1,720,958 research outputs found
Site specificity in 1,3-dipolar cycloadditions to a polycyclic polyene induced by complexation with tricarbonyliron
No full textTreatment of di-Me tricyclo[4.2.2.02'5]deca-3,7,9-triene-7,8-dicarboxylate (1) with nonacarbonyldiiron led to di-Me 7-10-tetrahaptotricyclo[4.2.2.02,5]deca-3,7,9-triene-7,8-dicarboxylate tricarbonyliron (I). This deriv. reacted smoothly with several 1,3-dipoles (nitrones, nitrile oxides, nitrile imines and diazoalkanes) at the cyclobutene double bond to give adducts from which the tricarbonyliron group could be easily removed by oxidative decomplexation with trimethylamine N-oxide
Crystal structures of 2,4,9-trimethyl-(3ar,4c,9c,9ac)-3a,4,9,9a-tetrahydrobenzo[f]isoindole-1,3-dione(endo), C15H17NO2 (1), and 2-methyl-(3ac,9ac)-3a,4,9,9a-tetrahydro-4r,9c-ethanobenzo[f]isoindole-1,3-dione(endo), C15H15NO2
No full textThe title compd. 1 is triclinic, space group P1, with a 7.198(2), b 7.965(2), c 12.707(2) Å, α 72.05(1), β 87.65(2), γ 65.18(2)° ; the title compd. 2 is orthorhombic, space group and Pbca, with a 8.156(3), b 13.177(3), c 22.742(4) Å. The chiral centers in both compds. C3a, C4, C9, C9a are either R, S, R, S or S, R, S, R (compd. 1) and either R, R, S, S or S, S, R, R (compd. 2) because of the centric space grou
Reactions between Triazolindiones and Equilibrating Forms of Cycloheptatriene Derivatives Featuring 7,7-Spiro and 1,7-Fused Heterocyclic Rings
No full textNitrile oxide-azaheptafulvene adducts consist of rapidly
equilibrating mixtures of fused (1) and spiro (2) isomers, the
relative stabilities of which are nicely reproduced by B3LYP/
6-31G* calculations. The reaction between these compounds
and MTAD affords only two diastereomeric adducts. These adducts are formal Diels-Alder product
DFT computational study of the epoxidation of olefins with dioxiranes
No full textTransition structures (TSs) of the reactions of dioxirane and dimethyldioxirane with ethylene, propene, cis-2-butene and trans-2-butene were located with the B3LYP/6-31G* method. The TSs of the reactions of ethylene and cis-2-butene exhibit a sym. spiro butterfly structure with synchronous formation of the two new C-O bonds and with substantial alignment of π bond axis with the breaking O-O bond
Peroxy Acid Epoxidation of Acyclic Allylic Alcohols. Competition between s-trans and s-cis Peroxy Acid Conformers
No full textRB3LYP calculations, reported here, indicate that peroxy acid s-cis conformer is more stable than its s-trans counterpart, in agreement with experimental data. Difference in stability is the highest in the gas phase, but it falls considerably on going from the gas phase to moderately polar solvent. In the case of peroxy formic acid, the enthalpy (free energy) difference is about 3.4 (2.5) kcal/mol, respectively, in the gas phase but decreases to 1.2 (0.6) kcal/mol in dichloromethane solution.
Introduction of an alkyl or aryl substituent on the peroxy acid, that is, on passing to peroxy acetic, peroxy benzoic (PBA), and m-chloroperoxy benzoic acid (MCPBA), adds a further significant (1.0-1.5 kcal/mol) favor to the s-cis isomer. RB3LYP/6-31+G(2d,p) calculations on the epoxidation of 2-propenol with peroxy formic and peroxy benzoic acids, respectively, suggest that the less stable peroxy acid s-trans conformer can compete with the more stable s-cis form in epoxidation reaction of these substrates. Transition structures arising from s-trans peroxy acids (“trans” TSs) retain both the well-established, for “cis” TS, perpendicular orientation of the O-H peroxy acid bond relative to the CdC bond and the one-step oxirane ring formation. These TSs collapse to the final epoxide via a 1,2-H shift at variance with the 1,4-H transfer of the classical Bartlett’s “cis” mechanism. The “trans” reaction pathways have a higher barrier in the gas phase than the “cis” reaction channels, but in moderately polar solvents they become competitive. In fact, the “trans”
TSs are always significantly more stabilized than their “cis” counterparts by solvation effects. Calculations also suggest that going from peroxy formic to peroxy benzoic acid should slightly disfavor the “trans” route relative to the “cis” one, reflecting, in an attenuated way, the decrease in the peroxy acid s-trans/s-cis conformer ratio. The predicted behavior for MCPBA parallels that of PBA acid
Facial selectivity in 1,3-dipolar cycloadditions to cis-3,4-dimethylcyclobutene. An experimental and computational study
No full textFacial selectivity in 1,3-dipolar cycloaddn. of diazomethane (2a), 3,4-dihydroisoquinoline N-oxide (2b), pyrroline N-oxide (2c), 5,5-dimethylpyrroline N-oxide (2d) and several nitrile oxides with cis-3,4-dimethylcyclobutene (1) has been investigated. The stereochem. of the cycloaddn. of 2a, 2b-2d and encumbered nitrile oxides (2i and 2j) is controlled by steric interactions with dominant formation of the anti diastereoisome
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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