1,720,960 research outputs found
Transition Metal Complex Catalyzed Photo- and Electrochemical (De)hydrogenations Involving C=O and C=N Bonds
No full textAbstract Herein, we summarize the photo- and electrochemical protocols for dehydrogenation and hydrogenations involving carbonyl and imine functions. The three basic principles that have been explored to interconvert such moieties with transition metal complexes are discussed in detail and the substrate scope is evaluated. Furthermore, we describe some general thermodynamic and kinetic aspects of such electro- and photochemically driven reactions. 1 Introduction 2 Dehydrogenation Reactions 2.1 Electrochemical Dehydrogenations Using High-Valent Metal Species 2.2 Electrochemical Dehydrogenations Involving Metal Hydride species 2.3 Photochemically Driven Dehydrogenation 3 Hydrogenation Reactions 3.1 Electrochemical Protocols 3.2 Photochemical Protocols 4 Conclusion 5 AbbreviationsAbstract Herein, we summarize the photo- and electrochemical protocols for dehydrogenation and hydrogenations involving carbonyl and imine functions. The three basic principles that have been explored to interconvert such moieties with transition metal complexes are discussed in detail and the substrate scope is evaluated. Furthermore, we describe some general thermodynamic and kinetic aspects of such electro- and photochemically driven reactions. 1 Introduction 2 Dehydrogenation Reactions 2.1 Electrochemical Dehydrogenations Using High-Valent Metal Species 2.2 Electrochemical Dehydrogenations Involving Metal Hydride species 2.3 Photochemically Driven Dehydrogenation 3 Hydrogenation Reactions 3.1 Electrochemical Protocols 3.2 Photochemical Protocols 4 Conclusion 5 Abbreviation
Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well‐Defined Base‐Metal Catalyst
No full textAbstract Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Here, we introduce an electrochemical method for the hydrogenation of ketones and aldehydes by in situ formation of a Mn‐H species. We utilise protons and electric current as surrogate for H2 and a base‐metal complex to form selectively the alcohols. The method is chemoselective for the hydrogenation of C=O bonds over C=C bonds. Mechanistic studies revealed initial 3 e− reduction of the catalyst forming the steady state species [Mn2(H−1L)(CO)6]−. Subsequently, we assume protonation, reduction and internal proton shift forming the hydride species. Finally, the transfer of the hydride and a proton to the ketone yields the alcohol and the steady state species is regenerated via reduction. The interplay of two manganese centres and the internal proton relay represent the key features for ketone and aldehyde reduction as the respective mononuclear complex and the complex without the proton relay are barely active.An electrochemical protocol is herein presented for the hydrogenation of ketones forming selectively the corresponding alcohols using protons and electric current. A dinuclear manganese catalyst is used with an internal proton relay to facilitate the reaction. Investigation by means of electrochemical methods and IR spectroelectrochemistry shed light on the catalytic cycle. imageDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165
The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis
No full textHerein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO)3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H2 formation forming [Mn2(H-1L1)(CO)6Br] and [Mn(H-1L3)(CO)3], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn-H species. 2 dimerizes during reduction, forming [Mn2(L2)(CO)6]2, but 1 and 3 do not. Reduction of 1, 2, and 3 is accompanied by bromide loss, and the final species represent [Mn2(H-1L1)(CO)6]3-, [Mn2(L2)(CO)6]2-, and [Mn(H-1L3)(CO)3]2-, respectively. 1 and 2 are active catalysts in the electrochemical CO2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H+/2e- reduced products of CO2 in dmf/water mixtures. For 2, CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1. Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s-1 are higher with 1 than with 2, that is, the OH group is beneficial for catalysis due to the internal proton transfer
Electroreduction of Carbonyl Compounds Catalyzed by a Manganese Complex
No full textChemical conversions are nowadays evaluated not only according to their functionality and originality but also regarding their atom and redox economy. Excess energy consumption, the origin of that energy, and waste production are becoming increasingly important factors in chemical synthesis development. This led to a re-emerging of electroorganic synthesis after decades of dormancy. Inspired by this, we combined organometallic catalysis and electrochemistry to develop a versatile and broadly applicable method for hydrogenation of ketones and aldehydes by electrons and trifluoroethanol as proton source under ambient conditions. Aromatic, aliphatic, and further functionalized ketones and aldehydes were converted to the corresponding alcohols in decent to excellent yields, with a catalyst loading of only 1%. The protocol is selective toward ketones and aldehydes over non-conjugated C=C bonds, esters, and carboxylic acids. As a base metal catalyst, we utilized a manganese complex with a proton relay, which was previously shown to electrohydrogenate CO2 via an Mn-H species. Mechanistic analysis showed that this hydride is also pivotal for the electrohydrogenation of C=O bonds in organic scaffolds and fosters ionic hydrogenations over radical-type PCET reactions, which leads to the observed selectivity toward polar substrates. Further mechanistic analysis shed light on the pKa of the metal hydride species, the kinetic rate of its formation, as well as the all-over catalysis. Chemical formation of the MnH species via H2 splitting under ambient conditions failed, which emphasizes that merging electrochemistry and organometallic catalysis can open different pathways for chemical catalysis under mild conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
