Development of chiral heteroleptic magnesium amides; asymmetric deprotonations mediated by six-membered metallocyclic amidomagnesium naphtholates

A series of enantioenriched six-membered metallocyclic amidomagnesium naphtholates were prepared and used to probe the structure–reactivity/selectivity relationships of heteroleptic magnesium base complexes within asymmetric deprotonation reactions. An effective complex was identified and applied within enantioselective enolisation processes, delivering good levels of enantioselectivity and also revealing key structural requirements for achieving such selectivity.     

Characterizing and Generating Bivariate Empirical Rank Distributions Satisfying Certain Positive Dependence Concepts

Abstract

This article introduces an approach for characterizing the classes of empirical distributions that satisfy certain positive dependence notions. Mathematically, this can be expressed as studying certain subsets of the class S_N of permutations of 1, …, N, where each subset corresponds to some positive dependence notions. Explicit techniques for it-eratively characterizing subsets of S_N that satisfy certain positive dependence concepts are obtained and various counting formulas are given. Based on these techniques, graph-theoretic methods are used to introduce new and more efficient algorithms for constructively generating and enumerating the elements of various of these subsets of S_N. For example, the class of positively quadrant dependent permutations in S_N is characterized in this fashion.     

Constraint-Based Representations of Statistical Graphs

Abstract

Pictor is an environment for statistical graphics that promotes simple commands for common uses and offers the ability to experiment with whole new paradigms. Pictor describes graphs as graphical objects whose component pieces are related by several sorts of constraints. This article describes in detail the constraint system that Pictor uses.     

Cyclopolymerization. XIII. Cyclopolymerization of Diallylaminomethylphenols

New highly substituted benzyl diallylamino monomers were prepared from substituted phenols by use of the Mannich reaction. Their ability to cyclopolymerize as the free phenols and the structures and properties of the pomers formed were investigated.     

Structures and properties of solvated and unsolvated isopropyl functionalised calix[4]arenes

The tetra-isopropyl ethers of calix[4]arene and p-t-butylcalix[4]arene have been isolated in the cone conformation, and structurally characterised as chloroform solvates. Thermogravimetric analysis demonstrated that the parent isopropylcalix[4]arene solvate is significantly more stable than the p-t-butylcalix[4]arene analogue, retaining the solvent up to a temperature of 125°C. It was found that the calix[4]arene ether sublimes at atmospheric pressure, and solvent-free crystals appropriate for structure determination were produced at reduced pressure. The p-t-butylcalix[4]arene ether was also isolated without solvent in the lattice, but in this case the calixarene was crystallised from acetone, as sublimation did not produce crystals of sufficient quality.     

Aryl Nitro Reduction with Iron Powder or Stannous Chloride under Ultrasonic Irradiation

The selective reduction of aryl nitro compounds in the presence of sensitive functionalities, including halide, carbonyl, nitrile, and ester substituents, under ultrasonic irradiation at 35 kHz is reported in yields of 39–98%. Iron powder proved superior to stannous chloride with high tolerance of sensitive functional groups and high yields of the desired aryl amines in relatively short reaction times. Simple experimental procedure and purification also make the iron reduction of aryl nitro compounds advantageous over other methods of reduction.     

Hydrated Rare Earth Structural Networks containing the Phenylene‐1, 2‐dioxydiacetate (PDDA) Ligand

Reaction of LnCl₃ with Na₂(PDDA) (PDDA = phenylene‐1, 2‐dioxydiacetate) in a 1 to 2 mol ratio in aqueous solution yielded [Ln₂(PDDA)₃(H₂O)₆] · 2H₂O, structurally characterized for Ln = Ce ( 1 ), Sm ( 2 ) (redetermination), Tb ( 3 ) and Y ( 4 ) in a monoclinic C2/c array, a second related structural form [orthorhombic, Pbcn] being obtained for Tb ( 5 ), Ho ( 6 ) and Er ( 7 ). The ‘domains of existence' of these two previously described forms are now extended to Ce–Dy, Y, and Eu–Er, respectively. Reaction under the same conditions for the heavier Yb³⁺ ion yielded [Yb₂(PDDA)₃(H₂O)₆]_(∞|∞) · 4H₂O ( 8 ), orthorhombic, Pbca. In the case of Ln = La the bimetallic species [NaLa(PDDA)₂(H₂O)₂]_(∞|∞) · 4H₂O ( 9 ) was obtained, while reaction of LnCl₃ with Na₂(PDDA) in a 1 to 3 mol ratio led to the isolation of the isotypic (monoclinic, P2₁/c) [NaLn(PDDA)₂(H₂O)₂]_(∞|∞) · 4H₂O) for Ln = Ce ( 10 ) and Sm ( 12 ). With the smaller Ln = Yb, the more definitively bimetallic [NaYb(PDDA)₂(H₂O)₂]_(∞|∞) · 3H₂O ( 13 ) (triclinic, P$\bar{1}$ )) was obtained, the trihydrate solvation ascribed differing from that recorded (dihydrate) in a cosynchronous report.     

A study of indoor radon in greenhouses in Mexico City, Mexico

Enclosed spaces in contact with soil, the main source of radon, like greenhouses have potentially high radon (²²²Rn) concentrations. Greenhouses are frequented by visitors and also are workplaces. The study of radon concentrations in greenhouses is, thus, a relevant concern for public health and environmental radiation authorities. For this study, the radon concentrations in 12 greenhouses in different locations within Mexico City were measured using nuclear track methodology. The detectors used for the study consisted of the well-known closed-end cup device, with CR-39 Lantrack^® as detector material. The measurements were carried out over a period of one year, divided into four three-month sub-periods. The lowest and highest annual mean radon concentrations found in individual greenhouses were 17.0 and 45.1 Bq/m³, respectively. The annual mean averaged over all 12 greenhouses was 27.3 Bq/m³. No significant seasonal variation was observed. Using the highest annual mean radon concentration found in an individual greenhouse, and an equilibrium factor of 0.4, the effective dose from ²²²Rn and its progenies was calculated to be 339.9 nSv/h. This corresponds to an annual dose rate of 679.8 μSv/y (0.057 WLM/y) for a worker spending 4 h a day, 5 days a week, 50 weeks a year, inside the greenhouse. For a visitor spending 12 h a year inside the greenhouse the annual dose is 2.469 μSv/y. The study of indoor radon concentrations in closed buildings such as greenhouses, which are both workplaces and open to visitors, is an important public health consideration.