Diastereoselective alkylation reactions employing a new camphor-based 2-phenylimino-2-oxazolidine chiral auxiliary

A new camphor-based 2-phenylimino-2-oxazolidine chiral auxiliary was prepared and it was shown to be a particularly effective chiral auxiliary for asymmetric alkylations affording high yields and diastereoselectivities. The alkylation products were readily cleaved by simple alkaline hydrolysis to give a-alkylated carboxylic acids in good yield and in almost enatiomerically pure form.     

Unbridged bidentate aluminum complexes supported by diaroylhydrazone ligands: Synthesis,structure and catalysis in polymerization of ε-caprolactone and lactides

A series of novel diaroylhydrazone aluminum complexes have been synthesized and well-defined structurally, and their catalytic performance in the polymerization of ε-caprolactone and lactides have also been evaluated. Complexes [(L^1–4)₂AlMe] ( 1 – 4 ) {[L¹ = (3,5-^tBu₂–2-OMe-C₆H₂)CH=NNCOC₆H₅], [L² = (3,5-^tBu₂–2-OMe-C₆H₂)CH=NNCO(C₆H₄–4-OCH₃)], [L³ = (3,5-^tBu₂–2-OMe-C₆H₂)CH=NNCO(C₆H₄–4-Br)] and [L⁴ = (2-OMe-C₆H₄)CH=NNCO(C₆H₄–4-^tBu)]} were prepared through treatment of AlMe₃ with the corresponding proligands L^1–4H in molar ratios of 1: 1 or 1: 2. Chemical structures of all the complexes were well-defined by elemental analysis, NMR spectra as well as single-crystal X-ray study. Complexes [(L^1–4)₂AlMe] ( 1 – 4 ) in this work represent the first examples of aluminum complexes of aroylhydrazone ligands with crystallographic characterization. Specifically, they are all in monomeric form with a penta-coordinated aluminum center, including two approximately co-planar five-membered metallacycles with aluminum. Introduced bulky tert-butyl substituents in aroylhydrazone ligands could affect the geometry around the central metal which is a distorted square-based pyramid in complexes 1 – 3 while being a trigonal bipyramidal in complex 4 , thus affecting their catalytic behaviors. The complexes can successfully catalyze the ring-opening polymerization of ε-caprolactone and _L-lactide under mild conditions without any activator. In addition, complexes 1 – 4 could also polymerize rac-lactide, affording atactic polylactides with high conversions and good controllability in relatively short reaction time.     

Theoretical investigation into coordination and selectivity of uranyl-unilateral benzotriazole salophens (X = O/S) for R/S-triadimefons

The design of new uranyl-ligands (uranyl-Ls) is of great significance for the separation and utilization of uranium. In this paper, the triazole group was introduced into uranyl-salophen (uranyl-S) to form new asymmetric uranyl-unilateral benzotriazole salophen (uranyl-UBS); we further replaced two oxygen atoms of uranyl-UBS with two sulfur atoms to generate uranyl-unilateral benzotriazole thio-salophen (uranyl-UBTS) as a new receptor. Then, we comprehensively explored coordination models of uranyl-UBS and uranyl-UBTS with R/S-triadimefons (R/S-TDFs) enantiomers as the guests using density functional theory calculations at the B3LYP//RECP/6-311G** level; we then investigated enantioselectivity recognition of the new receptors to the guests R/S-TDFs. The results indicated that the uranium atoms of the receptors uranyl-S, uranyl-UBS and uranyl-UBTS could coordinate with the carbonyl oxygens in guests R/S-TDFs to form complexes of guest-receptors R/S-TDFs-uranyl-Ls that exhibited two stable V-shaped structures with quite different properties. It was found that the coordination ability to the guests R/S-TDFs is uranyl-UBTS > uranyl-UBS > uranyl-S, while the enantioselectivity for the guests is uranyl-UBTS > uranyl-S > uranyl-UBS and, when the receptor is the same, R-TDF has stronger coordination ability than S-TDF. These results provide information and theoretical supports for the experiments of asymmetric uranyl-UBS with R/S-TDFs, and produce a reference for further exploring the coordination characteristics of asymmetric uranyl-salophen with the triazole derivatives.     

A Robust and Efficient Method for Steady State Patterns in Reaction-Diffusion Systems

An inhomogeneous steady state pattern of nonlinear reaction-diffusion equations with no-flux boundary conditions is usually computed by solving the corresponding time-dependent reaction-diffusion equations using temporal schemes. Nonlinear solvers (e.g., Newton's method) take less CPU time in direct computation for the steady state; however, their convergence is sensitive to the initial guess, often leading to divergence or convergence to spatially homogeneous solution. Systematically numerical exploration of spatial patterns of reaction-diffusion equations under different parameter regimes requires that the numerical method be efficient and robust to initial condition or initial guess, with better likelihood of convergence to an inhomogeneous pattern. Here, a new approach that combines the advantages of temporal schemes in robustness and Newton's method in fast convergence in solving steady states of reaction-diffusion equations is proposed. In particular, an adaptive implicit Euler with inexact solver (AIIE) method is found to be much more efficient than temporal schemes and more robust in convergence than typical nonlinear solvers (e.g., Newton's method) in finding the inhomogeneous pattern. Application of this new approach to two reaction-diffusion equations in one, two, and three spatial dimensions, along with direct comparisons to several other existing methods, demonstrates that AIIE is a more desirable method for searching inhomogeneous spatial patterns of reaction-diffusion equations in a large parameter space.     

Synthesis of polyphenol microsphere‐supported palladium complex and evaluation of its catalytic performance for Heck reaction

Polyphenol microspheres were prepared by template polymerization of phenol in the presence of horseradish peroxidase as bio‐enzyme catalyst and aqueous 1,4‐dioxane as solvent. The morphology of polyphenol microspheres thus obtained was observed with a scanning electron microscope. Further, polyphenol microspheres as obtained were loaded with palladium to generate polyphenol microsphere‐supported Pd complex. Resultant Pd complex catalyst supported by polyphenol microspheres was characterized by means of Fourier transformation infrared spectrometry, X‐ray diffraction, X‐ray photoelectron spectroscopy and scanning electron microscopy, and its thermal stability was examined. Moreover, the catalytic activity of polyphenol microsphere‐supported Pd catalyst as synthesized for the Heck reactions of acrylic acid with aryl iodides was evaluated. Results indicate that the polyphenol microsphere as obtained has a diameter of about 500 nm. Polyphenol microsphere‐supported Pd catalyst, as synthesized, at a dosage of 0.37 mol% Pd, possesses good catalytic activity for the Heck reactions of acrylic acid with aryl iodides in air at a low temperature of 50°C, and it also exhibits catalytic activity for bromide and activated chlorobenzene. The polyphenol microsphere‐supported Pd complex has good thermal stability, and it can be readily separated and reused; the yield of the reaction of iodobenzene with acrylic acid remains at 82% after five recycle runs, showing promising potential as a catalyst for Heck reactions. Copyright © 2012 John Wiley & Sons, Ltd.     

Certifying convergence of Lasserre’s hierarchy via flat truncation

Consider the optimization problem of minimizing a polynomial function subject to polynomial constraints. A typical approach for solving it globally is applying Lasserre’s hierarchy of semidefinite relaxations, based on either Putinar’s or Schmüdgen’s Positivstellensatz. A practical question in applications is: how to certify its convergence and get minimizers? In this paper, we propose flat truncation as a certificate for this purpose. Assume the set of global minimizers is nonempty and finite. Our main results are: (1) Putinar type Lasserre’s hierarchy has finite convergence if and only if flat truncation holds, under some generic assumptions; the same conclusion holds for the Schmüdgen type one under weaker assumptions. (2) Flat truncation is asymptotically satisfied for Putinar type Lasserre’s hierarchy if the archimedean condition holds; the same conclusion holds for the Schmüdgen type one if the feasible set is compact. (3) We show that flat truncation can be used as a certificate to check exactness of standard SOS relaxations and Jacobian SDP relaxations.     

非经典碳正离子简介

非经典碳正离子作为一种特殊的反应中间体,具有重要的理论和实际研究的意义。本文以碳正离子理论的发展为主线,以历史上非经典碳正离子(特别是降冰片基正离子)之争为中心,简要综述了Winstein、Brown和Olah三人的观点,并结合最新进展进行评述。此外,还对除降冰片基正离子之外的非经典碳正离子进行了适当的讨论。