1,721,516 research outputs found
Gaetano Minnucci (1896-1980). Vita, opere e scritti Gaetano Minnucci (1896-1980. Life, works and writings
No full textGaetano Minnucci (1896-1980) è stata una figura molto importante per la storia dell'architettura italiana del Novecento. Attivo ininterrottamente dal 1920 fino al 1980, l'ingegnere ha dato un importante contributo alla diffusione in Italia dell'architettura moderna e nel secondo dopoguerra ha realizzato opere di grande valore dal punto di vista ingegneristico e tecnologico in particolare nel Lazio e nelle Marche, la sua regione di origine
Radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethane
No full textThe radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethane was investigated on the basis of kinetic and product distribution data. Whereas the ionic process followed a third-order rate law (second order in Br-2), the radical bromination was second order in Br-2 and zero order in olefin in the reagent concentration range examined. Significant inverse kinetic isotope effects were found for the bromination of 3,4'-bis(trifluoromethyl)-1,1-diphenylethylene and cis-1,2-diphenylethylene under these conditions. At variance with the ionic bromination, which gave mixtures of meso- and d,l-dibromides only the meso-dibromides were obtained both from the cis- and trans-1,2-diphenylethylenes. The kinetic and product distribution data for the radical process are discussed in terms of a mechanism in which caged radical intermediates are formed. (C) 1998 John Wiley & Sons, Ltd
A kinetic, product and kinetic isotope effect investigation of the bromination of 1,1-diphenylethylenes and of their 2,2-dideuterio derivatives
No full textThe kinetics of bromination of 1,1-diphenylethylene(1a), 4-trifluoromethyl-1,1-diphenylethylene(1b), and 1-(3-trifluoromethylphenyl)-1-(4-trifluoromethylphenyl)ethylene(1c) and of their 2,2-dideuterio derivatives have been investigated in 1,2-dichlorodthane. The rate law was always second-order in Br-2 and first-order in olefin, with the following k(3): 1a (L=H), 1.7(0.1) x 10(7); 1b (L = H), 8.0 (0.1) x 10(4); 1c (L = H), 52 (2) dm(6) mol(-2) s(-1). Olefin 1a (L = H) gave dibromide 4a and vinyl bromide 3a in a ratio changing from 99:1 at 10(-2) mol dm(-3) to 5:95 at 10(-4) mol dm(-3) Br-2 and olefin. The k(3) was independent of the reagent concentrations and of the extent of proton loss from the intermediate, showing that the last step was not rate limiting and the formation-of the intermediate-was completely rate determining. In the whole investigated concentration range, only dibromides 4b and 4c were instead obtained from Ib and Ic,:respectively. A negligible KIE, 0.97 (0.01) was found for 1a, whether the dibromide or the vinyl bromide was the main product, while significant inverse KIEs were obtained for 1b, 0.75 (0.05), for 1c, 0.70 (0.05), on deuteriation of the methylene group, and for cis-1,2-diphenylethylene,2. The kinetic and product distribution data are discussed in terms of nature of the intermediates,: depending on substituent effect and reagent concentration
KINETIC EVIDENCE FOR RATE DETERMINATION DURING THE NUCLEOPHILIC STEP OF OLEFIN BROMINATION - THE CASE OF 5H-DIBENZ[B,F]AZEPINE-5-CARBONYL CHLORIDE
No full textThe bromination of 5H-dibenz[b,f]azepine-5-carbonyl chloride has been investigated in 1,2-dichloroethane at 5, 25, and 50-degrees-C. Working at reagent concentrations where the bromination was very slow, the formation of a 1:1 charge-transfer complex (CTC) was shown spectrophotometrically by the presence of a large difference absorption with respect to the olefin and to Br2 alone. Although the formation constant of this CTC was too low (K(f) less-than-or-equal-to 0.1 M-1) to be determined, the products (K(fepsilonCT) at the three temperatures were obtained from the slopes of the linear plots of the difference absorbances against the olefin concentrations, and a value of the formation enthalpy, DELTAH = -0.9 (0.05) kcal mol-1, was obtained from a plot of In (K(fepsilonCT) against 1/T. The kinetics of bromination were measured at variable temperature and reagent concentrations. At 50-degrees-C and [Br2] ranging between 2 x 10(-2) and 5 x 10(-1) M the usual third-order kinetics were observed. The same rate law was obeyed at 25-degrees-C and [Br2] less-than-or-equal-to 5 x 10(-2) M, whereas neither a third-order nor a fourth-order rate law was followed at 5 x 10(-1) M Br2 and olefin. At 5-degrees-C and [Br2] less-than-or-equal-to 5 x 10(-2) M the third-order rate law was again observed, but at 5 x 10(-1) M Br2 and olefin an overall fourth-order (third-order in Br2) rate law was cleanly obeyed. A very small apparent activation energy, E(a(obsd)) = 3.45(0.1) kcal mol-1, was found for the third-order process. A significant conductivity, which was highest at the lowest temperature, was found during the course of the reactions. trans-10,11-Dibromo-10,11-dihydro-5H-dibenz[b,flazepine-5-carbonyl chloride was the only reaction product. It was shown by D NMR measurements to exist in 1,2-dichloroethane solution in two forms, both having anti-oriented bromine atoms and being nonequivalent because of different bond angles and bond lengths at C(10) and C(11). They are interconverted through a seven-membered ring inversion by torsion about the C(4a)-N(5)-C(5a) bonds, with a free activation energy DETAG(double dagger) = 16.6(0.2) kcal mol-1. Introducing the values of E(a(obsd)) for the third-order bromination and of DELTAH for the CTC formation in the equation E(a(obsd)) = E(a) + DELTAH[1/ (1 + K(f)[Ol])] gives a true activation energy, E(a), too small for a reaction as slow as the investigated one. This shows that the rate-determining step of this reaction cannot be the CTC ionization, but is rather the collapse of bromonium-tribromide intermediates having a large and negative formation enthalpy. Low temperatures and high Br2 concentrations favor the transformation of the tribromide counteranion into pentabromide, and the third-order dependence of the rate on Br2 results from the fact that the rate-determining step involves a species containing three Br2 and one olefin molecules. This shift from an overall third-order to a fourth-order rate law with decreasing temperature and increasing [Br2] could provide a mechanistic criterium for rate determination during the nucleophilic step of olefin bromination
KINETICS AND STEREOCHEMISTRY OF THE MICROSOMAL EPOXIDE HYDROLASE-CATALYZED HYDROLYSIS OF CIS-STILBENE OXIDES
No full textThe microsomal epoxide hydrolase (mEH)-catalyzed hydrolysis of cis-4,4'-dimethylstilbene oxide (1a), cis-4,4'-diethylstilbene oxide (1b), cis-4,4'-diisopropylstilbene oxide (Ic), and cis-4,4'-dichlorostilbene oxide (1d) have been investigated using rabbit liver microsomal preparations. The kinetic parameters, K-m and V-max, and the absolute stereochemistry of the reactions have been determined and compared with those of cis-stilbene oxide (1e). All epoxides 1a-d are hydrolyzed by mEH with high product enantioselectivity to give (R,R)-(+)-diols with ee greater than or equal to 90%. The presence of the substituents on the phenyl rings markedly reduces the rates of mEH catalyzed hydrolysis with respect to cis-stilbene oxide, by increasing K-m and reducing V-max in the cases of 1a, 1b, and 1d, or reducing only the V-max in the case of 1c. The very low V-max, together with a persistent ability to fit into the mEH active site, make all these epoxides, and particularly 1c, inhibitors of cis-stilbene oxide hydrolysis. The kinetic and stereochemical results are interpreted on the basis of the proposed topology of the mEH active site. (C) 1994 Wiley-Liss, Inc
Crown ether catalyzed stereospecific synthesis of Z- and E-stilbenes by Wittig reaction in a solid-liquid two-phases system
No full textPotassium hydroxide and a catalytic amount of 18-crown-6 are used, in alternative to the classical Wittig conditions, to prepare very rapidly and stereoselectively Z- and E-stilbenes. In particular, the use of benzyltriphenylphosphonium iodides always leads to a complete Z-stereospecificity, while benzyldiphenylchlorophosphonium salts give a complete E-stereospecificity. (C) 1996 Elsevier Science Lt
Enantioconvergent transformation of racemic cis-beta-alkyl substituted styrene oxides to (R,R) threo diels by microsomal epoxide hydrolase catalysed hydrolysis
No full textBoth enantiomers of cis-beta-ethyl, beta-n-propyl, beta-n-butyl, and beta-n-hexyl substituted styrene oxides undergo microsomal epoxide hydrolase catalysed hydration at the (S) carbon to give the corresponding (R,R) three diols in a > 90% e.e. A complete kinetic resolution of the racemic epoxide is also obtained with the beta-ethyl substituted substrate, but not with its higher homologues
Crown ether catalyzed stereoselective synthesis of vinyl ethers in a solid liquid two-phase system
No full textPotassium hydroxide and a catalytic amount of 18-crown-6 provide a simple and stereoselective access to Z vinyl ethers in high yields by addition of primary, secondary and tertiary alcohols to phenylacetylene
La collezione degli strumenti di anestesiologia
No full textCollana del patrimonio storico scientifico dell'Università di Siena "Mater.iali
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