Kinetic H/D isotope effect

No kinetic H/D isotope effect in benzene oxidation by Oa was observed, and the mechanism shown below was proposed [120, 121] ... 

The temperature dependence of this rate constant was measured by Al-Soufi et al. [1991], and is shown in Figure 6.17. It exhibits a low-temperature limit of rate constant kc = 8x 105 s 1 and a crossover temperature 7 C = 80K. In accordance with the discussion in Section 2.5, the crossover temperature is approximately the same for hydrogen and deuterium transfer, showing that the low-temperature limit appears when the low-frequency vibrations, whose masses are independent of tunneling mass, become quantal at Tisotope effect increases with decreasing temperature in the Arrhenius region by about two orders of magnitude and approaches a constant value kH/kD = 1.5 x 103 at T[Pg.174]

The ee values in the enantioselective deprotonation are independent of the size of the attached alkyl residue. The method tolerates several substituents, e.g. 2- or 3-dibenzylamino, 3- or 4-(A(A-dialkylcarbamoyloxy), or 4-TBDMSO. Essentially enantiopure 2-hydroxy acids, 3-amino alkanols, 7-amino alkanols, cyclopropyl carbamates, and 2-hydroxy-4-butanolides " were obtained. Extraordinary high (>70) kinetic H/D isotope effects were observed in the deprotonation of chiral 1-deuteroalkyl carbamates. Kinetic resolution of racemic alkyl carbamates was achieved. ... 

In this section first different models of single H-transfer will be reviewed, including primary kinetic H/D isotope effects, where the focus is on the Bell-Limbach model. Then formal kinetics will be used to describe multiple hydrogen transfers and their kinetic isotope effects. 

In the theory of Bigeleisen [6], a combination of the theory of equilibrium isotope effects with Eyrings transition state theory [5], kinetic H/D isotope effects can be expressed by... 

The reaction rates observed are substantially increased as compared to TPP the increase is larger for the /1-form as compared to the a- form (Fig. 6.22(b)). Kinetic H/D isotope effects have not yet been studied. The difference in the reaction kinetics of the two forms has been explained as follows. The observed tautomerism in the a- form was interpreted with a circular tautomerism as illustrated in Fig. 6.21(a), with similar transfer rates for the formation of all intermediates. However, the observed transfer in the /1-form was assigned to a local HH-transfer within the two intramolecular hydrogen bonds which led to an extra increase in the rate constants. 

Al-Soufi et al. [83] have followed the kinetics of the intramolecular H- and D-transfer between the keto and the enol form of 2-(2 -hydroxy-4 -methylphenyl) benzoxazole (MeBO) dissolved in alkanes using optical methods. No dependence of the rate constants on the solvent viscosity could be found. The Arrhenius diagram obtained over a very wide temperature range is depicted in Fig. 6.48. At low temperatures, the very rare regime of temperature-independent rate constants is obtained, exhibiting a very large temperature independent kinetic H/D isotope effect of about... 

The larger kinetic H/D isotope effects in the parent radical can be explained in terms of the higher symmetry of the parent radical as compared to the di-fert-butyl radical. In the latter, the methyl groups on both sides of the ring are not ordered, leading to effective asymmetric double well potentials of the H-transfer. These examples show how subtle structural effects can lead to very different H-transfer properties. 

Figure 6.54 Arrhenius curves (a) and kinetic H/D isotope effects (b) of the intrinsic H-transfer in a thermophiiic aicohoi dehydrogenase (ADH) according to Kohen et ai. [91]. The soiid iines were caicuiated using the parameters iisted in Tabie 6.4.

The Kinetic H/D Isotope Effect in Electroless Copper Deposition... 

The (1-deuterated) diastereomers 135 are accessible via deuteration and TMEDA-assisted removal of the remaining proton [Eq. (38)] [79]. In other words, the extraordinarily high kinetic H/D isotope effect permits the utilization of deuterium as a protecting group against deprotonation. 

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