1,720,974 research outputs found
Versatility of Silicon-based compounds in organic synthesis
No full textOrganosilicon compounds exhibit a multitude of functions in organic synthesis, therefore a plenty of
research has been performed in this field and the interest in these important compounds is still growing.
Organosilanes own many attractive properties. They generally are much more moisture- and air-stable
than various organometallic reagents and can be readily prepared from a wide range of starting
materials. Silicon derivatives have low toxicity and display rich and diverse chemistry that can usually
be rationalized by understanding a relatively small number of fundamental properties of silicon.
The reactivity of organosilanes depends both on steric requirements and electronic contributions.
Silicon is below carbon in the periodic table, but it is capable of very different bonding characteristics.
It is less electronegative than carbon and hydrogen and it forms unusually strong bonds with
electronegative elements of the second period bearing a lone pair of electrons; therefore most of
organosilicon chemistry is driven by the formation of these bonds at the expense of weaker bonds.
In addition, the availability of relatively low energy empty 3d AOs lets Si to allow higher coordination
numbers, forming the so called hypervalent silicon compounds. The ability to expand its valence state
has consequences on the mechanisms of many reactions proceeding at Si.
Silicon can stabilize a positive charge in β-position, and this effect is known as the β-Si effect.3
Moreover, silicon provides stabilization of α-negative charge,4 in fact carbanions with an α-silicon
group are more stable than their carbon analogues.
Since the versatility in reactivity, a wide range of reaction-types involving silicon compounds have
been developed over the years. An exhaustive overview is almost impossible and during this lecture
only some classes of reactions will be covered.
From a synthetic point of view, organosilanes possess many desirable characteristics. In fact, besides
their green character, they can survive under a large variety of reactions and purification conditions,
such as air, moisture, silica-gel, reducing or oxidizing conditions.5 The silyl moiety can be introduced at
almost any stage in the synthesis and can remain unaltered until the appropriate conditions make it
reactive
Isomeric alkyl cation/arene complexes in the gas phase
No full textThe kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D-1-3-(p-tolyl)butane have been investigated in the, gas phase in the 100-160degreesC range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-3l1++G**//HF/6-31 + G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions
Acid-catalyzed aldehyde-ketone rearrangements in the gas phase. Cyclopentane- and cyclohexanecarboxaldehyde.
No full textProtonation by gaseous H+3, CnH+5 (n=1,2), s-C3H+7 and t-C4H+9 cations promotes rearrangement of cyclopentane- and cyclohexanecarboxaldehyde to cyclohexanone and cyclopentylmethyl ketone, respectively. The reaction was investigated by radiolytic and mass spectrometric methods
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Crucial Role of Elusive Isomeric h-Complexes in Gas-Phase Electrophilic Aromatic Alkylations
No full textThe kinetics and stereochem. of the protonation-induced unimol. isomerization of (R)-1-D1-3-(p-fluorophenyl)butane have been investigated in the gas phase at 40-100 C and 70-760 Torr. This process leads to the formation of the relevant meta and ortho isomers with partial racemization of the migrating sec-Bu moiety. Complete racemization is obsd., instead, when the isomerization reaction involves a 1,2-H shift in the moving alkyl group. These results, together with the relevant activation parameters, fully confirm the previous evidence of the occurrence in the alkyl cation/arene PES of noncovalent .eta.-type intermediates of defined structure and stability, lying well below the classical .pi.-complexes, as confirmed by ab initio calcns. Their crucial role in detg. the positional selectivity of gas-phase electrophilic arom. substitutions clearly emerges from the comparison of the present results with the site selectivity measured in the corresponding bimol. arene alkylation carried out at the same temps. and pressures
Critical Surveys Covering the Year 2011: Organometallic Compounds in Organic Synthesis
No full textDiscusses some of the major developments that have taken place in the field of organic synthesis, analytical methods and synthetic applications in (bio)material sciences. This volume serves as a useful reference source for students and researchers in both academia and industry
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