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81.
[reaction: see text] Enantiomerically pure gamma-fluoroalkyl beta-amino sulfones are readily synthesized in three steps starting from fluorinated imidoyl chlorides and arylmethyl sulfones. A complementary two-step sequence starting from chiral fluorinated beta-amino sulfoxides has also been developed. To illustrate the application of this procedure, a new method for the synthesis of alpha-fluoroalkyl allylic amines in optically pure form involving a Julia methylenation-desulfonylation reaction is presented. 相似文献
82.
Transformation
of dynamic DSC results into isothermal data for the curing kinetics study
of the resol resins 总被引:1,自引:0,他引:1
M. V. Alonso M. Oliet J. García F. Rodríguez J. Echeverría 《Journal of Thermal Analysis and Calorimetry》2006,86(3):797-802
Kinetics of thermosetting polymers curing is difficult to study by isothermal
methods based on the differential scanning calorimetry (DSC) technique. The
difficulty is due to the low sensitivity of the equipment for total reaction
heat measurements during high temperature process. The aim of this paper is
to display the equivalence between a dynamic model, the Ozawa method, and
an isothermal isoconversional fit, which allows predicting the isothermal
behavior of the resol resins cure through dynamic runs by DSC. In this work,
lignin–phenol–formaldehyde and commercial phenol–formaldehyde
resol resins were employed. In addition, the isothermal kinetic parameters
for both resins were performed by means of transformation of the data obtained
from the dynamic Ozawa method. 相似文献
83.
J. Hernández-Borges M. A. Rodríguez-Delgado F. J. García-Montelongo 《Chromatographia》2006,63(3-4):155-160
Several improvements in sample pretreatment for the determination of organic pollutants (i.e. n-alkanes, polycyclic aromatic hydrocarbons, PAHs) in marine biota (mussels) are presented. The use of liquid nitrogen and
homogenization of the samples are shown to be an alternative to the time consuming liophilization step required for the analysis
of biota samples. Microwave-assisted hydrolysis and extraction are combined to isolate organic pollutants (19 n-alkanes and 27 PAHs) from biota matrices. Experimental design (ED) and artificial neural networks (ANNs) were used to optimize
the experimental conditions. NIST-CRM 2978 was used to test the validity of the developed method which shows a good agreement
with certified values. 相似文献
84.
Cadierno V García-Garrido SE Gimeno J Nebra N 《Chemical communications (Cambridge, England)》2005,(32):4086-4088
A general and efficient method for the deprotection of N-allylic substrates in aqueous media, using catalytic amounts of the bis(allyl)-ruthenium(IV) complexes [Ru(eta3:eta2:eta3-C12H18)Cl2] and [{Ru(eta3:eta3-C10H16)(micro-Cl)Cl}2], has been developed. 相似文献
85.
A simple reverse flow-injection (rFIA) manifold for the direct determination of aluminum in drinking water is proposed. This rapid and sensitive method is based on the formation of an Al(3+) complex with salicylaldehyde picolinoylhydrazone (SAPH), which shows a maximum blue-green fluorescence (lambda(ex)=384 nm, lambda(em)= 468 nm) at pH 5.4. Operative conditions both for batch and rFIA procedures were investigated including reagent concentration, buffer solutions, injection loop, reacting coil and wavelengths used for the fluorimetric detection. The tolerance limits of foreign ions have been also evaluated, before and after the addition of masking agents. The reverse flow-injection procedure allows determination of Al(3+) at ppb level (LOD: 1.9 mug l(-1)) within a working range of 5-30 mug l(-1). The proposed method was successfully employed for the determination of Al(3+) in several commercial drinking, soft drinking (as certified reference material), and tap water samples. 相似文献
86.
Hevia E Pérez J Riera V Miguel D Campomanes P Menéndez MI Sordo TL García-Granda S 《Journal of the American Chemical Society》2003,125(13):3706-3707
As depicted in the scheme, the alkylidenamido complex 1, a N-rhenaimine, reacts with ketenes to afford the beta-lactams 2-4, which possess a {Re(CO)3(bpy)} fragment as substituent at nitrogen. Clean demetalations using HOTf or MeOTf yield the free beta-lactams or N-methyl-beta-lactams along with [Re(OTf)(CO)3(bpy)]. DFT calculations help to rationalize why the reaction is faster than those of non transition metal N-substituted imines. 相似文献
87.
Cabaleiro-Lago EM Rodríguez-Otero J García-López RM Peña-Gallego A Hermida-Ramón JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(20):5966-5974
A comprehensive B3LYP/6-31+G* study on the electrocyclization of 1,2,4,6-heptatetraene analogues was conducted. Starting from the cyclization of (2Z)-2,4,5-hexatrienal, a pericyclic disrotatory process favored by the assistance of a electron lone pair, we incorporated small modifications in its molecular structure to obtain a truly pseudopericyclic process. To this purpose electronegative atoms (fluorine and nitrogen) were added to give a more electrophilic character on the carbon atom which is attacked by the electron lone pair of the oxygen atom. The complete pathway for each reaction was determined, and changes in magnetic properties were monitored with a view to estimating the aromatization associated with each process. This information, together with the energetic and structural results, allowed us to classify the reactions as pseudopericyclic or pericyclic. Among all studied reactions only one was a truly pseudopericyclic process and another was a borderline case. The features of this unequivocally pseudopericyclic case were analyzed in depth. 相似文献
88.
Torres-Lapasió JR García-Alvarez-Coque MC Bosch E Rosés M 《Journal of chromatography. A》2005,1089(1-2):170-186
The problems associated to the modelling and optimisation of the chromatographic resolution of mixtures involving ionisable solutes at varying pH and acetonitrile content are discussed. Several retention models that separate the contributions of solute, column and stationary phase, were used. The retention was predicted with low errors in large pH domains (2-12), which was an essential requirement to face the optimisation of resolution. The selected mixture was particularly problematic under the viewpoint of resolution, owing to the excessively diverse acid-base behaviour of solutes. This variety led to sudden drops in retention at different pH for each solute, yielding numerous peak crossing, which made finding shared regions of high resolution especially difficult. Conventional resolution diagrams for these situations are scarcely informative, since both the overall and the worst elementary resolutions drop to zero if at least two compounds remain overlapped, even when all the others are baseline resolved. A new chromatographic objective function is proposed to address this drawback. This function, called "limiting peak count", is based on the limiting peak purity concept, and measures the success in the resolution focusing on the resolved solutes, in contrast to conventional resolution assessments that attend mainly to the least resolved solutes. Limiting peak count yields the same result as conventional assessments when full resolution is possible, but it is also able to discriminate the maximal resolving power in low-resolution situations. It offers a different perspective to that given by the complementary mobile phases approach, and the computation is far simpler. 相似文献
89.
The preparation of ethyl 4-(2-bromoethyl)thieno[2,3-b]pyrrole-5-carboxylate and ethyl 6-(2-bromoethyl)thieno[3,2-b]pyrrole-5-carboxylate by reaction of t-butyl 2-(2-thienyl)carbazate and t-butyl 2-(3-thienyl)carbazate with ethyl-5-bromo-2-oxopentanoate are described. 相似文献
90.
García-Río L Leis JR Moreira JA Araujo E Norberto F Ribeiro L 《The Journal of organic chemistry》2003,68(11):4330-4337
A kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl-3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H(2)PO(4)(-)/HPO(4)(2)(-), H(3)BO(3)/H(2)BO(3)(-), and HCO(3)(-)/CO(3)(2)(-) regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of p = 11.5. The reaction rate for the process through the monoanion, k(2), decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k(3) = 10(10) s(-)(1) to estimate the value of p for the formation of the dianion of the N-nitrosoguanidine, obtaining values of p < 24. 相似文献