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Methylphosphonium methylcarbonate, ylide precursor for halyde- and base-free Wittig reactions
No full textThe phosphonium salt triphenylmethylphosphonium methylcarbonate [PΦ,Φ,Φ,1][OCOOCH3] was obtained by methylation of triphenylphosphine (Ph3P) with dimethylcarbonate, adopting a green and sustainable procedure1.
The [PΦ,Φ,Φ,1][OCOOCH3] phosphonium salt was observed to possess significant P-CH3 proton acidity, and deuterium exchange experiments showed the formation of the analogous PhP3-CD3 phosphonium salt. Spontaneous deprotonation of the methyl group lead therefore to formation of the corresponding phosphorus ylide, Ph3P=CH2.
This Ph3P=CH2 ylide was tested for the Wittig reaction with benzaldehyde PhCHO, generating the desired PhC=CH2 olefination product. It was noteworthy that this Wittig reaction protocol did not require an alkyl halide or a strong base for the formation of the ylide, and could be conducted in air, making it a greener procedure.
The scope of the olefination reaction was extended to a number of carbonyl substrates, both aldehydes and ketones, with high conversions and selectivity. It was performed under mild conditions (34 – 80 °C), using a ratio ylide:carbonyl between 1.0 -3.0, in 2-methyl tetrahydrofuran (2-Me-THF) as solvent.
The study was also extended to other alkylphosphonium methylcarbonate ionic liquids ([P8,8,8,1][OCOOCH3] and [P4,4,4,1][OCOOCH3]). It was demonstrated that, depending on the reaction conditions, it was possible to achieve not only the transfer of a =CH2 fragment, but also the selective transfer of the bulkier alkyl group e.g. =CH(CH2)nCH3, giving access to a variety of olefins. Cis-trans selectivity was in the range 20-80
Halide- and base-free Wittig reaction: phosphonium methylcarbonate salts as ylide precursors
No full text
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
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