RESUMEN
In this work we report the first cycloaddition reactions between a diazido diborane(4) and terminal alkynes, providing unique access to bis-1,2,3-triazoles connected by a B-B bridge. The catalyst-free reactions are highly selective, yielding exclusively the thermodynamically disfavored bis-1,4-triazoles. The reactions are enabled by the high thermal stability of the diazido-diborane [B(hpp)(N3 )]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate). Due to the tetra-coordinate boron atoms in this reagent, the reactions are tolerant with respect to the introduction of Lewis-basic groups at the alkyne. The scope and limitations of the new reactions are discussed.
RESUMEN
Starting with electron-rich ditriflato-diborane B2(hpp)2(OTf)2 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate), novel symmetric and unsymmetric diboranes B2(hpp)2X2 and B2(hpp)2XY with X,Y = Br, NCS, N3 or OTf are synthesized by substitution reactions with SN1 mechanisms. The stability of the unsymmetric diboranes with respect to dismutation equilibria is evaluated.
RESUMEN
In modern democracies, large societal crises like the COVID-19 pandemic are accompanied by intensified public discourse about which policies and strategies are adequate to fight the crisis. In such times, the public sphere switches to crisis mode with fundamentally different communicative dynamics compared to routinised periods. Data from social media platforms like Twitter offers new possibilities to study such dynamics. However, comprehensive studies on how crises affect discourse in distinct national publics are missing up to now. Based on 1,762,262 tweets referring to COVID-19 written between 1 January and 30 April 2020 by 56,418 validated Swiss users, we illustrate how the lockdown of public life in Switzerland affected the discourse in the Swiss Twitter-sphere. Based on public sphere theories, we identify four crisis-related dimensions for our analysis. We show that the pandemic led to a narrowing of the topic agenda and to a more inwardly oriented public sphere with increased Twitter activity by experts. Furthermore, actors from the social periphery were able to reach the centre of public discourse with their tweets. Overall our study shows how methodological innovation allows us to better connect an empirical analysis with the concept of a public sphere as a communication network.
RESUMEN
Due to their combined Lewis acidity and electron-donor capability, BII cations exhibit an interesting reactivity, which is almost unexplored so far. In this work, we compare the reduction in a dicationic diborane of a series of vicinal diones with different redox potentials, namely 3,5-di-tert-butylbenzoquinone, 3,4,5,6-tetrachlorobenzoquinone, 1,2-naphthalene-dione, 9,10-phenanthrene-dione, 2,2'-dichlorobenzil, benzil and 1,2-acenaphthylene-dione. The experimental work is complemented by quantum-chemical calculations, illuminating the electron-transfer step in the reactions.
RESUMEN
Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late-transition-metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π-acceptor character of the neutral σ-donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron-donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.
RESUMEN
A novel type of fiber-optic cavity sensor for hydrogen diffusion into and out of fibers is presented. The sensor is an implementation of a cavity ringdown scheme in a silica-based single-mode fiber that has been exposed to gaseous hydrogen at normal pressure. The measured ringdown times during the H2 diffusion show good agreement with a theoretical diffusion model. This model allows the determination of the diffusion coefficient of hydrogen in silica, resulting in D = (3.02 +/- 0.07) x 10(-15) m2/s at 30 degrees C.
