Kinetic isotope effects, catalysis rates

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

Some diazonium couplings are subject to base catalysis and in these cases kinetic isotope effects are observed but since the rate of catalysed reaction is not linearly related to the base concentration (see p. 7), the SE3 mechanism is ruled out and the Se2 mechanism must operate, viz. 

Jacobs, G., Khalid, S., Patterson, P.M., Sparks, D.E., and Davis, B.H. 2004. Water-gas shift catalysis Kinetic isotope effect identifies surface formates in rate limiting step for Pt/ceria catalysts. Appl. Catal. A Gen. 268 255-66. 

Lectka and co-workers (252) subsequently extended this system to the catalysis of the imino ene reaction. This reaction proceeds in low conversion albeit good selectivity in dichloromethane. The optimal solvent proved to be benzotrifluoride (BTF), possessing solubility properties similar to dichloromethane while accelerating the rate of the ene reaction presumably due to its aromaticity. A variety of 1,1-disubstituted alkenes participated in the ene reaction, providing amino acid derivatives in high yields and selectivities (85-99% ee). Evidence for the concerted nature of this reaction was provided by a high primary kinetic isotope effect (ku/kr) = 4.4). 

Using the various simplifications above, we have arrived at a model for reaction 11.9 in which only one step, the chemical conversion occurring at the active site of the enzyme characterized by the rate constant k3, exhibits the kinetic isotope effect Hk3. From Equations 11.29 and 11.30, however, it is apparent that the observed isotope effects, HV and H(V/K), are not directly equal to this kinetic isotope effect, Hk3, which is called the intrinsic kinetic isotope effect. The complexity of the reaction may cause part or all of Hk3 to be masked by an amount depending on the ratios k3/ks and k3/k2. The first ratio, k3/k3, compares the intrinsic rate to the rate of product dissociation, and is called the ratio of catalysis, r(=k3/ks). The second, k3/k2, compares the intrinsic rate to the rate of the substrate dissociation and is called forward commitment to catalysis, Cf(=k3/k2), or in short, commitment. The term partitioning factor is sometimes used in the literature for this ratio of rate constants. 

Any observable effect of isotopic substitution on the rate or extent of a chemical/physical process. Equilibrium isotopic perturbation measurements can provide valuable information about kinetic isotope effects on enzymic catalysis. NMR shift difference measurements are also useful in detecting the effects of isotopic substitution on a fast (degenerate) equilibrium between two species differing only in their specific isotopic substitution . The... 

Isotope effects have also been applied extensively to studies of NAD+/NADP+-linked dehydrogenases. We typically treat these enzymes as systems whose catalytic rates are limited by product release. Nonetheless, Palm clearly demonstrated a primary tritium kinetic isotope effect on lactate dehydrogenase catalysis, a finding that indicated that the hydride transfer step is rate-contributing. Plapp s laboratory later demonstrated that liver alcohol dehydrogenase has an intrinsic /ch//cd isotope effect of 5.2 with ethanol and an intrinsic /ch//cd isotope effect of 3-6-4.3 with benzyl alcohol. Moreover, Klin-man reported the following intrinsic isotope effects in the reduction of p-substituted benzaldehydes by yeast alcohol dehydrogenase kn/ko for p-Br-benzaldehyde = 3.5 kulki) for p-Cl-benzaldehyde = 3.3 kulk for p-H-benzaldehyde = 3.0 kulk for p-CHs-benzaldehyde = 5.4 and kn/ko for p-CHsO-benzaldehyde = 3.4. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

In Guo, after the very fast protonation of the electron adduct by water at the heteroatom [k > 107 s 1, von Sonntag 1991 Candeias et al. 1992 at 0(6), N(3) or N(7), cf. reaction (180)], a rapid transformation occurs [reaction (181) k in H20) = 1.2 x 106 s k(in D20) = 1.5 x 10s s 1] which is also catalyzed by phosphate buffer (k = 5.9 x 107 dm3 mol-1 s 1) which has been attributed to a protonation at C(8) (Candeias et al. 1992). This assignment is based upon solid-state EPR data, where C(8)-H--adduct is the thermodynamically most stable H -adducl radical (Rakvin et al. 1987 for DFT calculations see Naumov and von Sonntag, unpubl. results). The high solvent kinetic isotope effect of ku/ko = 8 is a strong indication that a proton is transferred in the rate-determining step. The magnitude of the rate of phosphate buffer catalysis points to a protonation at carbon (for a similar reaction observed with the Thy radical anion see Table 10.20). The C(8)-H -ad-duct has a pKa value of 5.4 [equilibrium (182)]. 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

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... 

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