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51.
Hernández-Reyes CX Angeles-Beltrán D Lomas-Romero L González-Zamora E Gaviño R Cárdenas J Morales-Serna JA Negrón-Silva GE 《Molecules (Basel, Switzerland)》2012,17(3):3359-3369
New azanucleosides were obtained using sulphated zirconia (ZS) as catalyst in the nucleophilic oxirane ring opening reaction of 1-allyl-3-(oxiran-2-ylmethyl) pyrimidine-2,4(1H,3H)-dione and 1-allyl-5-methyl-3-(oxiran-2-ylmethyl)-pyrimidine-2,4(1H,3H)-dione, with (S)-prolinol. The new templates were obtained with good yields following a route which exploits the reactivity of epoxides in the presence of sulphated zirconia as catalyst. The key step was carried out using microwave and solvent-free conditions and proceeds with high selectivity. 相似文献
52.
Marina Pérez-Jiménez Dr. Natalia Curado Dr. Celia Maya Dr. Jesús Campos Prof. Dr. Eliseo Ruiz Prof. Dr. Santiago Álvarez Prof. Ernesto Carmona 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(21):6569-6578
This contribution focuses on complex [Mo2(H)2(μ-AdDipp2)2] ( 1 ) and tetrahydrofuran and pyridine adducts [Mo2(H)2(μ-AdDipp2)2(L)2] ( 1⋅thf and 1⋅py ), which contain a trans-(H)Mo≣Mo(H) core (AdDipp2=HC(NDipp2)2; Dipp=2,6-iPr2C6H3). Computational studies provide insights into the coordination and electronic characteristics of the central trans-Mo2H2 unit of 1 , with four-coordinate, fourteen-electron Mo atoms and ϵ-agostic interactions with Dipp methyl groups. Small size C- and N-donors give rise to related complexes 1⋅L but only one molecule of P-donors, for example, PMe3, can bind to 1 , causing one of the hydrides to form a three-centered, two-electron (3c-2e) Mo-H→Mo bond ( 2⋅PMe3 ). A DFT analysis of the terminal and bridging hydride coordination to the Mo≣Mo bond is also reported, along with reactivity studies of the Mo−H bonds of these complexes. Reactions investigated include oxidation of 1⋅thf by silver triflimidate, AgNTf2, to afford a monohydride [Mo2(μ-H)(μ-NTf2)(μ-AdDipp2)2] ( 4 ), with an O,O’-bridging triflimidate ligand. 相似文献
53.
Angel Alberola Celia Andrés Alfonso González Ortega Rafael Pedrosa 《Journal of heterocyclic chemistry》1984,21(5):1575-1576
A new regiospecific synthesis of 1-aryl-substituted pyrazoles by reaction of aryl-hydrazines with β-aminoenones is reported. 相似文献
54.
We discuss the use of MS and XRD in the study of an iron-poor soil. We have performed Mössbauer spectroscopy and careful XRD analysis on < 2 μm, < 50 μm and <2 mm sized samples, treated with different Fe-extraction methods. Since the iron content of the coarser fractions is comparable to that of the clay fractions, we show that the usual Mössbauer sample preparation procedures used to overcome the texture effects and to make background absorption corrections are insufficient and that additional homogenization is necessary. By milling the samples for a short interval we achieved spectral area values which are in agreement with Atomic Absorption results and allow to follow the behaviour of the extraction methods. 相似文献
55.
Celia Hoyles Lulu Healy 《International Journal of Computers for Mathematical Learning》1997,2(1):27-59
In this paper, we analyse the processes through which students come to negotiate mathematical meanings for reflective symmetry. We describe a microworld, Turtle Mirrors, designed to provide tools to help students focus simultaneously on actions, visual relationships and symbolic representations. Through a detailed case study of one students thinking-in-change, we examine how the interactions with her partner and with the machine support a fusion of spontaneous and scientific concepts. Other examples of students work further illustrate how the microworld tools offer a means to supplement local understandings of symmetry with those with more explicit, mathematical formulations.This revised version was published online in September 2005 with corrections to the Cover Date. 相似文献
56.
Cámpora J Maya CM Palma P Carmona E Gutiérrez-Puebla E Ruiz C 《Journal of the American Chemical Society》2003,125(6):1482-1483
The often facile C-/O-tautomerization of transition metal enolates is severely hindered in the cyclic Ni complexes 1 and 2, allowing the study of their individual reactivities. At room temperature only the O-bound tautomer, 2, reacts with aldehydes, giving rise to the corresponding addition products. 相似文献
57.
Santo M Giacomelli L Cattana R Silber J Blanco MM Schapira CB Perillo IA 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(7):1399-1407
The solvatochromism in 8-hydroxy-1,6-naphthyridin-5(6H)-one-7-carboxylic acid methyl ester (1), 5-hydroxy-1,7-naphthyridin-8(7H)-one-6-carboxylic acid methyl ester (2), and 4-hydroxy-2-methyl-1(2H)-isoquinolone-3-carboxylic acid methyl ester (3), has been studied in solvents of different polarity and hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) ability. The relative stabilities of isomers for these naphthyridine derivatives and their interaction with the solvent are reported. Two intramolecular hydrogen-bonded structures contribute to the ground state of compound 1. Temperature effects on the absorption bands were recorded to analyse the possible equilibrium between covalent and zwitterionic forms. The formation of zwitterionic species was observed only in HBD solvents, from which is inferred the solvent assistance in the proton transference. AM1 and PM3 semi-empirical calculations were used in support of the proposed interpretations. 相似文献
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