摘要:

Ostwald solubility coefficients, as logL, for solutes in water and ethanol have been combined to give logPEtOHfor partition between the two pure solvents. Sixty-four such values have been correlated through our solvation equation, the coefficients of which lead to the conclusion that ethanol and water solvents are equally strong hydrogen-bond bases, but that ethanol is much weaker as a hydrogen-bond acid. A slightly different solvation equation has been used to correlate 68 values of logLEtOH; the coefficients in this equation yield the same conclusions as to the hydrogen-bond acidity and basicity of bulk ethanol. In addition, an analysis of the various terms in the logLEtOHcorrelation equation allows the elucidation of the various chemical factors that govern the solubility of gaseous solutes in ethanol solvent at 298 K.Key words: solubility, partition, hydrogen-bonding, ethanol, water.

ISSN:0008-4042    

DOI:10.1139/v98-029

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The aminolysis reactions of ethyl aryl carbonates with benzylamines in acetonitrile at 25.0°C are investigated. The base-catalyzed path,k2, disappears when strong nucleophiles (X =p-CH3O andp-CH3) react with a substrate activated by a strong nucleofuge (Z =p-NO2). The large magnitude of rhox(-1.7 to -2.5), rhoz(3.4 to 4.3), and rhoxz(1.4) values, and relatively largekH/kD(1.6 to 1.8) found for the uncatalyzed path (k1) can be accounted for in terms of a stepwise mechanism with rate-limiting expulsion of the phenoxide leaving group. The catalyzed process (k2) is characterized by the much smaller magnitude of rhox(-1.0 to -1.7), rhoz(0.4 to 0.7), and rhoxz(0.2), the largerkH/kD(2.1 to 2.5) values, and the lower &Dgr;Hdouble daggervalues (1.8-1.9 kcal mol-1) than those of the uncatalyzed process (k1) with large negative &Dgr;Sdouble daggervalues (-65 to -67 cal K-1mol-1). These results are consistent with four- and six-centered transition states for the two processes,k1andk2, respectively.Key words: ethyl aryl carbonates, aminolysis mechanism, base catalysis, stepwise mechanism, cross-interaction constant.

ISSN:0008-4042    

DOI:10.1139/v98-038

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The vibrational circular dichroism (VCD) spectra of 2,5-dioxabicyclo[2.2.1]heptane-3,6-dione1(R = H) and the 1,4-dialkyl derivatives,1(R = Me) and1(R =t-Bu), were calculated by the ab initio magnetic field perturbation (MFP) procedure with a B3LYP/6-31G* force field, and the VCD spectra of the dimethyl and di-t-butyl derivatives were measured in the region 800-1500 cm-1. While the absolute configurations of1(R = Me) and1(R =t-Bu) could be assigned unambiguously by comparison of the experimental and predicted spectra, there is little obvious correspondence between the spectra of the series of compounds. The VCD spectra are analysed on the basis of operation of a coupled oscillator mechanism involving motions of the polar bonds. In the case of the parent dihydro system, bridgehead C-H bond motions appear to dominate contributions to the VCD of vibrational transitions into which they are mixed.Key words: vibrational circular dichroism (VCD), 2,5-dioxabicyclo[2.2.1]heptane-3,6-dione, dilactones, optical activity.

ISSN:0008-4042    

DOI:10.1139/v98-015

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The benefits of using microwaves to accelerate the rates of hydrogen isotope exchange reactions are illustrated by reference to heterogeneous and homogeneous metal-catalysed reactions as well as to homogeneous acid-catalysed reactions. The results show that good incorporation of deuterium is obtained in very short time intervals, typically <20 min, and that the approach has considerable potential for the labeling of a wide range of compounds with both deuterium and, by implication, tritium.Key words: microwave enhanced deuteriations, hydrogen isotope exchange.

ISSN:0008-4042    

DOI:10.1139/v98-045

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines in H2O containing 20 mol% DMSO at 25.0 ± 0.1°C. The reactivity increases as the substituent (X and Y) becomes a stronger electron-withdrawing group. The sigma+constants give better Hammett correlation than sigma constants for the reactions of 4-nitrophenyl X-substituted benzoates with glycylglycine (glygly) and hydrazine (NH2NH2), indicating that the ground-state stabilization effect is unusually significant on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2appear to proceed through the same mechanism, but the degree of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more significant for the glygly system than the NH2NH2system based on ßlgand rhoXvalues. The magnitude of the alpha -effect is observed to be not always dependent on the ßnucvalue but dependent on the electronic nature of the substituent X and Y, i.e., an electron-donating substituent increases the alpha -effect, while an electron-withdrawing one decreases the alpha -effect. The present study has led to the conclusion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha -effect, and the intramolecular H-bonding interactions (4) are proposed for the cause of the increasing or decreasing alpha -effect trends observed in the present system.Key words: alpha -effect, intramolecular H-bonding interaction, ground-state stabilization effect.

ISSN:0008-4042    

DOI:10.1139/v98-043

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The aziridination of olefins with {(4-nitrophenylsulfonyl)imino}phenyl-lambda3-iodane, NsN==IPh (1c), in the presence of [Rh2(OAc)4] proceeds in yields of up to 85% when the olefin is used in large excess. Under optimized conditions, styrene (4a) is aziridinated with 1 equiv. of NsN==IPh (1c) in 64% yield with 2 mol% of catalyst. The aziridines derived from electron-rich olefins undergo ring-opening under the conditions of the aziridination and afford rearrangement products or pyrrolidines. The aziridination is sterospecific with 1,2-dialkyl- and 1,2-arylalkyl-disubstituted olefins, but nonstereospecific with stilbene.The rho -value for aziridination of substituted styrenes is -0.61. No ring-opened products are observed upon aziridination of vinylcyclopropanes. In the presence of chiral RhIIcatalysts, the aziridination is enantioselective, affording an ee of 73% withcis- &bgr; -methylstyrene (4k) and Pirrungs [Rh2{(R)-(-)-bnp}4] catalyst. The experimental results are consistent with a one-step mechanism for transfer of the nitrenoid moiety from the catalyst to the olefin.Key words: aziridination, nitrene transfer, rhodium catalysis.

ISSN:0008-4042    

DOI:10.1139/v98-058

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The specific rates of solvolysis of benzylp-toluenesulfonate and nine benzylic-ring-substituted derivatives are satisfactorily correlated usingNTandYOTsscales within the extended Grunwald-Winstein equation. Addition of a third term, governed by the aromatic ring parameter, shows the additional term to be statistically significant. As one incorporates increasingly more electron-withdrawing substituents, the sensitivities towards changes in solvent nucleophilicity (l) increase, and those towards changes in ionizing power and changes in the aromatic ring parameter decrease. The trend oflvalues can be very nicely shown, with very precise correlations, using an internally generated scale ofYvalues. However, it is demonstrated that there is no overall advantage to use of an internally generated scale if one ultimately wishes to make a comparison with sensitivity values generated using an external scale ofYvalues.Key words: Grunwald-Winstein equation, aromatic ring parameter, solvent nucleophilicity, solvolysis, arylmethylp-toluenesulfonates.

ISSN:0008-4042    

DOI:10.1139/v98-059

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The effect of ion-pairing in an SN2 reaction is very different when the nucleophilic atom is changed from sulfur to oxygen, i.e., changing the nucleophile from thiophenoxide ion to phenoxide ion. When the nucleophile is sodium thiophenoxide, ion-pairing markedly alters the secondary &agr; -deuterium kinetic isotope effect (transition state structure) and the substituent effect found by changing theparasubstituent on the nucleophile. When the nucleophile is sodium phenoxide, ion-pairing does not significantly affect the secondary &agr; -deuterium or the chlorine leaving group kinetic isotope effects (transition state structure) or the substituent effects found by changing aparasubstituent on the nucleophile or the substrate. The different effects of ion-pairing may occur because the electron density on the hard oxygen atom of the sodium phenoxide is not affected significantly by ion-pairing.Key words: nucleophilic substitution, SN2, kinetic isotope effect, transition state, substituent effects, ion-pair.

ISSN:0008-4042    

DOI:10.1139/v98-056

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The lithium pKscale has been extended to 25 indicators with a pKrange of 9.7-24.4. The resulting scale is compared with the cesium ion pair acidities and to ionic pK's in DMSO and aqueous DMSO.Key words: ion pair acidity, organolithium compounds, indicator, acidity scale.

ISSN:0008-4042    

DOI:10.1139/v98-069

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The efficiency of various metallo-phtalocyanines (Pht) and -tetraphenylporphyrins (TPP) as catalysts for the H2O2oxidations of dibenzylsulfide, phenylchloroethylsulfide, and thioanisole is investigated in ethanol and acetonitrile, using imidazole as a cocatalyst. Neither PhtNiIInor TPPCoIIexhibits any catalytic activity. PhtMnIIand TPPMnIIICl accelerate markedly these reactions but do not promote quantitative oxidations, at most 70% of the sulfides being transformed into sulfoxides. In contrast, with PhtFeIIsulfoxides are obtained with a 100% yield from sulfides. Finally, the only catalyst able to oxidize sulfides rapidly (<5 min), completely and quantitatively (100% sulfone) is TPPFeIIICl in EtOH. The absence of any by-product, disulfide in particular, suggests that a free sulfenium radical cation is not an active intermediate in these reactions. The marked differences in the behaviour of TPPMnIIICl and TPPFeIIICl are analyzed by comparing the rates of the catalyst decomposition, of the sulfoxide and sulfone formation as a function of the hydrogen peroxide concentration. The results are discussed in terms of a competition between the several oxidative pathways and a possible mechanism for the oxygen transfer to sulfides.Key words: sulfide, oxidation, H2O2, manganese(III) or iron(III) tetraphenylporphyrin.

ISSN:0008-4042    

DOI:10.1139/v98-050

出版商:NRC Research Press    

年代:1998

数据来源: NRC