Rate constants, hydride transfer

We parameterize the hydride transfer steps in the direction of the reaction with isobutane. Accordingly, we define the hydride transfer rate constant, kn, from collision theory as... 

Hydride transfer rate constants were measured only recently by Stomkowski and the first published values of kn were later confirmed and complemented by other authors It is interesting to note that kn determined spectrophoto-metrically (the rate of disappearance of (C6H5)3C agrees well with the results obtained by the polarographic method elaborated by Plesch ° ... 

DR. NORTON The answer is, as yet, no. I think the experiment to do would be to take a weak base with all three of those hydrides as acids and look at the line broadening. By varying the concentration of the weak base over an enormous range, one should be able to obtain the proton transfer rate constants of all three of those hydrides to the same weak base. That is the next experiment on our list. 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

Another reaction mode of the nitronium ion towards rr-donors also exists where NO acts as an oxidant, resulting in the formation of carboca-tions via hydride abstraction. Rate constants and efficiencies of the gas phase hydride transfer reactions from alkanes to NOr have been... 

Stone CL, Bosron WF, Dunn MF. Amino acid substitutions at position 47 of human beta 1 beta 1 and beta 2 beta 2 alcohol dehydrogenases affect hydride transfer and coenzyme dissociation rate constants. J Biol Chem 1993 268 892-899... 

Table 7.5 Rate constants for hydride transfer from metal hydrides to Ph3C+ BF4 (CH2CI2 solvent at 25°C) [58, 93].

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

We estimate a rate constant of 10 -10 M s for the electron-transfer reaction and an E° for the rhodium-hydride couple that is similar to, or slightly less negative than, the E° value for the substrate. Our mechanism is summarized in Scheme I. 

The deuterium KIE values are generally in the range expected for linear three-center hydrogen transfer reactions,44107 and they track nicely with the rate constants for the reactions with the faster, more exothermic reactions displaying smaller KIEs. The large KIE value for reaction of the benzyl radical is noteworthy in that it exceeds the theoretical maximum for the classical model in a manner apparently similar to that seen with tin hydride (see below). 

Seymour and Klinman (entry 6 in Table 2 see Fig. 5 for a schematic mechanism) measured relative rate constants for the hydride-transfer reaction (H, D, T) from Cl of 2-deoxyglucose to the cofactor FAD. To explore the model for enzymically enhanced mnneling, according to which the enzyme conducts a fluctuational search for an efficient mnneling sub-state, five variants of glucose oxidase, anticipated to have differing capacities for the fluctuational search, were generated. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

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