Intramolecular kinetic isotope data

For comparison, let us compare the methanol data with those obtained by Horsewill et al. [80] who have reported an intramolecular quadruple proton transfer in the solid state between the four OH groups of solid calix[4]arene. Almost temperature independent rate constants were observed, which are again indicative of tunneling. An Arrhenius curve can be calculated using reasonable parameters (Table 6.4) which can reproduce both the pure methanol and the calix[4]arene data. It would be interesting to know more about the kinetic isotope effects in both systems. 

Further evidence to exclude the triplet radical pathway includes the use of cyclopropyl substrates, which serve as a radical clock. In all cases, the reaction proceeds with no indication of ring fragmentation. The nature of the transition state of the C—H insertion step has been analyzed, via a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes. A negative q value of 0.73 is obtained for the intermolecular reaction with trichloroethylsulfamate [71]. Such data indicate that there is a small, but significant, preference for electron-rich substrates, thus the resonance does contribute to the stabilization of a partial positive charge at the insertion carbon in the transition state. A kinetic isotope value of 1.9 is observed for competitive intramolecular C—H amination with a deuterated substrate (Eq. (5.21)). 

Intramolecular Isotope Effects. The data in Figure 2 clearly illustrate the failure of the experimental results in following the predicted velocity dependence of the Langevin cross-section. The remark has been frequently made that in the reactions of complex ions with molecules, hydrocarbon systems etc., experimental cross-sections correlate better with an E l than E 112 dependence on reactant ion kinetic energy (14, 24). This energy dependence of reaction presents a fundamental problem with respect to the nature of the ion-molecule interaction potential. So far no theory has been proposed which quantitatively predicts the E l dependence, and under these circumstances interpreting the experiment in these terms is questionable. 

Several papers in this volume deal with transport in various kinds of liquids others examine critically the fundamental statistical-mechanical theory determining isotope effects for both equilibrium and kinetic processes in condensed as well as gaseous systems. These studies are of interest not only because they serve as a framework for comparing the merits of different isotope separation processes, but they provide powerful tools for using isotope effect data to obtain an understanding of inter-molecular forces in condensed and adsorbed phases and changes in intramolecular forces in isolated molecules. The title of this volume has accordingly been broadened from that of the symposium to reflect the wider scope of its contents. 

The kinetic data together with isotope exchange and solvent isotope elfect data are consistent with a mechanism involving non-interconverting cis and trans allylic carbanion intermediates, with partially intramolecular proton transfer,... 

The base-catalysed isomerization of /3-epoxy-sulphones (126) to y-hydroxy-ajS-unsaturated sulphones (127) was shown by kinetic data, an isotope effect, and stereochemistry to involve an intramolecular suprafacial 1 -> 3 hydrogen migration, with a carbanion-like transition state. " 4-Hydroxy-2-sulpholen, 3-hydroxy-4-chlorosulpholan, and 3,4-epoxysulpholan were found to react with thiolates to give mixtures of cis- and tran5-3-hydroxy-4-(alkylthio)sulpholans in a ratio of 2 3." ... 

On the other hand, Hurst s group proposed another mechanism of water oxidation catalyzed by 2, based on structural [102], kinetic [101], and isotopic distribution data of O2 evolution [103]. Their mechanism involves a Ru ORu intermediate with a terminal ruthenyl O atom on each center that is considered to be either an O2 evolving species or its intermediate precursor. They proposed two pathways for a catalytic mechanism the elements of H2O are added as OH and H onto the adjacent terminal ruthenyl O atoms [as pathway (i). Fig. 14a], and OH and H are added to a bpy ring and one of the ruthenyl O atoms for incorporation of two water molecules [as pathway (ii). Fig. 14b] [103]. Their isotopic distribution data can be quantitatively accounted for by two pathways. It excludes a direct 0-0 bond formation either by the intramolecular coupling of Ru = O groups within a single complex or by the intermolecular mechanism by bimolecular reactions of the complexes. 

The reaction of pyridine-1-oxide with acetic anhydride to form 2-acetoxy-pyridine, which cannot proceed through an analogous anhydro base, exhibits pseudo-first-order kinetics and a secondary isotope effect 2,6-d2-, k fkiy = 0.92). An ionic process is consistent with the kinetic data and with the effects of 3-substituents on product distribution. The pathway involves attack of acetate ion within the ion pair (XII440) or attack of acetate at C-2 of the cation, but does not involve intramolecular rearrangement of the free cation. ... 

The aquation kinetics of the chloropentaamminecobalt(III) ion in water-ethanol mixtures has been studied. The rate constants correlate well with the Grunwald-Winstein Y parameter and with the dielectric constant of the medium. The data supports a D mechanism for the reaction. The loss of chloride from the complexes cw-[Co(en)2(NH2CH2CH20H)Cl] and cw-[Co(en)2(NH2(CH2)3 0H)Cl] has been studied in aqueous ethyleneglycol at 40-65 °C in acidic media and at 20-35 °C in basic media.The rate constants decreased linearly with the increasing mole fraction of the cosolvent. The loss of chloride resulted in the formation of the chelated amino-alcohols as the main product. The observed solvent isotope effect (A h2oAd2o) = 112 at 50 °C, [HCIO4] =0.01 moldm for chloride release is lower than the value reported for the aquation of the cw-[Co(en)2(alkylamine)Cl] complexes (1.38-1.44). This result may indicate the lack of direct solvent intervention in the act of substitution at the cobalt(III) center, as expected for a true intramolecular reaction. 

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