Secondary solvent kinetic isotope effect

The carboethoxy stabilized secondary enamines, 25 and 26, were studied by Guthrie and Jordan68. In the absence of buffer, and at pH 5 to 6, acid catalysis is evident and a solvent kinetic isotope effect, (kH+/kD+) = 2.3, is found for 25. These results clearly support rate-controlling C-protonation of the enamine the catalytic constants are included in Table 9. Both 25 and 26 show general-acid catalysis of hydrolysis in the pH... 

Denu, J. M., and Fitzpatrick, P. F., 1994, Intrinsic primary, secondary, and solvent kinetic isotope effects on the reductive half-reaction of D-amino acid oxidase evidence against a concerted mechanism. Biochemistry 33 400194007. 

The secondary deuterium kinetic isotope effect is comprised of two secondary p-deuterium kinetic isotope effects of 1.22/jg-D and a secondary -deuterium kinetic isotope effect of 1.02. This kinetic isotope effect is identical to those found in several protic and dipolar aprotic solvents such as sulphuric acid-water, trifluoroethanol and methylene chloride. As a result, it has been concluded that the phenyl diazonium salt decomposes by the same mechanism in all solvents. 

The low values obtained for the secondary deuterium kinetic isotope effects in the comparative solvolyses of (141) and (142) in formic acid (kn/ko = 1.154 at 29.9 °C) and in aqueous ethanol kji/ko = 1.116 at 64.4 °C) are regarded as evidence for extensive bridging (143) in the solvolytic transition state the deuterium atom at the C-1 bridgehead in (142) obviates the scrambling problem.As the contribution from the P-deuterium is unknown, then assuming the effects of a- and -deuterium are cumulative the tt-kinetic isotope effect is further decreased from the value expected for reaction occurring without nucleophilic assistance. The products of solvolysis of ent-methyl-12p-p-tolylsulphonyloxybeyeran-19-oate, which is a related bicyclo [3,2,1 ] -octan-2-yl system, have been discussed in terms of intermediate carbocation stabilities and lifetimes in the solvent employed. ... 

Kinetic isotope effects primary and secondary deuterium kinetic isotope effects. Heavy atom isotope effects. Solvent isotope effects. SnI and Sn2 mechanisms. 

The kinetic solvent-isotope effects on these reactions are made up of primary and secondary kinetic isotope effects as well as a medium effect, and for either scheme it is difficult to estimate the size of these individual contributions. This means that the value of the isotope effect does not provide evidence for a choice between the two schemes (Kresge, 1973). The effect of gradual changes in solvent from an aqueous medium to 80% (v/v) Me2SO—H20 on the rate coefficient for hydroxide ion catalysed proton removal from the monoanions of several dicarboxylic acids was interpreted in terms of Scheme 6 (Jensen et al., 1966) but an equally reasonable explanation is provided by Scheme 5. 

The nucleophile in the S.v2 reactions between benzyldimethylphenylammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C (equation 42) can exist as a free thiophenoxide ion or as a solvent-separated ion-pair complex (equation 43)62,63. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects for these Sjv2 reactions were determined to learn how a substituent on the nucleophile affects the structure of the S.v2 transition state for the free ion and ion-pair reactions64. 

Hydrogen abstraction from propan-2-ol and propan-2-ol- /7 by hydrogen and deuterium atoms has been studied by pulsed radiolysis FT-ESR. A secondary kinetic isotope effect was observed for H (D ) abstraction from the C—H (C—D) bonds. The results were compared with ab initio data. In similar work, the kinetic isotope effects in H and D abstraction from a variety of other alcohols in aqueous solvents have been measured. It was found that, compared with the gas phase, the reactions exhibit higher activation energies in agreement with the ability of solvation to decrease the dipole moment from the reactant alcohol to the transition state. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

Hammett relations, secondary kinetic isotope effects ", pressure studies and viscosity-independent rates ° have all pointed to a two-bond homolysis taking place via TS 3. The rates are, however, insensitive to change of the solvents = —1.18 in CDCI3, p+ = —1.17 in CCLt and p+ = —1.0 in C6H5CI at 80 °C). This may indicate that there is little solvent interaction taking place via 3. The variation of the activation parameters with the substituent can be solely dependent on the formation and cleavage of the bonds a and b in 3. 

The experimental data for oxidation of benzyl alcohol,1 aliphatic primary and secondary alcohols,2 and cholesterol3 with cetyltrimethylammonium (CTA) dichromate indicated that the reactions occur in a reverse micelle system produced by the oxidant. Michaelis-Menten-type kinetics were observed with respect to the reductants. The product of the oxidation of cholesterol depends on the solvent. In dichloromethane, the product is 7-dehydrocholesterol, whereas with dichloromethane containing acetic acid the product is 5-cholesten-3-one. A low kinetic isotope effect, k /ku = 2.81, was observed in the oxidation of methanol- this, combined with the rate data and the reverse solvent isotope effect [ (H20)/fc(D20) = 0.76], suggests that these reactions... 

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