-based computations.     

Use of cyclobutyl derivatives as intermediates in the synthesis of 1,2a,3,8b-tetrahydro-2H-cyclobuta[c]chromenes

Starting from cyclobutanones and tertiary cyclobutanols several new chromenes containing a cyclobutane ring have been prepared by acid catalyzed intramolecular alkylation of an oxygen-carrying aromatic ring. Interestingly one of the cyclobutanols leads to 3-[(4-methoxyphenoxy)methyl]-2,2,3-trimethylcyclopentanone that is a precursor in the synthesis of the carotenoid pigment capsorubin.     

Thermal analysis of apatite structure

Two types of thermal effects, caused by substitutions (Ca²⁺↔ Na⁺, ↔ CO₃ ^2-, SO₄ ^2-, OH^-↔ F^-) in synthetic precipitated apatites as well as by sorption of Cd²⁺, Zn²⁺, and Cr³⁺ ions from the solutions were studied by TG/DTA, XRD and FTIR analysis. The temperatures of exothermic effect at 330-340°C and of decomposition of carbonate and sulfate apatites at 650-950°C were shown to depend on the substitutions in the apatite structure. This revised version was published online in July 2006 with corrections to the Cover Date.