Hydrogen kinetic isotope effect studie

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

We will use reaction 10.15 to illustrate two important concepts of kinetic isotope effect studies. The first concerns the relation between isotope effects of different isotopes of the same element, say D and T. We denote the rate constant of reaction 10.15 by kn and consider isotope effects when one hydrogen in the a-position is substituted by deuterium or tritium ... 

Current Issues in Enzymatic Hydrogen Transfer from Carbon Tunneling and Coupled Motion from Kinetic Isotope Effect Studies... 

An insight into the nature of the transition state was provided by deuterium kinetic isotope effect studies in which 1 /1 for conversion of 5-methyl-CPD to 1-methyl-CPD using the perdeuterio compound was used to determine A . Here, log /l = -1.0 - 2450/2.3RT, which indicates hydrogen, and not deuterium, tunneling although the large temperature dependence was also attributed to a non-linear transition state with nearly complete loss of one C-H bending mode (Scheme 6.2). ... 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

The first term was found to correspond to the rate of enolisation (measured by an NMR study of hydrogen-deuterium exchange at the methylene group). The second term predominates at [Cu(II)] > 10 M and is characterised by a primary kinetic isotope effect of 7.4 (25 °C) and a p value of 1.24. Addition of 2,2 -bipridyl (bipy) caused an increase in 2 up to a bipy Cu(II) ratio of 1 1 but at ratios greater than this 2 fell gradually until the enolisation term dominated. The oxidation of a-methoxyacetophenone is much slower but gives a similar rate... 

The hydrogenation of nitroacetophenones has been studied and considerable kinetic and mechanistic information obtained. Differences in reaction rate, bonding and selectivity have been observed. The formation of 1-indolinone from 2-NAP was unexpected and revealed the presence of a surface nitrene. This intermediate has not been postulated in nitroaromatic hydrogenation previously. Hydrogenation in the presence of deuterium revealed, as well as a kinetic isotope effect, that it is likely that... 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

Three mechanisms have been proposed for this reaction (Scheme 21). The reaction is first order in each of the reactants. In another study, Reutov and coworkers159 found a large primary hydrogen-deuterium kinetic isotope effect of 3.8 for the reaction of tri-(para-methylphenyl)methyl carbocation with tetrabutyltin. This isotope effect clearly demonstrates that the hydride ion is transferred in the slow step of the reaction. This means that the first step must be rate-determining if the reaction proceeds by either of the stepwise mechanisms in Scheme 21. The primary hydrogen-deuterium kinetic isotope effect is, of course, consistent with the concerted mechanism shown in Scheme 21. 

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