In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine‐5‐carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high‐resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively

Оценка воздействия дамбы в д. Босоногово Бердюжского района на окружающую среду Тюменской области

В статье рассмотрена положительная и отрицательная оценка воздействия дамбы в д. Босоногово Бердюжского района на окружающую среду Тюменской области.The article considers a positive and negative assessment of the impact of a dam in the village of Bosonogovo, Berdyuga district, on the environment of the Tyumen region

Advancements in the mechanistic understanding of the copper-catalyzed azide–alkyne cycloaddition

The copper-catalyzed azide–alkyne cycloaddition (CuAAC) is one of the most broadly applicable and easy-to-handle reactions in the arsenal of organic chemistry. However, the mechanistic understanding of this reaction has lagged behind the plethora of its applications for a long time. As reagent mixtures of copper salts and additives are commonly used in CuAAC reactions, the structure of the catalytically active species itself has remained subject to speculation, which can be attributed to the multifaceted aggregation chemistry of copper(I) alkyne and acetylide complexes. Following an introductory section on common catalyst systems in CuAAC reactions, this review will highlight experimental and computational studies from early proposals to very recent and more sophisticated investigations, which deliver more detailed insights into the CuAAC’s catalytic cycle and the species involved. As diverging mechanistic views are presented in articles, books and online resources, we intend to present the research efforts in this field during the past decade and finally give an up-to-date picture of the currently accepted dinuclear mechanism of CuAAC. Additionally, we hope to inspire research efforts on the development of molecularly defined copper(I) catalysts with defined structural characteristics, whose main advantage in contrast to the regularly used precatalyst reagent mixtures is twofold: on the one hand, the characteristics of molecularly defined, well soluble catalysts can be tuned according to the particular requirements of the experiment; on the other hand, the understanding of the CuAAC reaction mechanism can be further advanced by kinetic studies and the isolation and characterization of key intermediates

Palladium-Catalyzed Cyclopropanation of Alkenyl Silanes by Diazo Alkanes: Evidence for a Pd(0) Mechanism.

Alkenyl silanes are efficiently converted to the corresponding silyl cyclopropanes in the presence of a slight excess of diazomethane (2-4 equiv) and a low loading of Pd(OAc)(2) (<0.5 mol %). Diazoethane and diazobutane can also be employed and yield silyl cyclopropanes with diastereoselectivities of up to 10:1 for the trans isomer. When conducted on a 4 g scale, the reaction only required a catalyst loading of 5x10(-3) mol %, which corresponds to a turnover frequency of 40 000 h(-1). Competition experiments revealed that vinyl silanes can be selectively cyclopropanated in the presence of an aliphatic terminal alkene and styrene. The complex [Pd(0) (2)(DVTMS)(3)] (38, DVTMS=divinyltetramethyldisiloxane) proved to be an exceptionally active catalyst for the cyclopropanation reaction, giving complete conversion at -35 degrees C in 1 min. Intermolecular and intramolecular competition experiments with DVTMS (36), both with Pd(OAc)(2) and 38, provided strong evidence for a Pd(0)(alkenyl silane)(3) resting state. Detailed density functional calculations on the reaction pathways for the cyclopropanation of trimethylvinylsilane and DVTMS by diazomethane with Pd(0) corroborated the experimental observations

Organometallic chemistry

Tesis (Master en Ingeniería Civil con mención en Ingeniería Vial)--Universidad de Piura. Facultad de Ingeniería. Programa Académico de Ingeniería Civil, 2008.La presente tesis tiene como objetivo modificar el asfalto convencional mediante la adición de polímeros SBS con el fin de mejorar sus características mecánicas de resistencia a las deformaciones presentadas por factores climatológicos y de tránsito (ahuellamiento y fatiga respectivamente). Para ello, se describen cada uno de los procesos necesarios en el cemento asfáltico modificado con polímeros SBS. Finalmente, la modificación del cemento asfáltico con polímeros SBS fabricado en obra ha permitido contar con un ligante de mejores características de resistencia a las deformaciones permanentes y mejor comportamiento a loa fatiga