Isotope effects hydride abstraction

Kinetic isotope effect. The oxidation of benzaldehyde by permanganate ions is believed to occur by hydride abstraction. What value of k /kD do you predict for C6H5CHO/ C6H5CDO For C6H5CHO/C6D5CHO ... 

The kinetic parameters are E = 6.3 kcal.mole" and AS = —38.4 eu, and at 25 °C the reaction exhibits a primary kinetic isotope effect of 6.6. When 0-labelled MnO was employed, no labelled oxygen appeared in the benzophenone. The mechanism involves abstraction of hydrogen, either as a hydride ion or a hydrogen atom, from the anion of the alcohol... 

A few results have been reported on the oxidation of cyclohexanol by acidic permanganate In the absence of added fluoride ions the reaction is first-order in both alcohol and oxidant , the apparent first-order rate coefficient (for excess alcohol) at 25 °C following an acidity dependence k = 3.5-1-16.0 [H30 ]sec fcg/A , depends on acidity (3.2 in dilute acid, 2.4 in 1 M acid) and D2o/ H20 is f-74. Addition of fluoride permitted observation of the reaction for longer periods (before precipitation) and under these conditions methanol is attacked at about the same rates as di-isopropyl ether, although dioxan is oxidised over twenty times more slowly. The lack of specificity and the isotope effect indicates that a hydride-ion abstraction mechanism operates under these conditions. (The reactivity of di-isopropyl ether towards two-equivalent oxidants is illustrated by its reaction with Hg(II).) Similar results were obtained with buffered permanganate. 

Two studies have been performed by Littler on the oxidation of cyclohexanol by Hg(II), the second leading to more detailed and reliable data. The reaction is first-order in both oxidant and substrate but the rate is independent of acidity. E is 24.8 kcal.mole AS is 1 eu, Ath/Acd is 3.0 and ko ol HzO 1-30-At 50 °C di-isopropyl ether is attacked at about one-half the rate of isopropanol, which implies that hydride ion abstraction is occurring in both cases. This is supported in the case of cyclohexanol by the isotope effects. 

The results from these experiments also allowed Hannon and Traylor to determine the primary and secondary hydrogen deuterium kinetic isotope effects for the hydride abstraction reaction. If one assumes that there is no kinetic isotope effect associated with the formation of 3-deutero-l-butene, i.e. that CH2=CHCHDCH3 is formed at the same rate (k ) from both the deuterated and undeuterated substrate (Scheme 25), then one can obtain both the primary (where a deuteride ion is abstracted) and the secondary deuterium... 

The parent ion 5 and its naphtho[6] analogue have been generated by treatment of 1 and 11, respectively, with trityl tetrafluoroborate. The cations so formed are captured by water and the final product of reaction is the ring opened aryl aldehyde (equation 24)230 231. Kinetic measurements have shown230 that abstraction of hydride ion from 1 is first order and displays a deuterium isotope effect of 6.5. To the best of the author s knowledge, the isolation and spectroscopic characterization of an unsubstituted cycloproparenyl cation has yet to be achieved. 

For oxidation of ethylbenzene in FeCl3-catalysed reaction with ferf-butyl hydroxyperoxide isotope effect fcHn//C id = 2.02(4) was found in analysis of deuterium content in ethylbenzene at natural abundance. Mixture of ethylbenzene and 1,1-dideuteroethylbenzene (45 55) was used for the determination of (hh/ fcDD = 5.0(1). Isotope effects KIEr- = 3.5(2) and KIE2- = 1.41(6) were calculated from formulas (26). The value of primary isotope effect is consistent with a C-H bond breakage in the rate-limiting step, but its value does not allow distinguishing among a hydrogen radical abstraction, C-H insertion or a hydride abstraction processes. Secondary isotope effect is consistent with formation of benzylic radical. 

Two other systematic solution phase tritium isotope effects studies have been made that will only be mentioned here. In the reduction of alkyl halides by trialkyl-tin hydrides, there is an abstraction step... 

How does structure determine organic reactivity, 35, 67 Hydrated electrons, reactions of, with organic compounds, 7,1 15 Hydration, reversible, of carbonyl compounds, 4, 1 Hydride shifts and transfers, 24, 57 Hydrocarbons, small-ring, gas-phase pyrolysis of, 4, 147 Hydrogen atom abstraction from O—H bonds, 9, 127 Hydrogen bonding and chemical reactivity, 26, 255 Hydrogen isotope effects in aromatic substitution reactions. 2, 163... 

That alkenes are formed from those alkyl groups containing a jS-hydrogen atom strongly implies that the mechanism of alkene formation involves a /3-hydride abstraction step. There is a very pronounced kinetic isotope effect when C6D5CD2CH2COCI is decarbonylated, which indicates that not only does a jS-deuteride abstraction take place but that it is also rate determining. Further evidence for the participation of a /3-hydride abstraction comes from the decarbonylation of erythro- or t/ reo-2,3-diphenylbutanoyl chloride, where the former yields the (ii)-alkene and the latter the (Z)-isomer. 

Allylic alcohols are more easily oxidised than saturated alcohols and in this case rates are greatest for the equatorial isomers [42]. Activated manganese dioxide is commonly used for selective oxidations of allylic alcohols, although it will also attack saturated alcohols slowly under special conditions [44]. Its mechanism of action is not understood, although some relevant observations have been reported [43]. 2,3-Dichloro-5,6 dicyanobenzoquinone (DDQ) is a very mild and selective oxidant for allylic alcohols [43]. Kinetic studies [46] on 3a- and 3 9 hydroxy-A -systems (8) revealed a higher rate of oxidation of the pseudo-equatorial 3 -alcohol K jK a = 6), and a large primary isotope effect Kj)IKb ca. 1/5) when the C(3) H was replaced by deuterium. These results indicate a rate-determining hydride abstraction from C(3>, with preferred removal of the pseudo-axial 3C1-H as a result of optimum a-7i overlap in the transition state (9). However, a detailed analysis of thermodynamic parameters shows that the cause of the rate difference appears mainly in the term (A5 ) rep-... 

Firstly, let us discuss the example of a thermophilic alcohol dehydrogenase from Bacillus stearothermophilus (bsADH) studied by Kohen et al. [91, 92]. This enzyme catalyzes the abstraction of a hydride to the nicotinamide cofactor NAD+ as depicted in Fig. 6.54. The Arrhenius diagram is depicted in Fig. 6.54(a) a sudden decrease in the apparent slope and the apparent intercept of the Arrhenius curves is observed around room temperature (Fig. 6.54(b)). The puzzling observation is that the kinetic isotope effects are independent of temperature in the high-temper-ature regime but dependent on temperature in the low-temperature regime. 

There was a significant isotope effect in the conversion of [3a- H]7a-hydroxy-cholesterol into 7 -hydroxy-4-cholesten-3-one, indicating that abstraction of a hydride ion from the 3a position may be rate limiting in the reaction. 

---