In this work, we report that organocatalyst 1-Bcat-2-PPh2–C6H4 ((1); cat = catechol) acts as an ambiphilic metal-free system for the reduction of carbon dioxide in presence of hydroboranes (HBR2 = ...HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo3.3.1nonane), BH3·SMe2 and BH3·THF) to generate CH3OBR2 or (CH3OBO)3, products that can be readily hydrolyzed to methanol. The yields can be as high as 99% with exclusive formation of CH3OBR2 or (CH3OBO)3 with TON (turnover numbers) and TOF (turnover frequencies) reaching >2950 and 853 h–1, respectively. Furthermore, the catalyst exhibits “living” behavior: once the first loading is consumed, it resumes its activity on adding another loading of reagents.
Metal‐free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active ...catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition‐metal‐based catalysts.
Less is more: Metal‐free systems, including frustrated Lewis pairs (FLPs), have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition‐metal‐based catalysts (see scheme).
Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the ...costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.
The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. ...The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.
Metallomimetic Chemistry of Boron Légaré, Marc-André; Pranckevicius, Conor; Braunschweig, Holger
Chemical reviews,
07/2019, Letnik:
119, Številka:
14
Journal Article
Recenzirano
The study of main-group molecules that behave and react similarly to transition-metal (TM) complexes has attracted significant interest in recent decades. Most notably, the attractive idea of ...replacing the all-too-often rare and costly metals from catalysis has motivated efforts to develop main-group-element-mediated reactions. Main-group elements, however, lack the electronic flexibility of TM complexes that arises from combinations of empty and filled d orbitals and that seem ideally suited to bind and activate many substrates. In this review, we look at boron, an element that despite its nonmetal nature, low atomic weight, and relative redox staticity has achieved great milestones in terms of TM-like reactivity. We show how in interelement cooperative systems, diboron molecules, and hypovalent complexes the fifth element can acquire a truly metallomimetic character. As we discuss, this character is powerfully demonstrated by the reactivity of boron-based molecules with H2, CO, alkynes, alkenes and even with N2.
1406 I. 1407 II. 1408 III. 1410 IV. 1411 V. 1413 VI. 1416 VII. 1418 1418 References 1419 SUMMARY: Almost all land plants form symbiotic associations with mycorrhizal fungi. These below‐ground fungi ...play a key role in terrestrial ecosystems as they regulate nutrient and carbon cycles, and influence soil structure and ecosystem multifunctionality. Up to 80% of plant N and P is provided by mycorrhizal fungi and many plant species depend on these symbionts for growth and survival. Estimates suggest that there are c. 50 000 fungal species that form mycorrhizal associations with c. 250 000 plant species. The development of high‐throughput molecular tools has helped us to better understand the biology, evolution, and biodiversity of mycorrhizal associations. Nuclear genome assemblies and gene annotations of 33 mycorrhizal fungal species are now available providing fascinating opportunities to deepen our understanding of the mycorrhizal lifestyle, the metabolic capabilities of these plant symbionts, the molecular dialogue between symbionts, and evolutionary adaptations across a range of mycorrhizal associations. Large‐scale molecular surveys have provided novel insights into the diversity, spatial and temporal dynamics of mycorrhizal fungal communities. At the ecological level, network theory makes it possible to analyze interactions between plant–fungal partners as complex underground multi‐species networks. Our analysis suggests that nestedness, modularity and specificity of mycorrhizal networks vary and depend on mycorrhizal type. Mechanistic models explaining partner choice, resource exchange, and coevolution in mycorrhizal associations have been developed and are being tested. This review ends with major frontiers for further research.
The ability of fungi to form mycorrhizas with plants is one of the most remarkable and enduring adaptations to life on land. The occurrence of mycorrhizas is now well established in c. 85% of extant ...plants, yet the geological record of these associations is sparse. Fossils preserved under exceptional conditions provide tantalizing glimpses into the evolutionary history of mycorrhizas, showing the extent of their occurrence and aspects of their evolution in extinct plants. The fossil record has important roles to play in establishing a chronology of when key fungal associations evolved and in understanding their importance in ecosystems through time. Together with calibrated phylogenetic trees, these approaches extend our understanding of when and how groups evolved in the context of major environmental change on a global scale. Phylogenomics furthers this understanding into the evolution of different types of mycorrhizal associations, and genomic studies of both plants and fungi are shedding light on how the complex set of symbiotic traits evolved. Here we present a review of the main phases of the evolution of mycorrhizal interactions from palaeontological, phylogenetic and genomic perspectives, with the aim of highlighting the potential of fossil material and a geological perspective in a cross-disciplinary approach.
Social media has been established in many larger emergencies and crises. This process has not started just a few years ago, but already 15 years ago in 2001 after the terrorist attacks of 9/11. In ...the following years, especially in the last 10, sometimes summarized under the term crisis informatics, a variety of studies focusing on the use of ICT and social media before, during or after nearly every crisis and emergency has arisen. This article aimed to recapitulate 15 years of social media in emergencies and its research with a special emphasis on use patterns, role patterns and perception patterns that can be found across different cases to point out what has been achieved so far, and what future potentials exist.
Nitrogen fixation and reduction at boron Légaré, Marc-André; Bélanger-Chabot, Guillaume; Dewhurst, Rian D ...
Science,
02/2018, Letnik:
359, Številka:
6378
Journal Article
Recenzirano
Odprti dostop
Currently, the only compounds known to support fixation and functionalization of dinitrogen (N
) under nonmatrix conditions are based on metals. Here we present the observation of N
binding and ...reduction by a nonmetal, specifically a dicoordinate borylene. Depending on the reaction conditions under which potassium graphite is introduced as a reductant, N
binding to two borylene units results in either neutral (B
N
) or dianionic (B
N
) products that can be interconverted by respective exposure to further reductant or to air. The
N isotopologues of the neutral and dianionic molecules were prepared with
N-labeled dinitrogen, allowing observation of the nitrogen nuclei by
N nuclear magnetic resonance spectroscopy. Protonation of the dianionic compound with distilled water furnishes a diradical product with a central hydrazido B
N
H
unit. All three products were characterized spectroscopically and crystallographically.
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