Moderator isotope effect

With [2H4]ethylene glycol as a substrate one observes a moderate isotope effect kn/k1H = 2) on the Kmax of the dioldehydratase reaction. This is much less than the value of ca. 10 observed with 1,2-propanediol. The application of partially de-uterated substrates allows one to measure an isotope effect arising through intramolecular competition between otherwise homotopic groups. Such intramolecular isotope effects can be substantiated by appropriate product analysis for deuterium. 

Unlike most other electrophilic substitution reactions, sulfonation shows a moderate isotope effect ordinary hydrogen (protium) is displaced from an aromatic ring about twice as fast as deuterium. Docs this mean that sulfonation takes place by a different mechanism than nitration, one involving a single step Almost certainly not. 

For the thermolysis of tra s -[PdEt2(PR3)2] the results are very different The reactions are retarded only slightly by added phosphine, and no H-D scrambling occurs, suggesting that the /1-H elimination is irreversible in this case. Thus the decomposition of tra 5-[Pd(CH2CD3)2(PR3)2] evolved only a 1 1 mixture of CH2=CD2 and CD3CH2D, and showed a moderate isotope effect (%/fco =... 

Moderator isotope effect due to different moderation efficiency on different molecules... 

Of these isotope effects, (1) moderator isotope effect and (3) average reaction energy isotope effect can be eliminated when experiments are performed at a high concentration of rare gas moderator. Therefore, use of a large amount of rare gas moderator (e.g., 90% Ar) is recommended to observe (2) the probability integral isotope effect, which is the most interesting one for hot atom chemists. 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

Oxidation of Se(0) is unlikely to involve Se isotope fractionation. Se(0) is extremely insoluble and elemental Se precipitates are often found in moderately reducing environments. As solid Se(0) is consumed by an oxidation reaction, any kinetic isotope effect is ultimately negated by mass balance effects. For example, if a strong kinetic isotope effect preferentially removes lighter isotopes from the surface of the solid, the surface becomes enriched in heavier isotopes. Ultimately, the removal of successive layers from the solid requires 100% oxidation of the Se(0) and thus there is no opportunity for any kinetic isotope effect to be expressed. 

Intermolecular reactions of propargylic alcohols with a-methylstyrene gave the corresponding 1-hexene-5-ynes in moderate yields with complete regioselectivity (Scheme 7.30). The incorporation of a deuterium atom at the C-6 position (acetylenic terminal carbon) of the product and a substantial isotope effect (kH/fco = 4) were observed when a-methylstyrene-methyl-dj was used in place of a-methylstyrene. It is considered that the Cp-Cy double bond of an allenylidene complex reacts with a-methylstyrene, where the allenylidene complex works as an enophile, to afford the corresponding vinylidene complex via an allenylidene-ene reaction, as shown in Scheme 7.30. 

Another method of seeking evidence of the EIcBirr mechanism is to exam heavy-atom isotope effects in the leaving group. Of course, these should be much more significant in an E2 process because the bond is breaking in the transition state. For example, Thibblin and co-workersfound that in the base-induced elimination of an alkyl halide in which the p-carbon is unusually acidic (indene derivative, 12), moderately strong bases (triethylamine and methoxide) lead to a significant Cl/ Cl isotope effect = 1.010 1.009, where a maximum effect of... 

Problem 11.6 Sulfonation resembles nitration and halogenation in being an electrophilic substitution, but differs in being reversible and in having a moderate primary kinetic isotope effect. Illustrate with diagrams of enthalpy H) versus reaction coordinate. ... 

One caveat that must be observed when comparing computed and experimental isotope effects is that experimental measurements can sometimes be for multistep reactions. When a particular elementary reaction is not rate-determining, that is, it is not the bottleneck in die overall process, then it does not matter whether or not that reaction has associated widi it a large KIE it will not influence the observed overall rate. A separate caveat with light atoms at low to moderate temperatures is that tunneling effects may play a significant role. 

The decarboxylation of 3-carboxybenzisoxazole (225 R = H, NO2) gives CO2 and (226). This reaction has been studied using 13C and 15N kinetic isotope effects.201 The isotope effects were modelled theoretically at the semiempirical and ab initio levels, but comparison of experimental and theoretical results shows that die former cannot be successfidly predicted by theory at the level of calculation employed. The kinetics of decarboxylation and deamination of DL-leucine by acidic permanganate in die presence of silver ion in moderately concentrated sulfuric acid is a two-stage process.202 The... 

The kinetics and the products of bromination of several substituted stilbenes with Bu4N+Br3 have been investigated in aprotic solvents at different temperatures and concentrations. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products the reaction followed a second-order rate law and inverse kinetic isotope effect Ap/Ap = 0.85 ( 0.05) was... 

It had been established several decades ago that the reaction of 1-chloro-l-ni-troethane with sodium nitrite in aqueous-alcohol medium is second order overall and first order in each reactant (Hawthorne 1956). 1-Deutero-l-chloro-l-nitroethane reacts more slowly than its lighter isotopomer. This means that the kinetic isotopic effect is observed. The reaction proceeds only in moderately alkaline media in strongly alkaline media it does not take place. Only those geminal halo nitro compounds, which carry hydrogen in the geminal position, can undergo the conversion. Based on these facts, Hawthorne (1956) suggested the Sn2 substitution preceded by the isomerization of the initial substrate into the aci-nitro form ... 

The erythro compound shows little or no kinetic isotope effect, but the threo compound has a moderate one, H/fcD 2-3-3-3, for both syn and anti processes. This suggests that an E2 process is involved. Eliminations from the cyclic bromides may produce trans alkenes by syn eliminations or by anti eliminations, if n 8 in(189). 

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